Trophodynamics of the plankton community at Dogger Bank: predatory impact by larval fish

The trophodynamics of a coastal plankton community were studied,focusing on fish larvae and their copepod prey. The major objectiveswere to describe distributional overlap and evaluate the predatoryimpact by larval fish. The study was carried out across DoggerBank in the North Sea, August-September 1991. Sampling transectscrossed tidal fronts off the Bank and plankton at all trophiclevels showed peak abundance within frontal zones. Also Verticallythere was a significant overlap in distributional patterns ofthe plankton. Seven species of fish larvae were abundant, ofthese sprat (Sprattus sprattus) dominated. The abundance ofone group of fish larvae peaked in the shallow water close tothe Bank, whereas other species, including sprat, were foundin deeper water. Prey preference and predation pressure of fishlarvae were assessed using information on prey sizes and growthrates of larvae and the copepod prey. We estimated larval removalof preferred prey sizes to 3–4% day^–1, counterbalancedby a 3–7% day^–1' replenishment from copepod productionand growth. Additional predation pressure on copepods by aninvertebrate predator was estimated to 1–3%day^–1.In conclusion, the dynamics of fish larvae and other zooplankterswere closely linked. At peak abundances of fish larvae (>35mg dry weight m^–2), the accumulated predation on specificsize ranges of copepods, exerted by larvae and other predators,could exceed the ability of copepod replenishment and intra-/interspecificcompetition among predators might take place.     

Plankton distribution across a slope current-induced front in the southern Bay of Biscay

Relatively warm (12.50–12.75°C) and high-salinity[<35.640 practical salinity units (PSU)] water flowing eastwardwas detected at the shelf-break during a cruise carried Outin the southern Bay of Biscay in Spring 1987. The slope currentinduced the formation of a convergent front separating well-mixedoceanic waters from haline-stratified coastal waters. Very highconcentrations of dissolved oxygen (295 µmol kg^–1)and chlorophyll a(>4.5 mg m^–3) were found at the outeredge of the frontal boundary. Small autotrophic flagellatesdominated the phytoplankton community. Primary production peakedat the boundary region. Estimated phytoplankton growth ratesindicated that active growth was taking place, with lower turnovertimes integrated over the water column at the frontal station(2.5–5 days) than at coastal (1.5–2.8 days) or oceanic(1.5–3.5 days) stations. The lowest doubling times (1–2days) were calculated for surface frontal populations. Accumulationof zooplankton was also observed associated with the convergentphysical structure, although this relationship was less markedthan for phytoplankton. Copepods, mainly Paracalanus parvus,Acartia clausi and Oithona helgolandica, formed the bulk ofthe mesozooplankton biomass. Compatibility between the sizeof phytoplankton cells and copepod size spectra indicate highfood availability for these animals, particularly in the vicinityof the front. The distribution of fish eggs and fish larvaewas also coupled with the slope current-induced front. Sardinelarvae were more abundant at the coastal side of the front,whereas larval stages of blue whiting reached the highest densitiesat off-shelf stations. Larvae of lamellibranch molluscs andbryozoa were restricted to nearshore waters, as the frontalboundary prevented larval dispersion to the open ocean. Theresults presented in this paper suggest that the Iberian slopecurrent and its associated shelf-break frontal structure werecrucial in controlling phytoplankton primary production, activityof grazers, distribution of larvae of fishes and benthic invertebrates,and ultimately in determining the structure of the pelagic foodweb in the southern Bay of Biscay during the seasonal periodof vertical mixing.     

