Showing papers in "Limnology and Oceanography in 1998"

A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature

Abstract: A new regulatory model can describe acclimation of phytoplankton growth rate (p), chlorophyll a : carbon ratio and nitrogen: carbon ratio to irradiance, temperature and nutrient availability. The model uses three indices of phytoplankton biomass-phytoplankton carbon (C), phytoplankton nitrogen (N), and chlorophyll a (Chl). The model links the light-saturated rate of photosynthesis to N: C, requires that Chl a synthesis be coupled to nitrogen assimilation, and includes several regulatory features. These include feedback inhibition of the nitrogen assimilation rate by increases in the N: C ratio, as well as regulation of Chl a synthesis by the balance between light absorption and photosynthetic carbon fixation. The model treats respiration as the sum of the maintenance metabolic requirement and the cost of biosynthesis. In addition, the model can account for accumulation and mobilization of energy reserves (i.e. variability of N: C) and photoacclimation (i.e. variability of Chl : N and Chl : C) in response to variations in irradiance and nutrient availability. The assumptions of the model are shown to be in agreement with experimental observations and the model output compares favorably with data for cultures in balanced and unbalanced growth. The light-, nutrient-, and temperature-dependencies of phytoplankton growth rate have often been modeled separately. Phytoplankton growth rate has been treated as a function of the dissolved nutrient concentration using the Monod equation or as a function of the cellular content (or quota) of limiting nutrient using the Droop equation (McCarthy 1980; Droop 1983; Morel 1987). Under conditions of balanced growth, the Droop and Monod equations are consistent with each other and with Michaelis-Menten nutrient uptake kinetics (Morel 1987). The temperature-dependence of growth rate has been treated as an exponential dependence or an Arrhenius equation, although other functions have also been used (Eppley 1972; Li 1980; Ahlgren 1987). The lightdependencies of growth and photosynthesis rates have been treated by a number of equations, including a modification of the Monod equation, a hyperbolic tangent, and a Poisson function

Atmospheric exchange of carbon dioxide in a low-wind oligotrophic lake measured by the addition of SF6

Abstract: Many freshwater lakes are supersaturated in CO 2 with respect to the atmosphere. This concentration gradient implies a net flux of CO 2 from the water to the air. The actual rate of gas exchange is governed by both this concentration gradient and the gas transfer coefficient, k, To directly measure k, we added the chemically and biologically inert gas, sulfur hexaflouride (SF 6 ), to the epilimnion of Mirror Lake, New Hampshire, a small (15 ha), low-wind softwater lake. k was independent of wind speed over the 50-d summer stratification period and averaged 2.65 ± 0.12 cm h -1 (95% CI; normalized to a Schmidt number of 600); k 800 was better correlated to precipitation events than it was to wind speed. Our data support the idea that gas exchange across the air-water interface is largely independent of wind at low wind speeds. The surface water of Mirror Lake was persistently supersaturated in CO 2 with respect to the atmosphere. During a 3.5-year period the partial pressure of CO 2 in the surface waters of the lake averaged 726 ± 39 μatm (95% CI) and showed substantial seasonal variation (360-2,000 μatm). Diel and day-to-day variation in CO 2 were very small compared to the CO 2 pool. We combined our estimates of k with weekly measurements of the partial pressure of CO 2 to estimate CO 2 gas exchange in the lake. Mirror Lake released from 26 to 50 g C m -2 to the atmosphere each year, depending on the method of calculating k. Atmospheric CO 2 exchange is a large term in the C economy of the lake-the most conservative gas flux estimate is about four times as large as outflow plus seepage of total dissolved inorganic carbon and 1.5 times as large as the export of dissolved organic C from the lake.

Interpretation of measured concentration profiles in sediment pore water

Abstract: A robust numerical procedure for biogeochemical interpretation and analysis of measured concentration profiles of solutes in sediment pore water has been developed. Assuming that the concentration-depth profile represents a steady state, the rate of net production or consumption as a function of depth can be calculated, together with the flux across the sediment-water interface. Three kinds of vertical transport can be included in the analysis: molecular diffusion, bioturbation, and irrigation. The procedure involves finding a series of least square fits to the measured concentration profile, followed by comparisons of these fits through statistical F-testing. This approach leads to an objective selection of the simplest production-consumption profile that reproduces the concentration profile. Because the numerical procedure is optimized with respect to speed, one prediction can typically be done in a few minutes or less on a personal computer. The technique has been tested successfully against analytical solutions describing the transport and consumption of 0, in sediment pore water. In other tests, measured concentration profiles of O,, NO;, , NH:, and ZCO, have been interpreted using the new procedure.

Linking nitrogen in estuarine producers to land-derived sources

Abstract: It is clear that anthropogenic nitrogen inputs from watersheds to estuaries stimulate eutrophication. It has been difficult, however, to explicitly link anthropogenic N entering estuaries to N found in estuarine producers. To explore this link, we compared stable isotope ratios of N in groundwater and producers from the Waquoit Bay watershedestuary system, Cape Cod, Massachusetts. The 615N values of groundwater nitrate within the Waquoit Bay watershed increase from -0.9%0 to + 14.9%0 as wastewater contributions increase from 4 to 86% of the total N pool. As a result, the average S’“N of dissolved inorganic nitrogen (DIN, nitrate + ammonium) received by different estuaries around Waquoit Bay increases from +0.5%0 to +9.5%0. This increase is strongly correlated to increases in S”N of eelgrass, macroalgae, cordgrass, and suspended particulate organic matter. The increase of all producers examined in Waquoit Bay with increasing S15N of DIN in groundwater demonstrates a tight coupling between N contributed to coastal watersheds and N used by primary producers in estuaries. The ability to identify effects of increasing wastewater N loads on 615N of estuarine producers may provide a means to reliably identify incipient eutrophication in coastal waters.

