Composition and distribution of the pelagic and sympagic algal assemblages in the Laptev Sea during autumnal freeze-up

The phytoplankton and ice algal assemblages in the SiberianLaptev Sea during the autumnal freeze-up period of 1995 aredescribed. The spatial distribution of algal taxa (diatoms,dinoflagellates, chrysophytes, chlorophytes) in the newly formedice and waters at the surface and at 5 m depth differed considerablybetween regions. This was also true for algal biomass measuredby in situ fluorescence, chlorophyll (Chl) a and taxon-specificcarbon content. Highest in situ fluorescence and Chl a concentrations(ranging from 0.1 to 3.2 µg l^–1) occurred in surfacewaters with maxima in Buor Khaya Bay east of Lena Delta. Thealgal standing stock on the shelf consisted mainly of diatoms,dinoflagellates, chrysophytes and chlorophytes with a totalabundance (excluding unidentified flagellates <10 µm)in surface waters of 351–33 660 cells l^–1. Highestalgal abundance occurred close to the Lena Delta. Phytoplanktonbiomass (phytoplankton carbon; PPC) ranged from 0.1 to 5.3 µgC l^–1 in surface waters and from 0.3 to 2.1 µg Cl^–1 at 5 m depth, and followed the distribution patternof abundances. However, the distribution of Chl a differed considerablyfrom the distribution pattern shown by PPC. The algal assemblagein the sea ice, which could not be quantified due to high sedimentload, was dominated by diatom species, accompanied by dinoflagellates.Thus, already during the early stage of autumnal freeze-up,incorporation processes, selective enrichment and subsequentgrowth lead to differences between surface water and sea icealgal assemblages.     

Respiratory ETS activity of plankton in the northwestern Alboran Sea: seasonal variability and relationship with hydrological and biological features

Respiratory electron transport system (ETS) activity was measuredin plankton samples (<200 µm) collected in the NW AlboranSea. Sampling was carried out during seasonal cruises (summerand autumn 2003 and winter and spring 2004) in 12 stations locatedin transects off the coast of Malaga (southern Spain). Thiswork reports for the first time seasonal variations of the Arrheniusactivation energy (Ea) as well as being the first study to addressCO₂ balance in the NW Alboran Sea. These variations were relatedto changes in the phytoplankton community assemblage, whichcould ultimately be caused by the seasonal variability of hydrologicalconditions. ETS activity was significantly higher in summer,coinciding with a higher chlorophyll a (Chl a) concentrationand relatively high levels of particulate organic matter. TheETS:Chl a_total ratios were low during the four seasons, suggestinga high contribution of autotrophic phytoplankton to the respiratoryactivity of planktonic community. Respiratory CO₂ production(RCP) calculated from ETS activity ranged from 4.6 to 28.1 mgC m^–3 day^–1 during the four cruises. Chl a-specificRCP was lower than the maximum photosynthetic rates reportedin the literature for the studied area, suggesting that primaryproduction (PP) and respiration in the water column might beunbalanced.     

Interannual variability in the distribution of the phytoplankton standing stock across the seasonal sea-ice zone west of the Antarctic Peninsula

The spatial distribution of phytoplankton cell abundance, carbon(C) biomass and chlorophyll a (Chl a) concentration was analysedduring three summers (1996, 1997 and 1999) in a seasonal sea-icearea, west of the Antarctic Peninsula. The objective of thestudy was to assess interannual variability in phytoplanktonspatial distribution and the mechanisms that regulate phytoplanktonaccumulation in the water column. Phytoplankton C biomass andChl a distributions were consistent from year to year, exhibitinga negative on/offshore gradient. The variations in C concentrationhad a close and non-linear relationship with the upper mixedlayer depth, suggesting that the vertical mixing of the watercolumn is the main factor regulating phytoplankton stock. Themagnitude of C gradients was 5-fold higher during 1996 thanduring 1997 and 1999. This was ascribed to interannual variationsin the concentration of diatom blooms in the region influencedby sea-ice melting. Vertical distribution of the phytoplankton,as estimated from Chl a profiles, also varied along an on/offshoregradient: Chl a was distributed homogeneously in the upper mixedlayer in coastal and mid-shelf stations and concentrated inthe deep layer (40–100 m) occupied by the winter waters(WW, remnants of the Antarctic surface waters during summer)in more offshore stations. The region with a deep Chl a maximumlayer (DCM layer) was dominated by a phytoplankton assemblagecharacterized by a relatively high concentration of diatoms.The extent of this region varied from year to year: it was restrictedto pelagic waters during 1996, extended to the shelf slope during1997 and occupied a major portion of the area during 1999. Itis hypothesized that iron depletion in near surface waters dueto phytoplankton consumption, and a higher concentration inWW, regulated this vertical phytoplankton distribution pattern.Furthermore, we postulate that year-to-year variations in thespatial distribution of the DCM layer were related to interannualvariations in the timing of the sea-ice retreat. The similaritybetween our results and those reported in literature for otherareas of the Southern Ocean allows us to suggest that the mechanismsproposed here as regulating phytoplankton stock in our areamay be applicable elsewhere.     

