Centropages Hamatus Research Articles

Seasonal and spatial dynamics of Centropages typicus and C. hamatus in the western North Atlantic

Centropages typicus and Centropages hamatus are two of the most abundant copepods on the continental shelf in the mid-Atlantic region of the western North Atlantic. Their range extends from the Scotian Shelf ( C. typicus) and the Grand Banks ( C. hamatus) in the north to Cape Hatteras in the south. South of Cape Hatteras they have only been observed in inshore waters of North and South Carolina and not offshore on the continental shelf or in coastal waters of eastern Florida. However, C. hamatus has been observed in western Florida. Abundances of both species are greatest in inshore regions in the mid-Atlantic Bight with C. hamatus tending to have a more coastal distribution. In this region seasonal variability is low with high abundances from late fall through mid-summer. In the north year-round presence of both species is confined to inshore areas and offshore banks such as Georges Bank, and Browns Bank, Emerald Bank and Western Bank on the Scotian Shelf. In this northern region there is a pronounced seasonal cycle in abundance with high abundances during late summer and fall. Periods of high reproductive rates are closely linked to blooms of large phytoplankton and food availability rather than temperature appears to be controlling population abundances.

Production and biochemical composition of eggs from neritic calanoid copepods reared in large outdoor tanks (Limfjord, Denmark)

Populations of Acartia tonsa (Dana) and Centropages hamatus (Lilljeborg) were monitored in outdoor enclosures imitating the natural estuarine environment in northern Denmark from August 2003 to February 2004. A. tonsa was predominant in summer while C. hamatus dominated from October and onwards. Mean egg production was normally higher in C. hamatus as compared to A. tonsa and continued until late December before a decrease was observed. Hatching success remained high even in February at temperatures below zero degree indicating that no diapause eggs were produced. Further, the egg quality evaluated from the fatty acids (AA, EPA and DHA) and amino acid contents did not change markedly during the study period. From this observation it is strongly suggested that nauplii originated from copepod eggs produced throughout the monitored sampling period constitute an adequate live feed for fish larvae probably even year round.

Top-down impact by copepods on ciliate numbers and persistence depends on copepod and ciliate species composition

Experiments with the copepods Acartia clausi, Centropages hamatus and Pseudocalanus sp. were performed to assess the species-specific effect of these copepods on the development of monospecific algae (Nephroselmis pyrifomis) and ciliate communities (Strombidium vestitum, Strombidium conicum, Strombidium sp. and Lohmanniella oviformis). It was hypothesized that potentially switching copepods like A. clausi will stabilize the algal community by switching between ciliate and algal food, in contrast to copepods with stereotypic filter feeding behaviour (Pseudocalanus sp.). In treatments with Pseudocalanus sp. and C. hamatus, all ciliate species were wiped out in 2 days, resulting in blooms of N. pyriformis. In treatments with A. clausi, two of the ciliate species were able to persist, but the combined ciliate and copepod community was not able to control the algal bloom. Ciliates became abundant in control treatments without copepods, but only S. vestitum and S. conicum seemed able to establish grazing control. Hence, when evaluating the role ofciliates in food webs, their actual numbers and species composition should be taken into account. Likewise, the species composition of copepods may be crucial; these experiments demonstrate that small filter feeding copepods may have tremendous impact on ciliate numbers.

Linking foraging strategies of marine calanoid copepods to patterns of nitrogen stable isotope signatures in a mesocosm study

