Showing papers in "Limnology and Oceanography in 2004"

Phytoplankton growth, microzooplankton grazing, and carbon cycling in marine systems

Abstract: We present an analysis of the global impact of microplanktonic grazers on marine phytoplankton and its implications for remineralization processes in the microbial community. The data were obtained by an extensive literature search that yielded 788 paired rate estimates of autotrophic growth (mu) and microzooplankton grazing (m) from dilution experiments. From studies in which phytoplankton standing stock was measured in terms of carbon equivalents, we show that the production estimate from dilution experiments is a reasonable proxy (r = 0.89) for production determined by the standard C-14 method. The ratio m: mu, the proportion of primary production (PP) consumed by micrograzers, shows that microzooplankton consumption is the main source of phytoplankton mortality in the oceans, accounting for 67% of phytoplankton daily growth for the full data set. This ratio varies modestly among various marine habitats and regions, with data averages ranging from 60% for coastal and estuarine environments to 70% for the open oceans, and from similar to59% for temperate-subpolar and polar systems to 75% for tropical-subtropical regions. Given estimates for the metabolic requirements of micrograzers and assuming they consume most bacterial production, regionally averaged estimates of the protistan respiration are 35-43% of daily PP for the first level of consumer or 49-59% of PP for three trophic transfers. The estimated contributions of microbial grazers to total community respiration are of the same magnitude as bacterial respiration. Consequently, potential ecosystem differences in micrograzer activity or trophic structure are a large uncertainty for biogeochemical models that seek to predict the microbial community role in carbon cycling from bacterial parameters alone.

Assessing the dynamics and ecology of marine picophytoplankton: The importance of the eukaryotic component

Abstract: We assess population dynamics of picophytoplankton groups (#2 mm diameter; Prochlorococcus, Synechococcus, and picoeukaryote) at a Pacific Ocean coastal site in the Southern California Bight. Weekly sampling (August 2000 to January 2002), dilution experiments, and flow cytometric analysis were combined with an instrument-specific calibration for cell size determination, allowing biovolume and carbon biomass estimation. Synechococcus was almost always numerically dominant, accounting for 60 6 12% of the total picoplankton cells over time. It had moderately high growth rates (0.52‐0.86 d 21 21 ).

Coupled nitrogen and oxygen isotope fractionation of nitrate during assimilation by cultures of marine phytoplankton

Abstract: We report the first measurements of coupled nitrogen (N) and oxygen (O) isotopic variations of nitrate (NO ) 2 3 during its assimilation by laboratory cultures of marine phytoplankton and derive the N and O kinetic isotope effects for nitrate assimilation by three species of diatoms ( Thalassiosira weissflogii, Thalassiosira oceanica,and Thalassiosira pseudonana) and a coccolithophorid (Emiliana huxleyi). Large interspecies and intraspecies variations in the N isotope effects were observed. The O isotope effect associated with nitrate consumption was consistently close to the N isotope effect, such that the 18 O/ 16 O and 15 N/ 14 N of nitrate varied in a ratio of ;1 : 1, regardless of species or of the magnitude of the isotope effect. In addition, the 18 O/ 16 O and 15 N/ 14 N of internal nitrate of T. weissflogii grown under various environmental conditions were elevated relative to the medium nitrate by a proportion of ; 1: 1. These findings are consistent with a nitrate isotopic fractionation mechanism that involves nitrate reduction as the chief fractionating step. The observed N : O isotopic coupling during nitrate assimilation suggests that combined N and O isotopic measurements of water column nitrate can provide new constraints on the ocean N cycle.

Quantitative detection of chlorophyll in cyanobacterial blooms by satellite remote sensing