Chlorophyll <Emphasis Type="Italic">a</Emphasis> as a measure of seasonal coupling between phytoplankton and the monsoon periods in the Gulf of Oman

We present data from a long time-series study to describe the factors that control phytoplankton population densities and biomass in the coastal waters of Oman. Surface temperature, salinity, nutrients, dissolved oxygen, chlorophyll a (Chl a), and phytoplankton and zooplankton abundance of sea water were measured as far as possible from February 2004 through February 2006, at two stations along the southern coast of the Gulf of Oman. The highest concentrations of Chl a (3 mg m⁻³) were recorded during the southwest monsoon (SWM) when upwelling is active along the coast of Oman. However, results from our study reveal that the timing and the amplitude of the seasonal peak of Chl a exhibited interannual variability, which might be attributed to interannual differences in the seasonal cycles of nutrients caused either by coastal upwelling or by cyclonic eddy activity. Monthly variability of SST and concentrations of dissolved nitrate, nitrite, phosphate, and silicate together explained about 90% of the seasonal changes of Chl a in the coastal ecosystem of the Gulf of Oman. Phytoplankton communities of the coastal waters of Oman were dominated by diatoms for most part of the year, but for a short period in summer, dinoflagellates were dominant.     

Comparison of east and west chlorophyll a standing stock and oceanic habitat along the Transition Domain of the North Pacific

Chlorophyll (Chl) a was measured every 10 m from 0 to 150 min the Transition Domain (TD), located between 37 and 45°N,and from 160°E to 160°W, in May and June (Leg 1) andin June and July (Leg 2), 1993–96. Total Chl a standingstocks integrated from 0 to 150 m were mostly within the rangeof 20 and 50 mg m^–2. High standing stocks (>50 mg m^–2)were generally observed westof 180°, with the exceptionof the sporadic high values at the easternmost station. Thetotal Chl a standing stock tended to be higher in the westernTD (160°E–172°30'E) than in the central (175°E–175°W)and eastern (170°W–160°W) TD on Leg 1, but thesame result was not observed on Leg 2. It was likely that largephytoplankton (2–10 and >10 µm fractions) contributedto the high total Chl a standing stock. We suggest that thehigh total Chl a standing stock on Leg 1, in late spring andearly summer, reflects the contribution of the spring bloomin the subarctic region of the northwestern Pacific Ocean. Thedistribution of total Chl a standing stock on Leg 2 was scarcelyaffected by the spring phytoplankton bloom, suggesting thattotal Chl a standing stock is basically nearly uniform in theTD in spring and summer. Moreover, year-to-year variation inthe total Chl a standing stock was observed in the western TDon Leg 1, suggesting that phytoplankton productivity and/orthe timing of the main period of the bloom exhibits interannualvariations.     

Microzooplankton grazing in a coastal embayment off Concepcion, Chile, (~36{degrees} S) during non-upwelling conditions

The impact of grazing by natural assemblages of microzooplanktonwas estimated in an upwelling area (Concepción, Chile)during the non-upwelling season in 2003 and 2004. Seawater dilutionexperiments using chlorophyll a (Chl a) as a tracer were usedto estimate daily rates of phytoplankton growth and microzooplanktongrazing. Initial Chl a concentrations ranged from 0.4 to 1.4mg Chl a m^–3 and phytoplankton prey biomass and abundancewere numerically dominated by components <20 µm. Phytoplanktongrowth and microzooplankton grazing rates were 0.19–0.25day^–1 and 0.26–0.52 day ^–1, respectively.These results suggest that microzooplankton exert a significantremoval of primary production (>100%) during the non-upwellingperiod.     

