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Previous field observations suggest that the composition of intracellular polyamine pools in phytoplankton determines the profile of polyamines released to the surrounding environ- ment; thus, knowing how these pools vary among species and in response to factors affecting phytoplankton growth provides a basis for understanding fluctuations in dissolved polyamines. Our analyses of the polyamine content of axenic marine phytoplankton cultures show that intra- cellular polyamine pools are large (100s to 1000s µmol l �1 of biovolume) and that putrescine and spermidine are the major compounds present; however, composition varied with species. Norsper- midine and norspermine were the dominant compounds found in the diatom Thalassiosira pseudonana and the dinoflagellate Amphidinium carterae, respectively. We explored the effects of temperature, salinity and nutrient limitation on polyamine pools in T. pseudonana, and found that either increasing temperature or decreasing salinity increased polyamine concentrations in cells and in the growth medium. Nutrient (N, P or Si) limitation caused reductions of intracellular polyamine concentrations, but release was enhanced under N or Si limitation. Polyamine ratios in the media were not the same as in intracellular pools, suggesting selective release or uptake of polyamines by phytoplankton. Thus, the composition of the dissolved polyamine pool in seawater can vary significantly and on short time scales, depending on phytoplankton community composi- tion and environmental factors affecting phytoplankton physiology. Our work provides experi- mental verification that biological mechanisms support inferences derived from environmental correlations about the factors controlling polyamine distributions in the sea.

Carbon dioxide (CO2) is the primary substrate for photosynthesis by the phytoplankton that form the base of the marine food web and mediate biogeochemical cycling of C and nutrient elements. Specific growth rate and elemental composition (C:N:P) were characterized for 7 cosmopolitan coastal and oceanic phytoplankton species (5 diatoms and 2 chlorophytes) using low density, nutrient-replete, semi-continuous culture experiments in which CO2 was manipulated to 4 levels ranging from post-bloom/glacial maxima (2900 ppm). Specific growth rates at high CO2 were from 19 to 60% higher than in low CO2 treatments in 4 species and 44% lower in 1 species; there was no significant change in 2 species. Higher CO2 availability also resulted in elevated C:P and N:P molar ratios in Thalassiosira pseudonana (~60 to 90% higher), lower C:P and N:P molar ratios in 3 species (~20 to 50% lower), and no change in 3 species. Carbonate system-driven changes in growth rate did not necessarily result in changes in elemental composition, or vice versa. In a subset of 4 species for which fatty acid composition was examined, elevated CO2 did not affect the contribution of polyunsaturated fatty acids to total fatty acids significantly. These species show relatively little sensitivity between present day CO2 and predicted ocean acidification scenarios (year 2100). The results, however, demonstrate that CO2 availability at environmentally and geologically relevant scales can result in large changes in phytoplankton physiology, with potentially large feedbacks to ocean biogeochemical cycles and ecosystem structure.

With progressive future global change, marine phytoplankton in surface oceans will be subjected to ocean acidification, as well as to increased solar exposures and decreased vertical transport of nutrients from depth due to increasing stratification. We employed a simultaneous multivariate treatment approach to investigate the physiological and biochemical responses of the diatoms Thalassiosira pseudonana and Skeletonema costatum to these projected ocean changes. Diatoms were grown under different 'clustered' regimes of solar radiation, nutrients, and pCO2 (pH), reflecting present-day (2011) and potential mid-century (2050) and end-of-century (2100) scenarios. Growth rates, chlorophyll a contents and maximal photochemical quantum yield all decreased from the present to 2100 scenarios. While cellular particulate organic carbon signifi- cantly increased in both species along with increased cellular organic C:N ratios, biogenic silica content showed species-specific differences among the cluster treatments. Our results suggest that reduced thickness of the upper mixed layer with enhanced stratification may interact with ocean acidification to influence diatom-related biogeochemical processes by affecting their growth and biochemical composition in species-specific ways.