Abstract: The extent of cyanobacterial blooms has been mapped using different satellite sensors from weather satellites to synthetic aperture radars. Quantitative detection of chlorophyll in cyanobacterial blooms by remote sensing, however, has been less successful. The first civilian hyperspectral sensor in space, Hyperion, acquired an image of cyanobacterial bloom in the western part of the Gulf of Finland on 14 July 2002. A chlorophyll concentration map was produced from this image using a spectral library that was created by running a bio-optical model with variable concentrations of chlorophyll. The results show that chlorophyll concentrations in the bloom area were much higher than reported by conventional water-monitoring programs, ships-of-opportunity, and satellite measurements. The reason why both in situ and satellite methods underestimate the amount of phytoplankton during cyanobacterial blooms is vertical and horizontal distribution of cyanobacteria, because cyanobacteria can regulate their buoyancy and are not uniformly distributed within the top mixed layer of water column in calm weather conditions. Aggregations of cyanobacteria form dense subsurface blooms and surface scums during extensive blooms. This study demonstrates that it is difficult to collect representative water samples from research vessels using standard methods because ships and water samplers destroy the natural distribution of cyanobacteria in the sampling process. Flowthrough systems take water samples from the depths at which the concentration of cyanobacteria is not correlated with the amount of phytoplankton that remote sensing instruments detect. The chlorophyll estimation accuracy in cyanobacterial blooms by many satellites is limited because of spatial resolution, as significant changes in chlorophyll concentration occur even at a smaller spatial scale than 30 m.

Direct evidence of a biophysical retention mechanism for coral reef fish larvae

Abstract: We examine the hypothesis that reef fish larvae have some direct influence on their own dispersal and ability to recruit to their natal reef by tracking cohorts of bicolor damselfish (Stegastes partitus) from hatching to settlement onto the reef, about 30 d later. We conducted high-resolution sampling during two consecutive years in a small area (15 km 3 20 km) off the west coast of Barbados, extending from depths of 0 to 100 m. Observations of discrete stage-specific larval patches of mean size of 29.4 and 13.2 km 2 for preflexion (1‐5-d old) and flexion/postflexion (.5-d old) stages extending ca. 30 m in the vertical indicated that larvae initially dispersing as patches tend to stay in coherent patches throughout their pelagic duration. Highest concentrations of preflexion larvae within a patch were in the upper 20 m, while those of older larvae were always deeper. Downward migration of about 60 m throughout ontogeny within stratified currents represented a retention mechanism for locally spawned larvae. Most of the variability in estimated retention rates between daily cohorts occurred during the earliest stages as a result of the dynamic nature of surface currents experienced by larvae prior to the onset of vertical migration. Differences in residence time between experiments were consistent with observed intermonthly variability in recruitment strength, implying that pelagic processes can explain recruitment rates. These results provide empirical evidence for larval retention of coral reef fishes and stress the role of active behavior in larval transport.

Denitrification and anammox activity in Arctic marine sediments

Abstract: We measured rates of N2 production through anaerobic NH oxidation with NO (anammox) and denitrification 12 42 in permanently cold (from 21.78 Ct o 4 8C) sediments off the east and west coasts of Greenland. The investigated sites (36- to 100-m water depth) covered sediments in which carbon contents ranged from 0.3 to 3.2 dry weight %, O2 uptake rates ranged from 3.4 to 8.3 mmol m 22 d 21 ,O 2 penetration depths ranged from 0.25 to 1.70 cm, and bottom-water NO concentrations ranged from 0.3 to 15.3 mmol L 21 . Total N2 production was 34‐344 mmol N m 22 2 3 d 21 , of which anammox accounted for 1‐92 mmol N m 22 d 21 (1‐35% of total) and denitrification for 33‐265mmol Nm 22 d 21 . At one of the high-Arctic sites, anammox activity had an optimum temperature (Topt )o f 12 8C, while that of bacterial denitrification was 248C. According to the classical temperature scheme for metabolic growth, the anammox response was psychrophilic, while denitrification was psychrotrophic. Although Topt was considerably higher than in situ temperatures, rates of denitrification and anammox were still high at 21.38C, reaching 17% and 40%, respectively, of those found at Topt. The activation energies, Ea, of anammox and denitrification were 51.0 and 60.6 kJ mol 21 , respectively, and the corresponding Q10 values were 2.2 and 2.4. Rates of anammox were linearly correlated with bottom-water NO concentrations (r 2 5 0.96, p , 0.0001, n 5 11) at the investigated sites. We 2 3 suggest that the slow-growing anammox bacteria are favored in sediments with high and stable NO conditions. 2 3

