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Arctic sea ice algae produce extracellular organic products, which, as bioconditioners of seawater, may stimulate early summer growth of pelagic, under-sea-ice phytoplankton in low light and low temperature conditions. Sea ice algae are inhibited or decline in numbers if prematurely exposed to high light conditions, thereby reducing their ability to produce bioconditioners. As climate change creates an early reduction or removal of snow and sea ice cover, the result may be a decrease in primary phytoplankton production. Key words: ice algae; Arctic phytoplankton; bioconditioning; sea ice decline Les algues de la glace de mer de l'Arctique produisent des matieres organiques extracellulaires. A titre de bioconditionneurs de l'eau de mer, elles peuvent stimuler la croissance estivale precoce de phytoplancton pelagique sous la glace de mer par basse temperature et faible luminosite. La quantite d'algues de glace de mer est freinee ou diminuee en presence prematuree de forte luminosite, ce qui diminue leur aptitude a produire des bioconditionneurs. Puisque le changement climatique donne lieu a la reduction ou au retrait precoce de la couverture de neige et de glace de mer, cela pourrait entrainer la diminution de la production du phytoplancton primaire. Mots cles: algues de mer; phytoplancton de l'Arctique; bioconditionnement; diminution de la glace de mer, INTRODUCTION As the reduction of Arctic snow and sea ice thickness, extent, and duration accelerates (e.g., Stroeve et al., 2012), scientists have speculated on the consequences for marine parameters, particularly [...]

The ocean is teeming with organisms so small you can't see them, populations of microorganisms called phytoplankton. Tiny they may be, but over recent decades these microscopic plant-like organisms have [...]

The acidification caused by the dissolution of anthropogenic carbon dioxide (C[O.sub.2]) in the ocean changes the chemistry and hence the bioavailability of iron (Fe), a Limiting nutrient in Large oceanic regions. Here, we show that the bioavailability of dissolved Fe may dectine because of ocean acidification. Acidification of media containing various Fe compounds decreases the Fe uptake rate of diatoms and coccolithophores to an extent predicted by the changes in Fe chemistry. A slower Fe uptake by a model diatom with decreasing pH is also seen in experiments with Atlantic surface water. The Fe requirement of model phytoplankton remains unchanged with increasing C[O.sub.2]. The ongoing acidification of seawater is likely to increase the Fe stress of phytoplankton populations in some areas of the ocean. 10.1126/science.1183517

Phytoplankton--the microalgae that populate the upper lit layers of the ocean--fuel the oceanic food web and affect oceanic and atmospheric carbon dioxide levels through photosynthetic carbon fixation. Here, we show that multidecadal changes in global phytoplankton abundances are related to basin-scare oscillations of the physical ocean, specifically the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. This relationship is revealed in ~20 years of satellite observations of chlorophyll and sea surface temperature. Interaction between the main pycnocline and the upper ocean seasonal mixed layer is one mechanism behind this correlation. Our findings provide a context for the interpretation of contemporary changes in global phytoplankton and should improve predictions of their future evolution with climate change. 10.1126/science.1177012

Atmospheric aerosol deposition is an important source of nutrients and trace metals to the open ocean that can enhance ocean productivity and carbon sequestration and thus influence atmospheric carbon dioxide concentrations and climate. Using aerosol samples from different back trajectories in incubation experiments with natural communities, we demonstrate that the response of phytoplankton growth to aerosol additions depends on specific components in aerosols and differs across phytoplankton species. Aerosol additions enhanced growth by releasing nitrogen and phosphorus, but not all aerosols stimulated growth. Toxic effects were observed with some aerosols, where the toxicity affected picoeukaryotes and Synechococcus but not Prochlorococcus. We suggest that the toxicity could be due to high copper concentrations in these aerosols and support this by laboratory copper toxicity tests preformed with Synechococcus cultures. However, it is possible that other elements present in the aerosols or unknown synergistic effects between these elements could have also contributed to the toxic effect. Anthropogenic emissions are increasing atmospheric copper deposition sharply, and based on coupled atmosphere-ocean calculations, we show that this deposition can potentially alter patterns of marine primary production and community structure in high aerosol, low chlorophyll areas, particularly in the Bay of Bengal and downwind of South and East Asia.