The chemistry, fluxes, and sources of carbon dioxide in the estuarine waters of the Satilla and Altamaha Rivers, Georgia

Abstract: The pH, alkalinity (Alk), and dissolved inorganic carbon (DIC) in estuarine waters of several rivers of Georgia in the southeastern United States are reported. Although they discharge into a narrow area in the South Atlantic Bight, the rivers along the coast of Georgia differ significantly in their drainage area (Piedmont rivers vs. coastal plain rivers), chemical composition (contents of carbonate and humics), and discharge rates. Large differences in pH, DIC, and Alk between these rivers clearly reflect the differences of material inputs to the rivers. Dramatic pH increases in the early stage of mixing (more obvious for the coastal plain rivers) reflect the low buffering capacity of the river waters despite the high content of humic substances. Flux of riverine DIC to the South Atlantic Bight is estimated to be 52.5 X lo9 mol yr-*. We suggest that the presently available world total riverine DIC flux to the ocean could have much uncertainty if the small river fluxes are important. Calculated pC0, values from the pH and DIC measurements are extremely high in low-salinity areas (1,000 to >6,000 patm at salinity 250 mol m-* yr-I at salinity

Relationship between microcystin production and cell division rates in nitrogen‐limited Microcystis aeruginosa cultures

Abstract: Analysis of changes over time in microcystin content of nitrogen-limited Microcystis aeruginosa (Kutzing) Lemmermann batch cultures (strain MASH01-A19) showed that net microcystin production was limited to the phase of growth when cell concentration was increasing. The net microcystin production rate decreased as the specific cell division rate (μ c ) decreased, but, more importantly, the specific toxin production rate (μ MCYST ) decreased at an identical rate to that of μ c when the culture became nitrate-limited. The actual size of the microcystin pool (total culture microcystin concentration) increased while cells were dividing, then remained constant or decreased only slightly during the stationary and death phases, even when the cultures were severely nitrate-starved. These findings demonstrate conclusively that the processes of cell division and microcystin production are tightly coupled under nitrogen-limited cell division. Our findings suggest that microcystin production is controlled by environmental effects on the rate of cell division, not through any direct effect on the metabolic pathways of toxin production. Reevaluation of data presented by others shows this to be the case for two other cyanobacterial species producing nine different microcystins over a wide range of environmental variables. We believe these relationships now provide a unifying view of environmental control of microcystin production in hepatotoxic cyanobacteria. We conclude that there is a direct linear correlation between cell division and microcystin production rates in all microcystin-producing cyanobacteria regardless of the environmental factor that is limiting cell division. We also conclude that microcystin is not a secondary metabolite, as is currently thought, but that it displays many of the attributes of essential intracellular nitrogenous compounds in toxigenic cyanobacteria.

Amino acids and hexosamines as indicators of organic matter degradation state in North Sea sediments

Abstract: Sediment cores from six stations in the eastern North Sea were analyzed for protein amino acids, the nonprotein amino acids beta-alanine and gamma-aminobutyric acid and the hexosamines galactosamine and glucosamine, and bulk parameters (organic carbon, nitrogen, total hydrolyzable amino acids and carbohydrates) in order to establish the degradation state of sedimentary organic matter. The study sites were selected on the basis of their different physical settings and macrofaunal communities so that a broad quality range in the organic matter would likely be covered. To test if the molecular parameters provide a robust matrix for quality determination we integrated our results with complementary literature data ranging from marine source organisms to deep-sea environments. A principal component analysis based on the mole percent contribution of amino acids showed that there are systematic variations in the amino acid spectra as a consequence of degradation of organic matter. Comparison with more established quality parameters such as hexosamines confirmed that amino acids reflect the degradation state of the organic matter. The amino acids glycine, serine, and threonine were enriched in the more degraded material, and others, such as phenylalanine, glutamic acid, tyrosine, leucine, and isoleucine, became depleted with increasing degradation state. Selective preservation of structural compounds (diatom cell walls, chitinous organic matter) vs. preferential breakdown of cell plasma material appears to be the reason for the contrasting behavior of these molecular compounds. Some of the essential amino acids for macrofauna nutrition (arginine, methionine, and histidine) occurred in lower concentrations in the North Sea sediments compared to organism tissue and therefore may be limiting to growth of deposit-feeders. [KEYWORDS: Liquid-chromatographic determination; coastal marine-environment; early diagenesis; surface sediments; sugar composition; suspended matter; picomole range; trap material;deep ocean; particulate]

Production, sedimentation, and isotopic composition of organic matter in Lake Ontario

Abstract: Organic matter and its carbon and nitrogen isotopic composition were measured in sequential sediment trap and core samples from the Rochester Basin of Lake Ontario to evaluate their usefulness in reconstructing historic changes in lake productivity. The greatest flux of organic matter from the epilimnion occurred during late summer and coincided with whiting events, indicating that calcite precipitation is an effective mechanism for sedimenting organic matter. Carbon isotopes of organic matter were low prior to the onset of stratification, increased to maximum values in late summer, and then decreased following fall overturn. This pattern is controlled mainly by the timing of stratification and primary productivity, which preferentially removes 12CO2 from the epilimnion. The physiological effect of decreased carbon isotopic fractionation with decreasing supplies of [CO2]aq may have also contributed to increased δ13CorgC. Nitrogen isotopes showed a seasonal pattern opposite to that of carbon, whereby δ15N values were low during the summer stratified period and high for the remainder of the year. Seasonal variability in δ15NorgN probably reflects changes in the source of sedimented organic particles, which is dominated by isotopically depleted phytodetritus during the stratified period and isotopically enriched organic matter from heterotrophic or detrital sources during the mixed period. A comparison of organic carbon accumulation rates and δ13CorgC between sediment cores collected in 1987 and 1993–1994 confirms earlier predictions that diagenetic processes reduce the mass accumulation of organic carbon in the zone of oxic pore waters, but will not change the δ13CorgC values. All cores analyzed for δ13CorgC display the reproducible pattern of a progressive increase in the 19OOs, peaking in the early to mid-1970s, and then decreasing to the present. This pattern matches the historical trends of phosphorus loading to the basin, suggesting that δ13C of organic carbon is a reliable proxy for paleoproductivity and responds to spring phosphorus supplies in the water column. The δ15N of sedimentary organic matter increased linearly from 1840 to 1960 at a rate of 0.3ppt per decade, and remained relatively constant thereafter except for an increase in the upper few centimeters of sediment. The increase in δ15NorgN reflects a combination of factors, including early forest clearance by Europeans, increased sewering by municipalities after 1940, and increased nitrate utilization as productivity increased in the lower Great Lakes. Increased rates of denitrification in the central basin of upstream Lake Erie from the 1930s to the early 1970s may have also contributed to the rise in δ15NorgN values.