Community structure, biomass and productivity of size-fractionated summer phytoplankton populations in lower Narragansett Bay, Rhode Island

Seventeen size-fractionation experiments were carried out duringthe summer of 1979 to compare biomass and productivity in the< 10, <8 and <5 µm size fractions with that ofthe total phytoplankton community in surface waters of NarragansettBay. Flagellates and non-motile ultra-plankton passing 8 µmpolycarbonate filters dominated early summer phytoplankton populations,while diatoms and dinoflagellates retained by 10 µm nylonnetting dominated during the late summer. A significant numberof small diatoms and dinoflagellates were found in the 10–8µm size fraction. The > 10 µm size fraction accountedfor 50% of the chlorophyll a standing crop and 38% of surfaceproduction. The <8 µm fraction accounted for 39 and18% of the surface biomass and production. Production by the< 8 µm fraction exceeded half of the total communityproduction only during a mid-summer bloom of microflagellates.Mean assimilation numbers and calculated carbon doubling ratesin the <8 µm (2.8 g C g Chl a^–1 h^–1; 0.9day^–1)and<5 µm(1.7 g C g Chl a^–1h^–1; 0.5day^–1)size fractions were consistently lower than those of the totalpopulation (4.8 g C g Chl a^–1 h^–1; 1.3 day^–1)and the <10 µm size fraction (5.8 g C g Chl a^–1h^–1; 1.4 day ^–1). The results indicate that smalldiatoms and dinoflagellates in fractionated phytoplankton populationscan influence productivity out of proportion to their numbersor biomass. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia.     

Phytoplankton pigments at the Weddell-Scotia confluence during the 1993 austral spring

During a 1993 austral spring cruise, a complex biomass was encounterednear South Orkney Island that ranged from a low-biomass, Chaetocerostortissimus assemblage south of the front towards the ice edge,to a high-biomass, Thalassiosira gravida-dominated assemblageat the northern edge. The maximum levels of chlorophyll (Chl)a (up to 6 mg m^–3) were higher than those observed inprevious high-performance liquid chromatography-based studiesof pigments in the pelagic Southern Ocean. The non-photosyntheticpigment chlorophyllide a comprised up to 75% of the chlorophyllouspigments in the southern assemblage, but < 5% in the northernassemblage. Concentrations of the xanthophylls diadinoxanthin(DD) and diatoxanthin (DT), used as indicators of mean irradiance,indicated low-light-adapted populations. Low-light DD + DT/Chla ratios in surface waters indicated that vertical mixing limitedphytoplankton residence time in the near-surface layer, andthus limited exposure to maximum irradiance. Deck incubationsof natural assemblages indicated that the dark epoxidation reaction(i.e. the return of DT to DD) was a two-step reaction with theinitial rate being more rapid (t^1/2 = 9.5 min) than the second(t^1/2 = 55 min). Fucoxanthin, a major diatom pigment, was morestable chemically in the water column than Chl a, and the verticalprofiles of fucoxanthin followed those of chlorophyllide a insome cases. The formation and apparent stability of chlorophyllidea and fucoxanthin are important considerations when estimatingphotosynthetically active biomass over large regions of theocean.     

Nutrient and phytoplankton dynamics in the Queen Charlotte Islands (Canada) during the summer upwelling seasons of 2001-2002

The Queen Charlotte Islands, Canada, lie at the northern extremeof the coastal upwelling system of the eastern North PacificOcean (51–54°N). In this study, the first observationsof spatial and inter-annual patterns in nutrient inventories,chlorophyll (Chl) a and phytoplankton assemblages are reportedand related to oceanographic conditions in near-shore watersof the island archipelago. Stronger and more persistent upwellingin 2002 coincided with higher nutrient and Chl a standing stockscompared to 2001 and a higher proportion of diatoms. Dinoflagellateswere more prevalent in 2001, including several potentially harmfulspecies. At sub-seasonal scales, Chl a concentrations were greatestduring downwelling conditions and smallest during upwellingconditions. On the west coast, weak water column stratification,high relative proportions of diatoms and large nutrient inventoriesaccompanied upwelling-favourable conditions, whereas on theeast coast, there was no direct relationship between the BakunUpwelling Index and water column stability. According to redundancyanalysis, variability in species composition was best explainedby sea surface temperature, the depth of the euphotic zone andnutrient inventories. The east coast supported blooms of coccolithophoreswithin protected bays, confirming previous satellite observationsthat showed bright patches in these areas. The data illustratethat moderate upwelling can have an important influence on near-shorealgal standing stocks and species composition at the northernextreme of the upwelling system off the west coast of NorthAmerica, and that topographical complexity may be importantfor the development of phytoplankton blooms.     

