Seasonal variations in upwelling and in the grazing impact of copepods on phytoplankton off A Coruña (Galicia, NW Spain)

The impact of grazing by copepods on phytoplankton was studied during a seasonal cycle on the Galician shelf off A Coruña (NW Spain). Grazing was estimated by measuring the chlorophyll gut content and the evacuation rates of copepods from three mesh-size classes: 200-500 (small), 500-1000 (medium), and 1000-2000 μm (large). Between February 1996 and June 1997, monthly measurements of water temperature, chlorophyll concentration, primary production rates, and copepod abundance, chlorophyll gut content, and evacuation rates were taken at an 80-m-deep, fixed shelf station. Additionally, the same measurements were collected daily during two bloom events in March and in July 1996. Small copepods were the most abundant through the seasonal cycle. The highest grazing impact, however, was due to the medium and large size classes. Grazing by small copepods exceeded grazing by medium and large copepods only during phytoplankton spring blooms. The impact of copepod grazing (considering all size fractions) was generally low. On average, 2% of the phytoplankton biomass and 6% of the primary production were removed daily by the copepod community. Maximum grazing impact values (9% of the phytoplankton biomass and 39% of the primary production) were found in mid-summer. These results suggest that most of the phytoplankton biomass would escape direct copepod grazing in this upwelling area.     

Mesozooplankton distribution patterns and grazing impacts of copepods and Euphausia crystallorophias in the Amundsen Sea, West Antarctica, during austral summer

The rapid melting of glaciers as well as the loss of sea ice in the Amundsen Sea makes it an ideal environmental setting for the investigation of the impacts of climate change in the Antarctic on the distribution and production of mesozooplankton. We examined the latitudinal distribution of mesozooplankton and their grazing impacts on phytoplankton in the Amundsen Sea during the early austral summer from December 27, 2010 to January 13, 2011. Mesozooplankton followed a latitudinal distribution in relation to hydrographic and environmental features, with copepods dominating in the oceanic area and euphausiids dominating in the polynya. Greater Euphausia crystallorophias biomass in the polynya was associated with lower salinity and higher food concentration (chlorophyll a, choanoflagellates, and heterotrophic dinoflagellates). The grazing impact of three copepods (Rhincalanus gigas, Calanoides acutus, and Metridia gerlachei) on phytoplankton was low, with the consumption of 3 % of phytoplankton standing stock and about 4 % of daily primary production. Estimated daily carbon rations for each of the three copepods were also relatively low (<10 %), barely enough to cover metabolic demands. This suggests that copepods may rely on food other than phytoplankton and that much of the primary production is channeled through microzooplankton. Daily carbon rations for E. crystallorophias were high (up to 49 %) with the grazing impact accounting for 17 % of the phytoplankton biomass and 84 % of primary production. The presence of E. crystallorophias appears to be a critical factor regulating phytoplankton blooms and determining the fate of fixed carbon in the coastal polynyas of the Amundsen Sea.     

Distribution of biogenic silica and particulate organic matter in coastal and oceanic surface waters off the South Shetland Islands in summer

Biogenic silica (BSi), lithogenic silica (LSi), particiulate organic carbon (POC) and nitrogen (PON), and chlorophyll a (Chl a) concentration levels were measured in the surface waters (<100 m) off the northern coast of the South Shetland Islands in summer 1991. High concentration levels of BSi and LSi were recorded in the oceanic area and the coastal area, respectively. However, marked regional differences were not observed for POC, PON and Chl a concentrations. The mean BSi/POC atomic ratio (±SD) in the oceanic area (0.27±0.17) was 6 times that in the coastal area (0.045±0.020), except for the bloom situation (0.19±0.029). In contrast, the mean POC/PON atomic ratio was not significantly different in the coastal area (5.9±1.4) and the oceanic area (5.2±1.7). Nitzschia spp. were the dominant diatoms in the oceanic area but not in the coastal area. High BSi/POC ratios have been reported for blooms dominated by Nitzschia spp. even in the coastal regions of the Antarctic Ocean. The area difference in the BSi/POC ratios was probably related to the difference in species composition of phytoplankton and not to regional contrast. This species contributes significantly to high BSi/POC ratios in the Antarctic Ocean.     