The concentration of carbon dioxide in seawater may affect phytoplankton physiol- ogy and ecology and their role in marine biogeochemical cycles. In order to assess the effects of CO2 on the elemental composition of marine phytoplankon, carbon, nitrogen, and phosphorus quotas were measured in 4 species of marine phytoplankton acclimated to 150 to 1500 ppm CO2 (5 to 50 µM) in semi-continuous cultures. Nitrogen quotas declined steeply with increasing CO2 in the centric diatoms Thalassiosira pseudonana and T. weissflogii acclimated to 150 to 380 ppm (5 to 13 µM), but more slowly as the CO2 increased from 380 to 1500 ppm (13 to 50 µM). Nitrogen demand varied little with CO2 in the pennate diatom Phaeodactylum tricornutum, but was posi- tively correlated with CO2 over the range of 150 to 770 ppm in the prymnesiophyte Isochrysis galbana. Based on the nitrogenCO2 trends in centric diatoms, relief from carbonnitrogen co- limitation could lead to 2-fold larger cells as CO2 increases from 150 to 380 ppm, but only 15% larger cells from 380 to 770 ppm CO2. Phosphorus quotas in the 3 diatoms decreased as CO2 increased from 150 to 380 ppm. As previously observed in these and other species, C:N, C:P, and N:P ratios increased with increasing CO2, but the present results show that much of this variation was due to differences in nitrogen and phosphorus rather than carbon quotas. Marine phyto - plankton could provide a negative feedback against increasing CO2 over the pCO2 range of 150 to 380 ppm by supporting larger cells or higher biomass, but would support a smaller carbon sink as atmospheric CO2 rises above 380 ppm.

The stoichiometric NPZ model included a limiting nutrient (nitrogen), 3 species of phytoplankton, and a single species of zooplankton. The 3 species of phytoplankton were modeled after the diatoms Thalassiosira pseudonana (T.p.) and T. weissflogii (T.w.), and the brown tide species Aureoumbra lagunensis (A.l.). The zooplankton species was assumed to be a general large ciliate (cell volume 6.9 × 10 to 21 × 10 μm; Hansen et al. 1997). In parameters and equations, phytoplankton were indexed as i = 1 for A.l., i = 2 for T.p., and i = 3 for T.w. All model simulations used the base parameter values listed below unless otherwise stated (see 'Scenarios' section of the table in Supplement 1). Simulations were programmed in R using the deSolve package (R Development Core Team 2011).

Although mathematical models suggest that competition between primary producers in response to dynamical changes in the availability of a limiting nutrient is non-linear, experimental data supporting this basic hypothesis are sparse. Using continuous culture systems with nitrate as a single limiting nutrient, we present results of competition experiments between 2 species of marine phytoplankton, a diatom Thalassiosira pseudonana and a coccolithophore Coccolithus braarudii. These 2 organisms of similar size represent biogeochemically and ecologically distinct functional groups. Consistent with classical resource competition theory, under steady-state nitrate limitation (i.e. continuous flow chemostats), the diatom was outcompeted by the coccolithophore. However, when pulses of nitrate were provided to the chemostats (i.e. non-equilibrium, dynamical conditions) the diatom outcompeted the coccolithophore. The rate of exclusion was a linear function of the fre- quency of nitrate pulses. These results experimentally demonstrate that dynamical nutrient supply allows co-existence of 2 primary producers competing for a single limiting nutrient and may help us to understand phytoplankton succession in the ocean.

Mathematical models are a useful tool for predicting the responses of marine phyto- plankton to changes in nutrient inputs and other environmental factors. Two modeling approaches — Monod and Droop — have been traditionally used. These 2 model types were fitted to empirical data for specific growth rate, cellular N:C ratio, cellular ammonium uptake rate, and ammonium concen- tration measured in N-limited cyclostats at different dilution rates and in nutrient-saturated batch cultures. The modeled data were for a small, fast-growing coastal diatom Thalassiosira pseudonana (~4.5 μm diameter) and for a larger, slower growing diatom T. weissflogii (~11 μm diameter) cultured in seawater medium at 20°C and 14 h d -1 of light. The observed data did not conform well to the clas- sic Monod equation, but could be fit to a modification of this equation in which the maximum growth rate was assigned a value higher than the observed maximum rate. Likewise, data for cellular N uptake rate versus ammonium concentration did not conform well to the standard saturation equa- tion, but could be fit to a modification of the equation in which the maximum uptake rate was set above the empirically measured value and the x-intercept was shifted from the origin to a finite pos- itive value. Both modified models accurately fit the observed steady-state relationships between ammonium concentrations and specific growth rates of the 2 species. However, the 2 models showed different transient dynamics in response to a change in the concentration of inflowing nutrients in time-course simulations. Such differences suggest that the choice between Droop and Monod approaches, when used as part of larger food web models, could lead to widely divergent predictions of algal blooms and other nonequilibrium dynamics of ecosystems.

