MARINE ECOLOGY PROGRESS SERIES Published December 16 Mar. Ecol. Prog. Ser.
Growth and grazing rates of the herbivorous dinoflagellate Gymnodinium sp. from the open subarctic Pacific Ocean
Suzanne L. Strom'
School of OceanographyWB-10, Universityof Washington. Seattle. Washington98195, USA
ABSTRACT: Growth, grazing and cell volume of the small heterotroph~cdinoflagellate Gyrnnodin~um sp. Isolated from the open subarctic Pacific Ocean were measured as a funct~onof food concentration using 2 phytoplankton food species. Growth and lngestlon rates increased asymptotically with Increas- ing phytoplankon food levels, as did grazer cell volume; rates at representative oceanic food levels were high but below maxima. Clearance rates decreased with lncreaslng food levels when Isochrysis galbana was the food source; they increased ~vithlncreaslng food levels when Synechococcus sp. was the food source. There was apparently a grazlng threshold for Ingestion of Synechococcus: below an initial Synechococcus concentration of 20 pgC 1.' ingestion rates on this alga were very low, while above this initial concentratlon Synechococcus was grazed preferent~ally Gross growth efficiency varied between 0.03 and 0.53 (mean 0.21) and was highest at low food concentrations. Results support the hypothesis that heterotrophic d~noflagellatesmay contribute to controlling population increases of small, rap~dly-grow~ngphytoplankton specles even at low oceanic phytoplankton concentrations.
INTRODUCTION as Gymnodinium and Gyrodinium is difficult or impos- sible using older preservation and microscopy tech- Heterotrophic dinoflagellates can be a significant niques; experimental emphasis has been on more component of the microzooplankton in marine waters. easily recognizable and collectable microzooplankton In the oceanic realm, Lessard (1984) and Shapiro et al. groups such as tintinnid ciliates. Swimming and feed- (1989) found heterotrophic dinoflagellates to be abun- ing behaviors of heterotrophic dinoflagellates, how- dant in the North Atlantic, while these organisms can ever, differ substantially from those of ciliated proto- be numerous at least seasonally in the subarctic North zoans, and the 2 groups of organisms may have very Pacific (Taylor & Waters 1982, Strom & Welschmeyer different responses to a given set of environmental 1991, B. Booth pers. comm.). Nearshore waters can also conditions. support sizable populations of heterotrophic dino- Many species of heterotrophic dinoflagellates are flagellates (e.g.Gifford 1988, Weisse & Scheffel-Moser known to feed herbivorously (e.g. Lessard & Swift 1990, Hansen 1991) which at times can be as large as 1985, Jacobsen & Anderson 1986, Gaines & Elbrachter populations of ciliated protozoans (Smetacek 1981, 1987, Buck et al. 1990). When abundant, such species Garrison & Buck 1989, Lessard 1991). may have a significant impact on phytoplankton popu- Despite their potential importance, there have been lation~.Lessard (1991) calculated that the dinoflagel- few experimental laboratory studies of heterotrophic late Oblea rotunda could have cleared up to 53 % of dinoflagellates. Distinguishing photosynthetic from the daily primary production following a spring bloom heterotrophic species of common marine genera such in Chesapeake Bay, while Sherr et al. (1991) deter- mined that > 5 pm flagellates (probably primarily dino- Present address: Marine Science Inst~tute,Un~versity of flagellates) cleared a larger proportion of the water in Texas at Austln, Port Aransas, Texas 78373-1267, USA a salt marsh estuary than did the ciliate population in a
O Inter-Research/Printed In Germany 104 Mar. Ecol. Prog. Ser. 78: 103-113.1991