Dimethylsulfoxide (DMSO) is an abundant but poorly understood methylated sulfur compound in the marine environment. One potentially significant loss pathway for DMSO is through its biological reduction to dimethylsulfide (DMS), which has been documented in a number of organisms, most notably bacteria. Here we present the first detailed study of DMSO reduction by several marine phytoplankton in axenic cultures. Reduction of DMSO was observed in four algal classes, with in vivo reduction rates ranging from 0.006 to 1.5 [micro]mol [[L cell volume].sup.-1] [s.sup.-1] at 1.0 mmol [L.sup.-1] DMSO. Corresponding turnover times for measured intracellular DMSO pools varied from hours to days. Michaelis-Menton kinetic parameters were estimated for Isochrysis galbana, Thalassiosira pseudonana, and Amphidinium carterae. The half-saturation constant ([K.sub.m]) and maximal rate ([V.sub.max]) for DMSO reduction ranged between 0.96 and 2.7 mmol [[L cell volume].sup.-1] and 17-118 nmol [[L cell volume].sup.-1] [s.sup.-1], respectively. Our results suggest that DMSO reduction is a universal activity in marine phytoplankton, even in algae with no detectable dimethylsulfoniopropionate (DMSP). Although reduction of DMSO by marine eukaryotes may not contribute significantly to removal of DMSO from the dissolved phase, this reduction is likely to be a major source of DMS in species lacking detectable DMSP lyase activity. The ability of marine phytoplankton to reduce DMSO to DMS should allow algae to cycle these compounds as part of an antioxidant system.

To study the role of phytoplankton in mercury cycling, we measured methylmercury (MeHg) and total mercury ([Hg.sub.T]) in surface waters during the spring 2003 phytoplankton bloom in San Francisco Bay. Conditions that described the peak of the bloom, the amount of sorbent, and decay of the bloom were summarized by principal component analysis (PCA). Multivariate analyses conducted with the PCA factors demonstrated that the bloom accounted for a significant (p = 0.03) decrease in dissolved (

Abstract: Models of ocean colour rely on information about phytoplankton absorption, which varies according to community composition and photoacclimation. Here we show that pigment packaging, which is strongly determined by [...]

Abstract: In 2003, the Georges Bank stock of haddock (Melanogrammus aeglefinus) experienced the largest recruitment event recorded during its assessed history. Several hypotheses have been advanced to explain recruitment variability [...]

During the passage of a cold front in March 2002, bio-optical properties examined in coastal waters impacted by the Mississippi River indicated that westward advective flows and increasing river discharge containing high concentrations of nonalgal particles contributed significantly to surface optical variability. A comparison of seasonal data from three cruises indicated spectral models of absorption and scattering to be generally consistent with other coastal environments, while their parameterization in terms of chlorophyll (Chl) a concentration showed seasonal variability. The exponential slope of the colored dissolved organic matter (CDOM) averaged 0.0161 [+ or -] 0.00054 [nm.sup.-1] and nonalgal absorption averaged 0.011 [nm.sup.-1] with deviations from general trends observed due to anomalous water properties. Although the phytoplankton specific absorption coefficients varied over a wide range [0.02 to 0.1 [m.sup.2] [(mg Chl).sup.-1] at 443 nm] being higher in offshore surface waters, values of phytoplankton absorption spectra at the Sea-Viewing Wide Field-of-View Sensor (Sea WiFS) wave bands were highly correlated to modeled values. Particulate scattering characteristics were similar to observations for other coastal waters, while backscattering ratios were on average lower in phytoplankton-dominated surface waters (0.011 [+ or -] 0.003) and higher in low Chl near-bottom waters (0.0191 [+ or -] 0.0045). Average percent differences in remote sensing reflectance [R.sub.rs] derived from modeled and in-water radiometric measurements were highest in the blue wave bands (52%) and at locations with more stratified water columns. Sea WiFS estimates of Chl and CDOM absorption derived using regional empirical algorithms were highly correlated to in situ data. OCIS codes: 010.4450, 290.1350, 120.0280.

Chl a and C-normalized pigment ratios were studied in two dinophytes (Prorocentrum minimum and Karlodinium micrum), three haptophytes (Chrysochromulina leadbeateri, Prymnesium parvum cf. patelliferum, Phaeocystis globosa), two prasinophytes (Pseudoscourfieldia marina, Bathycoccus prasinos) and the raphidophyte Heterosigma akashiwo, in low (LL, 35 [micro]mol photons [m.sup.-2] [s.sup.-1]) and high light (HL, 500 [micro]mol photons [m.sup.-2] [s.sup.-1]). Pigment ratios in LL and HL were compared against a general rule of photo-acclimation: LL versus HL ratios [greater than or equal to]1 are typical for light-harvesting pigments (LHP) and < 1 for photo-protective carotenoids. Peridinin, prasinoxanthin, gyroxanthin-diester and 19'-butanoyloxy-fucoxanthin were stable chemotaxonomic markers with less than 25% variation between LL versus HL Chl a-normalized ratios. As expected, Chls exhibited LL/HL to Chl a ratios > 1 with some exceptions such as Chl [c.sub.3] in P. globosa and MV Chl [c.sub.3] in C. leadbeateri. LL/HL to Chl a ratios of photosynthetic carotenoids were close to 1, except Hex-fuco in P. globosa (four-fold higher Chl a ratio in HL vs LL). Although pigment ratios in P. globosa clearly responded to the light conditions the diadinoxanthin-diatoxanthin cycle remained almost unaltered at HL. Total averaged pigment and LHP to C ratios were significantly higher in LL versus HL, reflecting the photoacclimation status of the studied species. By contrast, the same Chl a-normalized ratios were weakly affected by the light intensity due to co-variation with Chl a. Based on our data, we suggest that the interpretation of PPC and LHP are highly dependent on biomass normalization (Chl a vs. C)., Introduction The assessment of phytoplankton populations constitutes a major task in many oceanographic studies, due to their important role in the pelagic food webs )Fenchel 1988) and their implications in [...]

