Ammonium excretion and glutamate dehydrogenase activity of zooplankton in Great South Bay, New York

In Great South Bay, nanoplankton, (<20 sµm) accountedfor the largest fraction (56%) of zooplankton glutamate dehydrogenase(GDH) activity over a one year period. Microzooplankton (20–200µm) and macrozooplankton (>200 µm) accountedfor 20% and 24%, respectively. Total zooplankton ammonium regenerationin Great South Bay could account for 74% of the ammonium requirementby phytoplankton in winter, but in summer when phytoplanktondemand was greater, and zooplankton population was low, it suppliedless than 5%. This study suggests that the smallest zooplanktonfraction, less than 20 µm, can be the most important asregards nitrogen regeneration in estuarine environments. MacrozooplanktonGDH activity in Great South Bay ranged from 0.18 mg atoms NH⁺₄-Nm^–3 d^–1 in winter to 3.34 mg atoms NH⁺₄-N m^–3d^–1 in spring. Over an annual period, the averaged GDH/excretionratio was 20.4 3.5 (n = 10), and this ratio agrees well withobservations by other investigators. Observed macrozooplanktonexcretion rates showed a strong correlation with the excretionrates indirectly estimated from GDH activities. The GDH/excretionratio seems to vary depending on the internal physiologicalstates of zooplankton as well as food availability.     

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     

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.     

Implications of zooplankton stoichiometry on distribution of N and P among planktonic size fractions

Pooled samples from the upper 20 m at five stations in the Oslofjord(Norway) were size fractionated and analysed for particulatedry weight, carbon, nitrogen and phosphorus. The nano fraction(0.7–20 µm) dominated in biomass throughout thesampling period. The C:N ratios of the fractions did not differmuch from each other. The C:P ratios of the nano- and microfraction(20–200 µm) were considerably higher than the ratioof the mesofraction (200–2000 µm) throughout thesampling period. High C:P ratios and low phosphate concentrationsabove the pycnocline suggest that the system was P-limited.The stoichiometry of mesozooplankton was more constant thanthe stoichiometry of the other fractions, and the zooplanktonconstituted consistently a higher percentage of the phosphoruspool than of the carbon pool. This suggests that the mesozooplanktoncan act as a sink of nutrients due to its invariable stoichiometry.     

Uptake of dissolved nitrogen by phytoplankton in a eutrophic subtropical lake

Uptake rates for ananonium, nitrate, urea and dinitrogen byphytoplankton in Lake Okeechobee ranged from 0.58 to 1.52 µmol1^–1 h^–1 among four representative stations duringa short-term study period. Ammonium accounted for 53% of theuptake rates, followed by nitrate (19%), urea (16%) and dinitrogen(12%). Half-saturation constants for nitrogen (N) uptake rangedfrom 8.70 µmol 1^–1 for ammonium, 2.07 iimol 1^–1for urea and 2.21 µmol 1^–1 for nitrate at Southstation. This study reveals spatially varying N uptake rates,particularly N fixation, within a large eutrophic lake.     

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.     

Grazing of copepod assemblages in the north-east Atlantic: the importance of the small size fraction

The North Atlantic was the site for the 1989 JGOFS Pilot Study,an international study of ocean fluxes in relation to the carboncycle. In this paper we present preliminary estimates of thegrazing pressure by copepod assemblages at four stations 60,56, 52 and 47°N, along the JGOFS 20°W transect, duringJune–July. Three major size fractions of mesoplanktoniccopepods were considered, small (200–500 µm), medium(500–1000 µm) and large (1000–2000 µm).At each station, copepod composition and abundance were analysedand the gut fluorescence method was used to estimate ingestionrates. The results support the importance of the small sizefraction relative to the other fractions, in terms of numericalabundance and their grazing impact. However, the total grazingpressure of copepods on phytoplankton was relatively minor duringthe period of sampling since the fraction of phytoplankton standingstock and primary production consumed by the copepods was onaverage <1 and 2% respectively. The implications of theseresults as well as the potential sources of bias involved inthese types of measurements and estimations are discussed.     