third of their experiments. In Antarctic waters, herbi- Trace metals and chelator were then added (Gifford vory by Gymnodinium sp. accounted for nearly all 1985) and the medium was autoclaved and stored in grazing losses to the natural phytoplankton assem- 2 l polycarbonate bottles. blage in microcosm experiments (Bj~rnsen& Kupa- Cultures were maintained in 250 m1 polycarbonate nnen 1991). Given that heterotrophic dinoflagellates flasks that had been acid-cleaned (10 % HCI), Milli-Q may be at least seasonally abundant and active as rinsed, and autoclaved while containing about 150 m1 herbivores, their behavioral responses comprise an Milli-Q water. The dinoflagellate was kept at 12 "C important part of the behavior of the entire microzoo- (= late summer mixed layer water temperature at plankton community. Station P) on a 15 h light: 9 h dark cycle; irradiance In nutrient-rich areas of the open ocean, the mecha- was dim (1 5 pEin m-' S-'). The food supply was a mix- nisms that control phytoplankton biomass are not well ture of 4 phytoplankton species (Isochrysis galbana, understood (e.g. Banse 1990, Martin et al. 1990). For Emiliania huxleyi, Micromonaspusilla, Synechococcus the open subarctic Pacific in particular, a long history sp. [strain DC-2))on which the grazer grew best during of oceanographic research centered on Weather isolation in multi-well plates. Protozoa were fed every Station P (50' N. 145' W) has indicated that phyto- 5 to 6 d with exponential-phase phytoplankon cultures plankton biomass remains low and nearly constant and were transferred every 2 to 3 wk. Care was taken year-round, in spite of high nutrient concentrations to maintain Gymnodinium sp. stocks on relatively low and seasonal increases in primary production (Miller et levels of food to prevent possible adaptation to unnatu- al. 1988 and references therein). Recently, ithas been rally high phytoplankton concentrations. hypothesized that microzooplankton grazing is re- Phytoplankton were grown in 250 m1 borosil~cate sponsible for maintaining phytoplankton standing flasks on IMR medium (Eppley et al. 1967), with the stocks at observed low levels (Evans & Parslow 1985, exception of Synechococcus sp. and Emiliania huxleyi, Frost 1987, Miller et al. 1988). If true, individual micro- which were grown on IMR medium diluted 1 :4 with zooplankton species must have significant grazing sterile filtered seawater. Cultures were maintained at rates at low natural food concentrations. Potential 13 'C (Synechococcus at 18 "C) on a 15 h light : 9 h dark population growth rates must be nearly equivalent to cycle and were transferred every 2 wk to maintain those of phytoplankton to maintain low plant standing exponential-phase growth. stocks in the face of increases in primary production. In Experimental design. Prior to the experiment, a this study, a herbivorous heterotrophic dinoflagellate large stock (2.5 1) of dinoflagellates was grown on the was isolated from the open subarctic Pacific, and food species Isochrysis galbana (cell diameter 4.5 pm) growth and grazing rates of this species were meas- and Synechococcus sp. (strain DC-2) (cell dimensions ured over a range of food concentrations. Grazer cell 1.2 X 2.4 pm). Protozoan culture medium (140 ml) was size and gross growth efficiency were also determined. added to each of 18 polycarbonate flasks (250 ml), precleaned as above. Aliquots of late exponential phase I. galbana and Synechococcus cultures were MATERIALS AND METHODS added to each flask for nominal total phytoplankton concentrations ranging from 10 to 300 pgC 1-' (3 flasks Culture methods. The dinoflagellate was isolated per food concentration). At the time of experiment set- from water collected at Station P in late September up, Synechococcus contributed about one-third and I. 1987. Water collected from the mixed layer using a galbana about two-thirds the total phytoplankton car- Teflon-lined 30 1 Go-Flo bottle was placed in acid- bon. (This combination of food species yielded a higher cleaned (10 % HCI), Milli-Q-rinsed 2 1 polycarbonate dinoflagellate growth rate than any other paired bottles covered with one layer of neutral density combination of I. galbana, Synechococcus, Emiliania screening and stored in an incubator cooled by flowing huxleyi, and Micromonaspusilla in preliminary experi- surface seawater during transit to shore (ca 3 d). Most ments. The dinoflagellate would not grow when offered bottles were enriched by additions of 2 or 3 species of only a single food species.) Subsamples (10 ml) of the cultured phytoplankton to encourage growth of herbi- Gyn~nodiniumstock culture were then added to each vorous protozoans. flask for an initlal grazer concentration of ca 40 cells Laboratory isolation and culture maintenance proce- ml-l An additional 4 control flasks containing phyto- dures in general follocved Gifford (1985). Protozoan plankton only were set up as above: 2 each at total culture medium was made by passlny seawater phytoplankton concentrations of 75 and 350 pgC 1-l. (collected from the mixed layer at Station P) through a The experiment was carried out under conditions of Gelman A/E filter followed by a 0.45 pm Nuclepore light and temperature specified for grazer culture filter in a 147 mm Plex~glasfilter holder A peristaltic maintenance. Flasks were incubated on slowly rota- pump fitted with silicone tubing was used for filtration. ting turntables (1 rpm) to ensure an even light field for Strom: Dinoflagellate growth and grazing 105
