Species-specific differences in phytoplankton responses to N and P enrichments and the N:P ratio in the Archipelago Sea, northern Baltic Sea

A nutrient enrichment experiment was conducted in order to studythe role of nitrogen (N), phosphorus (P) and the N:P ratio onthe early summer phytoplankton community in the ArchipelagoSea, northern Baltic Sea. The phytoplankton community was, interms of chlorophyll a and total biomass, primarily N-limited,but the individual species varied in their responses to thenutrient supply. The recorded overall N limitation was due tofast growth responses of a few N-limited species such as thediatom Chaetoceros wighamii (Brightwell) and the mixotrophicchrysophyte Uroglena sp. Another dominating diatom, Skeletonemacostatum (Greville) Cleve was most clearly P-limited. The N:Pratio had the strongest effect on Uroglena sp., which grew exponentiallyin the enrichments with a high N:P ratio. This can be explainedby the ability of the species to feed on P-rich bacteria, whichgives it a competitive advantage in P-limited conditions. Thespecies-specific differences in the responses to the nutrientenrichments can generally be explained by differences in thespecies physiology and they were consistent with the theoryof resource competition.     

Clearance of picoplankton-sized partides and formation of rapidly sinking aggregates by the pteropod, Limacina reiroversa

Findings from experiments showed that the web-feeding euthecosomatouspteropod, Limacina retroversa, can produce rapidly sinking,mucous aggregates. It is suggested that, by adhesion, theseaggregates scavenged picoplankton-sized particles, which werethus effectively cleared from the medium. In contrast, Calanusfinmarchicus was not able to clear these particles in our experiments.Sedimentation velocities of 10 aggregates measured in vivo wereup to 1000 m day^–1, with an average of {small tilde}300m day^–1 (not including two aggegates with neutral buoyancy).Mean velocities measured for feces of C.finmarchicus, Calanushyperboreus and Thyssnoessa sp. were consider ably lower. Wesuggest that the sedimentation of L retroversa aggregates wasthe source of mucous flocs collected in sediment traps (Bathmannet al., Deep-Sea Res., 38,1341–1360,1991) and at the seafloor at 1200 m depth in the southern Norwegian Sea. This processmay be an important mediator of sedimentation to the deep sea,when these pteropods are present in surface waters in largeabundance.     

Photosynthetic parameters, pigment composition and respiration rates of the marine diatom Skeletonema costatum grown in continuous light and a 12:12 h light-dark cycle

Nutrient-sufficient cultures of a Trondheimsfjord (Norway) cloneof the marine centric diatom Skeleionema costatum (Grev.) Clevewere grown at 75 µmol m^–2 s^–1 and 15C at24 and 12 h daylength to study diurnal variations and the effectof daylength on pigment and chemical composition, photosyntheticparameters, dark respiration rates and scaled fluorescence excitationspectra (F), the latter used as estimates for the absorptionof energy available to Photosystem II. Specific growth rateswere 1.06 and 0.56 day^– in 24 and 12 h daylength, respectively,while dark respiration rates were generally 85% of the net growthrate. The Chla-normalized photosynthetic coefficients P^B_m anda^B were {small tilde}20–25% higher in continuous lightthan at 12 h daylength, while the Chla:C ratio was {small tilde}15%lower (0.051 versus 0.061 w:w). Thus, the carbon-normalizedcoefficients P^c_m and a^c were <11% lower at 24 h than at 12h daylength. The maximum quantum yield _max, the Chla:C ratioand F differed negligibly, as did the light saturation indexl_k, the N:C ratio and the ratios Chlc:Chla and Fucoxanthin:Chla. P^B_m and l_k did not exhibit diurnal variations at 24 hdaylength, and varied within 23% of the daily mean at 12 h daylength.Predictions of the daily gross photosynthetic rate based ondata for a given time of the day should thus not be >10%in error relative to an integrated value based on several datasets collected through 24 h. _max was 0.084–0.117 mol O₂(mol photons)^– for gross oxygen evolution. However, ifused in mathematical models for predicting the gross and netgrowth rates (i.e. the gross and net carbon turnover rates),‘practical’ values of 0.076 and 0.040 g-at C (molphotons)^–, respectively, should be employed. Correspondingly,values for a^B and P^B_m should be adjusted pro rata. ¹Present address: College of Marine Studies, Sjmannsveien 27,N-6008 lesund, Norway     

