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.     

Response of Sargasso Sea phytoplankton biomass, growth rates and primary production to seasonally varying physical forcing

The response of phytoplankton biomass, growth rates and primaryproduction to seasonally varying physical forcing was studiedat a station southeast of Bermuda over an 18 month period. Phytoplanktongrowth rates and primary production were measured using thepigment-labeling method, and phytoplankton biomass was calculatedfrom these measurements. Phytoplankton carbon biomass variedsystematically over the year. Highest values were observed duringthe winter and spring. Seasonal variations of chlorophyll (Chi)a in the surface layer could primarily be attributed to variationsin phytoplankton biomass and secondarily to photoacclimation.During the summer period, average values of carbon (C)/Chl ratios(g C g^–1 Chi) ranged from 160 at the surface to 33 atthe 1.6% light level, changes attributed to photoacclimationof the phytoplankton, consistent with the observation that phytoplanktonbiomass did not vary as a function of depth. Phytoplankton growthrates in the surface layer did not vary systematically overthe year, ranging from 0.15 to 0.45 day^–1, in spite ofseasonally varying concentrations of nitrate. Growth rates variedas a function of depth from average values of 0.3 day^–1in the surface layer to <0.1 day¹ at the 1.6% light level.Thus, the primary response of the phytoplankton community tonutrient enrichment during the winter period was an increasein phytoplankton biomass rather than an increase in growth rates.A simple nutrient-phyto-plankton-zooplankton model was usedto explore this phenomenon. The model demonstrated that theobserved response of the phytoplankton to nutrient enrichmentis only possible when phytoplankton growth is not severely limitedby nutrients.     

Plankton in the River Rhine: structural and functional changes observed during downstream transport

The growth dynamics of phytoplankton, zooplankton and bacterioplanktonin the River Rhine were analysed simultaneously with a numberof environmental factors in order to identify environmentalsteering factors and to describe some of their interrelations.Observations on the metabolic activity (for algae and bacteria)and density (for all organisms) were carried Out three timesin 1990 using successive sampling of the same water parcel duringits transport in the lower 660 km reach of the river. High algaldensities (up to 170.5 µg chlorophyll a l^–1), rotifers(up to 1728 l^–1), crustaceans (up to 65 l^–1) andbacteria (up to 16x10⁹ l^–1) were found. Algae and rotifersshowed a rapid successive development during transport, whilecrustaceans were only abundant in the tidal reach of the river.In May, a vigorous growth of phytoplankton, zooplankton andbacteria was found. The diatom-dominated phytoplankton depletedthe dissolved silicate in the river water and this led to acollapse of the populations, indicated by a decreased specificrate of photosynthesis (measured by the ¹⁴C method) and vigorousbacterial growth (measured by [³H]thymidine incorporation).Subsequently, the remaining phytoplankton diminished to verylow levels near the river mouth. In July and September, it seemedthat biological interactions within the plankton populationsor between plankton and benthos balance the population densitiesso that separate developmental stages, as in spring, were lessprominent. Estimates of the growth rates and loss rates of thephytoplankton were made. Phytoplankton exerted a substantialinfluence on the partitioning of nutrients (nitrogen, phosphorus,silicate) over water and suspended matter (as analysed by elementanalysis). It seems likely that only the reduction of phosphate,as planned under the Rhine Action Programme, and not that ofnitrogen, may restrict the peaks of plankton growth describedhere.     

Photosynthetic algal production, accumulation and release of phytoplankton storage carbohydrates and bacterial production in a gradient in daily nutrient supply

In a mesocosm experiment providing a gradient of semi-continuousaddition of mineral nutrient, production rates and mortalityof phytoplankton were estimated. Heterotrophic bacterial biomassand production rates and their responses to the mineral nutrientsadditions were also estimated. The purpose of the experimentwas to establish responses of the major biological factors asa function of nutrient amendments. Initial primary productionwas 0.47 µg C L^–1 day^–1. In the most fertilizedmesocosm, phytoplankton biomass increased at a specific rateof 0.4 day^–1 during the first week of the experiment,and on day 9 primary production reached a peak at 1027 µgC L^–1 day^–1. The responses in the other fertilizedmesocosms were intermediate, and in an unfertilized controlthe variables measured stayed almost constant throughout theexperiment. The termination of the blooms in the fertilizedmesocosms was a consequence of nitrogen limitation, and nitrogenlimitation subsequently induced storage of intracellular organicmaterial in the phytoplankton. In the mesocosm receiving thehighest daily dose of nutrients, strong post-bloom nutrientlimitation resulted in high phytoplankton mortality, and releaseof organic material from the algae supported the gradient’shighest heterotrophic bacterial production.     

