Contribution of ciliated protozoa to the planktonic biomass in a series of Ontario lakes: quantitative estimates and dynamical relationships

The plankton of nine Ontario lakes spanning several physiographicregions was sampled every two weeks during the ice-free periodof 1981, and one lake was studied in the three previous years.Phytoplankton, zooplankton, and ciliated protozoa were sampled,counted and sized. The size data were converted to biomass estimatesto yield quantitative comparisons of the relative allocationof biomass among different functional compartments. This isthe first study to look simultaneously and quantitatively atthe total plankton system of lakes (including ciliates, pbytoplanktonand net zooplankton) over a broad physiographic region. Ciliatesconstitute –10% of the non-algal biomass and 5% of thetotal planktonic biomass of these lakes. Ciliate standing cropsamong lakes are significantly corrrelated with total organicand total inorganic carbon concentrations in the water column,while the dynamics of ciliate biomass fluctuations are significantlycorrelated with variations in total phosphorus concentration,in conductivity, in Kjeldahl nitrogen concentration, and ininorganic carbon content. There appears to be a significantdynamical relationship between ciliates as a proportion of thetotal planktonic biomass, exclusive of filamentous and large(>30 µm) spherical algae, and the relative biomassof small algae (2–5 µm) as a fraction of total algalbiomass, again exclusive of filaments and large (>30 µm)algae. The hypothesis is advanced that ciliates primarily functionas bacterial grazers in planktonic ecosystems and that theirprimary competitors in this role are rotifers.     

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.     

Seasonal and interannual variability of size-fractionated phytoplankton biomass and community structure at station Kerfix, off the Kerguelen Islands, Antarctica

Time series of phytoplankton biomass and taxonomic compositionhave been obtained for the 3 years 1992, 1993 and 1994 in thenorthern part of the Southern Ocean (station Kerfix, 5040'S,6825;E) Autotrophic biomass was low throughout the year (<0.2mg m^–3 except during a short period in summer when a maximumof 1.2 mg chlorophyll (Chl) a m– was reached. During winter,the integrated biomass was low (<10 mg m^–2) and associatedwith deeply mixed water, whereas the high summer biomass (>20mg m^–2) was associated with increased water column stability.During summer blooms, the >10 µ;m size fraction contributed60% to total integrated biomass. Large autotrophic dinoflagellates,mainly Prorocentrum spp., were associated with the summer phytoplankton maxima and accounted for >80% of the total autotrophcarbon biomass. In November and December, the presence of thelarge heterotrophic dinoflagellates Protoperidinium spp. andGyro dinium spp. contributed a high proportion of total carbonbiomass. During winter, the <10 µm size fraction contributed80% of total Chi a biomass with domination of the picoplanktonsize fraction. The natural assemblage included mainly nakedflagellates such as species of the Prasinophyceae, Cryptophyceaeand Prymnesiophyceae. During spring, picocyanobacteria occurredin sub-surface water with a maximum abundance in September of106 cells 1^–1     

Size-fractionated phytoplankton carboxylase activities in the Indian sector of the Southern Ocean

During the ANTARES 3 cruise in the Indian sector of the SouthernOcean in October–November 1995, the surface waters ofKerguelen Islands plume, and the surface and deeper waters (30–60m) along a transect on 62°E from 48°36'S to the iceedge (58°50'S), were sampled. The phytoplankton communitywas size-fractionated (2 µm) and cell numbers, chlorophyllbiomass and carbon assimilation, through Rubisco and ß-carboxylaseactivities, were characterized. The highest contribution of<2 µm cells to total biomass and total Rubisco activitywas reported in the waters of the Permanent Open Ocean Zone(POOZ) located between 52°S and 55°S along 62°E.In this zone, the picophytoplankton contributed from 26 to 50%of the total chlorophyll (a + b + c) with an average of 0.09± 0.02 µg Chl l^–1 for <2 µm cells.Picophytoplankton also contributed 36 to 64% of the total Rubiscoactivity, with an average of 0.80 ± 0.30 mg C mg Chla^–1 h^–1 for <2 µm cells. The picophytoplanktoncells had a higher ß-carboxylase activity than largercells >2 µm. The mixotrophic capacity of these smallcells is proposed. From sampling stations of the Kerguelen plume,a relationship was observed between the Rubisco activity perpicophytoplankton cell and apparent cell size, which variedwith the sampled water masses. Moreover, a depth-dependent photoperiodicityof Rubisco activity per cell for <2 µm phytoplanktonwas observed during the day/night cycle in the POOZ. In thenear ice zone, a physiological change in picophytoplankton cellsfavouring phosphoenolpyruvate carboxykinase (PEPCK) activitywas reported. A species succession, or an adaptation to unfavourableenvironmental conditions such as low temperature and/or availableirradiance levels, may have provoked this change. The high contributionof picophytoplankton to the total biomass, and its high CO₂fixation capacity via autotrophy and mixotrophy, emphasize thestrong regeneration of organic materials in the euphotic layerin the Southern Ocean.     