The major aim of this study was to evaluate the relationships between the rates of microbial activities (phytoplankton primary production, bacterial secondary production, bacterial utilization of organic matter, enzymatic activities, protozoan grazing on bacteria), bacterial numbers, and dissolved organic carbon concentrations and the trophic state index (TSI) of lakes in the upper trophogenic water layer in the pelagial zone along the trophic gradient (from oligo/mesotrophy to hypereutrophy) in 19 lakes of the Mazurian Lake District (northeastern Poland). Multiple regression analysis (analysis of variance--ANOVA) on all collected data and the TSI along eutrophication gradient showed that all studied microbial processes and parameters were very tightly coupled to the trophic conditions of the studied lakes. All studied microbial processes involved in utilization and enzymatic degradation of organic matter were strongly positively dependent on the intensity and rates of photosynthetic organic matter production and exudation that markedly increased along the eutrophication gradient of lakes. [V.sub.max] of alkaline phosphatase, aminopeptidase, and nonspecific esterase showed significant correlation with the TSI of the studied lakes. Protozoans removed a significant portion of bacterial production, i.e., from ~20% to 75-85% of newly produced bacterial biomass was simultaneously consumed by protozoans along the eutrophication gradient. These observations suggest that the importance of protozoan grazing on bacteria on regulation of bacterial production depends on lake productivity. The general working hypothesis that the intensity of microbial processes of organic matter can be tightly coupled to increasing eutrophication was proven in these studies.

We propose a new operational definition for summer phytoplankton blooms based on station-specific chlorophyll a distributions. This definition has been applied to a large monitoring data set from the Kattegat, a shallow marginal sea affected by man-induced eutrophication, in order to describe spatial and temporal variations of summer blooms as well as their underlying cause. Blooms were associated with relatively higher salinity, nutrient concentrations, and wind speed prior to observation as well as relatively lower temperature, which suggests that entrainment and the upwelling of nutrient-rich bottom water are the main causes of summer blooms. The station-specific frequency of blooms was, on average, 8.7%, with higher frequencies in the frontal area toward the Skagerrak as well as in the shallow western area. Blooms generally had a limited extent in localized regions, and we suggest that blooms could be initiated in hydrodynamically active regions and subsequently transported to other parts of the Kattegat. There were no trends in the bloom frequency over the study period (1989-1999), but interannual variations were linked to external nitrogen input, through upwelling of regenerated nutrients, which suggests that blooms are increasing in frequency over longer timescales. Summer blooms were dominated by large species (Ceratiurn spp. and Rhizosolenia spp.), which indicates that size-dependent grazing affects the development and probably also the fate of summer phytoplankton blooms. This bloom definition can be generally applied to other data sets to investigate the properties of high phytoplankton biomass.

We assess population dynamics of picophytoplankton groups ([less than or equal to] 2 [micro]m diameter; Prochlorococcus, Synechococcus, and picoeukaryote) at a Pacific Ocean coastal site in the Southern California Bight. Weekly sampling (August 2000 to January 2002), dilution experiments, and flow cytometric analysis were combined with an instrument-specific calibration for cell size determination, allowing biovolume and carbon biomass estimation. Synechococcus was almost always numerically dominant, accounting for 60 [+ or equal] 12% of the total picoplankton cells over time. It had moderately high growth rates (0.52-0.86 [d.sup.-1]) and was subject to low grazing mortality (-0.14 to -0.39 [d.sup.-1]). Prochlorococcus growth and mortality rates were roughly balanced (0.33 [+ or -] 0.14 [d.sup.-1] and -0.36 [+ or -] 0.06 [d.sup.-1], respectively). Picoeukaryotes had the highest growth rates (0.71-1.29 [d.sup.-1]) and were responsible for, on average, 76% of net carbon production (NCP), amounting in up to 32.05 [+ or -] 1.31 [micro]g C [L.sup.-1] [d.sup.-1] produced and 28.31 [+ or -] 2.61 [micro]g C [L.sup.-1] [d.sup.-1] consumed. In order to better define the eukaryotic component of these populations, an isolate was characterized via small subunit rRNA gene sequencing, transmission electron microscopy, and growth experiments and was identified as the prasinophyte Ostreococcus, not previously known to the Pacific Ocean. Our results show that although picoeukaryotes do not stand out as particularly important players in this system on the basis of cell abundance, they dominate in terms of picophytoplankton biomass and trophic transfer potential of carbon in this size class.