P limitation of heterotrophic bacteria and phytoplankton in the northwest Mediterranean

Abstract: Surface-water microbial populations were investigated in the northwest Mediterranean for possible indicators of phosphate deficiency and limitation. Low phosphorus availability was suggested by short turnover time (min. observed 0.68 h), high-alkaline phosphatase activity (V,,,,, = 28 nM hydrolyzed h-l), subsaturation of phosphate uptake (2.6-9% of V,,,), and high-pulse uptake capacity of added orthophosphate. Based on high pulse uptake capacity and subsaturated uptake in both the >l-pm and in the 0.2-l-pm size fractions, P deficiency is suggested for both phytoplankton and heterotrophic bacteria. P limitation of heterotrophic bacteria was also supported by fast positive responses after phosphate addition in both thymidine incorporation in whole-water samples and increased bacterial cell numbers in predator-free water. No effects were found after addition of carbon or nitrogen sources alone. Combined with other published evidence, we suggest that the growth rates of not only phytoplankton, but also of heterotrophic bacteria, are P limited in this environment in summer. The finding has important implications for the dynamics of accumulation of dissolved organic carbon in the photic zone and thus for the carbon cycle of oceans. The generally oligotrophic state of the Mediterranean Sea has traditionally been explained as a consequence of its antiestuarine circulation; the net evaporation from the Mediterranean produces an inflow of low-nutrient surface water through the Gibraltar and an eastward surface current in the Mediterranean (Redfield et al. 1963). If one combines an assumption of nitrogen-limited growth in the inflowing Atlantic water with the usual assumption of a faster recycling of phosphorus than of nitrogen, one might expect a strong nitrogen limitation in the upper photic zone during Mediterranean summer stratification. Contrary to such an expectation, accumulated experimental data seem to indicate that parts of the Mediterranean are characterized by a nitrate : phosphate ratio below Redfield (Krom et al. 1991). This seems to be particularly prominent in the eastern parts of the Mediterranean (Krom et al. 1991) and in the Adriatic Sea (Vukadin and Stojanski 1976). In an investigation of orthophosphate uptake in surface water from Villefranche Bay on the French Mediterranean coast, Dolan et al. (1995) found relatively short turnover times (down to 1.6 h). Estimates of bioavailable orthophosphate concentrations in this area (Thingstad et al. 1995) are also very low (0.8 nM), indicating, but not proving, that P may be the limiting nutrient also in this part of the western Mediterranean. The underlying biogeochemical mechanisms behind such an apparent shift toward P deficiency seem still to be unknown,

Isotopic composition of nitrate in the central Arabian Sea and eastern tropical North Pacific: A tracer for mixing and nitrogen cycles

Abstract: The δ15N composition of nitrate and N2 gas was measured in the eastern tropical North Pacific (ETNP) and central Arabian Sea (AS) suboxic regions. The δ15N of nitrate increased from 6‰ at 2,500 m to 15‰ at 250–350 m in both regions, while the δ15N of N2 concurrently decreased from 0.6‰ to 0.25‰. The denitrification isotopic fractionation factor (edenit) for each region was estimated using both advection-reaction and diffusion-reaction models. Values for edenit in the ETNP ranged from 25 ± 2 (advection-reaction) to 30 ± 3 (diffusion-reaction). Values for edenit in the central AS varied from 22 ± 3 (advection-diffusion) to 25 ± 4 (reaction-diffusion) using a starting nitrate isotopic composition of 6‰ but were indistinguishable from calculated values from the ETNP when an initial value of 5‰ was employed. Based upon the model results, an average global edenit of 27 ± 3 is proposed for marine suboxic water columns. Isotopic enrichment of nitrate in oxic waters beneath the active denitrification regions was observed and indicates the presence of significant cross-isopycnal ventilation at depth. The isotopic composition of nitrate decreased above 250 m to −80 m, and this pattern is hypothesized to be caused by the input of isotopically light nitrogen from nitrogen fixation in the euphotic zone. A simple isotopic mass balance indicates that a significant percentage of primary productivity in the central AS may be fueled by nitrogen fixation.

