Food Selection and Feeding Behaviour of Baltic Sea Mysid Shrimps

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Food selection and feeding behaviour of Baltic Sea mysid shrimps

MAIJU VIHERLUOTO

Academic dissertation in Hydrobiology, to be presented, with the permission of the Faculty of Science of the University of Helsinki, for public criticism in the Lecture hall of the Department of Ecology and Systematics, P. Rautatienkatu 13, Helsinki, on March 16th 2001, at 12 noon.

HELSINKI 2001

This thesis is based on the following papers, which are referred to by their Roman numerals:

I Viherluoto, M., Kuosa H., Flinkman, J. & Viitasalo, M. 2000: Food utilisation of pelagic mysids, Mysis mixta and M. relicta, during their growing season in the northern Baltic Sea. – Mar. Biol. 136: 553-559.

II Viherluoto, M., Viitasalo, M. & Kuosa, H.: Growth rate variation in the pelagic mysid, Mysis mixta (Mysidacea); effect of food quality? – Submitted manuscript.

III Viherluoto, M. & Viitasalo, M. 2000: Temporal variability in functional responses and prey selectivity of the pelagic mysid, Mysis mixta, in natural prey assemblages. – Mar. Biol. (In press.)

IV Viherluoto, M. & Viitasalo, M.: Effect of light on the feeding rates of pelagic and littoral mysid shrimps: a trade-off between feeding success and predation avoidance. – Submit- ted manuscript.

V Engström, J., Viherluoto, M. & Viitasalo, M. 2000: Effects of toxic and non-toxic cyanobacteria on grazing, zooplanktivory and survival of the mysid shrimp Mysis mixta. – J. Exp. Mar. Biol. Ecol. (In press.)

Papers I and III are reproduced by the kind permission of Springer-Verlag and paper V of Elsevier Science. 4 5

Food selection and feeding behaviour of Baltic Sea mysid shrimps

MAIJU VIHERLUOTO

Viherluoto, M. 2001: Food selection and feeding behaviour of Baltic Sea mysid shrimps. – W. & A. de Nottbeck Foundation Sci. Rep. 23: 1-35. ISBN 951-98521-2-3 nid.; ISBN 951-45-9828-8 PDF Mysids are an important link in the energy flow between primary and secondary producers and fish in the Baltic Sea. The present work contributes to mysid research by investigating the feeding and diet change of pelagic mysids (Mysis mixta and M. relicta) during their most intensive growth period during summer and autumn. The effects of light on the feeding rates of the pelagic (M. mixta) and the littoral (Praunus flexuosus) mysids and the effects of cyanobacteria on the feeding efficiency and survival of M. mixta were also studied. Pelagic mysids fed on various food items during their growth period and the diet clearly changed from phytoplankton and benthic material to a more carnivorous and pelagic diet towards autumn. Both the size of the mysids and the availability of food influenced the diet composition. Mysids of less than 7 mm in length were inefficient in capturing and handling larger zooplankters. Thus, 7-8 mm was a threshold size for zooplankton feeding. The mysids, which had attained this size, increased their zooplankton utilisation and grew faster than the mysids which grazed mostly on phytoplankton. Thus, omnivorous feeding habit may reduce intraspecific competition and therefore reduce juvenile mortality. Different zooplankton taxa are important at different stages of the mysids’ life cycle. Small mysids fed mostly on rotifers and gradually shifted to feed on cladocerans and copepods. Al- though they are omnivorous, they did have some prey preferences. The most selected species were the cladoceran Evadne nordmanni, and the copepods, Eurytemora affinis and Temora longicornis. The preference for E. affinis appeared to be dependent upon true selection, since E. affinis has good escape ability and is therefore a difficult prey to capture. Acartia sp. was mostly rejected although it was abundant throughout the study period. Ingestion rates followed sigmoidal functional response curves (Holling type III), with saturation levels at between 400 and 500 mg C l-1 depending on the month. This indicates that mysids cannot saturate their zooplankton feeding in natural feeding conditions, unless they are able to detect and forage in denser zooplankton patches. Light had a strongly negative effect on the feeding rates of pelagic mysids compared to feeding in total darkness, whereas no such effect was found on the feeding rates of littoral mysids. The habitats of mysids and thus their adaptation to natural light conditions, differ, which explains their different feeding rates. Light increases the predation risk and pelagic mysids migrate to deeper water when light increases, while littoral mysids hide among the macroalgal vegetation. The behavioural patterns of pelagic mysids in the presence of light influenced their feeding. Mysids fed on cyanobacteria and were partly able to avoid the toxic strain, Nodularia spumigena. This might be an evolutionary adaptation in areas where cyanobacteria blooms are common. However, filaments of the cyanobacteria clogged the feeding appendages of the mysids and thus hampered their zooplankton feeding efficiency. Changes in the state of the Baltic Sea, such as eutrophication and changes in salinity level, may affect the plankton community and hence, the quality of food available to the mysids. Decreased salinity favours the prey species that mysids prefer, such as E. affinis and some cladocerans, though the increased occurrence of cyanobacteria blooms may alter their feeding success and decrease the quality of available food.

Maiju Viherluoto, Department of Ecology and Systematics, Division of Hydrobiology, P.O. Box 17, FIN-00014 University of Helsinki, Finland. 6

CONTENTS INTRODUCTION

INTRODUCTION ...... 6 Mysids – a link between lower trophic levels and fish Mysids – a link between lower trophic levels and fish ...... 6 Pelagic mysids: Mysis mixta and M. Mysid shrimps (Malacostraca, Peracarida, Mysi- relicta ...... 8 dacea) are common crustaceans which inhabit Littoral mysids: Praunus flexuosus ...... 8 various aquatic environments, including oceans, Northern Baltic plankton community and estuaries and other brackish water ecosystems food availability of mysids ...... 9 as well as freshwater lakes. They are highly adap- Previous studies on the feeding of tive species and therefore also good invaders of Baltic mysids ...... 9 Growth of mysids ...... 11 new areas (Ketelaars et al. 1999). Most of the Conceptual background ...... 12 species are marine (~95 %), some live in brack- Optimal foraging theory ...... 12 ish water and a few species occur in freshwater Functional responses ...... 12 environments. Furthermore, some have become Predation cycle...... 13 adapted to live in caves and wells and a few live Trade-offs ...... 14 in commensal association with other animals. OBJECTIVES OF THE STUDY ...... 15 Some species burrow into the sediment, live just above it or migrate between bottom and surface MATERIAL AND METHODS ...... 16 waters, a few are strictly pelagic species and Study area ...... 16 some live in shallow water in the littoral zone Sampling ...... 17 (Mauchline 1980). Field studies ...... 18 In the Baltic Sea, there are currently at least Laboratory experiments ...... 18 Statistical analyses and equations ...... 18 20 species of mysids, of which over half live only in the area near the entrance to the Baltic RESULTS AND DISCUSSION ...... 19 Sea, where the salinity is close to oceanic salin- Food utilisation during growth ...... 19 ity levels (Köhn 1992). Only 7 species occur east C:N ratio ...... 21 or northwards of the Arkona Sea. Mysis mixta Selective feeding ...... 22 Lilljeborg and the two sibling species of M. rel- Effects of environmental factors on feeding success ...... 24 icta Lovén (I and II; Väinölä 1986), are pelagic Abiotic factors: the effect of light ...... 24 species. The other four, Neomysis integer Biotic factors: the effect of (Leach), Praunus flexuosus (Müller), P. inermis cyanobacteria ...... 25 (Rathke) and Hemimysis anomala G.O. Sars, which is a recent invader from the Pontocaspian CONCLUSIONS ...... 26 region to the northern Baltic (Salemaa & Hieta- ACKNOWLEDGEMENTS ...... 28 lahti 1993), live more or less in the littoral zone, among macroalgae, in crevices along rocky REFERENCES ...... 29 shores, or on sandy beaches (Fig. 1). The distribution of mysids is mainly regulated by salinity, temperature and the depth of the water column, and they seem to avoid areas where oxygen concentration is low at the bottom (Ackefors 1969, Salemaa et al. 1986). Mysids utilise a diversity of foods during their life cycle, which spans from a few months to two years (e.g. Lasenby & Langford 1973, 7

Fig. 1. Northern Baltic Sea mysid species in their natural habitats as constructed by H. Salemaa, University of Helsinki. Art by J. Flinkman.

Mauchline 1980, Grossnickle 1982, Rudstam et populations (Hansson et al. 1990a, Rudstam et al. 1989, Toda & Wada 1990, Kjellberg et al. al. 1992, Thiel 1992, 1996, Johannsson et al. 1991, Hakala et al. 1993, Cartes & Sorbe 1998, 1994, Aaser et al. 1995, Almond et al. 1996). In Chapman & Thomas 1998, Branstrator et al. the northern Baltic, during autumn, mysids and 2000). They have species-specific feeding modes planktivorous fish have been shown to consume (Mauchline 1980) and some species can switch over 50% of the zooplankton production (Hans- from one feeding mode to another when food son et al. 1990a, Rudstam et al. 1992). Further- availability changes (Viitasalo & Rautio 1998). more, in autumn they compete for food with fish They feed on small particles such as phytoplank- and may thus have the potential to influence the ton, rotifers, small cladocerans and detritus, by food gain of other pelagic zooplanktivores (Rud- creating a suspension feeding current, or feed stam & Hansson 1990). raptorially, i.e. actively capturing selected prey Mysids are prey for many larger predators from the environment. By utilising both pelagic globally, such as invertebrates, various fish (Thiel and benthic food sources, they provide an ener- 1996, Hostens & Mees 1999), birds and seals gy link between these environments. Together (Mauchline 1980), thereby linking primary and with planktivorous fish, e.g. herring (Clupea secondary production to higher trophic levels. harengus) and sprat (Sprattus sprattus), mysids In the Baltic Sea, mysids are eaten, for exam- have a strong influence on Baltic zooplankton ple, by adult herring (Aneer 1980, Aro et al. 8

1986, Rudstam & Hansson 1990, Flinkman et of Finland, while M. relicta is dominant in the al. 1991, Arrhenius & Hansson 1993), perch Bothnian Bay (Salemaa et al. 1986, 1990, Simm (Perca fluviatilis), smelt (Osmerus eperlanus) & Kotta 1992). In the northern Baltic, both spe- (Thiel 1996) and also by benthic fish, such as cies favour deep (>50 m) and cold water with a turbot (Scophthalmus maximus) (Aarnio et al. high oxygen content. In the southern parts, M. 1996). In the Baltic pelagic mysids are a good mixta is also found in more shallow areas (Sale- food resource due to their high abundance and maa et al. 1990). M. relicta is sensitive to sud- energy content (Wiktor & Szaniawska 1988). On den temperature changes and therefore stays in the other hand, their distribution is apparently deeper, more stable water (Holmquist 1962). patchy (Salemaa et al. 1986), which may affect Mysis-species are nectobenthic crustaceans their availability to predators. which perform diurnal vertical migrations. They Mysids are excellent experimental organisms. remain near the bottom during daytime and rise They are easy to collect with a net or an epiben- at dusk towards surface waters to forage. At thic sled, if the areas where they are abundant dawn, the mysids descend to escape visual pre- are known. They are large and durable and rela- dation by fish such as herring (Mauchline 1980, tively easy to handle and remain in good condi- Bowers & Vanderploeg 1982, Grabe & Hatch tion in the laboratory for a long period. Their 1982, Rudstam et al. 1986, 1989). Pelagic omnivorous feeding habits also make them po- mysids are adapted to living in a dark environ- tentially good species for food selection and prey ment and their eyes are easily damaged by strong switching studies. My studies concentrate on the light (Lindström 2000). Thus, the main regulat- common mysids in the Baltic, the pelagic Mysis ing factor for this vertical migration is light and mixta and M. relicta and the littoral Praunus flex- mysids are shown to avoid light levels exceed- uosus. ing 10-4 lux (Rudstam et al. 1989).

Pelagic mysids: Mysis mixta and M. relicta Littoral mysids: Praunus flexuosus

The present distribution of the species reflects Praunus flexuosus is of north-Atlantic origin and their biogeographical history. Mysis relicta is a belongs to the marine-euryhaline and eurytherm glacial relict (Segerstråle 1957, Holmquist species. It can tolerate salinities from 3.5 to 37 1962), inhabiting both brackish and freshwater ‰ (McLusky 1979) and temperatures from 3 environments in the northern hemisphere. The to 22 ºC (Välipakka 1990). P. flexuosus are com- Baltic M. relicta have been subdivided, on the monly found from the southern Baltic to the basis of electrophoretic findings, into two sib- northern parts, with the exception of Bothnian ling species, that partly co-occur in the northern Bay (Köhn 1992). In the northern Baltic Sea, Baltic Sea (Väinölä 1986). The M. relicta that they live in salinities from 3 to 7 ‰ and in are found in the study area belong to sibling spe- temperatures between 4 and over 20 ºC. They cies II (Väinölä 1986). M. relicta is most abun- occur in shallow water, in inshore habitats, main- dant in the northern Baltic and is not regularly ly among Fucus vesiculosus and Zostera mari- found to the south of 56°N nor to the west of na vegetation, where they form small shoals 18°30´E (Salemaa et al. 1990). M. mixta is of (Hällfors et al. 1975, Välipakka 1990). P. flexu- Atlantic origin and favours more saline water osus migrates horizontally in late summer from than M. relicta. In the Baltic, M. mixta is widely shallow (0-1 m) to deeper water (5-15 m), to distributed except for in the Bothnian Bay, where avoid warm temperatures (>20 ºC). They over- low salinity limits its distribution (Köhn 1992). winter in deeper areas and migrate back to in- It dominates the Mysis-populations in the Gulf shore habitats in spring after the ice break-up. 9