We present an analysis of the global impact of microplanktonic grazers on marine phytoplankton and its implications for remineralization processes in the microbial community. The data were obtained by an extensive literature search that yielded 788 paired rate estimates of autotrophic growth ([micro]) and microzooplankton grazing (m) from dilution experiments. From studies in which phytoplankton standing stock was measured in terms of carbon equivalents, we show that the production estimate from dilution experiments is a reasonable proxy (r = 0.89) for production determined by the standard [sup.14]C method. The ratio m:[micro], the proportion of primary production (PP) consumed by micrograzers, shows that microzooplankton consumption is the main source of phytoplankton mortality in the oceans, accounting for 67% of phytoplankton daily growth for the full data set. This ratio varies modestly among various marine habitats and regions, with data averages ranging from 60% for coastal and estuarine environments to 70% for the open oceans, and from ~59% for temperate-subpolar and polar systems to 75% for tropical-subtropical regions. Given estimates for the metabolic requirements of micrograzers and assuming they consume most bacterial production, regionally averaged estimates of the protistan respiration are 35-43% of daily PP for the first level of consumer or 49-59% of PP for three trophic transfers. The estimated contributions of microbial grazers to total community respiration are of the same magnitude as bacterial respiration. Consequently, potential ecosystem differences in micrograzer activity or trophic structure are a large uncertainty for biogeochemical models that seek to predict the microbial community role in carbon cycling from bacterial parameters alone.

Variability in upper ocean optical properties is often driven by changes in the particle pool. We investigated the effects of such changes by characterizing individual particles. For particles in natural assemblages, we used a combination of Mie theory and flow cytometry to determine diameter (D), complex refractive index (n + in'), and optical cross-sections at 488 nm. Particles were grouped into categories of eukaryotic pico/nanophytoplankton, Synechococcus, heterotrophic prokaryotes, detritus, and minerals to interpret variability in concurrently measured bulk inherent optical properties (IOPs) in New England continental shelf waters during two seasons. The summed contributions of individual particles to phytoplankton absorption and particle scattering were close to values for these properties measured independently using bulk methods (87% and 107%, respectively). In surface waters during both seasons, eukaryotic phytoplankton were responsible for the majority of both total particle absorption and total particle scattering. Mineral particles contributed the most to backscattering ([b.sub.b]) in the spring, whereas in the summer both mineral and detrital particles were important. Synechococcus and heterotrophic prokaryotes never contributed more than 14% to IOPs. Our findings emphasize that the measurement of nonliving particles, including detritus and minerals, is necessary for understanding variability in [b.sub.b] in the ocean, an important quantity in the interpretation of satellite ocean color.

In the Gulf of Aqaba, the northeasternmost segment of the Red Sea, phytoplankton blooms are more intense than in other oligotrophic regions (e.g., the Sargasso Sea). In this study, we use multiyear in situ (1988-2000) and Sea viewing Wide Field of View Sensor (SeaWiFS) (1999-2001) chlorophyll a (Chl a) data to describe the dynamics of phytoplankton biomass throughout the Gulf of Aqaba. The temporal pattern of phytoplankton biomass in the Gulf of Aqaba includes a strong spring bloom and a somewhat weaker autumn bloom, the length, intensity, and timing of which vary from year to year. In addition, highly positive west-to-east (W to E) gradients in Chl a were found throughout the Gulf of Aqaba. A corresponding negative gradient in sea surface temperature (SST) obtained from MODerate resolution Imaging Spectroradiometer (MODIS) in 2001 indicates that the gradient of Chl a across the Gulf of Aqaba is an outcome of an Ekman-driven upwelling along the eastern side. Calculations of the upwelling and convective fluxes that are based on meteorological data from Eilat, Israel, indicate that upwelling is comparable to or exceeds convection during much of the year. We present a conceptual model demonstrating how upwelling and convection can either support or oppose each other, thereby jointly controlling mixed-layer depth and the development of phytoplankton blooms. Coastal upwelling plays a larger role in controlling phytoplankton dynamics than was previously thought, and it explains much of the observed spatial and temporal variability in phytoplankton distributions.