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.     

Grazing impact of copepod assemblages and gravitational flux in coastal and oceanic waters off central Chile during two contrasting seasons

In evaluating carbon flux in coastal and oceanic waters offcentral Chile (~36°S), the grazing pressure by copepod size-assemblagesand the gravitational flux from the surface layer were estimatedduring two contrasting seasons: spring upwelling (October 1998)and winter (July 1999) periods. Grazing pressure upon phytoplanktonbiomass was small (<5%) during both periods at all stations.It was, however, an important proportion of primary productionat the coast during the spring (17–43%) but minimal inwinter (<5%) while, at the oceanic station, it was significantduring both seasons (13–46%). Similarly, the downwardflux of particulate organic carbon was a significant percentageof primary production at the coast during the spring (31%) andless so in winter (15%); at the oceanic station, it was a higherfraction during both seasons (46–47%). Copepod pelletswere only a minor component of the faecal flux, suggesting thatthey were rapidly recycled in the water column. The seasonaldifferences in these carbon fluxes in the coastal upwellingzone were related directly to the larger quantities of organicmatter in the water column during the spring upwelling period,including higher chlorophyll a concentrations and primary productionrates. During the winter, the particulate organic carbon appearsto be mainly recycled in the water column. In the oceanic zone,in contrast to what is expected, a large percentage of the primaryproduction appears to be exported during both periods, thoughconsumption by copepod assemblages was also important, suggestingsignificant horizontal carbon export from the coast to the openocean.     

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.      

Copepod grazing in a summer cyanobacteria bloom in the Gulf of Finland

Blooms of the cyanobacteria Nodularia spumigena and Aphanizomenon flos-aquae dominated the phytoplankton assemblages of the western Gulf of Finland and the eastern side of the northern Baltic Sea in late July–August, 1992. The bloom overlapped the peak seasonal contributions of the dominant mesozooplankton herbivores in the region, the copepods Acartia bifilosa and Eurytemora affinis and the cladoceran Bosmina longispina maritima. Using radio-labelling techniques; the copepods were offered one of the cyanobacteria, Nodularia, as well as the 10–54 µm fraction of the natural phytoplankton assemblage. In general, incorporation rates of the labelled phytoplankton into the copepods declined with increasing contributions of the cyanobacteria. For both copepods, incorporation was inversely related to total phytoplankton biomass, whether measured as chlorophyll, total cells or cyanobacteria biomass. The very low rates for Acartia (< 0.8 µl [copepod h]^–1) indicated that this copepod was likely starving in the cyanobacteria bloom, consistent with the generally poor condition of the animal observed in the laboratory. The other major mesozooplanktor, B. longispina maritima, ingested substantially more cyanobacterial biomass than the two copepods, based on HPLC-identified cyanobacteria-specific pigment echinenone in the gut. Bloom carbon provided < 1% and < 4% of the daily rations for Acartia and Eurytemora, respectively. Total copepod demand in the cyanobacteria blooms was trivial, < 1% of bloom biomass consumed daily. These results suggest that copepod herbivory is relatively unimportant in dissipating summer cyanobacteria blooms in the Gulf of Finland.     