Macroalgal vegetation offers Praunus spp. a all zooplankters can use (Reinikainen et al. 1995, good feeding ground with various phyto- and Koski et al. 1999a, Engström et al. 2000). There- zooplankton species (Nordström 1997). P. flex- fore, the abundance of cyanobacteria in late sum- uosus swim in small swarms and rest in an up- mer does not improve the food availability for right position among algae (Mauchline 1980). mysids in the Baltic. They follow algal vegetation zones in their dis- In early summer, after the spring bloom, the tribution but they are also to be found on bare first zooplankton taxa which increase in num- sand and sandy mud bottoms (Välipakka 1990). bers are the rotifers (Lignell et al. 1993). Ther- mal stratification during summer leads to an increase in zooplankton biomass in the pelagic Northern Baltic plankton community and food zone. The rotifers are followed by cladocerans availability of mysids and copepods, which are most abundant in warmer waters (Viitasalo et al. 1995, Koski et The plankton community, including both phy- al. 1999c). For zooplanktivorous mysids the food to- and zooplankton, changes with the seasons. availability is thus good throughout the summer Most of the phytoplankton species show great period, until waters start to cool down in late year-to-year variations, which cannot be direct- autumn. ly associated with changes in the hydrography The most abundant mesozooplankton species and nutrient levels (Kononen & Niemi 1984). in the northern Baltic are the rotifer Synchaeta However, some trends are obvious in the suc- baltica, the cladoceran Bosmina longispina mar- cession of species. In spring, the phytoplankton itima and the copepods Acartia spp., Eurytemo- is composed of large diatoms and dinoflagellates, ra affinis and Temora longicornis (Hernroth & which form strong spring blooms in the surface Ackefors 1979, Viitasalo 1992, Viitasalo et al. waters (Niemi 1975, Kononen & Niemi 1984, 1995, Uitto et al. 1997, Koski et al. 1999c). Heiskanen 1995). After the bloom, vegetative These zooplankters perform vertical migrations cells and resting cysts of diatoms and dinofla- within the upper water layer during summertime. gellates settle (Heiskanen & Kononen 1994, In the Gulf of Finland, the migration is mainly Kremp & Heiskanen 1999) and constitute a regulated by light (Burris 1980). The grazing major food source for benthic animals (Kupari- activity of the dominant cladocerans and cope- nen et al. 1984), including mysids. During the pods also shows variation between day and summer, sedimentation is at its lowest, while nighttime. It is most active during night, in the autotrophic and heterotrophic pico- and nano- upper water layer, where edible food for zooplankton become dominant in the pelagial (Nie- plankters is abundant (Uitto 2000). The vertical mi 1975). Thus, summertime is favourable for migration of zooplankters affects pelagic mysids pelagic feeding of mysids, whereas suspension that also migrate in search of food. feeding on detritus at the bottom is more difficult, due to low sedimentation. In late summer, the occurrence of filamentous cyanobacteria in- Previous studies on the feeding of Baltic creases, of which the most common species are mysids Aphanizomenon flos-aquae, Nodularia spumigena (Sivonen et al. 1989) and N. sphaerocarpa Most of the studies on mysid feeding have dealt (Lehtimäki et al. 2000). When the weather is with freshwater M. relicta (e.g. Cooper & Gold- calm, water warm and phosphorus available, man 1980, Lasenby & Fürst 1981, Bowers & cyanobacteria may form massive, potentially Vanderploeg 1982, Johannsson et al. 1994, Al- toxic blooms (Kononen et al. 1996). Cyanobac- mond et al. 1996), which have been introduced teria are known to be poor quality food that not to many large lakes to increase fish production 10

Table 1. Earlier studies on the diet and prey selection of mysids (Mysis mixta, M. relicta and Praunus flexuosus) in the Baltic Sea. Only results concerning feeding of mysids are included in the table, other parts of the papers’ results are excluded. zpl = zooplankton, phytopl = phytoplankton, NBp = Northern Baltic proper, GF = Gulf of Finland.

(Lasenby et al. 1986). In the Baltic Sea, the stud- tasalo et al. (1998) investigated the predatory ies have mostly concentrated on M. mixta. The abilities of N. integer and Viitasalo and Rautio first studies analysed the diet from dissected (1998) the functional responses and prey selec- stomach samples (Rudstam et al. 1989, 1992, tion of P. flexuosus. Hansson et al. 1990b) and subsequent ones have The main conclusion of studies on mysid di- mainly dealt with experimental work on func- ets is that they are omnivorous and capable of tional responses (Mohammadian et al. 1997) and utilising a wide variety of food sources, depend- on factors affecting feeding rates of M. mixta ing on food availability. On the other hand, at (Gorokhova & Hansson 1997, Hamrén & Hans- least M. mixta and P. flexuosus are predominantly son 1999). The next phase in the diet studies were carnivorous and discriminate between cladocer- investigations with stable isotope analyses and ans and copepods. Table 1. shows the previous isotope fractionation, which enable reconstruc- Baltic feeding studies of the three mysid spe- tion of the diet from muscle, exuvia and faeces cies: M. mixta, M. relicta and P. flexuosus. My samples (Rolff et al. 1993, Hansson et al. 1997, studies add to the previous body of knowledge Gorokhova & Hansson 1999). the aspect of seasonal change in the diet, prey Feeding studies on other Baltic mysid species selection and in the effect of environmental fac- are scarce. Uitto and co-workers (1995) studied tors on feeding success. In addition, the influ- the predation rates of Neomysis integer and ence of cyanobacteria on the feeding and sur- Nordström (M. Sc. thesis, 1997) the diet of N. vival of mysids is studied for the first time. integer, Praunus inermis and P. flexuosus. Vii- 11

Growth of mysids do not carry broods during the winter (Salemaa, H., University of Helsinki, personal communi- Crustaceans periodically moult their whole exo- cation). skeleton during growth. The moult-cycle has In the northern Baltic, winter is an unproduc- profound effects on many aspects of the func- tive time and the mysid population is at its min- tion of the animal. In northern regions, such as imum (Salemaa et al. 1986, Rudstam & Hans- in the Baltic, low temperatures may inhibit son 1990). The maximal growth rate is only 1% moulting, and hence growth, during late autumn of the body weight for M. mixta and this decreas- and winter (Mauchline 1980). Juvenile mysids es continuously during autumn and winter until grow by shedding their exoskeleton at intervals April, when it is only 0.05% of the body weight which become progressively longer as they ap- for females (Gorokhova 1998). proach maturity (Clutter & Theilacker 1971). Mysids can utilise many food sources and food Ingested food is partitioned to various com- abundance might be a less important factor reg- ponents of growth. In freshwater Mysis relicta, ulating their growth than food quality. There is about 14 % of the ingested food goes to somatic no information available on the effects of food growth, 4 % to reproduction, 67 % to respira- quality on the growth of mysids but general tion and 15 % to moulting and egestion (Mauch- trends can be derived from studies on copepods line 1980). Partitioning depends upon the ambi- and cladocerans. The effects of food quality upon ent temperature as well as on the species and the growth rates of marine invertebrates can be sex of the animal. When the bodyweight of the measured by many different criteria, for exam- mysid increases, the energy consumption also ple, by the chemical (C:N or C:P ratio) or min- increases. The energy consumption is 1.5 times eral content of food or by its toxicity (Kiørboe higher for adults than for juveniles (Gorokhova 1989, Jónasdóttir 1994, McKinnon 1996, Sand- 1998). As the mysid grows, however, the energy ers et al. 1996, Lindley et al. 1997, Koski et al. losses to metabolism increase faster than con- 1999b). sumption, resulting in a smaller proportion of Maintenance, growth and reproduction de- the energy being available for growth (Gorokho- mand different food qualities. Maintenance me- va 1998). tabolism requires primarily energy, while growth In the northern Baltic Sea, pelagic mysids requires many other essential elements (Sterner (Mysis mixta and M. relicta) usually have a one- & Robinson 1994). Food quality affects the year life cycle (Rudstam et al. 1986). The young growth of marine crustaceans via moulting and are released in early spring, after the ice break- weight (Chen & Folt 1993, McKinnon 1996). up. New juveniles grow during summer and au- Poor quality food is noted as being almost as tumn and start to breed during late autumn. bad as starving in regard to growth, if no other Ovigerous females carry their brood for 4 to 5 supplementary food is available (Chen & Folt months and begin to release them during early 1993). Food that is known to be toxic or of poor spring. The growth of the juveniles is most rap- quality, if offered alone, may however, be a use- id during summer, when adults are scarce and ful supplement in mixed diets. Koski et al. food is abundant (Salemaa et al. 1986, Rudstam (1999b) found that a toxic prymnesiophyte con- & Hansson 1990). tained some specific, nutritionally important In the northern Baltic, the littoral mysid Prau- components which were lacking from other al- nus flexuosus usually produces one summer gen- gae and thus copepods produced more eggs on eration per year. However, several broods are a mixed diet than on any of the algae alone. Sim- produced, because embryogenesis in littoral ilar results are also shown with mixed diets con- mysid marsupium only takes about three weeks. taining cyanobacteria and diatoms (Schmidt & In contrast to the Mysis species, littoral mysids Jónasdóttir 1997). Thus, omnivory may be a 12 good strategy to optimize nutritional needs for Mott & Moxter 1991). Optimal foraging in- both growth and reproduction. cludes, in addition to the selection of the best prey items, also the choice of the best feeding techniques and foraging locations. To maximise Conceptual background the net energy gain throughout its life cycle, the forager should modify all these choices accord- The function of an aquatic system depends on a ing to changing conditions (Hughes 1980). complex web of interactions that includes both Mysids are omnivorous and may therefore se- numerical and behavioural responses of both lect prey items and feed according to the opti- predator and prey species. Predation directly mal foraging theory. influences the next trophic level and its effects may ‘cascade’ to the trophic levels that are further away (Carpenter et al. 1985). In feeding Functional responses theory, the most important aspect is optimisa- tion; how can animals optimise their feeding Changes in prey densities affect predator’s con- towards the maximum energy gain, which can sumption rates and this relationship is known as then be allocated to maintenance, growth and the predator’s functional response (Solomon reproduction, which all demand different food 1949). Different responses were classified into qualities? The concepts which are relevant to my three types by Holling (1959). The feeding re- studies are briefly reviewed below, mainly from sponse adopted by a predator in relation to the the point of view of a predator. abundance of prey is important for the stability of predator-prey relationships. These three types of functional response may have either stabilis- Optimal foraging theory ing or destabilising effects on the population dynamics of prey species. All the functional re- Most animals have the capability to consume a sponse types have a phase of increasing inges- wider range of prey than they actually choose. tion and at a certain prey concentration, the feed- According to the optimal foraging theory ing saturates (Fig. 2). Type II functional response (MacArthur & Pianka 1966, Hughes 1980), a occurs if the time spent handling the prey deter- predator should maximise the overall net ener- mines the maximum ingestion rate (Chigbu & gy intake per unit of time. This is a question of Sibley 1994). Ingestion rate, therefore, smooth- whether to invest energy for searching for the ly approaches a plateau, determined by the most profitable prey or to eat everything ‘in the number of handling times that can be fitted into way’ and spend no energy on searching. The the total time available. At high prey densities, optimal forager balances these two alternatives type II and type III responses are similar. At low and, depending on the availability of different densities, the type III curve has an accelerating prey, selects the best prey (Landry 1981). Large phase, where an increase in prey density leads prey may be the best energetically, but they may to a more than linear increase in ingestion rate. also be the most difficult to catch, handle and A type III functional response may occur if there ingest (Pastorok 1981). Nutritional benefit can is switching between prey species (Gismervik also be maximised by food selection based on & Andersen 1997) or if the ability of the preda- the nutritional quality, which includes digesti- tor to capture prey increases with the number of bility and nutritional value. The optimal forager encounters with the prey (prey density) (Landry should discriminate against toxic food, e.g. cy- 1981). The optimal forager switches prey when anobacteria, since ingestion would be deleteri- the abundance of the most profitable prey de- ous even when other food is not available (De- creases, and switches to another prey, which has 13

