Showing papers in "Limnology and Oceanography in 2002"

Stable carbon and nitrogen isotope composition of aquatic and terrestrial plants of the San Francisco Bay estuarine system

Abstract: We report measurements of seasonal variability in the C-N stable isotope ratios of plants collected across the habitat mosaic of San Francisco Bay, its marshes, and its tributary river system. Analyses of 868 plant samples were binned into 10 groups (e.g., terrestrial riparian, freshwater phytoplankton, salt marsh) to determine whether C-N isotopes can be used as biomarkers for tracing the origins of organic matter in this river‐marsh‐estuary complex. Variability of d 13 C and d 15 N was high (;5‐10‰) within each plant group, and we identified three modes of variability: (1) between species and their microhabitats, (2) over annual cycles of plant growth and senescence, and (3) between living and decomposing biomass. These modes of within-group variability obscure any sourcespecific isotopic signatures, confounding the application of C-N isotopes for identifying the origins of organic matter. A second confounding factor was large dissimilarity between the d 13 C-d 15 N of primary producers and the organicmatter pools in the seston and sediments. Both confounding factors impede the application of C-N isotopes to reveal the food supply to primary consumers in ecosystems supporting diverse autotrophs and where the isotopic composition of organic matter has been transformed and become distinct from that of its parent plant sources. Our results support the advice of others: variability of C-N stable isotopes within all organic-matter pools is high and must be considered in applications of these isotopes to trace trophic linkages from primary producers to primary consumers. Isotope-based approaches are perhaps most powerful when used to complement other tools, such as molecular biomarkers, bioassays, direct measures of production, and compilations of organic-matter budgets.

A comment on the use of flushing time, residence time, and age as transport time scales

Abstract: Applications of transport time scales are pervasive in biological, hydrologic, and geochemical studies yet these times scales are not consistently defined and applied with rigor in the literature. We compare three transport time scales (flushing time, age, and residence time) commonly used to measure the retention of water or scalar quantities transported with water. We identify the underlying assumptions associated with each time scale, describe procedures for computing these time scales in idealized cases, and identify pitfalls when real-world systems deviate from these idealizations. We then apply the time scale definitions to a shallow 378 ha tidal lake to illustrate how deviations between real water bodies and the idealized examples can result from: (1) non-steady flow; (2) spatial variability in bathymetry, circulation, and transport time scales; and (3) tides that introduce complexities not accounted for in the idealized cases. These examples illustrate that no single transport time scale is valid for all time periods, locations, and constituents, and no one time scale describes all transport processes. We encourage aquatic scientists to rigorously define the transport time scale when it is applied, identify the underlying assumptions in the application of that concept, and ask if those assumptions are valid in the application of that approach for computing transport time scales in real systems.

Utilization of different nitrogen sources by the marine cyanobacteria Prochlorococcus and Synechococcus

Abstract: Prochlorococcus is the most abundant phytoplankter throughout the photic zone in stratified marine waters and experiences distinct gradients of light and nitrogen nutrition. Physiologically and genetically distinct Prochlorococcus ecotypes partition the water column: high-B/A (low-light adapted) ecotypes are generally restricted to the deep euphotic zone near or at the nitracline. Low-B/A (high-light adapted) ecotypes predominate in, but are not limited to, NO -depleted surface waters, where they outnumber coexisting Synechococcus populations. The niche parti2 3 tioning by different Prochlorococcus ecotypes begs the question of whether they also differ in their nitrogen (N) utilization physiology, especially with respect to NO utilization. To explore this possibility, we studied the capa2 3 bilities of different Prochlorococcus and Synechococcus strains to grow on a variety of N sources. We found that all the isolates grew well on NH and all were capable of urea utilization, occasionally at a lower growth rate. 1 4 None of the Prochlorococcus isolates were able to grow with NO . Four high-B/A Prochlorococcus isolates grew 2 3 on NO , but all others did not. Whole genome analysis of the low-B/A Prochlorococcus MED4 revealed that the 2 2 genes required for NO uptake and reduction were absent. The genome of the high-B/A ProchlorococcusMIT 9313 2 3 also lacked the NO utilization genes but has homologs of genes required for NO utilization consistent with its

Assessment of the relationships between dominant cell size in natural phytoplankton communities and the spectral shape of the absorption coefficient