Capelin larvae (Mallotus villosus) and community structure of zooplankton off the coast of northern Norway

Capelin larvae (Mallotus villosus) and mesozooplankton werecollected in May 2001 in the southern part of the Barents Sea.During the period of hatching and first feeding, capelin larvaeexperience high mortality rates. In order to understand theunderlying mechanisms influencing larval survival, we monitoreda selected site for 10 days using a variety of different samplingapproaches. Hydrological parameters conductivity–temperature–depth–fluorescence(CTDF) were recorded and zooplankton samples obtained simultaneouslyusing MOCNESS at three time periods. The mesozooplankton consistedessentially of copepods (85), dominated by Calanus finmarchicus.The zooplankton community consisted of different assemblagesderived from different areas (i.e. different water masses; Atlanticand coastal). Capelin larvae were found in surficial coastalwaters. Copepod nauplii and C. finmarchicus were less abundantin coastal waters where small copepodites (such as Acartia spp.and Temora longicornis) were found together with early stagesof coastal invertebrates that are possible prey for capelinlarvae. We suggest that capelin larvae base their foraging onsmall prey organisms frequently found among the coastal zooplanktonand that these larvae are less dependent on the propagationof the recruiting generation of C. finmarchicus present fromApril to June.     

The role of copepods in the planktonic ecosystems of mixed and stratified waters of the European shelf seas

The European shelf seas can be divided into regions which have tidally mixed waters and thermally stratified waters. The tidally mixed near shore environments support zooplankton communities dominated by smaller copepods and having large meroplankton contributions. These small copepods (Centropages spp., Temora spp., Acartia spp., Paral Pseudo/Microcalanus spp.) together with the microzooplankton component form a different and more complex food web than the larger copepod/diatom link associated with thermally stratified waters. The copepods Calanus finmarchicus and C. helgolandicus account for over 90% of the copepod dry weight biomass in stratified waters. Although occurring in lower numbers in mixed waters they can still make significant contributions to the biomass. A 31 year time series from the European shelf shows the inter- and intea-annual variability of these species. The basic biology and food web that these two systems support, and the transfer of energy, can result in marked differences in quantity and quality of particulates available as food for fish larvae. Calanus dominated systems allow the primary production to be directed straight through the trophic food chain (diatoms/Calanus/fish larvae) while the near shore communities of smaller copepods limit the amount of energy being transferred to the higher trophic levels. Eighty-two Longhurst Hardy Plankton Recorder hauls were used as the data base for this study. In all cases the zooplankton was dominated by copepods both in numbers and biomass accounting for > 80% of total zooplankton dry weight in the Irish Sea, Celtic Sea, shelf edge of the Celtic Sea and the northern and southern North Sea in Spring.     

Weak 'top-down' control of dinoflagellate growth in the coastal Skagerrak

Effects of planktivorous fish (Sprattus sprartus), copepods(mainly Oithona sp.) and benthic filter feeders (Mytilus edulis)on the biomass and species composition of a natural marine planktoncommunity, dominated by dinoflagellates, was studied experimentallyin September 1988. Plastic 300 I cylinders were filled withSkagerrak surface water, filtered through a 90 µm nylonnet in order to remove mesozooplankton. Copepods were addedin three different concentrations (0, 1x and 6x the ambientdensity) with and without nutrient additions (nitrate, silicateand phosphate). In two nutrient-enriched cylinders with addedcopepods (1x and 6x), 5 cm sprats were also added (10 per cylinder).In two nutrient-enriched cylinders without added copepods. bluemussels (20 per cylinder. 3 cm length) were introduced. Chlorophylla. inorganic nutrients and densities of phytoplankton, ciliatesand copepods were measured throughout the experimental periodof 10 days. Gyrodinium aureolum increased in cell numbers inall but the mussel cylinders, while the other dominating dinoflagellate,Ceratium furca, decreased. This decrease was not due to grazing,but probably a container artefact. Diatoms, Rhizosolenia fragilissinia,Leptocylindrus danicus and Chaetoceros spp., increased slowlyin treatments without fish. In the fish treatments, diatomshad appreciably higher growth rates and cell numbers, probablydue to predation by fish on the copepods. Gyrodiniun aureolunalso had a somewhat higher growth rate in the presence of fish.although this was probably not due to reduction of copepod grazing.The results show that planktivorous fish can affect the biomassand species composition of natural phytoplankton communitiesin marine waters, but that the effect on the late summer-autumndinoflagellate dominated community is weak. Blue mussels areefficient grazers of the whole phytoplankton community and arenot particularly selective with respect to either size or species.     