Fig. 2. Idealised functional responses (types I, II, III and IV) (Wootton 1999). N = Number of prey ingested, N = Density A of prey. become the more abundant (Abrams 1986). A The predation cycle begins with the location of type IV functional response also exists (Fig. 2, the prey item. Visually hunting predators, such Wootton 1999), which was not included in the as fish, locate prey from a distance, whereas non- original classification by Holling (1959). Type visual predators such as mysids, locate their prey IV response is similar to type II but after the by mechano-reception, i.e. from hydromechan- plateau level has been reached, the type IV in- ical signals that the prey create when moving gestion curve starts to decrease (Wootton 1999). through the water (Zaret & Kerfoot 1975, Dren- This could be the type of response, for example, ner et al. 1978, Gardner 1981). Several physical for planktivores feeding on cyanobacteria, when and behavioural traits influence the location the continual feeding clogs the feeding append- process. Pelagic prey may reduce the possibili- ages of animals and thus starts to decrease the ty of being located by being small (O’Brien et feeding efficiency. al. 1976, Gardner 1981, Pastorok 1981, Gerrit- I studied the functional responses of M. mixta sen 1984, Greene 1986) and transparent (Thet- in relation to the changing natural prey assem- meyer & Kils 1995), by decreasing ingestion and blages from summer to autumn. Functional re- thus gut pigmentation (Tsuda et al. 1998, Cieri sponses for the total prey community indicate & Stearns 1999) and by moving smoothly and the overall feeding patterns of the studied pred- slowly through the water (Gerritsen 1984, ators but the influence on individual prey spe- Tiselius et al. 1993). Vertical migration to dark- cies cannot be elucidated. er water layers also decreases the risk of being detected by predators (e.g. O’Brien 1986, Lei- bold 1991). Predation cycle From the predator’s point of view, the encounter rate of prey is important and varies with The predation cycle consists of several steps, swimming speed (Evans 1989, Kiørboe & Viss- which combine to produce the outcome of the er 1999). Fast swimming prey are encountered predation trial (O’Brien 1986). Differences in more often and therefore they are also detected predation may be due to optimal choice by the more frequently (Gerritsen & Strickler 1977, predator or differing vulnerabilities of prey or Gerritsen 1984, Tiselius et al. 1993). When a both (Pastorok 1981). The optimal strategies and prey has been located, the predator has the choice prey also differ for predators having different of either pursuing it or continuing to search for feeding strategies (cruising vs. ambush preda- a better prey (Hughes 1980). If the predator de- tors; Gerritsen & Strickler 1977, Hughes 1980). cides to try to capture the prey, the pursuit and 14 attack phase start. After attacking the prey, the benefits for fitness gained from one process at escape response of the prey determines the suc- the expense of another (Colinvaux 1986, Stearns cess of the attack. Escape responses of prey dif- 1989, Begon et al. 1990). The most prominent fer very much (e.g. Lonsdale et al. 1979, Greene life-history trade-offs involve the cost of repro- & Landry 1985, Browman et al. 1989, Viitasalo duction. The trade-off may be intra-individual, et al. 1998). Cladocerans and rotifers generally between reproductive effort made by a female have weak escape responses compared to cope- in one season and the probability that she will pods and, therefore, they usually rely more on survive to the next season to breed again, or it being undetectable (Greene 1986). Capturing is may be intergenerational; between a female’s followed by handling and finally ingestion of reproductive effort and the probability that her the prey item. Prey may make it impossible for offspring will survive to the next season (Stearns the predator to handle and ingest it by, for ex- 1989). My study concentrated on intra-individ- ample, being spiny (Walls & Ketola 1989), ex- ual trade-offs. creting a mucus sheet (Kitchell & Carpenter Predators can influence prey communities 1986), growing a helmet (Havel 1986), or by through selective predation, affecting the behav- producing chemical defences (Scrimshaw & ioural patterns of prey and forcing them to avoid Kerfoot 1986). Chemical factors are also impor- predation by hiding or escaping more vigorous- tant when the predator suspension feeds on ally or by changing their habitats (Sih 1986). Pre- gae and tries to avoid toxic species. Avoidance dation avoidance is costly for prey (Power 1986) happens when an algal cell, filament or colony and thus less energy is left for other functions, has been captured and the predator discriminates such as growth, which in turn affects feeding against toxic forms and avoids their ingestion through body size. In aquatic systems some prey (DeMott & Moxter 1991). This is beneficial for species perform diel vertical migration (DVM) both the predator and the toxic algal population. to avoid predation (e.g. Zaret & Suffern 1976, Ramcharan et al. (1985) showed that the con- Ghan et al. 1998), which is energetically costly trolling factor of a mysid’s prey preference is (Lampert 1989, Dodson 1990, Fiksen & Carlot- the capture success of different prey. Mysids ti 1998). The costs of DVM are reduced growth prefer prey that move slowly and are therefore and fecundity (Pastorok 1981, Lampert 1989). easy to capture (Nero & Sprules 1986). Three However, DVM may also benefit migratory another factors in addition to mechanical capture imals. It has been suggested that DVM provides and handling efficiency are known to affect the a metabolic or demographic advantage and also prey selection of M. relicta: the vigour of prey that, by migration from the food rich surface escape response, predator-prey encounter fre- waters, predators give the phytoplankton com- quency and the availability of prey (Cooper & munity an opportunity to grow and recover from Goldman 1980). In my work, predation efficien- intensive foraging (Lampert 1989, review). cy and prey selection of the pelagic M. mixta Pelagic mysids migrate vertically through the were studied using natural prey assemblages. water column to minimise the risk of being eat- Also, the avoidance of toxic algae was studied en by visually hunting fish and to maximise food with toxic and non-toxic cyanobacteria. intake (Mauchline 1980, Bowers & Vanderploeg 1982, Rudstam et al. 1986, 1989). In my thesis the effect of light on the feeding rates of both Trade-offs pelagic and littoral mysids was studied and the probable trade-off between the minimisation of Life-history traits are often compromises. Indi- predation risk and the maximisation of feeding viduals need to decide whether to invest more was discussed. energy in one trait or another. Trade-offs are 15

OBJECTIVES OF THE STUDY on laboratory experiments on animals collected from the field. Study III deals with prey selec- Mysids are an important part of the Baltic food tion and functional responses in natural prey web and the zooplankton community. Their feed- assemblages during summer and autumn. I want- ing and ecology are studied mainly because of ed to know how the change in natural prey com- their importance as a food source for various fish position affects the predation rates and respons- species (Aneer 1980, Bowers & Vanderploeg es of mysids during their growth period. 1982, Aro et al. 1986, Rudstam et al. 1986, 1989, Light is an important environmental factor, Rudstam & Hansson 1990, Arrhenius & Hans- which clearly influences mysid behaviour by son 1993, Aarnio et al. 1996). Previous studies increasing the risk of predation by visual preda- have investigated their feeding and also their tors. In the fourth study (IV), the aim was to effect on the zooplankton community, upon determine the effects of light on both pelagic which they feed. This thesis contributes to our and littoral mysids’ feeding rates. I wanted to knowledge of mysids by taking into account the know if the response of mysids to increasing light seasonal aspect, which greatly affects feeding and predation risk would be different in differ- through the growth of the mysids and through ent habitats. the seasonal succession of plankton communi- In the fifth study (V) the effects of both non- ties. Therefore, I have concentrated my studies toxic (Aphanizomenon flos-aquae and Nodularia on the three seasons which cover the most effi- sphaerocarpa) and toxic strains (Nodularia cient growth period for both pelagic and littoral spumigena) of Baltic cyanobacteria on mysid mysids: spring, summer and autumn (Salemaa feeding and survival were studied. This has not et al. 1986, Rudstam & Hansson 1990, Aaser et been previously undertaken, and because cyano- al. 1995). bacteria blooms are a common phenomenon in I studied the diet, prey selection and growth late summer and seem to be increasing (Kahru of pelagic mysids, and also the effects of some et al. 1994), it is important to investigate their environmental factors (light and cyanobacteria) effects on common planktivores, including on mysid feeding rates by collecting samples mysids. A feeding experiment with better qual- from the field and by conducting experiments ity food (green flagellate Brachiomonas subma- in the laboratory. The first two studies are based rina) was also conducted, to see if the mysids on field data. Paper I is about the diet change of feed more actively on high quality food, than on the pelagic mysids, M. mixta and M. relicta dur- cyanobacteria. The approaches and experimen- ing their growth from spring to autumn. This was tal set-ups are summarised in Table 2. undertaken in order to gain knowledge of the food items actually consumed in natural conditions and how the diet changes. Stomach analyses have been done previously from Baltic M. mixta (Rudstam et al. 1989, Hansson et al. 1990b, Rudstam et al. 1992) but not from M. relicta. In study II, the influence of food quality (phyto- vs. zooplankton and benthic vs. pelagic food) on the growth rate of M. mixta was studied. It is important to take food quality into con- sideration when studying growth, because it also has a major influence on growth at the population level. The other three manuscripts are based 16

Table 2. Experimental designs and study purposes of the field and laboratory studies presented in the thesis. Exp. = experiment, Zpl = zooplankton, Phytopl. = phytoplankton.

MATERIAL AND METHODS es from the North Sea (Ackefors 1969, Seg- erstråle 1969). Saline water pulses occur irregu- Study area larly and quite rarely, depending upon meteorological conditions in the Danish Straits (Hän- The Baltic Sea is one of the largest brackish ninen et al. 2000). The large salinity gradient water areas in the world. It is a semi-enclosed between the Bothnian Bay in the north and the and shallow (mean depth 55 m) sea, surrounded Danish Straits in the south results in the estab- by a large catchment area. It is characterised by lishment of different species compositions. Spe- strong seasonality and vertical thermal and sa- cies inhabiting the Baltic Sea are mainly either linity stratification, partial ice-cover during win- of marine or fresh water origin, even though true ter and lack of tidal movements. Salinity is reg- brackish water species are also to be found. In ulated by river discharge and saline water puls- the brackish water, most of the species live at 17

halocline, the average salinity of the water is 6 ‰ (Kuparinen et al. 1984), and a thermocline is formed during the summers. In the sampling area, hydrographical variations are regulated by meteorological conditions and mesozooplankton community dynamics are regulated by changes in water temperature and salinity (Vii- tasalo et al. 1995). Pelagic mysids living in deeper water, mainly below the thermocline (Rudstam et al. 1989), were sampled from open exposed sea areas from the Ajax deep (59°43N, 23°13E) (depth 80 m) and Längden (depth 60 m), situated to the south of the Tvärminne Zoological Station (TZS), on the Hanko Peninsula (Fig. 3). The bottom is mainly soft, with a high organic content in the surface sediment. Littoral mysids were collected from a shallow, more sheltered area (mean depth 1-2 m), a rocky shore near the Zoological Station. The shoreline is rocky and the hard bottom mainly covered with Fucus vesiculosus vegetation.

Sampling

Sampling of the pelagic mysids, M. mixta and Fig. 3. Map of the study area showing the mysid sampling M. relicta, was done at nighttime, during dark- stations at Längden and Ajax and the zooplankton sam- ness, to prevent possible eye damage (Lindström pling station at Storfjärden, in the Gulf of Finland, north- 2000). Pelagic mysids were collected with a large ern Baltic Sea. TZS = Tvärminne Zoological Station. plankton net, with a mesh size of 0.5 mm, diam- eter of 0.8 m and length 3 m, which was lowered near the bottom and then lifted slowly to the surface. In studies IV and V, mysids were the limits of their distribution and often suffer also collected using an epibenthic sled, which from osmotic stress (Aniansson 1990). There- was drawn along the bottom for 10 minutes and fore, there are only a few species of both algae then slowly lifted up. Littoral mysids were col- and animals that live permanently in the Baltic lected with an arm net, which was pulled through and thus food webs are often shorter and less F. vesiculosus algae in the littoral zone (depth complex than in the oceans. 1-2 m) (IV). Studies for this thesis were undertaken at the The samples for the diet analyses (I and II) entrance to the Gulf of Finland, in the northern were preserved in 4 % buffered formaldehyde Baltic Sea (Fig. 3). The coastal area is charac- (final conc.) immediately after sampling. Mysids terised by thousands of islands and a very com- for the experiments (III, IV and V) were placed plex shore and bottom topography (Pitkänen into insulated containers with cold seawater from 1999). In the study area, there is no permanent below the thermocline. Within an hour, the 18 mysids were transported to a temperature-con- tribution of the food particles throughout the trolled room (13 °C), maintained in darkness duration of the experiments (III, IV and V). from 22.00 to 06.00. The mysids were gently Mortality experiments with toxic cyanobacteria transferred to 0.2 mm filtered seawater with a (Nodularia spumigena) were conducted in 2.2 l sieve and a pipette. The mysid species were iden- aquaria (V). The experiments were performed tified and kept in aerated filtered seawater within a temperature controlled room at 12-13 °C, out food, for 24 h before the experiments. with a 16:8 h light:dark cycle. The average light Zooplankton for the studies was collected us- level in the experimental bottles (IV) was 12 µE ing a 100 or 200 mm mesh zooplankton net from (1 µE = 6.02 × 1017 qu/m²/s), which corresponds the same place as the mysids (III) or from Stor- to the level near the thermocline in our study fjärden (I, IV and V), a 35 m deep archipelago area during summertime (Lindström, M., area (Fig. 3). A larger mesh-size net was used Tvärminne Zoological Station, personal commu- for the last studies (IV and V), because only nication). In the experimental bottles and aquar- copepods were needed in the experiments. ia, there was always one mysid per bottle and a counted/measured amount of zooplankton or algae (Table 2). Field studies

Field data was used in studies I and II for the Statistical analyses and equations stomach and growth analyses. First, all mysids were measured from the tip of the rostrum to Parametric tests were used when the assump- the end of the telson and their stage of sexual tions of normal distribution, homogeneity of maturity was recorded. Second, to identify the variances and independency of observations food particles in the stomachs, the mysids were were fulfilled or when the data could be trans- carefully dissected, the stomachs and their con- formed and thus meet these assumptions (Zar tents transferred onto a glass slide (Nordström 1999). These tests include two factor analysis 1997), and observed with an inverted microscope of variance (ANOVA) on log (x+1) transformed (100× to 400× magnification). 50 individual food data (IV), 1-way and 2-way ANOVA and regres- items were identified from each stomach. Mysis sion analysis (V). When parametric tests could mixta was abundant in every sample and 10 not be used, the analyses were done using non- stomachs were examined for each size class from parametric tests: Mann-Whitney U-test (I, V), June to September. In contrast, M. relicta was Wilcoxon signed ranks test (I, II, V), Fisher’s rare throughout the summer and therefore, all exact test (III), Spearman correlation test (III) of the M. relicta stomachs were studied. Alto- and the Scheirer-Ray-Hare test (V). gether 180 M. mixta and 74 M. relicta were an- In study II, the length distributions of mysid alysed. populations were separately studied for every sampling day, to elucidate their different growth lines. The best-fit distributions were counted for Laboratory experiments the mysid population using the MIX programme (an interactive program for fitting mixtures of Experiments were conducted in the laboratory distributions; Macdonald & Green 1988). The to reveal the prey selection patterns and the ef- program analyses histograms as mixtures of sta- fects of light and cyanobacteria on the feeding tistical distributions, that is, by finding a set of rates of mysids. Experiments were performed overlapping component distributions that gives in 1.18 l glass bottles in a slowly rotating (0.5 the best fit to the histogram. RPM), plankton wheel, to maintain random dis- 19

The equations, which were used in this thesis, The selectivity index was calculated for each are the following: prey group, to determine the selection intensity

The percentage overlap of diets (Pjk) of the for different prey, using the average abundance mysid species (I) was counted with the Schoen- percentages derived from the carbon contents of er overlap index (Schoener 1970): prey in the diet and in the environment.