Previous mesocosm experiments and field studies have indicated a central role of major nutrients (e.g., N, P, and Si) in regulating phytoplankton abundance and species composition and influencing the rate of uptake of bioactive metals by phytoplankton. We monitored the changes in bioactive metals (Cu, Zn, and Mn) concentration and speciation that occurred over the course of a bloom of the coccolithophore Emiliana huxleyi under three different nutrient treatments. In each treatment, the electrochemically labile fraction of Cu decreased from 1.1 to 0.3 nmol [L.sup.-1] during the formation of the E. huxleyi bloom. This decrease was likely due to the active release of organic ligands by phytoplankton, which shut off as soon as the level of labile Cu reached 0.3 nmol [L.sup.-1] ([Cu.sup.2+] = 0.02 nmol [L.sup.-1]). Organic Zn-binding ligands showed elevated concentrations in bag 1 (the N- and P-replete bag) only. The release of these ligands coincided with a 15-fold increase in the concentration of dead E. huxleyi cells in bag 1 but only a threefold increase in the P- or N-depleted bags. This suggests that the Zn-binding ligands may have originated from dead or decaying E. huxleyi cells. In both the P- and N-limited bags, labile Mn concentrations showed a succession of two peaks and troughs over the 2-week period; these were synchronized---but inversely correlated--with bacterial abundance. Labile Mn showed weaker fluctuations and no clear synchronization with bacterial abundance in the N- and P-replete bag. We hypothesize that the passive release of organic Zn-binding ligands under the nutrient-replete conditions may have helped restrict the detrimental effect of [Zn.sup.2+] on Mn uptake by phytoplankton cells, thus diverting part of the dissolved Mn away from sequestration or oxidation by bacteria.

The availability of different forms of nitrogen in coastal and estuarine waters may be important in determining the abundance and productivity of different phytoplankton species. Although urea has been shown to contribute as much as 50% of the nitrogen for phytoplankton nutrition, relatively little is known of the activity and expression of urease in phytoplankton. Using an in vitro enzyme assay, urease activities were examined in laboratory cultures of three species: Aureococcus anophagefferens Hargraves et Sieburth, Prorocentrum minimum (Pavillard) Schiller, and Thalassiosira weissflogii (Grunow) Fryxell et Hasle. Cultures of P. minimum and T. weissflogii were grown on three nitrogen sources (N[O.sub.3.sup.-], N[H.sub.4.sup.+], and urea), while A. anophagefferens was grown only on N[O.sub.3.sup.-] and urea. Urease was found to be constitutive in all cultures, but activity varied with growth rate and assay temperature for the different cultures. For A. anophagefferens, urease activity varied positively with growth rate regardless of the N source, while for P. minimum, urease activity varied positively with growth rate only for cultures grown on urea and N[H.sub.4.sup.+]. In contrast, for T. weissflogii, activity did not vary with growth rate for any of the N sources. For all species, urease activity increased with assay temperature, but with different apparent temperature optima. For A. anophagefferens, in vitro activity increased from near 0-30[degrees]C, and remained stable to 50[degrees]C, while for P. minimum, increased in vitro activity was noted from near 0-20[degrees]C, but constant activity was observed between 20[degrees]C and 50[degrees]C. For T. weissfloggii, while activity also increased from 0[degrees]C to 20[degrees]C, subsequent decreases were noted when temperature was elevated above 20[degrees]C. Urease activity had a half-saturation constant of 120-165 [micro]g atom N [l.sup.-1] in all three species. On both an hourly and daily basis, urease activity in A. anophagefferens exceeded nitrogen demand for growth. In P. minimum, urease activity on an hourly basis matched the nitrogen demand, but was less than the demand on a daily basis. For T. weissflogii, urease activity was always less than the nitrogen demand. These patterns in urease activity in three different species demonstrate that while apparently constitutive, the regulation of activity was substantially different in the diatom. These differences in the physiological regulation of urease activity, as well as other enzymes, may play a role in their ecological success in different environments.

Mesoscale eddies have been shown to be a common feature of open ocean regions such as the Sargasso Sea. By lifting nutrients up to the lower portions of the euphotic zone, these eddies can cause episodic phytoplankton blooms that can lead to substantial new production. In the Sargasso Sea, it has been estimated that such blooms can account for 35%-50% of annual new production. In the present study, it was shown that a common large diatom Stephanopyxis palmeriana is capable of growing in a low-light environment typical of the bottom 50 m of the euphotic zone at rates sufficiently high to sustain the contemporary estimates of new production that have been attributed to mesoscale eddies. The diatom was grown in laboratory batch experiments at irradiance levels from 11 to 79/[micro]mol photons [m.sup.-2] [s.sup.-1], equivalent to irradiance levels occurring in the Sargasso Sea during the summer at depths from ~100 m (the 1% light level) up to 50 m. Resulting growth rates were compared with growth rates from the literature for similar large diatoms, and a simple model of new production was developed to show the dependency of new production on specific growth rate. Included in the model were other important parameters such as the depth of nutrient incursion resulting from lifting of the thermocline when an eddy passes, the duration of the bloom, and the size of the prebloom diatom population. Using realistic ranges for these parameters, it was evident from the model that there is no physiological constraint on these large diatoms from growing fast enough at very low light levels to meet the new production estimates resulting from eddies.