Estimates of phytoplankton sticking efficiency (α) were made in the laboratory within a 1500 l annular flume meso- cosm over the initiation, maintenance and senescence phases of a bloom of the diatom Thalassiosira pseudonana. The spatially weighted mean turbulence kinetic energy dissipation rate in the annular flume was comparable to values found at the ocean's sur- face on a calm day. The α of T. pseudonana varied as a result of physiological state, and ranged from 0 ± 0.08 during the bloom ini- tiation to 0.26 and 0.73 ± 0.16, respectively, during bloom mainte- nance and senescence phases. During the periods of high α, physiological changes included (1) diminished phytoplankton photosynthetic quantum efficiency, (2) an increase in super-oxide dismutase protein expression, reflecting oxidative stress, and (3) the induction of a biochemical cascade initiating autocatalytic pro- grammed cell death. Additionally, during the period of high physi- ological stress on the diatoms, there was an increase in the pres- ence of transparent exopolymer particles and bacteria. Applying a variable α to a 1-dimensional export flux model shows that carbon export can be increased by at least 2-fold compared to simulations assuming a typically modeled value of α = 1 (100% sticking effi- ciency). The model using a physiologically dependent α had a low initial sticking efficiency that allowed a significant increase in the critical concentration of algal cells. Such an increase in the number of cells during bloom initiation followed by an increase in α during the maintenance and senescence phases resulted in enhanced export fluxes during the latter, 'stickier', stages of the bloom.

Growth responses of the brown tide organism Aureococcus anophagefferens and 4 co- occurring microalgal species to varying concentrations of nitrate, ammonium, urea, and glutamate were assayed in laboratory experiments. Analogous to seasonal shifts in nutrient regimes in local bays, growth functional responses were used to predict relative species abundances in simulated communities supported by a reduced nitrogen source after cultivation on NO3 - . Simulations were based on Monod kinetic parameters, lagged growth responses and threshold nutrient concentrations. Presented with NO3 - only, rank order of biomass production was Thalassiosira pseudonana > Nan- nochloris atomus > Synechococcus bacillaris > Prorocentrum minimum > A. anophagefferens at all concentrations. In contrast, model communities offered NH4 + or glutamate at most environmental concentrations (1 to 50 µM N) tended to be dominated by A. anophagefferens and P. minimum. Over environmental ranges of urea concentrations (1 to 50 µM N), T. pseudonana grew fastest, followed by A. anophagefferens. In field validation experiments, bay water with bloom concentrations of A. anophagefferens (10 5 cells ml -1 ) amended with equimolar (total N) nitrate-rich groundwater or reduced N-rich sediment porewater supported equal increases in total chlorophyll a through time. However, porewater selectively stimulated A. anophagefferens growth more than groundwater, and the converse was observed for cyanobacteria. Extrapolation of all experimental results to conditions in Long Island embayments suggests that phytoplankton communities supplied primarily with NO3 - will be dominated by diatoms, such as T. pseudonana, and perhaps by chlorophytes and cyanobacte- ria like N. atomus and S. bacillaris. Conversely, phytoplankton communities primarily supplied with low-to-moderate concentrations of reduced N-species, NH4 + , and dissolved organic nitrogen (DON) will be dominated by A. anophagefferens and, to a lesser extent, dinoflagellates like P. minimum. Our results are consistent with published field observations of A. anophagefferens bloom dynamics in Long Island estuaries.