Biomass, feeding and production of Noctiluca scintillans in the Seto Inland Sea, Japan

The abundance and biomass of the large heterotrophic dinoflagellateNoctiluca scintillans, together with the changes in its potentialprey items, were monitored in the Seto Inland Sea, Japan, duringsummer 1997 (17 July-11 August). Growth and grazing rates ofNscintillans fed natural plankton populations were also measuredeight and seven times, respectively, during the survey period.The abundance and biomass of N scintillans averaged over thewater column (19 m) were in the range 1–345 cells 1^–1(temporalaverage = 93 cell1^–1) and 0.1–49.6 µg C l^–1(temporalaverage = 13.8 µg C l^–1; three times higher thanthat of calanoid copepods during the same period). Noctilucascintillans populations followed the changes in phytoplankton:N.scintillans biomass was increasing during the period of diatomblooms and was at a plateau or decreasing during periods oflow chlorophyll a. The growth rates of N.scintillans (µ)were also consistent with the wax and wane of the N.scintillanspopulation: N.scintillans showed highest growth rates duringdiatom blooms. A simple relationship between µ and chlorophylla concentration was established, and the production of N.scintillanswas estimated using this relationship and the measured biomass.The estimated production averaged over the water column wasin the range >0.1–5.2 µg C l^–1 day^–1(temporalaverage = 1.4 µg C l^–1 day^–1; 64% of the productionof calanoid copepods during the same period). Diatom clearancerates by N.scintillans were in the range 0.10–0.35 mlcell^–1 day^–1, and the phytoplankton population clearanceby N.scintillans was >12% day^–1. Thus, although thefeeding pressure of N.scintillans on phytoplankton standingstock was low, N.scintillans was an important member of themesozooplank-ton in terms of biomass and production in the SetoInland Sea during summer.     

Pigment-specific rates of phytoplankton growth and microzooplankton grazing in a subtropical lagoon

Phytoplankton growth and microzooplankton grazing rates wereevaluated in one station in Bahía Concepción,located in the middle region of the Gulf of California, México.We used high-performance liquid chromatography (HPLC) estimationsof phytoplankton pigment signatures to evaluate the annual variationof taxon-specific grazing and growth rates obtained with thedilution technique. Chlorophyll-a (Chl-a) concentrations variedwidely (0.34–3.32 µg L^–1) and showed two maxima,during late spring and autumn, associated with the transitionbetween mixed and stratified conditions. Phytoplankton growthrates varied seasonally with the lowest rates during summer(range: 0.01–2.55 day^–1 for Chl-a; 0.00–3.84day^–1 for Chl-b; 0.26–3.29 day^–1 for fucoxanthin;0.00–6.27 day^–1 for peridinin; 0.00–4.35 day^–1for zeaxanthin). Microzooplankton grazing was an important lossprocess (range: 0.0–1.89 day^–1 for Chl-a; 0.00–3.12day^–1 for Chl-b; 0.26–3.29 day^–1 for fucoxanthin;0.00–2.03 day^–1 for peridinin; 0.00–3.51 day^–1for zeaxanthin). Average grazing rates accounted 68–89%of estimated average phytoplankton pigment-specific growth rates.The analysis of pigment signatures indicates that diatoms anddinoflagellates were the dominant groups, and contrary to expectationfor typical subtropical lagoons, the specific growth rates inBahía Concepción showed a pronounced seasonalvariability, linked to transitional hydrographic conditions.Our results indicate a close coupling between the communitymicrozooplankton grazing and phytoplankton growth rates, withoutselective feeding behavior. These results suggest that microzooplanktonplay a critical role and may significantly modify the availabilityand efficiency of transfer of energy to higher trophic levels.     