( 1 ) RESULTS AND DISCUSSION where Pij and Pik are proportions of resource i (i.e. a certain food item/particle) of the total re- Food utilisation during growth sources used by (mysid) species j and species k or size classes, and n is the total number of re- Generally, mysids feed omnivorously on phyto- source states (i.e. all food particles). and zooplankton and also on benthic material Probable nitrogen limitation of mysids (II) was (I) when staying near the bottom during day- calculated according to Urabe & Watanabe light hours. Pelagic mysids (Mysis mixta and M. (1992) who showed how to estimate a theoreti- relicta) grow rapidly from being a few millime- cal maximum for food C:N ratio, above which ters in length in spring, to two centimeters in the consumer is nitrogen limited. The equation autumn and their nutrition changes along with is as follows: growth (I, II and III). Diet change may be due to two important reasons. First, the availability of

Q*c-e = Qz-e / Kc ( 2 ) different plankton groups in the Baltic Sea changes during the course of the year (Niemi where Q*c-e is the maximum elemental ratio of food 1975, Viitasalo et al. 1995, Uitto et al. 1997, (here C:N), Qz-e is the elemental ratio of the con- Koski et al. 1999c). Second, small mysids are sumer (C:N) and Kc is the gross growth efficiency less able to capture evasive zooplankton species of the consumer in carbon. We used a Kc of 0.22, than larger individuals (e.g. Cooper & Goldman derived from annual production and consumption 1980). Therefore the small size of the mysids, estimates (g C m-2 yr-1) for Mysis mixta in the north- together with the early summer’s plankton com- ern Baltic proper (Rudstam et al. 1986). munity, forces the diet to differ compared to that The chi-squared based selectivity index C by of large sized mysids’ with late summer/autumn Yate’s correction for continuity (Pearre 1982) plankton availability. During the first months of (III) was calculated for every prey group in the the life of a mysid, the food available mainly natural zooplankton assemblage, to find out the consists of diatoms, dinoflagellates and rotifers. selection intensity for different prey: The seasonal stomach content analyses showed clear changes in the utilised food. In c2 1/2 C = ± ( y/n) or June, the 4 to 7 mm long M. mixta foraged al- 2 1/2 C = ±[(|adbe-bdae| - n/2) / abde] ( 3 ) most exclusively on phytoplankton, mainly settled diatoms and other benthic phytoplankton where particles (I). This is consistent with the stomach analyses of M. relicta in Stony Lake (Lasenby Species & Langford 1973), which showed that small in- A Others Total dividuals eat only algae and detritus. Generally,

Diet ad bd ad + bd = d phytoplankton biomass is at its minimum in July

Environment ae be ae + be = e (Niemi 1976), whereas the abundance of

Total ad + ae = a bd + be = b ad + ae + bd + be = n cladoceran and copepod species is close to their maxima (Viitasalo et al. 1995). Therefore, 20 zooplankton availability is high for the mysids Comparison of the diets of pelagic Mysis-spe- that are capable of capturing them. Notably, the cies revealed a distinct difference (I). M. relicta share of copepods in the diet increased strongly utilised more phytoplankton and benthic food in August, simultaneously with their increased than M. mixta (Fig. 4). The difference was evi- abundance in the water (I). Also, the utilisation dent throughout the study period, but it was larg- of pelagic food increased steadily from early sum- est in the middle of the summer, in July and mer to autumn. This was probably due to the August, when M. relicta fed on average 90 % change in food availability, the growth of mysids on benthic material. The reason for M. mixta’s and the change in light conditions during the sea- more carnivorous diet could be its larger size in sons. In early summer, the water column is more the study area and the reason for its more pelag- illuminated and settled diatoms offer a good food ic feeding habits, its vertical migration to upper supply for small mysids. Towards autumn, the waters compared to the vertical migration of M. light level decreases, which further reduces the relicta (Salemaa et al. 1986). predation risk and the pelagic plankton community is feasible for large mysids.

Fig. 5. Frequency distributions of mysid lengths in Mysis mixta populations at the Ajax deep (80 m) from June to September 1997. The best-fit curves are drawn by hand according to the MIX programme (Macdonald and Green, 1988).

The growth rate of M. mixta varied during the study period (II). In June, the juvenile population had a unimodal size distribution but, in the middle of July, a part of the juvenile population started to grow faster (Fig. 5). These two different parts of the population had different diets; Fig.4. The monthly averages of (A) zooplankton: the smaller cohort fed on average 50 % on zoo- phytoplankton ratio and (B) pelagic:benthic ratio in the plankton and 6 % on pelagic material, and the diet of Mysis mixta and M. relicta. Symbols denote the larger ones 75 % and 27 %, respectively. Thus, means and vertical lines denote standard deviations. the difference is clear in both diet components. 21

At the beginning of July, when the two cohorts ture larger prey than smaller mysids have (Coop- started to grow at different rates, both cohorts er & Goldman 1980), which further separates changed their diets to a more zooplanktivorous their size distributions and, hence, their diets. In composition. However, the larger and more August, the mysids of the smaller cohort also zooplanktivorous cohort grew more rapidly than reached the threshold size for zooplankton feed- the smaller, less zooplanktivorous cohort (II). ing (freshwater, Mysis relicta >7 mm, Grossnick- The larger cohort kept its growth rate steady until le 1982), their growth rate consequently in- September, after which their growth slowed creased and in mid-September the two cohorts down. again united. Thus, at the beginning of Septem- The growth of mysids seems to be associated ber, the size frequency distribution was again with the amount of zooplankton in their diet. unimodal (Fig. 5). There was a fairly close rela- Animal food may nutritionally be of better qual- tionship between the pelagic feeding habits on ity for mysids than phytoplankton or detritus, as zooplankton and mysid size. Since the pelagic has been suggested for copepods (e.g. Conover particles in late summer and autumn were most- & Corner 1968, Corner et al. 1976, Heinle et al. ly zooplankton, we suggest that M. mixta need- 1977, Stoecker & Capuzzo 1990, a review). For ed to attain a threshold size in order to start ef- instance, it is known that green algae (Dunstan fective feeding on zooplankton. et al. 1992) and cyanobacteria (Reinikainen et This diversified feeding may be beneficial for al. 1995, Koski et al. 1999a) are not high quali- the mysids, because it is likely to reduce intraspe- ty foods compared to zooplankton or dinofla- cific competition for food (Hughes 1980) and gellates. Also decomposing benthic material, for thus increase survival during the growth period. example, detritus and diatoms, can be low quality Omnivorous feeding habits of mysids may also food compared to fresh pelagic material (e.g. benefit the plankton community since, as many Stoecker & Capuzzo 1990, Dittel et al. 1997, different species are fed upon, it is unlikely that Lehtonen 1997). Therefore, it is suggested that any of the prey species is foraged too intensive- the mysids that fed on pelagic food and zoo- ly for a long time. If certain prey become scarce, plankton grew more rapidly than the benthic mysids probably switch to other more abundant feeders and phytoplankton grazers. prey. I suggest that, at the beginning of July, mysids at the larger end of the size distribution started to be large enough to capture zooplankters, and C:N ratio thus gain more protein and amino acid rich animal food (Stoecker & Capuzzo 1990, a review). Generally, a low C:N ratio of food indicates good In contrast, the rest of the population continued food quality (Kiørboe 1989, McKinnon 1996, to feed mainly on phytoplankton (decomposing Lindley et al. 1997). However, while some stud- diatoms, cyanobacteria and green algae) and ies show a strong effect of both mineral and their growth rate remained lower than that of chemical composition of food on reproductive individuals already feeding raptorially, which success (Kiørboe 1989, Jónasdóttir 1994, Klep- may provide more energy per unit time than pel et al. 1998, Koski et al. 1998) or growth phytoplankton grazing. This may be because of (Sterner 1997, Schulz & Sterner 1999) of zoo- differences in migration behaviour. Some mysids plankton, in other studies such an effect has not have spent more time near the bottom, whereas been observed (e.g. Sanders et al. 1996). other mysids have migrated to the upper water A few studies have investigated nitrogen and column, where zooplankters are available. Af- carbon content of mysids. Donnelly et al. (1993) ter gaining this growth advantage, the larger measured the C:N ratio of three mysid species mysids have continually better chances to cap- (Eucopia sculpticauda, E. unguiculata, Gnath- 22 ophausia ingens) in the Gulf of Mexico. Com- in the same deep-water areas in the northern pared to these values (5.9 to 7.3), my values for Baltic Sea. Baltic M. mixta are very low (3.3 to 4.0) but similar to Gorokhova’s (1999) results for juvenile M. mixta (3.8) in the northern Baltic proper. I Selective feeding also attempted to estimate the nitrogen limitation of M. mixta. According to Urabe and Wa- By selective predation, invertebrate predators can tanabe (1992), it is possible to estimate a theo- influence zooplankton communities by control- retical maximum for the food C:N ratio, above ling population sizes and relative abundances of which the consumer is nitrogen limited. My re- prey (e.g. Dodson 1974, Murtaugh 1981, Bran- sults indicate that mysids are not limited by ni- strator 1995, Spencer et al. 1999). In the Baltic trogen in the northern Baltic and that the C:N Sea, mysid predation is considered to be an im- ratio of food does not explain the different portant factor affecting zooplankton communi- growth rates of the two cohorts of pelagic mysids ties (Rudstam & Hansson 1990, Rudstam et al. during summer (II). Food quality, and thus 1992, Thiel 1996). Mysids are omnivorous and growth, may also depend upon other essential their diet usually reflects the availability of dif- components, such as unsaturated fatty acids ferent food items (I), but consistent patterns of (Tang & Dam 1999, Anderson & Pond 2000). prey preference have also been detected (III, Knowledge of the role of essential fatty acids Rudstam et al. 1992). M. mixta selected differ- for mysids is lacking and therefore conclusions ent prey taxa during their growth period (III). about their influence on mysid growth cannot Small mysids do not have the capability of cap- be drawn. However, omnivory, which confers a turing the most evasive prey and therefore their higher probability of obtaining all the required ‘preference’ is probably based on apparent se- nutrients, probably provides a better quality diet lectivity, i.e. the escape ability of prey regulates for mysids than phytoplankton or detritus alone, their foraging (Greene 1986). Furthermore, cap- as several studies have shown with other marine ture of large prey requires faster swimming invertebrates (e.g. Conover & Corner 1968, speed, which requires more energy (Buskey Heinle et al. 1977, Gifford & Dagg 1988, 1998), therefore it is not beneficial for small Stoecker & Capuzzo 1990, review). mysids to try to capture large prey if the proba- The main conclusions of the studies I and II, bility of success is low. Small mysids fed main- are that both food availability and mysid growth ly on rotifers during early summer (III), which probably affect the diet composition of Mysis is probably due to the undeveloped predatory species in the northern Baltic Sea. The mysids abilities of these mysids (Lasenby & Langford that feed on pelagic food and zooplankton grow 1973) and could also be a consequence of rotif- more rapidly than the benthic feeders and phy- ers being the most abundant taxa in the water. toplankton grazers, which is consistent with ear- Rotifers do not perform strong escape jumps and lier findings concerning copepods (e.g. Heinle are probably captured by filter feeding current et al. 1977, Stoecker & Capuzzo 1990, review). (Viitasalo & Rautio 1998). Although the diverse Both of the pelagic mysid species are omnivo- phytoplankton mainly forms the diet of small rous and the same diet shift from phytoplankton juveniles (I), providing essential nutrients and and benthic material to zooplankton and pelag- fatty acids (Tang & Dam 1999, Anderson & Pond ic material occurs during mysid growth. How- 2000), rotifers are an important additional food ever, there is a clear difference between the di- (III) in regard to energy, when intensive growth ets of these species. M. mixta utilises more zoo- of the juveniles starts (II). plankton and pelagic food than M. relicta. This During summer and autumn, the main com- may reduce competition between mysids living ponent of the diet was copepods (I, III). Cope- 23 pods were abundant and after mysids had at- observation) and is therefore more easily detect- tained the threshold size of ~7 mm (Grossnickle ed by the mysid. 1982), also constituted feasible prey. Thus, Cladocerans were neither very abundant nor mysids feed on larger prey as they grow and their selected, with the only exception being the spe- physical capabilities develop, which is in accord- cies Evadne nordmanni, which is large compared ance with the optimal foraging theory which to the other cladocerans available (Bosmina long- states that, most of the time in nature, the net ispina maritima, Pleopsis polyphemoides). Our energy gain is of central importance (Hughes results indicate that, firstly, the predation suc- 1980). Mysids may also change their diet when cess mostly depends on prey escape capabilities a certain prey population becomes too scarce and and mysids’ ability to capture and handle prey find some other, more abundant prey instead but also that true selection exists for certain prey (Fulton 1982), thus optimising their energy in- species. Secondly, that different prey species and take. In lakes Tahoe and Michigan, M. relicta groups are important during different phases of changes its prey preference depending on the the mysids’ growth period. relative abundance of prey species available Changes in prey densities also affect the con- (Bowers & Vanderploeg 1982, Folt et al. 1982). sumption rates of predators, as described by M. mixta feeding was not solely based on the Solomon (1949) and Holling (1959). Some stud- availability of prey items (III). Selection was ies on the functional responses of mysids have evident during the summer, but in September and been performed in freshwater lakes (e.g. Folt et October there was not much difference between al. 1982, Chigbu & Sibley 1994) and in the Bal- the preferences for different prey species (Fig. 5 tic Sea (Mohammadian et al. 1997, Viitasalo & in III). During autumn, large mysids are appar- Rautio 1998), mostly concentrating on a few prey ently capable of capturing almost anything in species at a time. We studied the functional re- the water and this may explain the low degree sponses and ingestion rate of M. mixta with a of selection observed. The copepod, Temora natural zooplankton assemblage (III). The vari- longicornis, was selected from natural zooplank- ation in ingestion rates was best explained by ton assemblage even when it was relatively in- the sigmoidal functional response (type III, abundant. Reasons for this selection could be Holling 1959) curve, with explanatory levels of the large size of this copepod, which makes it 86 to 97 %. The sigmoid functional response interesting as a food item and also creates strong- may occur if the ability of the predator to cap- er hydrodynamic signals that non-visual preda- ture prey increases with the number of encoun- tors can detect (Drenner et al. 1978). The other ters with the prey (prey density, Landry 1981). positively selected copepod was Eurytemora The month of June was the only exception, when affinis, although it performs strong escape jumps the saturation levelled out already at a food con- and is considered a difficult prey to capture (Vii- centration of 50 mg C l-1 and the functional re- tasalo et al. 1998). The preference of M. mixta sponse did not fit properly to any of the types of for E. affinis shows true selection (Greene 1986), functional response curves. In June, the mysids despite the expectation of rejection due to its were small (average 5 mm) and their natural diet good escape ability. The third most common mainly consisted of phytoplankton (I). This was copepod, Acartia sp., was mostly rejected, which not offered in these experiments and therefore may also indicate true selection, i.e. the deci- the ingestion rate stayed at a very low level de- sion not to pursue. Acartia sp. are quite fast es- spite the increased zooplankton concentration. capers, which might be the reason for their re- The ingestion rate increased with increasing jection (Viitasalo & Rautio 1998). E. affinis zooplankton concentration, until the saturation swims more abruptly, creating larger hydro-me- level was reached. This level occurred at between chanical signals compared to Acartia (personal 400 and 500 mg C l-1, depending upon the month. 24