The colonial cyanobacterium Trichodesmium as a physical and nutritional substrate for the harpacticoid copepod Macrosetella gracilis

The pelagic harpacticoid copepod Macrosetella gracilis usesthe colonial cyanobacterium Trichodesmium not only as a physicalsubstrate for juvenile development, but also as a food source.By associating itself with a buoyant colonial cyanobacterium,M.gracilis has developed a successful mode of life for existencein the plankton. Further evidence of M.gracills' dependenceon Trichodesmium as a physical substrate is demonstrated bypreviously undescribed microscopic observations of a gravidM.gracilis female attaching eggs to a Trichodesmium colony.Shipboard experiments investigating the ingestion and assimilationof Trichodesmium carbon (C) were conducted in September 1991and January/February 1992 in waters of the Bahamas and the Caribbean,respectively. Macrosetella gracilis not only ingested, but rapidlyincorporated, cyanobacterial organic matter into its own cellularmaterial. Utilization of ingested Trichodesmium by M.graciliswas investigated by assessing the metabolic partitioning andincorporation of ¹⁴C-labelled Trichodesmium into copepod lipids,proteins, polysaccharides and low-molecular-weight (LMW) compoundsusing sequential biochemical fractionation techniques. Despitevariations in grazing rates between the two sites and times(September 1991,0.017 µg C^* µg^–1 C h^–1;January 1992, 0.134 µg C ^* µg^–1 C h^–1,the partitioning of incorporated C into the different biochemicalfractions was relatively consistent. There was rapid assimilationof ingested C into the LMW ({small tilde}60%) and polysaccharidefractions ({small tilde}30%) in the first few hours, with asubsequent increase in the percent C incorporated into protein.On average, {small tilde}21% of the Trichodesmium C ingestedby M.gracilis was assimilated. Therefore, M.gracilis is an importantsecondary link in the food web of oligotrophic waters whereTrichodesmium is abundant.     

Growth kinetic parameters of two planktonic desmid species under fluctuating phosphorus conditions in continuous-flow culture

Two planktonic desmid taxa, Staurastrum chaetoceras and Cosmariumabbreviatum var. planctonicum, were examined under pulsed phosphorus(P)-limited conditions in continuous culture. Two pulse regimeswere applied, i.e. 2.5 µM P twice a week and 10 µMP once every 2 weeks. Under both pulse regimes, S.chaetocerasshowed a higher maximum P uptake rate (V^max) and affinity forP uptake (V^max/affinity constant K^m) than C.abbreviatum. Affinityfor P uptake in S.chaetoceras just before pulsing was higherthan shortly after pulsing, whereas C.abbreviatum did not showany significant difference. Cell densities and cellular P quotain S.chaetoceras and C.abbreviatum fluctuated in a comparableway, but fluctuations in C.abbreviatum were less pronouncedthan in S.chaetoceras. The mean cell volume of both specieswas greater under the bimonthly than under the biweekly pulseregime, and the lowest under continuous P-limited conditions.Competition between S.chaetoceras and C.abbreviatum under thebimonthly pulse regime was won by S.chaetoceras, but displacementwas less fast than under more frequent pulsing (equalling thesame total P supply). The above-mentioned results are discussedin relation to possible ecological strategies.     

