MARINE ECOLOGY PROGRESS SERIES Published November 14 Mar Ecol Prog Ser
Long-term distribution patterns of mobile estuarine invertebrates (Ctenophora, Cnidaria, Crustacea: Decapoda) in relation to hydrological parameters
Martin J. Attrill'.', R. Myles ~homas~
'Marine Biology & Ecotoxicology Research Group, Plymouth Environmental Research Centre, University of Plymouth, Drake Circus, Plymouth PL4 8.4.4, United Kingdom 'Environment Agency (Thames Region), Aspen House, Crossbrook St., Waltham Cross, Herts EN8 8HE, United Kingdom
ABSTRACT: Between 1977 and 1992, semi-quantitative samples of macroinvertebrates were taken at fortnightly intervals from the Thames Estuary (UK)utilising the cooling water intake screens of West Thurrock power station. Samples were taken for 4 h over low water, the abundances of invertebrates recorded in 30 min subsamples and related to water volume filtered. Abundances of the major estu- arine species have therefore been recorded every 2 wk for a 16 yr period, together wlth physico- chemical parameters such as temperature, salinity and freshwater flow. Annual cycles of distribution were apparent for several species. Carcinus rnaenas exhibited a regular annual cycle, with a peak in autumn followed by a decrease in numbers over winter, relating to seasonal temperature patterns. Conversely, abundance of Crangon crangon was consistently lowest in summer, responding to seasonal changes in salinity, whilst Liocarcinus holsatus, Aurelia aurita and Pleurobrachia pileus were only pre- sent in summer samples, with P. pileus often in vast numbers (>l00000 per 500 million I).The estuar- ine prawn Palaemon longirostris showed no obvious sustained annual pattern, but evidence for a longer cycle of distribution was apparent. During 1989-1992 severe droughts in southeast England severely disrupted annual salinity patterns and coincided with a large increase in the Chinese mitten crab Eriocheir sinensis population. This included the first synchronised migration of adults in the UK. Settlement of young crabs during low-flow periods is suggested as an explanation for this population increase.
KEY WORDS: Thames Power station Sallnity Freshwater flow Crangon . Carcinus . Eriochejr Palaenlon . Aurelia . Pleurobrachia
INTRODUCTION 1971). Much work has been undertaken on the diurnal rhythmic behaviour of estuarine organisms (e.g. Patterns of organism distribution along the length of Janssen & Kuipers 1980, McCaw & Naylor 1992, La- an estuary are determined primarily by the longitu- prise & Dodson 1994), but less is known about the dinal salinity profile (Remane & Schlieper 1971), a effect of the seasonal variation on organism distribu- marine organism's penetration up the estuary being tion. Long-term data sets with seasonal components dependent on its tolerance to reduced salinities. The tend to concentrate on plankton (Carter & Dadswell ambient salinity at any one point can also vary on a 1983, Caste1 1993) and benthic invertebrates (Chap- daily basis (due to tidal movement) and on a seasonal man & Brinkhurst 1981, Henderson 1984). basis (due to variations in freshwater input), this vari- The intake of cooling water by power stations pro- ability increasing the stress organisms are subjected to vides a convenient and comparatively efficient method and thus affecting their realized distribution (McLusky of sampling estuarine organisms (Boreman et al. 1981, Summers 1989),the majority of studies focusing on fish communities entrained on intake screens (e.g. Turn- penny 1983, Robin 1988, 1991, Claridge & Potter 1994).