all replicates and to minimize aggregation of phyto- (
1400 h 1200 (CI V5. looo g 800 0 600 1> $ 200 0 100 200 300 0 100 200 total phytoplankton (~gCl-0
Fig. 2. Gymnodinium sp. Cell volume as a function of average food concentration (data fit to Monod model]
evidence that heterotrophic dinoflagellates may have higher carbon densities than photosynthetic flagel- lates. Carbon-specific ingestion calculated using a carbon density of 0.3 pg pm-3, determined for the heterotrophic dinoflagellate Protoperidinium parvum (Jacobson 1988), gave a maximum of 0.19 h-'. -1.0 Clearance and ingestion rates on Isochrysis galbana 0 100 200 from Days 1 and 2 of the experiment are grouped total phytoplankton OLgCP) a Fig. 1. Gymnodinium sp. Growth rate as a function of average food concentration. (A) Day 1 (data fit to Monod model); (B)Day 2
Cell volume
Gymnodinium sp. cell volume exhibited the same hyperbolic relationship to food concentration as did Day l growth rate (Fig. 2), ranglng from a low of 600 pm3 to a maximum of 1200 pm3. Maximum cell volume as predicted by a Monod model fit to the data was 1111 pm3.
Grazing rate
When fed a mixture of Isochrysis galbana and Synechococcus sp., Gymnodinium sp. exhibited total clearance rates ranging from 0.19 to 1.64 pl ind.-' h-' and total ingestion rates ranging from 1.0 to 51.0 pgC ind. h-'. Maximum volume-specific clearance (body -J ' volumes cleared body volume-' h-') was 1.8 X 106 h-', 0 100 200 while maximum carbon-specific ingestion (phyto- Isochrysis (pgCA) plankton carbon ingested body carbon-' h-') was 0.41 Fig. 3. Gymnodinium sp. Clearance (A)and ingestion (B)rates : h-', the latter calculated using the volume carbon as a function of dverage food concentration for individuals conversion factor of Strathmann (1967) for determina- grazing on Isochrysis galbana. (0) Day 1; (U) Day 2 (B]Data tion of Gymnodinium carbon content. There is some fitto Monod model Strorn: Dinoflagellate growth and grazing 107
(there were no obvious differences between days). Clearance rates decreased sharply with food concen- tration up to ca 25 pgC 1-',then decreased gradually with increasing food levels, reaching a minimum of ca 0.1 p1 ind:' h-' (Fig. 3A).Ingestion of I. galbana (Fig. 3B) increased to a maximum of 12.1 pgC h-' as predicted by a Monod model fit to the data, with K,, = 19.2 pgC I-'. Grazing by Gymnodinium sp. on Synechococcus sp. differed on Days 1 and 2 of the experiment, so data are presented separately. Clearance rates on Day 1 varied at the lowest food concentrations but generally were near 0 below 20 pg phytoplankton C I-', then in- creased linearly to 0.5 p1 ind.-' h-' at the highest food concentrations (Fig. 4A). On Day 2, clearance rates from flasks with low initial Synechococcus concentra- tions were still near 0, while rates from flasks with high initial Synechococcus concentrations ranged from 0.6 to 0.8 p1 ind:' h-' (Fig. 4B).This was in spite of the fact that average Synechococcus concentrations in these latter flasks were very low (< 15 pgC 1-l) on Day 2. The same pattern was observed for Gymnodinium sp, ingestion of Synechococcus sp. (Fig. 5). Ingestion
10 1 I I I I l I I 0 10 20 30 40 50 60 70 Synechococcus (pgCP)
Fig 5 Gyrnnodinium sp. Ingestion rate as a functlon of aver- age food concentration for individuals grazing on Synecho- coccus sp. on Day 1 (A) and Day 2 (B).Day 2: (0) initial Synechococcus concentration < 20 ygC I-'; (U) initial Synechococcus concentration > 20 pgC I-'
rates were very low in any flask in which the initial Synechococcus concentration was below 20 pgC 1-'. Above this initial food level, ingestion rate increased linearly with food concentration on Day 1, reaching a maximum of 30.4 pgC ind:' h-'. Ingestion ranged from 5.4 to 9.2 pgC ind:' h-' on Day 2.
