Total Mesozooplankton Research Articles

In situ feeding physiology and grazing impact of the appendicularian community in temperate waters

The physical and biological factors affecting the abundance and ingestion rates of dif- ferent appendicularian species were investigated from April 1999 to May 2000 at a coastal station in the English Channel and on a transect of 3 stations across the shelf in the central Cantabrian Sea. Individual gut chlorophyll and gut food volume contents were used to determine the ingestion rates on autotrophic prey and on total particulate material. Body size was the variable explaining most of the variability in gut contents. For most species, over 60% of the ingested material came from non- chlorophyll-containing prey. Appendicularian community grazing impact was higher at the oceanic stations during early spring and autumn, with maximum values close to 10% of the total phytoplank- ton biomass removed daily. Oikopleura longicauda and O. fusiformis were the species with the high- est grazing impact. During the study period, appendicularians removed an average of 8% of the pri- mary production (PP) measured at the only station for which we have concurrent measurements. Our results suggest that the bulk of appendicularian populations resides in the surface mixed layer and that their grazing rates increase with increasing PP. However, the percentage of the PP removed by the appendicularian community decreases with increasing productivity, indicating that their grazing impact is relatively more important under oligotrophic conditions. Appendicularians could account for close to 40% of the total mesozooplankton grazing.

Plankton distribution associated with frontal zones in the vicinity of the Pribilof Islands

We studied the effect of four types of fronts - the coastal front, the middle front, the shelf partition front and the shelf break front on the quantitative distribution and the composition of plankton communities in the Pribilof area of the eastern Bering Sea shelf in late spring and summer of 1993 and 1994. The coastal fronts near St. Paul and St. George Islands and the coastal domains encircled by the fronts, featured specific taxonomic composition of planktonic algae, high abundance and production of phytoplankton, as well as large numbers of heterotrophic nanoplankton. The coastal fronts were also characterized by high values of total mesozooplankton biomass, high concentrations of Calanus marshallae, as well as relatively high abundances of Parasagitta setosa and Euphausiacea compared to surrounding shelf waters. We hypothesize that wind induced erosion of a weak thermocline in the inner part of the coastal front as well as transfrontal water exchange in subthermocline layers result in nutrient enrichment of the euphotic layer in the coastal fronts and coastal domains in summer time. This leads to prolonged high primary production and high phytoplankton biomass. In this paper a new type of front ‐ the shelf partition front located 45-55 km to the north-east off St. Paul Island is described which is assumed to be formed by the flux of oceanic domain waters onto the shelf. This front features a high abundance of phytoplankton and a high level of primary production compared to the adjacent middle shelf. Near the southwestern periphery of the front a mesozooplankton peak occurred, composed of C. marshallae, with biomass in the subthermocline layer, reaching values typical for the shelf break front and the highest for the area. High abundance of phyto- and zooplankton as well as heterotrophic nanoplankton and elevated primary production were most often observed in the area adjacent to the shelf break front at its oceanic side. The phyto- and mesozooplankton peaks here were formed by oceanic community species. The summer levels of phytoplankton numbers, biomass and primary production in the shelf break frontal area were similar to those reported for the outer and middle shelf during the spring bloom and the coastal domains and coastal fronts in summer. In the environment with a narrow shelf to the south of St. George Island, the mesozooplankton peak was

Mesozooplankton community structure in the Southern Ocean upstream of the Prince Edward Islands