Light scattering and chlorophyll concentration in case 1 waters: A reexamination

Abstract: An analysis is presented based on a large dataset (N = 2,787) made up of recent measurements of the beam attenuation coefficient at 660 nm and of the chlorophyll concentration by using the SeaTech transmissometer and the high-pressure liquid chromatography technique, respectively. This analysis, restricted to case 1 waters, aims at reassessing a previous nonlinear relationship established between the particle scattering coefficient, b,, (very close to the particle attenuation coefficient, c,,), and the chlorophyll concentration, [Chl]. As a first result, nonlinearity is fully confirmed over the whole range of oceanic chlorophyll concentration (about 3 orders of magnitude). Despite more accurate measurements, the scatter in this relationship remains large and is actually comparable to that observed within the old dataset. Rather than establishing a single relationship between c,, (or b,,) and [Chl] for the entire upper water column, the deep layer and the near-surface layer (important for remote-sensing application) have been studied separately. This separation has led to two distinct expressions. A more appropriate parameterization is thus proposed when dealing specifically with, and modeling, the near-surface layer. As a consequence, a modified criterion is also suggested with a view to identifying turbid case 2 waters. Understanding or predicting the propagation of radiant energy within a water body requires that the boundary conditions (at the interface and bottom) and the inherent optical properties (IOP) within the medium are both known or prescribed. Strictly speaking, these properties (IOP) within the medium are both known or prescribed. Strictly speaking, these properties comprise the absorption coefficient, a, and the volume scattering function p(0); the scattering coefficient, b, derives from p(0) by integrating over the whole space, and the attenuation coefficient, c, represents the sum of a and b. These last three coefficients are expressed as m--l. The IOP result from the presence in a water body of colored dissolved organic substances and of scattering as well as absorbing particulate matter. In the open ocean, far from notable terrigenous influence, these optically active materials are locally and permanently Acknowledgments OMEX data were made available with the originators’ permission from the EU-MAST OMEX 1 Programme by the British Oceanographic Data center (BODC). The HPLC chlorophyll measurements for this project were undertaken by R. Barlow and R. E C. Mantoura at the Plymouth Laboratory. They are gratefully acknowledged for having made their data available to us. The transmissometer data for the same cruise were calibrated and quality controlled by the BODC. We thank R. Lowry for his help in the control of, and access to, these data. The BOFS data, published on a CD-ROM, were also produced under the BODC, and are duly appreciated here. The HPLC data for the EUMELI and OLIPAC (French JGOFS) cruises and those (unpubl.) for the MlNOS campaign were performed by H. Claustre with the collaboration of J. C. Marty, C. Cailliau, and E Vidussi. We thank them for these data. We also thank H. Claustre for helpful suggestions and discussions on a first draft of this paper: Substantial help with fieldwork and maintenance-calibration of the transmissometers used during the French cruises were provided by D. Tailliez, who is gratefully thanked. We acknowledge and thank our colleagues who worked at the two U.S. JGOFS-WOCE timeseries and during the EqPac cruises or were involved in the related databanks-these colleagues rendered this study feasible through their continuous effort. This work is a contribution to the French JGOFS Programme “Prosope.”

Effects of phosphorus‐deficient diets on the carbon and phosphorus balance of Daphnia magna

Abstract: We used laboratory growth and feeding experiments to study the balance of carbon (C) and phosphorus (P) in Daphnia magna. Daphnia were fed high-concentration mixtures of P-sufficient and P-deficient green algae (Scenedesmus acutus; molar C:P 80 and 900, respectively) or mixtures of P-deficient Scenedesmus and a P-rich cyanobacterium (Synechococcus elongatus; C:P = 60). P-deficient diets resulted in rapid declines in the growth rates of Daphnia and unexpected declines in Daphnia's P-to-dry- weight ratio. P-deficient Scenedesmus and P-rich Synechococcus supported poor growth as sole foods but improved growth in mixtures. A 2-d experiment with a dietary C: P gradient from 120 to 900 revealed a linear relation between Daphnia's final P-to-dry-weight ratio and its growth rate (r(2) = 0.92). Growth in mass (mean +/- SE) ranged from 0.54 +/- 0.01 to 0.17 +/- 0.02 d(-1) whereas final specific P content ranged from 1.47 +/- 0.01% to 1.08 +/- 0.02%. Supporting stoichiometric theory, experiments with dual-labeled (C-14/P-32) Scenedesmus showed that C assimilation efficiency declines as the C:P ratio of the diet increases. Adding unlabeled P-deficient algae reduced the C assimilation efficiency for labeled P-sufficient algae, whereas adding unlabeled P-rich algae improved the C assimilation efficiency for labeled P-deficient algae. C gross growth efficiency (production/ingestion) steadily decreased with declining dietary P. In contrast, P gross growth efficiency exhibited a unimodal pattern with reduced values for both P-rich and strongly P-deficient resources. The unexpected decline with P-deficient diets was apparently due to low but consistent P-release rates by strongly P-limited Daphnia. Mass- balance calculations confirmed that Daphnia was a strong sink for P when resources exhibited intermediate levels of P deficiency. Our results support stoichiometric theory but show that P-deficient diets lead to significant declines in Daphnia's P content. [KEYWORDS: Algal nutrient limitation; mineral limitation scenedesmus-acutus; varying quality; food quality; zooplankton; assimilation; phytoplankton; ingestion; release]

Photochemical reactivity of dissolved lignin in river and ocean waters

Abstract: The photochemical reactivity of dissolved lignin and photobleaching of dissolved organic matter (DOM) were examined in riverine and open-ocean water samples. Approximately 75% of the total dissolved lignin in Mississippi River water was lost during 28 d of incubation in sunlight, mostly due to photooxidation. The remaining fraction of dissolved lignin was much less susceptible to photooxidation. About 90% of the dissolved lignin in river water was present as high-molecular-weight (HMW, >1,000 Dalton) DOM. However, after exposure to sunlight, about 80% of the remaining lignin was present as low-molecular-weight (LMW, <1,000 Dalton) DOM. An absolute increase in concentration of LMW lignin provided direct evidence for the phototransformation of macromolecular DOM to smaller molecules. The composition of riverine dissolved lignin also changed dramatically during photooxidation. The abundance of syringyl relative to vanillyl phenols decreased twofold, while concentrations of vanillic acid relative to vanillin, (Ad/Al)v, increased fourfold. The increase in (Ad/Al)v was found exclusively in the photooxidized LMW DOM. The photochemically altered composition of riverine dissolved lignin was consistent with the composition of oceanic dissolved lignin, The dissolved lignin in HMW DOM from the equatorial Pacific Ocean was highly resistant to photooxidation. These findings suggest that photochemical reactions play a prominent role in determining the composition and reactivity of terrigenous DOM in the ocean,