Seasonal variations of bacterial abundance and biomass and their relation to phytoplankton in the hypertrophic tropical lagoon Cienaga Grande de Santa Marta, Colombia

The seasonal development of bacteria was studied in the hypertrophiccoastal lagoon Ciénaga Grande de Santa Marta (Caribbeancoast of Colombia). This large but only 1.5 m deep lagoon issubject to strong seasonal variations of salinity from almostfully marine (April/May) to brackish conditions in October/November.Chlorophyll ranged from 6 to 182 µg L^–1, and grossprimary production amounted to 1690 g C m^–2 per year.Total bacterial number (TBN) ranged from 6.5 to 90.5 x 10⁹ cellsL^–1 and bacterial biomass (BBM) from 77 to 1542 µgC L^–1, which are among the highest ever reported for naturalcoastal waters. Neither TBN nor BBM varied significantly withsalinity, phytoplankton or seston concentrations. Only the bacterialmean cell volume showed a significant relation to salinity,being highest (0.066 µm³) during the period of increasingand lowest (0.032 µm³) during decreasing salinity. Bacterialprotein accounted for 24% (19–26%) and phytoplankton proteinfor 57% (53–71%) of total seston protein. The ratio (annualmean) of bacterial carbon to phytoplankton carbon was 0.44 (range0.04–1.43). At low phytoplankton abundance [chlorophylla (Chl a) < 25 µg L^–1], bacterial carbon wasalmost equal to phytoplankton biomass (i.e. the mean ratio was1.04). In contrast, at Chl a > 100 µg L^–1, BBMwas low compared to phytoplankton biomass (the mean ratio was0.16). In general, BBM varied less than phytoplankton biomass.Most probably, the missing correlation between bacterial andphytoplankton variables was due to (i) organic material partlyderived from allochthonous sources serving as food resourcefor bacteria and (ii) a strong resuspension of bacteria fromthe sediment caused by frequent wind-induced mixing of the veryshallow lagoon.     

The productive and optical status of the oligotrophic waters of the Southern Aegean Sea (Cretan Sea), Eastern Mediterranean

Primary production, pigment concentrations and spectral measurementsof downwelling irradiance were made at four stations in fourseasons (spring, summer, autumn, winter) during 1994 in thewaters of the South Aegean Sea (Cretan Sea), Eastern Mediterranean.Rates of production were determined using in Situ incubationtechniques and included measurements at the surface microlayer.Depth-integrated values averaged over season were 5.66 mg Cm^–2 h^–1 for primary production and the correspondingchlorophyll (Ch1) a and phaeophytin (Phaeo) a values had meansof 4.87 and 1.21 mg m^–3 respectively. The assimilationratio remained very low (mean over season: 1.19 mg C mg^–2Chl a h^–1 as did the Phaeo a/Chl a ratio (mean over season:0.24). The annual production for the area was estimated to yield24.79 g C m^–2 year^–1. Primary production and Chla estimates showed statistically significant seasonal, spatialand depth variations. The spectral values of the attenuationcoefficient Kd (     

Size and species-specific primary productivity and community structure of phytoplankton in Tokyo Bay

Combined methods of size fractionation and single-cell isolationwere used to investigate the seasonal variation of phytoplanktondynamics in Tokyo Bay with an emphasis on primary productivity.Red tides occurred in Tokyo Bay from spring to autumn; a diatom,Skeletonema costatum, and a raphidophycean, Heterosigma akashiwo,were the most important primary producers. Small diatoms andflagellates, including these species, were dominant and showedrapid changes of phytoplankton community structure within severaldays in summer. The nanoplankton (3–20 µm) fractioncontributed most to chlorophyll a concentration and primaryproductivity during spring to autumn, whereas the microplankton(>20 µm) contribution was remarkable in winter. Picoplankton(<3 µm phytoplankton) remained relatively constantthroughout the year. A significant reverse relationship wasobtained between assimilation rate and chlorophyll a contentfor the total and nanoplankton population; the assimilationrate was high at the initial phase of the bloom, then decreasedto a minimum level at the peak of the bloom. Factors controllingthe reduction of assimilation rates at the peak, and changesin phytoplankton community structure, are discussed.     