Size-fractionated chlorophyll a biomass and picophytoplankton cell density along a longitudinal axis of a temperate estuary (Southampton Water)

Size-fractionated chlorophyll a biomass and picophytoplanktoncell number distributions were investigated along a longitudinalaxis of Southampton Water estuary during autumn. Chlorophylla concentration in the >5µm and the 1–5 µmsize fractions was highest midway down the estuary, and decreasedboth in the landward and seaward directions. In contrast, chlorophylla biomass in the 0.2–1 µm size fraction showed nodecline towards the seaward end of the estuary. In agreementwith this observation, phycoerythrin-containing picocyanobacteriacell concentration showed a positive exponential-like relationshipwith salinity and eukaryotic picophytoplankton were also highestat high salinities. Expressed as a percentage of total, chlorophylla standing stock in both the 1–5 µ.m (4.4–28.7%)and the 0.2–1 µm size fractions (1.7–8.6%)was inversely correlated with total chlorophyll a concentration.Both these two fractions made a greater input to the total phaeopigmentconcentration than to the total pool of active chlorophyll a.     

Microzooplankton and macrozooplankton glutamate dehydrogenase as indices of the relative contribution of these fractions to ammonium regeneration in the Gulf of Maine

Ammonium regeneration by micro- (35–153 µm) andmacrozooplankton (> 153 µm) was determined in the Gulfof Maine by measuring the activity of the excretory enzyme glutamatedehydrogenase (GDH) in various size fractions. GDH maxima weregenerally observed to correspond to the depth of the chlorophyllmaximum as previously reported in the Gulf of Mexico and inthe vicinity of the Nantucket Shoals. GDH activity of the microzooplanktonwas considerably lower than the macrazooplankton, suggestingthe microzooplankton made only a minor contribution (1–11%) to the total ammonium regenerated. These results were confirmedby biomass estimates made from counts of individual species.Ammonium excretion by both zooplankton fractions was estimatedto supply 5–31 % of the nitrogen requirements for primaryproduction, with an estimated 59–63% supplied by the verticaltransport of nitrate (new nitrogen) into the euphoric zone.     

Annual Zooplankton Succession in Coastal NW Mediterranean Waters: The Importance of the Smaller Size Fractions

Zooplankton abundance, biomass (biovolume) and taxonomic compositionwere studied within an annual cycle (August 1995–October1996) in the Bay of Blanes (northwest Mediterranean). Weeklyzooplankton sampling included oblique tows made with a 200 µmJuday–Bogorov net, and vertical tows made with a 53 µmnet, to adequately sample both mesoplankton and the smallerzooplankton fractions. Total zooplankton abundance showed highvariability, lacking any clear seasonal pattern. However, thedifferent species within the zooplankton community displayeda clear succession throughout the year. In general, cyclopoidcopepods (Oithona spp.) and cladocerans (Peniliaavirostris)dominated the summer and autumn communities, whereas in winterand spring, calanoid copepods (Clausocalanus spp., Paracalanussp. and Centropages typicus) were predominant. The zooplanktonannual cycle in the Bay of Blanes does not resemble those ofother Mediterraneanlittoral areas, probably due to the inherentparticularity and variability associated with open coastal environments.On average, the abundance of organisms estimated with a traditional200 µm Juday–Bogorov net was 8.1 times lower thanthe values obtained with a 53 µm net. Even if only organisms>200 µm collected in the 53 µm tows were considered,the total abundance within the 53 µm net was still 4.4times higher than the estimates from the Juday–Bogorovnet. These results suggest the need for accurate samplings ofthe entire zooplankton assemblage when characterizing the structureand dynamics of zooplanktonic communities.     