Abstract: Size-fractionated chlorophyll concentration and phytoplankton absorption spectra were compared for a wide variety of natural communities. We found that, in general, when phytoplankton abundance increases, larger sizeclasses are added incrementally to a background of smaller cells. Natural phytoplankton communities from surface waters were explicitly characterized according to their dominant cell size and taxonomic group, and the relationships between this classification and the spectral shape of the phytoplankton absorption coefficient for the whole assemblage was described. By specifying the cell size of the dominant organism (pico-, ultra-, nano-, or microplankton), more than 80% of the variability in spectral shape of the phytoplankton absorption coefficient from 400 to 700 nm could be explained. This is a result of the strong covariation of the size of dominant organisms and several factors controlling the spectral shape of the phytoplankton absorption coefficient, such as pigment packaging and concentration of accessory pigments. Consequently, the shapes of phytoplankton absorption spectra can be reproduced using a spectral mixing model, where two spectra, representing the normalized phytoplankton absorption coefficients for the smallest and the largest cells found in our data set, are combined additively, using a single parameter to specify the complementary contribution of each. The differences between reproduced and measured spectra contain taxonomic and physiological information. This parameterization provides a simple tool for extracting ecological information from optical measurements. It can also be used in sensitivity analyses to describe the influence of the dominant cell size of phytoplankton on optical properties of surface waters. Changes in phytoplankton species composition are a central feature of marine ecosystem dynamics. Description and prediction of these changes are important goals to many fields in oceanography. In recent years, great effort has been made to understand how changes in phytoplankton species composition can affect optical properties of surface waters (e.g., Morel 1997; Kahru and Mitchell 1998; Stuart et al. 1998; Stramski et al. 2001). A major application of these results is the use of in situ optical instruments or remote sensing to observe variability of phytoplankton continuously or synoptically. This is a complicated topic because phytoplankton communities include species differing in size, shape, external and internal structures, and pigment composition. All these characteristics influence their interaction with the light field to some degree, so many factors must be considered to completely describe the optical properties of different communities of phytoplankton. A central goal is 1

Compositional changes in free-living bacterial communities along a salinity gradient in two temperate estuaries

Abstract: There is now clear evidence for major differences in heterotrophic bacterial composition between freshwater and marine ecosystems. A fundamental question that remains unresolved is whether the compositional succession occurs by a gradual replacement of the major phylotypes, or whether there are drastic compositional shifts in discrete areas along the gradient from one system to another. The aim of this study is to examine the change in the phylogenetic composition of the free-living bacterioplankton across the salinity gradient in both the Choptank and the Pocomoke Rivers, both subestuaries of the Chesapeake Bay, using fluorescence in situ hybridization (FISH). The proportion of free-living cells that could be detected using FISH varied widely (3 to 80%), and the majority of cells detected belonged to bacteria, whereas archaea represented less than 3%. The distribution of the different members of bacteria exhibited a clear pattern along the salinity gradient, with the dominance of a-proteobacteria in the lower, saltwater regions and of b-proteobacteria in the upper freshwater regions. The cytophaga-flavobacterium cluster prevailed in the turbidity maximum located in the middle estuary, and the g-proteobacteria showed sporadic peaks along the transect that may have been related to local events. The replacement of a- for b-proteobacteria along the salinity gradient was not gradual but rather occurred rapidly within the turbidity maximum region of the estuaries where the fresh and salt waters mix. The pattern of phylogenetic succession was linked to the development of the turbidity maximum, which is related to rainfall and the ensuing hydrological conditions. There is indication that the phylogenetic succession is accompanied by strong physiological changes in the bacterial assemblage, expressed as a decline in bacterial growth efficiency and community production. The transition appears not to be simply the result of conservative mixing of riverine and estuarine bacterial assemblages, but rather appears to involve cell inactivation and/or death.

Iron-sulfur-phosphorus cycling in the sediments of a shallow coastal bay: Implications for sediment nutrient release and benthic macroalgal blooms

Abstract: We conducted a study to determine the seasonal relationship between iron, sulfur, and phosphorus in the upper sediments and pore waters of a shallow intercoastal bay. From April 1999 to September 2000, sediment cores were collected from Rehoboth Bay, Delaware. Analyses of the sediments in the upper 4 cm revealed that redox conditions controlled Fe-S-P concentrations in the sediments, pore waters, and overlying water. Monthly sampling showed a marked decrease in the reactive solid phase P pool (ascorbate leachable fraction, ASC-P) and sharp increases in soluble P (measured as PO ) in pore waters and overlying waters, as the conditions became more reducing through32 4 out the summer months. These changes were paralleled by decreases in the amorphous Fe(III) (ascorbate leachable fraction, ASC-Fe) and total Fe(III)oxyhydroxide pools [dithionite extracted fraction, Fe(III) oxide] and increases in solid FeS/FeS2. The release of soluble P from sulfidic sediments to oxygenated overlying waters only occurred during periods of solid FeS/FeS2 production, which indicates that Fe(III) oxides act as a barrier to diffusive P flux. During these anoxic conditions, the regenerative P appears to induce secondary benthic algal blooms and promotes eutrophication in these inland bays through late summer. By the late fall and into early spring, sulfide production diminished and oxic conditions were reestablished as indicated by increases in solid amorphous and crystalline Fe(III) oxides and decreases in FeS/FeS2 concentrations. During this period, increasing ASC-Fe concentrations correlated with increases in ASC-P concentrations and decreases in pore-water PO . The seasonal correlations 32 4 between Fe-S-P indicate that Fe redox chemistry controls sediment P flux to the overlying water column.

Nitrogen Fixation and Nitrogen Isotope Abundances in Zooplankton of the Oligotrophic North Atlantic

Abstract: Deep-water nitrate is a major reservoir of oceanic combined nitrogen and has long been considered to be the major source of new nitrogen supporting primary production in the oligotrophic ocean. 15N:14N ratios in plankton provide an integrative record of the nitrogen cycle processes at work in the ocean, and near-surface organic matter in oligotrophic waters like the Sargasso Sea is characterized by an unusually low 15N content relative to average deep-water nitrate. Herein we show that the low dΔ15N of suspended particles and zooplankton from the tropical North Atlantic cannot arise through isotopic fractionation associated with nutrient uptake and food web processes but are instead consistent with a significant input of new nitrogen to the upper water column by N2 fixation. These results provide direct, integrative evidence that N2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean.