Annual variability in ciliate community structure, potential prey and predators in the open northern Baltic Sea proper

Biomass of ciliates, bacteria and mesozooplankton, as well asbiomass estimates of phytoplankton from chlorophyll a values,were studied in the mixed layer of the northern Baltic Sea proper,between February and December 1998. Production of phytoplanktonand bacteria was measured, and production of ciliates and mesozooplanktonwas estimated. The phytoplankton spring bloom in late Marchwas dominated by diatoms and dinoflagellates. Ciliates had abiomass peak shortly after the spring bloom, while mesozooplanktonpeaked in July. Thus, the predation pressure on ciliates waslow in spring, and ciliates were major predators, potentiallyconsuming up to 15% of the primary production. In summer, therewas a shift from larger to smaller ciliates coinciding witha shift from larger to smaller primary producers, an increasein bacterial production, and also an increase in mesozooplanktonabundance, mainly copepods. Elevated mesozooplankton predationand selective removal of larger ciliate species and/or a shiftto smaller prey size presumably caused these changes. The potentialcarbon consumption from ciliates and mesozooplankton was highestin summer and autumn, reaching 55 and 40% of the primary productionin summer and autumn, respectively. Ciliates consumed twiceas much as mesozooplankton, thus acting as important regenerators.     

Primary production by benthic microalgae in nearshore marine sediments of Signy Island,Antarctica

Summary During the austral summer of 1987/1988, three 24 h in situ primary productivity measurements were made at a nearshore sublittoral site on the east coast of Signy Island, Antarctica. The first experiment in December, coincided with the peak of the benthic algal bloom as shown by benthic chlorophyll measurements and a primary productivity rate of 700.9 mg carbon m^–2 day^–1. In January, the experiment was undertaken during the peak of the phytoplankton bloom when light intensities reaching the benthos were greatly reduced. A rate of 313.4 mg carbon m^–2 day^–1 was measured, half that of the previous month. In March the phytoplankton bloom had died off, benthic light intensities had increased and production was 391.8 mg m^–2 day^–1. The experiments indicate changes in benthic microalgal activity during the summer, linked to changes in the benthic light climate. Compared with previous measurements of phytoplanktonic activity at Signy, the microphytobenthos seems to be an important source of primary production. A production estimate of 100.9 mg carbon m^–2, for the ice-free summer period, lies within the range of values of results from other polar studies.     

Zooplankton diversity and the predatory impact by larval and small juvenile fish at the Fisher Banks in the North Sea

The biomass and diversity of the mesozooplankton and fish larvaecommunity were investigated across a frontal zone in the centralNorth Sea in the early summer, to investigate whether larvalfish predation is a regulator of mesozooplankton production.Pronounced changes in the mesozooplankton community were observedacross the front off the Jutlandic coast. The biomass and thediversity of copepods changed across the front as the populationof Calanus finmarchicus became abundant in the deeper water.The crustaceans (Acartia spp. and Evadne spp.) and polychaetelarvae dominated the mesozooplankton in the coastal water. Thebiomass of fish larvae was dominated by gadoid larvae. As inthe copepods, a shift in fish diversity was observed in thefrontal zone. On the coastal side of the front, whiting (Merlangiusmerlangus) dominated the biomass, while offshore from the frontwhiting were absent and cod (Gadus morhua) was the dominantlarval fish species on the deeper stations. The present investigationdemonstrates two different trophic pathways related to hydrographyin the central North Sea. First, in the shallow coastal water,the abundant small neritic copepods are predominately predatedby whiting, while in the offshore region the larger oceaniccopepods are predated by cod larvae. However, the predationpressure by the fish larvae was in general low (<10%) relativeto copepod production per day. Consequently, in the early summer,the copepod production potentially results in a build-up ofcopepod biomass on both sides of the front.     

Protozooplankton in the Weddell Sea,Antarctica: Abundance and distribution in the ice-edge zone