0 -0.2 ! I I I I I I I 0 10 20 30 40 50 60 70 Synechococcus @gC/l) 0 100 200 300 Fig. 4. Gymnodinium sp. Clearance rate as a function of total phytoplankton average food concentration for individuals grazing on (cL~CA) Synechococcus on Day 1 (A) and Day 2 (B).Day 2: (0) initial Synechococcus concentration < 20 pgC 1.'; (U) initial Fig. 6. Gymnodinium sp. Gross growth efficiency as a func- Synechococcus concentration > 20 ygC I-' tion of average food concentration 108 Mar. Ecol. Prog. Ser. 78: 103-113, 1991
Gross growth efficiency phytoplankton concentration were larger during Day 2 than during Day 1 and it is less certain that the grazers Gymnodinium sp. GGEs exhibited a great deal of were in a state of balanced growth. If real, the in- variability (Fig. 6), ranging from 0.03 to 0.53 with a creased growth rate in low food treatments between mean of 0.21 (SD = 0.11; n = 29). Maximum GGE Days 1 and 2 (Fig. 1) suggests that the grazer may have appeared to be a function of food concentration, with become gradually conditioned to low food concentra- th.e highest value of 0.53 observed at a total phyto- tions over the course of the experiment. During Day 2, plankton concentration of 20 ygC 1-Iand low values of growth rates were maximal even at average phyto- 0.09 to 0.14 observed at total phytoplankton concen- plankton concentrations of 10 pgC 1-l. Upper water- trations > 200 pgC 1-'. column algal carbon concentrations at Station P ranged from 1 to 24 pg 1-' in May and August 1984 (Booth et al. 1988),and during 1987- 88 ranged up to 50 DISCUSSION pg I-' (Booth pers. comm.). Cells i 5 pm in size ac- counted for a large fraction of this biomass and thus Gymnodinium sp. growth rates reported here must be may have been available to small protozoan grazers. In interpreted very cautiously. Rates were based on only as much as Gymnodinium is representative of open 2 time points, initial and final, and could easily be bias- subarctic microzooplankton species, sufficient concen- ed by experimental error. Also, the 24 h time interval trations of phytoplankton are present in the open between samples may have been too long, such that subarctic Pacific to support high rates of protozoan critical portions of the exponential growth curve could growth. If phytoplankton biomass begins to increase, have been missed. Given the paucity of data on hetero- high growth rates of individual protozoan species trophic dinoflagellates, however, it was decided that should enable these grazer populations to keep pace the data should be presented, at least for comparison with incipient blooms. with similarly-collected data for other protist species. The cell volume of the heterotrophic dinoflagellate Gymnodinium exhibited a maximum growth rate of 0.7 Gymnodinium was observed to vary by a factor of d-l, reached at a concentration (critical concentration) 2 over the range of phytoplankton concentrations of ca 80 pgC 1-' on Day 1 of the experiment. Growth employed in this experiment. While the relationship rates reported for other herbivorous dinoflagellates are between grazer cell volume and food concentration similar, but critical concentrations are much higher (Fig. 2) resembled that between growth rate and food (Table 1). Critical concentration differences may be concentration (Fig. lA), there was no direct relation- due to differences in feeding mechanism, suitability of ship between grazer cell volume and growth rate. The prey species, or isolation location (oceanic species may same finding has been reported for the microflagellate be adapted to lower food levels). When adjusted to Pseudobodo sp. grazing on the prasinophyte Micro- 12 "C, reported maximum growth rates of mlcroflagel- monaspusilla (Parslow et al. 1986). Goldman & Dennet lates and ciliates are consistently