Ultraviolet (UV) radiation is a significant stressor in aquatic environments and can inhibit primary productivity of phytoplankton. The effects of UV depend on many factors, including phytoplankton community composition and acclimation status. Using spectrally resolved biological weighting functions (BWFs), we determined sensitivity of photosynthesis and acclimation to UV in a common estuarine diatom, Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, and cryptomonad, Cryptomonas sp. Cryptomonas sp. grown under high PAR (photosynthetically active radiation) (250 µmol quanta m -2 s -1 ) was significantly more sensitive to photoinhibition in the UV-B part of the spectrum (280 to 320 nm) than T. pseudonana under high PAR. Growth under low irradiance (25 µmol quanta m -2 s -1 ) increased sensitivity of T. pseudonana. After a week-long exposure to moderate UV radiation, sensitivity of Cryptomonas sp. declined, while sensitivity of T. pseudonana did not change. Growth rates and chlorophyll a-specific absorption decreased in both species. Based on the BWFs obtained in this study, we predict 11 to 26% UV inhibition of depth-integrated primary production by these species under summer conditions in a shallow, turbid temperate estuary.

Despite the fact that the influence of metal chemistry on metal uptake in marine phyto- plankton has been well studied, the effects of physiological processes such as cellular growth remain less well known. In this study, the cell-specific growth rate of the coastal diatom Thalassiosira pseudonana (Clone-3H) was modified by 3 environmental factors: temperature, irradiance, and the light-dark (LD) cycle. Uptake of Cd and Zn was subsequently quantified using short-term exposure. These environmental factors significantly affected metal uptake. Cd and Zn uptake rates increased with increasing irradiance, temperature, and illumination period. We demonstrate that the metal uptake rate initially increased in proportion with an increase in cell-specific growth rate, and then remained constant. Uptake of Cd and Zn by the diatoms was not affected by 2 photosynthetic inhibitors, suggesting that their uptake was not affected by the photosynthetic activity. There was no obvious diel trend for metal uptake. Thus in this study, cell size, photosynthetic activity, and cell cycle were unlikely to have accounted for the variations in metal uptake at different temperatures, irradi- ances, and LD cycles. The dependence of metal uptake on cell-specific growth rate may have impor- tant implications for the prediction of metal accumulation by marine phytoplankton in different ecosystems or during phytoplankton blooms. Further studies are needed to examine the underlying mechanisms for such relationships at cellular and subcellular levels.

Radiotracer experiments were performed to quantify the carbon assimilation and efflux in a subtropical coastal copepod Acartia spinicauda, and the importance of copepod grazing on the release of Cd, Fe, Se and carbon, from diatoms of different sizes (4 to 100 µm) at different food con- centrations (0.04 to 9.0 mg C l -1 ). Carbon assimilation was not significantly affected by the diatom food concentration (Thalassiosira pseudonana, T. weissflogii and T. rotula) or the amount of food ingested, but was lower for the large diatom T. rotula than for the small diatom T. pseudonana. No significant relationship between carbon assimilation and gut passage time of food materials was observed. The efflux rate-constant (physiological turnover rate) of carbon ranged between 0.134 and 0.372 d -1 and was not significantly affected by diatom concentration (T. weissflogii). During the phys- iological turnover period, over 50% of the copepod's carbon metabolic loss was in the form of dis- solved organic carbon, whereas only a small fraction of carbon was lost in the form of feces. A signif- icant fraction of the copepod's carbon was also lost through respiration, the relative importance of which decreased with increasing period of depuration. The retention of carbon, Cd, Fe, and Se by the diatoms (T. pseudonana, T. weissflogii, T. rotula, and Cosinodiscus sp.) was further compared in the presence and absence of the copepods. In general, there was no increase in the release of metals and carbon from the diatoms in the presence of grazing copepods regardless of diatom concentrations, suggesting that copepod grazing does not trigger significant releases of diatom metals and carbon directly into the ambient environment. Our results suggested that the excretion by zooplankton as well as leakage from fecal pellets presumably account for the majority of the zooplankton-mediated production and cycling of dissolved organic matter in the ocean.