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

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

Phytoplankton and nutrients in the waters north-west of Spitsbergen in the autumn of 1979

Phytoplankton biomass, primary production rates and inorganicnutrients were measured in the uppermost layer of the ice-edgeregion and in open water and compared with environmental factorsduring a three-week cruise in September – October 1979.Biomass and production values were low (maximum 2.2 µgchl a l^–1, 2.5 mg C m^–3 h^–1). A post-bloomcommunity of diatoms, consisting mainly of representatives ofChaetoceros, Leptocylindrus, Nitzschia and Thalassiosira, waspredominant. Concentrations of phosphate were quite low (maximum0.55 µM I^–1). Nitrate and silicate ranged from nomeasurable quantities to 5.7 µM l^–1 and 3.8 µMl^–1, respectively. The possibility of light and nutrientlimitation on phytoplankton growth is discussed.     

Copepod egg production, moulting and growth rates, and secondary production, in the Skagerrak in August 1988

Measurements of hydrography, chlorophyll, moulting rates ofjuvenile copepods and egg production rates of adult female copepodswere made at eight stations along a transect across the Skagerrak.The goals of the study were to determine (i) if there were correlationsbetween spatial variations in hydrography, phytoplankton andcopepod production rates, (ii) if copepod egg production rateswere correlated with juvenile growth rates, and (iii) if therewas evidence of food-niche separation among co-occumng femalecopepods The 200 km wide Skagerrak had a stratified water columnin the center and a mixed water column along the margins. Suchspatial variations should lead to a dominance of small phytoplanktoncells in the center and large cells along the margins; however,during our study blooms of Gyrodinium aureolum and Ceratium(three species) masked any locally driven differences in cellsize: 50% of chla was >11 µm, 5% in the 11–50µm fraction and 45% <50 µm. averaged for allstations. Chlorophyll ranged from 0.2 to 2.5 µg l^–1at most depths and stations. Specific growth rates of copepodsaveraged 0.10 day^–1 for adult females and 0.27 day^–1for juveniles The latter is similar to maximum rates known fromlaboratory studies, thus were probably not food-limited. Eggproduction rates were food-limited with the degree of limitationvarying among species: 75% of maximum for Centropages typicus, 50% for Calanus finmarchicus, 30% for Paracalanus parvus and 15% for Acartia longiremis and Temora longicornis. Thedegree of limitation was unrelated to female body size suggestingfood-niche separation among adults. Copepod production, summedover all species, ranged from 3 to 8 mg carbon m^–3day^–1and averaged 4.6 mg carbon m^–1 day^–1. Egg productionaccounted for 25% of the total.     

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.     

Phytoplankton community structure derived from HPLC analysis of pigments in the Faroe-Shetland Channel during summer 1999: the distribution of taxonomic groups in relation to physical/chemical conditions in the photic zone

A study of the phytoplankton community in the Faroe-ShetlandChannel was conducted in July 1999. Samples were collected atvarious depths in the photic zone along three transects (thenorthern entrance, the center and the southern entrance). Exceptfor a few easterly stations where nitrate and silicate werebelow 1 µM, all nutrients (phosphate, silicate, ammonium,nitrite and nitrate) were non-limiting for phytoplankton growth.HPLC pigment analysis revealed a pronounced (>50%) dominanceof Prymnesiophyceae at all stations. Their pigment ratio ofdiatoxanthin + diadinoxanthin/Chl a (DDX/Chl a) indicated thatthe phytoplankton community was controlled by light. Primaryproduction in the delayed spring bloom varied from 1.2 to 1.8g C m^–2 day^–1 along the northern transect. Alongthe other two transects, primary production ranged from 1.6to 3.8 g C m^–2 day^–1. Associated with the characteristicsindicating the establishment of a bloom, the relative contributionof diatoms and Prymnesiophyceae increased, whereas that of Prasinophyceae,Cryptophyceae, Chrysophyceae and Cyanobacteriaceae decreased.With respect to their vertical distribution, Cyanobacteriaceae,Chrysophyceae and Dinophyceae tended to have a higher abundance,relative to other taxonomic groups, in the surface layers. Therelative abundance of diatoms and Chlorophyceae increased withdepth. The DDX/Chl a ratio of the Prymnesiophyceae decreasedwith depth, indicating that vertical mixing in the upper 30m of the photic zone occurred less frequently than the timespan of physiological acclimation of cellular pigment composition.     