Nitrogen isotope fractionation during the uptake and assimilation of nitrate, nitrite, ammonium, and urea by a marine diatom

Abstract: Knowledge of the fractionation of nitrogen isotopes by phytoplankton is a key requirement for the calibration of the new δ 15 N paleotracer. An essential part of information required in this calibration concerns the magnitude of isotopic fractionation during the incorporation of N substrates by phytoplankton. To this end, the δ 15 N of batch cultures of Thalassiosira pseudonana grown on nitrate, nitrite, ammonium, and urea was determined. This paper reports the first δ 15 N study of phytoplankton growth on urea (e.g. organic N substrate). The δ 15 N of the particulate nitrogen (PN) collected during the logarithmic growth phase, thus for N-sufficient cells, was lower than the δ 15 N of the source due to kinetic isotope fractionation. With increasing drawdown of the N substrate, the δ 15 N of the accumulating PN increased in accordance with the Rayleigh distillation model. Enrichment factors (e) derived from a least-squares analysis of the accumulated δ 15 N pN data were 5.2 ± 0.2‰, 0.9 ± 0.6‰, 20 ± 1‰, and 0.8 ± 0.6‰ for NO 3 - , NO 2 , NH 4 + , and urea incorporation, respectively. Overall, e values for nitrate incorporation were consistent with field estimates of ∼6‰, and could be used to estimate past relative nitrate utilization as estimated from the δ 15 N of bulk sedimentary organic matter.

An iron limitation mosaic in the California upwelling regime

Abstract: Although Fe limitation is well documented in open ocean high -nutrient, low -chlorophyll (HNLC) areas, little is known about the potential for Fe limitation in coastal environments. We present a series of four Fe addition experiments that demonstrate varying degrees of Fe limitation in the central California coastal upwelling area. Fe concentrations vary widely here ( 8.0 nM) because inputs from rivers and resuspended shelf sediments are unevenly distributed. The biological response to Fe availability is also extremely variable. Fe-replete areas experience extensive blooms of large diatoms and almost complete depletion of nutrients. In slightly Fe-stressed areas, Fe limits the growth of large diatoms but does not control nutrient biogeochemistry or growth of other planktonic organisms. In severely Fe-limited waters, Fe exerts a fundamental control on nitrate and silicic acid drawdown, particulate organic carbon production, and the growth of phytoplankton, zooplankton, and bacteria. We propose a four -level classification scheme for Fe limitation in coastal California waters. Each level is characterized by a set of specific biological and biogeochemical responses to Fe. Parameters that show a characteristic response to Fe addition and thus define a region's Fe limitation status include particulate Si:N and Si:C production ratios, NO3− and H2SiO3 drawdown, C fixation, large diatom and picoplankton growth, bacterial production, and zooplankton grazing and biomass. Fe limitation of coastal upwelling regions needs to be recognized as an important biogeochemical process that could profoundly affect global new production and carbon cycling. The physical, chemical, and biological complexity of coastal upwelling regimes requires that Fe limitation effects be addressed with a more sophisticated approach than has generally been used to describe oceanic HNLC regimes.

Physiological stress and cell death in marine phytoplankton: Induction of proteases in response to nitrogen or light limitation

Abstract: The physiological processes of natural phytoplankton mortality due to environmental stress (vs. that caused by sedimentation and predation) are poorly understood. Cell survival was examined in batch cultures of the diatom Thalassiosira weissjogii and the chlorophyte Dunaliella tertiolecta during deprivation of fixed nitrogen or light. Despite severe impairment of photosynthetic efficiencies, both species remained viable during 2 weeks of N starvation. Under N stress, a specific protease was induced in the diatom, overall activity of proteases doubled, and there was gradual, selective loss of certain proteins, especially ribulose 1,5bisphosphate carboxylase/oxygenase. Light-deprived diatoms were virtually unaffected, but the chlorophyte underwent catastrophic cell death after about 6 d of darkness. Cell death coincided with a large increase in protease activity and the induction of a specific protease. Although we cannot completely rule out roles for viruses or bacteria in the losses of cells, the consistent timing, the unique response to stress, and the coincident expression of a specific protease strongly suggest that the process is a form of autocatalyzed cell death, such as apoptosis. While of uncertain adaptive significance, phytoplankton cell death may have implications for species succession and cycling of organic matter in aquatic ecosysterns.

The Caspian Lake: History, biota, structure, and function

Abstract: The elongate, endorheic Caspian Lake has a north-south orientation and its main freshwater inflow, the Volga River, enters at the shallow north end. Two deep basins occupy its central and southern regions. These facts lead to horizontal differences in temperature, salinity, and ecology. Nutrient levels and primary production are low. Historically, lake level has fluctuated by -6 m, but on the geological timescale, fluctuations of >200 m have occurred. The lake formed in the late Miocene, first went through a long shrinking phase, expanded to three times its present size in the late Pliocene, and repeatedly rose and fell throughout the Pleistocene, with corresponding freshenings and salinizations. The lake surface has remained below sea level since the last pleniglacial. This dynamic history led to the assemblage of a set of euryhaline biota of Tethyan and freshwater origin, besides Baltic elements, that invaded with glacial meltwater. Endemism is as high as in Lake Baikal, but typical marine groups are absent. Because the lake is oxygenated to the bottom, its biota show a vertical zonation, but without a true abyssal community, suggesting cat&o pollution of the Volga, Ural, and Kura Rivers; and destruction of the spawning grounds of sturgeon, signalling a forthcoming collapse of the world caviar market (Dumont 1995). The Caspian is the endorheic terminus of the Volga River, which contributes >80% of its total inflow, 237 km3 yr-’ on average (Butorin 1979), with a range of 200-450 km3. The Kura River is second in line and discharges only 16.8 km’ yr-I; the Ural River is third, with a discharge of 8.1 km” yr I. Lake surface area, situated below sea level, is close to 400,000 km2 and lake volume is -78,000 km3, or -40% of the world’s continental surface water. Here, I attempt to provide insight into the nature of the Caspian ecosystem, including its eventful past. I outline gaps in our knowledge and suggest research for the future.