O Inter-Research 1996 Resale of full article not permitted Mar Ecol Prog Ser 143: 25-36, 1996
Continual sampling from the intakes has provlded items of debris entering the system. The final cooling some long-term, seasonal information on these fish pipes leading to the power station are 2.5 cm in dia- communities (Van den Broek 1979, Andrews 1984, meter, so all suspended solid matter and organisms Claridge et al. 1986), together with fewer data on >l cm in size are filtered and removed by 6 rotating entrained invertebrates (Huddart & Arthur 1971a, band (or Brackett) screens. At the top of the filter, the Karas 1992, Bamber & Henderson 1994). Knowledge screen is washed clean by water jets, and the filtered remains llmlted, however, on the consistency of cycli- material (including organisms) is flushed via a gutter cal seasonal patterns in estuarine populations due to system to a trash pit and then through a small opening the inherent difficulties of compiling such long-term back to the estuary. To collect organisms, square data sets. framed nets with mesh size of 1 mm were fitted to the Fish populations entrained on power station intake end of each screen's gutter. Sampling periods started screens were used during the 1960s and 1970s to 1 h before low water and continued for a period of 4 h. document the recovery of the polluted Thames Estuary, The nets were emptied and repositioned every 30 min, UK (Huddart Si Arthur 1971b, Wheeler 1979, Andrews the organisms caught in that sample being identified & Rickard 1980), with a diverse fish community being and enumerated. The total catch for the 4 h period can present by the end of the 1970s. Huddart & Arthur be summed for temporal comparison. (1971a, c) undertook supplementary studies on the Seasonal variability in the demand for electricity dic- brown shrimp Crangon crangon (L.) from power station tates the amount of water needed to cool the power intake screens, including seasonal abundance patterns station's generators, and hence the number of intake and their response to the low oxygen environment pumps in operation (up to 6). Due to this variation, the present In the Thames at the time. Other studies have number of organisms collected is expressed in terms of provided information on the benthic macrofaunal com- 500 million 1 to allow comparison between surveys. munity (Birtwell & Arthur 1980, Andrews et al. 1982, The number of pumps in operation can also vary dur- Attrill et al. 1996a),with Attrill et al. (1996b) describing ing a survey as demand fluctuates, the calculation for the response of upper estuary invertebrate com- number of individuals per 500 million 1 being: munities to seasonal changes in low freshwater flow. lOOA The aims of this paper are therefore to detail the popu- E = (P,X L,) +(P, X L2)+ ... + (P" X L,) lation dynamics of the most abundant invertebrates entrained on power station screens in the Thames Estu- where E is the expected number of individuals of a ary over a period of 16 yr and, where possible, to relate species in 500 million 1, A the actual number of indi- these patterns to changes in the major hydrological viduals recorded in the 4 h sample, P,, is the number of parameters. pumps in operation and L, the length of time (in hours) pumps P, are in operation.
SITE DESCRIPTION AND SURVEY METHOD
The Thames Estuary extends 110 km from Tedding- ton Weir (west London), the limit of tidal influence, through Greater London and out into the southern North Sea (Fig. 1). Approximately 10 million people inhabit the Thames catchment. The pressure of this large metropolitan population has resulted m periods of extremely poor water quality in the Thames Estuary (Wheeler 1979), with large anoxic stretches of the mid- estuary present during the 1950s. Recovery of the estu- ary since the 1960s has been documented by analysis of the fish communities entrained at power stations, particularly the coal-fired power station at West Thur- West ThurrockPower rock (Fig. l), situated on the north bank of the estuary Station 66.5 km seawards of Teddington Weir (National Grid Reference: TQ 590 770). The power station extracts up to 150 million 1 h-' of Fig. 1.Map of the Thames Estuary (UK)~ndicatlng surround- water from the Thames estuary for cooling purposes, ing metropolitan arcas and the pos~tionof West Thurrock the intake opening being situated well below the low power station. L: Lots Road power stat~on,Hamrnersmlth; tide mark. Vertical slats 0.5 m apart prevent large T: Teddington Weir Attrill & Thomas: Estuarine invertebrate distribution patterns