Summary Protozooplankton were sampled in the iceedge zone of the Weddell Sea during the austral spring of 1983 and the austral autumn of 1986. Protozooplankton biomass was dominated by flagellates and ciliates. Other protozoa and micrometazoa contributed a relatively small fraction to the heterotrophic biomass. During both cruises protozoan biomass, chlorophyll a concentrations, phytoplankton production and bacterial biomass and production were low at ice covered stations. During the spring cruise, protozooplankton, phytoplankton, and bacterioplankton reached high concentrations in a welldeveloped ice edge bloom 100 km north of the receding ice edge. During the autumn cruise, the highest concentrations of biomass were in open water well-separated from the ice edge. Integrated protozoan biomass was <12% of the biomass of phytoplankton during the spring cruise and in the autumn the percentages at some stations were >20%. Bacterial biomass exceeded protozooplankton biomass at ice covered stations but in open water stations during the fall cruise, protozooplankton biomass reached twice that of bacteria in the upper 100m of the water column. The biomass of different protozoan groups was positively correlated with primary production, chlorophyll a concentrations and bacterial production and biomass, suggesting that the protozoan abundances were largely controlled by prey availability and productivity. Population grazing rates calculated from clearance rates in the literature indicated that protozooplankton were capable of consuming significant portions of the daily phyto- and bacterioplankton production.     

127 Climate Variability and Phytoplankton Production in PNW Coast Estuaries

The west coast of North America receives strong forcing from climate modes such as El Niño-Southern Oscillation and the Pacific Decadal Oscillation. Estuaries are poised at a sensitive interface because estuarine biota and habitat will be affected by variability in properties and processes associated with the ocean, the watershed, and the local weather. Observations from the Washington coast and Willapa Bay illustrate these three arenas of influence. Variation in ocean upwelling and ocean thermocline depth associated with the 1997–98 El Niño versus the 1999 La Niña affected temperature and nutrient availability in Willapa Bay. Variation in river flow associated with the 2000–01 drought affected estuarine circulation and residence time. And, variation in prevailing wind direction and/or cloudiness was highly correlated with phytoplankton biomass. This situation is responsible for the complexity of understanding climate impacts on estuarine systems. In order to help evaluate which mechanisms, remote oceanic processes or local watershed/estuarine characteristics, most affected Willapa Bay's phytoplankton production, several phytoplankton species were used as indicators of water mass origin and compared with primary productivity data to assess whether phytoplankton blooms were dominated by endemic or imported species. Our analysis resolved that the highest primary production (and the appearance of Pseudo-nitzschia spp.) was associated with oceanic intrusions of phytoplankton biomass into Willapa Bay. This result underscores the dominant role that variation in ocean and climate play in controlling Pacific Northwest estuarine production. However, while the largest blooms were oceanic in origin, numerous medium-sized production events were from either oceanic or estuarine sources, indicating a complex picture.     

The inshore marine ecosystem off the Vestfold Hills,Antarctica

The planktonic, ice/water interface, and benthic communities at three sites off the coast of the Vestfold Hills, Antarctica, were examined over a complete year.The planktonic flora and fauna were composed predominantly of oceanic species with diatoms and copepods the numerically dominant groups. Primary production was largely restricted to the summer months except for epontic algae which developed in spring. The zooplankton exhibited a similar seasonal cycle but lagged some months behind that of the phytoplankton.The ice/water interface (epontic) fauna consisted of species from the plankton and benthos. Copepods were major contributors; however, two amphipod species dominated. Seasonality of the fauna in this habitat was determined by ice formation and breakout, and development of ice algae.Each of the benthic substrates supported a characteristic macrofaunal assemblage, although infaunal amphipods and tanaids were similar at each site. Infauna exhibited a distinct seasonal cycle related to that of the primary producers whereas macrofauna showed no seasonal changes in abundance.Species composition of each community in this coastal antarctic region was comparable with that of similar habitats in other antarctic coastal areas, supporting the circumpolarity of antarctic marine communities.     