higher than those of (1990) determined that the microflagellate Paraphyso- dinoflagellates (Table 1). It is not known whether this monas imperforata could adjust its cell volume about observation is generally true, perhaps owing to differ- 5-fold when fed a range of prey types, becoming very ences in feeding mechanism among the 3 groups of small when bacteria were the food source and larger organisms, or whether the discrepancy is due to the when large phytoplankton species were offered. They small number of species thus far investigated. theorized that feeding by direct interception is most The maximum observed Gymnodinium growth rate efficient when the difference between predator and of 0.7 d-' is approximately equal to growth rates deter- prey size is minimized. This hydrodynamic explana- mined for mixed layer phytoplankton at Station P. tion, however, does not seem to apply in the present Average phytoplankton growth rates during spring case: Gymnodinium cell volume apparently increased and summer 1984, measured using 2 different tech- as Synechococcus (the smaller of the 2 food species) niques, ranged from 0.63 to 0.68 d-' (Booth et al. 1988, made up a progressively larger fraction of the diet Miller et al. 1988). During September 1987, when (Fig ?A). Alternatively, since a positive relationship Gymnodinium was collected, pigment-specific phyto- was observed between Gymnodinium cell volume and plankton growth rates as measured by the seawater total phytoplankton ingestion (Fig. 7B),cell-volume dilution technique ranged from 0.07 to 0.59 d" (Strom changes in this species may simply reflect the amount & Welschmeyer 1991). of food contained within grazer food vacuoles at any Dinoflagellate growth rates were high at relatively given time. The average number of cells contained in a low phytoplankton concentrations, especially during grazer should increase with food concentration until Day 2 of the experiment. Again, these data must be digestion, rather than encounter rate or rate of interpreted cautiously, especially as decreases in phagocytosis, limits ingestion. Strom: Dinoflagellate growth and grazing 109
Table 1. Growth rates and critical concentrations for varlous species of protozoans feeding herbivorously. Taxonomic affiliat~on: D = heterotrophic dinoflagellate;MF = microflagellate;T = tintinnid ciliate; A = aloricate ciliate. nd = not determined
Grazer Taxonomic Temp. Food(s) p ,,,,,\ p mdr; Critical Reference affiliation ('C) (d-l) adjusted' conc. -l (WC 1.')
Flagellates Gymnodinium sp. D 12 lsoch~ysisgalbana + 0.7 0.7 80 This study Synechococcus sp
Gymnodinium sp. D 1 Naturally occurring 0.3 0.6 250 Bjerrnsen & (fieldpopulation) phytoplankton Kuparinen (1991)
Ox yrrhis D 20 Phaeodactylum tricornutum 1.3 0.8 nd Goldman et al. (1989) marina I. galbana 0.8 0.5 Dunaliella tertiolecta 0.8 0.5
Protoperidinium D 20 Leptocyl~ndncusdanlcus 1.2 0.7 1250 Jacobson (1987) hirobls Polykrikos D nd Scrippsiella trochoidea 0.7 - 450 Gaines (1988) koloidl~ Oxyrrh~s D nd D.tertiolecta 0 7 - 175 manna
Paraphysomonas MF 24 D. tertiolecta 2.4 1.1 nd Goldman & Caron imperforata Chlorella stlgmataphora 2.4 1.1 (1985) C. capsulata 2 3 1 .O 1, galbana 2.4 1.1 Porphyridium sp 2.4 1.1 l? tricornutum 2.5-3.5 1.1-1.6
Pseudobodo sp. MF 18 Micromonas pusilla 2.0 1.4 nd Parslow et al. (1986)
Ciliates Tin tinnopsis cf. T 18 1. galbana + 1.4 acuminata Monochrysis lutheri Eutintinnus T 18 I. galbana t M. lutheri + 1.4 pectinis D. tertiolecta Helicostomella T 18 1. galbana + D. tertiolecta 0.8 subulata
Favella sp. T 15 Heterocapsa tnquetra 1.1 95 Stoecker et al (1983)
Tin t~nnopsis T 15-25 1. galbana 1.3-2.0 100 Verity (1985) acumjnata Tin tinnopsis T 5 - 15 Dicra tend inorna ta 0.5-1 .l 120 vasculum
Lohmaniella A 15-23 Naturally occurring 1.O nd Rassoulzadegan (1982) spiralis particulate matter