Mechanisms, effects, and scales of dispersal in freshwater zooplankton

Abstract: The distribution of organisms can be regulated by local environmental factors and regional processes such as dispersal. Here, we review recent work on the role of dispersal for generating population and community structure in freshwater zooplankton. We examine evidence for different mechanisms of dispersal among lakes, for the effects of dispersal limitations on populations and communities, and for the effects of spatial scale on dispersal rates. Zooplankton move via human or animal vectors, flowing surface waters, and wind; the relative importance of the different modes of transport is poorly understood. Several lines of evidence suggest that dispersal among lakes separated over short spatial scales (,10 km) is sufficiently rapid that local interactions should limit species diversity and composition more than the supply of colonists. However, dispersal limitation over broad scales (tens to thousands of kilometers) might constrain geographic ranges and influence community structure. The current explosion in the incidence of exotic species indicates that such global- or continental-scale dispersal was limiting in the past. The spread of exotic species also provides opportunities to study the scale dependence of zooplankton dispersal. We show how patterns of range expansion can be used to estimate the change in invasion likelihood with distance to a source population. Such dispersal functions provide a crucial link between small-scale experimental studies and broad-scale geographic patterns. Theories of geographic structure in communities and populations come from two broad schools of thought. The regional approach examines the importance of movement of individuals and the colonization of isolated patches of habitat separated over broad spatial scales. Parallel theories have been developed for regional control of both genetic variation in populations (Slatkin 1985) and species diversity in communities (MacArthur and Wilson 1963). The regional approach often emphasizes neutral or drift processes such as colonization and extinction and ignores differences among species or environments (Hubbell 2001). By contrast, the local approach examines the response of species to conditions that influence population growth rates within relatively small, homogeneous patches of habitat (MacArthur 1972). Local-scale ecology focuses on differences among organisms or habitats that allow coexistence of multiple species or genotypes. The local and regional views have both enjoyed 1

Essential fatty acids in the planktonic food web and their ecological role for higher trophic levels

Abstract: We measured concentrations of essential fatty acids (EFAs) in four size categories of planktonic organisms— seston (10‐64 mm), microzooplankton (100‐200 mm), mesozooplankton (200‐500 mm), and macrozooplankton (.500 mm)—and in rainbow trout ( Oncorhynchus mykiss) in coastal lakes. Size-dependent patterns in concentrations of specific fatty acids (FAs) are important for ecosystem function, because planktivorous fish and some invertebrates are size-selective predators. We demonstrate that the retention of individual FAs differs among the four size categories of planktonic organisms in our study systems. Changes in individual EFA concentrations within the planktonic food web were similar in all coastal lakes sampled, which indicates the generality of our findings. Although concentrations of arachidonic acid, eicosapentaenoic acid (EPA), and linoleic acid increased steadily with plankton size, the concentration of a-linolenic acid decreased slightly in larger size fractions of zooplankton. Concentrations of another EFA, docosahexaenoic acid (DHA), declined sharply from mesozooplankton to the cladoceran-dominated macrozooplankton size class. Our results indicate that the retention of EFAs, as a function of plankton size, is related, in part, to the taxonomic composition of planktonic food webs. We suggest that, in general, zooplankton exhibit an EPA-retentive metabolism with increasing body size, whereas different taxonomic groups within the planktonic food web retain DHA differently. Finally, we conclude that EPA is highly retained in zooplankton, whereas in rainbow trout DHA is highly retained.

Phytoplankton growth and stoichiometry under multiple nutrient limitation

Abstract: Phytoplankton growth and stoichiometry depend on the availability of multiple nutrients. We use a mathematical model of phytoplankton with flexible stoichiometry to explain patterns of phytoplankton composition in chemostat experiments and nutrient drawdown dynamics that are found in the field. Exponential growth and equilibrium represent two distinct phases, each amenable to mathematical analysis. In a chemostat at a fixed dilution (growth) rate, phytoplankton stoichiometry matches the nutrient supply stoichiometry over a wide range at low growth rates and over a narrow range at high growth rates. In a chemostat with a fixed nutrient supply stoichiometry, phytoplankton stoichiometry varies with dilution rate nonlinearly, between the supply stoichiometry at low dilution rates and a species-specific optimal ratio at high dilution rates. The flexible-stoichiometry model we study predicts low equilibrium concentrations of two nutrients over a wide range of supply ratios, contrary to the predictions of a traditional fixed-stoichiometry model. The model is in quantitative agreement with experimental data, except at extreme nutrient supply ratios, which require a negative feedback from quota to uptake to fit the data. Our analysis points to the importance of better understanding the regulation of uptake rates in determining phytoplankton stoichiometry and incorporating this knowledge into phytoplankton models.