Bioavailability of DON from natural and anthropogenic sources to estuarine plankton

Abstract: Utilization of dissolved organic nitrogen (DON) from natural (forests) and anthropogenic (animal pastures, urban/ suburban storm water runoff) sources (three sites per source) by estuarine plankton communities was examined in spring, summer, and fall. The proportion of DON utilized ranged from 0 to 73%. Overall, urban/suburban storm water runoff had a higher proportion of bioavailable DON (59% 6 11) compared to agricultural pastures (30% 6 14) and forests (23% 6 19). DON bioavailability varied seasonally; however, the seasonal pattern differed for the three sources. Bacterial production increased linearly with the amount of DON utilized across all sources and seasons; the rate of increase was approximately five times greater per micromole of N as DON used relative to dissolved inorganic N (DIN) used. Although phytoplankton production generally increased with DON addition, the increased production was not correlated with the amount of DON utilized, suggesting that a variable portion of dissolved organic matter (DOM)-N was directly or indirectly available to the phytoplankton. This indicates that phytoplankton production is not a good measure of the amount of bioavailable DON, and measurements of the amount of bioavailable DON based on bacterial responses alone might not reflect N available to phytoplankton. Preliminary seasonal budgets of bioavailable N (DIN plus bioavailable DON) as a function of land use suggest that ;80% of the total dissolved N (TDN) from urban/suburban runoff is bioavailable, whereas a lower proportion (20‐ 60%) of TDN is bioavailable from forests and pastures. N budgets for aquatic ecosystems based on only DIN loading underestimate bioavailable N loading, whereas total N or TDN budgets overestimate bioavailable N inputs.

Determination of limiting polyunsaturated fatty acids in Daphnia galeata using a new method to enrich food algae with single fatty acids

Abstract: A new method that allows the highly reproducible supplementation of free fatty acids to planktonic microalgae was used to investigate the role of particular highly unsaturated fatty acids (PUFAs) in somatic growth limitation of Daphnia galeata feeding on Scenedesmus obliquus or Stephanodiscus hantzschii. No evidence for biotransformation of the supplemented fatty acids into other fatty acids by the algae was found. Using the algal cell itself as a transfer vehicle, the supplemented fatty acids were incorporated by D. galeata. In standardized growth experiments with juvenile D. galeata, growth on S. obliquus was improved by supplementation with the PUFAs a-linolenic acid (a-LA, 18:3n-3), eicosapentaenoic acid (EPA, 20:5n-3), and docosahexaenoic acid (DHA, 22:6n-3), but not by arachidonic acid (ARA, 20:4n-6), which illustrates that PUFAs should not be regarded as a single resource. Corresponding changes in the fatty acid pattern of D. galeata indicated that EPA is the limiting PUFA during growth on S. obliquus and that D. galeata converts DHA and C18-PUFAs into EPA. Growth on S. hantzschii was not improved by supplementation with EPA and ARA but was with a-LA, which indicates that a-LA is the limiting PUFA and that EPA cannot be converted into a-LA. These results suggest that the availability of EPA determines which PUFA is limiting for growth. Because of the ability of the daphnids to convert a-LA into EPA, both PUFAs are substitutable resources under EPA limitation, but because EPA cannot be converted into a-LA, both PUFAs are nonsubstitutable resources under a-LA limitation. In aquatic food webs, the factors that regulate energy transfer between primary producers and consumers are crucial in understanding the transfer of energy across the plant‐ herbivore interface. It has been clear for many years that variation in the carbon transfer efficiency from primary to secondary production is quite large. This variation can be attributed to variation in food quality, but the determinants of food quality might be of a different nature, such as morphology, digestive resistance, toxicity, and nutritional inadequacy. From a nutritional point of view, not all units of carbon are equal. Nutrient-limited algae (in freshwater systems mostly P-limited) are widely accepted to be a food source of low quality (Sterner and Schulz 1998). However, at C : P ratios ,300, food quality for Daphnia might become constrained by factors others than P (Sundbom and Vrede 1997). Unless mineral limitation, toxins, or algal morphology constrain the utilization of algal biomass, the quality of algal carbon determines carbon transfer efficiency. Low quality of carbon can be due to a shortage of essential biochemicals in the diet, since such nutrients cannot be synthesized or are synthesized by a consumer in amounts inadequate to sustain growth. Polyunsaturated fatty acids (PUFAs, fatty acids with two or more double bonds) are essential for many vertebrates and invertebrates (Stanley-Samuelson et al. 1988), and the importance of PUFAs in freshwater zooplankton nutrition has recently been articulated (Gulati and DeMott 1997). Re1

Interpreting stable isotopes in food webs: Recognizing the role of time averaging at different trophic levels

Abstract: Lake Tanganyika, East Africa, has a simple pelagic food chain, and trophic relationships have been established previously from gut-content analysis. Instead of expected isotopic enrichment from phytoplankton to upper level consumers, there was a depletion of 15 N in August 1999. The isotope signatures of the lower trophic levels were an indicator of a recent upwelling event, identified by wind speed and nitrate concentration data, that occurred over a 4-d period several days prior to sampling. The isotope structure of the food web suggests that upwelled nitrate is a nutrient source rapidly consumed by phytoplankton, but the distinctive signature of this nitrate is diluted by time averaging in the upper trophic levels. This time averaging is a consequence of the fact that the isotopic signature of an organism is related to variable nitrogen sources used throughout the life of the organism. This study illustrates the importance of recognizing differences in time averaging among trophic levels.