A meso-scale oceanographic grid survey was conducted during the first cruise of the Marion Offshore Ecosystem Variability Study in the upstream region of the Prince Edward Islands in austral autumn (April/May) 2001. Mesozooplankton samples, collected using a Bongo net (fitted with 200-µm and 300-µm mesh nets), were separated into three size fractions, 200–500 µm, 500–1,000 µm, 1,000–2,000 µm, by reverse filtration. Total surface (depth<5 m) chlorophyll-a concentration during the study ranged between 0.11 and 0.34 µg l–1 and was always dominated by picophytoplankton (0.45–2.0 µm). Total mesozooplankton abundance and biomass during the survey ranged between 49 and 1,512 ind. m–3 and between 0.7 and 25 mg Dwt. m–3, respectively. Throughout the survey, the 200 to 500 µm class numerically dominated the mesozooplankton community, with an average of ~69% (SD=±12.3%). The dominant species in the 200- to 500-µm size fraction were the copepods, Oithona similis, Calanus simillimus and Metridia lucens, and the pteropod, Limacinaretroversa. However, in terms of biomass, the 1,000- to 2,000-µm group was predominant, with dry weight values constituting an average of ~66% (SD=±10.2%). The most well-represented species in this group were the carnivorous Euphausia vallentini, Thysanoessa vicina, Sagitta gazellae and Eukrohnia hamata. Three distinct groupings of stations were identified by numerical analysis. The different station groupings identified reflect changes in the relative contributions of the dominant species, as opposed to the presence/absence of species.

Mesozooplankton community structure, abundance and biomass in the central Arctic Ocean

During the "International Arctic Ocean Expedition 1991" (20 August–21 September 1991) mesozooplankton was sampled at six stations in the Nansen, Amundsen and Makarov Basins of the central Arctic Ocean from 1,500 m depth to the surface by multiple opening/closing net hauls. Total mesozooplankton abundance decreased from 268 ind. m–3 in the surface layer (0–50 m) to <25 ind. m–3 below 200 m depth. The small copepods Oithonasimilis and Microcalanuspygmaeus, as well as copepod nauplii, were most abundant close to the surface, while Oncaeaborealis and Spinocalanus spp. frequently occurred at greater depth. Mesozooplankton dry mass (DM) integrated over the upper 1,500 m of the water column was surprisingly stable throughout the investigation area and measured 2.0±0.3 g DM m–2. Dry mass in the upper 50 m measured 20.9 mg m–3 and was dominated by Calanushyperboreus (57.4%) and C. glacialis (21.1%). C. finmarchicus was very abundant only in the Nansen Basin. Below 200 m the calanoid copepods Metridialonga, Microcalanuspygmaeus and Pareuchaeta spp., the decapod Hymenodoraglacialis and chaetognaths of the genus Eukrohnia were the principal contributors to biomass values of <1 mg DM m–3. Hence, vertical changes in abundance, biomass and species composition were much more pronounced than regional differences between the basins. Three different mesozooplankton communities were differentiated according to their faunistic composition and are discussed in context with the major water masses: Polar Surface Water, Atlantic Layer and Arctic Deep Water.

Predation impact of carnivorous macrozooplankton in the vicinity of the Prince Edward Island archipelago (Southern Ocean) in austral autumn 1998

The composition, biomass, feeding and predation impact of carnivorous macrozooplankton (>2 cm) on mesozooplankton (0.2–2 cm) in the sub-Antarctic waters of the southwest Indian Ocean were investigated at 19 stations in austral autumn (April/May) 1998. Zooplankton abundances and biomass ranged from 6.9 to 95.2 ind m−3 and between 1.8 and 18.1 mg Dwt m−3, respectively. Throughout the investigation, mesozooplankton comprising mainly copepods numerically and by biomass dominated net samples. Among the copepods, Calanus simillimus, Clausocalanus brevipes, Ctenocalanus vanus, and Oithona spp. dominated. The carnivore component of the macrozooplankton consisted mainly of five groups: decapods, amphipods, chaetognaths, euphausiids and gelatinous zooplankton. Among these, chaetognaths (Eukrohnia hamata and Sagitta gazellae) and euphausiids (Nematoscelis megalops and Euphausia longirostris) were the most prominent. Collectively, the carnivorous macrozooplankton comprised between 11% and 72% of total zooplankton biomass. Total predation impact of the carnivorous macrozooplankton varied considerably but generally accounted for <5% (range 0.7–44%) of the total mesozooplankton standing stock. Tentative calculations suggest that carnivorous macrozooplankton may contribute, via vertical migrations and production of fast sinking faecal pellets, to a downward flux of carbon equivalent to up to 9% of the total mesozooplankton stock per day within the Antarctic Polar Frontal Zone. As a consequence, carnivorous macrozooplankton may increase the localised efficiency of the biological pump.