Size-fractionated Primary Production in the South Atlantic and Atlantic Sectors of the Southern Ocean

Results are presented from size fractionated chlorophyll a (Chla) and primary production studies along a transect between Antarcticaand southern Africa during the second South African AntarcticMarine Ecosystem Study (SAAMES II), conducted in late australsummer (January to February) 1993. Total integrated Chl a alongthe transect was highest in the vicinity of the Marginal IceZone (MIZ) and Antarctic Polar Front (APF). At these stations,integrated Chl a biomass was always >25 mg Chl a m^–2and was dominated by microphytoplankton. Although nominal increasesinChl a biomass were also associated with the Subantarctic Front(SAF) and Subtropical Convergence (STC), total Chl a biomassin these regions was dominated by nanophytoplankton. Withinthe inter-frontal regions, total integrated Chl a biomass waslower, generally <25 mg Chl a m^–2, and was always dominatedby nanophytoplankton. An exception was found in the AgulhasReturn Current (ARC) where picophytoplankton dominated. Totaldaily integrated production along the transect ranged between60 and 436 mg C m^–2 day^–1. Elevated production rateswere recorded at stations occupied in the vicinity of the MIZand at all the major oceanic frontal systems. The contributionsof the various size fractions to total daily production displayedthe same spatial pattern as integrated biomass, with microphytoplanktonbeing the most important contributor in areas characterizedby elevated phytoplankton biomass. Outside these regions, nanophytoplanktondominated the total phytoplankton production. Again, an exceptionwas found in the ARC north of the STC where picophytoplanktondominated total production. There, the lowest production alongthe entire transect was recorded, with total daily integratedproduction always <90 mg C m^–2 day^–1. The increasedproduction rates recorded in the MIZ appeared to result fromincreased water column stability as indicated by a shallow mixed-layerdepth. Within the inter-frontal regions, the existence of adeep mixed layer appeared to limit phytoplankton production.Low silicate concentrations in the waters north of the APF mayalso have limited the growth of large microphytoplankton.     

Size-fractionated mesozooplankton biomass, metabolism and grazing along a 50{degrees}N-30{degrees}S transect of the Atlantic Ocean

Size-fractionated mesozooplankton grazing and metabolism wereinvestigated along the wide latitudinal range (50°N–30°S)covered during the Atlantic Meridional Transect (AMT) 11 cruise.Five different oceanic provinces were traversed in this cruise:North Atlantic Drift (NADR), North Atlantic Subtropical Gyral(NAST), Canary Coastal (CNRY), Eastern Tropical Atlantic (ETRA),and South Atlantic Gyral (SATL). CNRY and ETRA were affectedby the upwelling Mauritanian and equatorial respectively andprimary production in these provinces was higher than in theoligotrophic subtropical gyres (NAST and SATL). Both mesozooplanktonand phytoplankton biomass were highest around the equator. Theamount of chlorophyll a ingested daily by copepods was noticeablyhigher in mesotrophic than in oligotrophic provinces as shownby the spatial distribution of gut content values and the highabundances of copepods recorded at the equator. Grazing impactalong the transect ranged from 0.2 to 5.6% of the phytoplanktonstanding stock and from 1.6 to 14.5% of primary production.If only phytoplankton >2 µm are considered, the rangesare 1.0–19.4% (stock) and 3.4–44.7% (primary production).Grazing impact upon both phytoplankton biomass and primary productionfollowed a spatial distribution similar to that of chlorophylla ingestion, with higher values in upwelling zones than in thegyres. Weight-specific rates of respiration and NH₄⁺ and PO₄^3–excretion showed large variability both along the transect andwithin provinces, but did not differ between provinces. Therefore,zooplankton assemblages inhabiting the different provinces visitedin the AMT 11 seem to be adapted to the prevailing thermal conditions.Given the substantial proportion of nitrogen and phosphorusthat are supplied to primary producers through the excretoryactivity of mesozooplankton (the percentage of nitrogen andphosphorus requirements of phytoplankton accounted for by mesozooplanktonexcretion was >30% in all the provinces) it follows thatthey may play a crucial role as nutrient regenerators, especiallyin the oligotrophic gyres where regenerated production dominates.     