Copepod abundance in the western Irish Sea: relationship to physical regime, phytoplankton production and standing stock

The distinct patterns of stratification in the North Channeland stratified region of the western Irish Sea influence theseasonal abundance of phytoplankton. The 3–4 month productionseason in the stratified region was characterized by productionand biomass peaks in the spring (up to 2378 mg C m² day^–1and 178.4 mg chlorophyll m^–2) and autumn (up to 1280 mgC m^–2 day^–1 and 101.9 mg chlorophyll m^–2).Phytoplankton in the North Channel exhibited a short, late productionseason with a single summer (June/July) peak in production (4483mg Cm^–2 day^–1) and biomass (–160.6 mg chlorophyllm^–2). These differences have little influence on copepoddynamics. Both regions supported recurrent annual cycles ofcopepod abundance with similar seasonal maxima (182.8–241.810³ind. m^–2) and dominant species (Pseudocalanus elongatusand Acartia clausi). Specific rates of population increase inthe spring were 0.071 and 0.048 day¹ for the North Channel andstratified region, respectively. Increased copepod abundancein the stratified region coincided with the spring bloom, andwas significantly correlated with chlorophyll standing stock.Increased copepod abundance preceded the summer production peakin the North Channel. This increase was not correlated withchlorophyll standing crop, suggesting that a food resource otherthan phytoplankton may be responsible for the onset of copepodproduction prior to the spring bloom. Hetero-trophic microplanktonas an alternative food source, and advection of copepods fromthe stratified region, are proposed as possible explanationsfor copepod abundance increasing in advance of the summer peakin primary production.     

Trophic links in the lowland River Meuse (Belgium): assessing the role of bacteria and protozoans in planktonic food webs

Trophic interactions within the plankton of the lowland RiverMeuse (Belgium) were measured in spring and summer 2001. Consumptionof bacteria by protozoa was measured by monitoring the disappearanceof ³H-thymidine-labelled bacteria. Metazooplankton bacterivorywas assessed using 0.5-µm fluorescent microparticles (FMPs),and predation of metazooplankton on ciliates was measured usingnatural ciliate assemblages labelled with FMPs as tracer food.Grazing of metazooplankton on flagellates was determined throughin situ incubations with manipulated metazooplankton densities.Protozooplankton bacterivory varied between 6.08 and 53.90 mgC m^–3 day^–1 (i.e. from 0.12 to 0.86 g C^–1bacteria g C^–1 protozoa day^–1). Metazooplankton,essentially rotifers, grazing on bacteria was negligible comparedwith grazing by protozoa (1000 times lower). Predation of rotiferson heterotrophic flagellates (HFs) was generally low (on average1.77 mg C m^–3 day^–1, i.e. 0.084 g C^–1 flagellatesg C^–1 rotifers day^–1), the higher contribution ofHF in the diet of rotifers being observed when Keratella cochleariswas the dominant metazooplankter. Predation of rotifers on ciliateswas low in spring samples (0.56 mg C m^–3 day^–1,i.e. 0.014 g C^–1 ciliates g C^–1 rotifers day^–1)in contrast to measurements performed in July (8.72 mg C m^–3day^–1, i.e. 0.242 g C^–1 ciliates g C^–1 rotifersday^–1). The proportion of protozoa in the diet of rotiferswas low compared with that of phytoplankton (<30% of totalcarbon ingestion) except when phytoplankton biomass decreasedbelow the incipient limiting level (ILL) of the main metazooplantonicspecies. In such conditions, protozoa (mainly ciliates) constituted50% of total rotifer diet. These results give evidence thatmicrobial organisms play a significant role within the planktonicfood web of a eutrophic lowland river, ciliates providing analternative food for metazooplankton when phytoplankton becomesscarce.     