El Nio (1992) in the equatorial Pacific: low biomass with a few dominating species in the microphytoplankton

As a part of the US Joint Global Ocean Flux Studies, the microphytoplanktoncell numbers. volumes and biomass from eight stations on a transect(12S–12N) on or near 140W from the cruise of the R/V‘Thomas G.Thompson’ (Cruise TT007) February-March,1992, are integrated with previously reported counts. Althoughthese large cells (>15 µm) were from a diverse population,with many species (81–137) in this size range noted fromeach station, only a few (2–7) species made up 50% ofthe cell abundance of the totals of the diatoms, dinoflagellates,coccolithophorids and other algal cells from discrete watersamples taken in the upper 200 m. Even during the 1992 El Nio,reports indicate that surface nitrate was not depleted nearthe equator, but the low numbers of cells in this size fractionindicate that an unknown factor (other than nitrate or light)limited the growth. This synthetic analysis shows high diversity(Margalef's D > 10.4 at the maxima of each station), andlow cell numbers (1.4.6–3.73 10⁸ cells m^–2) andlow biomass (42.8–97.2 µg C m^–2). The integratednumbers of larger coccolithophorids and diatoms showed somereduction near the equator, but the large reduction noted inthe total phytoplankton from the equator to 2N was largelydue to the dip in dinofiagellate numbers, coupled with a shallowmixed layer. Biomass had much the same latitudinal profile.During these El Nio conditions, this integrated study acrossa total of 24 latitude shows an anomaly of low equatorial phytoplanktonbiomass.     

First report of the cyanobacterium Aphanizomenon ovalisporum Forti in two Greek lakes and cyanotoxin occurrence

Aphanizomenon ovalisporum is reported for the first time inGreece, in two warm, monomictic lakes. Aphanizomenon ovalisporumwas dominant constituting 99 and 58% of the total cyanobacterialbiomass in lakes Lysimachia and Trichonis, respectively. Trichomeswere solitary (length 60–700 µm), were narrowedslightly at the ends, had a few terminal hyaline cells and hadcells containing gas vesicles (length 2.5–6.9, width 2.4–5.1µm). Heterocytes, spherical or ellipsoidal (length 4.4–10.5,width 2.41–5.1 µm) and akinetes (length 16.0–27.8,width 6.0–15.9 µm) were located in the middle ofthe trichome. High performance liquid chromatography (HPLC)analysis detected microcystin–LR (MC–LR) and a putativeanabaenopeptin in the L. Lysimachia sample. The sestonic MC–LRconcentration was 0.9 µg L^–1. The origin of MC–LRin L. Lysimachia is discussed. The other cyanobacteria presentwere Pseudanabaena sp. and Planktothrix mougeotii (1% of thetotal cyanobacterial biomass).     

Why are there no picoplanktonic O2 evolvers with volumes less than 10-19 m3?

Non-scalable components occupy an increasing fraction of thebiomass of cells as their size decreases, thus decreasing thefraction of the biomass available for other scalable, essentialactivities. The non-scalable components include the genome,and membranes such as the plasrnalemma and the outer membraneof cyanobacteria and the plastid envelope membranes of chlorophytes.The predicted influences of the increasing fraction of non-scalablecomponents in small cells (0.5 µm radius spherical cellsrelative to 5 µm radius cells) are threefold. One predictionis a decreased maximum specific growth rate (biomass increaseper unit biomass per unit time) due to a decreased fractionof biomass occupied by scalable catalysts. This in turn givesa lower catalytic activity per unit biomass leading to a lowermaterial or energy conversion rate per unit biomass. A secondprediction is a reduced fraction of the biomass occupied bylight-harvesting material, thus partly or entirely removingthe photon-harvesting advantage of smaller cells and diminishingspecific growth rate at low incident photon flux densities.A third prediction is that the different C:N:P ratio in non-scalablecomponents compared to ‘average’ biomass means avariation in C:N:P ratio at low cell sizes, with a lower C:Nratio in smaller cells and a higher C:P ratio in small cyanobacterialcells, but a lower C:P ratio in smaller chlorophyte cells. Thedifference in C:P ratio predictions is a function of the higherDNA content of chlorophyte than cyanobacterial cells. Comparisonsof these predictions with the observed effects on cell propertiesand behaviour of variations in cell size in the range 0.3–2µm radius for cyanobacteria and 0.5–9 µm radiusfor chlorophytes yields the following results. The general trendfor an increase in maximum specific growth rate with decreasingcell size (a finding with little theoretical explanation) appearsto be reversed as cell size decreases below     

Spatial heterogeneity of zooplankton biomass and size structure in southern Quebec lakes: variation among lakes and within lake among epi-, meta- and hypolimnion strata