Mesozooplankton distribution and grazing during the productive season in the Northwest Antarctic Peninsula (FRUELA cruises)

Abstract Mesozooplankton distribution and community structure in the Bellinghausen-Bransfield sector of the Antarctic Ocean were investigated during the FRUELA cruises (December 1995–February 1996). Total mesozooplankton biomass ranged between 0.015 and 1.43 g C m −2 . Biomass was higher in the Southern boundary of the Antarctic Circumpolar Current (SbyACC) area and in coastal waters of the Antarctic Peninsula. Total mesozooplankton abundance ranged from 0.4×10 3 to 1.3×10 5 individuals m −2 , of which 41.6–99.5% corresponded to copepods, mainly families Oithonidae, Oncaeidae, Pseudocalanidae, Calanidae and Metrididae. There was no evidence of coupling between mesoscale physical features and biomass or community structure. While coastal stations mainly at the Gerlache Strait remained in a highly productive state through the spring–summer, oceanic stations experienced a marked shift from a productive condition during FRUELA 95 to a low biomass, pteropod-dominated situation during FRUELA 96, possibly due to changing weather conditions. The median ingestion rates of herbivorous crustaceans during the FRUELA cruises were 0.7 mg Chl a m −2 day −1 . Measured ingestion rates represented only 0.1% of the chlorophyll standing stock or 10% of the daily primary production. Thus, crustacean mesozooplankton had little control on the development of phytoplankton blooms in the area.

Gelatinous zooplankton assemblages associated with water masses in the Humboldt Current System, and potential predatory impact by Bassia bassensis (Siphonophora: Calycophorae)

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 210:13-24 (2001) - doi:10.3354/meps210013 Gelatinous zooplankton assemblages associated with water masses in the Humboldt Current System, and potential predatory impact by Bassia bassensis (Siphonophora: Calycophorae) F. Pagès1,*, H. E. González2, M. Ramón1, M. Sobarzo3, J.-M. Gili1 1Institut de Ciències del Mar (CSIC), Plaça del Mar s/n, 08039 Barcelona, Catalunya, Spain 2Instituto de Biología Marina, Universidad Austral de Chile, Casilla 567, Valdivia, Chile 3Centro EULA-Chile, Universidad de Concepción, Casilla 160-C, Concepción, Chile *E-mail: fpages@icm.csic.es ABSTRACT: Large numbers of gelatinous zooplankton were collected off Mejillones Peninsula, Chile (Humboldt Current System) in January 1997 during an oceanographic cruise. The area was characterized by the mixing of 3 water masses and the development of coastal upwelling. Siphonophores were the predominant group at most of the stations and the calycophoran Bassia bassensis was overwhelmingly the most abundant species. Five group associations were distinguishable in relation to the water masses identified. Siphonophores were associated with Subtropical Surface Water, the ctenophore Pleurobrachia sp. with Subantarctic Water, the pelagic tunicate Salpa fusiformis with Equatorial Subsurface Water, an assemblage of all gelatinous groups with mixed waters, and a low occurrence of gelatinous groups with upwelled Equatorial Subsurface Water. Molluscs were the group least associated with any water mass. The potential percentage of small copepods removed by B. bassensis ranged between 2.9 and 69.3%. Our results indicate that B. bassensis was the most important secondary predator in the top 50 m of the water column, and could therefore have had a significant trophic impact on the population of small copepods off the Mejillones Peninsula during the sampling period, where small copepods constituted 80.6% of the total mesozooplankton community. This siphonophore potentially ingested an average of 17.3% of the total copepod biomass. KEY WORDS: Siphonophores · Ctenophores · Salps · Molluscs · Water masses · Humboldt Current · Bassia · Copepods · Predation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 210. Online publication date: January 26, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Grazing by mesozooplankton from Kiel Bight, Baltic Sea, on different sized algae and natural seston size fractions