The contribution of picophytoplankton to community structure in a Mediterranean brackish environment

The seasonal composition of phytoplankton communities was investigatedin a Mediterranean brackish area (Varano lagoon). Twelve stationswere sampled monthly from March 1997 to February 1998. Numbersof prokaryotic and eukaryotic picophytoplankton cells were estimatedby epifluorescence microscopy, while larger phytoplankton (nanoand micro fractions) were enumerated by the Utermöhl settlingtechnique. Picophytoplankton densities ranged from 0.7 to 448.6cells x 10⁶ l^–1. Nano- and microphytoplankton abundancesvaried between 0.2 and 7.9 cells x 10⁶ l^–1. The picoplanktonfraction was represented mainly by cyanobacteria and the Utermöhlfraction by nano-sized phytoflagellates (56.2%) and diatoms(20.1%). The phytoflagellates had a greater abundance over timewhile diatoms reached the highest densities in summer and fall.In Varano lagoon, phytoplankton development is related to ‘nitrogen-poor'waters and to phosphorus availability. Suspension-feeding bivalves(Mytilus galloprovincialis) are sufficiently abundant to filtera volume equivalent to the volume of Varano lagoon at leastonce daily. These observations suggest that grazing exerts animportant influence on phytoplankton dynamics, mainly on themicro fraction, and that diatoms seem to play an important rolein the food web dynamics of this coastal fishery.     

Seasonal and interannual variability of chlorophyll a and primary production in the Equatorial Atlantic: in situ and remote sensing observations

The seasonal variability of phytoplankton in the EquatorialAtlantic was analysed using Sea-viewing Wide Field-of-view Sensor(SeaWiFS)-derived chlorophyll a (Chl a) concentration data from1998 to 2001, together with in situ Chl a and primary productiondata obtained during seven cruises carried out between 1995and 2000. Monthly averaged SeaWiFS Chl a distributions werein agreement with previous observations in the Equatorial Atlantic,showing marked differences between 10° W in the EasternTropical Atlantic (ETRA) and 25° W in the Western TropicalAtlantic (WTRA) provinces (Longhurst et al. 1995. J. PlanktonRes., 17, 1245–1271). The seasonal cycle of SeaWiFS-derivedChl a concentration calculated for 0–10° S, 0–20°W (ETRA) is consistent with in situ Chl a measurements, withvalues ranging from 0.16 mg m^–3, from February to April,to 0.52 mg m^–3 in August. Lower variability was observedin 10° N–10° S, 20–30° W (WTRA) whereminimum and maximum concentrations occurred in April (0.15 mgm^–3) and in August (0.24 mg m^–3), respectively.A significant empirical relationship between depth-integratedprimary production and in situ measured sea surface Chl a wasfound for ETRA, allowing us to estimate the seasonal cycle ofdepth-integrated primary production from SeaWiFS-derived Chla. As for Chl a, this model was verified in a small area ofthe Eastern Equatorial Atlantic (0–10° S, 0–20°W), although in this instance it was not completely able todescribe the magnitude and temporal variability of in situ primaryproduction measurements. The annual euphotic depth-integratedprimary production rate estimated for ETRA by our empiricalmodel was 1.4 Gt C year^–1, which represents 16% of theopen ocean primary production estimated for the whole AtlanticOcean.     

Size-fractionated primary production studies in the vicinity of the Subtropical Front and an adjacent warm-core eddy south of Africa in austral winter

Results are presented of size-fractionated primary productionstudies conducted in the vicinity of the Subtropical Front (STF),an adjacent warm-core eddy, and in Sub-antarctic waters duringthe third South African Antarctic Marine Ecosystem Study (SAAMESIII) in austral winter (June/July) 1993. Throughout the investigation,total chlorophyll (Chl a) biomass and production were dominatedby small nano- and picophytoplankton. No distinct patterns intotal Chl a were evident. At stations (n = 7) occupied in thevicinity of the STF, total integrated biomass values rangedfrom 31 to 53 mg Chl a m^–2. In the vicinity of the eddy,integrated biomass at the eddy edge (n = 3) ranged from 24 to54 mg Chl a m^–2 and from 32 to 43 mg Chl a m^–2 inthe eddy (n = 2). At the station occupied in the Sub-antarcticwaters, total integrated biomass was 43 mg Chl a m^–2.Total daily integrated production was highest at stations occupiedin the vicinity of the STF and at the eddy edge. Here, totalintegrated production ranged from 150 to 423 mg C m^–2day^–1 and from 244 to 326mg C m^–2 day^–1, respectively.In the eddy centre, total integrated production varied between134 and 156 mg C m^–2 day^–1. At the station occupiedin the Sub-antarctic waters, the lowest integrated production(141 mg C m^–2 day^–1) during the entire survey wasrecorded. Availability of macronutrients did not appear to limittotal production. However, the low silicate concentrations duringthe survey may account for the predominance of small nano- andpicophytoplankton. Differences in production rates between theeddy edge and eddy core were related to water column stability.In contrast, at stations occupied in the vicinity of the STF,the control of phytoplankton production appears to be relatedto several processes, including water column stability and,possibly, iron availability.     