Environmental control of zooplankton biomass size structure(53–100, 100–202, 202–500 and >500 µm)was investigated in the three limnetic strata of 25 southernQuébec Shield lakes, Canada. Among-lake differences werethe greatest source of variation of zooplankton biomass, whereasthe strong lake–by–stratum interaction observedindicated that the vertical variations of zooplankton biomassand its size fractions were not constant from lake to lake.The analysis of spatial and local factors based on thermal stratais consistent with conceptual models of predation and nutrientcontrol on the biomass and size structure of the zooplankton.Productivity of the aquatic systems, which was driven by lakedepth, flushing rate and total phosphorus concentration, wasthe primary factor influencing total zooplankton biomass andsize structure at among-lake scale in epilimnetic waters. Theeffects of the planktivorous fish on the large zooplankton biomass(>500 µm) was more clearly perceived when the effectof lake depth was removed by partial redundancy analysis. Thisstudy showed that although bottom-up and top-down forces arecomplementary in structuring of zooplankton communities, theycan also act differently on the community attributes (e.g. biomassand size structure). Among-lake zooplankton biomass is predictablefrom lake trophy, but the size structure and vertical distributionof zooplankton communities appear to be controlled by lake stratificationand by inference to interactions with size selective predationby fish. In metalimnetic waters, the 53–100 and 100–202µm zooplankton biomass fractions were primarily dependenton abiotic factors, while the 202–500 and >500 µmfractions were related to planktivory and picophytoplanktonconcentrations. The well-oxygenated and cold hypolimnetic watersof some lakes offered a refuge from surface turbulence and planktivoryto large zooplankton size fractions (202–500 and >500µm).     

A comparison of performance of WP2 and MOCNESS

Zooplankton biomass in the Barents Sea was monitored during1988–97 using WP2 and MOCNESS plankton nets. These twosampling gears differ in their size and mode of operation. Theplankton samples were size fractionated into three categoriesand the dry weight per square metre was calculated. The smallestand the medium size fractions (< 2000 µm) representedmainly copepods, and the larger size fraction (> 2000 µm)consisted mainly of macrozooplankton such as krill and amphipods.WP2 biomass values were higher for the smallest size fraction,whereas the MOCNESS tended to give higher values for the largestsize fraction However, the total amount of zooplankton biomass(g m^–1) obtained by these two methods was not significantlydifferent.     

Copepod grazing impact on the trophic structure of the microbial assemblage of the San Pedro Channel, California

In August 2002 and March 2003 the trophic structure of the microbialassemblage from the San Pedro Channel, California was studiedfollowing the experimental alteration of the number of copepods.Changes in the abundance/biomass of microorganisms <80 µmduring 3-day incubations were monitored in (i) the absence ofmetazoa >80 µm, (ii) the presence of natural abundancesof metazoa and (iii) the presence of an elevated number of copepods.Prokaryotes and small-sized eukaryotes (<4 µm) dominatedplankton biomass during both experimental months. Diatoms numericallydominated the 10–80 µm plankton in August 2002,but ciliate and heterotrophic dinoflagellate biomass generallyexceeded diatom biomass on both dates. Ingestion of protozooplankton(predominantly ciliates) contributed substantially to copepoddaily carbon rations. The adult copepod assemblage removed 4.6and 36% per day of the microzooplankton standing stocks (10–80µm size fraction) in August and March, respectively. Elevatedcopepod grazing pressure on protozooplankton resulted in increasedbiomass of nanoplankton (<5 µm) presumably via a trophiccascade. Accordingly, the copepod–protozoan trophic linkappears to be a key factor structuring the planktonic microbialassemblage in the San Pedro Channel. This paper is one of six on the subject of the role of zooplanktonpredator–prey interactions in structuring plankton communities.     

Use of the Plastochron Index to Evaluate Effects of Light, Temperature and Nitrogen on Growth of Soya Bean (Glycine max L. Merr.)