Changes in primary productivity and chlorophyll a in response to iron fertilization in the Southern Polar Frontal Zone

Abstract: EisenEx�the second in situ iron enrichment experiment in the Southern Ocean�was performed in the Atlantic sector over 3 weeks in November 2000 with the overarching goal to test the hypothesis that primary productivity in the Southern Ocean is limited by iron availability in the austral spring. Underwater irradiance, chlorophyll a (Chl a), photochemical efficiency, and primary productivity were measured inside and outside of an iron-enriched patch in order to quantify the response of phytoplankton to iron fertilization. Chl a concentration and photosynthetic rate (14C uptake in simulated in situ incubations) were measured in pico-, nano-, and microphytoplankton. Photochemical efficiency was studied with fast repetition rate fluorometry and xenon-pulse amplitude modulated fluorometry. The high-nutrient low-chlorophyll waters outside the Fe-enriched patch were characterized by deep euphotic zones (63-72 m), low Chl a (48-56 mg m-2), low photosynthetic efficiency (Fv/Fm ~ 0.3), and low daily primary productivity (130-220 mg C m-2 d-1). Between 70 and 90% of Chl a was found in pico- and nanophytoplankton. During the induced bloom, Fv/Fm increased up to ;0.55, primary productivity and Chl a reached the maximum values of 790 mg C m-2 d-1 and 231 mg Chl a m-2, respectively. As a consequence, the euphotic depth decreased to ~41 m. Picophytoplankton biomass hardly changed. Nano- and microphytoplankton biomass increased. In the first 2 weeks of the experiment, when the depth of the upper mixed layer was mostly 80 m.

Influence of simulated bivalve biodeposition and microphytobenthos on sediment nitrogen dynamics: A laboratory study

Abstract: Suspension-feeding eastern oysters, Crassostrea virginica, were once abundant in Chesapeake Bay and may then have exerted top-down control on phytoplankton and also reduced turbidities, thereby increasing light available to benthic plants. Alternatively, oysters may have simply recycled inorganic nutrients rapidly back to the water column, with no long-lasting reduction in phytoplankton biomass resulting from oyster feeding activity. To help distinguish between these scenarios, we explored changes in nitrogen fluxes and denitrification in laboratory incubations of sediment cores held under oxic and anoxic conditions in response to loading by pelletized phytoplankton cells, an experimental analog for oyster feces and pseudofeces. When organics were regenerated under aerobic conditions, typical of those associated with oyster habitat, coupled nitrification‐denitrification was promoted, resulting in denitrification of;20% of the total added nitrogen. In contrast, under anoxic conditions, typical of current summertime conditions in main-stem Chesapeake Bay where phytoplankton is microbially degraded beneath the pycnocline, nitrogen was released solely as ammonium from the added organics. We postulate that denitrification of particulate nitrogen remaining in oyster feces and pseudofeces may enhance nitrogen removal from estuaries. In aerobic incubations with sufficient light (70 mmol m 22 s 21 ), a benthic microalgal/cyanobacterial community grew that not only absorbed the inorganic nitrogen released from the added organics but also fixed N 2. This result suggests that an ecosystem dominated by benthic primary production may develop in shallow waters when reduced turbidity associated with bivalve feeding increases light penetration to a level that can sustain benthic microalgal production.

Relating nutrient uptake with transient storage in forested mountain streams

Abstract: Streams control the timing and delivery of fluvial nutrient export from watersheds, and hydraulic processes such as transient storage may affect nutrient uptake and transformation. Although we expect that hydraulic processes that retain water will increase nutrient uptake, the relationship between transient storage and nutrient uptake is not clear. To examine this relationship, we injected a conservative tracer and nutrients (ammonium and phosphate) into 13 streams for a total of 37 injections at Hubbard Brook Experimental Forest (HBEF), New Hampshire. Transient storage was estimated by fitting conservative solute data to a one-dimensional advection, dispersion, transient storage model. To correct for variation in depth and velocity among streams, we considered nutrient uptake as a masstransfer coefficient (Vf), which estimates benthic demand for nutrients relative to supply. Transient storage decreased with increasing specific discharge (discharge per unit stream width). Transient storage explained only 14% of variation in ammonium Vf during the entire year and 35% of variation during summer months. Phosphate uptake was not related to transient storage, presumably because P uptake is predominantly by chemical sorption at HBEF. At HBEF, surface water pools can store water but were not modeled as such by use of the transient storage model. These pools were probably not important areas of nutrient uptake; further variation in the relationship between nutrient uptake and transient storage may be explained by biological demand.