Estimated copepod production rate and structure of mesozooplankton communities in the coastal Barents Sea during summer–autumn 2007

The southern Barents Sea is considered to be the most productive area in the Arctic Ocean; however, there are no assessments of daily production rates in the coastal waters. During the summer and autumn of 2007, we investigated the variation of mesozooplankton community structure relative to environmental conditions at 12 coastal stations. Copepods dominated the total zooplankton biomass and abundance during both periods. Diversity indices and the total biomass of zooplankton communities differed significantly between the two seasons. Cluster analyses revealed two distinct groups of stations which were associated with Ura Bay and the adjacent open sea, respectively. Daily production rates of the copepod species examined were calculated using three methods based on: (1) a temperature-dependent equation and (2) two multiple regressions that consider temperature, body weight, and chlorophyll a concentration. Significant seasonal differences for daily production rates were found using all three model equations (p?<?0.05): 358?±?188–1,775?±?791 versus 198?±?85–1,584?±?559?μg?dry?mass?m^?3?day^?1. Results of principal components analyses demonstrated that the abundance and biomass of herbivorous species were related to variation in chlorophyll a concentration while the abundance and biomass of other species (omnivorous copepods and Ctenophora) were related mainly with water temperature and salinity. Mesozooplankton biomass was higher during this study relative to previous studies. Computed copepod production rates were higher compared with other Arctic seas confirming a high productive potential of the coastal southern Barents Sea.     

Distribution and dietary relationships of the Japanese temperate bass Lateolabrax japonicus juveniles with two contrasting copepod assemblages in estuarine nursery grounds in the Ariake Sea, Japan

Early juvenile Japanese temperate bass Lateolabrax japonicus samples were collected during four cruises to study the spatial and temporal patterns of distribution and feeding habits of the fish in Chikugo estuary, Ariake Sea, Japan. Gut contents were studied by separating, identifying and counting the prey organisms. Plankton samples were collected during each cruise to study the numerical abundance of copepods in the water. Copepod dry biomass and gut content dry mass were estimated. Juveniles were distributed over wide estuarine areas in salinities ranging from as low as 0·37 to as high as 28·81. Considerable spatial and temporal variations were observed in the copepod distribution in the environment and in the food habits of the fish. Two distinctly different copepod assemblages were identified along the estuary: one in the upper river, dominated by a single species Sinocalanus sinensis, and the other a multi‐specific assemblage in the lower estuary dominated by common coastal copepods such as Acartia omorii, Oithona davisae, Paracalanus parvus and Calanus sinicus. The gut content composition corresponded strongly with the copepod composition in the environment, i.e. a single species (S. sinensis) dietary habit at the upper river and a multi‐specific dietary habit dominated by the common coastal copepods in the lower estuary. Ivlev's electivity index showed that the fish strongly preferred larger copepods and avoided smaller ones. Higher dry biomass of copepods in the water as well as higher dry masses of the gut contents were recorded in the low‐to‐medium saline upper river areas, indicating that these areas are of particular importance as nursery grounds for the juvenile Japanese temperate bass. It was speculated that ascending to the upstream nursery areas to utilize S. sinensis, which is the single dominant copepod in these areas, is one of the most important survival strategies of the Japanese temperate bass juveniles in the Chikugo estuary.     