Phytoplankton are an important source of selenium (Se) for aquatic invertebrates, which accumulate Se primarily through dietary ingestion. The extent to which Se bioavailability varies among different phytoplankton species could help explain different bioaccumulation patterns observed for invertebrates in nature. We measured the efficiency with which 3 benthic invertebrates assimilated 75 Se from 5 phytoplankton species using standard pulse-chase techniques. The inverte- brates included the amphipod Leptocheirus plumulosus and the bivalves Macoma balthica and Pota- mocorbula amurensis. The phytoplankton species included Cryptomonas sp. (Cryptophyceae), Gymnodinium sanguinem (Dinophyceae), Phaeodactylum tricornutum (Bacillariophyceae), Syne- chococcus sp. (Cyanophyceae) and Thalassiosira pseudonana (Bacillariophyceae). The range of Se assimilation efficiency (AE) by L. plumulosus (32.1 ± 1.8 to 69.5 ± 7.1%) was the lowest of the 3 organ- isms. No relationship was observed between the proportion of Se in algal cell cytoplasm and Se AE by L. plumulosus, which is consistent with findings for assimilation of other trace elements by this organism. Se AE by M. balthica (range: 58.0 ± 3.2 to 92.3 ± 6.0%) varied according to the proportion of cytoplasmic Se in algal cells (p < 0.0001, r 2 = 0.868). P. amurensis assimilated between 78.3 ± 2.0 and 88.9 ± 3.6% of Se from algal cells, and the relationship between cytoplasmic Se and Se AE was described by the following equation: Se AE = 69.2 + 0.22 · (% cytoplasmic Se) (p = 0.003, r 2 = 0.405). This relationship suggests that P. amurensis assimilated non-cytoplasmic Se from phytoplankton, perhaps through utilization of the glandular digestive pathway. Consistently high Se assimilation from algae by P. amurensis may contribute to elevated Se concentrations observed for this organism.

Although temperature effects on phytoplankton growth and photosynthesis can be clearly demonstrated in the laboratory, their relevance in the field is much harder to establish. Recently, however, it has been recognized that temperature has a significant influence on nitrogen uptake. In particular, temperate marine diatom species may be limited by their ability to acquire nitrate at temperatures above approximately 16°C. In order to explore this idea, we grew the diatom Thalassiosira pseudonana at 8, 17 and 25°C, and compared cell composition, and rates of growth (µ), 15 N incorporation, calculated nitrate incorporation (the product of µ and cell N content), and the activ- ity of nitrate reductase (NR), a key enzyme involved in nitrate incorporation. Cell N content, protein and volume remained relatively constant across different temperatures, but cell C, chlorophyll a (chl a), and C:N ratio increased with increasing temperature, suggesting that C metabolism was affected more strongly than N metabolism. Classical temperature models suggested that growth and various indices of nitrate metabolism all responded to temperature, with Q10 values of close to 2 over the whole temperature range. However, Q10 values over the interval from 8 to 17°C were higher than 2 and much lower than 2 between 17 and 25°C. Limitations to the Q10 concept are considered. Tem- perature effects on different measures of nitrate metabolism were very similar, supporting the hypothesis that the effects of temperature on diatom nitrate metabolism are mediated at the level of NR activity. Recent biochemical data for NR also supports this idea.

Biological processes (uptake, trophic transfer, regeneration/excretion, and decomposition) are critical in controlling the fate of bioactive metals in the ocean. Whether nutrient conditions can affect metal uptake in marine phyto- plankton and its subsequent trophic transfer along planktonic food chains is little known. We demonstrate that Cd accumu- lation in marine diatoms is dependent on ambient nitrogen conditions and thus the physiological status of phytoplankton cells. Diatom cells Thalassiosira pseudonana and T. weiss- flogii inoculated under nitrogen-limited conditions accumu- lated considerably less Cd than cells maintained under nitro- gen-enriched conditions. The intracellular partitioning of Cd in the diatoms was also positively related to the level of ambi- ent N. Phosphate and silicate starvation did not affect Cd uptake in diatoms. The trophic transfer, quantified by mea- surement of the Cd assimilation efficiency (AE) from ingested diatoms in copepods Calanus sinicus, increased with an increase in the N quota of the diatom cells maintained in a semi-continuous culture. AEs were however comparable in copepods feeding on N-starved and N-enriched diatoms. There was a linear relationship between the Cd AE and the Cd distribution in the cytoplasm of diatoms. The Cd regener- ation rate in copepods was independent of the N status of ingested diatoms. Our study suggests that the cycling of Cd in marine planktonic food chains may be depressed under nitrogen-limited conditions. Conversely, nitrogen enrichment leads to an increase in Cd uptake in phytoplankton cells and zooplankton, elevating the potential exposure of plankton to toxic metals. Thus, nitrogen-stimulated eutrophication may not only affect the cycling of carbon, nitrogen and phosphorus in aquatic ecosystems, but can also result in changes in the cycling of toxic metals in marine food webs.