Regional, vertical and seasonal distribution of phytoplankton and photosynthetic pigments in Lake Baikal

A 3-year phytoplankton study was carried out in Lake Baikal(Siberia) as part of the CONTINENT project and in conjunctionwith a 60-year long monitoring programme by the Irkutsk StateUniversity. A combination of microscopy and high performanceliquid chromatography (HPLC) pigment analysis was used. Allover the lake, the dominant functional group (by biovolume)was the vernal diatom blooms, due to the dominance of endemicCyclotella species. Chlorophyll a (Chl a) was significantlyhighest at the Selenga and Barguzin inflows (2.39 ± 0.34and 2.49 ± 0.18 nmol L^–1, mean ± 95% CI,respectively) and higher in the South than in the North (1.43± 0.26 and 0.96 ± 0.13 nmol L^–1). This variationof Chl a reflected changes in the phytoplankton composition.Diatoms and Chrysophyceae were the major contributors to thetotal Chl a except in the South (Chlorophyceae) and SelengaDelta (cyanobacterial picoplankton). There were also indicationsof species composition changes due to enhanced P-loading fromthe Selenga River. However, canonical analyses indicated thattemperature and stratification were the major driving forcesfor regional distribution patterns and seasonal succession.It seems likely that further global warming will cause a shiftin the species and group composition towards small cells atthe expense of the large endemic diatom flora.     

Size structures of microplankton biomass and production in Jiaozhou Bay, China

Seasonal investigations of size-fractionated biomass and productionwere carried out from February 1992 to May 1993 in JiaozhouBay, China. Microplankton assemblages were separated into threefractions: pico- (0.7–2 µm), nano- (2–20 µm)and netplankton (20–200 µm). The biomass was measuredas chlorophyll a (Chi a), paniculate organic carbon (POC) andparticipate organic nitrogen (PON). The production was determinedby ¹⁴C and ¹⁵N tracer techniques. The seasonal patterns in biomass,though variable, were characterized by higher values in springand lower values in autumn and summer (for Chi a only). Theseasonal patterns in production, on the other hand, were moreclear with higher values occurring in summer and spring, andlower values occurring in autumn and winter. Averaged over thewhole study period, the respective proportions of total biomassaccounted for by net-, nano- and picoplankton were 26, 45 and29% for Chi a, 32, 33 and 35% for POC, and 26, 32 and 42% forPON. The contributions to total primary production by net-,nano- and picoplankton were 31, 35 and 34%, respectively. Therespective proportions of total NH₄⁺–N uptake accountedfor by net-, nano- and picoplankton were 28, 33 and 39% in thedaytime, and 10, 29 and 61% at night. The respective contributionsto total NO₃^–-N uptake by net-, nano- and picoplanktonwere 37, 40 and 23% in the daytime, and 13, 23 and 64% at night.Some comprehensive ratios, including C/N biomass ratio, Chla/C ratio, C uptake/Chl a ratio, C:N uptake ratio and the f-ratio,were also calculated size separately, and their biological andecological meanings are discussed.     