The plastochron index (PI) has been compared with leaf growthand biomass accumulation in young soya bean plants of severalcultivars that were grown in controlled environments with differentirradiance levels and durations, temperatures, and nitrogen(N) regimes. Increasing the photoperiod from 10 to 16 h day^–1 increasedthe plastochron rate (PR) and the proportion of axillary growth.Doubling the photosynthetic photon flux density (PPFD) to 1000µmol m^–2S^–1, increased PR and the proportionof roots to total plant weight, but decreased the proportionof stems plus petioles to total. In a series of experiments,the plants were grown in an 8 h photoperiod at constant temperaturesof 17, 20, 26 or 32 °C. As temperature increased, PR increased,but the duration of leaf expansion decreased. Leaves were largestat 20 and progressively smaller at 26, 32 and 17 °C. Biomasswas greatest for a given PI at 20 °C and decreased in theorder of 26, 32, and 17 °C. The proportion of axillary growthalso was greatest at 20 °C. When plants were grown in a15 h photoperiod at temperatures from 17.1 to 26.6 °C, leafsize continued to increase up to the highest temperature. At17 °C, the PR in the 15 h photoperiod (PPFD 390 µmol;m^–2S^–1) was about threefold greater than in 8 h(500 µmol m^–2 S^–1); biomass accumulation perday was about fivefold greater. Increasing N from 3 to 36 mMincreased PR about 10 per cent, altered biomass partitioningamong plant parts, and increased the biomass of the plants.The NO₂ form of N markedly stimulated axillary growth as comparedwith the NH₄⁺ form. Environment or cultivar had little influenceon the duration of leaf expansion in terms of PI. Cultivarsdid not differ consistently in biomass production and allocationin the different environments. Glycine max (L.) Merrill, soybean, soya bean, plastochron index, leaf development, growth analysis, partitioning, light, nitrogen, temperature     

Carbon dynamics in the 'grazing food chain' of a subtropical lake

Studies were conducted over a 13 month period at four pelagicsites in eutrophic Lake Okeechobee, Florida (USA), in orderto quantify carbon (C) uptake rates by size-fractionated phytoplankton,and subsequent transfers of C to zooplankton. This was accomplishedusing laboratory ¹⁴C tracer methods and natural plankton assemblages.The annual biomass of picoplankton (<2 µm), nanoplankton(2–20 µm) and microplankton (<20 µm averaged60, 389 and 100 µg C 1^–1 respectively, while correspondingrates of C uptake averaged 7, 51 and 13 µg C1^–1h^–1. The biomass of microzooplankton (40–200 µm)and macrozooplankton (<200 µm averaged 18 and 60 µgC 1^–1, respectively, while C uptake rates by these herbivoregroups averaged 2 and 3 µg C 1^–1 h^–1. Therewere no strong seasonal patterns in any of the plankton metrics.The ratio of zooplankton to phytoplankton C uptake averaged7% over the course of the study. This low value is typical ofthat observed in eutrophic temperate lakes with small zooplanktonand large inedible phytoplankton, and indicates ineffectiveC transfer in the grazing food chain. On a single occasion,there was a high density (<40 1^–1) of Daphnia lumholrzii,a large-bodied exotic cladoceran. At that time, zooplanktoncommunity C uptake was <20 µg C 1^–1 h^–1and the ratio of zooplankton to phytoplankton C uptake was near30%. If D.lumholrzii proliferates in Lake Okeechobee and theother Florida lakes where it has recently been observed, itmay substantially alter planktonic C dynamics.     

Nitrogen uptake by size-fractionated plankton in permanently well-mixed temperate coastal waters

Nitrogen uptake by net- (15–200 µm), nano- (1–15µm) and picoplankton (<1 µm) was measured overseasonal cycles at two stations with different patterns of biologicaland chemical cycles in the Morlaix Bay (western English Channel).Though assimilable dissolved N nutrient pool at both stationswas nitrate-dominated, characteristics of biomass and N uptakeby netplankton differed from conventional patterns in two respects.In the first, biomass (26–30%) and N uptake (36–43%)were less important than those of nanoplankton. In the second,the netplankton did not show any marked preference for nitrateover ammonium (nitrate to ammonium uptake ratios of 0.98 and1.08). In contrast, nanoplankton had a preference for ammoniumover nitrate (ammonium to nitrate uptake ratios of 2 and 1.2).N uptake by picoplankton was only 8% of total N uptake at bothstations and was supported mainly by regenerated N (66% ammoniumand 17% urea), with nitrate uptake detectable in only one instanceand nitrite uptake in none. Substrate-dependent uptake of ammoniumin all fractions and a higher ammonium uptake in the nanoplanktonfraction in summer at both stations when ambient ammonium concentrationswere high indicated that while nitrate may satisfy a part ofN requirements, availability of ammonium and its flux throughnanoplankton determine the magnitude of total N uptake in thesewaters. Most of the N uptake in picoplankton appears to be autotrophic,suggesting that a substantial part of heterotrophic uptake,if any, could be localized in the fractions >1 µm,and mediated by free-living and particle-bound bacteria.     