Mesozooplankton biomass and indices of grazing and metabolic activity in Antarctic waters

Mesozooplankton abundance, body area spectrum, biomass, gut fluorescence and electron transfer system (ETS) activity were studied in the Antarctic Peninsula during the post-bloom scenario in these waters. Values of abundance and biomass were rather low and decreased sharply from the slope waters to the coastal area. In contrast, specific gut fluorescence and ETS activity were high in the coastal area and decreased through the shelf-break. Large copepods were very scarce, similarly to the post-bloom conditions in phytoplankton where large cells are not abundant and small cells such as flagellates dominate the water column. The vertical distribution showed two well defined layers by day, one at the surface which corresponded to krill organisms and a second at depth (>300 m) formed mainly by the large copepod Metridia gerlachei. During the short night, this layer ascended at the time that krill at the surface migrated to deeper waters as observed from acoustics and net sampling. This observation and the absence of large copepods over the shelf suggest that krill consumption on large phytoplankton cells during the bloom is followed by an increase in predation upon mesozooplankton. It also suggests that krill decrease the abundance and biomass of mesozooplankton over the shelf and continues their predation upon mesopelagic copepods during the post-bloom in Antarctic waters. This behaviour could explain the long ago described impoverishment in mesozooplankton south of the Antarctic Circumpolar Current.     

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

Long-term dynamics (1959–1997) of the copepod speciesPseudocalanus elongatus, Temora longicornis, Acartia spp. andCentropages 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 Deepand Gotland Basin, as well as between 5 year periods, were investigatedby means of Analysis of Variance (ANOVA). No significant differencesin mesozooplankton biomass between areas were found. On theother hand, clear time-trends could be demonstrated and relatedto salinity and temperature, with P.elongatus biomass mainlydependent on salinity and T.longicornis, Acartia spp. and cladoceransbiomasses dependent, to a large extent, on thermal conditions.Decreasing salinities since the early 1980s due to a lack ofmajor inflows of highly saline water from the North Sea andincreased 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 andcladocerans followed, to a large degree, the temperature developmentand showed, in general, an increase. The shift in species compositionduring this period is considered to be a reason for decreasinggrowth rates of Baltic herring (Clupea harengus) since the early1980s, and for sprat (Sprattus sprattus) since the early 1990s.Generally, it is suggested that low mesozooplankton biomassesin the 1990s were caused, at least partially, by amplified predationby clupeid fish stocks.     

Vertical distribution of mesozooplankton in the central and eastern Arabian Sea during the winter monsoon

The vertical distribution of mesozooplankton in the centraland eastern Arabian Sea was investigated during the winter monsoonin 1995. Samples were analysed from discrete depth zones definedaccording to oxygen and temperature profiles of the water column.Zooplankton had higher standing stock in the mixed layer comparedto the strata below. The mixed layer had 78.5% of the totalcolumn biomass, while the deepest (500–1000 m) layer accountedfor only 0.9%. The stratum between 500 and 1000 m had the lowestabundance of copepods as well as other zooplankton. A notablefeature was that zooplankton biomass and density did not showmuch variations between coastal and offshore regions. Copepodawere the dominant group. Herbivores were generally more abundantat all depths. A total of 94 species of calanoid copepods wereidentified. Based on vertical distributions, they were assignedto four groups: (i) species restricted to the upper 200 m; (ii)predominantly surface-living species with tails to deeper waters;(iii) sparser deeper-living species generally confined below300 m; (iv) species occurring throughout the water column. Diversitywas fairly high in all strata with equitability being higherin the deeper strata.     

Across-shelf predatory effect of Pleurobrachia bachei (Ctenophora) on the small-copepod community in the coastal upwelling zone off northern Chile (23{degrees} S)