Synchronous variation of dissolved organic carbon and color in lakes

Abstract: Temporal variation in dissolved organic carbon (DOC) and water color (light absorption at 440 nm) was measured in 20 lakes in northern Michigan that varied in DOC, pH, morphometry, and relative productivity as indicated by chlorophyll and total phosphorus (TP). Monthly observations during May‐August over 6 yr revealed that DOC and color varied by 6- and 28-fold among lakes and varied substantially through time within lakes. The pattern of temporal variation differed among years but was synchronous among lakes. Of the 190 possible correlations among lake time series, most were positive for DOC (158) and color (160), and 50% of the positive correlations were significant (P , 0.05). Other variables, such as total phosphorus, chlorophyll, and pH, were less synchronous and had fewer significant positive correlations (13‐25%). Temporal dynamics in DOC and color were related to ice-out date as well as spring and summer precipitation. Years of late ice-out and high spring rain were associated with high DOC and color in spring. A summer drought in one year lead to declines in color (up to 40%) and DOC (up to 38%) in nearly all of the lakes. The common temporal dynamics of DOC and color were most likely the result of climatic conditions that affected loading of allochthonous carbon as well as losses due to photodegradation. The variations in DOC were sufficient to cause large changes in light penetration, standing stocks of carbon, and ecosystem metabolism.

The effect of Fe and Cu on growth and domoic acid production by Pseudo-nitzschia multiseries and Pseudo-nitzschia australis

Abstract: Toxigenic pennate diatoms of the genus Pseudo-nitzschia produce domoic acid (DA), the neurotoxin linked to amnesic shellfish poisoning. We investigated how Fe and Cu affect growth and DA production byP. multiseries and P. australis clones isolated from Monterey Bay, California. Growth rates of these species could be inhibited by both Fe limitation (pFe 20.5, m 5 50% mmax) and Cu toxicity (pCu 10.5, m 5 30‐50% mmax). The rate of DA production during the exponential phase was a significant inverse function of cellular growth rates (P , 0.02). Voltammetric measurements of dissolved DA-equivalents in short-term experiments indicated that the cellular production of DA increased from ;5 amol DA cell 21 h 21 under optimal growth conditions to ;40 amol DA cell 21 h 21 under Fe-deficient conditions and;105 amol DA cell 21 h 21 under Cu-stressed conditions. The DA was released to the medium under metal stress conditions, with intracellular DA concentrations decreasing relative to nonstressed cells. Fe uptake rates by P. multiseries were slow compared to other marine diatoms in the absence of dissolved DA but were enhanced threefold (P , 0.03) by adding DA to the medium. DA addition also partially alleviated toxic Cu conditions. Our findings suggest that DA production during exponential growth of these two toxigenic Pseudo-nitzschia species is directly induced by Fe-deficient or Cu stress conditions and that 95% of this DA is actively released into the medium. Changing trace metal conditions in coastal waters therefore may have a profound effect upon intracellular DA concentrations and thereby influence the toxic effect of these harmful bloom events.

Absorbance, absorption coefficient, and apparent quantum yield: A comment on common ambiguity in the use of these optical concepts

Abstract: Several important optical terms, such as “absorbance” and “absorption coefficient,” are frequently used ambiguously in the current peer-reviewed literature. Because these terms are important when deriving other quantities, such as the apparent quantum yield of photoproduction, ambiguity in the application of these concepts leads to results that are difficult or impossible to interpret correctly. Such ambiguity also hinders comparison of results between studies and ultimately harms proper parameterization of numerical models of oceanic processes, as well as the refinement of remote sensing algorithms. We review these concepts and the implications of such ambiguities. A few simple recommendations that follow conventions developed by optical oceanographers are provided to authors dealing with these concepts. In particular, the symbol a is recommended for the absorption coefficient (in Napierian form, m−1), which is also preferred over absorbance (dimensionless) when data are presented; the symbol a is not recommended for absorbance; A should be used with caution because, although it has been used widely for absorbance in photochemistry and photobiology, it has also been used for absorptance in physics and optical oceanography; the term “absorptivity” is not recommended because of conflicting definitions in the current literature; the pathlength should always be given whenever absorbance data are presented; and normalization of photoproduction rates to absorbance or absorption coefficient should be performed only on optically thin samples, unless the inner filter effects are accounted for and corrected.

Sedimentological evidence of an increase in Pseudo-nitzschia (Bacillariophyceae)abundance in response to coastal eutrophication

Abstract: Pseudo-nitzschia H. Peragallo, a marine planktonic diatom genus containing some species capable of producing the neurotoxin domoic acid, is often documented in extremely high concentrations in the northern Gulf of Mexico in the plume of the Mississippi River, especially when river flow and nutrient inputs are high. Limited historical data suggest that Pseudo-nitzschia abundance has increased in the northern Gulf of Mexico since the 1950s. Five sediment cores were collected and analyzed to test whether Pseudo-nitzschia increases coincided with increasing nutrient concentrations in the Mississippi River, thereby suggesting a cause-effect relationship. Pseudo-nitzschia abundance increased in all five cores, correlating significantly with increasing nitrate fluxes and decreasing silicate to nitrate ratios. A diatom dissolution index, based partly on scanning electron microscopic analysis of the fine structure of Pseudo-nitzschia and other lightly silicified diatom valves preserved in the sediment, indicates that the increase in Pseudo-nitzschia abundance appears to reflect a response to eutrophication rather than diagenesis. This study provides evidence for a possible link between coastal eutrophication and harmful algal blooms.