Samples of fish and invertebrates were taken by Ctenophora and Cnidaria staff from the National Rivers Authority (NRA), and precursory organisations, every 2 wk at spring tide The ctenophore Pleul-obrachia pileus (0.F Miiller) between 1977 and 1992. The power station was closed was consistently recorded in summer (Fig. 5), in vary- in 1993. ing numbers ranging from a maximunl of l0 (1985) to Daily flow rates of freshwater over Teddington Weir 100000 (1989). During such blooms P. pileus was the were measured by a NRA gauging station. Salinity and dominant organism in catches, resulting in sampling temperature measurements were obtained during nets rupturing under their weight in 1989. In the weekly water analysis runs by the NRA sampling ves- majority of years, peak abundances occurred during sel along the length of the estuary, the measurements June or July. The moon jellyfish Aurelia aurita (L.)also corresponding to a zone off West Thurrock power formed discrete summer peaks in abundance at West station. All salinity measurements were half-tide Thurrock, generally being recorded in samples in corrected to allow spatial and temporal comparison. July/August (Fig. 6). Large numbers of A. aurita were a regular occurrence during the first half of the survey, where abundances exceeded 1200 per 500 million 1, RESULTS but between 1985 and 1992 it was comparatively scarce, being absent during 1987. Physico-chemical data
As would be expected for a temperate estuary, there Crustacea: Decapoda: Natantia was a strong seasonal pattern in temperature over the 16 yr period surveyed, with summer peaks and winter Six species of natant decapod were recorded in sam- troughs (Fig. 2).However, there was a distinct increase ples from West Thurrock: Crangon crangon, Palaenlon in the maximum summer temperature between 1977 longirostrjs hdilne Edwards, Palaemon serratus (Pen- and 1984, this maximum temperature being main- nant), Pandalus montaguj Leach, Palaernon elegans tained above 20°C over the ensuing 8 yr. Consistently Rathke and Palaenionetes varjans (Leach), the latter 2 cold winters occurred between 1982 and 1988, with being rare occurrences, generally as single individuals. minimum temperature falling to around or below 5OC. P. serratus and P. montagui were regularly encountered In 1984 both the highest and lowest water tempera- during summer months, but in low numbers. C. crangon tures during the survey were recorded. was the most consistently abundant species recorded at Salinity followed a similar, although less predictable, West Thul-rock (Fig. 7), and displayed a distinct cyclical pattern to temperature (Fig. 3), particularly between distribution, regularly reaching peak abundances of 1977 and 1988. Annual patterns became far more vari- 10000 to 100000 per 500 million 1 at the end of each able in the ensuing years, with high summer salinities year. A dramatic decrease in numbers was evident, and, at the end of the survey period, no distinguishable generally during May and June, C. crangon being winter minimum. Consecutive droughts occurred in absent from samples in 4 years of the survey at this time. southeast England from 1989 to 1992 (Attrill et al. The estuarine prawn P. longirostris was also compara- 1996b), thus reducing the freshwater flow over Ted- tively abundant in the Thames, regularly occurring in dington Weir (Fig. 4) and disrupting the typical salinity significant numbers (peak of 50 000 per 500 milllon 1)on patterns within the estuary. High rainfall occurred at the screens at West Thurrock (Fig. 8). There is some the start of 1990, which is reflected in the reduced evidence that P. longirostris exhibits an annual cycle salinities at West Thurrock, but precipitation across the of abundance, with peak numbers in summer and catchment was minimal after this point, resulting in lower numbers during winter months, although this saline water being present all year off the power pattern was extremely weak, particularly over the station. second half of the survey. The population appears to have gone through phases of abundance in this part of the estuary, with larger scale peaks of abundance (e.g. Invertebrates 1982- 1985.1990- 1992) followed by several years when the species was comparatively scarce (1986-1990). A wide taxonomic range of invertebrates was recorded in samples from the intake screens at West Thurrock (Appendix l), but only 3 main groups (cteno- Crustacea: Decapoda: Brachyura phores, scyphozoan cnidarians and decapod crus- taceans) were regularly present in large enough Four species of crab were recorded at West Thurrock: numbers to warrant further analysis. Carcinus maenas (L.),Liocarcjnus holsatus (Fabricus), Degrees Celsius