Center for Limnology, University of Wisconsin, 680 N. Park St., Madison, Wisconsin 53717

Abstract: Phytoplankton biomass and production in lakes tend to be increased by phosphorus input and decreased by grazing or high levels of colored, dissolved organic carbon (DOC). We estimated and compared the effects of these three factors by using data from three lakes that were manipulated during 1991-1995, and data from a reference lake. Multivariate probability distributions of chlorophyll or primary production, as predicted by P input rate, DOC, and grazer length, were fit to the data. All three factors had substantial effects on chlorophyll, primary production, and their variability. Comparable reductions in the mean and variance of chlorophyll and primary production were achieved by reducing P input rate from 5 to 0.5 mg me2 d-l, increasing DOC from 5 to 17 mg C liter I, or increasing mean crustacean length from 0.2 to 0.85 mm. The negative effect of mean crustacean length (an index of size-selective predation) results from grazing by herbivorous zooplankton. The negative effect of DOC on primary producers could be explained by shading. The results suggest that natural variation in colored DOC concentrations is a major cause of variation in primary production.

The role of surface‐active carbohydrates in the formation of transparent exopolymer particles by bubble adsorption of seawater

Abstract: Experiments were performed to examine the role of surface-active polysaccharides in the formation of transparent exopolymer particles (TEP) by bubble adsorption in seawater. Filtered (1 .O pm, 0.45 pm) and ultrafiltered (0.1 pm, 100 kDa, and 10 kDa) seawater samples were bubbled in a glass foam tower. The neutral sugar composition, concentration of TEP, and concentration of surface-active carbohydrates in generated foam samples were determined. Three different surface seawater samples (Monterey Bay, California; Shannon Point, Anacortes, Washington; and East Sound, Orcas Island, Washington) were used in the experiments. Significant concentrations of pre-existing and new TEP were extracted into foam by bubble adsorption. Newly generated TEP accounted for 28-52% of the TEP collected in the foam samples by bubbling l.O-pm-filtered samples. Neutral sugar composition analyses of foam samples derived as a function of bubbling time indicated that two types of surface-active carbohydrates were extracted by bubble adsorption: highly surface-active carbohydrates, which were extracted initially, and less surfaceactive carbohydrates. As in our previous mesocosm study (Mopper et al. 1995), highly surface-active polysaccharides were enriched in deoxysugars (fucose and rhamnose), whereas the less surface-active polysaccharides and residual (bubble-stripped) water were glucose rich. In addition, the highly surface-active fraction was strongly enriched in covalently bound sulfate. The concentrations of TEP and surface-active carbohydrates that were extracted into the foam both decreased sharply with decreasing filter size used to filter samples prior to bubbling, in agreement with theoretical considerations. The results of this study suggest that bubble adsorption of sulfate-rich surface-active carbohydrates is an important pathway for the formation of TEP in surface waters, especially during algal blooms.

Photochemical transformations of surface and deep marine dissolved organic matter: Effects on bacterial growth

Abstract: The effects of photochemical transformations on the bioavailability of marine dissolved organic matter (DOM) were investigated in surface and deep water from the Gulf of Mexico. Seawater samples were collected from eight depths (15–1,000 m), passed through 0.2-µm pore-size filters, and exposed to sunlight in quartz bottles in a flowing seawater deck incubator for 5-9 h. Following sunlight exposure, samples were inoculated (1:10) with unfiltered seawater from 15-m depth, and bacterial growth rates were estimated from rates of 3H-leucine incorporation in dark incubations. Exposure of surface-water DOM to sunlight resulted in a 75% reduction in bacterial production, whereas exposure of deep-water DOM resulted in a 40% enhancement in bacterial production. Photomineralization of bioreactive DOM likely contributed to the reduction of bacterial growth in surface water, but the photoproduction of biorefractory DOM also appeared to contribute to reduced bacterial growth. Enhanced bacterial growth in irradiated deep water was consistent with previous studies demonstrating the photoproduction of bioavailable substrates from deep-water DOM. Phototransformations of DOM appeared to be multifaceted and to play a critical role in the cycling of DOM in the ocean.

How important are rare species in aquatic community ecology and bioassessment

Abstract: We examined the effects of excluding rare species on the comparison of species richness. A river benthic data set and a randomization resampling procedure were used to show the importance of this consideration in aquatic bioassessment in particular. The data set was manipulated by deleting species at three levels of rarity as defined by occurrence frequency: once in all 24 replicates from each of three sites, no more than twice, and no more than five times. We focused on differences in species richness because many other bioassessment metrics are dependent on species richness and species composition. Species abundance patterns at the three sites were very different, with many more rare species at the least impacted site than at the more impacted sites. As sample size increased, the diffcrcnces in species richness among the three sites markedly increased in the original data set. However, the exclusion of rare species at the same level of rarity substantially reduced species richness at the least impacted site but had little effect on the most impacted site. This result led to a serious underestimation of the differences in species richness among the sites in terms of both absolute numbers and species loss percentages. Deleting rare species can damage the sensitivity of community-based methods to detect ecological changes; rare species are critical for bioassessment.