Dynamics of inorganic phosphorus in pelagic communities of the Sea of Okhotsk

Inorganic phosphorus uptake and regeneration in the OkhotskSea waters were investigated in July–August 1994 withthe use of radioisotopic techniques. The rates of PO₄-P uptakeby microplankton in the upper mixed layer were between 1.5 and6.6 µg P l^-1 day^-1 (average 2.75) in areas of diatom dominance,and between 0.68 and 1.68 µg P l^-1 day^-1 (average 1.16)in areas of intense warming and summer phytoplankton minimum.The residence time of PO₄-P standing stock in water at differentstations varied between 1.5 and 24 days (mean 9 days). The shareof bacterioplankton contributing to total PO₄-P uptake was 50%in areas of the summer phytoplankton minimum and 20–30%in areas of diatom dominance. The PO₄-P regeneration rate wasmeasured first time experimentally in the temperate sea. Itsrates varied from 0.30 to 1.65 µg P l^-1 day^-1. In areasof diatom dominance, it compensated with 30–60% of PO₄-Puptake. In zones of summer phytoplankton minimum and in thelayers of deep chlorophyll maxima at 10–25 m depths, thePO₄-P regeneration rate often exceeded its uptake. Primary phytoplanktonproduction correlated well with PO₄-P uptake values in the uppermixed layer, while no correlation was found between primaryproduction and the ambient PO₄-P content in water.     

Sinking rates of heterogeneous, temperate phytoplankton populations

Throughout the summer of 1978, the sinking rates of phytoplanktonwithin the Controlled Experimental Ecosystems (CEE's) were monitoredusing a technique based upon measurement of the transit timeof radioactively (¹⁴C) labeled cells. The experimental frameworkof FOODWEB 1 offered an unprecedented opportunity to documentthe sinking rates of heterogeneous phytoplankton of diversetaxonomic composition, growing under a variety of nutrient regimes;the absence of advective exchange in the CEE's provided knowledgeof the preconditioning history of the phytoplankton sampledat any given time. Sinking rates of whole phytoplankton assemblages (not size-fractioned)ranged from 0.32 – 1.69 m·day^–1; the averagerate (± s.d.) observed was 0.64 ± 0.31 m·day^–1.The most notable deviations from the mean value occurred whenthe population size distribution and taxonomic composition shifteddue to blooms. The relationship between phytoplankton sinkingand ambient nutrient levels was studied by following the ratesof a given size fraction (8–53 µm) for ten daysfollowing nutrient enrichment of a CEE. Over this time sinkingrates ranged from 1.08– 1.53 m·day^–1; decreasedrates occurred after nutrification, yet over the course of theentire trial sinking rates were not significantly (p >0.05)correlated to the ambient levels of any single nutrient species. The sinking rate implications of spore formation were also studied,and showed that sinking rates of Chaetoceros constrictus andC. socialis spores (2.75 ± 0.61 m·day^–1)were ca 5-fold greater than rates measured when the vegetativestages of these species dominated the population, reflectingthe influence of physiological mechanisms in controlling phytoplanktonbuoyancy. An example of the potential influence of colony formation uponbuoyancy was noted in observations of C. socialis which occasionallywas found to exist in large spherical configurations made ofcoiled cell chains and having low (0.40 m·day^–1)sinking rates. A hydrodynamic rationale is presented to showhow such a colony together with enveloped water may behave asa unit particle having lower excess density, and therefore lowobserved sinking rate, despite its conspicuously large size. Overall, sinking rates were not significantly correlated withany of the measured nutrient or photic parameters. There were,however, trials and comparisons which showed evidence for: (1)higher sinking rates in populations dominated by large cells,(2) decreased sinking rates after nutrient enrichment, and (3)buoyancy response to light levels. It is suggested that correlationof sinking rates with synoptic environmental measurements atany given time is not explicit because the associations mayinvoke lag times of physiological response. The interpretationmade from these findings is that the preconditioning historyof the population, rather than the prevailing conditions atthe time of a given measurement, is most closely associatedwith population buoyancy modifications.     

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.     

Productivity of Daphnia longispina in a highly humic boreal lake

The development of the Daphnia longispina (O. F. Müller)population in a highly humic boreal lake was followed throughoutone growing season, and the amount of secondary production wasestimated in relation to primary production and available foodresources. The growth rate method was applied in the secondaryproduction measurements. Daphnia longispina did not appear inthe water column until 16 May, after which the animals werepresent throughout the growing season. The population showedthree density peaks; the first appeared in early June, and thesecond and third in mid-July and at the beginning of September,respectively. Somatic production followed a seasonal pattern,with highest production rates in midsummer. The maximum valueof 127 mg C m^–2day^–1 was recorded at the beginningof July. The total annual net production of D. longispina was7.9 g C m^–2. During most of the growing season, the primaryproductivity in the lake was well below 100 mg C m^–2 day^–1and the total annual productivity of photosynthetic algae was5.0 g C m^–2. We conclude that in this lake the zooplanktonpopulation did not rely on phytoplankton primary productionas a sole carbon source, but that most of the carbon must haveoriginated from bacterial production either directly or througha microbial loop.     