Spring bloom dynamics in Kongsfjorden,Svalbard: nutrients,phytoplankton, protozoans and primary production

The marine ecosystem in Kongsfjorden (79°N), a glacial fjord in Svalbard, is to a large extent well known with regard to hydrography, mesozooplankton and higher trophic levels. Research on primary production and lower trophic levels is still scare and especially investigations from winter and spring periods. The spring bloom dynamics in Kongsfjorden were investigated in 2002. The development in nutrient conditions, phytoplankton, protozoans and primary production were followed from 15 April until 22 May. The winter/spring in 2002 was categorized as a cold year with sea ice cover and water masses dominated by local winter-cooled water. The spring bloom started around 18 April and lasted until the middle of May. The bloom probably peaked in late April, but break-up of sea ice made it impossible to sample frequently in this period. Diatoms dominated the phytoplankton assemblage. We estimated the total primary production during the spring bloom in 2002 to range 27–35 g C m⁻². There was a mismatch situation between the mesozooplankton and the phytoplankton spring bloom in 2002.     

Grazing effect of the invasive reef-forming polychaete Ficopomatus enigmaticus (Fauvel) on phytoplankton biomass in a SW Atlantic coastal lagoon

In this work we evaluate the potential grazing impact of the invasive reef-forming polychaete Ficopomatus enigmaticus in a SW Atlantic coastal lagoon (Mar Chiquita coastal lagoon; 37° 40' S, 57° 23' W; Argentina). This gregarious species feeds on suspended detritus and phytoplankton. Given the large area dominated by reefs, suspension feeding by this species might reduce the lagoon phytoplankton concentration and even hinder local eutrophication. To evaluate this hypothesis in situ replicated mesocosm experiments were performed in spring 2005 and, summer and winter 2006. Mesocosms enclosing reefs and without reefs were installed and grazing intensity was measured as the difference in chlorophyll a concentration and turbidity between the reef-treatment and the treatment without reefs. Reefs of F. enigmaticus decreased the mean chlorophyll a concentration, more during summer (56% decrease) than spring (25% decrease) and winter (19% decrease). Reefs also decreased water turbidity during summer (54% decrease) and spring (23% decrease). While previous studies indicate that the physical structure of these reefs alters water flow increasing water turbidity, our evidences show that their suspension-feeding activity can counteract this effect. Water turbidity was positively correlated with chlorophyll a concentration, which suggests that phytoplankton grazing by F. enigmaticus decreased light attenuation through the water column, with a potential for enhancement of benthic primary productivity. Therefore, our results suggest that grazing by this polychaete affects overall estuarine primary production as well as the relative importance of planktonic and benthic carbon sources to higher trophic levels.     

Effects of the ice-edge bloom and season on the metabolism of copepods in the Weddell Sea,Antarctica

The metabolic responses of several species of Antarctic copepods to primary productivity and changes between seasons were investigated. To examine the influence of the spring ice-edge bloom on the metabolism of copepods, oxygen consumption rates were determined on specimens from three zones of widely different ice coverage and chlorophyll biomass: pack ice (pre-bloom), ice edge (bloom) and open water (post-bloom). Summer metabolic rates were compared with published winter rates. Field work was done in the Weddell Sea in the region of 60 °S, 36°W in late November and December 1993. Oxygen consumption rates were determined by placing individuals in syringe respirometers and monitoring the oxygen partial pressure for 10–20 hours. Higher metabolic rates were observed in the primarily herbivorous copepods, Calanoides acutus, Rhincalanus gigas and Calanus propinquus in regions of higher primary production: ice edge and open water. The carnivorous Paraeuchaeta antarctica showed a similar pattern. The omnivorous copepods Metridia gerlachei and Gaetanus tenuispinus showed no changes in metabolism between zones. Data on routine rates of copepods from the winter were available for C. propinquus and P. antarctica. In P. antarctica, rates were higher in the summer. Calanus propinquus showed a higher metabolic rate in the summer than in the winter, but the difference was not significant at the 0.05 level. It was concluded that copepods near the ice zone in the ice zone in the Antarctic rely on the spring ice-edge bloom for growth and completion of their life cycle.     