30
0 1977 1978 1979 1980 1981 1982 1983 1984 Date Date
30
0 0 1985 1986 1987 1988 1989 1990 1991 1992 1985 1986 1987 1988 1989 1990 1991 1992 Date Dale
Fig. 2. Water temperature of the Thames Estuary off West Thurrock power Fig. 3. Salinity of the estuarine zone off West Thurrock power station, stallon, 1977-1992 1977-1992 Abundance (11500 million I +l) Cumecs 350 1,000,000 1
Date Dale Cumecs 350
0 l 1985 1986 1987 1988 1989 1990 1991 1992 1985 1986 1987 1988 1989 1990 1991 1992 Date Date
Fig. 4. Gauged freshwater flow of the River Thames over Teddington Weir, Fig. 5. Abundance of Pleurobrachia pileus recorded in fortnightly samples from 1977-1992 West Thurrock power station, 1977-1992 Abundance (11500 million I) Abundance (11500 million I +l) 1,400 1,000,000
0 1977 1978 1979 1980 1981 1982 1983 1984 Dale Dale
1985 1986 1987 1988 1989 1990 1991 1992 Dale Dale
Fig 6. Abundance of Au~eliaacrrYta recorded in fortnightly samples from West Fig. 7. Abundance of Crangon <.rangon recorded In fortnlyhlly samples from West Thurrock power stallon, 1977-1992 Thurrock power statlon, 1977-1992 Attr~ll& Thomas:Estuar~ne invertebrate distribution patterns
Eriocheir sinensis Milne Edwards and Macropodia ros- rock, but occurrences became more regular after 1988 trata (L.).M. rostrata was only recorded on 1 occasion, (Fig. 11). Abundances increased during 1991-1992 probably associated with drifting clumps of the hydroid culminating in 32 crabs per 500 million 1 being re- Sertularia cupressina L. The shore crab C. maenas ex- corded in November 1992. During October 1992, hibited a strong annual cycle (Fig g),with peak abun- hundreds of crabs were reported in the intake tanks of dances being recorded during October or November in Lots Road power station (Hammersmith), 45 km up- the majority of years, the size of this peak varying from stream of West Thurrock (Fig. 1).All crabs recorded at
1977 1978 1979 1980 1981 1982 1983 1984 1977 1978 1979 1980 1981 1982 1983 1984 Date Date
1985 1986 1987 1988 1989 1990 1991 1992 Date Date
FI~.9.Abundance of Carclnc~smaercas recorded In fortnightly samples from West Fiy, 10 Abundance of Llocarcinus holsatus recorded In fortnightly samples from Thurrock poiver slation, 1977-1992 West Thurrock power stat~on,1977-1992 Attrill & Thon~as:Estuarine invertebrate distribution patterns
Abundance (11500 milllion I) dence for this in the tidal channels of the Wadden Sea. Seasonal migrations within estuaries have also been recorded (Henderson & Holmes 1987), peak numbers occurring in the Severn Estuary (UK)between Novern- ber and January with annual abundance minima dur- ing the summer. This pattern is similar to that recorded in the Thames, suggesting that populations migrate seawards during winter to avoid reduced salinities and again later in the year to reproduce (Lloyd & Yonge 1947, Henderson & Holmes 1987). At these times C. Dale crangon are entrained on the screens of West Thurrock power station in large numbers, migrating further up Fig. 11 Abundance of Erjocheir sinensls recorded in fort- nightly samples from West Thurrock power station, the estuary during late spring/summer to exploit the 1977-1992. Data have been aggregated to quarters: Jan-Mar, patterns of encroaching salinity. It would therefore Apr-Jun, Jul-Sep, Oct-Dec appear that this pattern of migration is indicative of estuarine populations of C. crangon; the Severn and additional competitor to resident Carcinus maenas, as Thames are considered to hold separate populations of their diets and feeding behaviour are similar (Choy 1986, this species (Henderson et al. 1990). In turn, these Lee & Seed 1992).However, the population peak of C. patterns are distinct from intertidal populations, which maenas at West Thurrock occurs later in the year. reach peak numbers in summer and lowest abun- Longer-term population trends are evident for L. holsa- dances during winter (Beukema 1992). tus, with the species being absent, or recorded in very The shore crab Carcjnus maenas also demonstrates a low numbers, between 1984 and 1989, with no associ- cyclical pattern of distribution within the Thames, with