Exopolymer production by intertidal epipelic diatoms

Abstract: Epipelic diatoms are motile, the locomotive force being generated by the extrusion of carbohydrate-rich exopolymers (EPS). Production rates of EPS, colloidal (water-soluble) carbohydrates, and glucan (a photosynthetic storage product of diatoms) were investigated in axenic cultures, in sediments under laboratory conditions, and in the field. In stationary-phase axenic monocultures of Cylindrotheca closterium, Navicula perminuta, and Nitzschia sigma, EPS concentrations significantly increased during darkness. EPS concentrations in darkness corresponded to decreases in glucan concentration. Glucan concentrations increased significantly in illuminated cultures, Identical patterns of concentration changes were observed in intact diatom-dominated sediments. Inorganic 14C incorporation was used to determine rates of production and flux of photoassimilates through the different carbohydrate fractions in intact sediments. Biofilms exhibited endogenous rhythms of primary and carbohydrate production under laboratory conditions in the absence of tidal influence and under constant illumination (200 µmol m−2 s−1). Maximum production (27.7 mg C m−2 h−1) occurred during periods when sediments would have been emersed had they remained in the field, with 8.0, 3.0, and 0.8% of photoassimilates being incorporated into colloidal-S, glucan, and EPS, respectively. Higher rates of primary production were measured under natural illumination (617 µmol m−2 s−1) in the field (82.7 mg C m−2 h−1), but the maximum incorporation into colloidal-S, glucan, and EPS was similar (10.9, 2.8, and 1.0%, respectively). Pulse experiments revealed that 14C initially incorporated into glucan was subsequently reallocated into EPS. Increases in labeled EPS occurred up to 4 h after 14C incorporation, coinciding with periods of migratory activity linked to impending tidal immersion. Production of EPS by epipelic diatoms therefore appears to be closely linked to migratory rhythms, During periods of photosynthesis, assimilated carbon is stored as intracellular glucan, as well as being utilized to produce EPS. A number of stimuli increase the production of EPS: dim light, darkness, and migration linked to tidal immersion. Such increases in EPS production can be met either by increasing the allocation of photoassimilated carbon from current photosynthesis or by reallocating an internal carbon source. This short-term variability in EPS production has implications for our understanding of estuarine carbon budgets and biogenic stabilization of sediments.

Organic carbon partitioning during spring phytoplankton blooms in the Ross Sea polynya and the Sargasso Sea

Abstract: In this study we evaluate the partitioning of organic carbon between the particulate and dissolved pools during spring phytoplankton blooms in the Ross Sea, Antarctica, and the Sargasso Sea. As part of a multidisciplinary project in the Ross Sea polynya we investigated the dynamics of the dissolved organic carbon (DOC) pool and the role it played in the carbon cycle during the 1994 spring phytoplankton bloom. Phytoplankton biomass during the bloom was dominated by an Antarctic Phaeocystis sp. We determined primary productivity (PP; via H’CO, incubations), particulate organic carbon (POC), bacterial productivity (BP; via [‘Hlthymidine incorporation), and DOC during two occupations of 76”3O’S from 175”W to 168”E. Results from this bloom are compared to blooms observed in the Sargasso Sea in the vicinity of the Bermuda Atlantic Time-Series Study station (BATS). We present data that demonstrate clear differences in the production, biolability, and accumulation of DOC between the two ocean regions. Despite four- to fivefold greater PP in the Ross Sea, almost an order of magnitude less DOC (mmol m ?) accumulated during the Ross Sea bloom compared to the Sargasso Sea blooms. In the Ross Sea 89% (- 1 mol C m ‘) of the total organic carbon (TOC) that accumulated during the bloom was partitioned as POC, with the remaining 11% (- 0.1 mol C rn’) partitioned as DOC. In contrast, a mean of 86% (0.7.5-1.0 mol m ‘) of TOC accumulated as DOC during the 1992, 1993, and 1995 blooms in the Sargasso Sea, with as little as 14% (0.08-0.29 mol C m-?) accumulating as POC. Although a relatively small portion of the fixed carbon was produced as DOC in the Ross Sea, the bacterial carbon demand indicated that a qualitatively more labile carbon was produced in the Ross Sea compared to the Sargasso Sea. There are fundamental differences in organic carbon partitioning between the two systems that may be controlled by plankton community structure and food-web dynamics.

Mean mixed depth of sediments: The wherefore and the why

Abstract: The activities and consequently the bioturbational effects of deposit-feeding organisms are largely restricted to a narrow surficial zone of marine sediments with a worldwide, environmentally invariant mean of 9.8 cm with a standard deviation of 4.5 cm. Currently available theories of infaunal behavior cannot predict quantitatively this observation. A new simple model that accounts for the feedback between resource (food) abundance, its reactivity, and the intensity of bioturbation leads to a quantitative estimate of 9.7 cm. This model constitutes a fundamental advance in our understanding bioturbation.

Minireview: The importance of benthic-pelagic coupling and the forgotten role of life cycles in coastal aquatic systems

Abstract: The classical models of production and plankton community dynamics in coastal waters include an important role for benthic-pelagic coupling in the form of biogcochemical cycling (the turnover of nutrients in the form of either living matter or its decomposed constituents). We think, however, that biogeochemical explanations of ecosystem functioning underrepresent the actual complexity of the studied phenomena. We suggest that models will be more complete if they incorporate life-cycle patterns of planktonic and benthic organisms. The inclusion of such biological information into an ecological context will enhance the understanding of ecological patterns of global importance. What we presently know provides compelling evidence that new research directions, ranging from sedimentology to systematics and from physiology to molecular biology, are needed.