Strom bidium A 12 Pyranumonas sp. 0.9 450 Jonsson (1986) reticula tum Lohmaniella A 12 Pyramimonas sp 1.0 450 spiralis
Balanion sp. A 15 H. triquetra 2.2 nd Stoecker et al. (1986) 'h,, adjusted to 12 "C, assuming a Q,, = 2 ' 'hax adjusted to 12 "C,using Q,, values determined for these organisms 110 Mar. Ecol. Prog. Ser. 78: 103-113, 1991
vorous protozoa (Fig. 8 references), given the smaller size of this grazer. The maximum carbon-specific ingestion rate of 0.19 h-' (based on a relatively high Gymnodiniurn cell carbon content) is compatible with a growth rate of 0.7 d-' and a GGE of 0.15, both typical values for this grazer feeding at high food concen- trations (Figs. 1 & 6). Clearance and ingestion of Isochrysis galbana by Gynlnodiniurn exhibited a classic relationship to food concentration (Fig. 3),with clearance reaching a mini- mum and ingestion a maximum at 40 to 50 pg 4001 I. galbana C I-'. Similar functional response curves -1 0 1 2 I (Synechococcus) / I (total) have been obtained for a number of ciliate species (Heinbokel 1978, Fenchel 1980, Scott 1985, Verity 1985, Jonsson 1986). Maximum clearance rates on I. galbana and Synechococcus were similar (0.8 to 1.2 p1 ind.-' h-'), but maximum ingestion rates on Synecho- coccus were higher than rates on I. galbana (30 and 20 clgC ind.-' h-', respectively). Clearance and ingestion rates as a function of food concentration for Gyrnnodinium sp. feeding on Synechococcus sp. exhibited an unusual pattern. Apparently there was no grazing on Synechococcus until initial concentrations of this species were > 20 pgC 1-' (Fig. 5). At initial concentrations above this I I I 1 1 i -10 0 10 20 30 40 50 60 level, ingestion gradually increased with food concen- I (pgC/ind*hr) tration; rates remained high even when Synecho- coccus concentrations were reduced well below the Fig 7. Cell volume of Gymnod~niumsp. versus Synecho- initial threshold level of 20 pgC 1-' on Day 2 of the coccus sp. as a fraction of d~et(A) and total phytoplankton ingestion (B)
Maximum volume-specific clearance rates for the heterotrophic dinoflagellate Gymnodinium sp. are higher than maximum rates observed for larger her- bivorous protozoans, but appear reasonable given the small size of this grazer (Fig. 8) Clearance rates for the dinofIagellate Oxyrrhis marina, shown for comparison, appear anomalously low. Reported clearance rates for 0. manna may not represent true maxima achievable by the organism at lower food concentrations. Alternatively, this species may employ a different feeding mech.an.ism than Gymno- dinium. 0. marina appears to seek out and capture prey cells by extension of a th1.n fi.larnent (Goldman et al. 1989). Calculation of clearance rates may not be appropriate for this class of grazer, in which the cell volume (pm3) capacity to ingest relatively large prey cells compen- Fig. 8. Max~mumvolume-specific clearance rates of herb]- sates for the inability to process large volumes of vorous Protozoa. 1, Tint~nnopsisvasculum, 2, T acuminata (+ water. It is not known whether Gymnodlnium feeds Verity 19851; 3. Lohman~ella spiral~s;4. Strombidlurn reti- by utilizing a capture filament or, like chrysomonad culatum; 5. S. vestitum (X: Jonsson 1986);6, Oxyrrhis marina microflagellates, by entrainment of food cells into a fed Isochrysis galbana; 7, 0. marina fed Phaeodactylum water current (e.g. Fenchel 1982). tncornutum (W:Goldman et al. 1989);8, Lohman~cllaspiralis (V:Rassoul7adcgan 19821, 9, 7: cf. acuminata; 10, Hel~co- Maximum carbon-specific ingestion for Gymno- stomella subulala (a: Hcinbokel, 1978); 11, Gymnodjnjum sp dinium is comparable to rates reported for other herbi- (A. thts study) Strom: Dinoflagellate growth and grazlng 111