ated change in the overall salinity reqime for these years. lowest abundances being recorded at West Thurrock The gelatinous planktonic species also recorded during January-February and peak numbers during peaks in abundance during the summer months, Aure- October-November. The vast majority of individuals lia aurita being present for a much shorter length of were green type, the orangehed crabs being compa- time than Pleurobrachia pileus and being much less ratively intolerant of reduced salinity and thus rarer common over the second half of the survey period. An- in estuaries (McGatv & Naylor 1992). Aagaard et al. nual population blooms have been widely recorded for (1995)recorded peak numbers of C. maenasduring Au- both species (e.g. Moller 1980, Van der Veer & Sadee gust in a Danish fjord, and associated this with a peak 1984, Frank 1986, Schneider & Behrends 1994) and in foraging activity rather than migrations of the popu- have been reported to have a significant impact on the lation. Lowest abundances were between February and zooplankton community (Kuipers et al. 1990, Behrends May and related to temperature-dependent activity & Schneider 1995). However, peak abundances as patterns, the threshold for active foraging being 8°C recorded at West Thurrock (Figs. 5 & 6) are later in the (Naylor 1962, Atkinson & Parsons 1973).Water temper- year for both species than in surrounding North Sea atures in the Thames Estuary fell below this value for areas. Van der Veer & Sadee (1984) recorded maxi- only a short period during January-February in most mum abundances of P. pileus in the Dutch Wadden Sea years (Fig. 2),coincident with minimum abundances of during May, whilst A. aurita populations generally C. maenas, on the intake screens of the power station. reach a peak in late spring (Norway: Olesen et al. 1994; It would therefore appear that reduction in catches of Southampton Water: Lucas & Williams 1994). This C. maenas is due to an inactive population rather than delayed peak in the Thames may reflect the period migration of adults. Peak numbers of crabs in the required for the population to move up the estuary. Thames occurred later than those recorded by Aagaard Several of the crustacean species recorded at West et al. (1995),but this does not necessarily suggest any Thurrock are present in the estuary over the majority differences in foraging behaviour. Peaks on the intake of the year, but the abundances of 2 species on the screens tended to be due to a large, and short-term, intake screens show distinct annual, cyclical fluctua- influx of small crabs which masked any other cycles of tions. The brown shrimp Crangon crangon is perhaps activity that may have been present. the most common mobile macrofaunal species in large Two other crustaceans, Palaemon longirostris and north European estuaries with strong tidal currents Eriocheir sinensis, did not show distinct annual cycles (Tiews 1970) and is present in very large numbers in sustained throughout the length of the survey. Between the Thames. Tidal migrations have been recorded for 1977 and 1982, however, peak abundances of P. Ion- the species (Hartsuyker 1966, Al-Adhub & Naylor girostris were recorded during the summer, with cor- 1975),although Janssen & Kuipers (1980) found no evi- responding minima in winter. This pattern is in direct Mar Ecol Prog Ser 143: 25-36, 1996
contrast to other studies on the species in European from the power station for the previous 7 yr, suggesting estuaries (e.g. Marchand & Alliot 1981, Van den Brink an aggregation of crabs, perhaps on their spawning mi- & Van der Velde 1986, Cartaxana 1994) which have gration. This is supported by the fact that all females recorded movements of P. longirostns in response to were ovigerous (so must have spawned in the upper changes in salinity. The prawns migrate up to freshwater half of the estuary) and particularly by the observation during periods of reduced river flow and thus should have of the large number of crabs at Lots Road power station been absent at the mid-estuary site at West Thurrock dur- during October. The estuary at West Thurrock is wider ing summer if this pattern was repeated in the Thames. and deeper than at Lots Road, allowing the crabs to dis- During subsequent years this annual pattern was not so perse. As 30 were still caught on the power station in- evident, the appearance of P. longirostris on the screens take screens on the north bank, it would suggest