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 199:43-53 (2000) - doi:10.3354/meps199043 Grazing by mesozooplankton from Kiel Bight, Baltic Sea, on different sized algae and natural seston size fractions F. Sommer1,*, H. Stibor2, U. Sommer3, B. Velimirov1 1Institut für Medizinische Biologie, Universität Wien, Abteilung Allgemeine Mikrobiologie, Währinger Straße 10, 1090 Wien, Austria 2Zoologisches Institut, Ludwig-Maximilians-Universität, Abteilung Aquatische Ökologie, Karlstraße 23-25, 80333 München, Germany 3Institut für Meereskunde, Christian-Albrechts-Universität, Abteilung Meeresbotanik, Düsternbrooker Weg 20, 24105 Kiel, Germany *E-mail: a9101292@unet.univie.ac.at ABSTRACT: Grazing experiments were conducted with natural mesozooplankton from Kiel Bight, Germany, using radioactive labelled phytoplankton cultures and seston size fractions. The results of experiments using phytoplankton cultures indicated that bivalve veligers performed highest clearance of particles within a size range of 4.7 to 6.3 µm, whereas optimum particle size for copepods was 15 µm. The results of experiments using labelled natural seston size fractions identified bivalve veligers and appendicularians as those responsible for the removal of particles within the smallest size class (<2 µm). Seston size fractions larger than 5 µm were mainly cleared by copepods and nauplii. As particle size increased, the contribution of copepod clearance to total zooplankton clearance within size classes increased from 57% (<5 µm size class) to more than 81% (30 to 100 µm size class). When the nauplii clearance rates were included, the total copepod clearance accounted for 90 to 97.6% of the total volume cleared of particles bigger than 10 µm. Despite low abundances of bivalve veligers and appendicularians in Kiel Bight at the time of the experiment, we calculated that approximately 10 and 8.5%, respectively, of the carbon ingested by total mesozooplankton was due to veliger and appendicularian grazing. The importance of bivalve veligers might be seen in their grazing on seston particles that escape predation by copepods and on the amount of energy that is therefore directed from the water column to the benthos when larvae settle. KEY WORDS: Bivalve veligers · Copepods · Size-selectivity · Community grazing Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 199. Publication date: June 26, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

The structure of zooplankton communities, in the 2 to 2000 μm size range, in the Arabian Sea during and after the SW monsoon, 1994

Zooplankton communities, studied in the surface mixed layer on a 1600 m transect across the Arabian Sea, were found to differ in their temporal and spatial response to seasonal forcing. The transect studied, spanned seasonally eutrophic upwelling, mesotrophic downwelling and aseasonal oligotrophic waters. The nano- and microzooplankton communities constituted a relatively constant compartment in the tropical monsoon ecosystem, whilst the mesozooplankton showed a clear response to both upwelling and season. The heterotrophic nanoflagellates were concentrated in the surface mixed layer, except in the eutrophic upwelling waters of the SW monsoon. They reached maximum cell concentrations of 855 ml -1 during the SW monsoon and a maximum biomass of 8.4 mg C m -3 during the intermonsoon. Nanozooplankton standing stocks, in the surface mixed layer, ranged between 7 and 333 mg C m -2, with highest stocks found during the intermonsoon. The microzooplankton community was dominated by Protozoa, particularly aloricate ciliates and heterotrophic dinoflagellates, which accounted for up to 99% in terms of numbers and up to 71% of the biomass. Sarcodines and metazoan nauplii were recorded in lower numbers (<400 l -1). The microzooplankton were also concentrated in the surface mixed layer during both periods, except in the eutrophic coastal waters during the SW monsoon, when relatively high biomass values were found below the mixed layer depth. Their standing stocks, in the surface mixed layer, ranged between 50 and 182 mg C m -2, with the highest concentration found in the mesotrophic offshore waters during the late monsoon period. Total mesozooplankton standing stocks, in the surface 100 m, decreased with distance from the coastal to offshore waters and between seasons, decreasing from 1248 to 238 mg C m -2 during the late SW monsoon and 656–89 mg C m -2 during the following intermonsoon. The largest size class, of 1000–2000 μm sized organisms, dominated throughout except at the oligotrophic station during the intermonsoon period, when the smallest class, of 200–500 μm, were more important. The shift in size structure from large to small zooplankton occurred in response to a shift in dominance from large to small phytoplankton cells both spatially, along a eutrophic–oligotrophic gradient, and seasonally. These responses are a result of the physical forcing associated with the monsoon seasons in the Arabian Sea.