Competition between biological and photochemical processes in the mineralization of dissolved organic carbon

Abstract: The photo- and bioreactive components of dissolved organic matter (DOM) from three different environments were determined during long-term decomposition experiments. Terrigenous DOM was collected from a black-water system, plankton DOM was harvested from phytoplankton cultures, and lake water served as a DOM source with both terrigenous and plankton components. Photomineralization accounted for the removal of 46 and 7% of terrigenous and lake-dissolved organic carbon (DOC), respectively, while no loss in DOC was observed when plankton DOM was exposed to irradiation. Biomineralization accounted for the removal of 27% each of terrigenous and lake DOC and 74% of plankton DOC. Phototransformations of terrigenous and lake DOM resulted in 7% and 2% increases in biodegradable DOC, respectively, while no increase in biodegradable DOC was observed for irradiated plankton DOM. In two different experimental approaches, terrigenous DOM was exposed to sequential and alternating bio- and photodegradation, respectively, to determine the fractions of DOC that were bioreactive and photoreactive. About 15% of terrigenous DOC was susceptible to both biomineralization and photomineralization. These results demonstrate that biological and photochemical processes compete in the mineralization of DOC. Photomineralization of bioreactive DOC is likely an important factor determining the net effect of irradiation on the bioreactivity of DOM.

The demise of the marine cyanobacterium, Trichodesmium spp., via an autocatalyzed cell death pathway

Abstract: We present experimental laboratory evidence and field observations of an autocatalyzed, programmed cell death (PCD) pathway in the nitrogen-fixing cyanobacteriumTrichodesmium spp., which forms massive blooms in the subtropical and tropical oceans. The PCD pathway was induced in response to phosphorus and iron starvation as well as high irradiance and oxidative stress. Transmission electron microscopy revealed morphological degradation of internal components including thylakoids, carboxysomes, and gas vesicles, whereas the plasma membranes remained intact. Physiologically stressed cells displayed significantly elevated endonuclease activity and terminal d-UTP nick-end labeling. Nucleic acid degradation was concordant with increased immunoreactivity to human caspase-3 polyclonal antisera and enhanced cleavage of a caspase-specific substrate, DEVD. Caspase activity was positively correlated with mortality and was inhibited by the irreversible caspase inhibitor Z-VAD-FMK. A search of the Trichodesmium erythraeumgenome identified several protein sequences containing a conserved caspase domain structure, including the histidine- and cysteine-containing catalytic diad found in true caspases, paracaspases, and metacaspases. Induction of PCD by caspase-like proteases in a bacterial photoautotroph with an ancient evolutionary history requires a reassessment about the origins and roles of cell death cascades. This process is a previously unappriciated mortality mechanism that can lead to the termination of natural Trichodesmium blooms and that can influence the fluxes of organic matter in the ocean.

Light driven seasonal patterns of chlorophyll and nitrate in the lower euphotic zone of the North Pacific Subtropical Gyre

Abstract: The euphotic zone below the deep chlorophyll maximum layer (DCML) at Station ALOHA (a long-term oligotrophic habitat assessment; 228459N, 1588009W) transects the nearly permanently stratified upper thermocline. Hence, seasonal changes in solar radiation control the balance between photosynthesis and respiration in this lightlimited region. Combining profiles of radiance reflectance, algal pigments, and inorganic nutrients collected between January 1998 and December 2000, we explore the relationships between photosynthetically available radiation (PAR), phytoplankton biomass (chlorophyll a), and the position of the upper nitracline in the lower euphotic zone. Seasonal variations in the water-column PAR attenuation coefficient displace the 1% sea-surface PAR depth from approximately 105 m in winter to 121 m in summer. However, the seasonal depth displacement of isolumes (constant daily integrated photon flux strata) increases to 31 m due to the added effect of changes in sea-surface PAR. This variation induces a significant deepening of the DCML during summertime with a concomitant increase in chlorophyll a and the removal of 36 mmol m 22 inorganic nitrogen [NO 1 NO ] in the 90‐200-m depth range, 22 32 equivalent to approximately 34% of the annual flux of particulate nitrogen collected in sediment traps placed at 150 m. We conclude that in this oceanic region the annual light cycle at the base of the euphotic zone induces an increase in the phototrophic biomass analogous to a spring bloom event.