Cobalt limitation and uptake in Prochlorococcus

Abstract: Processes that enable marine phytoplankton to acquire trace metals are fundamental to our understanding of primary productivity and global carbon biogeochemical cycling. Here we show that the abundant marine cyanobacterium, Prochlorococcus strain MED4-Ax, has an absolute cobalt requirement and that zinc cannot substitute for cobalt in the growth medium, as is the case in some other phytoplankton species. When resuspended into fresh medium, uptake of cobalt into the cell occurs as free cobalt (Co 21 ). In contrast, cultures augmented with conditioned medium assimilated cobalt significantly faster than those in fresh medium, leading to the hypothesis that Prochlorococcus produced organic cobalt ligands in the conditioned medium. This work suggests that the availability of cobalt might influence the composition of phytoplankton assemblages in the open ocean.

Annual primary production: Patterns and mechanisms of change in a nutrient-rich tidal ecosystem

Abstract: Although nutrient supply often underlies long-term changes in aquatic primary production, other regulatory processes can be important. The Sacramento‐San Joaquin River Delta, a complex of tidal waterways forming the landward portion of the San Francisco Estuary, has ample nutrient supplies, enabling us to examine alternate regulatory mechanisms over a 21-yr period. Delta-wide primary productivity was reconstructed from historical water quality data for 1975‐1995. Annual primary production averaged 70 g C m 22 , but it varied by over a factor of five among years. At least four processes contributed to this variability: (1) invasion of the clam Potamocorbula amurensis led to a persistent decrease in phytoplankton biomass (chlorophyll a) after 1986; (2) a long-term decline in total suspended solids—probably at least partly because of upstream dam construction—increased water transparency and phytoplankton growth rate; (3) river inflow, reflecting climate variability, affected biomass through fluctuations in flushing and growth rates through fluctuations in total suspended solids; and (4) an additional pathway manifesting as a long-term decline in winter phytoplankton biomass has been identified, but its genesis is uncertain. Overall, the Delta lost 43% in annual primary production during the period. Given the evidence for food limitation of primary consumers, these findings provide a partial explanation for widespread Delta species declines over the past few decades. Turbid nutrient-rich systems such as the Delta may be inherently more variable than other tidal systems because certain compensatory processes are absent. Comparisons among systems, however, can be tenuous because conclusions about the magnitude and mechanisms of variability are dependent on length of data record.

Pathways of organic carbon utilization in small lakes: Results from a whole-lake 13C addition and coupled model

Abstract: In many small aquatic ecosystems, watershed loading of organic C exceeds autochthonous primary production. Although this allochthonous organic C has long been thought of as refractory, multiple lines of evidence indicate that substantial portions are respired in the receiving aquatic ecosystem. To what extent does this terrestrial C support secondary production of invertebrates and fish? Do current models adequately trace the pathways of allochthonous and autochthonous C through the food web? We evaluated the roles of allochthonous and autochthonous organic C by manipulating 13 C content of dissolved inorganic C in a small, softwater, humic lake, thereby labeling autochthonous primary production for about 20 d. To ensure rapid and sufficient uptake of inorganic 13 C, we enriched the lake with modest amounts of N and P. We constructed a carbon flow model based on the ambient and manipulated levels of 13

Carbon and nutrient deposition in a Mediterranean seagrass (Posidonia oceanica) meadow

Abstract: The annual depositional flux of carbon (C), nitrogen (N), and phosphorous (P) to the sediments under a northeast Spain Posidonia oceanica meadow was evaluated, and the sources and fate of the material deposited elucidated. The annual deposition of carbon represented 198 g C m 22 yr 21 , 72% of which was derived from the seston and 28% from P. oceanica detritus. The depositional flux was poor in nitrogen (13.4 g N m 22 yr 21 ) and phosphorous (2.01 g P m 22 yr 21 ), although comparable to the nutrient inputs required to support the growth of P. oceanica. Remineralization in the sediment only returned 15. 6gCm 22 yr 21 , yielding a net carbon accumulation of 182 g C m 22 yr 21 . Our results show that in the Mediterranean littoral P. oceanica meadows are important sites of net organic carbon burial, derived from sedimented sestonic particles and seagrass detritus.

Phytoplankton iron limitation in the Humboldt Current and Peru Upwelling

Abstract: We investigated phytoplankton Fe limitation using shipboard incubation experiments in the high-nutrient South American eastern boundary current regime. Low ambient Fe concentrations (;0.1 nM) in water collected from the Humboldt and Peru Currents were supplemented with a range of added Fe levels up to 2.5 nM. Phytoplankton chlorophyll a, photosystem II photosynthetic efficiency, and nitrate and phosphate drawdown increased in proportion to the amount of Fe added. The Humboldt Current algal community after Fe additions included colonial and flagellated Phaeocystis globosa and large pennate diatoms, whereas the Peru Upwelling assemblage was dominated by coccolithophorids and small pennate diatoms. Apparent half-saturation constants for growth of the two communities were 0.17 nM Fe (Humboldt Current) and 0.26 nM Fe (Peru Upwelling). Net molar dissolved Si(OH)4 : NO drawdown ratios were low in both experiments (;0.2‐0.7), but net particulate silica to nitrogen production 2 3 ratios were higher. Fe limitation decreased net NO : PO utilization ratios in the Humboldt Current incubation to 2 32 34 well below Redfield values. Phytoplankton community sinking rates were decreased by Fe additions in the Peru Upwelling, suggesting potential Fe effects on carbon export. Our results confirm that primary producers in the Peru Upwelling/Humboldt Current system can be limited by Fe, but the biological and biogeochemical consequences of Fe limitation differ from Fe-limited California coastal upwelling waters that have been previously studied.