Fluctuating silicate:nitrate ratios and coastal plankton food webs.

Marine diatoms require dissolved silicate to form an external shell, and their growth becomes Si-limited when the atomic ratio of silicate to dissolved inorganic nitrogen (Si:DIN) approaches 1:1, also known as the "Redfield ratio." Fundamental changes in the diatom-to-zooplankton-to-higher trophic level food web should occur when this ratio falls below 1:1 and the proportion of diatoms in the phytoplankton community is reduced. We quantitatively substantiate these predictions by using a variety of data from the Mississippi River continental shelf, a system in which the Si:DIN loading ratio has declined from around 3:1 to 1:1 during this century because of land-use practices in the watershed. We suggest that, on this shelf, when the Si:DIN ratio in the river decreases to less than 1:1, then (i) copepod abundance changes from >75% to <30% of the total mesozooplankton, (ii) zooplankton fecal pellets become a minor component of the in situ primary production consumed, and (iii) bottom-water oxygen consumption rates become less dependent on relatively fast-sinking (diatom-rich) organic matter packaged mostly as zooplankton fecal pellets. This coastal ecosystem appears to be a pelagic food web dynamically poised to be either a food web composed of diatoms and copepods or one with potentially disruptive harmful algal blooms. The system is directed between these two ecosystem states by Mississippi River water quality, which is determined by land-use practices far inland.

Mesozooplankton biomass in the upper 1000 m in the Arabian Sea: overall seasonal and geographic patterns, and relationship to oxygen gradients

Mesozooplankton biomass distributions in the upper 1000 m of the Arabian Sea were studied as part of the US JGOFS Arabian Sea Process Study. Samples were collected in vertically stratified MOCNESS tows during four seasons in 1995 at six stations spanning the Arabian Sea. This paper describes results from the size-fractionated dry weight and wet weight biomass profiles and their relationship to the seasonal monsoon cycle and the pronounced oxygen minimum zone (OMZ). The total mesozooplankton biomass and most size fractions exhibited a significant onshore/offshore biomass gradient during most seasons and at most depths. The gradient was strongest in August during the late Southwest Monsoon and weakest in March during the Spring Intermonsoon, when maxima of some size-fractions occurred offshore. Seasonal changes in zooplankton biomass were small. The late Southwest Monsoon was the time of higher biomasses for nearshore stations, while the Spring Intermonsoon was the time of higher biomasses for offshore stations. Most of the zooplankton biomass present in the upper 1000 m of the water column occurred in the upper 200 m, but there was substantial diel vertical migration, especially in the large size class, down to 300–400 m, well within the suboxic water of the OMZ. Subsurface biomass increased in the lower OMZ within the depth range of the 0.05–0.1 ml/l oxygen gradient (about 500–600 m). The enhanced zooplankton biomass, higher potential food levels, and possible short food chains suggested active biological modification of the sinking flux in this depth zone. Biomass depth profiles showed close relationships to oxygen profiles. The shapes of the zooplankton biomass and oxygen profiles varied with geographic location across the Arabian Sea but were remarkably consistent over the year at each station. Geographic differences in the shapes of the profiles were dependent on the vertical extent of the suboxic zone and may correspond to broad regional differences in the processing of the export flux in midwater. However, the consistency of the shapes over time implies long-term stability in the structure and function of the midwater ecosystem at each location.