In Vitro Phosphorylation of Proteins in IAA-Treated Mesocotyls in Zea mays

Five-mm sections of elongation zones of Zea mesocotyls wereincubated for designated periods with various concentrationsof IAA. In vitro protein phosphorylation in the soluble fraction(85,000 x g supernatant) prepared from the sections was analyzedby sodium dodecyl sulfate-polyacrylamide gel electrophoresis.The phosphorylation of proteins in the soluble fraction thathad been prepared from sections incubated for 20 min in thepresence of 10{small tilde}^s M IAA was greater than that inthe sections incubated for 20 min without IAA. The amount ofphosphorylation of proteins per protein became higher when higherconcentrations increased (10{small tilde}⁸—10{small tilde}⁵M).The growth of sections incubated in the presence of 10{smalltilde}⁸ M IAA or higher concentrations was greater than thatof sections incubated in the absence of IAA. The promotion ofgrowth by IAA was greater at higher concentrations of IAA. Proteinsin the soluble fraction, prepared from sections incubated for20 min in the presence of 10{small tilde}⁵ M IAA, were phosphorylatedin the presence of either 10 fM cAMP, 10 µM cGMP, 100µM W-7, 100 µM W-5, 20 µM H-7 or 20 µMHA1004. The calmodulin antagonist, W-7, and the inhibitor ofprotein kinase C, H-7, inhibited the phosphorylation of proteinsstimulated by incubation with IAA. These results suggest thatIAA promotes cell elongation via protein phosphorylation thatdepends on calmodulin-dependent protein kinase and protein kinaseC. (Received November 29, 1995; Accepted May 20, 1996)     

Effect of Substituted Pyridazinones on Chloroplast Structure and Lipid Metabolism in Greening Barley Leaves

The effect of the substituted pyridazinone herbicides, Sandoz9785 and Sandoz 6706, on lipid metabolism was studied in greeningbarley leaves. The herbicides had no effect on chlorophyll formationbut caused an altered chloroplast morphology during greening.In leaves supplied with {¹⁴C} acetate, Sandoz 9785 decreasedincorporation of radioactivity into linolenate while Sandoz6706 decreased incorporation into both linolenate and trans-3-hexadecenoate.Decreased linolenate labelling was accompanied by an accumulationof {¹⁴C}linolenate in diacylgalactosylglycerol. {¹⁴C}Palmitateaccumulated in phosphatidylglycerol when synthesis of trans-3-hexadecenoatewas inhibited. The results are discussed in relation to thefunction of acyl lipids in fatty acid desaturation and the roleof lipids in chloroplast morphology. Key words: Chloroplast structure, Lipid synthesis, Substituted pyridazinones, Fatty acid desaturation     

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.     

Growth responses, P-uptake and loss of flagellae in Chlamydomonas reinhardtii exposed to UV-B

The flagellated, planktonic green alga Chlamydomonas reinhardtiiwas exposed to UV-B doses (peak intensity at 312 nm) from 0.6to 36 kJ m^-2. Flagellar status, uptake of ³³P and growth rateswere judged in both P-limited and P-saturated algae. A linearrelationship was found between loss or withdrawal of flagellaeand UV exposure. Even 10 min (0.6 kJ m^-2 exposure initiatedinactivation of flagellae and total loss as well as signs ofcell wall destruction were found at the highest doses Within72 h after irradiation, flagellar recovery had occurred in 34%of non-limited and 87% of P-limited cells. Uptake of P was apparentlyenhanced at low doses, but fell to almost zero at the higherdoses. In P-saturated cells, an initial high uptake was followedby a pronounced P leakage upon radiation. A similar patternwas found for growth rates, where notably P-limited cells gainedincreased growth at low doses, while growth was drasticallyreduced at 16 kJ m^-2. The data show that major functional parametersin this freshwater flagellate are affected by moderate dosesof UV-B, and that reduced P uptake is well correlated with inactivationof flagellae and growth inhibition.     

Effect of light regime upon growth rate and chemical composition of a clone of Skeletonema costatum from the Trondheimsfjord, Norway

Nutrient-saturated cultures of Skeletonema costatum were grownat 15C and 42 combinations of photon flux density (PFD) anddaylength. The growth rate increased with the daylength andPFD up to 460–630 µE m^–2 s^–1 (maximum2.5 doublings day At 2000 µE m^–2 s^–1 the growthrate was reduced by 45%. The chlorophyll (chl) content of thecells and the rate of production of carbon per unit chlorophylland ambient light increased for declining light regimes as didcellular nitrogen and carbon. The N/C ratio, cellular phosphorusand ratios between in vivo fluorescence, with and without DCMU,and chlorophyll varied negligibly. The ATP/C ratio was linearlyrelated to the growth rate. The results were described mathematically.The chl/C ratio was low both in strong light and in marginallylow light, corresponding to low cellular chlorophyll and highcellular carbon, respectively. The observed increase in cellularnitrogen and carbon at shade adaptation probably represent anincrease in the size of internal stores of organic nitrogenand may imply that Skeletonema cells become enriched with organicnitrogen when staying in nitrate-rich subsurface layers, e.g.in or below a nutricline. However, close to zero growth in marginallight the cells become greatly enriched with respect to everymeasured factor. Such cells may be physiologically resting stageswhich may ensure survival during dark periods and promote rapiddevelopment during the initial phase of blooms. Cultures andnatural blooms of Skeleronema in the Trondheimsfjord exhibitvery similar patterns of variation.     

Density and distribution of bacterioplankton and planktonic ciliates in the Bering Sea and North Pacific

The total number of planktonic bacteria in the upper mixed layerof the Bering Sea during the late spring-early summer periodranged between 1 and {small tilde}4 x 10⁶ ml^–1 (biomass10–40mg C m^–3). In the northern Pacific, along 47–52⁶N,the corresponding characteristics of the bacterioplankton densityin the upper mixed water layer were: total number 1–2x 10⁶ cells ml^–1 and biomass 15–46mg C m^–3Below the thermocline at 50–100 m, the density of bacterioplanktonrapidly decreased. At 300 m depth, it stabilized at 0.1–0.2x 106 cells ml^–1. The integrated biomass of bacterioplanktonin the open Bering Sea ranged between 1.2 and 3.6 g C m^–2(wet biomass 6–18 g m^–2) Its production per dayvaried from 2 to 23 mg C m^–3 days^–1 in the upper0–100 m. The numerical abundance of planktonic ciliatesin this layer was estimated to be from 3 to l0 x 10³ cells l^–1,and in the northern Pacific from 0.4 to 4.5 x 10³ l^–2.Their populations were dominated by naked forms of Strombidium,Strombilidium and Tontonia. In some shelf areas, up to 40% ofthe total ciliate population was represented by the symbioticciliate Mesodinium rubrum. The data on the integrated biomassof basic groups of planktonic microheterotrophs are also presented,and their importance in the trophic relationships in pelagiccommunities of subarctic seas is discussed.     

Carbon and nitrogen content of Noctiluca scintillans in the Seto Inland Sea, Japan

The carbon and nitrogen content of Noctiluca scintillans cellsfrom the Seto Inland Sea, Japan was investigated in order toestimate its biomass in natural samples. The carbon contentof N.scintillans ranged from 123 to 627 ng C cell^–1 witha mean value of 353 ng C cell^–1, or 1.12 to 2.67 fg Cµm^–3 with a mean value of 1.98 fg C µm^–3.The nitrogen content ranged from 36.0 to 232 ng N cell^–1with a mean value of 131 ng N cell^–1, or 0.499 to 0.910fg N µm^–3 with a mean value of 0.694 fg N µm^–3.Total cell carbon and nitrogen increased but the carbon andnitrogen per cell volume decreased with increasing cell volume.The C/N ratio of the cells ranged from 2.3 to 4.4, which wasrelatively low compared with the Redfield ratio. The carbonand nitrogen content was extremely low (91.2 ng C cell^–1,41.8 ng N cell^–1) for starved cells, whereas it was extremelyhigh (528 ng C cell^–1, 205 ng N cell^–1) for cellswhich had ingested the large diatom, Coscinodiscus wailesii.Our results suggest that the carbon and nitrogen content ofN.scintillans varies depending on its physiological conditionand the type of food that it has recently consumed.     

Pyrosoma atlanticum (Tunicata, Thaliacea): grazing impact on phytoplankton standing stock and role in organic carbon flux

Pyrosomas are the large group of pelagic tunicates whose trophicrole in pelagic communities has not yet been sufficiently studied.We ran across a local area of high concentration of the mostwidespread and commonest species of pyrosomas, Pyrosoma atlanticum,450 miles off the Congo river mouth. The following was estimated:gut pigment content, defecation rate, organic carbon and pigmentcontent of fecal pellets, and sinking rate. Based on these dataand the measured number of pyrosomas colonies the grazing impacton phytoplankton and the fecal pellet flux were calculated.During the night swarms of 50–65 mm P.atlanticum removed53% of phytoplankton standing stock in the 0–10 m layer;sparsely distributed pyrosomas consumed only 4%. The grazingimpact in the 0–50 m layer was only 12.5 and <1% respectively.The fecal pellet flux resulting from nocturnal feeding of P.atlanticumwhile swarming made up 1.4–1.6 x 10⁶ pellets m^–210 h^–1 or 305–1035 mg C m^–2 10 h^–1 and1.4 x 10⁵ pellets m^–2 10 h^–1 or 87.4 mg C m^–210 h^–1 while non-swarming. Incubation experiments showedthe rapid degradation of fecal pellets at 23°C: the lossof pigment and carbon content was {small tilde}60–70%after 45 h. We believe that given the sinking rate of 70 m day^–1the main part of fecal material does not leave the upper watercolumn and is retained in the trophic web of the epipelagiclayer.     

Use of immobilized algae for estimating bioavailable phosphorus released by zooplankton

Cells of the green alga Selenastrum capricomuxum were immobilizedin permeable alginate beads to prevent them from being grazedby zooplankton. Algae were able to grow in these beads and wereused as a new technique to estimate bioavailable phosphorus(P) released by zooplankton. P-limited algal cells were encapsulatedin alginate beads and used to measure P-release by Daphnia pulexfeeding on P-saturated and P-limited free algal cells. Daphnidsgrazing on P-saturated cells released 20 times more P availablefor the immobilized algae than animals grazing on P-limitedcells (0.06 versus 0.003 µg P mg^–1 Daphnia-DW h^–1).     

Effects of light intensity and quality on the relative N and P requirement (the optimum N:P ratio) of marine planktonic algae

The relative requirement of N and P (the optimum N:P ratio)by Dunaliella tertiolecta, Phaeodactylum tricornutum, Prymnesiumparvum and Thalassiosira pseudonana was studied under variouslight intensities and spectra. The ratio was determined as theratio of the minimum cell N and P concentrations (q₀N and q₀pwhen either nutrient was limiting. The ratio varied widely amongspecies; under light-saturation for growth (116 µEin ^m–2s^–1 it ranged from 11.8 in ^D. tertiolecta to 36.6 in P.tricornutum. The ratio appeared to be higher at a sub-saturatingintensity (24 µEin m^–2 s^–1 in all except P.tricornutum, mainly because of higher q_oN with little changein q_oP. In T. pseudonana Q_oP also increased, resulting in aninsignificant change in the ratio. The ratio varied little withinthe range of saturation intensity. Light quality affected q_oNand q_oP as well as the ratio, and the pattern of change variedfrom species to species. The optimum ratio of individual specieswas linearly correlated to their q_oN except in P. tricornutum.q_oN for all species showed a linear correlation with cell proteinconcentrations irrespective of light conditions. The changeof optimum N:P ratios in the three species thus appears to berelated to changes in cell protein contents. The ratio of carbohydratesto protein remained constant regardless of light intensity orquality and was higher in P-limited cultures. We conclude thatchanges in light regime can strongly influence algal nutrientrequirements and species interrelationships by altering theoptimum cellular N:P ratio.     