The concentration of Se in algal cells is a primary determinant of Se tissue contents in higher level consumers. Differences in the ability of algal species to concentrate selenite and vari- ability in ambient selenite concentrations may be important determinants of algal Se contents. We compared uptake of 75 Se-labeled selenite by 14 algal species at 2 environmentally relevant selenite concentrations. Se content per unit cell volume (Se/Vc) varied by almost 4 orders of magnitude when algae were exposed to 4.5 nM selenite and almost 5 orders of magnitude when exposed to 0.15 nM selenite. Chlorophytes typically exhibited the lowest Se enrichments, while prymnesiophytes, prasinophytes and dinoflagellates exhibited the highest. The Se/Vc of diatoms and cryptophytes var- ied by >2 orders of magnitude. Even at the lowest selenite concentration, about half the species con- centrated Se to an extent that might cause toxicity at higher trophic levels. Within species, the Se cell concentration typically varied by only 2- to 3-fold when exposed to selenite concentrations that dif- fered by 30-fold. The Se cell concentration of only 1 species, the diatom Skeletonema costatum, var- ied in proportion to ambient selenite concentrations. A more detailed study of the dependence between Se cell concentration on ambient selenite concentrations in the diatom Thalassiosira pseudonana revealed an asymptotic approach to a maximum Se cell concentration at high ambient selenite concentrations (> 0.1 nM). Given the results for other species exposed to 4.5 and 0.15 nM selenite, such saturation is likely to be a common feature. Our results indicate that the composition of the phytoplankton community could have a pronounced impact on Se concentrations in marine food webs.

The uptake of colloid-bound Cd, Cr, and Zn by marine plankton (including the diatom Thalassiosira pseudonana, the dinoflagellate Prorocentrum minimum, and the copepod Paracalanus aculeatus) was studied using the radiotracer technique. Natural organic colloids (operationally defined as particles between 1 kDa and 0.2 µm) were isolated from Hong Kong coastal seawater using cross-flow ultrafiltration, and radiolabeled with 109 Cd, 51 Cr, and 65 Zn. The uptake of colloid- bound metals by plankton was then compared with the uptake of metals in the low molecular weight fraction (LMW

Tracer experiments using 13 C-labeled diatoms Thalassiosira pseudonana were carried out at two 850 m sites (I off Cape Fear and III off Cape Hatteras) on the North Carolina, USA, slope to examine patterns of macrofaunal consumption of fresh phytodetritus. Experiments examined the influence of taxon, feeding mode, body size and vertical position within the sediment column on access to surficial organic matter. δ 13 C measurements were made on macrofaunal metazoans and agglutinating protozoans from background sediments and from sediment plots in which 13 C-labeled diatoms were deposited and then sampled 0.3 h, 1 to 1.5 d, 3 mo and 14 mo later. Significant between-site differences were observed in background δ 13 C signatures of sediments, metazoans, and large, agglutinating protozoans, with values 2 to 3‰ lower at Site III than at Site I. Background δ 13 C signatures also varied as a function of taxon and of vertical position in the sediment column at Site III. The background δ 13 C value of carnivores was higher than that of surface-deposit feeders among Site I annelids, but no annelid feeding-group differences were observed at Site III. δ 13 C data from short-term (1 to 1.5 d) experiments revealed rapid diatom ingestion, primarily by agglutinated protozoans and annelids at Site I and mainly by annelids at Site III. Selective feeding on diatoms was exhibited by paraonid polychaetes, especially Aricidea spp. Exceptionally high uptake and retention of diatom C also was observed in the maldanid Praxillella sp., the nereid Ceratocephale sp. and several other surface-deposit feeding polychaetes. After 14 mo, little of the diatom 13 C remained at Site III, but high concentrations of the tracer were present in annelids and agglutinating protozoans at Site I. At both sites, non-annelid metazoans and subsurface-deposit feeding annelids exhibited the least uptake and retention of diatom C Our hypotheses that large-bodied taxa and shallow-dwelling infauna should have greatest access to freshly deposited organic matter were not borne out. Some small, deep-dwelling taxa acquired label more readily than large or near-surface forms. Differences in tracer fates between sites reflected greater vertical mixing at Site III. These results indicate heterogeneity in benthic processes along the Carolina margin, but suggest that labile organic matter is consumed quickly at both sites. Because most of the taxa found to consume freshly deposited diatoms in these experiments are typical of bathyal settings, we infer that phytodetritus reaching the seabed in margin environments is rapidly processed by protozoan and metazoan components of the benthic fauna.