Phytoplankton seasonal dynamics in a Mediterranean coastal lagoon: emphasis on the picoeukaryote community

The dynamics of the phytoplankton community were investigatedin a marine coastal lagoon (Thau, NW Mediterranean) from February1999 to January 2000. Dilution experiments, chlorophyll a (Chla) size-fractionation and primary production measurements wereconducted monthly. Maximum growth and microzooplankton grazingrates were estimated from Chl a biomass fractions to separatepico- from nano- and microphytoplankton and by flow cytometryto distinguish between picoeukaryotes and picocyanobacteria.In spring, the phytoplankton community was dominated by Chaetocerossp. and Skeletonema costatum, which represented most of biomass(B) and primary production (P). Nano- and microphytoplanktongrowth was controlled by nutrient availability and exceededlosses due to microzooplankton grazing (g). Picoeukaryote andcyanobacteria growth was positively correlated with water temperatureand/or irradiance, reaching maximum values in the summer (2.38and 1.44 day^–1 for picoeukaryotes and cyanobacteria, respectively).Picophytoplankton accounted for 57% of the biomass-specificprimary productivity (P/B). Picophytoplankton was strongly controlledby protist grazers (g = 0.09–1.66 day^–1 for picoeukaryotes,g = 0.25–1.17 day^–1 for cyanobacteria), and microzooplanktonconsumption removed 71% of the daily picoplanktonic growth.Picoeukaryotes, which numerically dominate the picoplanktoncommunity, are an important source of organic carbon for theprotistan community and contribute to the carbon flow to highertrophic levels.     

Horizontal distribution of Thetys vagina Tilesius (Tunicata, Thaliacea) in the Japan Sea during spring 2004

The occurrence of the salp Thetys vagina was observed in theJapan Sea during spring 2004. Catches up to 187 kg wet weight(WW) per 2.18 x 10⁵ m³ (equal to 0.9 g WW m^–3) were collectedwith 10-m diameter surface-water otter trawl nets. The horizontaldistribution indicated that the high biomass was related tothe area with high chlorophyll a (Chl a) concentration, whichwas located around the subarctic front with the warm TsushimaCurrent. Five prey taxa were identified from the gut contentsof individuals from the high Chl a area. The diatom Coscinodiscusspp. (13–55 µm in diameter) dominated numerically.Another significant prey was the large diatom Coscinodiscuswailesii (219–313 µm) that is an indicator of thespring bloom in this area. The mass occurrence of T. vaginathus appears related to phytoplankton availability, though themechanisms remain uncertain.     

RELATING PHYTOPLANKTON DYNAMICS AND PRODUCTION TO SEDIMENT RESUSPENSION IN SOUTHERN LAKE MICHIGAN

Millie, D. F.¹, Fahnenstiel, G. L.², Carrick, H. J.³, Lohrenz, S. E.⁴, & Schofield, O. M. E.^{5 1}USDA-Agricultural Research Service, Sarasota, FL 34236, USA, ²NOAA-Lake Michigan Field Station, Muskegon, MI 49441, USA, ³Institute of Marine Science, University of Southern Mississippi, Stennis Space Center, MS 39529, USA, ⁴Department of Biological Sciences, State University of New York at Buffalo, Buffalo, NY 14060, USA; ⁵Isttitute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road New Brunswick NJ 08901 USA, Sediment resuspension is an annually recurrent feature during spring holomixis in southern Lake Michigan. Relationships between resuspension events and phyt-oplankton biomass, compositional dynamics, and pro-duction were evaluated during 1998 and 1999. Increased water-column light attenuation (K_PAR) and suspended particulate matter (SPM) concentrations corresponded with resuspension events within nearshore regions. However, neither K_PAR nor SPM corresponded with chlorophyll (Chl) a concentrations, indicating no impact of resuspension on instantaneous biomass accumulation. Diatoms and cryptophytes dominated phytoplankton assemblages and together typically comprised greater than 85% of the Chl a. The associations of SPM/K_PAR with diatom Chl a, and the inverse relationship between relative diatom and crypto-phyte Chl a corresponded with the dominance of diatoms and cryptophytes in near- and offshore waters, respectively. Moreover, a spatial variation in species composition occurred during resuspension events; small, centric diatoms exhibiting meroplanktonic life histories and large, pennate diatoms considered benthic in origin were associated with sediment resuspension whereas large, net diatoms and cryptophytes typically comprising phytoplankton of the annual spring bloom and of optically-clear, offshore waters were not. The presence of viable diatom photopigments and the abundance of small centric diatoms within the surficial sediments, established this layer as the source of meroplankton. Integral production was dramatically reduced within sediment-impacted waters; however, nearshore assemblages appeared to have greater photosynthetic capacities than offshore assemblages. Although resuspension dramatically influenced near-shore phytoplankton assemblages, it appeared to have little, if any relationship with the compositional development of the annual spring bloom.