There is considerable interest in the biogeochemical cycling of cadmium (Cd) and phosphate (P[O.sub.4]) in surface waters, driven in part by the ongoing development of a paleonutrient proxy that utilizes Cd preserved in fossil planktonic foraminifera to determine past P[O.sub.4] utilization efficiencies in ocean surface waters. The present article reports the results of a field study into the effects of Fe limitation on the Cd: P composition of natural assemblages of marine phytoplankton in the Antarctic Zone of the Pacific sector of the Southern Ocean. Iron enrichment to shipboard incubation bottles led to increases in community growth rate and final biomass. After 10.7 d of incubation, the climax community was dominated by large diatoms of the genus Fragillariopsis, Pseudonitzschia, and Nitzschia. Direct measurements of phytoplankton metal:P ratios from controlled shipboard experiments indicate that Cd:P, Co:P, and Zn:P ratios decreased from control values with increasing initial dissolved Fe concentrations in the incubation bottles, by factors of ~2-10 at highest Fe additions. We suggest that the effect of Fe limitation on resident diatoms is to decrease growth rate, leading to elevated cellular Cd content. The dissolved Cd:P ratio in iron-limited surface waters of the Southern Ocean may, therefore, respond to the supply of Fe to the resident phytoplankton community, which has implications for the developing paleonutrient proxy. We suggest that the biological uptake of Cd and P is independent of the dissolved Cd:P[O.sub.4] ratio. As a consequence, the results argue against the use of empirical Rayleigh fractionation models or models with fixed phytoplankton uptake ratios to account for regional variability in surface water dissolved Cd:P[O.sub.4].

During the 1995 monsoons, phytoplankton biomass was low over large areas of the Arabian Sea in spite of often high concentrations of inorganic nutrients, characteristics typical of high-nutrient, low-chlorophyll (HNLC) areas. The objective of this study, a part of the Arabian Sea U.S. JGOFS program, was to elucidate controls of phytoplankton biomass and community structure in these areas, using primarily pigment-based methods. Unlike other HNLC areas, phytoplankton biomass and growth rates in the monsoonal Arabian Sea were not likely limited by the availability of trace nutrients but rather controlled by grazers. Evidence for this conclusion is high concentrations of iron, the nutrient likely limiting phytoplankton biomass in other HNLC areas; phytoplankton growth rates clearly not limited by concentrations of inorganic nutrients; blooms during grow-out experiments in the absence of mesozooplankton; rates of microzooplankton and mesozooplankton grazing that approached rates of primary production; and the absence of blooms in situ in spite of persistently high concentrations of inorganic nutrients. A more detailed look at shifts of phytoplankton community structure with increasing levels of phytoplankton biomass, in situ and during grow-out experiments, shows that diatoms were the only group of organisms capable of blooming in the absence of mesozooplankton, i.e., their abundance in situ was likely controlled by mesozooplankton grazing. Changes in the abundance of different picoautotroph taxa with increasing levels of phytoplankton biomass suggest that their biomass was controlled within tight bounds by microzooplankton grazing, rather than by the availability of resources. Thus, these results are consistent with the hypothesis that phytoplanton abundance and community structure in the HNLC areas of the monsoonal Arabian Sea are controlled by top-down forces, grazing, rather than bottom-up forces, availability of resources.

Satellite-derived chlorophyll estimates from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Coastal Zone Color Scanner (CZCS), a large archive of in situ near-surface chlorophyll data, and satellite sea surface temperature (SST) measurements were used to quantify spatial and seasonal variability of near-surface chlorophyll and SST in middle shelf to slope waters off the coast of the U.S. Northeast. The results of empirical orthogonal function (EOF) analysis on normalized monthly fields (after temporal and spatial means were removed) show that all three chlorophyll climatologies have similar mode 1 temporal and spatial patterns in these waters. Mode 1, which explains about half of the total variability in monthly climatological images, shows that shelf waters in the Gulf of Maine (GOM) are out of phase with the mid-Atlantic bight (MAB), with seasonally high chlorophyll concentrations in winter in the MAB and in summer in the GOM. The three chlorophyll climatologies begin to differ at higher modes (modes 2 and 3), although SeaWiFS and in situ climatologies keep similar features through mode 3. Higher modes in both SST and chlorophyll are related to the effects of tidal mixing on Georges Bank, differences in seasonal stratification between the southwestern and northeastern GOM, and the importance of the spring bloom in MAB outer shelf waters and western GOM. SST patterns during the CZCS and SeaWiFS eras are very similar, and this indicates that the observed differences between results obtained with these two sensors are probably not caused by differences in physical processes during the two satellite eras.