If we compare the ingestion rate of M. mixta of the risk of predation (e.g. Zaret & Suffern and the average zooplankton density in the Bal- 1976, Loose & Dawidowicz 1994). In contrast, tic (~40 mg C l-1, Mohammadian et al. 1997), littoral mysids are used to a very broad light spec- we can conclude that mysids cannot saturate their trum in shallow water and are therefore well feeding, unless they are able to detect and for- adapted to the light level (Lindström 2000). age in denser zooplankton patches. In dense The difference between mysids living in the patches, however the saturation is possible, since pelagial and in the littoral was clear from their zooplankton densities as high as 850 ind. l-1 have feeding rates in light and in darkness (IV). Lit- been observed in the southern Baltic (Kils 1992). toral mysids fed at the same rate despite the pre- vailing light conditions and, in addition, the feeding was not affected by changes in the natural Effects of environmental factors on feeding light conditions during the course of the seasons. success The treatments did not thus have any influence on the feeding rates of P. flexuosus, whereas the Abiotic factors: the effect of light body mass of mysids affected their feeding efficiency. Littoral mysids do not perform vertical Many physical factors, such as salinity, temper- migrations to avoid bright light but escape visu- ature (DeGraeve & Reynolds 1975, McLusky ally hunting fishes by hiding among the mac- 1979, Mauchline 1980) and oxygen concentra- roalgal vegetation. Predation avoidance does not tion (Ackefors 1969, Salemaa et al. 1986), have necessarily, therefore, interfere with the feeding a strong influence on the survival, distribution of these mysids, because they can continue cap- and behaviour of mysids. Environmental factors turing prey and suspension feeding among the also affect predation rates and prey-capture abil- algae. ity. Increased temperature is shown to increase In contrast, pelagic mysids were clearly affect- the movement and feeding rate of mysids up to ed by light during the experiments. They sup- a certain limit, after which their mortality starts pressed their feeding rate in light and fed at a to increase (DeGraeve & Reynolds 1975, Chipps higher rate in total darkness. M. mixta is shown 1998). However, the most important physical to be able to detect a chemical substance released environmental factor which governs the behav- by herring and then decrease its feeding (Ham- iour and distribution of mysids, is light (Mauch- rén & Hansson 1999). In our study, this could line 1980). In general, mysids are attracted to not be the reason for suppressed feeding, because weak sources of light but avoid bright light. no predators were kept in the experimental wa- Bright light often inhibits the swimming activi- ter. The reason for this suppressed feeding could ty (Mauchline 1980) and swarming behaviour be an endogenous reaction to avoid moving in (Steven 1961), and may damage their large, sen- well-lit water, even when predators are not sitive eyes (Lindström 2000). Light is an impor- present. tant factor controlling the vertical migration of The ingestion rate of M. mixta differed signif- pelagic mysids (e.g. Rudstam et al. 1989). It is icantly between the three experimental periods, usually assumed that mysids do not require light being lowest during early summer and highest to capture prey but rather use mechano-recep- in the autumn. However, the differences in feed- tion to locate moving plankton (Cooper & Gold- ing rates of pelagic mysids between experimen- man 1980, Murtaugh 1981, Viitasalo et al. 1998). tal periods were small when the ingestion rates If mysids benefit from hunting in the more illu- were calculated per dry weight of mysids. This minated, upper water column, there should be a shows that pelagic mysids feed at the same rate trade-off between the maximising of feeding rate relative to their body mass throughout their in the upper water column and the minimising growth period, despite the change in natural light 25 conditions with one exception. In July, the in- (N. sphaerocarpa and Aphanizomenon flos- gestion rate of M. mixta in light differed consid- aquae) and the effects of aggregate-forming cy- erably from other feeding rates, because no feed- anobacteria on the raptorial feeding success and ing occurred in this experiment. survival of pelagic mysids. Our studies show that At the Baltic latitudes, light conditions change M. mixta feed on both toxic and non-toxic cy- from the beginning of summer to late autumn anobacteria. The different strains of cyanobac- and so mysids are adapted to a decrease in light teria were morphologically similar, all occurring as the season changes. The duration of the verti- as single filaments since they were grown in cal migration changes through the course of the culture. Thus, it seems that mysids can recog- seasons. Pelagic mysids ascend later and descend nise the toxins, since the feeding rate on toxic earlier relative to sunrise and sunset during ear- cyanobacteria was always lower than that on ly summer than in the autumn (Teraguchi et al. non-toxic ones (V). Reduction of feeding rate 1975, Rudstam et al. 1989). This was expected when alternative food is not available seems to to be seen in the results of the pelagic mysids, be an adaptive behaviour, which is also found in as an increase in the difference of feeding rates copepods (Engström et al. 2000). Copepods are in dark and in light towards the autumn. How- known to select less strongly against low quali- ever, no such a trend was observed. ty food when nothing else is available, i.e. high In short, pelagic mysids are more vulnerable quality food is scarce or absent (DeMott 1995), to changes in light conditions than littoral mysids which may also explain the behaviour of the and neither of the mysid species benefited from mysids. Only rarely, is only a single food spe- increased light. This suggests that pelagic and cies available in nature. Even though cyanobac- littoral mysids rely on mechano-reception for teria is often the dominant species during inten- locating and capturing their prey, regardless of sive blooms, there is always something else to the habitat and its natural light conditions. feed upon. Only few filaments of cyanobacteria were found in the stomachs of mysids in 1997 (I), when cyanobacterial blooms were especial- Biotic factors: the effect of cyanobacteria ly intense. This shows that mysids can select between toxic cyanobacteria and other food and Pelagic mysids that migrate to the upper water concentrate on foraging for high quality food. column (Rudstam et al. 1989) frequently en- Nevertheless, the feeding rate on better quality counter cyanobacteria blooms which are com- food (the green flagellate, Brachiomonas sub- mon phenomena in the Baltic during late sum- marina) was not different from that on the cy- mer (Kahru et al. 1994, Kononen et al. 1996). anobacteria in the feeding experiment with M. As omnivorous animals, they probably ingest, mixta. The small B. submarina cells may also either actively or passively, cyanobacteria fila- be suboptimal food for mysids, as they have been ments, in addition to other algae and zooplank- observed to utilise larger algae, e.g. filamentous ton. Cyanobacteria blooms are often toxic (Sivo- diatoms in natural conditions (Bowers & Gross- nen et al. 1989) and therefore feeding on these nickle 1978, Mauchline 1980). strains may have significant effects on the mysid The avoidance of toxic food was also support- populations. Generally, mysids are shown to be ed by the mortality experiment, in which the tox- very sensitive to toxins such as dredge spoil, ic cyanobacteria, N. spumigena did not increase industrial waste (Nimmo & Hamaker 1982) and the mortality of adult mysids (V). The mortality pesticides (Robinson 1999) and therefore cyano- of mysids fed with daphnids in filtered seawater bacterial toxins may also reduce their survival. was not different from the mortality in the water We studied the feeding on both toxic (Nodu- with a bloom concentration of toxic cyanobac- laria spumigena) and non-toxic cyanobacteria teria. This supports the idea that mysids can ac- 26 tively avoid cyanobacteria filaments in the wa- they avoid toxic food when other food is availa- ter when better food is available. Another possible but it may also be because of tolerance bility is that they are resistant to the toxin, which against cyanobacterial toxins. Mysids can reduce has been demonstrated for the cladocerans Bos- their intake of toxins in nature by switching, se- mina longirostris and Moina macrocopa, as well lective feeding and by avoiding algal bloom ar- as the rotifer Brachionus calyciflorus (Stark- eas. Future studies should focus on toxin resist- weather & Kellar 1983, Hanazato & Yasuno ance in combination with feeding selectivity and 1987, Fulton 1988). Tolerance to toxins in Bal- the potential effect of cyanobacteria on the re- tic mysids could be an evolutionary adaptation production of mysids. in areas where toxic blooms are frequent and animals cannot totally avoid waters with high densities of cyanobacteria filaments. In order to CONCLUSIONS prove this hypothesis, we would need a comparison of mortality rates of animals in toxic The most important findings of my thesis are cyanobacteria water from an area where cyano- linked to the seasonal change in both food com- bacteria blooms are common (e.g. the Baltic Sea) position and mysid growth. The pelagic mysids, and from an area where toxic blooms do not M. mixta and M. relicta, fed actively on both occur (e.g. the Atlantic). phyto- and zooplankton and their diet changed Cyanobacteria also have other negative effects. remarkably from small juveniles to mature We observed that M. mixta fed on Acartia sp. at adults, though being omnivorous at all stages of a lower rate in the presence of aggregate-form- their life cycle (I). The same shift in the diet also ing N. sphaerocarpa than in clear filtered sea- took place when the predatory behaviour of M. water. The reason for this decreased feeding on mixta was studied. Prey selection changed, con- copepods in the presence of cyanobacteria could centrating at first on rotifers and, after the mysids be that the mysids also fed on algae. However, had attained the threshold size for effective zoo- this is unlikely because mysids tend to shift their plankton capturing, copepods became the most diet to a more carnivorous one when they are selected prey (III). The growth of M. mixta was large enough to capture, e.g. copepods (I, II). clearly associated with the proportion of zoo- Therefore, the studied mysids (~14 mm long) plankton food (I, II, III). Increased feeding on concentrate their feeding on copepods instead copepods in particular increased the growth rate. of cyanobacteria. Cyanobacteria filaments have Two other important findings were firstly, that a tendency to form aggregates in the water. These the response to light differed between pelagic clumps may clog the feeding appendages of and littoral mysids, probably due to their differ- mysids and thus hamper their feeding. We ob- ent ways of predation avoidance, and secondly, served cyanobacteria filaments in the feeding that M. mixta discriminated between common appendages of every mysid which had been kept non-toxic and toxic cyanobacteria and thus, re- in the cyanobacteria water, which shows that duced toxin intake. However, cyanobacteria ag- aggregates probably interfere with both filter gregates clogged the feeding appendages of feeding and raptorial feeding during intensive mysids and therefore lowered their feeding effi- blooms. ciency. Thus, it seems that mysids utilise both toxic Although the pelagic mysid M. mixta is om- and non-toxic cyanobacteria but also avoid the nivorous, it shows some prey preference. The intake of toxin by decreasing ingestion when prey selection may be strongest when many dif- only toxic cyanobacteria are available. Survival ferent zooplankton groups are available and se- of mysids in the presence of toxic cyanobacte- lection between them is possible. Otherwise, ria was high, which further supports the idea that mysids probably feed on other plankters, includ- 27 ing phytoplankton and do not suffer from se- Mysids utilise several trophic levels during vere food shortage (Adare & Lasenby 1994). The their life cycle, including primary producers, influence of mysid predation on the zooplank- herbivorous and carnivorous secondary produc- ton community is strongest during summer and ers and decomposers from the sediments. In ad- autumn, when zooplankton is most abundant, dition, they have an influence on fish popula- and minimal in spring, when the zooplankton tions both in pelagial and in bottom habitats abundance is lower and mysids are small. When (Aneer 1980, Rudstam & Hansson 1990, Thiel the abundance of mysids is sufficiently high, they 1996). Thus, they have a major influence on other may stabilise the seasonal fluctuations of zoo- trophic levels by acting as both top-down and plankton population sizes in the northern Baltic bottom-up regulators (Fig. 6). Sea. The regulating influence on zooplankton The changing state of the Baltic Sea, includ- populations is especially large when the effect ing lowered salinity (Hänninen et al. 2000) and of all planktivores (including planktivorous fish, increased eutrophication with cyanobacteria e.g. herring and sprat) is taken into account blooms (e.g. Kahru et al. 1994, Bianchi et al. (Hansson et al. 1990a, Rudstam et al. 1992). 2000), has a strong influence on the plankton communities. For mysids, the most crucial change would be a large change in salinity both in the surface (littoral mysids) and deep waters (pelagic mysids), because species distribution is strongly limited by salinity (Mauchline 1980, Köhn 1992). Changes in hydrographical conditions would also have an influence on the food availability and quality, through changes in plankton communities. Decreasing salinity in the northern Baltic would favour species such as the copepod, E. affinis, and some cladoceran and rotifer species (Viitasalo et al. 1995). This would also favour mysids, which feed on rotifers as juveniles and select for E. affinis (III, Hansson et al. 1990a) and cladocerans (Rudstam et al. 1992) as adults. The changes in the state of the Baltic Sea also have effects on higher predators, such as fish, which further influence mysid populations through top-down regulation. Increased nutrient loading (Karjalainen 1999) may have severe effects on both phyto and zooplankton communities. Mysids are also affected by eutrophication of the Baltic. Increasing primary production increases the amount of set- tling material and thus oxygen consumption near the bottom. In addition, the occurrence of fila- Fig. 6. The role of pelagic mysids in the food web of the mentous algae increases, which leads to oxygen northern Baltic Sea. Light grey arrows in the background indicate energy flows from lower trophic levels, through depletion in the littoral zone. Mysids are nekto- mysids to fish. Dark grey arrows indicate environmental benthic animals and lowered oxygen concentra- factors studied in this thesis, which affect the feeding effi- tions near the bottom may force them to change ciency of mysids. their habitats for better oxygenated areas, since 28 they avoid anoxic bottoms (Salemaa et al. 1986). am also thankful to Make for accepting me onto Decreasing oxygen near the bottom forces pe- the project, despite my Turku-past (!) and for lagic mysids to feed more in the pelagial and the time he spent with me discussing the exper- less on benthic material. Furthermore, increas- iments, results (logic or unexpected), applica- ing time spent in the upper water column and tions or whatever else that mattered. I would also decreased possibility to migrate to deeper and like to thank Ilppo Vuorinen for introducing me darker waters, exposes them even more to fish to the field of marine ecology on a course on the predation. western coast of Sweden. That course turned my In addition, eutrophication decreases light pen- mind from terrestrial to aquatic ecology and gave etration through the water due to increased phy- me an idea of how fun the work of collecting toplankton abundance. This may harm littoral seashells and shore animals could be. I am grate- mysids, because decreased light reduces the ful to Prof. Emeritus Åke Niemi for accepting growth of Fucus vesiculosus (Ruuskanen 2000), me as a PhD student to the division of Hydrobi- which provides important shelter for mysids ology. Special thanks go to Tvärminne Zoolog- avoiding predators. Decreased light in the water ical Station: Jouko and Marita Pokki, Raija may benefit pelagic mysids by reducing preda- Myllymäki, Totti and Ulla Sjölund, Lallu Key- tion risk, as well as increasing intensity of verti- näs, Bebbe Åström, Svante Degerholm, Antti cal migration and thus better access to the food Nevalainen, Magnus Lindström, Eva Sandberg- supply. Kilpi and others for excellent working facilities The increased phytoplankton production is and indispensable help during my work. Espe- another consequence of increased nutrient load- cially, I want to thank Totti for the patience and ing. Nitrogen-fixing cyanobacteria in particular expertise on many nighttime samplings; with- are favoured by increased phosphorous loading out his endless patience and endurance, the (Kononen et al. 1996). Cyanobacteria blooms amount of mysids in my experiments would have affect many trophic levels. The effect ranges been remarkably smaller! I am very grateful to from increased competition with other algae to Sirkka-Liisa Nyéki in the department library for potential toxic effects on top predators such as kindly helping me to find the articles I needed. birds (Bianchi et al. 2000), as well as effects During the last months, the help of Carl-Adam upon the mysids themselves. They may feed on Hæggström and Markku Viitasalo with final cyanobacterial filaments (I, V) primarily, if noth- preparations of this thesis and the dissertation ing else is available, or incidentally, while filter have been indispensable. feeding on other algae. The discrimination be- The following people have provided valuable tween toxic and non-toxic strains is an impor- comments on my thesis, which improved the tant way to increase survival during intensive manuscript considerably: Marja Koski, Eva blooms (V). Cyanobacteria, especially Nodular- Sandberg-Kilpi, Roope Flinkman, Jonna Eng- ia spp., which form aggregates, may hamper ström and of course the supervisors. Sture Hans- feeding efficiency of all mysids by clogging their son and Heikki Salemaa revised the thesis and feeding appendages and therefore reducing feed- Stephen Venn checked the English language. ing success. Thank you all! I wish to thank Heikki and Sture also for the discussions on the Baltic ecosystem and especially mysids as a part of it. My co- ACKNOWLEDGEMENTS authors: Jonna Engström, Roope Flinkman, Har- ri Kuosa and Markku Viitasalo are acknowledged My most sincere gratitude goes to my supervi- for fruitful co-operation. sors Markku Viitasalo and Harri Kuosa; thank I want to express my thanks to Jonna Engström you for your guidance, support and criticism. I for sharing a room (putting up with me) and for 29 the many discussions on ecology, statistics and Adare, K. I. & Lasenby, D. C. 1994: Seasonal changes in life at large, to Marja Koski, Anke Kremp, Tomi the total lipid content of the opossum shrimp, Mysis relicta (Malacostraca:Mysidacea). – Can. J. Fish. Aquat. Hakala, Sandra Green, Miina Karjalainen and Sci. 51: 1935-1941. other colleagues at the division for coffee/lunch Almond, M. R. J., Bentzen, E. & Taylor, W. D. 1996: Size discussions and a good working atmosphere. I structure and species composition of plankton commu- would also like to thank those dear EZECO-peo- nities in deep Ontario lakes with and without Mysis relicta and planktivorous fish. – Can. J. Fish. Aquat. Sci. ple: Make, Roope, Tarja, Marja, Jonna, Tomi, 53: 315-325. Sandra and Miina for unforgettable get-togeth- Anderson, T. R. & Pond, D. W. 2000: Stoichiometric theo- ers, crazy humour and best of all, for inspiring ry extended to micronutrients: Comparison of the roles of essential fatty acids, carbon, and nitrogen in the nu- Finnish music! It has been fun to work with you trition of marine copepods. – Limnol. Oceanogr. 45: all! Special thanks go to Sanna Sopanen, Riitta 1162-1167. Autio and Janne Rintala for being there. I am Aneer, G. 1980: Estimates of feeding pressure on pelagic also grateful for all the friends in Turku, Kuo- and benthic organisms by Baltic herring (Clupea harengus v. membras L.). – Ophelia 1: 265-275. pio, Helsinki, Göttingen and Hawaii for your Aniansson, B. 1990: Pohjois-Euroopan meret, Pohjois- encouragement, care and for providing so much Euroopan ympäristö. – Norstedts Tryckeri, Stockholm. in my life outside work. 247 pp. My humble thanks go to my mother, father Aro, E., Uitto, A., Vuorinen, I. & Flinkman, J. 1986: The food selection of Baltic herring in late summer in the and sister and her family for (almost) always northern Baltic Sea. – ICES 1986/Baltic Fish Commit- believing in me and in my choice of career, al- tee/No. 261-19. beit first curiously asking what are those strange Arrhenius, F. & Hansson, S. 1993: Food consumption of animals that I wish to study. The biggest thanks larval, young and adult herring and sprat in the Baltic Sea. – Mar. Ecol. Prog. Ser. 96: 125-137. belong to Teemu, my life companion, for his Begon, M., Harper, J. L. & Townsend, C. R. 1990: Ecolo- endless understanding and support, without gy, individuals, populations and communities. Second which completing this thesis would have been edition. – Blackwell Science, USA. 945 pp. so much harder and life so much more boring. Bianchi, T. S., Engelhaupt, E., Westman, P., Andrén, T., Rolff, C. & Elmgren, R. 2000: Cyanobacterial blooms This thesis was financed by Walter and An- in the Baltic Sea: Natural or human-induced? – Limnol. drée de Nottbeck Foundation. Oceanogr. 45: 716-726. Bowers, J. A. & Grossnickle, N. E. 1978: The herbivorous habits of Mysis relicta in Lake Michigan. – Limnol. Oceanogr. 23: 767-776. REFERENCES Bowers, J. A. & Vanderploeg, H. A. 1982: In situ predatory behavior of Mysis relicta in Lake Michigan. – Hydro- Aarnio, K., Bonsdorff, E. & Rosenback, N. 1996: Food biologia 93: 121-131. and feeding habits of juvenile flounder Platichthys fle- Branstrator, D. 1995: Ecological interactions between By- sus (L.), and turbot Scophthalmus maximus L. in the thotrephes cederstroemi and Leptodora kindtii and the Åland Archipelago, northern Baltic Sea. – J. Sea Res. implications for species replacement in Lake Michigan. 36: 311-320. – J. Great Lakes Res. 21: 670-679. Aaser, H. F., Jeppesen, E. & Søndergaard, M. 1995: Sea- Branstrator, D., Cabana, G., Mazumder, A. & Rasmussen, sonal dynamics of the mysid Neomysis integer and its J. B. 2000: Measuring life-history omnivory in the opos- predation on the copepod Eurytemora affinis in a shal- sum shrimp, Mysis mixta, with stable nitrogen isotopes. low hypertrophic brackish lake. – Mar. Ecol. Prog. Ser. – Limnol. Oceanogr. 45: 463-467. 127: 47-56. Browman, H. I., Kruse, S. & O’Brien, W. J. 1989: Forag- Abrams, P. 1986: Indirect interactions between species that ing behavior of the predaceous cladoceran, Leptodora share a predator: varieties of indirect effects. – In: Ker- kindtii, and escape responses of their prey. – J. Plankton foot, W. C. & Sih, A. (eds.), Predation. Direct and indi- Res. 11: 1075-1088. rect impacts on aquatic communities, pp. 38-54. Uni- Burris, J. E. 1980: Vertical migration of zooplankton in the versity press of New England. Gulf of Finland. – Am. Midland Nat. 103: 316-322. Ackefors, H. 1969: Ecological zooplankton investigations Buskey, E. J. 1998: Energetic cost of position-holding be- in the Baltic proper 1963-1965. – Series Biology, Insti- havior in the planktonic mysid Mysidium columbie. – tute of Marine Research, Lysekil, Sweden. No. 18: 1- Mar. Ecol. Prog. Ser. 172: 139-147. 139. 30