Excretion of nitrite during nitrate assimilation by phytoplankton is examined in relation to estimates of new production. Evidence from the literature indicates that nitrite excretion is exhibited by a wide variety of phytoplankton species in cultures and can represent up to 25, 45 and 50"0 of nitrate uptake in Thalassiosira pseudonana, Prorocentrum minimum and Skeletonema costa turn respectively, at irradiance levels which are typical of the primary nitrite maximum in oligotrophic oceans. In natural populations, nitrite excretion can represent up to 60% of nitrate uptake in surface waters of upwelling areas. Conversely, nitrite uptake can equal or exceed nitrate uptake in several species of marine phytoplankton in cullures as well as in natural populations. The first phenomenon can lead to large. \ driations of the net/gross new production estimates, as well as widely biased estimates of the f ratio. For example, a nitrite excretion of 40% of nitrate uptake will lead to a 33";, underestimate of the f ratio in a range of values typical of oligotrophic oceans. The effects of nitrite uptake taking place concurrently with nitrate uptake are more difficult to assess, but the overall result is that f ratios in stratified oceanic areas are probably underestimated by the ''N tracer technique.

It has been shown that for dead marine diatom cells or diatom cells which are severely stressed metabolically, larger cells sink faster than small cells as dictated by Stokes' Law. In these cases, the slope of the sinking rate versus cell volume relationship within a culture reaches a maximum. Within cultures of rapidly dividing cells, larger cells sinking rate is reduced physiologically to that of smaller cells and the slope of this relationship approaches zero. In several marine diatom species between 5 and 100 μm in diameter, deviations from the maximum slope of the volume versus sinking rate relationship could be used to quantify the physiological reduction of sinking rates. This allowed us to differentiate 2 different components of sinking rate control, the ballasting component (driven by changes in cell composition and volume) which, when dominant, causes sinking rates to be proportional to cell volume and the energy-requiring, protoplast and vacuolar component which, when active, allows sinking rates to become independent of cell volume. Across the 9 species of diatoms examined, including the 3 single-celled species (Ditylum brightwellii, Thalassiosira pseudonana, and T. weissflogii), 4 chain-forming coastal bloom diatoms (T. aestivalis, Skeletonema costatum, Chaetoceros debilis and C. compressum) and 2 large floating open ocean species (Ethmodiscus sp. and entire Rhizosolenia spp. mats), there was a strong correlation between log cell volume and sinking rate only for cells that were metabolically inactivated either through extended dark treatment or through treatment with the respiratory inhibitor KCN. This was true both within and between cultures. However, no correlation between sinking rate and cell volume was found for rapidly growing cells maintained at saturating irradiances. This supports the notion that there is no obligate correlation between cell volume and sinking rate for metabolically active cells. This potential for cellular modification of the sinking rate versus volume relationship suggests that physiological state may be an important feature to include in models where carbon flux is predicted on the basis of particle size spectra. We suggest that the minimum cell volume necessary for active sinking rate control is ca 200 μm 3 , and that this represents a lower limit for Villareal's (1988; Deep Sea Res 35:1037-1045) theoretical minimum volume necessary for positive buoyancy.