Factors influencing the short-term variation in phytoplankton composition and biomass in coral reef waters

The short-term temporal dynamics of phytoplankton composition was compared among coral reef waters, the adjacent ocean and polluted harbour water from July until October along the south-western coast of Curaçao, southern Caribbean. Temporal variations in phytoplankton pigment 'fingerprints' (zeaxanthin, chlorophyll b, 19'-hexanoyloxyfucoxanthin, fucoxanthin, 19'-butanoyloxyfucoxanthin, chlorophyll c₂ and c₃ relative to chlorophyll a) in the ocean were also observed in waters overlying the reef. However, with respect to specific pigments and algal-size distribution, the algal composition in reef waters was usually slightly different from that in the oceanic water. Phytoplankton biomass (chlorophyll a) was either higher or lower than in the oceanic water. The relative amount of fucoxanthin and peridinin was usually higher, and the relative and absolute amount of zeaxanthin was significantly lower than in oceanic water. Zeaxanthin-containing Synechococci were significantly reduced in reef water. Average algal cell size increased from the open water to the reef and the harbour entrance. Large centric diatoms (>20 m Ø) were better represented in reef than in oceanic water. In reef-overlying waters, the nitrate and nitrite concentrations were higher than in oceanic water. In front of the town, anthropogenic eutrophication (sewage discharge and ground water seepage) resulted in higher NH₄, NO₃ and PO₄ concentrations than at other reef stations. This concurred with significantly enhanced phytoplankton biomass (chlorophyll a), chlorophyll c₂ and peridinin amounts at Town Reef compared with the other reef stations. Polluted harbour water usually showed the highest phytoplankton biomass of all stations, dominated by diatoms and dinoflagellates. Conditions in reef waters and harbour water promoted the occurrence and the relative abundance of diatoms and dinoflagellates. Harbour water did not influence the phytoplankton composition and biomass at reef stations situated >5 km away from the harbour entrance. We conclude that phytoplankton undergoes a shift in algal composition during transit over the reef. The dominant processes appear to be selective removal of zeaxanthin-containing Synechococcus (by the reef benthos) and (relative) increase in diatoms and dinoflagellates. The difference in the phytoplankton composition between reef and oceanic waters tends to increase with decreasing dilution of reef water with ocean water.     

Trophic control of biogenic carbon export in Bransfield and Gerlache Straits, Antarctica

Size-fractionated chlorophyll a and photosynthetic carbon incorporation,microbial oxygen production and respiration and particulatevertical flux were measured in January 1996 at three regions,characterized by distinct hydrographic fields and planktoniccommunities, of the Antarctic Peninsula: (1) a diatom-Phaeocystissp., dominated community associated with the relatively stratifiedwaters of the Gerlache Strait, (2) a nanoplankton-Cryptomonassp. dominated assemblage at the Gerlache–Bransfield confluence;and (3) a nano- and picoplankton community in mixed waters ofthe Bransfield Strait. Despite the marked differences in bothcommunity structure and total phytoplankton biomass and primaryproduction, and against predictions from models about trophiccontrol of C export, the lowest respiration rates were measuredat Bransfield (pico- and nanoplankton), and no difference wasobserved between the Gerlache (large diatoms) and Bransfieldstations in relative vertical particle flux (6.4 vs. 5.1 % ofsuspended C; 14.9 vs. 10.4 % of net community production, respectively).Growth and loss rates of the phytoplankton population studiedfor each community indicate that microbial populations can beexplained by in situ growth, but spatial (diatom-Phaeocystissp., bloom) and temporal (diatom-Phaeocystis sp. bloom and nanoplanktoncommunities) scales of study were shown to be insufficient foraddressing the coupling between primary production and biogeniccarbon export, especially after the appreciation of the accumulationof dissolved organic carbon in the water column. This wouldexplain the unexpected results and highlights the necessityof including the mechanisms controlling accumulation and consumptionof dissolved organic matter into conceptual models about thetrophic control of C export.