Nutrient budget for the Eastern Mediterranean: Implications for phosphorus limitation

Abstract: The eastern Mediterranean has a high nitrate to phosphate (N : P) ratio (~28 : 1) in the deep water and a highly unusual P limitation of the primary productivity. We present a detailed nutrient budget of inputs to the basin, which shows that there is a high N: P ratio (>16 : 1) in all the input sources, particularly from the atmospheric source, where the N: P ratio was 117 : 1. The high N: P ratio is retained within the system because there is no significant denitrification in either the sediments or intermediate water. This is because of the extreme oligotrophic nature of the system, which is caused by the unusual anti-estuarine flow at the Straits of Sicily. Support for this conclusion is provided by the observation that the only area of the eastern Mediterranean where the N: P ratio in deeper water is ~16 : 1 is the northern Adriatic, which is also the only area with significant denitrification. The N budget (total input to basin vs. net output at the straits of Sicily) balances closely. This N balance suggests that N fixation is an insignificant process in this P-limited system. The unusually light 15N values in the deep water nitrate and particulate organic nitrogen can be explained by processes other than nitrogen fixation. These processes include a lack of significant denitrification in the basin and by particulate organic matter exported from surface waters during the Plimited winter plankton bloom.

Autochthonous versus allochthonous carbon sources of bacteria: Results from whole-lake 13C addition experiments

Abstract: Organic substrates for pelagic bacteria are derived from dissolved organic carbon (DOC) in the water column. DOC is a heterogeneous mixture of molecules, some of which are imported from the watershed (allochthonous DOC) and others that are produced by autotrophs within the system (autochthonous DOC). We examined the importance of autochthonous versus allochthonous DOC in supporting the growth of pelagic bacteria by manipulating the 13 C content of autochthonous sources in a whole-lake experiment. NaH 13 CO3 was added daily to two small forested lakes for a period of 42 d, thereby strongly labeling autochthonous primary production. To obtain bacterial carbon isotopes, bacteria were regrown in vitro in particle-free lake water and in situ in dialysis tubes; little difference was found between the two methods. The contribution of autochthonous versus allochthonous carbon to the bacterial biomass was estimated by applying a two-member mixing model using a 13 Co f228‰ as the

Oxygen dynamics in permeable sediments with wave-driven pore water exchange

Abstract: The effects of advective pore water exchange driven by shallow water waves on the oxygen distribution in a permeable (k 5 3.3 3 10 212 to 4.9 3 10 211 m 2 ) natural sediment were studied with a planar oxygen optode in a wave tank. Our experiments demonstrate that pore water flow driven by the interaction of sediment topography and oscillating boundary flow changes the spatial and temporal oxygen distribution in the upper sediment layer. Oxygenated water intruding in the ripple troughs and deep anoxic pore water drawn to the surface under the ripple crests create an undulating oxic‐anoxic boundary within the upper sediment layer, mirroring the topographical features of the sediment bed. Anoxic upwelling zones under ripple crests can separate the oxic sediment areas of neighboring ripple troughs with steep horizontal oxygen concentration gradients. The optode showed that migrating wave ripples are trailed by their pore water flow field, alternately exposing sediment volumes to oxic and anoxic pore water, which can be a mechanism for remobilizing particulate oxidized metal precipitates and for promoting coupled nitrification‐denitrification. More rapid ripple migration (experimental threshold;20 cm h 21 ) produces a continuous oxic surface layer that inhibits the release of reduced substances from the bed, which under slowly moving ripples is possible through the anoxic vertical upwelling zones. Swift, dramatic changes in oxygen concentration in the upper layers of permeable seabeds because of surface gravity waves require that sediment-dwelling organisms are tolerant to anoxia or highly mobile and enhance organic matter mineralization. The dominant boundary layer flows in shallow marine environments are those generated by surface gravity waves. This dominance is reflected by the presence of wave ripples structuring large areas of shallow sandy seabeds that are abundant in coastal, estuarine, and shelf environments. Most of these sandy sediments are permeable (k . 10 212 m 2 ) and thus allow interstitial water motion. Pressure differences at the sediment‐water interface might drive interfacial solute transport through the surface layers of these beds. This advective transport can exceed transport by molecular diffusion by several orders of magnitude (Huettel and Webster 2001). In contrast, the major transport mechanisms in fine-grained muddy sediments are molecular diffusion and locally bioturbation (Berner 1980; Aller 1982). Increased fluid exchange between sediment and overlying water affects the oxygen dynamics in permeable sediments and therefore also affects biogeochemical processes. Booij