Diurnal variations in photosynthetic rates: comparisons of ultraphytoplankton with a larger phytoplankton size fraction

We compared the composition and photosynthetic activity of twosize fractions of phytoplankton during a cruise from the Gulfof Maine to the Sargasso Sea in August 1983. At every station,and at every depth, ultraplankton (defined here as cells passingthrough 3 µm pores in filters) made a major contributionto both the standing crop of chlorophyll and the rate of primaryproduction. Ultraphytoplankton assemblages were dominated byphycoerythrin-rich cyanobacteria. Overall, the ultraplanktoncontribution to total primary production was greatest at lowphoton fluxes: (i) at the beginning and end of the photoperiod;(ii) with increasing depth in the euphotic zone; and (iii) whendaily irradiance was low. The composition of ultraphytoplanktonvaried with depth. Surface (2 m) ultraphytoplankton assemblageswere almost exclusively composed of phycoerythrin-rich cyanobacteriawith smaller (0.2–0.8 µm) cyanobacteria predominating.Below the surface mixed-layer, the proportion of larger (0.8–30 µm) to smaller cyanobacteria increased and the eukaryoticcomponent of the ultraphytoplankton often became important.At two Sargasso Sea stations, the greatest numbers of cyanobacteriawere below the mixed layer at the 1% light level, while themaximum numbers of eukaryotic ultraphytoplankters occurred deeperstill, at the 0.5% light level, coincident with the chlorophyllmaximum. At the bottom of the euphotic layer in the SargassoSea. eukaryotes numerically dominated the ultraphytoplanktonand made a major contribution to the chlorophyll maximum.     

The size distribution of phytoplankton and participate organic matter in the Equatorial Atlantic Ocean: importance of ultraseston and consequences

The size fractionation of paniculate matter (<200, <35,<3 and <1 µm) has been measured in the EquatorialAtlantic Ocean at different stations. Chlorophyll a, phaeophytin,particulate carbon, nitrogen and phosphorus have been analysed.Primary production by ¹⁴CO₂ uptake was also measured with prescreeningtechnique. It appears from this study, that the pariculate matter has avery small size: 40–60% of the chlorophyll passed through1 µm Nucleopore filter, and 75–90% of the paniculatecarbon and nitrogen passed through 3 µm Nucleopore filterin offshore waters. From the atomic ratio C/N, C/P and C/chla, and primary productionvalues, the <3 µm fraction would be mainly constitutedby inactive photosynthetic organisms or partides of detritus.The 3–35 µm fraction, in contrast, would be principallyactive phytoplankton.     

Phytoplankton size fractions in a tropical neritic ecosystem near Kingston, Jamaica

Temporal patterns in phytoplankton size fractions were studiedfor >1 year on the shelf south of Kingston, Jamaica. On average,in the East Channel, total chlorophyll was partitioned as 42%netplankton (>20 µm), 30% nanoplankton (2–20µm) and 28% picoplankton (0.2–2 µm); correspondingaverages for size-fractionated primary production were 27, 30and 43%. Netplankton was more variable than the other size fractions,and it predominated at times of high total chlorophyll concentration.When total chlorophyll was low, picoplankton predominated. Regressionmodels using wind and rainfall parameters could explain up to46% of the variation in chlorophyll concentrations. In theseregression models, the variation accounted for was always greatestfor the net- and least for the picoplankton. A suggested mechanismfor meteorological-phytoplankton coupling is that during periodsof easterly winds (90–150°), offshore water (in whichpicoplankton dominates) is advected onto the shelf, either displacingor diluting the nearshore water. At times of weaker westerlywinds (230–290°) there is a persistence of the phytoplanktoncommunity in nutrient-rich shelf waters and net- and nanoplanktonbiomass increases.     

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.     

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).     

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.     

Phytoplanktonic excretion and bacterial reassimilation in an oligomesotrophic lake: molecular weight fractionation

The release of photosynthetically produced compounds by phytoplanktonand their reassimilation by bacteria were studied in the oligomesotrophicLake Pavin (Massif Central of France). Samples were collectedmonthly from April to November 1993 at the central station ofthe lake at depths of 1, 10 and 15 m. Release of dissolved organicproducts and bacterial reassimilation were assayed by the 14Cmethod which permits the quantitative and qualitative studyof phytoplankton exudates. Gel-permeation chromatography wasused to separate phytoplankton exudates into different molecularweight products. Size fractionation was used to measure 14Cfixation in three fractions: 160–1 µm (mainly phytoplankton),1–0.2 µm (mainly bacteria) and <0.2 µm.The average per cent release of photosynthetically produceddissolved organic compounds by chlorophyllian protists is 10%of algal primary production (range 2–45%). Gel-permeationchromatography showed that low-molecular-weight products (<700Da) dominated this excreted organic matter pool and were animportant source of carbon for heterotrophic microorganisms,allowing the establishment of a microbial loop, owing to theheterotrophic bacterial reassimilation of exudates constitutingup to 90% (mean 42 ± 22%) of excreted products.     