Net zooplankton and the biological pump: a comparison between the oligotrophic and mesotrophic equatorial Pacific

Structure and functioning of the zooplankton community and their consequences on the export fluxes of carbon, nitrogen and phosphorus were studied in two contrasting systems of the equatorial Pacific: the oligotrophic TTS (Typical Tropical Structure) to the west, and the mesotrophic HNLC (High Nutrient-Low Chlorophyll) in the central Pacific. Data were collected during the FLUPAC cruise equatorial transect (September-October 1994) of R.V. L'Atalante and four 6–8-day long time-series stations made between 165°E and 150°W. Along the equator, a sharp 2.5-fold increase in mesozooplankton biomass (200–2000 pm) was observed between 174° and 172°W, with a simultaneous change in surface salinity and chlorophyll concentration, corresponding to the shift between the TTS and HNLC systems. No significant zonal trend was observed within the two systems. Compared with TTS, HNLC presented a significantly greater contribution of the (500–2000 μm) size class to total mesozooplankton biomass, less diel variations, and a shallower vertical distribution. Lower metabolic rates in HNLC were accounted for by different taxonomic compositions in the two areas. Microzooplankton (35–200 gm) had a rather uniform biomass in TTS and HNLC, presented no significant diel variations in the 0–100 or 0–200 m layers, and displayed a shallower vertical distribution than the mesozooplankton. Consequences of such zooplanktonic features on the “biological pump” are assessed. Dissolved nitrogen (DN) and phosphorus (DP) active fluxes, resulting from excretion of interzonal mesozooplankton migrants were 2.1 times higher in the TTS than in the HNLC. However, dissolved inorganic carbon active fluxes were equal in the TTS and HNLC systems, due to differences in C, N, P metabolisms. Combined mesozooplankton and estimated micronekton nitrogen active fluxes represented 40% of the passive flux as measured by sediment traps in the TTS, and 9% in the HNLC. Estimates of mesozooplankton fecal production in the pbotic zone lead to a 2-fold increment between the oligotrophic and mesotrophic stations, and a larger contribution of the fecal production to the sinking flux. It is, therefore, concluded that the mesozooplankton role in the biological pump is mainly passive in the HNLC system, in contrast to the TTS site.

Mesozooplankton associations with medium to large marine snow aggregates in the northern Gulf of Mexico

Large and diverse mesozooplankton communities were observed on marine snow particles collected in coastal and oceanic waters of the northern Gulf of Mexico. Mesozooplankton were col- lected from seven phyla, including ostracods, cladocerans, pelecypods and ascidian larvae not previ- ously recorded as being associated with marine snow. Copepod nauplii were the most common, sometimes at concentrations >100 per aggregate. Oncaea spp., Oithona spp. and Microsetclla norvcgica were the most common copepod species. Total mesozooplankton abundance ranged between 2 and 278 organisms per aggregate. Organisms varied markedly in their distribution across the aggregate surface and in their behaviour towards the snow matrix. Comparison of snow communities with zoo- plankton abundance determined from net tows suggests that some species are concentrated on snow particles. Snow particles and their associated microbial communities may be a significant source of nutrition for these mesozooplankton. Mesozooplankton may contribute significantly to the degra- dation and decomposition of large snow particles as they sink through the upper water column.