Research is presented on phytoplankton growth for four regions of the North Pacific Ocean, including the Bering Sea, the Western Gyre, the Aleutian Trench and the Gulf of Alaska gyre. Growth rates for picoplankton as determined by flow cytometric analysis were typically lower than growth rates for the total phytoplankton community.

Natural populations of diatoms were incubated for 4-12 h with [H.sup.13]C[O.sub.3.sup.-1]. The production of particulate and dissolved fractions of organic carbon and neutral aldoses (NAld) was followed by gas chromatography/mass spectrometry. The extracellular production rate of dissolved organic carbon (DOC) ranged from 4.1% to 6.4% of total (particulate and dissolved) production rate. Glucose was a major component of the excreted dissolved neutral aldoses (DNAld), and galactose, rhamnose, fucose, xylose, and mannose were found as secondary components of the excreted DNAld. The comparison of NAld composition with cellular products suggests that glucose in the excreted DNAld is composed mainly of storage glucan. On the other hand, the high ratios of dissolved production to total (dissolved and particulate) production of galactose, rhamnose, fucose, xylose, and mannose probably reflect the active excretion of heteropolysaccharides by diatoms. By assuming steady-state concentrations, turnover rates of DOC and DNAld can be estimated from the phytoplankton production of [sup.13]C-labeled material. The estimated turnover rates of DNAld through phytoplankton photosynthesis is 4.2-5.1 times higher than that of total DOC, which indicates the bioreactive nature of DNAld. This high turnover rate of DNAld mainly resulted from the high turnover rate of glucose, and it is likely that dissolved glucan is important as a carbon and energy carrier in the marine food web. The fact that the turnover rates of DNAld, which is considered to constitute heteropolysaccharides, are lower than that of glucose, would suggest that heteropolysaccharides are more resistant to biological degradation than glucan.

Research on the Agulhas Current, located between the Indian and Atlantic Oceans, indicates that seasonal and spatial variation in chlorophyll distribution can be attributed to changes in the strength of subtropical gyre and cross-frontal mixing, changes in density of the juxtaposed fronts, and changes in light availability and mixed-layer depth.

Dimethylsulfide (DMS) released from phytoplankton produces cloud condensation nuclei in the marine troposphere, thereby providing a climate-regulating mechanism by increasing cloud albedo. A 200-k.y.-long sedimentary record from the equatorial Atlantic of DMS-producing phytoplankton suggests that DMS production increased during the glacial stages, thus potentially working to further cool the glacial world. Important factors limiting the operation of this possible feedback mechanism for planetary cooling may be the strength of trade winds and the availability of fine, iron-rich dust needed to fertilize growth of DMS producers. Keywords: dimethylsulfide, phytoplankton, climate-regulation, equatorial Atlantic.

The phytoplankton community appears to have a structure that is very resistant to change, according to a study conducted in the Sargasso Sea off Bermuda. The biomass of phytoplankton varied over the year, reaching a maximum in the winter, but there was no change in composition of the eucaryotic phytoplankton community or the relative biomass of cyanobacteria in the surface layer. The biomass and primary production distribution across five size classes of phytoplankton also did not vary. Cyanobacteria on the surface layer had higher growth rates in the winter.

Intracellular structures of unicellular marine phytoplankton affect the angular distribution of total scattered light intensity, especially for the medium range of scattering angles. Experiments were conducted with living cultures of the phytoplankton species Chlorella vulgaris, Chlorella kesleri and Chroococcus minor, and with cultures treated chemically by boiling in 5% NaOH and physically by ultrasound. The treatments change the cells' internal structures and the scattering matrix functional also changes.

A reassessment of the relationship between the particle scattering coefficient b sub p and the chlorophyll concentration has been conducted using a large dataset of recent measurements in case 1 waters, in the open ocean. The relationship is nonlinear over the whole range of chlorophyll concentration. The relationship still shows a large amount of noise, however, despite the greater accuracy of measurements. Studying the deep layer and the near-surface layer separately is recommended.

Information regarding the feeding demands and the effectiveness of filter feeders have been used to explain variations in plankton biomass. In a study involving hydrodynamic conditions in a Danish fjord, it was observed that phytoplankton biomass variations can be explained by the interaction between hydraulic processes and filter-feeding organisms. This phenomenon is of utmost importance in marine monitoring programs.

The binding of copper ions (Cu2+) to the cell surfaces of the marine alga Dunaliella tertiolecta and to organic matter excreted by the alga in seawater was examined as a function of salinity, pH and temperature. The results showed that the rate of Cu2+ uptake occur in two steps, which correlates with high- and low-affinity binding sites on the algal surface. High-affinity binding constant had the same order of magnitude as the binding capacity of algal exudates. Changes in salinity, pH and temperature caused changes in Cu2 adsorption and amount Cu2+ in complexed form.