Carpenter, S. R., Kitchell, J. F. & Hodgson, J. R. 1985: Dodson, S. 1974: Adaptive change in plankton morpholo- Cascading trophic interactions and lake productivity. – gy in response to size-selective predation: a new hypoth- BioScience. 35: 634-639. esis of cyclomorphosis. – Limnol. Oceanogr. 19: 721- Cartes, J. E. & Sorbe, J. C. 1998: Aspects of population 729. structure and feeding ecology of the deep-water mysid Dodson, S. 1990: Predicting diel vertical migration of zoo- Boreomysis arctica, a dominant species in western Med- plankton. – Limnol. Oceanogr. 35: 1195-1200. iterranean slope assemblages. – J. Plankton Res. 20: Donnelly, J., Stickney, D. G. & Torres, J. J. 1993: Proxi- 2273-2290. mate and elemental composition and energy content of Chapman, M. A. & Thomas, M. F. 1998: An experimental mesopelagic crustaceans from the eastern Gulf of Mex- study of feeding in Tenagomysis chiltoni (Crustacea, ico. – Mar. Biol. 115: 469-480. Mysidacea). – Arch. Hydrobiol. 143: 197-209. Drenner, R. W., Strickler, J. R. & O’Brien, W. J. 1978: Chen, C. Y. & Folt, C. L. 1993: Measures of food quality Capture probability: the role of zooplankter escape in as demographic predictors in freshwater copepods. – J. the selective feeding of planktivorous fish. – J. Fish. Res. Plankton Res. 15: 1247-1261. Bd Can. 35: 1370-1373. Chigbu, P. & Sibley, T. H. 1994: Predation by Neomysis Dunstan, G. A., Volkman, J. K., Jeffrey, S. W. & Barrett, S. mercedis: effects of temperature, Daphnia magna size M. 1992: Biochemical composition of microalgae from and prey density on ingestion rate and size selectivity. – the green algae classes Chlorophyceae and Prasinophyc- Freshwat. Biol. 32: 39-48. eae. 2. Lipid classes and fatty acids. – J. Exp. Mar. Biol. Chipps, S. R. 1998: Temperature-dependent consumption Ecol. 161: 115-134. and gut-residence time in the opossum shrimp Mysis Engström, J., Koski, M., Viitasalo, M., Reinikainen, M., relicta. – J. Plankton Res. 20: 2401-2411. Repka, S. & Sivonen, K. 2000: Feeding interactions of Cieri, M. D. & Stearns, D. E. 1999: Reduction of grazing the common copepods Eurytemora affinis and Acartia activity of two estuarine copepods in response to the bifilosa with the cyanobacteria Nodularia sp. – J. Plank- exudate of a visual predator. – Mar. Ecol. Prog. Ser. 177: ton Res. 22: 1403-1409. 157-163. Evans, G. T. 1989: The encounter speed of moving preda- Clutter, R. I. & Theilacker, G. H. 1971: Ecological effi- tor and prey. – J. Plankton Res. 11: 415-417. ciency of a pelagic mysid shrimp; estimates from growth, Fiksen, Ø. & Carlotti, F. 1998: A model of optimal life energy budget and mortality studies. – Fishery Bulletin history and diel vertical migaration in Calanus finmar- 69: 93-115. chicus. – Sarsia 83: 129-147. Colinvaux, P. 1986: Ecology. – John Wiley & Sons, Inc., Flinkman, J., Aro, E., Vuorinen, I. & Kotilainen, P. 1991: USA. 725 pp. The annual changes in food selection of Baltic herring. Conover, R. J. & Corner, E. D. S. 1968: Respiration and – ICES CM 1991/J: 14. nitrogen excretion by some marine zooplankton in rela- Folt, C. L., Rybock, J. T. & Goldman, C. R. 1982: The tion to their life cycles. – J. Mar. Biol. Ass. U. K. 48: 49- effects of prey composition and abundance on the pre- 75. dation rate and selectivity of Mysis relicta. – Hydrobio- Cooper, S. D. & Goldman, C. R. 1980: Opossum shrimp logia 93: 133-143. (Mysis relicta) predation on zooplankton. – Can. J. Fish. Fulton, R. S. III 1982: Preliminary results of an experi- Aquat. Sci. 37: 909-919. mental study of the effects of mysid predation on estua- Corner, E. D. S., Head, R. N., Kilvington, C. C. & Penny- rine zooplankton community structure. – Hydrobiologia cuick, L. 1976: On the nutrition and metabolism of zoo- 93: 79-84. plankton X. Quantitative aspects of Calanus helgolan- Fulton, R. S. III 1988: Resistance to blue-green algal tox- dicus feeding as a carnivore. – J. Mar. Biol. Ass. U. K. ins by Bosmina longirostris. – J. Plankton Res. 10: 771- 56: 345-358. 778. DeGraeve, G. M. & Reynolds, J. B. 1975: Feeding behav- Gardner, M. B. 1981: Mechanisms of size selectivity by ior and temperature and light tolerance of Mysis relicta planktivorous fish: a test of hypotheses. – Ecology 62: in the laboratory. – Trans. Am. Fisheries Soc. 2: 394- 571-578. 397. Gerritsen, J. 1984: Size efficiency reconsidered: a general DeMott, W. R. 1995: Optimal foraging by a suspension- foraging model for free-swimming aquatic animals. – feeding copepod: responses to short-term and seasonal Am. Nat. 123: 450-467. variation in food resources. – Oecologia 103: 230-240. Gerritsen, J. & Strickler, J. R. 1977: Encounter probabili- DeMott, W. R. & Moxter, F. 1991: Foraging on cyanobac- ties and community structure in zooplankton: a mathe- teria by copepods: responses to chemical defenses and matical model. – J. Fish. Res. Bd Can. 34: 73-82. resource abundance. – Ecology 72: 1820-1834. Ghan, D., McPhail, J. D. & Hyatt, K. D. 1998: The tempo- Dittel, A. I., Epifanio, C. E., Cifuentes, L. A. & Kirchman, ral-spatial pattern of vertical migration by the freshwa- D. L. 1997: Carbon and nitrogen sources for shrimp post- ter copepod Skistodiaptomus oregonensis relative to pre- larvae fed natural diets from a tropical mangrove sys- dation risk. – Can. J. Fish. Aquat. Sci. 55: 1350-1363. tem. – Estuar. Coastal Shelf Sci. 45: 629-637. 31