concentrations. Grazing thresholds of the latter type may not be apparent when grazers are offered only a single prey species. Euphotic zone concentrations of Synechococcus spp. in the natural environment of the subarctic Pacific are consistently lower than the Gyrnnodinium threshold level of 20 pgC I-',ranging from < 1 to about 15 pgC 1-' (Booth et al. 1988, Booth pers. comm.). Once grazing is initiated, Gyrnnodiniunl can effectively consume Synechococcus at these low levels (Fig. 4B); perhaps transient peaks in prey abundance occur which suffice to trigger the grazing response. Alternatively, Gymno- dinium may not be an important consumer of Synecho- coccus in this ocean region. Prey selection by Gymnodinium was dependent on initial prey concentration (Fig. 10). In flasks with an initial phytoplankton concentration < 75 pgC 1-' (initial Synechococcus concentration < 25 pgC 1-l) Isochrysis galbana was always ingested in greater proportion than would be predicted by its relative abundance. In flasks with higher initial food concentrations, Synecho- coccus was generally the preferred prey species. In other words, once significant grazing on Synechococ- cus commenced, this species was grazed preferential- 0 20 40 60 80 100 120 initial conc (pgC/l) ly. These findings contrast with those of Heinbokel (1978), who observed no selective grazing by Eutintin- Fig. 9. Initial versus final concentration of Isochrysis galhdna nus pectinnus under conditions of low food concentra- [A)and Synechococcus sp. (B).Solid lines represent equality tion, preferential ingestion of I. galbana occurring only [no change in concentration dur~ngexperiment). I. galbana when average food concentration exceeded 60 pgC 1-l. was grazed at all initial concentratlons (all points fall below line), while Synechococcus was not grazed until ~nitial The dynamics of prey selection by protozoan grazers concentrations exceeded 20 pC I-' may depend on whether there exists a threshold feeding response for any of the prey species available. Additionally, as suggested by Verity (1991), selective experiment (Fig. 9). An identical result was obtained ingestion may be determined by the nutritional quality for the flagellate Paraphysomonas imperforata feeding of the entire spectrum of potential prey species. on Dunaliella tertiolecta both alone and in a food mix- ture (Goldman & Dennett 1990, their Fig. 6B),although the apparent threshold concentration for P imperforata was very high (11 700 pgC 1-l). The classic definition of a grazing threshold is the food concentration below which it is energetically unrewarding for a grazer to continue attempting to gather food (Frost 1975).The energy required to create feeding currents outweighs that gained from ingestion of the occasional phytoplankter, and the grazer goes into a reduced-activity or non-feeding mode. However, alternative types of grazing thresholds may be en- Concentration visioned in which it is energetically unrewarding to (Synechococcus/ total) either ingest or digest cells of certain species at low Fig. 10. Gyrnnodinium sp. Ingestion of Synechococcus as a food concentrations. Perhaps certain structural or fraction of total ingestion versus initial Synechococcus enzymatic modifications are necessary before a herbi- availability. Solid line represents no feeding preference vore can feed on a given phytoplankton species (ingestion directly proport~onalto relative abundance of prey species).(0) Initi.al phytoplankton concentrationc 75 ygC 1-' effectively. Once these modifications are made, it (initial Synechococcus concentration < 25 ygC 1-'); (0) initial would be energetically feasible for the herbivore to phytoplankton concentration >75 pgC 1.' (initial Synecho- graze this phytoplankton species even at very low coccus concentration > 25 pgC 1-l) 112 Mar. Ecol. Prog. Ser.