Seasonal and interannual variability in sources of nitrogen supporting export in the oligotrophic subtropical North Pacific Ocean

Abstract: Over timescales of months to years, the export of organic nitrogen from the oceanic euphotic zone (principally as sinking particulate nitrogen, PN) is believed to balance the input of exogenous combined inorganic nitrogen (i.e., export production balances new production). In the oligotrophic North Pacific subtropical gyre, there are two significant sources of new nitrogen: the upward flux of nitrate from deep water and the biological fixation of dissolved N2 in near-surface waters. Because these sources have distinct stable isotopic signatures, we were able to use the total PN and d 15 N measurements of sinking particles and an isotopic mass balance model to deconvolute the relative and absolute contributions of the nitrate flux and nitrogen fixation to the gravitational export of PN. The sinking flux of PN and its isotopic composition both varied widely between 1989 and 2001. Seasonally, N 2 fixation correlated inversely with mixed-layer depth, reaching a maximum in June‐August, whereas nitrate-supported export correlated inversely with sea surface temperature, reaching a maximum in February‐March. These patterns were consistent with summertime increases in diazotroph biomass and water column N2 fixation rates, as indicated by phycoerythrin pigment concentrations and 15 N2 tracer studies. Annually, the relative contribution of N 2 fixation to N export varied from 36 to 69% (mean 5 48%) and showed a significant increasing trend over the period of observation. Although total PN export correlated with the Southern Oscillation Index, the nitrate- and nitrogen fixation‐based components appeared to respond to climate forcing in distinct ways, complicating our interpretation of the mechanisms of climate-induced changes in particle export.

The summer metabolic balance in the epilimnion of southeastern Quebec lakes

Abstract: On the basis of data collected in Quebec lakes, del Giorgio and Peters (1994) and Carignan et al. (2000) have come to opposite conclusions regarding the metabolic balance between heterotrophy and autotrophy in lakes in general. In the present study, epilimnetic oxygen and carbon dioxide saturation was measured in 33 lakes from the St. Lawrence Lowlands region of Quebec to examine the extent of epilimnetic net heterotrophy (i.e., O2:CO2 balance) in lakes of different characteristics. We found that ~75% of the lakes were undersaturated with oxygen and supersaturated with CO2. There was a strong negative relationship between the departures of O2 and CO2 from saturation. What has not been noted elsewhere is that oxygen concentrations were negatively related to dissolved organic carbon (DOC) concentration, and, therefore, metabolic gas balances could be predicted from DOC; a value between 4 and 6 mg L−1 DOC corresponds to metabolic equilibrium. Because most of the lakes in del Giorgio and Peters (1994) had DOC concentrations above this threshold and most lakes in Carignan et al. (2000) were below, their apparently contradictory conclusions can be reconciled within a larger general theory. Contrary to studies elsewhere, however, we found that the degree of oxygen undersaturation increased with lake trophic status, expressed either as total phosphorus or nitrogen concentrations.

Copper toxicity and cyanobacteria ecology in the Sargasso Sea

Abstract: The closely related cyanobacteria Synechococcus and Prochlorococcus have different distributions in stratified water columns in the northern Sargasso Sea. The abundance of Synechococcus is relatively uniform with depth, but Prochlorococcuscell numbers are low within shallow mixed layers and high in and below the thermocline. Because free cupric ion (free Cu 21 ) concentrations are high (up to 6 pM) in shallow mixed layers and lower in deeper water, there is an inverse relationship between Prochlorococcus densities and the free Cu 21 concentration. We explored the possibility of a causal underpinning for this relationship by examining the relative copper sensitivities of Prochlorococcus and Synechococcus in cultures and field populations. Prochlorococcus isolates from both the high- and low‐light adapted ecotypes were inhibited at free Cu 21 concentrations that had no effect on Synechococcus. However, the high‐light adapted strains were more copper resistant than their low‐light adapted counterparts. When copper was added to Prochlorococcus from environments where the in situ free Cu 21 was low (in deeply mixed water columns and below the mixed layer in stratified conditions), net growth rates were substantially reduced and cells arrested in the G1 and early S phases of the cell cycle. Prochlorococcus in shallow mixed layers were less sensitive to copper and were probably members of the copper-resistant high‐light adapted ecotype. Synechococcus were relatively copper resistant across a range of environments in the Sargasso Sea. These observations are consistent with our hypothesis that copper plays a role in cyanobacteria ecology in the Sargasso Sea.