Winter-spring variability of size-fractioned autotrophic biomass in Concepcion Bay, Chile

The temporal variability of size-fractioned autotrophic biomassat three depth levels (1, 8 and 25 m) was studied during thewinter-spring transition at two oceanographic stations in ConcepciónBay. Size spectra were obtained on eight occasions by two differentmethods: (i) determining the biomass of seven autotrophic sizefractions by in vivo fluorescence; and (ii) measuring the filamentlength of chain-forming diatoms through direct microscopy. Aclear vertical gradient of biomass was found in all profiles,with maximum values in the surface layer (1 and 8 m levels).Values of chlorophyll were on average 6.2 (range 1.08–25.67)times higher at 1 m than at 25 m, and 7.4 (range 1.15–26.83)times more at 8 m than at 25 m. On a temporal basis, total biomassincreased from low average values in winter (2.5 mg chl-a m^–3)to high values in late spring (11.6 mg chl-a m^–3). Duringthe whole sampling period (June 8-November 19), the nano- andnet-plankton (1.8–40 µm and 40–335 µmsize fractions respectively) were more abundant near the surface(1 and 8 m depth) than close to the bottom (25 m depth); however,the picoplankton fraction (<1.8 (µm) showed an inverserelationship, with a slight trend to increase near the bottomtoward spring. The highest absolute biomass was concentratedin the net-plankton fraction during the whole period and therelative importance of the picoplankton decreased from winter(6.50 and 15.5% for shallow and bottom levels) to spring (1.5and 10.3% for shallow and bottom levels). This relative effectis caused by the higher absolute values of biomass observedin the net-plankton fraction toward spring. These changing patternsshould have an impact in the size-composition and abundanceof higher trophic levels, mainly through grazing, in particularby modifying food availability to microfJagellates, ciliatesand filter-feeding zooplankton.     

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.     

Zooplankton biomass gradient off south-western Nova Scotia: nearshore ctenophore predation or hydrographic separation?

A persistent large-scale cross-shelf gradient in zooplanktonbiomass >1050 µm was evident off south-western NovaScotia during annual spring surveys between 1985 and 1987, withrelatively low levels inshore and higher levels offshore. Conversely,the abundance of the tentaculate ctenophore Pleurobrachia pileuswas the greatest inshore, and distributed reciprocally to zooplankton>1050 µm. The principle prey of both adult ctenophoresand post-larval cod is zooplankton >1050 µm (primarilycalanoid copepods), and cod growth rates are strongly influencedby prey biomass. Ctenophore predation appears to have been responsiblefor the low nearshore zooplankton biomass, whereas the influenceof hydrographic factors on the zooplankton gradient was minimal.On a smaller scale, persistent, abrupt changes in zooplanktonbiomass >1050 µm and ctenophore density existed 3–30km from shore, in contrast to linear gradients in water density(₁) during a 5 week sampling period in spring 1987. Ctenophoreswere confined to depths <55 m and zooplankton >1050 µmpredominantly occurred at depths >55 m. High concentrationsof chlorophyll and phaeopigment were evident at depths <55m also suggesting intense predation by ctenophores on largeherbivores. The relatively high proportion of smaller zooplankton(153–308 –m) in the nearshore is also consistentwith the predation hypothesis. The reduced growth experiencedby post-larval cod inshore appears generated by ctenophore predationof a common prey resource.     

Nitrogen dynamics in lower Narragansett Bay, Rhode Island. I. Uptake by size-fractionated phytoplankton populations

The contribution of nanoplankton (< 10 µm fraction)to winter – spring (1977 – 78) and summer (1978,1979) phytoplankton nitrogen dynamics in lower NarragansettBay was estimated from ammonium, nitrate and urea uptake ratesmeasured by ¹⁵N tracer methods. During the winter – spring,an average of 80% of chlorophyll a and nitrogen uptake was associatedwith phytoplankton retained by a 10 µm screen. In contrast,means of 51 – 58% of the summer chlorophyll a standingcrops and 64 – 70% of nitrogen uptake were associatedwith cells passing a 10 µm screen. Specific uptake ratesof winter – spring nanoplankton populations were consistentlylower than those of the total population. Specific uptake ratesof fractionated and unfractionated summer populations were notsignificantly different. Ammonium uptake averaged between 50and 67% of the total nitrogen uptake for both the total populationand the < 10µm fraction. The total population and the10 µm fraction displayed similar preferences for individualnitrogen species. Though composed of smaller cells, flagellatedominated nanoplankton assemblages may not necessarily takeup nitrogen at faster rates than diatom dominated assemblagesof larger phytoplankters in natural populations. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia