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.     

Planktonic nitrogen transformations during a declining cyanobacteria bloom in the Baltic Sea

The uptake of ¹⁵N-labelled nitrogen nutrients (ammonium, urea,nitrate) was studied during the decline of a bloom of nitrogen-fixingcyanobacteria in the Baltic Sea. This was done by sampling anorth-south transect of stations, representing different stagesof the bloom. Comparison with nitrogen fixation data showedthat this process was of minor importance, and that the nitrogenuptake was dominated by regenerated nitrogen, mainly ammonium.From time series incubations for studying nutrient uptake, itappears that the regeneration of ammonium was substantial, butthat the production of urea or nitrate was slow. The integrateddaily uptake was calculated for the 0–15 m interval atfour stations and values ranged between 6 and 21 mmol N m^–2day^–1, of which the regenerated nutrients, ammonium andurea, constituted 71–93%. Nitrate was of minor importanceand the highest nitrate uptake rates were found close to thethermocline (at 15 m) and in the southern part of the Baltic.Comparison with carbon fixation data reported from simultaneousmeasurements at two stations gave C/N uptake ratios of 4.9 and2.1 for integrated daily uptake. Contrary to earlier findings,the concentration of DON increased with increasing salinity(from 15 to 17 µmol l^–1). This was correlated withthe declination of the bloom and is suggested to be a resultof a gradual release of less easily utilized DON from the degradationof cyanobacteria. The C/N ratio of DOM was high, 21–23.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

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.     

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.     

Structure and Function of an Age Series of Poplar Plantations in Central Himalaya. II Nutrient Dynamics

This paper elucidates nutrient dynamics in 5- to 8-year-oldpoplar (Populus deltoides) clone D₁₂₁ plantations previouslyinvestigated for dry matter dynamics. The nutrient concentrationin different layers of the vegetation were in the order: tree> shrub > herb, whereas the standing state of nutrientswere in the order: tree > herb > shrub. Soil, litter andvegetation, respectively, accounted for 80-89, 2-3 and 9-16%of the total nutrients in the system. Considerable reductions(trees 42-54, shrubs 31-37 and herbs 15-23%) in concentrationof nutrients in leaves occurred during senescence. The uptakeof nutrients by the vegetation and also by the different components,with and without adjustment for internal recycling, has beencalculated separately. Annual transfer of litter nutrient tothe soil by vegetation was 113·7-137·6 N, 11·6-14·6P and 80·1-83·2 K kg ha^-1 year^-1. Turnover rateand time for different nutrients ranged between 0·72-0·89year^-1 and 1·12-1·39 years, respectively. Thehigh turnover rate of litter on the forest floor indicates thegreater productivity of the stands, which was due to the higherdry matter dynamics and nutrient release for the growing vegetation.The nutrient use efficiency in poplar plantations ranged from159-175 for N, 1405-1569 for P and 295-332 for K. Compared withEucalyptus, there was a higher proportion of nutrient retranslocationin poplars largely because of higher tissue nutrient concentrations;this indicates lower nutrient use efficiency as compared tothe eucalypt plantation. Compartment models for nutrient dynamicshave been developed to represent the distribution of nutrientpools and net annual fluxes within the system.Copyright 1995,1999 Academic Press Populus deltoides plantations (Clone D₁₂₁), nutrient retranslocation, net nutrient uptake, nutrient use efficiency, nutrient cycling, nutrient pool, nutrient fluxes     

Linking limitation to species composition: importance of inter- and intra-specific variation in grazing resistance

Short-term responses of producers highlight that key nutrients (e.g., N, P)—or combinations of these nutrients—limit primary production in aquatic and terrestrial ecosystems. These discoveries continue to provide highly valuable insights, but it remains important to ask whether nutrients always predominantly limit producers despite wide variation in nutrient supply and herbivory among systems. After all, predictions from simple food chain models (derived here) readily predict that limitation by grazers can exceed that by nutrients, given sufficient enrichment. However, shifts in composition of producers and/or increasing dominance of invulnerable stages of a producer can, in theory, reduce grazer limitation and retain primacy of nutrient limitation along nutrient supply gradients. We observed both mechanisms (inter- and intra-species variation in vulnerability to herbivory) working in a two-part mesocosm experiment. We incubated diverse benthic algal assemblages for several months either in the presence or absence of benthic macro-grazers in mesocosms that spread a broad range of nutrient supply. We then conducted short-term assays of nutrient and grazer limitation on these communities. In the “historically grazed” assemblages, we found shifts from more edible, better competitors to more resistant producers over enrichment gradients (as anticipated by the food web model built with a tradeoff in resistance vs. competitive abilities). However, contrary to our expectations, “historically ungrazed” assemblages became dominated by producers with vulnerable juvenile forms but inedible adult forms (long filaments). Consequently, we observed higher resource limitation rather than grazer limitation over this nutrient supply gradient in both “historically grazed” (expected) and “historically ungrazed” (not initially expected). Thus, via multiple, general mechanisms involving resistance to grazing (changes in species composition or variation in stage-structured vulnerability), producer assemblages should remain more strongly or as strongly limited by nutrients than grazers, even over large enrichment gradients. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

An ecosystem model of the phytoplankton competition in the East China Sea, as based on field experiments

An ecological dynamic model for the simulation of two pelagic phytoplankton groups is developed in this article. Model parameters were adjusted and validated based on the light-limited field culture experiments and the mesocosm experiments in the East China Sea (ECS). The calculation comparisons from the proposed model, along with field experiment observations, show that the model simulate the datasets very well, qualitatively and quantitatively. The parameters’ sensitivity analysis indicates that the competition between the diatoms and dinoflagellates is most sensitive to the photosynthetic process, followed by the exudation process of the phytoplankton, while the autolysis and respiration processes of phytoplankton and the grazing and exudation processes of zooplankton can also influence this competition to some extent. The sensitive parameters include: the photosynthetic optimal specific rate; the optimal irradiance and optimal temperature for phytoplankton growth; and the half-saturation constant for limiting nutrients, etc. Results of the sensitivity analysis also indicate that light, temperature and limiting nutrients are the controlling environmental factors for the competition between the diatoms and dinoflagellates in the ECS. In order to explore the effects of light and nutrients on the phytoplankton competition, simulations were carried out with varying light and nutrient conditions. Model simulations suggest that the diatoms favor higher irradiance, lower DIN/PO₄–P ratios, higher SiO₄–Si/DIN ratios and higher nutrient concentrations, as compared to the dinoflagellates. These results support the speculation that the increase in the DIN/PO₄−P ratio and the decrease in the SiO₄–Si/DIN ratio in the ECS may be responsible for the composition change in the functional Harmful Algal Bloom (HAB) groups from the diatom to the dinoflagellate communities over the last two decades. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: L. Naselli-Flores     

Nutrient Cycling in an Age Sequence of Western Washington Douglas-fir Stands

The cycling of nitrogen, phosphorus, calcium, magnesium andpotassium in a series of western Washington Douglas-fir [Pseudotsugamenziesii (Mirb.) Franco] stands ranging in age from 9 to 95years has been described. The stands were of relatively lowproductivity being limited by low nitrogen. The content of nitrogen,phosphorus, magnesium and potassium in tree foliage all tendedto stabilize at about 40 years whereas calcium continued toincrease. The content of all nutrients in the wood continuedto increase with stand age. Nitrogen in the forest floor accumulatedconstantly at about 5.7 kg ha^–1 year^–1 and thistogether with the above-ground tree accumulation meant about10.5 kg ha^–1 year^–1 nitrogen was immobilized. Calciumalso increased with time in the forest floor with age whereasthe other nutrients were fairly constant after about 30 years.Understorey nutrient content reached a peak at about 20 years,while understorey litter-fall was significant throughout theage sequence. Internal redistribution, especially of nitrogen,represented an increasingly greater proportion of stand requirementwith increasing stand maturity. Pseudotsuga menziesti (Mirb.) Franco, Douglas-fir, biomass, litter-fall, nutrient content, nutrient cycling     

Performance of the Redfield Ratio and a Family of Nutrient Limitation Indicators as Thresholds for Phytoplankton N vs. P Limitation

We aim to define the best nutrient limitation indicator predicting phytoplankton biomass increase as a result of nutrient enrichment (N, P, or both). We compare the abilities of different indicators, based on chemical measurements of nitrogen (N) and phosphorus (P) fractions in the initial plankton community, to predict the limiting factor for phytoplankton growth as inferred independently from short-term laboratory experiments on the same natural communities in a data set from NE Baltic Sea (Tamminen and Andersen, Mar Ecol Prog Ser 340:121–138, 2007). The best indicators had a true positive rate of about 80% for predicting both N and P limitation, but with a higher false positive rate for N than for P limitation (25 vs. 5%). Estimated threshold ratios for total nutrients (TN:TP) were substantially higher than the Redfield ratio, reflecting the relatively high amounts of biologically less available dissolved organic N in the study area. The best overall performing indicator, DIN:TP, had chlorophyll-response based threshold ratios far below Redfield, with N limitation below 2:1 and P limitation above 5:1 (by atoms). On the contrary, particulate N:P ratio was the overall worst predictor for N or P limitation, with values clustering around the Redfield N:P ratio (16:1, by atoms) independent of the limiting factor. Estimated threshold ratios based on inorganic nutrients (DIN:DIP) and so-called biologically available nutrients (BAN:BAP = (PON + DIN):(POP + DIP)) were also generally clearly above 16:1, indicating that the Redfield ratio rather reflects the transition from N limitation to combined N + P limitation, than to single limitation by P. Coastal systems are complex systems with regard to nutrient dynamics, historically considered to represent the transition from P-limited freshwater to N-limited marine systems. Our analysis shows that rather simple ratios reflect phytoplankton requirement for nutrients. Based on the high prediction performance, analytical considerations, and general data availability, the DIN:TP ratio appears to be the best indicator for inferring in situ N vs. P limitation of phytoplankton from chemical monitoring data.     

Structure and Function of Oak Forests in Central Himalaya. II. Nutrient Dynamics

This paper elucidates nutrient dynamics in oak forests previouslyinvestigated for dry matter dynamics. The nutrient concentrationsin different life forms were of the order: herb > shrub >tree, whereas the standing state of nutrients were of the order:tree > shrub > herb. Soil, litter and vegetation, respectively,accounted for 32·4–98·0 %; 0·3–3·5%, and 10·2–66·6 % of the total nutrientsin the system. Considerable reductions (8·5–41·7%)in concentrations of nutrients in leaves occurred during senescence.The uptake of nutrients by vegetation, and also by differentcomponents with and without adjustment for internal recycling,has been calculated separately. Annual transfer of litter (above+ below ground) to the soil by vegetation was 115·9–187N, 7·5–15·6 P, 122·7–195·1Ca, 36·1–48·8 K and 2·88–5·16Na kg ha^–1 yr^–1. Turnover rate and turnover timefor different nutrients ranged between 0·66–0·84yr^–1 and 1·19–1·56 yr^–1, respectively.Compartment models for nutrient dynamics have been developedto represent the distribution of nutrient contents and net annualfluxes within the system. Quercus leucotrichophora forest, Q.floribunda forest, Q. lanuginosa forest, Nutrient concentration, standing state, uptake, internal cycling, turnover     

Nutrient Movement in Litter Fall and Precipitation Components for Central Himalayan Forests

The annual total litter fall in six Central Himalayan forestsranged from 2.1 to 3.8 t C ha^–1, of which 54 to 82 percent was leaf litter, 9–20 per cent wood litter and 6–14per cent other litter. In all forests the order of relativeabundance of nutrients (kg ha^-1 year^-1) in litter fall was Ca(50.8–91.6) > N (47.7–72.2) > K (22.8–37.1)> P (4.1–6.4). Leaf litter accounted for 63–95per cent of the total nutrients returned through litter fall. In these forests throughfall ranged from 71.3 to 81.4 per cent,stemflow from 0.50 to 2.16 per cent and canopy interceptionfrom 17.7 to 28.2 per cent of the gross rainfall. In the incidentrainfall the concentration and annual input of Ca was the greatestand of P the least. Canopy precipitation was richer in all nutrientscompared to incident rainfall. Net gain of nutrients from thecanopy ranged from 0.16 kg ha^-1 year^-1, for P, to 17.77 kg ha^-1year^-1 for K. Leaching was greatest for K and least for N. Ofthe total quantity of nutrients returned to the soil, 11 to46 per cent was accounted for by precipitation components. Thusprecipitation inputs play a significant role in nutrient cyclingof these forests. Himalaya, forest, litter fall, precipitation components, nutrients     

Nutrient Dynamics in Himalayan Alder Plantations

Dynamics of four macro-nutrients were studied in an age series(7, 17, 30, 46 and 56 years) of Himalayan alder (Alnus nepalensisD. Don) plantations in the Kalimpong forest division of theeastern Himalayas. Concentrations of nutrients were in the orderN > K > Ca > P in most of the tree components and inunderstorey vegetation. There was an inverse relationship betweennutrient concentrations of perennial parts and diameter at breastheight. The relative contributions of standing state of nutrientsin different tree components of mature plantations were in theorder; bole > branch > below-ground part > twig andleaf > catkin. Sequential arrangement of nutrient storagein tree components was: N > K > Ca > P. Soil totalN and available P increased with plantation age. Annual inputsof nutrients (kg ha^-1) to the forest-floor via litterfall were:183-235 N, 4·9-7·0 P, 33·5-39·5K, and 9·2-10·8 Ca. Total annual accretion ofN through biological fixation ranged from 29 to 117 kg ha^-1in different plantations. Turnover rate and turnover time fordifferent nutrients in the age series of plantations fluctuatedbetween 0·10-0·55 year^-1 and 1·8-9·3years, respectively. Nutrient use efficiencies decreased withplantation age for all nutrients except for calcium. Uptakeof nutrients is a more energy consuming process than release.Copyright1993, 1999 Academic Press Alnus nepalensis D. Don, plantation age, nitrogen accretion, nutrient concentration, standing state, uptake, turnover     

Seasonal variations of phytoplankton photosynthate partitioning in two lakes of different trophic level

The aim of this study was to compare vertical and seasonal variationsof inorganic carbon allocation into macromolecules by the phytoplanktonpopulation in a eutrophic lake (Lake Aydat) and an oligo-mesotrophiclake (Lake Pavin). Biochemical fractionation was conducted byconsecu tive differential extractions in order to separate proteins,polysaccharides, lipids, and low molecular weight compounds(LMW). The ratio of light absorption at480 and 665 nm by acetoneextracts of phytoplankton pigments was used as an indicatorof the nutritional statusof natural phytoplankton populations.Our results show that in Lake Aydat, the main photosyntheticend productswere poly saccharides, whereas in Lake Pavin, radioactivitywas predominantly incorporated into the protein fraction. Moreover,the seasonal cycles of mixing and stratification in these twolakes affected the pattern of ¹⁴C incorporation into LMW andmacromolecules. An increase in the relative synthesis of proteinsoccurred during stratification periods. It was linked to anincrease in temperature and nutrient limitation further complicatedby the shift in species composition of the populations. Differences recorded both between the two lakes of different trophicstatus and between seasons confirm that the proportion of carbonincorporated into proteins might be a useful indicator of thephysio logical status of phvtoplankton communities.     

Photoadaptation in Antarctic phytopfankton: variations in growth rate, chemical composition and P versus I curves

The response of phytoplankton to variations in the light regimewas studied during the VULCAN and ACDA cruises in the Antarctic.Unenriched batch cultures of 12–19 days' duration reachedchl concentrations of 10–50 µg^–1 and exhibitedexponential growth rates, with the maximal rate being 0.41 doubl,day^–1. Ice edge algae exhibited maximum growth rates atphoton flux densities (PFD) of 30–100 µE m^–2S^–1and the growth rate was reduced by about 30% at 500–1000µE m^–2S^–1 The chl/C ratio ranged between 0.004and 0.018, with the lowest ratios at PFDs above 500 µEm^–2S^–1 chl/C ratios were also below maximum at PFDsbelow 40–50 µE m^–2S^–1 The C:N:P ratioswere close to the Redfield ratios; the Si/C ratio averaged 0.16(atoms), and the ATP/C ratio averaged from 0.0024 to 0.0050in different culture senes. When thawed after having been frozenfor 10 days, shade-adapted cultures were in a much better conditionthan sun-adapted ones. P versus I data showed that the maximumassimilation number varied from 0.75 to 4.4 µg C (µgchl)^–1h^–1. It varied inversely with the chl/C ratio;therefore the maximum carbon turnover rate varied little betweensamples (0.024/0.035 h^–1). Low biomass communities exhibitedrelatively high values for (the initial slope of P versus Icurves), low values for 1_sat (160–330 µE m^–2S^–1),and they were susceptible to photoinhibition. In contrast, communitiesdominated by Odontella weissflogii exhibited low values for, a high value for I_sat (560 µE m^–2S^–1 andthey tolerated high PFDs. The photo-adaptational status of thephytoplankton in natural water samples is discussed relativeto the profile of water column stability and mixing processes.     

Changes in particulate and dissolved organic matter in nutrient-enriched enclosures from an area influenced by mucilage: the northern Adriatic Sea

This study examined the partitioning of organic matter intoparticulate organic carbon (POC) and dissolved organic carbon(DOC) pools in nutrient-enriched enclosures containing naturalplankton from the Gulf of Trieste (northern Adriatic), an areaaffected by mucilage. The strategy of nutrient additions wasto introduce a pulse of new nutrients in concentrations thatmimic natural inputs and to survey community structure and organicmatter fluxes long enough so that plankton became nutrient-limited.Maximal bacterial biomass attained roughly double the initialvalue, while autotrophic carbon increased by nearly an orderof magnitude. The microflagellate-dominated community releasedmore DOC per unit biomass (5.5 ± 7.2 to 50.6 ±28.0 µg C µg Chl a^-1 day^-1 versus 3.4 ± 3.4to 10.8 ± 4.6 µg C µg Chl a^-1 day^-1 for diatom-dominatedphytoplankton), POC increase was modest (~300 µg C l^-1)and there was little change in DOC. Organic carbon partitioningduring two experiments in which diatoms prevailed was dominatedby POC (>800 µg C l^-1) in the exponential growth phasewith an increasing contribution of particulate carbohydratesthat paralleled gradual nutrient depletion. Transition to thestationary phase and the decay of autotrophic communities wereaccompanied by the net accumulation of a carbohydrate-rich DOC.     

The fate of the carotenoid pigment fucoxanthin during passage through the copepod gut: pigment recovery as a function of copepod species, season and food concentration

The amount of fucoxanthin, a taxonomically diagnostic carotenoid,recovered after passage through the guts of the copepods Acartiacalifomiensis and Calanus pacificus, was determined after thecopepods had fed on low (50 µg Cl^–1) and high (350µg C1^–1 for Acartia; 500 ug C H for Calanus) concentrationsof the diatom Thalassiosira weissftogii, during spring (May)and winter (December). Changes in pigment concentrations andcell abundances were assessed in experimental (with copepods)and control (without copepods) samples by standard incubationexperiments. Pigment recovery was assessed by (i) comparingthe amount of ingested pigment recovered in the experimentalgroups with that predicted to have been ingested from cell countdata and (ii) comparing fuco-xanthin/cell ratios in controland experimental samples. Both techniques suggested that pigmentloss is substantial (usually 60–100%), regardless of species,food availability or season. Patterns of pigment conservationdiffered between species, although pigment recovery was alwayshigher at high, than at low, food concentrations. Pigment recoveryin Acartia was higher (9.4–28.0%) in the spring than duringthe winter (0 recovery), regardless of food concentration. InCalanus, however, pigment recovery was always higher at high(34.9–67.8%) than at low (0 recovery) food concentrations,regardless of season.     

The effect of nutrient additions on the partitioning of nutrients in an experimental coastal Mediterranean system

The hypothesis that the importance of dissolved organic matter (DOM) as a reservoir of C, N, and P declines, relative to that of the particulate pool, with increasing nutrient inputs was tested using mesocosms exposed to a gradient of nutrient inputs in the Spanish Mediterranean. The nutrient additions included a treatment equivalent to the loading in the coastal ecosystem studied (5 mmol N m^–2 d^–1), and mesocosms receiving half , 2-, 4-, 8-, and 16-fold this value, as well as a mesocosm to which no nutrients were added. Nutrients were added at ratios of 20 N (as ammonium) : 7 Si : 1 P. The initial concentration of dissolved inorganic nutrients was very low (dissolved inorganic nitrogen < 0.05 M, phosphate < 0.01) and comprised, together with the particulate pool < 25% of the total N and P in the system, with the bulk N and P in the system present as DOM (> 75%). Particulate and dissolved organic matter was depleted in N (C/N ratio > 15) and, particularly, P (C/P ratio > 1000), indicative of a strongly nutrient, particularly phosphorus, deficient ecosystem. Experimental nutrient additions lead to a parabolic change in C/N and C/P ratios in the dissolved organic matter with increasing nutrient inputs, which approached the Redfield stoichiometry at nutrient inputs > 8 fold above the ambient loading. The relative size of the dissolved inorganic nutrient pools (about 20% of the N and P) did not vary, but there was a tendency towards an increase in the relative size of the particulate pool at the expense of a decrease in the relative importance of DOM as a reservoir of N, P and C, with increasing nutrient inputs. The production of nutrient-depleted organic matter at low nutrient inputs likely prevents efficient recycling, leading to the dominance of nutrients in DOM in the system.     

Bacterial community composition in an Arctic phytoplankton mesocosm bloom: the impact of silicate and glucose

In order to study interactions between microorganisms at different nutrient conditions in an arctic environment, a mesocosm experiment was performed in Kongsfjorden, Svalbard (79°N). A phytoplankton bloom was initiated by daily additions of mineral nutrients (ammonium and phosphate) to all mesocosm units. The addition of silicate and glucose, forming a factorial design (+Si/+C, +Si/−C, −Si/+C, −Si/−C), was intended to produce different types of growth rate limitation for the bacterial community. We here focus on the response in bacterial community composition to different nutrient situations. Phytoplankton, bacteria and viruses were enumerated by flow cytometry, while denaturing gradient gel electrophoresis (DGGE) was used to track changes in the bacterial community composition. Our results showed that both glucose and silicate addition affected the bacterial community composition, with the largest effect from glucose. The initial increase in bacterial abundance was most pronounced in the glucose units. After silicate addition, highest bacterial abundance was observed in the silicate treatments where mineral nutrient competition by diatoms was expected to be highest. The major effect of glucose was expressed by the significant separation of the +C and the −C samples at the end of the experiment, while silicate addition resulted in a more stable bacterial community structure. In the unit, given both silicate and glucose, the diatoms were totally outcompeted by the bacterial community. The competitive success of the heterotrophic bacteria in C-replete situations allows the conclusion that the bacteria were not more negatively affected by low temperatures than phytoplankton.