The foraging modes of calanoid copepods differ in that stationary suspension-feeding is more easily detected by prey with strong escape responses (ciliates) than is ‘cruising’ or ‘ambushing’ feeding. Thus, the ability of a copepod to include heterotrophic prey in its diet may be associated with its foraging mode and, further, with its nitrogen stable isotope signature (δ15N). This is because a more carnivorous diet may be expected to result in a higher δ15N. We tested this hypothesis in a mesocosm study using a density gradient (0 to 80 ind. l-1) of calanoid copepods. We expected copepod δ15N to generally increase with decreasing copepod density because of increased food availability, and predicted stronger increases for cruising than for stationary suspension-feeding species. As an assemblage, copepods had a pronounced impact on the food web: diatoms and ciliates decreased, whereas nanoflagellates increased with increasing copepod density. As expected, Centropages hamatus, a cruising species, showed the strongest isotopic increase and also highest population growth at low copepod density, suggesting that it was the most efficient species in capturing ciliates. Temora longicornis, a stationary suspension-feeder, showed a uniform isotopic increase in all mesocosms, which we believe resulted from nutritional stress arising from poor feeding on both ciliates (too fast for ingestion by T. longicornis) and nanoflagellates (too small). However, Pseudocalanus elongatus, a species equally categorised as a stationary suspension-feeder, showed increases in its δ15N similar to those for C. hamatus. While this may indicate potential switching in its foraging mode, alternative explanations cannot be ruled out, partly because qualitative and quantitative aspects of trophic enrichment in our experiment could not be clearly separated. This study shows that consumer δ15N are difficult to interpret, even if potential food sources and aspects of the species’ biology are known, and thus emphasises the necessity for further laboratory studies to help better interpret zooplankton δ15N in the field.

Estuarine calanoid copepod abundance in relation to season, salinity, and land-derived nitrogen loading, Waquoit Bay, MA

Calanoid copepod abundance and distribution were measured by monthly plankton tows from May through November in 1998 and March through August in 1999 in three subestuaries of the Waquoit Bay estuarine system. There was a dramatic seasonal change in the composition of calanoids in Waquoit Bay with a spring community consisting of Acartia hudsonica, Centropages hamatus, and Eurytemora affinis, replaced by Acartia tonsa in the summer. The abundance of A. hudsonica, A. tonsa, and C. hamatus was higher in the saltier reaches of the subestuaries, but the abundance of E. affinis and copepod nauplii was not affected by salinity. Even though different land-derived nitrogen loads created different concentrations of chlorophyll in the water of Waquoit Bay subestuaries, there was no apparent response in the abundance of most calanoid copepods to differences in nitrogen load to the subestuaries. The uncoupling of phytoplankton food from mesozooplankton consumers suggests that other factors such as water residence time, phytoplankton quality and composition, or variations in microzooplankton food play a role in the abundance dynamics of Waquoit Bay calanoid populations.

Seasonal variability and inter-specific differences in hatching of calanoid copepod resting eggs from sediments of the German Bight (North Sea)

Hatching experiments were conducted on sediment samples collected on seven cruises between March 2002 and February 2003, at four stations near Helgoland (54 � 11 0 N, 7 � 53 0 E). Samples were incubated for up to 12 months and examined weekly for calanoid copepod nauplii. 12807 nauplii hatched in total. Of these 36.43% were stage N1 (pooled), 44.08% were Temora longicornis (stage N2 and older), and 17.96% were Centropages hamatus (stage N2 and older). Some Acartia spp. and an unidentified species were found, too. Hatching varied significantly between sampling dates. At three stations, counts of all nauplii peaked in samples taken in April, while at one station the maximum was recorded in the sample collected in October. The seasonal pattern of hatching was similar in T. longicornis and C. hamatus. In March, April, October and February numbers of all nauplii were initially low, but increased over the following 2‐3 weeks. In June, August and December, however, average numbers were comparatively high at first, but declined thereafter. In three of the four samples that were incubated for 12 months nauplii hatched from the sediment for the entire experimental period. Temora longicornis and C. hamatus displayed clearly distinct patterns of hatching in these long-term incubations.

The effect of temperature and duration of incubation on the hatching of diapause eggs of Centropages hamatus (Lilljeborg) (Copepoda, Calanoida)

Diapause eggs of Centropages hamatus were used to investigate the effect of temperature and duration of incubation on egg hatching. Eggs were incubated for 10, 12, 14, 16, 20, 24, 28, 32, 36 and 40 h at 15°C and 14L–10D. After incubation for the designated period, eggs were transferred to 25°C and monitored periodically to determine egg hatching. Control eggs were incubated solely at 15°C and monitored for egg hatching. The greatest daily hatching success of eggs occurred within 1 or 2 days after transfer from 15°C to 25°C, while the controls required 3–4 days. The cumulative hatching success of eggs was significantly lower than the control, with the exception of eggs held for at least 36 h at 15°C before transfer to 25°C. These results indicate that overall time to hatching of diapause eggs of C. hamatus can be reduced by transferring the eggs to a higher temperature, for example, 25°C, following a minimum period of time (36 h) at reduced temperature, for example, 15°C. Exposure to 15°C for only 10 h does not appear to be sufficient to result in any subsequent hatching at higher temperature.