GGEs determined for Gymnodinium are within the ting them to reach a relatively high concentration range of values reported for other heterotrophlc proto- before being grazed to low levels. Maximum volurne- zoans, both herbivorous and bactenvorous (summa- specific clearance rates for Gymnodinium were as high rized by Caron & Goldman 1990). The tendency for or higher than comparable rates for herbivorous protozoan GGEs to decrease at high food concentra- ciliates, suggesting that heterotrophic dinoflagellates tions has been noted for a number of ciliate species have the potential to be major consumers of phyto- (Heinbokel 1978, Stoecker & Evans 1985, Verity 1985, plankton in marine systems Jonsson 1986). Verity (1985) calculated that assimi- lation efficiency (growth + respiration/ingestion) did Acknowledgements. I thank the members of the SUPER not decrease at high food levels during experiments program, in particular B. Frost, for encouragement and with herbivorous tintinnids; however, Stoecker & support. D. G~ffordassisted with isolation of Station P protists; Evans (1985) observed undigested phytoplankton cells D. Montagnes and F. J. R. Taylor gave taxonomic advice. The in the fecal material of ciliates feeding at high food manuscript was greatly improved by the comments of E. Lessard and an anonymous reviewer. This research was concentrations. This suggests that digestion may, in supported by N.S.F. grant OCE 86-13621 to B. W. Frost fact, be less efficient when food is very abundant. Lower assimilation efficiencies at high food levels have been reported for the herbivorous copepod Calanus LITERATURE CITED pacificus (Landry et al. 1984), although long acclima- tion times (> 1 wk) apparently are necessary for the Banse, K.(1990) Does iron really limit phytoplankton produc- required changes in digestive enzyme activity to occur tlon in the offshore subarctic Pacific? Llmnol. Oceanogr. (Hassett 1986). 35: 772-775 Bjarnsen, P. K., Kuparinen, J. (1991). Growth and herbivory Increased GGEs at low food concentrations mean by heterotrophic dinoflagellates in the Southern Ocean. that transfer of material from phytoplankton to higher studied by microcosm experiments Mar. Biol 109: trophic levels is most efficient when food is scarce. By 397-405 the same token, nutrient regeneration by grazers will Booth, B. C. (1988) Size classes and major taxonomic groups of phytoplankton at two locations in the subarctic Pacific be less efficient at low food concentrations. Changes in Ocean in May and August, 1984. Mar. Biol. 97: 275-286 digestive efficiency, which may underlie GGE varia- Booth, B. C., Lewin, J., Lorenzen, C. J. (1988). Spring and tion, indicate that the composition of fecal material summer growth rates of subarctic Paclflc phytoplankton produced by grazers can vary depending on their assemblages determined from carbon uptake and cell feeding regime. volumes estimated using epifluorescence rnicroscopy. Mar. Biol. 98: 287-298 Booth, B. C., Lewin. J., Norris, R. E. (1982). Nanoplankton species predominant in the subarctic Pacific in May and CONCLUSIONS June 1978. Deep-Sea Res 29: 185-200 Buck, K. R., Bolt, P. A., Garrison, D. L. (1990). Phagotrophy and fecal pellet production by an athecate dinoflagellate The herbivorous dinoflagellate Gymnodiniurn sp., in Antarctic sea ice. Mar. Ecol. Prog. Ser. 60: 75-84 isolated from Station P in the open subarctic Pacific, Caron, D. A., Goldman, J. C. (1990). Protozoan nutrient reached maximum growth rates at relatively low food regeneration. In: Capriulo, G M. (ed.) Ecology of marlne levels and grew rapidly even at low phytoplankton protozoa. Oxford Univ. 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This article was presented by E & B. She/-I; Corvallis, Oregon, Manuscript first received: March 11, 1991 USA Revised version accepted: October 1, 1991