Photochemically produced carboxylic acids as substrates for freshwater bacterioplankton

Abstract: . High-molecular-weight dissolved organic matter is abundant in humic lakes and is a large potential source of energy for heterotrophic organisms. These substances are hard to degrade enzymatically because of their high aromaticity and complex structure. However, there is increasing evidence that photochemical processes render the material more bioavailable. We demonstrate a substantial photochemical production of four carboxylic acids (oxalic, malonic, formic, and acetic acid) in a humic lake. The combined production rate in the surface water of these four acids was 19 pg C liter-l h-l with natural sunlight. Furthermore, based on radiotracer studies, we found that the amount of carbon assimilated and oxidized to CO, from malonic, formic, and acetic acid exceeded bacterial carbon production, sometimes by more than one order of magnitude. This implies that carboxylic acids were major bacterioplankton substrates. Nevertheless, under natural sunlight at the lake surface, microbial utilization of carboxylic acids was substantially lower than the photochemical production of the acids. Hence, photochemically produced carboxylic acids may accumulate in sunlight exposed environments and may also serve as bacterial substrates after mixing into deeper layers, or during night.

Burial efficiency of phosphorus and the geochemistry of iron in continental margin sediments

Abstract: We have examined the distributions of phosphorus and iron in sediments from well-oxygenated environments on the Atlantic Canadian and the Portuguese continental margins and from the anoxic region of the Chesapeake Bay. The measurements include total, citrate-dithionite-bicarbonate (CDB) extractable, ascorbate extractable, and dissolved P and Fe; acid volatile sulfide; and pyrite. A surface layer (varying in thickness between 2 and 4 cm) enriched in P and Fe was revealed by both the CDB and the ascorbate extractions in all sediments except those from the Chesapeake Bay. The amount of phosphate extracted by the two reagents was similar, but more iron was extracted by the CDB reagent, probably because of its ability to dissolve crystalline iron oxides. Within the Feand P-enriched surface layer, the Fe : P ratio in the ascorbate extract varied within a narrow range (6-14), as did the soluble-reactive phosphate (SRP) concentration (5-16 PM), suggesting that SRP is in sorption equilibrium with the solid phase. Our data are consistent with a dynamic cycling of P and strong interactions between the cycles of P, Fe, and sulfur in many marine environments. The reductive dissolution of amorphous Fe during burial and the formation of pyrite diminish the capacity of the sediment to sequester P, and only a portion of the P that arrives at the sediment-water interface actually gets buried with the sediment.

Denitrification in estuarine sediments determined by membrane inlet mass spectrometry

Abstract: Steady-state and transient-state denitritication rates were measured in sediment cores from a brackish river of the Chesapeake Bay using high-precision, membrane-inlet mass spectrometry. Denitrification was independent of salinity over the range of 1–13 ppt and was directly dependent on nitrate concentration over the range of 0–200 µM in the over-lying water. Denitrification was observed when the water-column nitrate concentration was

The role of dissolved organic matter release in the productivity of the oligotrophic North Pacific Ocean

Abstract: Based on a long-term set of observations and measurements at a station in the subtropical North Pacific Ocean, it now appears that contemporaneous rates of primary production in low-nutrient open ocean regions and perhaps in the ocean as a whole may be greater than had been considered in field studies conducted in previous decades. Data collected at the Hawaii Ocean Time-Series (HOT) Station ALOHA from October 1988 to July 1997 indicate that daytime particulate organic carbon (POC) production, based on 12-h 14C in situ incubations, averages 472 mg C m−2 d−1 (SD = 125 mg C m−2 d−1; n = 70). This carbon production rate is two- to three-fold greater than most of the pre-1980 estimates. We present evidence that particulate production rates may have been overestimated by up to 30% as a result of 14C-labeled dissolved organic carbon (14C-DOC) adsorption onto glass fiber filters. More importantly, when one considers the 14C-DOC that is produced but not adsorbed onto the filters, gross primary production rates (14C-POC plus 14C-DOC) in the subtropical North Pacific Ocean may approach 1 g C m−2 d−1. We hypothesize that the large flux of 14C-DOC may be a manifestation of decade-scale habitat changes resulting from variations in climate. The balance between POC and DOC production will ultimately influence the structure of the food web, especially the interactions of phytoplankton and heterotrophic bacterial populations, and the mechanisms and rates of carbon sequestration by the biological pump.

Hydrologic influence on methane and carbon dioxide dynamics at two north‐central Minnesota lakes

Abstract: Annual emissions of (CH4 + CO2) to the atmosphere were proportional to net hydrologic inputs of C, mostly by groundwater, at two lakes in the Shingobec River watershed in north-central Minnesota. Williams Lake (WL), a closed basin lake near the top of the watershed, had a hydraulic residence time of 2–4 yr and groundwater exchange of about +2 mol dissolved inorganic carbon (DIC) and −0.1 mol dissolved organic carbon (DOC) m−2 lake area yr−1. The Shingobee River flows through Shingobee Lake (SL) that had a hydraulic residence of 0.3–0.4 yr and received net groundwater plus surface-water inputs of +5.3 to +7.3 mol DIC and fewer than + 1.3 mol (DOC + particulate organic carbon) m−2 yr−1. Approximately 60–80% of net annual C input to SL was from groundwater. Lake storage of CH4 and CO2 was greatest in late winter, with maximum emissions to the atmosphere immediately following ice melt. The lakes emitted CH4 continuously during open water, having annual losses of −1.6 mol CH4 m−2 yr−1 at WL and −1.9 mol CH4 m−2 yr−1 at SL. Although the WL epilimnion was CO2 depleted throughout summer, net annual CO2 exchange with the atmosphere was near zero because springtime emission offset summertime uptake. CO2 supersaturation resulted in emission of −8.0 mol CO2 m−2 yr−1 at SL.