THE DIET, PREY SIZE AND CONSUMPTION OF PHILINE ORIENTALIS (OPISTHOBRANCHIA: PHILINIDAE) IN HONG KONG

Philine orientalis occurs on Hong Kong's beaches for {smalltilde}2 months each year, in May and June when it breeds, laysegg capsules and dies. It is believed to have a life span ofone year. Hoi Sing Wan, a sandy beach in the New Territoriesof Hong Kong is dominated at lower levels by the venerid bivalveTapes philippinarum, the population of which in May and June,following spring recruitment, mostly comprises juveniles. P.orientalis, occurring at a density of {small tilde} 1.m^–2,feeds voraciously upon these juveniles consuming, in laboratoryexperiments, a mean of 25.day^–1. At Hoi Sing Wan, P. orientalisappears to be highly prey specific. It is, however, known tofeed on other prey items and probably has a catholic diet, duringits annual migration from deeper to shallower sediments, accordingto the prevailing fauna. In terms of wet weight, P. orientalis consumes a mean of {smalltilde}4.4% (2.2–6.7%) of its body weight day^–1.In terms of dry weight, however, this figure increases to amean of 35.5% and one figure of 208% has been recorded. Althoughthis is because of the large body fluid content of this, essentiallyshell-less, gastropod, using a hydrostatic skeleton to achieveburrowing, such figures further attest to the voracity of thebreeding adults. This study joins others in questioning ourpresent understanding of the scale of energy budgets on suchshores, i.e., are our estimates of loss to predation grosslyunderestimated? ^*Present address: Department of Biology, Baptist College, WaterlooRoad, Kowloon, Hong Kong. (Received 20 May 1989; accepted 3 July 1989)     

The phycocyanin to chlorophyll {alpha} ratio and other cell components as indicators of nutrient limitation in two planktonic cyanobacteria subjected to low-light exposures

The cell composition of the planktonic cyanobacteria, Oscillatoriaagardhii (Gomont) and Oscillatona redekei (van Goor), was comparedfor cultures grown under nitrogen (N) and phosphorus (P) limitation,and light climates which were energy (E) limited (photoperiods3:21 and 6:18 light:dark (LD) and irradiances 12–153 µmolm^–2s^–1). Increases in carbohydrate/protein ratio(CHO/Prot) and declines in chlorophyll a (Cha) and phycocyanin(PC) resulted from N and P limitation. N-, P- and E-limitedcultures could be distinguished on the basis of P content andthe ratio of PC/Cha. The P content range of 0.1–0.55%of ash-free dry weight (AFDW) for P-limited cultures was lowerthan that for N- and E-limited cultures (0.56–2.2 %AFDW).Cultures limited by N were distinguishable from E-limited cellsby lower PC/Cha ratios, ranging from 0 to 4.08, compared to3.9–6.9 for E-limited cells. Under the 3:21 LD cycle,the minimum PC/Cha ratio of E-limited cells was 4.5. Increasesin the CHO/Prot ratios were proportional to the difference betweenthe nutrient-limited growth rate and the non-nutrient-limitedgrowth rate. A comparison of the composition of the two speciesshowed greater accumulation of carbohydrate by O.agardhii undernutrient-limiting conditions, but that O. redekei had higherlevels of protein, chlorophyll a and phycocyanin and, in theabsence of P limitation, higher P contents than O.agardhii.     