Pulse-chase feeding and multi-labeled radiotracer techniques were employed to measure the assimilation of 6 trace elements ( l l O m ~ g , 2 4 1 ~ m , '09Cd, 5 7 ~ o , '$Se and " ~ n ) from ingested diatoms in the mussel Mytilus edu l~s feeding at different rates (0.1, 0.49 and 1.5 mg dry wt h-'). Uniformly radiolabeled diatoms Thalassiosira pseudonana were fed to mussels for 0.5 h, and the behavior of the radiotracers in individual mussels was followed for 96 h in a depuration seawater system. Assimilation efficiency (AE) of each element declined with increasing ingestion rate and increased with gut passage time. The importance of extracellular digestion relative to intracellular digestion increased with ingestion activity, which, when coupled with a decline in AE, suggested that extracellular digestion is less efficient in metal absorption. Zn assimilation was most affected by ingestion rate, suggesting that AE may play a role in the physiological regulation of this metal in M. edulis. In an experiment to simulate the effects of an acidic gut, lowered pH (5.5) enhanced the release of elements from intact diatom cells, especially at low particle concentration. These results indicate that both feeding components of the mussel (i.e. gut passage time, digestive partitioning) and metal chemistry (i.e. metal release at lowered pH within the bivalve gut) are responsible for the difference in the assimilation of trace metals at different food quantities observed in mussels.

During 2 periods in the late spring and early summer 1989, blooms in the Rhode River, Maryland, USA, were triggered by nutrient inputs from local and remote watersheds. Parameters of the photosynthesis-irradiance relationship increased in May when the bloom was dominated by Thalassiosira pseudonana, and decreased in June when the bloom was dominated by large dinoflagellates. Relative variability of quantum efficiency was a s great a s that of chlorophyll-specific absorption coefficient. Light-saturated photosynthetic rates were predictable based on species counts and previously published size-dependent, nutrient-saturated growth rates for diatoms and dinoflagellates. Dinoflagellate blooms occur in years of high hydrologic nutrient input, but, due to the reduced photosynthetic efficiency of the dinoflagellate blooms, carbon fixation in the system varies proportionately less than biomass.

Microzooplankton, numerically dominated by phagotrophic ciliates and flagellates, can have a significant grazing impact on phytoplankton, especially on ultraplanktonic (< 5 pm) algal cells, which are responsible for a large part of primary production in many pelagic systems. However, there are few analyses of actual in situ clearance rates of this size class of phytoplankton by natural protozoan assemblages. We estimated clearance rates of < 6 pm sized algal prey by heterotrophic flagellates and ciliates present in tidal creek water in a salt marsh estuary during the months of December 1989 to March 1990 via their rates of uptake of fluorescently labeled algae (FLA). Three types of FLA, ranging in size from 1.9 to 5.4 pm, were made from cultures of Nannochloris atomis, Chlorella capsulata, and Thalassiosira pseudonana. Both ciliates and flagellates ingested the FLA at rates greater than they ingested similarly sized fluorescent microspheres. Flagellates 5 to 10 pm in size predominantly ingested the 2 pm FLA, while oval cells 10 to 15 p in size, likely athecate dinoflagellates, were important consumers of 3.4 and 5.4 pm FLA. Estuarine ciliates, mostly choreotrichs 15 to 60 pm in size, ingested all types of FLA, but with lower clearance rates for the 2 pm FLA than for larger sized FLA. Clearance rates determined for flagellates ranged from 0.004 to 0.83 pl cell-' h-', and for ciliates from 0.24 to 8.3 p1 cell-' h-'. Estimated daily clearance rate by algivorous protozoa over the 4 mo period averaged 45 % of the water volume based on uptake of the 2 pm sized FLA, and 107 % of the water volume based on uptake of the larger sized FLA. The average grazing impact of flagellates was about 33 % that of ciliates for 2 pm sized prey, and 50 and 85 % that of ciliates for 5.4 and 3.4 pm sized prey respectively. Two experiments were carried out to determine the functional feeding response of algivorous flagellates and ciliates over an order of magnitude range in concentration of each FLA type. The 2 groups of protozoa had differing clearance-rate and FLA-ingestion-rate responses; clearance rates generally decreased with increase in FLA concentration. The results support the ideas that (1) total rnicrozooplankton herbivory is a combination of partial grazing pressures by individual species on different size classes of phytoplankton, and (2) the importance of algivory by < 20 pm sized phagotrophic flagellates should be considered in future studies.

Caracterisation de la bioaccumulation des metaux lourds et leurs effets sur la croissance de Thalassiosira pseudonana, Dunaliella tertiolecta, Emiliania huxleyi et Oscillatoria woronichinii

Modele simple d'etude de la cinetique de l'absorption et de la liberation de 14 C par T.p. en culture en chimiostat avec l'azote comme facteur limitant