Assessment of Cryptophyceae ingestion by copepods using alloxanthin pigment: a caution

The accessory pigment alloxanthin is a well-known taxonomic marker for Crypto- phyceae in natural seawater. The use of alloxanthin to estimate in situ zooplankton grazing and selectivity on Cryptophyceae was studied using 2 dominant copepods of the southern North Sea, Temora longicornis and Centropages hamatus. High-performance liquid chromatography (HPLC) analyses of water samples and of freshly caught copepods, starvation experiments, and feeding experiments (shipboard bottle-incubations) were carried out. Although alloxanthin was always detected in copepod extracts, (1) no correlation was found with alloxanthin in seawater, and (2) no significant grazing on Cryptophyceae was observed. Results of our gut-evacuation experiments showed that at least 78% of the initial alloxanthin content of T. longicornis and C. hamatus remained after 90 min, whereas fucoxanthin and chloropigments decreased rapidly with time. Alloxanthin and astaxanthin esters were the only pigments remaining in the body of T. longicornis after several hours' starvation. It is concluded that most of the detected alloxanthin did not originate from the gut but from the body tissues. Our results suggest that alloxanthin is not suitable as a biomarker for quantitative or qualitative estimates of copepod grazing on Cryptophyceae.

Seasonal variation in egg morphology and hatching success in the calanoid copepods Temora longicornis, Acartia clausi and Centropages hamatus

We report here on the seasonal changes in ultramicroscopic morphology of the eggs of the calanoid copepod Temora longicornis. During a field study between 1996 and 1998 in the Menai Strait, eastern Irish Sea, T. longicornis produced two types of eggs; hatching or subitaneous eggs were spawned during autumn-winter, whereas a high proportion of non-hatching eggs (up to ∼8%) were found during spring-summer. Hatching and non-hatching eggs looked very similar under the light microscope, both being round in shape and with no apparent external ornamentation or spines. However, analysis under the scanning electron microscope showed clear differences in external morphology and in the thickness of the chorion of the two egg types. The surface of the subitaneous egg was smooth overall with circular pits and the egg chorion was thin (∼1 μm). The surface of the non-hatching egg was ornamented with rough ridges and the chorion was thick (∼5 μm). The diameter and the surface of the eggs spawned by T longicornis were also different from those of Acartia clausi and Centropages hamatus, two calanoid species commonly found in the Menai Strait. It is concluded that the non-hatching eggs produced by T longicornis are resting egg stages. The massive production of resting eggs by T. longicornis during peak reproduction may have profound implications for the population dynamics of this copepod species and the ecology of the area.

Phycotoxin accumulation in zooplankton feeding on Alexandrium fundyense--vector or sink?

Zooplankton can consume toxic Alexandrium spp. dinoflagellates in the Gulf of Maine and retain paralytic shellfish poisoning (PSP) toxins, potentially acting as toxin vectors. We performed experiments to determine toxin budgets for common species of copepods (Acartia hudsonica, Eurytemora herdmani, Centropages hamatus) feeding on toxic Alexandrium fundyense, offered as monocultures or in mixtures of algal prey, by comparing calculated toxin ingestion rates and toxin content of copepod body tissue and fecal pellets. When fed monocultures, both copepod tissue and fecal pellet fractions accounted for s 5% each of the calculated ingested toxin, and thus by difference ≥ 90% was lost as a dissolved fraction into the seawater medium. The presence of alternative food did not significantly alter the efficiency of toxin retention. Sloppy feeding or regurgitation are probable mechanisms for release of toxin to sea water. Experiments using varying concentrations of A. fundyense and alternative non-toxic species did not show significant acts of cell concentration on toxin retention efficiency. Total toxin retained and efficiency of retention varied among copepod species. Toxin profiles (% molar composition) of dinoflagellates, copepod tissues and fecal pellets differed slightly, suggesting some metabolic transformation. Because of their low retention efficiency, copepod grazers can effectively disperse PSP toxins produced by Alexandrium spp. into the environment, where they are much less likely to be harmful-zooplankton act as a sink for PSP toxins. Nevertheless, sufficient toxin body burdens are attained to contribute to propagation of PSP toxins to other trophic levels.

Nutrient composition and metamorphosis success of Atlantic halibut (Hippoglossus hippoglossus , L.) larvae fed natural zooplankton or Artemia

Atlantic halibut larvae were fed docosohexanoic acid- (DHA) selco enriched Artemia (RH-cysts) or wild zooplankton in duplicate tanks from first-feeding and 60 days onward. The zooplankton were collected from a fertilized sea water pond and consisted mainly of different stages of Eurytemora affinis and Centropages hamatus. There were no differences in survival, or in growth during the first 45 days of feeding, between larvae fed the two prey items, but the larvae fed Artemia showed much higher incidence of malpigmentation and impaired eye migration than larvae fed zooplankton. The prey organisms contained similar amounts of dry matter and protein, but Artemia was higher in lipid and glycogen than the zooplankton. Larvae fed Artemia were higher in both glycogen and lipid than the zooplankton-fed larvae towards the end of the feeding period. There were large differences between the prey organisms in the concentrations of essential fatty acids (% of total fatty acids) which was reflected in the fatty acid composition of the larval body. It is concluded that the macronutrient composition of Artemia in the present study was probably within the optimal range for promotion of growth and survival in young Atlantic halibut. The concentration of n-3 HUFA, and especially DHA, is however, very much lower in enriched Artemia than in copepods, and may be one of the factors triggering developmental errors in Atlantic halibut.

Survival and diapause egg production of the copepod Centropages hamatus raised on dinoflagellate diets

Copepods are often said to require a mixed diet to maximize their survival and reproduction potential. Copepods may be forced to feed nonselectively in food limited environments, but this does not mean they require a diverse diet. In this study, we show that the right single food item, at the right stage, can be as good or better than a mixed diet for development, survival, diapause egg production and viability. Centropages hamatus was reared on unialgal or mixed diets of five dinoflagellates: Amphidinium carterae, Prorocentrum micans, Scrippsiella trochoidea, Lingulodinium polyedra, and Gymnodinium sanguineum, ranging in size from 11×16 μm to 44×57 μm. Four experiments of 12–30 days were conducted, each testing five diets against a mixed Control diet of the four larger dinoflagellates. Algal volumes were adjusted so diets would have the same initial carbon content or cell volume at a level known to be in excess of the amount required for growth. Eggs were collected on the final 2 days of the experiments, the survivors were counted and female prosome length was measured. Diapause egg viability was measured after the eggs completed a refractory period. Nauplii had high survival on unialgal P. micans and S. trochoidea diets, but the adult females did not produce eggs. Naupliar survival was low on a unialgal diet of G. sanguineum, but the adult females had the highest rate of egg production. An optimal diet fosters growth and survival at all stages and maximizes viable egg production, to ensure high recruitment to the next generation. This can be achieved with single food items appropriate for the developmental stage. Copepods fed just two dinoflagellates, P. micans to foster juvenile growth and G. sanguineum to support final maturation and fecundity, had higher total egg production than copepods reared on the mixed diet of four dinoflagellates.

Copepod diapause eggs: a potential source of nauplii for aquaculture

The aim of this study was to evaluate the feasibility of producing, storing, and hatching copepod diapause eggs that could be used as a source of nauplii for rearing larval fish. The copepod Centropages hamatus was reared on a mixed diet of four dinoflagellates: Gymnodinium sanguineum, Lingulodinium polyedra, Prorocentrum micans, and Scrippsiella trochoidea, in 19-l carboys mounted on a plankton rotator in a walk-in environmental chamber, set at 15°C and a 12L–12D cycle. Eleven experiments were conducted. Following maturation to the adult stage, eggs were produced over a 16–33 day period. Total egg production was assessed in three of the experiments and ranged from 1.2 to 2.4 million eggs. Hatching success was generally >80% for eggs stored 4–17 months at 25°C. Diapause eggs shipped via overnight express remained viable and were used as a source of nauplii to feed larval fish.

Hydrodynamic Control of Late Summer Species Composition and Abundance of Zooplankton in Hudson Bay and Hudson Strait (Canada)

The spatial variation in zooplankton biomass, abundance and species composition in relation to hydrography and chlorophyll a (Chl a) was studied in the subarctic waters of Hudson Bay and Hudson Strait. Sampling was carried out in early September 1993 at 21 stations arranged along a transect following the Quebec coast from James Bay, in Hudson Bay, to the vicinity of Ungava Bay in Hudson Strait. Both the biomass and the abundance of total zooplankton were low along the lower part of Hudson Bay (averaging 1. 6g DM m -2 and 9432 ind. m -2 ) and increased sharply toward the upper end of the Bay and in Hudson Strait (averaging 6.0 g DM m -2 and 40 583 ind. m -2 ). A total of 80 zooplankton taxa was identified in the samples. Copepods were clearly numerically dominant at all sampling stations, accounting for more than 85% and 93% of the zooplankton community in the Bay and the Strait, respectively. Clustering samples by their relative species composition revealed four groups distributed along well defined environmental gradients characterizing the distribution of physical variables and Chl a. The first group, located in the most southern region of Hudson Bay and farther offshore, northwest of the Belcher and Sleeper Islands, was strongly influenced by freshwater run-off from James Bay and other major rivers around the Bay, and was characterized by the presence of two euryhaline copepod species (Acartia longiremis and Centropages hamatus). The second and the third groups occupied the largest region along the sampling transect, from the middle of Hudson Bay to the western region of Hudson Strait, and were characterized by a typical arctic zooplankton fauna related to the cyclonic circulation in central Hudson Bay. The fourth group was located in the easternmost part of the sampling transect in Hudson Strait where the highest phytoplankton biomass values were observed (Chl a ∼220 mg m -2 ). The zooplankton assemblage there showed an important increase in the abundance of the large herbivorous copepod Calanus glacialis/finmarchicus, which were numerically four times more abundant in the central Strait region (averaging 15 251 ind. m -2 ) than in the western side of Hudson Strait and in Hudson Bay (3629 ind. m -2 ). These results support the hypothesis that the structure in the local biological community is influenced by the local hydrodynamic features which, through their action on surface water temperature, salinity, stratification and mixing conditions, lead to spatial differentiation of the phytoplankton and zooplankton communities.

Reproductive cycles of dominant calanoid copepods in the North Sea

The seasonal cycles of reproduction of the dominant copepods Acartia clausi, Temora longicornis, Centropages hamatus, C. typicus and Pseudocalanus spp. were studied at Helgoland Roads, southern North Sea. Egg production rate, clutch size, number of spawning females and pro- some length were compared with temperature and phytoplankton concentration. Female A. clausi, T. longicornis and Pseudocalanus spp. were present all year round. T. longicornis and Pseudocalanus spp. bred continuously throughout the year, although egg production rates and the proportion of spawning females were low in winter. A. clausi did not spawn between the end of September and the end of January. Female C. hamatus were absent from the water column from November to May, and C. typicus from February to August. As the latter 3 species spawn year round in warmer regions, low temperature inhibition of reproduction is suggested for Helgoland, where winter temperatures are below 0°C. Maximum egg production was observed in all species, except the late-arriving C. typicus, in April/May, when females were largest. Significant correlations were only obtained between clutch size or egg production rate and prosome length, which in turn was correlated with temperature. It is therefore concluded that for Helgoland waters temperature controlled egg production by its effect on prosome length, and that food conditions were not limiting.

Long-term dynamics of main mesozooplankton species in the central Baltic Sea

Long-term dynamics (1959–1997) of the copepod species Pseudocalanus elongatus, Temora longicornis, Acartia spp. and Centropages hamatus, as well as the taxonomic group of cladocerans, are described for the open sea areas of the central Baltic Sea. Differences between areas, i.e. Bornholm Basin, Gdansk Deep and Gotland Basin, as well as between 5 year periods, were investigated by means of Analysis of Variance (ANOVA). No significant differences in mesozooplankton biomass between areas were found. On the other hand, clear time-trends could be demonstrated and related to salinity and temperature, with P.elongatus biomass mainly dependent on salinity and T.longicornis, Acartia spp. and cladocerans biomasses dependent, to a large extent, on thermal conditions. Decreasing salinities since the early 1980s due to a lack of major inflows of highly saline water from the North Sea and increased river run-off, both triggered by meteorological conditions, obviously caused a decrease in biomass of P.elongatus. Contrarily, the standing stocks of the other abundant copepod species and cladocerans followed, to a large degree, the temperature development and showed, in general, an increase. The shift in species composition during this period is considered to be a reason for decreasing growth rates of Baltic herring (Clupea harengus) since the early 1980s, and for sprat (Sprattus sprattus) since the early 1990s. Generally, it is suggested that low mesozooplankton biomasses in the 1990s were caused, at least partially, by amplified predation by clupeid fish stocks.

Copepod grazing selection and particle discrimination on the basis of PSP toxin content

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 181:163-176 (1999) - doi:10.3354/meps181163 Copepod grazing selection and particle discrimination on the basis of PSP toxin content Gregory J. Teegarden* Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA *E-mail: gregt@gsosun1.gso.uri.edu ABSTRACT: Omnivorous copepods are capable of discriminatory feeding using mechano- and chemosensory mechanisms. The presence of phycotoxins in phytoplankton often results in reduced consumption of such potential prey by copepods, though it has not been clear if this is the result of discriminatory feeding by either tactile (mechanosensory) or chemosensory recognition of toxic prey, or perhaps a physiological response to ingested neurotoxic compounds. In this study, experiments were performed to determine whether 3 species of marine copepods (Acartia tonsa, Centropages hamatus, and Eurytemora herdmani) that commonly co-occur with toxic Alexandrium spp. dinoflagellates were capable of discriminating between cultured Alexandrium spp. strains on the basis of paralytic shellfish poisoning (PSP) toxin content, i.e. by chemosensory means, using live fluorescently labeled cells. Additional experiments investigated whether toxic cells in mixtures with non-toxic alternate species of dinoflagellates affected either prey selection or total carbon consumption rates of copepods, and whether daily carbon rations could be maintained on both toxic and non-toxic Alexandrium spp. monoculture diets. Results indicated that all 3 copepod species could discriminate between toxic and non-toxic Alexandrium spp. cells by chemosensory means, suggesting that selective behavior, rather than physiological effects, governs the grazing response of copepods exposed to toxic prey. Prey selection in mixtures of several dinoflagellate species depended on whether the Alexandrium spp. cells present were toxic or non-toxic. C. hamatus and E. herdmani (but not A. tonsa) maintained daily carbon rations despite the presence of toxic Alexandrium spp., chiefly through increased consumption of alternate prey. For A. tonsa and C. hamatus, carbon rations were not equivalent between toxic and non-toxic Alexandrium spp. monoculture diets, indicating strong aversions to PSP toxins, and the potential for physiological effects when no other food is available. In all experiments feeding behavior varied among copepod species, suggesting that grazing pressure on toxic Alexandrium spp. is not uniform throughout the zooplankton community. The grazer-deterrent effects observed have implications for the function of PSP toxins. KEY WORDS: Alexandrium · Copepod · Grazing behavior · PSP toxins Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 181. Publication date: May 18, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Longevity of subitaneous and diapause eggs of Centropages hamatus (Copepoda: Calanoida) from the northern Gulf of Mexico

Copepod resting eggs are abundant in the seabed of many bays and estuaries where they provide a potential source of recruits for growth of planktonic populations. In the northeastern Gulf of Mexico the copepod Centropages hamatus (Lillejeborg) occurs in the water column only during the late fall, winter and early spring. The species produces subitaneous and diapause eggs, and both egg types have been found in the seabed. We determined the longevity of these two egg types to ascertain their potential for contributing to the growth of the planktonic population and for sustaining a persistent egg bank. Eggs were collected from females and incubated in the laboratory under temperature and oxygen conditions chosen to simulate field conditions. The diapause eggs were also exposed to sulfide. The total hatching success of subitaneous eggs in two experiments declined from highs of 78 and 97% to zero after 60 and 90 d of exposure to anoxia. The total hatching success of diapause eggs that were exposed to anoxia for 90 d however was typically greater than 80%. Some diapause eggs hatched after being incubated under anoxia for 437 d. Diapause eggs survived longer at ambient field temperatures when incubated under anoxia (437 d) compared to normoxia (118 d). Exposure to sulfide did not result in greater mortality of diapause eggs compared to anoxia alone. Diapause eggs that were incubated at ambient field temperatures did not hatch when exposed to normoxia until the temperature dropped to <20 °C. The results of this study suggest that C.␣hamatus sustain a short-term reserve of subitaneous eggs in the seabed that provides recruits for the current year's population. The greater longevity of diapause eggs suggests that they sustain the seasonal reappearance of the species year after year in the northeastern Gulf of Mexico. However, the contribution of diapause eggs of C. hamatus from the Gulf of Mexico to a persistent egg bank is questionable since hatching ceased after 437 d.

Short-term changes in the mesozooplanktonic community in the Seine ROFI (Region of Freshwater Influence)(eastern English Channel)

Discrete-depth, hourly mesozooplankton samples were collected over a 92 h period in May 1992 at an anchor station within the Seine Region of Freshwater Influence (ROFI) (English Channel). The mesozooplankton community defined as a euryhaline marine assemblage was dominated by the calanoid copepods Acartia spp., Temora longicornis and Centropages hamatus, the cladoceran Evadne nordmanni and the appendicularian Oikopleura dioica. The semi-diurnal tidal current was the dominant factor in determining the short-term temporal changes in the community in terms of density and species composition so that zooplankton patches displayed oscillating motion in relation to tidal advection. Although a few species (e.g. Pleurobrachia pileus) exhibited higher densities around low tide, maximum densities were observed for most species (e.g. T.longicornis and E.nordmanni) around high tide, according to salinity variations. Diumal changes were only reported for cyclopoid copepods (i.e. Halicyclops sp. and Cyclopina sp.) which were mainly endobenthic during the day and moved into the water column at night. Besides temporal changes in depth-averaged densities, most species exhibited vertical migrations at diel and/or tidal periods. Tidal vertical migrations were reported only for a few taxa and could be the result of passive mechanisms (e.g. vertical mixing) rather than of active behaviour. Diel vertical migrations were observed in most of the abundant taxa. While this migration pattern did not appear to be an adaptation to predator avoidance within the Seine ROFI, it could regulate horizontal transport of organisms and promote their retention. The consequences of the short-term mesozooplankton fluctuations for sampling designs are discussed.

Winter distribution of phytoplankton and zooplankton around some sandbanks of the Belgian coastal zone

The distribution of phytoplankton and zooplankton around three sandbanks (Gootebank, Westhinder and Buitenratel sandbank) off the Belgian Coast was investigated in February 1994. The abundance of phytoplankton taxa was significantly different between the sandbanks. Community analysis using TWINSPAN resulted in a clear separation of clusters corresponding to the different sandbanks. The zooplankton community analysis, on the contrary, showed a rather indistinctive division of the sandbank stations. This was due to the omnipresence of three dominant copepod species (Temora longicornis, Pseudocalanus elongatus and Centropages hamatus) . When these species were excluded from the analysis, a clearer distinction between the different sandbanks was found. The observed differences in phyto- and zooplankton species distribution could be explained by the position of the sandbanks. Westhinder is positioned further fromthe coast than Buitenratel, while Gootebank has an intermediate position. Buitenratel and Gootebank harbour typical coastal plankton communities, while the plankton community over Westhinder is clearly influenced by the Atlantic current penetrating the southern North Sea from the English channel. More phyto-benthic species were found at Buitenratel than at Gootebank, probably because of its limited depth. Thus, the Belgian coastal zone, which is considered as one box in most spatial descriptions of the North Sea plankton, in fact harbours heterogeneous plankton communities at the end of winter.