Growth and ingestion rates of larval fish populations in the coastal waters of Israel

Clupeoid larvae were collected on eight cruises between February1984 and February 1985 in the coastal waters of Israel. Fromanalysis of daily growth increments of otoliths, growth ratesof the abundant clupeoids, Engraulis encrasicolus, Sardina pilchardusand Sardinella aurita were found to be 0.55 mm day^–1,0.67 mm day^–1 and 0.60 mm day^–1, respectively, duringthe first month after hatching. Ingestion rates were estimatedusing an equation from the literature relating ingestion andgrowth of larval fish. Ingestion calculated for populationsof fish larvae in pelagic waters ranged from 0 to >23 mgC m^–2 day^–1 with maximum rates observed in April.Annual ingestion by larval fish at a pelagic station near Haifawas calculated to be 2.2 g C m^–2 year^–1, 10–20%of annual primary production estimated from ¹⁴C uptake.     

Basin-scale latitudinal patterns of copepod grazing in the Atlantic Ocean

Size-fractionated copepod abundance and ingestion rates wereinvestigated along a 50°S–50°Nlatitudinal transect,during the Atlantic Meridional Transect (AMT) 4, 5 and 6 cruises(boreal spring–autumn 1997, boreal spring–summer1998). Copepod abundance was higher at high latitudes in spring,near northwest Africa, in the equatorial and Benguela upwellingsystems, and in the Subtropical Convergence, and lower in oligotrophicgyres. Gut contents were not related to phytoplankton biomassor production. Gut evacuation rate averaged 0.03 min^-1, andwas not related to latitude or body size. Conservative estimatesof copepod community total ingestion rates ranged between 3.4and 173 mg C m^-2 day^-1 for AMT4, 1.6–252 mg C m^-2 day^-1in AMT5 and 10–160 mg C m^-2 day^-1 in AMT6. Maximum valueswere always in the upwelling regions, the subtropical convergenceand high latitudes in the Northern Hemisphere during borealspring. Calculated ingestion rates translate into average dailyminimal consumption values of 2.07%, 1.89% and 2.6% of totalchlorophyll stock, or 8.02%, 14.5% and 12.9% of total primaryproduction ingested daily on AMT4, 5 and 6 respectively. Grazingimpact increases considerably if we consider ingestion of phytoplanktonlarger than 2 µm, especially under the influence of theEquatorial and North African upwellings, where copepod ingestionrepresents up to 30% of the biomass and >100% of productionby large cells.     

Noctiluca scintillans MACARTNEY in the Northern Adriatic Sea: long-term dynamics, relationships with temperature and eutrophication, and role in the food web

The first ‘bloom’ of Noctiluca scintillans in theNorthern Adriatic Sea was recorded in 1977. The organism causedseveral red tides in the whole basin during the late 1970s,a period characterized by increasing nutrient loads. Duringthe 1980s and early 1990s, there was no ‘red tide’,but the species was an almost constant summer presence, associatedwith high temperatures. Noctiluca scintillans was almost completelyabsent from 1994 until May 1997, concurrent with a general planktondecrease. From summer 1997, N. scintillans was recorded againin the whole basin, although there was no other signal of increasingeutrophication. In contrast to all previous observations, duringwinter 2002–2003, N. scintillans was continuously sampledin the Gulf of Trieste. We estimated experimentally growth andgrazing rates of the dinoflagellate at 9–10°C in cultureand consuming the natural assemblage. Noctiluca scintillanswas able to reproduce actively at low temperatures, showingsimilar growth rates in both experiments (k = 0.2 day^–1).The values found were close to those reported in the literaturefor higher temperatures. The natural diet was mainly composedof phytoplankton (ingestion = 0.008 µg C Noctiluca ^–1day^–1), microzooplankton (ingestion = 0.008 µg CNoctiluca ^–1 day^–1) and bacteria (ingestion = 0.005µg C Noctiluca ^–1 day^–1) with an average carboncontent of 0.138 ± 0.020 µg C Noctiluca cell^–1.