Zooplankton community structure in the Western Mediterranean sea related to mesoscale hydrodynamics

In the Liguro-provencal basin (Northwestern Mediterranean sea), an heterogeneous hydrodynamic structure was observed in May 1986 (Pinca & Dallot, 1995). The hydrological system was characterized, as usual for this region, by the cyclonic Ligurian current with its associated thermohaline coastal fronts separating the coastal area from a central divergence zone, but was complicated in 1986 by the presence of an anticyclonic eddy in the middle of the central area. The sampling sites were consequently separated into four zones according to this physical structure. Mesozooplankton community structure related to the physical organization of the water masses is presented here. In order to obtain a description of the community structure, we studied both structure indices (species richness, evenness, dominance) and Rank-Frequency Diagrams. These descriptors showed a certain patchiness: species diversity and evenness were both minimal in the central area and increased towards the zones that surrounded the eddy and the coastal regions. However, almost everywhere we observed a stage 1 in the ecological succession as depicted by Frontier (1976), indicating that the development of the community structure (Margalef, 1967) was still at a juvenile state, else, that it was disturbed by the rapidly growing population of a single species. The most striking characteristic of the data set was actually the strong predominance of one or two copepod species. Centropages typicus prevailed: its population increased during the spring bloom, particularly in the area of the anticyclonic eddy. In order to better identify the organization of the region in terms of different ecological maturity stages and to search for a relation between mesoscale physical structure and community patterns, we grouped the sites on the basis of relative abundance of ranked species. We used an agglomerative cluster analysis with a new dissimilarity index, corrected to limit the weight of the most abundant species. We then tested for differences among the seven obtained groups in terms of structure indices, biological and environmental parameters. The physical environment could be related to the organization of the plankton community. In the central area, where the divergence and the eddy were located, a higher concentration of phytoplankton, total mesozooplankton abundance and biomass were recorded. The eddy was characterized by a higher concentration of relatively bigger and omnivorous copepods (Euchirella rostrata, Pleuromamma gracilis, Centropages typicus), probably favored by their stronger swimming ability in this highly energetic environment. However, the more specific organization of the community was regulated by more complicated biological interrelations that gave rise to a higher level of patchiness in the ecological structure.

Mesozooplankton in the northern Wadden Sea of Sylt: Seasonal distribution and environmental parameters

A statistical analysis was performed to determine environmental factors of significant influence on the mesozooplankton in the northern Wadden Sea of Sylt (German Bight) in 1991. The total mesozooplankton (>76 μm) was significantly correlated with water temperature as was the case with the calanoid copepodAcartia sp.;Pseudo- andParacalanus sp. were influenced by the amount of rainfall, whereas the amount of the meroplanktonic larvae of the cirripedia was related to the amount of particulate organic nitrogen in the water. A comparison of the years 1984 and 1991 demonstrates the positive effect of temperature on the copepod speciesAcartia sp. in summer.

Latitudinal variations in mesozooplankton grazing and metabolism in the central tropical Pacific during the U.S. JGOFS EqPac study

Pigment ingestion rates by three size classes of mesozooplankton (200–500 μm, 500–1000 μm and 1000–2000 μm) within the euphotic zone were measured during the Survey l (February/March) and Survey 2 (August/September) cruises of the 1992 United States Joint Global Ocean Flux Study (U.S. JGOFS) in the central equatorial Pacific (EqPac). Survey 1 was characterized by El Niño conditions while Survey 2 was characterized by typical climatological conditions. The small animals (200–500 μm) contributed more than 50% (range: 34–80%) to the total mesozooplankton grazing reported here. Mesozooplankton grazing was higher within the equatorial region (5°S–5°N) than at higher latitudes (5°S–12°S, 5°N–12°N). The carbon-specific ingestion rates of both the large and small animals tended to be higher during Survey 1 than during Survey 2. In contrast to the carbon-specific ingestion rates, the mesozooplankton biomass during Survey 1 was lower than that during Survey 2. Thus, the higher ingestion rates during Survey 1 were offset by lower biomass and the mesozooplankton grazing was quite similar during Surveys 1 and 2. Mesozooplankton removal of chi a was higher within the high-phytoplankton-biomass equatorial region than at higher latitudes during Survey 1, but regional differences were not significant during Survey 2. Mesozooplankton community grazing was equivalent to an average daily removal of 3.0% (range: 0.5–7.7%) and 2.2% (range: 0.8–3.5%) of the total chi a standing stock within the euphotic zone during Surveys 1 and 2, respectively. However, mesozooplankton grazing was equivalent to a daily removal of 47% (range: 15–91%) and 36% (range: 23–66%) of the > 5μm chl a standing stock within the euphotic zone during Surveys 1 and 2, respectively. Assuming a carbon-to-chi a ratio of 58, we estimate that mesozooplankton grazing removed an average of 6% (range: 2–12%) of 14C primary production during Survey 1, and 5% (range: 2–10%) during Survey 2. Removal rates were not significantly different between the equatorial region and the higher latitudes. Rates of respiration and ammonium excretion were estimated from empirical models based on animal weight and water temperature. Phytoplankton ingestion could not satisfy the estimated daily maintenance-carbon demands of mesozooplankton. The shortages were more pronounced in the large size fraction than in the small size fraction. The estimated ammonium excretion by the mesozooplankton could support 4–15% and 3–17% of 14C primary production during Surveys 1 and 2, respectively.