that being sporadic between 1986 and 1990 when the species there were very 1.arge numbers of crabs in the estuary was absent in samples for periods of time. P. longirostns at the time. It would appear that this movement of crabs does, however, show evidence of longer-term population during the end of 1992 was the first large scale breed- variation, with large numbers of prawns being present in ing migration of E. sinensis in the UK. the estuary between 1982-1984 and 1990-1992. During The question arises as to why, after 60 yr of occa- this latter period, drought conditions occurred in the sional records from the UK, the Thames population of Thames region, with consistent low freshwater flow Eriocheir sinensis has suddenly increased in size. Vast (Fig. 4). If the population was following the salinity front, numbers of these crabs are a familiar sight in other it would be expected that the majority of P. longirostris northern European countries, but they have never, would inhabit the upper reaches of the estuary for most of until now, reached such large abundances in the UK. the time, but this was clearly not the case. Ingle (1986) suggested that the UK rivers were much Ever since the first Chinese mitten crab was recorded faster flowing than their northern European equiva- from the Thames Estuary (Harold 1935) records have lents, so preventing any large scale settlement and been few and far between, despite the intensive sam- development of mitten crabs. From the evidence pre- pling undertaken during the 1960s and 1970s to docu- sented in this paper it would appear that this hypothe- ment the clean-up of the estuary. The intensity of sam- sis may be correct. The rise in the number of E. sinen- pling continued during the 1980s, with the use of the sis recorded in the Thames Estuary coincides with the intake screens at West Thurrock power station, but the overall long-term decrease in freshwater flows follow- status of the mitten crab as an occasional, interesting ing the drought period since 1989 (Fig. 4). It is possible, vagrant continued. The data presented in Fig. 11 therefore, that this decrease in flow allowed the estab- demonstrate how this pattern changed during 1991 and lishment of young crabs during 1989, these crabs 1992, with adult crabs appearing more regularly at developing to maturity and so formed the spawning West Thurrock culminating in 32 individuals being migration witnessed at the end of 1992. At this time the caught in November 1992. This figure for 1 survey is crabs would be approximately 3 to 4 yr old, the age of greater than the combined records of Eriocheir sinensis maturity suggested for the species (Ingle 1986).
Appendix 1. Taxonomic list of invertebrate species recorded from the intake screens at West Thurrock power station. Drifting colonial organisms have been excluded
Cnidaria Crustacea: Decapoda Aurelia aurita (L ) Crangon crangon (L.) Ctenophora Pandalus rnontagu~Leach Pleurobrachja pileus (O.F,bliiller) Palaernon elegans Rathke Amelida: Polychaeta Palaernon longirostns Milne Edwards Nere~sd~vers~color (O.F. Miiller) Palaemon serratus (Pennant) Nereis succlnea (Frey & Leuckart) Palaernonetes varians (Leach) Crustacea: Amphipoda Carcinus rnaenas (L.) Corophjurn volutator (Pallas) Erlochelr s~nensisMilne Edwards Garnrnarus salinus Spooner Liocarcinus holsatus (Fabncus) Gamrnarus zaddachi Sexton Macropodia rostrata (L.) Crustacea: Isopoda Mollusca: Bivalvia ldotea sp. Mytilus edulls L. Crustacea: Mysidacea Mollusca: Cephalopoda Mesopodopsis slabberi (Van Beneden) Alloteuthis subulata (Lamark) Neornysis integer (Leach) Sepia officianalisL. Praunus flexuosus (Miiller) Sepiola atlantica dtOrblgny Schistornysis spiritus (Norman) Urochordata: Ascidiacea .Sinelk armata (Milne-Edwards) Wfolgula rnanhattensis (de Kay) Attrill & Thomas: Estuar~neinvertebrate distribut~onpatterns
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Thisarficle was submitted to the editor Manuscript first received: June 23, 1996 Revised version accepted: September 3, 1996 Abundance (11500 million I) Abundance (11500 million I)
1977 1978 1979 1980 1981 1982 1983 1964 1977 1978 1979 1980 1981 1982 1983 1984 Dale Dale
1985 1986 1987 1988 1989 1990 1991 1992 Dale Dale
Fig. 9. Abundance or Carcini~srr~aenas recorclecl in fortnightly samples Fig. 10. Abundance of Liocarcinus holsatus recorded in fortnightly sam- from West Thurrock power stallon, 1977-1992 ples from West Thurrock power station, 1977-1992