Taxonomic richness of stream benthic algae: Effects of flood disturbance and nutrients

Abstract: We sampled benthic algae monthly for 15 months in 12 New Zealand gravel-bed streams to investigate amongstream differences in algal taxonomic richness and how this might relate to among-stream differences in flood disturbance and nutrient resource regimes. The mean number of benthic algal taxa per month ranged from 9.4 to 21.3 among streams. There were moderate month-to-month fluctuations in richness, partly in response to flood disturbances. Flow perturbations generally reduced richness in streams with unarmored bed sediments but had little or no effect on richness in streams with armored sediments. Richness was moderate within days of floods in most streams but then did not vary significantly for periods of up to 50 d. However, in streams with prolonged periods without bed-moving floods, richness slowly increased after ;100 d, and, in two of these streams, it peaked at 200‐ 300 d before declining. Unexpectedly, there was not a significant pattern in mean monthly richness among the streams as a function of annual flood frequency. Mean richness was strongly negatively related to soluble nutrient concentrations (particularly with soluble inorganic nitrogen [SIN]; R 2 5 0.511, P 5 0.018). However, this was not a direct relationship, because nitrogen-fixing taxa were common in the streams with low SIN, resulting in intermediate to high mat-scale nitrogen concentrations. This possibly shifted these communities to P-limited growth. A combination of disturbance frequency and mat P gave the most parsimonious model of among-stream variations in benthic algal richness (R 2 5 0.635, P 5 0.037). The highest richness occurred in streams with low to intermediate frequencies of flood disturbance (up to 10 bed-moving events yr 21 ) and intermediate to high concentrations of mat P( .0.6% P). We could not define an upper nutrient concentration that negatively affected benthic algal richness (perhaps because none of our streams were highly enriched). We discuss our results in relation to three contemporary models of biodiversity.

Does competition for nanomolar phosphate supply explain the predominance of the cyanobacterium Synechococcus

Abstract: Experimental work during a cruise along a W-E transect in the Mediterranean Sea suggests that (1) orthophosphate concentrations in the upper photic zone show a decreasing trend from the west to the east reaching levels well below 1 nM and (2) microorganisms in the 0.6-2-μm size fraction, probably Synechococcus, have, in addition to high affinity for orthophosphate, significantly higher maximum uptake rates than heterotrophic bacteria or eukaryotic algae. These specific advantages concerning orthophosphate uptake at low (<5 nM) as well as at relatively high (5-25 nM) concentrations could explain both general Synechococcus abundance in P-depleted environments and transient blooms of this species in the open ocean where episodic orthophosphate nanopulse events are likely to occur.

Increased sensitivity to ultraviolet radiation in nitrogen-limited dinoflagellates: Photoprotection and repair

Abstract: Nitrogen (N) limitation significantly increased the sensitivity of photosynthesis to inhibition by ultraviolet radiation (UV) in two estuarine dinoflagellates, Akashiwo sanguinea(5 Gymnodinium sanguineum ) and Gymnodinium (5 Gyrodinium) cf. instriatum. Biological weighting functions (BWFs) and the kinetics of photosynthetic response to UV indicated that the main mechanism for the increase in sensitivity was less efficient repair. A decrease in cell size and in the concentration of the photoprotective mycosporine-like amino acids also elevated sensitivity. The BWFs predict that increased UV-B due to ozone depletion would cause a more than 1.5-fold greater additional inhibition of N-limited compared to nutrient-sufficient dinoflagellates. The BWFs of the N-limited cultures are similar to those measured for natural assemblages of phytoplankton in the Chesapeake Bay under low N availability.

Isotopic evidence for chemosynthesis‐based nutrition of macrobenthos: The lightness of being at Pacific methane seeps

Abstract: The importance of chemosynthetic nutritional pathways was examined for macrofaunal invertebrates (.300 mm) from methane seeps in the Gulf of Alaska (4,413‐4,443 m), on the Oregon margin (590 m), and on the northern California slope [Eel River margin] (520 m) by use of natural abundance stable isotopic data. Seep macrofauna exhibited lighter d 13 C and d 15 N values than those in nonseep sediments, but isotopic signatures varied among seep sites. Macrofaunal isotopic signatures indicated chemosynthetically fixed carbon sources with a significant contribution from methane-derived carbon (MDC) in macrofauna from sediments of pogonophoran fields (averaged 13

Importance of acid polysaccharides for 234Th complexation to marine organic matter

Abstract: In order to study the role of polysaccharides (PS) in the colloidal organic matter (COM) pool for complexing 234 Th, controlled laboratory experiments were carried out to determine the chemical nature of the strong Th(IV) binding to macromolecular organic ligands (.1 kDa). The partition coefficient of 234 Th between marine COM and solution, Kc, is higher than that for any known marine mineral sorbent. PS-enriched fractions of COM had the highest partition coefficient (Kc) of any sorbent for 234 Th. Kc of 234 Th and other metals, including Fe, Mn, Zn, Pb, and Pu, were up to an order of magnitude higher than that for bulk COM. Most importantly, log Kc values correlated linearly with the fraction of PS-enriched carbon ( f PS) of marine COM, as Kc 5 Kc(0) 3 The log Kc value 2.2 f PS 10 . of ;7.9 of the pure PS end-member fraction was very similar to the highest values obtained for model acid PS (log Kc ; 8). A value for the conditional stability constant for Th binding to the pure PS end member of 10 7.8 could be determined from a concentration of strong acid binding sites (with a pKa #3) of 1.3 mmol g 21 COM. Through the novel use of gradient gel electrophoresis (including two-dimensional polyacrylamide gel electrophoresis), the strong Th(IV) binding ligand was shown to be ;13 kDa in size and to have strong acidic functional groups. We propose that the observed variability of OC : 234 Th ratios in suspended, and sinking matter in the ocean might be caused by the variability of PS content.