Diel variations in gut pigment content, diel vertical migration and estimates of grazing impact for copepods in the southern Benguela upwelling region in October 1987

The gut fluorescence technique was used to estimate ingestionand filtration rates of the adult female copepods Paracalanusparvus, Cenlropages brachiatus and Calanus austrails, and copepoditestages 3, 4 and 5 of C.australis in the southern Benguela upwellingregion. During the study period chlorophyll concentrations withinthe upper 20 m of the water column were high, 5 µg I^–1in mid-shelf waters and 15–30 µg I^–1 in innershelf waters. Copepod gut pigment content was low and constantduring the day then increased sharply during the first 2 h aftersunset. Gut pigment content was 2–6 times higher duringthe night compared with daytime values. Small non-migratingcopepods (Paracalanus parvus) showed the smallest diel differencein gut pigment content and large migrating copepods (Centropagesbrachiatus and Calanus australis) the largest difference. Eggproduction rates were 20 and 50% of maximum at the mid-shelfand inner shelf stations respectively, suggesting food-limitation.Comparison of ingestion rates calculated from egg productiondata with ingestion rates calculated from gut pigment data suggestedthat the copepods were feeding omnivorously at the inner shelfstations but herbivorously at the mid-shelf stations. Assumingthat all of the phytoplankton was available as food, the nearshorecopepod assemblage grazed {small tilde}1% of the standing cropeach day, and the mid-shelf assemblage grazed 5% day^–1.Because of errors and uncertainties associated with the gutfluorescence technique, the feeding impact could be underestimatedby 2–4-fold. We discuss several approaches which couldlead to more precise estimates of feeding rates. ³Present address: Marine Sciences, SUNY, Stony Brook, NY, 11794-5000,USA     

Turnover of the Intracellular Mannitol Pool of Fucus spiralis L. (Fucales, Phaeophyta) during Osmotic Shock

13C {¹H} nuclear magnetic resonance spectroscopy has been usedto study the turnover of the low-molecular weight carbohydratemannitol in the marine brown alga Fucus spiralis L. During incubationof plants in seawater medium with ¹³C enriched bicarbonate,all of the NMR-visible ¹³C appeared in mannitol; no significant¹³C-labelling of laminaran was observed in seawater, in a hypo-salineor in a hyper-saline medium. Pulse-chase experiments showedthat intracellular mannitol was subject to turnover (half-time20 h), the rate of carbon assimilation into mannitol being slightlygreater than the dissimilation rate, possibly due to net mannitolsynthesis during growth. Fucus spiralis was conservative in the use of mannitol as anintracellular osmoticum. In a hypo-saline medium, mannitol showedno substantial change while potassium, the major cellular cation,was reduced. In contrast, mannitol increased substantially overa 12 h incubation period in a hyper-saline medium, whereas potassiumcontent remained constant. Mannitol assimilation and dissimilation rates were not affectedsignificantly by transfer to a hypo-saline medium. The increasedmannitol content of plants incubated in a hyper-saline mediumappeared to be due to a significant increase in the mannitolassimilation rate. Key words: Mannitol, Fucus, Phaeophyta, ¹³C-NMR, osmotic adjustment     

Plankton community respiration in Bransfield Strait (Antarctic Ocean) during austral spring

Community respiration (R) was determined in Bransfield Straitfrom oxygen changes in water samples incubated in borosilicatebottles maintained at in situ temperature. The respiratory electrontransport system (ETS) activity of seawater communities wasalso measured from the same samples. Both data sets were relatedby the regression equation: log R (mg O₂ m^–3 day^–1)=0.462+0.730xlogETS activity mg O₂ m^–3 day^–1) (r=0.80, n=23). Fromthis equation and 37 ETS activity depth profiles, we calculatedthe integrated (0–100 m) community respiration as beingin the range 1.2–4.5 g O₂ m^–2 day^–1 (mean=2.2).These values do not differ significantly from other publishedresults for the Arctic and Antarctic Oceans. Assuming a respiratoryquotient of unity, the areal respiration ranges between 0.45and 1.69 g C m^–2 day^–1 (mean=0.8). This would representan important sink for the primary production reported for BransStrait. The spatial distribution of community respiration showedhigher values associated with the warmer and phytoplankton-richwaters outflowing from Gerlache Strait into Bransfield Strait,and with the front that separates Bellingshausen Sea watersfrom Weddell Sea waters. We suggest that this pattern of distributionmay be related to the transport of organic matter by the BransfieldCurrent along the front.