Spatial diversity gradients are a pervasive feature of life on Earth. We examined a global ocean circulation, biogeochemistry, and ecosystem model that indicated a decrease in phytoplankton diversity with increasing latitude, consistent with observations of many marine and terrestrial taxa. In the modeled subpolar oceans, seasonal variability of the environment Led to competitive exclusion of phytoplankton with slower growth rates and Lower diversity. The relatively weak seasonality of the stable subtropical and tropical oceans in the global model enabled Long exclusion time scales and prolonged coexistence of multiple phytoplankton with comparable fitness. Superimposed on the decline in diversity seen from equator to pole were 'hot spots' of enhanced diversity in some regions of energetic ocean circulation, which reflected Lateral dispersal. 10.1126/science.1184961

A method to measure the in situ growth rate of phytoplankton was used to measure cell cycle distributions of prochlorophytes in the north western Mediterranean Sea. The method involves measurement of cell DNA by flow cytometry. Measured DNA distributions are then used to estimate the cell fraction in S + G2, which is analogous to the frequency of dividing cells index used in bacterial populations. The results showed that nitrogen and light are the major factors affecting in situ cycle distributions in the prochlorophytes. The in situ growth rate is essential to an understanding of oceanic production.

Nitrate uptake by phytoplankton in the euphotic layer of the Gulf Stream warm-core ring 82B (WCR 82B) was investigated. Nitrate uptake was assessed using chemiluminiscent nitrate analysis of nitrogen-15-labeled nitrate. Nitrate turnover times ranging from four hours near the surface to 150 hours at the top of the nitracline were obtained from calculations from six profiles taken during two days of observation. The particular characteristics of WCR 82B, which was formed in Feb 1982, caused nitrate concentrations that were below the detection limits of conventional automated nutrient analysis, but exceeded the limit of detection of the chemiluminiscent technique.

The 100-meter thick deep scattering layer in the area of the the central hydrothermal vent field on Endeavor ridge was investigated. Analyses of surface-to-bottom profiles of temperature, salinity, light attenuation and acoustic backscatter intensity showed that the deep scattering layer consists of high concentrations of macrozooplankton over plume-contaminated bottom waters. Maximum species richness was estimated at 83 taxa and zooplankton biomass was estimated to be 21 milligrams/cubic meter. Zooplankton concentrate above the plume-contaminated waters to take advantage of abundant chemosynthetic bacteria and other nutrients carried upwards by the plume.

Mesoscale patches of high phytoplankton concentration, indicated by chlorophyll (Chl) concentration, within the deep Chl maximum were revealed by gyre-scale high resolution sections sampled by towing in the North Atlantic Ocean during summer. Mesoscale patches were characterized as having Chl concentrations that exceed that of spring phytoplankton blooms. They occur at sites where local upwelling events are indicated by the distributions of hydrographic variables or where hydrographic conditions favor dynamic upwelling events.

Research was conducted to examine bacterioplankton heterotrophic production, abundance and biomass in the northwestern Mediterranean during summer stratification in four successive stratification seasons. The main objectives were to derive accurate estimates of bacterial biomass and heterotrophic production and to investigate the relationship between bacterial and phytoplankton parameters at various time and space scales. Results indicate that summer values for biomass were similar for both northern and eastern Mediterranean but production estimates were higher in the northern portion.

The study focuses on the impact on phytoplankton growth and phytoplankton distributions due to submesoscale vertical motions by modeling a phytoplankton population in a baroclinically unstable submesoscale eddy. Results show that plankton distributions depend strongly on the light intensity and local vertical transport.

The response of phytoplankton community and carbon production and export to variations in physical and biochemical processes in the Cold Tongue region of the equatorial Pacific Ocean at 0N, 140 W is investigated using a one dimensional multi-component lower trophic level ecosystem model.

The phytoplankton community response to the variations in physical forcing and biological processes in the Cold Tongue region of the equatorial Pacific Ocean at 0N, 140W is investigated.

Chlorophyll has long functioned as the prominent field metric for phytoplankton biomass, but its variability can be strongly influenced by physiological shifts in intracellular pigmentation in response to changing growth conditions. Hence an alternative measure of phytoplankton carbon biomass is needed that is insensitive to intracellular changes in pigmentation. The most likely optical candidate to fulfill this requirement is the particular beam attenuation coefficient [c.sub.p].

The Geochemistry, Phytoplankton and color of the Ocean programme is set up to clarify and understand the variability of phytoplankton composition on large spatial scales under a multi-year sampling strategy. The results supported that tropical areas have low seasonal variability and are characterized by relatively high divinyl-chlorophyll a and zeaxanthin concentration and that the variability is strongest at high altitudes where phytoplankton biomass have large seasonal cycles.