Zooplankton of tikehau atoll (Tuamotu archipelago) and its relationship to particulate matter

The standing stock and taxonomic composition of zooplankton (>200 μm) were monitored in the lagoon of Tikehau atoll from April 1985 to April 1986. These data were supplemented by two 10 d studies on the variability, structure and functioning of the pelagic ecosystem. The biomass of animals >200 μm comprised 50% of the total biomass of all organisms from 35 to >2000 μm. The zooplankton populations were characterized by successive blooms of copepods, larvaceans, pteropods and salps, probably arising from the periodic input of detritus from the reef during windy periods. As a result, the ecosystem was not in a steady state and the data for the fluxes of organic matter are presented separately for April 1985 and April 1986. Using the C:N:P ratio method, net growth efficiencies, K2, were calculated for total mesozooplankton, mixed copepods, and two planktonic species, Undinula vulgaris and Thalia democratica. Combined with nitrogen and phosphorus excretion rates, these K2 values enabled the assessment of production rates. On a 24 h basis, P:B ratios (%) were close to 100 for the total zooplankton and 54, 34 and 800 for mixed copepods, U. vulgaris and T. democratica, respectively. These ratios are 5.7 times lower than that recorded for phytoplankton. High productivity may be ascribable to the high density of seston, the high temperature (29.5°C), and the kind of organisms present. Zooplankton production equalled 38 and 30% of 14C uptake during April 1985 and April 1986, respectively. Ingestion of animals >35μm was calculated by means of assimilation efficiencies and amounted to 17 and 7% of particulate organic carbon, 100 and 38% of living carbon, and 64 and 140% of primary production during the two periods, respectively. Finally, inorganic exduring was 32 and 18% of phytoplankton nitrogen and phosphorus requirements. A model based on the dimensional structure of the pelagic food-web, has been drawn to illustrate the biomass and carbon, nitrogen and phosphorus fluxes in the study area. The lagoon appears to export part of its planktonic biomass, which is 4.2 times lower one sea mile outside the main pass connecting the lagoon to the open ocean.

Daily ration and predation by medusae and ctenophores in Saanich Inlet, B.C., Canada

The daily ration and predation impact of medusae and ctenophores in the upper 25 m of Saanich Inlet were determined using data from gut contents analyses, digestion times, and predator/prey densities. The weight-specific daily rations of the 3 major gelatinous predators ( Pleurobrachia bachei, Phialidium gregarium, and Phialidium lomae) were a negative power function of their biomass. Small predators obtained a ration much in excess of maintenance needs, while the largest specimens had little excess carbon growth or reproduction. In the upper 25 m predation impact was small on the total mesozooplankton, (mostly <1%·d −1 of biomass cropped), but it was higher in the upper 5 m (5 to 10%·d −1 of biomass removed). Specific predation on Euphausia pacifica eggs and nauplii was much greater, with 10 to 40% of stocks consumed by gelatinous predators per day. In Saanich Inlet, medusae and ctenophores were significant consumers of commercially important euphausiids (eggs and larvae), and are probably competitors of juvenile salmon for these same prey.