To estimate the predation effect of the predominant ctenophorePleurobrachia bachei on the small-copepod community in the upwellingarea off Mejillones (23°S), northern Chile, a series ofoceanographic cruises and predation experiments were conductedin the austral springs 2000, 2001 and 2002. The daily consumptionrates and predatory effect of P. bachei on the small copepods(in terms of % of standing stock and biomass removed daily)were determined at three stations located in relation to theshelf-break (coastal, shelf-break and oceanic) reaching valuesup to 4.5% per day of the <1500 µm copepod standingstock. Our results indicate that the ctenophores were most abundantat the coastal station, that small copepods dominated the copepodcommunity (being more abundant nearshore), and that the relativefrequency of ctenophores with copepods in their guts was alsohigher near the coast. The predatory effect of P. bachei onthe small-copepod community was also higher in the coastal zone.However, the effect of this predation on the copepod biomassin terms of carbon did not decrease steadily seawards, whichmay be due to the larger sized copepods consumed at the offshorestations. Determinations of predatory effect on the secondaryproduction of the more abundant small-copepod populations (i.e26% daily in 2000) suggest that this single species of Pleurobrachiais modulating the population growth rate of the small copepods,the copepod community size structure, and maybe even the alternanceof key species in the Mejillones coastal upwelling zone.     

Picoplankton community structure at a coastal front region in the northern part of the South China Sea

Abundances of picoplankton groups were determined by flow cytometryin the Northern South China Sea (SCS) in winter 2004 to studythe dynamics of picoplankton at a coastal front region. Prochlorococcusis more abundant in relatively high temperature and salinitywaters than in nearshore area. Heterotrophic bacteria dominatein total picoplanktonic biomass but keep rather stable in biomassand surface/bottom biomass ratio on both sides of the front.Increases of picophytoplanktonic biomass and their surface/bottombiomass ratio are remarkable mainly owing to the contributionof Synechococcus on the offshore open SCS waters. Temperatureis found to limit the growth of Synechococcus and Prochlorococcus.Picoeukaryotes and heterotrophic bacteria are less sensitiveto the change in hydrographic conditions across the front. Theautotrophic/heterotrophic biomass ratio of picoplankton is lowerin eutrophic coastal waters on the nearshore side relative tothe offshore and oligotrophic open SCS.     

Environmental factors influencing diurnal distribution of zooplankton and ichthyoplankton

The diurnal vertical distribution of a large number of speciesof zooplankton, icbthyoplankton and micronekton were determinedin the top 150 m in three locations in the Shelf Water, on theNova Scotia Shelf, and Slope and on Georges Bank during springand fall periods. Species were categorized as to their trophiclevel and their type of diurnal migration behaviour. The influenceof temperature, salinity, and water density on the diurnal verticaldistribution of the species was examined. Temperature was foundto have the greatest influence on the distribution of the largestnumber of species. Diurnal migration behavior of the same speciesin Shelf and Slope water and at different times of the yearwas examined. Results showed that species changed their behaviorin the two water masses, while some species changed their migrationbehavior at different times of the year. During the night inApril the most abundant copepod species, Calanus finmarchicus,making up about 80% of the biomass, was found concentrated abovethe thermocline and the main chlorophyll layer. The majorityof the less abundant species of copepods were found below thethermocline and the chlorophyll layer. At night in August thetwo most abundant copepod species, Centropoger typicus and Paracalanusparvus, making up at least 80% of the zooplankton biomass, werealso concentrated above the thermocline and the main chlorophyllLayer. Three species of copepods were concentrated at the depthof the main chlorophyll layer and two species were concentratedbelow the chlorophyll layer and thermocline. The vertical distributionof other zooplankton and ichthyoplankton species was examinedin relation to the thermocline and chlorophyll layer. Relationshipsbetween concentrations of six species of fish larvae and allspecies of copepods in the same samples showed a general increasein the numbers of larvae m^–3 as the numbers of copepodsm^–3 increased in a range of 500–4000 m^–3.However, the concentration of Merluccius bilinearis decreasedas the concentration of copepods exceeded 4000 m^–3 suggestingthat high concentrations of copepods may not be a favourableenvironment for the larvae.     