Gifford, D. J. & Dagg, M. J. 1988: Feeding of the estua- Hansson, S., Rudstam, L. G. & Johansson, S. 1990b: Are rine copepod Acartia tonsa dana: carnivory vs. herbivo- marine planktonic invertebrates food limited? The case ry in natural microplankton assemblages. – Bull. Mar. of Mysis mixta (Crustacea, Mysidacea) in the Baltic Sea. Sci. 43: 458-468. – Oecologia 84: 430-432. Gismervik, I. & Andersen, T. 1997: Prey switching by Acar- Havel, J. E. 1986: Predator-induced defenses: a review. – tia clausi: experimental evidence and implications of In: Kerfoot, W. C. & Sih, A. (eds.), Predation. Direct intraguild predation assessed by a model. – Mar. Ecol. and indirect impacts on aquatic communities, pp. 263- Prog. Ser. 157: 247-259. 278. University press of New England. Gorokhova, E. 1998: Exploring and modelling the growth Heinle, D. R., Harris, R. P., Ustach, J. F. & Flemer, D. A. dynamics of Mysis mixta. – Ecol. Modelling 110: 45- 1977: Detritus as food for estuarine copepods. – Mar. 54. Biol. 40: 341-353. Gorokhova, E. & Hansson, S. 1997: Effects of experimen- Heiskanen, A.-S. 1995: Contamination of sediment trap tal conditions on the feeding rate of Mysis mixta (Crus- fluxes by vertically migrating phototrophic micro-organ- tacea, Mysidacea). – Hydrobiologia 355: 167-172. isms in the coastal Baltic Sea. – Mar. Ecol. Prog. Ser. Gorokhova, E. & Hansson, S. 1999: An experimental study 122: 45-58. on variations in stable carbon and nitrogen isotopes frac- Heiskanen, A.-S. & Kononen, K. 1994: Sedimentation of tionation during growth of Mysis mixta and Neomysis vernal and late summer phytoplankton communities in integer. – Can. J. Fish. Aquat. Sci. 56: 2203-2210. the coastal Baltic Sea. – Arch. Hydrobiol. 131: 175-198. Grabe, S. A. & Hatch, E. R. 1982: Aspects of the biology Hernroth, L. & Ackefors, H. 1979: The zooplankton of the of Mysis mixta (Lilljeborg 1852) (Crustacea, Mysida- Baltic proper. A long-term investigation of the fauna, its cea) in New Hampshire coastal waters. – Can. J. Zool. biology and ecology. – Report Fish. Bd Sweden, Inst. 60: 1275-1281. Mar. Res. 2: 1-60. Greene, C. G. 1986: Patterns of prey selection: implica- Holling, C. S. 1959: The components of predation as re- tions of predator foraging tactics. – Am. Nat. 128: 824- vealed by a study of small-mammal predation of the 839. European pine sawfly. – Can. Entomol. 91: 293-320. Greene, C. H. & Landry, M. R. 1985: Patterns of prey se- Holmquist, C. 1962: The relict concept – is it a merely lection in the cruising calanoid predator Euchaeta elon- zoogeographical conception? – Oikos 13: 262-292. gata. – Ecology 66: 1408-1416. Hostens, K. & Mees, J. 1999: The mysid-feeding guild of Grossnickle, N. E. 1982: Feeding habits of Mysis relicta – demersal fishes in the brackish zone of the Westerschel- an overview. – Hydrobiologia 93: 101-107. de estuary. – J. Fish Biol. 55: 704-719. Hakala, I., Ryabinkin, A. & Salemaa, H. 1993: Population Hughes, R. N. 1980: Optimal foraging theory in the ma- structure and life cycle of Mysis relicta in Lake Paana- rine context. – Oceanogr. Mar. Biol. Annual Rev. 18: järvi. – Oulanka Rep. 12: 115-118. 423-481. Hällfors, G., Kangas, P. & Lappalainen, A. 1975: Littoral Johannsson, O. E., Rudstam, L. G. & Lasenby, D. C. 1994: benthos of the northern Baltic Sea. III. Macrobenthos of Mysis relicta: assessment of metalimnetic feeding and the hydrolittoral belt of filamentous algae on rocky shores implications for competition with fish in lakes Ontario of Tvärminne. – Int. Rev. Ges. Hydrobiol. 60 (3): 313- and Michigan. – Can. J. Fish. Aquat. Sci. 51: 2591-2602. 333. Jónasdóttir, S. H. 1994: Effects of food quality on the re- Hamrén, U. & Hansson, S. 1999: A mysid shrimp (Mysis productive success of Acartia tonsa and Acartia hud- mixta) is able to detect the odour of its predator (Clupea sonica: laboratory observations. – Mar. Biol. 121: 67- harengus). – Ophelia 3: 187-191. 81. Hanazato, T. & Yasuno, M. 1987: Evaluation of Micro- Kahru, M., Horstmann, U. & Rud, O. 1994: Satellite de- cystis as food for zooplankton in a eutrophic lake. – tection of increased cyanobacteria blooms in the Baltic Hydrobiologia 144: 251-259. sea: natural fluctuation or ecosystem change? – Ambio Hänninen, J., Vuorinen, I. & Hjelt, P. 2000: Climatic fac- 23: 469-472. tors in the Atlantic control the oceanographic and eco- Karjalainen, M. 1999: Effect of nutrient loading on the logical changes in the Baltic Sea. – Limnol. Oceanogr. development of the state of the Baltic Sea – an over- 45: 703-710. view. – W. & A. de Nottbeck Found. Sci. Rep. 17: 1-35. Hansson, S., Hobbie, J. E., Elmgren, R., Larsson, U., Fry, Ketelaars, H. A. M., Lambregts-van de Clundert, F. E., B. & Johansson, S. 1997: The stable nitrogen isotope Carpentier, C. J., Wagenvoort, A. J. & Hoogenboezem, ratio as a marker of food-web interactions and fish mi- W. 1999: Ecological effects of the mass occurence of gration. – Ecology 78: 2249-2257. the Ponto-Caspian invader, Hemimysis anomala G. O. Hansson, S., Larsson, U. & Johansson, S. 1990a: Selective Sars, 1907 (Crustacea: Mysidacea), in a freshwater stor- predation by herring and mysids, and zooplankton com- age reservoir in the Netherlands, with notes on its aut- munity structure in a Baltic Sea coastal area. – J. Plank- ecology and new records. – Hydrobiologia 394: 233-248. ton Res. 12: 1099-1116. 32

Kils, U. 1992: The ecoSCOPE and dynIMAGE: microscale Kuparinen, J., Leppänen, J.-M., Sarvala, J., Sundberg, A. tools for in situ studies of predator-prey interactions. & Virtanen, A. 1984: Production and utilization of or- – Arch. Hydrobiol. Beih. Ergebn. Limnol. 36: 83-96 ganic matter in a Baltic ecosystem off Tvärminne, south- Kiørboe, T. 1989: Phytoplankton growth rate and nitrogen west coast of Finland. – Rapp. P. –v. Réun. Cons. int. content: implications for feeding and fecundity in a her- Explor. Mer. 193:180-192. bivorous copepod. – Mar. Ecol. Prog. Ser. 55: 229-234. Lampert, W. 1989: The adaptive significance of diel verti- Kiørboe, T. & Visser, A. W. 1999: Predator and prey per- cal migration by zooplankton. – Funct. Ecol. 3:21-27. ception in copepods due to hydromechanical signals. – Landry, M. R. 1981: Switching between herbivory and car- Mar. Ecol. Prog. Ser. 179: 81-95. nivory by the planktonic marine copepod Calanus pa- Kitchell, J. F. & Carpenter, S. R. 1986: Piscivores, fossils, cificus. – Mar. Biol. 65: 77-82. and phorbins. – In Kerfoot, W. C. & Sih, A. (eds.), Pre- Lasenby, D. C. & Fürst, M. 1981: Feeding of Mysis relicta dation. Direct and indirect impacts on aquatic commu- Lovén on macrozooplankton. – Rep. Inst. Freshwater nities, pp. 132-146. University press of New England. Res. Drottningholm 59: 75-80. Kjellberg, G., Hessen, D. O. & Nilssen, J. P. 1991: Life Lasenby, D. C. & Langford, R. R. 1973: Feeding and as- history, growth and production of Mysis relicta in the similation of Mysis relicta. – Limnol. Oceanogr. 18: 280- large, fjord-type Lake Mjosa, Norway. – Freshwat. Biol. 285. 26: 165-173. Lasenby, D. C., Northcote, T. G. & Fürst, M. 1986: Theo- Kleppel, G. S., Burkart, C. A. & Houchin, L. 1998: Nutri- ry, practise and effects of Mysis relicta introductions to tion and the regulation of egg production in the calanoid North American and Scandinavian lakes. – Can. J. Fish. copepod Acartia tonsa. – Limnol. Oceanogr. 43: 1000- Aquat. Sci. 43: 1277-1284. 1007. Lehtimäki, J., Lyra, C., Suomalainen, S., Sundman, P., Köhn, J. 1992: Mysidacea of the Baltic Sea – State of the Rouhiainen, L., Paulin, L., Salkinoja-Salonen, M. & art. – In: V. J. Köhn, M. B. Jones & A. M. Moffat (eds.), Sivonen, K. 2000: Characterization of Nodularia strains, Taxonomy, Biology and Ecology of (Baltic) mysids cyanobacteria from brackish waters, by genotypic and (Mysidacea:Crustacea), pp. 5-24. International Expert phenotypic methods. – Int. J. Syst. Evol. Microbiol. 50: Conference, Hiddensee, Germany, Rostock University 1043-1053. Press, Rostock. Lehtonen, K. K. 1997: Ecophysiology of two benthic am- Kononen, K. & Niemi, Å. 1984: Long-term variation of phipod species from the northern Baltic Sea. – Monogr. the phytoplankton composition at the entrance to the Gulf Bor. Envir. Res. 7: 1-33. of Finland. – Ophelia, Suppl. 3: 101-110. Leibold, M. A. 1991: Trophic interactions and habitat seg- Kononen, K., Kuparinen, J., Mäkelä, K., Laanemets, J., regation between competing Daphnia species. – Oeco- Pavelson, J. & Nômmann, S. 1996: Initiation of cyano- logia 86: 510-520. bacterial blooms in a frontal region at the entrance to the Lignell, R., Heiskanen, A.-S., Kuosa, H., Gundersen, K., Gulf of Finland, Baltic Sea. – Limnol. Oceanogr. 41: Kuuppo-Leinikki, P., Pajuniemi, R. & Uitto, A. 1993: 98-112. Fate of a phytoplankton spring bloom: sedimentation and Koski, M., Engström, J. & Viitasalo, M. 1999a: Reproduc- carbon flow in the planktonic food web in the northern tion and survival of the calanoid copepod Eurytemora Baltic. – Mar. Ecol. Prog. Ser. 94: 239-252. affinis fed with toxic and non-toxic cyanobacteria. – Mar. Lindley, J. A., John, A. W. G. & Robins, D. R. 1997: Dry Ecol. Prog. Ser. 186: 187-197. weight, carbon and nitrogen content of some calanoid Koski, M., Klein Breteler, W. & Schogt, N. 1998: Effect of copepods from the seas around southern Britain in win- food quality on rate of growth and development of the ter. – J. Mar. Biol. Ass. U. K. 77: 249-252. pelagic copepod Pseudocalanus elongatus (Copepoda, Lindström, M. 2000: Eye function of Mysidacea (Crusta- Calanoida). – Mar. Ecol. Prog. Ser. 170: 169-187. cea) in the northern Baltic Sea. – J. Exp. Mar. Biol. Ecol. Koski, M., Rosenberg, M., Viitasalo, M., Tanskanen, S. & 246: 85-101. Sjölund, U. 1999b: Is Prymnesium patelliferum toxic for Lonsdale, D. J., Heinle, D. R. & Siegfried, C. 1979: Car- copepods? – Grazing, egg production, and egestion of nivorous feeding behavior of the adult calanoid cope- the calanoid copepod Eurytemora affinis in mixtures of pod Acartia tonsa dana. – J. Exp. Mar. Biol. Ecol. 36: ‘good’ and ‘bad’ food. – ICES J Mar. Sci. 56 supple- 235-248. ment: 131-139. Loose, C. J. & Dawidowicz, P. 1994: Trade-offs in diel Koski, M., Viitasalo, M. & Kuosa, H. 1999c: Seasonal de- vertical migration by zooplankton: the costs of predator velopment of mesozooplankton biomass and production avoidance. – Ecology 75: 2255-2263. on the SW coast of Finland. – Ophelia 50: 69-91. MacArthur, R. H. & Pianka, E. R. 1966: On optimal use of Kremp, A. & Heiskanen, A.-S. 1999: Sexuality and cyst a patchy environment. – Am. Nat. 100: 603-609. formation of the spring-bloom dinoflagellate Scrippsiella Macdonald, P. D. M. & Green, P. E. J. 1988: User’s guide hangoei in the northern Baltic Sea. – Mar. Biol. 134: to program MIX: An interactive program for fitting mix- 771-777. tures of distributions. – Guenther printing, Ontario, Can- ada. 33