Phytoplankton production in subarctic lake and river ecosystems: development of a photosynthesis-temperature-irradiance model

We investigated the size-dependent temperature response of naturalphytoplankton communities from a lake and a river in the Canadiansubarctic. Photosynthesis by total, <2 m and >2 µmsize fractions was determined at 11 irradiances (1–109%of ambient solar radiation) and five temperatures (5–25C)in outdoor solar incubators. Temperature had no effect on photosynthesisat low irradiance, but strongly regulated the photosyntheticresponse at saturating and inhibiting irradiances. For the riverphytoplankton, low temperatures lowered EK values (onset oflight saturation) and shifted photosynthesis in the water columnfrom light dependence to temperature depend ence. A photosynthesis-temperature-irradiance(P-T-E) model was developed to describe the varied temperatureresponse of photosynthesis across the full range of limiting,saturating and inhibiting irra diances. The P-T-E model explained74–95% of the variation in photosynthesis for all sizefractions (total community, >2 µ fraction and <2µm fraction). Picoplankton (<2 µm) had greaterphotosyn thetic rates (P_max) at all temperatures than did thetotaland >2 µm communities. The picoplankton fraction wasalso more responsive to increasing temperature than larger cells,implying a greater sensitivity to diurnal or longer term changesin lake water temperature.     

Effect of Phytochrome and Kinetin on Nitrite Reductase Activity in Zea mays

Red light and kinetin (10 µm) increased nitrite reductase(NIR) activity by 85 and 47% respectively in excised leavesof etiolated Zea mays. The stimulatory effect of kinetin decayedslower than that of red light. Indoleacetic acid (10 µm)had no effect on NIR activity. In the presence of abscisic acid(10 µm), the kinetin stimulated increase in NIR activitywas totally nullified, however, the red light irradiated plantsretained 20–25% increase in NIR activity over the darkcontrol. If ABA was given 2 h after kinetin treatment or redlight irradiation, it totally blocked kinetin stimulation asnoticed earlier, but red light stimulation was inhibited byonly 11%. Kinetin-treated and the red light irradiated leavesshowed 20–25% increase in nitrate accumulation, whichwas totally nullified by ABA. The experiments presented suggestan independent mode of signal transduction by kinetin and phytochromein stimulating NIR activity. (Received December 2, 1986; Accepted February 7, 1987)     

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).     

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.     

A Comparison Between the ATPase and Proton Pumping Activities of Plasma Membranes Isolated from the Stele and Cortex of Zea mays Roots

Plasma membrane vesicles of high purity, determined by markerenzyme assays, were obtained by phase partitioning microsomalfractions from stelar and cortical tissues of Zea mays (cv.LG11) roots. ATP hydrolytic activities in both of the plasmamembrane fractions were inhibited by vanadate, SW26 and erythrosinB, but were insensitive to nitrate. Activity in both fractionsexhibited a marked pH optimum of 6·5 and displayed typicalMichaelis-Menten kinetics. A high substrate specificity wasapparent in both the stele and cortex plasma membrane fractions,while the lower fractions, after phase partitioning, showedlower specificity for nucleotide substrates. Specific activitiesof the stele (67·8 µmol Pi mg^–1 h^–1)and cortex (78·4 µmol Pi mg^–1 h–¹)plasma membrane H⁺ -ATPases were very similar. Proton pumping activities in microsomal membrane fractions fromstele and cortex were inhibited by nitrate and insensitive tovanadate. Homogenization of stele and cortex tissue in the presenceof 250 mol m^–3 KI resulted in microsomal fractions exhibitingvanadate-sensitive, nitrate-insensitive proton pumping activity,suggesting a plasma membrane origin for this activity. SW26was also an effective inhibitor of proton pumping activity,although results indicated an interaction between SW26 and thefluorescent probes quinacrine and acridine orange. The results are discussed in relation to models for the transportof ions into the stele and are consistent with a role for theH⁺ -ATPase activity in this process. Key words: ATPase, cortex, plasma membrane, stele, Zea mays     

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.