Diet and feeding niches of juvenile Gadus morhua, Melanogrammus aeglefinus and Merlangius merlangus during the settlement transition in the northern North Sea

A study on the feeding ecology of juvenile cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus during the pelagic to demersal transition was carried out on fishes sampled throughout their settlement season at a local nursery ground in the north-western North Sea, off the Scottish east coast. A comprehensive quantitative taxonomic analysis of the diets, as described in the paper, showed the emergence of distinctive feeding niches, minimizing the potential for competition between species and size categories. The diet of the juveniles changed with fish size, water depth, time of year and distance offshore. Small G. morhua were present in the study area earlier in the season, settled further inshore and ate a higher proportion of pelagic prey (copepods) and as size increased they moved into deeper waters and targeted larger, more benthic prey. As M. aeglefinus grew larger and moved into deeper waters, a diet of largely copepods, amphipods, pelagic Ammodytes spp., cyprids and pelagic gastropods evolved to one dominated predominantly by fishes and benthic invertebrates. In the case of M. merlangus, widespread ages and sizes throughout the sampling season, a consequence of their more protracted spawning season, resulted in dietary changes which were more likely to be influenced by seasonal changes in the prey field, in addition to developmental (size) changes, than the diets of the other two species.     

Seasonal Dynamics of Mesozooplankton in Brazilian Coastal Waters

The composition, numerical abundance and seasonal distribution of mesozooplankton, in addition to copepod biomass, were studied in the São Sebastião Channel (SSC) during different seasonal periods and hydrographic regimes. Two stations were sampled every 3 months from January 1996 to July 1997 and also in July 1998 and January 1999. Temporal differences in oceanographic conditions in the SSC were observed; there was clear seasonal variation in the thermohaline structure. Phytoplankton composition, standing-stock and biomass also showed consistent seasonal variation, peaking during summer due to advection of cold and nutrient-rich South Atlantic Central Water (SACW). Sixty-nine mesozooplankton taxa were identified and counted. Mesozooplankton density also increased during summer, ranging from 93 to 12,774 ind  m^?3. In contrast, a unimodal peak of copepod biomass was recorded during winter and under the influence of the oligotrophic Coastal Water (CW), suggesting that copepod biomass at SSC is driven by resource control. There was no significant evidence of seasonal pattern in overall community composition or evidence of changes due to water mass advection. The mesozooplankton community was continuously dominated by coastal and warm-water species, particularly the copepods Acartia lilljeborgi, Paracalanus aculeatus, P. quasimodo, Parvocalanus crassirostris, Corycaeus giesbrechti and Oithona oswaldocruzi, the cladocerans Penilia avirostris and Evadne tergestina, in addition to the appendicularians Oikopleura longicauda and O. dioica, all of which are perennial species in the SSC. This taxonomic composition may represent an efficient biotic structure for rapid recycling of primary production during periods of SACW influence.     

The zooplankton community structure in relation to its biological and physical environment on the Faroe shelf, 1989-1997

The Faroe shelf water is separated from the offshore water by a persistent tidal front, which surrounds the islands. This shelf water contains a neritic zooplankton community, which, regarding species composition, production, seasonal development and environmental conditions, is quite different from that in the surrounding ocean. While during spring and summer the zooplankton in the oceanic environment are dominated by the copepod Calanus finmarchicus, the zooplankton in the shelf water are largely dominated by neritic copepods, mainly Acartia longiremis and Temora longicornis. Calanus finmarchicus occurs in interannually highly variable abundance in the Faroe shelf ecosystem. Meroplanktonic larvae, mainly Balanus spp, and decapod larvae, are also common in the shelf water during spring and summer. During the period presented (1989-1997), the Faroe shelf ecosystem has undergone very large changes in abundance of different zooplankton species. The midsummer abundance of C.finmarchicus, which originally is advected into the shelf from the open ocean, fluctuated from 400 copepods m^-3 in 1989 to 25 copepods m^-3 in 1994, and at the same time the neritic zooplankton increased from 120 m^-3 in 1989 to 450 m^-3 in 1994. Consequently, the midsummer biomass in the shelf fluctuated by a factor of 10 during the same period. It is presumed that this variability between oceanic- and neritic-dominated zooplankton, their sizes and their biomass has greatly affected the entire pelagic ecosystem.      