Mauchline, J. 1980: The biology of mysids and euphausi- Reinikainen, M., Kiviranta, J., Ulvi, V. & Niku-Paavola, ids. – Adv. Mar. Biol. 18: 1-677. M.-L. 1995: Acute toxic effects of a novel cyanobacteri- McKinnon, A. D. 1996: Growth and development in the al toxin on the crustaceans Artemia salina and Daphnia subtropical copepod Acrocalanus gibber. – Limnol. Oce- pulex. – Arch. Hydrobiol. 133: 61-69. anogr. 41: 1438-1447. Robinson, P. W. 1999: The toxicity of pesticides and or- McLusky, D. S. 1979: Some effects of salinity and temper- ganics to mysid shrimps can be predicted from Daphnia ature on the osmotic and ionic regulation of Praunus spp. toxicity data. – Wat. Res. 33: 1545-1549. flexuosus (Crustacea, Mysidacea) from Isefjord. – Ophe- Rolff, J. C., Broman, D., Näf, C. & Zebühr, Y. 1993: Po- lia 18: 191-203. tential biomagnification of PCDD/Fs – New possibilites Mohammadian, M. A., Hansson, S. & De Stasio, B. T. 1997: for quantitative assessment using stable isotope trophic Are marine planktonic invertebrates food limited? The position. – Chemosphere 27: 461-468. functional response of Mysis mixta (Crustacea, Mysida- Rudstam, L. G., Danielsson, K., Hansson, S. & Johansson, cea) in the Baltic Sea. – Mar. Ecol. Prog. Ser. 150: 113- S. 1989: Diel vertical migration and feeding patterns of 119. Mysis mixta (Crustacea, Mysidacea) in the Baltic Sea. – Murtaugh, P. A. 1981: Size-selective predation on Daph- Mar. Biol. 101: 43-52. nia by Neomysis mercedis. – Ecology 62: 894-900. Rudstam, L. G. & Hansson, S. 1990: On the ecology of Nero, R. W. & Sprules, W. G. 1986: Predation by three Mysis mixta (Crustacea, Mysidacea) in a coastal area of glacial opportunists on natural zooplankton communi- the northern Baltic proper. – Ann. Zool. Fennici 27: 259- ties. – Can. J. Zool. 64: 57-64. 263. Niemi, Å. 1975: Ecology of phytoplankton in the Rudstam, L. G., Hansson, S., Johansson, S. & Larsson, U. Tvärminne area, SW coast of Finland. II. Primary pro- 1992: Dynamics of planktivory in a coastal area of the duction and environmental conditions in the archipela- northern Baltic Sea. – Mar. Ecol. Prog. Ser. 80: 159- go and the sea zone. – Acta Bot. Fennica 105: 1-73. 173. Niemi, Å. 1976: Växtplanktonets ekologi och miljö i Rudstam, L. G., Hansson, S. & Larsson, U. 1986: Abun- Tvärminneområdet. – Ph. D. thesis, University of Hel- dance, species composition and production of mysid sinki. 73 pp. shrimps in a coastal area of the northern Baltic proper. – Nimmo, D. R. & Hamaker, T. L. 1982: Mysids in toxicity Ophelia Suppl. 4: 225-238. testing – a review. – Hydrobiologia 93: 171-178. Ruuskanen, A. 2000: Ecological responses of Fucus vesic- Nordström, H. 1997: Rantavyöhykkeen halkoisjalka- ulosus L. along environmental gradients in the northern äyriäisten (Mysidacea) ravinnonkäyttö Itämeressä. – Baltic Sea. – W. & A. de Nottbeck Found. Sci. Rep. 21: M.Sc. thesis, University of Helsinki. 57 pp. 1-20. O’Brien, W. J. 1986: Planktivory by freshwater fish: thrust Salemaa, H. & Hietalahti, V. 1993: Hemimysis anomala and parry in the pelagia. – In: Kerfoot, W. C. & Sih, A. G. O. Sars (Crustacea: Mysidacea) – Immigration of a (eds.), Predation. Direct and indirect impacts on aquatic Pontocaspian mysid into the Baltic Sea. – Ann. Zool. communities, pp. 3-16. University press of New Eng- Fennici 30: 271-276. land. Salemaa, H., Tyystjärvi-Muuronen, K. & Aro, E. 1986: Life O’Brien, W. J., Slade, N. A. & Vinyard, G. L. 1976: Appar- histories, distribution and abundance of Mysis mixta and ent size as the determinant of prey selection by bluegill Mysis relicta in the northern Baltic Sea. – Ophelia Sup- sunfish (Lepomis macrochirus). – Ecology 57: 1304- pl. 4: 239-247. 1310. Salemaa, H., Vuorinen, I. & Välipakka, P. 1990: The dis- Pastorok, R. A. 1981: Prey vulnerability and size selection tribution and abundance of Mysis populations in the by Chaoborus larvae. – Ecology 62: 1311-1324. Baltic Sea. – Ann. Zool. Fennici 27: 253-257. Pearre, S. 1982: Estimating prey preference by predators: Sanders, R. W., Williamson, C. E., Stutzman, P. L., Moel- uses of various indices, and a proposal of another based ler, R. E., Goulden, C. E. & Aoki-Goldsmith, R. 1996: on c2. – Can. J. Fish. Aquat. Sci. 39: 914-923. Reproductive success of ‘herbivorous’ zooplankton fed Pitkänen, H. 1999: The Baltic waters around Southern Fin- algal and nonalgal food resources. – Limnol. Oceanogr. land – the threat of international nutrient loading – In: 41: 1295-1305. Luukkonen, J. (ed.), Baltic Bulletin. WWF-news 3/99 Schmidt, K. & Jónasdóttir, S. H. 1997: Nutritional quality Special edition for Baltic, pp.12-13. of two cyanobacteria: How rich is ‘poor’ food? – Mar. Power, M. E. 1986: Predator avoidance by grazing fishes Ecol. Prog. Ser. 151: 1-10. in temperate and tropical streams: importance of stream Schoener, T. W. 1970: Non-synchronous spatial overlap of depth and prey size. – In: Kerfoot, W. C. & Sih, A. (eds.), lizards in patchy habitats. – Ecology 51: 408-418. Predation. Direct and indirect impacts on aquatic com- Schulz, K. L. & Sterner, R. W. 1999: Phytoplankton phos- munities, pp. 333-352. University press of New England. phorus limitaton and food quality for Bosmina. – Lim- Ramcharan, C. W., Sprules, W. G. & Nero, R. W. 1985: nol. Oceanogr. 44: 1549-1556. Notes on the tactile feeding behaviour of Mysis relicta Lovén (Malacostraca: Mysidacea). – Verh. Internat. Verein. Limnol. 22: 3215-3219 34

Scrimshaw, S. & Kerfoot, W. C. 1986: Chemical defenses Thetmeyer, H. & Kils, U. 1995: To see and not to be seen: of freshwater organisms: beetles and bugs. – In: Ker- the visibility of predator and prey with respect to feed- foot, W. C. & Sih, A. (eds.), Predation. Direct and indi- ing behaviour. – Mar. Ecol. Prog. Ser. 126: 1-8. rect impacts on aquatic communities, pp. 240-262. Uni- Thiel, R. 1992: Quantitative estimation of mysids – Neo- versity press of New England. mysis integer (Leach, 1814) – and their production within Segerstråle, S. G. 1957: On immigration of the glacial rel- a typical southern Baltic Bay. – In: V. J. Köhn, M. B. icts of Northern Europe, with remarks on their prehisto- Jones & A. M. Moffat (eds.), Taxonomy, Biology and ry. – Soc. Sci. Fennica, Comment. Biol. 16: 1-117. Ecology of (Baltic) mysids (Mysidacea:Crustacea), pp. Segerstråle, S. G. 1969: Biological fluctuations in the Bal- 73-78. International Expert Conference, Hiddensee, tic Sea. – Progr. Oceanogr. 5: 169-184. Germany, Rostock University Press, Rostock. Sih, A. 1986: Predators and prey lifestyles: an evolution- Thiel, R. 1996: The impact of fish predation on the zoo- ary and ecological overview. – In: Kerfoot, W. C. & Sih, plankton community in a southern Baltic bay. – Limno- A. (eds.), Predation. Direct and indirect impacts on aquat- logica 26: 123-137. ic communities, pp. 3-16. University press of New Eng- Tiselius, P., Jonsson, P. R. & Verity, P. G. 1993: A model land. evaluation of the impact of food patchiness on foraging Simm, M. & Kotta, I. 1992: The abundance and distribu- strategy and predation risk in zooplankton. – Bull. Mar. tion of Mysis in the Gulf of Finland. – In: V. J. Köhn, M. Sci. 53: 247-264. B. Jones & A. M. Moffat (eds.), Taxonomy, Biology and Toda, H. & Wada, E. 1990: Use of 15N and 14N rations to Ecology of (Baltic) mysids (Mysidacea:Crustacea), pp. evaluate the food source of the mysid, Neomysis inter- 55-60. International Expert Conference, Hiddensee, media Czerniawsky, in a eutrophic lake in Japan. – Hy- Germany, Rostock University Press, Rostock. drobiologia 194: 85-90. Sivonen, K., Kononen, K., Carmichael, W. W., Dahlem, A. Tsuda, A., Saito, H. & Hirose, T. 1998: Effect of gut con- M., Rinehart, K. L., Kiviranta, J. & Niemelä, S. I. 1989: tent on the vulnerability of copepods to visual preda- Occurrence of the hepatotoxic cyanobacterium Nodu- tion. – Limnol. Oceanogr. 43: 1944-1947. laria spumigena in the Baltic Sea and structure of the Uitto, A. 2000: Diurnal and vertical grazing activity of toxin. – Appl. Envir. Microbiol. 55: 1990-1995. mesozooplankton during summer on the SW coast of Solomon, M. E. 1949: The natural control of animal popu- Finland. – Bor. Envir. Res. 5: 137-146. lations. – J. Anim. Ecol. 18: 1-35 Uitto, A., Heiskanen, A.-S., Lignell, R., Autio, R. & Paju- Spencer, C. N., Potter, D. S., Bukantis, R. T. & Stanford, J. niemi, R. 1997: Summer dynamics of the coastal plank- A. 1999: Impact of predation by Mysis relicta on zoo- tonic food web in the northern Baltic Sea. – Mar. Ecol. plankton in Flathead Lake, Montana, USA. – J. Plank- Prog. Ser. 151: 27-41. ton Res. 21: 51-64. Uitto, A., Kaitala, S., Kuosa, H. & Pajuniemi, R. 1995: Starkweather, P. L. & Kellar, P. E. 1983: Utilization of cy- Effect of nutrient addition and predation of mysid shrimp anobacteria by Brachionus calyciflorus: Anabaena flos- (Neomysis integer) on a plankton community in a short- aquae (NRC-44-1) as a sole or complementary food term enclosure experiment in the northern Baltic. – Aqua source. – Hydrobiologia 104: 373-377. Fennica 25: 23-31. Stearns, S. C. 1989: Trade-offs in life-history evolution. – Urabe, J. & Watanabe, Y. 1992: Possibility of N and P lim- Funct. Ecol. 3: 259-268. itation for planktonic cladocerans: An experiment test. Sterner, R. W. 1997: Modelling interactions of food quali- – Limnol. Oceanogr. 37: 244-251. ty and quantity in homeostatic consumers. – Freshwat. Väinölä, R. 1986: Sibling species and phylogenetic rela- Biol. 38: 73-481. tionships of Mysis relicta (Crustacea: Mysidacea). – Ann. Sterner, R. W. & Robinson, J. L. 1994: Thresholds for Zool. Fennici 23: 207-221. growth in Daphnia magna with high and low phospho- Välipakka, P. 1990: Zur Verbreitung, Biologie und Ökolo- rus diets. – Limnol. Oceanogr. 39: 1228-1232. gie von Mysidacea (Crustacea, Malacostraca) in der Steven, D. M. 1961: Shoaling behavior of a mysid. – Na- Mecklenburg Bucht und in den kustenfernen Gebieten ture 192: 280-281. der eigentlichen Ostsee (1985-1988). – Ph. D. thesis, Stoecker, D. K. & Capuzzo, J. M. 1990: Predation on Pro- Rostock University. 172 pp. tozoa: its importance to zooplankton. – J. Plankton Res. Viitasalo, M. 1992: Mesozooplankton of the Gulf of Fin- 12: 891-908. land and northern Baltic Proper – a review of monitor- Tang, K. W. & Dam, H. G. 1999: Limitation of zooplank- ing data. – Ophelia 35: 147-168. ton production: beyond stoichiometry. – Oikos 84: 537- Viitasalo, M., Kiørboe, T., Flinkman, J., Pedersen, L. W. & 542. Visser, A. W. 1998: Predation vulnerability of plankton- Teraguchi, M., Hasler, A. D. & Beeton, A. M. 1975: Sea- ic copepods: consequences of predator foraging strate- sonal changes in the response of Mysis relicta Loven to gies and prey sensory abilities. – Mar. Ecol. Prog. Ser. illumination. – Verh. Internat. Verein. Limnol. 19: 2989- 175: 129-142. 3000. 35

Viitasalo, M. & Rautio, M. 1998: Zooplanktivory by Prau- nus flexuosus (Crustacea: Mysidacea): functional responses and prey selection in relation to prey escape responses. – Mar. Ecol. Prog. Ser. 174: 77-87. Viitasalo, M., Vuorinen, I. & Saesmaa, S. 1995: Mesozoo- plankton dynamics in the northern Baltic Sea: implications of variations in hydrography and climate. – J. Plank- ton Res. 17: 1857-1878. Walls, M. & Ketola, M. 1989: Effects of predator-induced spines on individual fitness in Daphnia pulex. – Limnol. Oceanogr. 34: 390-396. Wiktor, K. & Szaniawska, A. 1988: Energy content in relation to the population dynamics of Mysis mixta (Liljeborg) from the southern Baltic. – Kieler Meeres- forsch., Sonderh. 6: 154-161. Wootton, R. J. 1999: Ecology of Teleost Fishes. Second edition. – Kluwer Academic Publishers, Dordrecht, The Netherlands. 386 pp. Zar, J. H. 1999: Biostatistical Analysis. Fourth edition. – Prentice Hall, New Jersey. 663 pp. Zaret, T. M. & Kerfoot, C. 1975: Fish predation on Bosmi- na longirostris: body-size selection versus visibility selection. – Ecology 56: 232-237. Zaret, T. M. & Suffern, J. S. 1976: Vertical migration in zooplankton as a predator avoidance mechanism. – Lim- nol. Oceanogr. 21: 804-813.