Productivity, photosynthetic pigments and hydrology in the coastal front of the Eastern English Channel

In the French area of the Eastern English Channel, the coastalwaters 3 or 4 miles wide are separated from offshore watersby a coastal front. The phytoplanktonic biomass (B) and production(P) are higher in coastal waters, and an increase of biomasshas been observed at the interface. This frontal biomass, lessproductive, resulted from hydrodynamic accumulation, not dueto local growth. The productivity (P/B) deficit (50%) spreadsover the external area of the front, probably because the biomassis partly constituted of exported inactive coastal phytoplankton.The productivity is higher again in offshore waters. The variationof the pigment pool indicates a stress on phytoplankton in thefront and this ergocline was not productive at the time of thetwo cruises.     

Copepod and barnacle nauplius distributions in the Irish Sea: relation to springtime hydrographic variability

The distributions and abundances of copepod and barnacle naupliiare described along a five-station transect in the Irish Seafrom Liverpool Bay, England, to Dundalk Bay, Ireland, in May1989. Correspondence analysis revealed that various hydrographicregions in the Irish Sea are characterized by distinct naupliusassemblages. Acartia, Centropages and Temora nauplii were associatedwith the Welsh coastal water east of the Liverpool Bay front.The western stratified water to the west of the seasonal, westernIrish Sea front was distinguished from the mixed water to theeast by abundances of Calanus, Pseudocalanus, Oithona, Microcalanusand harpacticoid nauplii. The Liverpool Bay and western IrishSea fronts are, apparently, boundaries between coastal and offshoreassemblages of crustacean nauplii.     

Seasonal changes in zooplankton composition in the Barents Sea, with special attention to Calanus spp. (Copepoda)

The Barents Sea is an important area with respect to fisheriesresources (i.e. capelin and cod). In May, June and August 1981zooplankton biomass was measured along a transect at 30°E,from the ice border southwards. A maximum was recorded in Atlanticwater by the end of June (>100 g wet weight m^–2 InAugust the biomass values were relatively low south of the Polarfront and increased northwards into Arctic water (–50g m^–2 The species composition was influenced by the distributionof cold Arctic water and warmer Atlantic water. The zooplanktonwas dominated by the copepods Calanus finmarchicus and C. glacialis;the former is regarded as an Atlantic species and C. glacialisas an Arctic species.     

Are copepods important grazers of the spring phytoplankton bloom in the western Irish Sea?

The spring phytoplankton bloom and copepod grazing were studied at a coastal and offshore station in the western Irish Sea during 1997. Maximum chlorophyll standing stocks of 132.8 mg m^-2 inshore and 199.4 mg m^-2 offshore were measured in late April. At that time, mean water column temperatures were 10 and 8C at the coastal and offshore station, respectively. Spring bloom production at the coastal station was estimated as 31.2 g C m^-2 and was dominated by the diatom Guinardia delicatula. Offshore, production was 28.2 g C m^-2 and the bloom was composed of small (10 m) phytoflagellates and the silicoflagellate Dictyocha speculum. Maximum copepod abundance (189 and 544 x 10³ individuals m^-2, inshore and offshore, respectively) coincided with the spring bloom. Pseudocalanus and Temora ingestion rates were derived from measurements of gut pigment fluorescence, and were found to vary during the course of the spring bloom as a result of changes in gut content. Grazing by late copepodite and adult Pseudocalanus and Temora was variable inshore, but overall accounted for 17% of bloom production. Offshore, 22% of bloom production was grazed with maximum grazing (76% of daily production) occurring at the end of the bloom. Large copepod species were not major grazers of the spring bloom. Greater utilization of spring bloom production by copepods in the western Irish Sea compared to regions of the North Sea is attributed to differences in population size at the time of the bloom.