Gas transfer velocities of CO2 in three European estuaries (Randers Fjord, Scheldt, and Thames)

Abstract: We measured the flux of CO 2 across the air‐water interface using the floating chamber method in three European estuaries with contrasting physical characteristics (Randers Fjord, Scheldt, and Thames). We computed the gas transfer velocity of CO2 (k) from the CO2 flux and concomitant measurements of the air‐water gradient of the partial pressure of CO2 (pCO2). There was a significant linear relationship between k and wind speed for each of the three estuaries. The differences of the y-intercept and the slope between the three sites are related to differences in the contribution of tidal currents to water turbulence at the interface and fetch limitation. The contribution to k from turbulence generated by tidal currents is negligible in microtidal estuaries such as Randers Fjord but is substantial, at low to moderate wind speeds, in macrotidal estuaries such as the Scheldt and the Thames. Our results clearly show that in estuaries a simple parameterization of k as a function of wind speed is site specific and strongly suggest that the y-intercept of the linear relationship is mostly influenced by the contribution of tidal currents, whereas the slope is influenced by fetch limitation. This implies that substantial errors in flux computations are incurred if generic relationships of the gas transfer velocity as a function of wind speed are employed in estuarine environments for the purpose of biogas air‐water flux budgets and ecosystem metabolic studies. Based on organic carbon flux budgets, the overall picture of the net ecosystem metabolism in the coastal ocean is that temperate open continental shelves (bordered by a continental margin) are net autotrophic (net exporters of carbon and thus potential sinks for atmospheric CO2) while near-shore

Biodiversity and ecosystem functioning: A complex adaptive systems approach

Abstract: Environmental factors regulate biodiversity through species sorting processes. Species distributions in communities affect ecosystem processes and environmental factors. These dynamics are determined by the properties (traits) of species in the community. The optimal temperatures for growth, the minimal amount of resource that sustains positive mass balance, and the amount of energy allocated to predator defenses are examples of such traits. Over time, the trait distributions in communities may change in response to environmental changes, which, in turn, changes the processes and consequently the structure of the system. The result of such processes is the focus of complex adaptive systems (CAS) theory. This paper gives an overview of how CAS theory can contribute to understanding the role of biodiversity on the ability of functional groups that make up the ecosystem to change their species compositions in response to changes in the environment. Any trait that requires investment of energy, mass, or time is subjected to a tradeoff for alternative use of this resource. Such interspecies tradeoff relationships can be used to make predictions about past environmental conditions, as well as the response of the properties of a group of species, e.g., total productivity and species distributions, to future changes in the environment. The traitbased framework presented here makes explicit predictions regarding the relation between the environment, trait distributions, and ecosystem processes. Trait variance, a measure of the width of the distribution of traits in the community, is proportional to the rate at which species within functional groups can replace each other in response to environmental changes. This adaptive capacity is crucial for the ecosystem’s ability to maintain certain processes under times of change. Examples of empirical tradeoffs are given as well as how to formalize them to use in the CAS framework.

Patterns and controls of lotic algal stable carbon isotope ratios

Abstract: Spatial and temporal variations in stable carbon isotope ratios (i.e., d 13 C) of primary producers are common but poorly understood features of isotopic characterizations of aquatic food webs. I investigated factors that control d 13 C of algae (concentration and d 13 C of inorganic carbon, algal fractionation, and growth rates) in riffle habitats across a gradient in stream size and productivity in northern California. There was considerable seasonal and spatial variation in d 13 C of the green alga Cladophora glomerata, microalgal-influenced epilithic biofilms, and their herbivores. Algal and herbivore d 13 C were depleted in 13 C in small, unproductive tributary streams (244‰ to 230‰) compared with more productive sites downstream (231‰ to 223‰). The majority of variation in algal d 13 Co f Cladophora and epilithic biofilms was determined by dissolved CO 2 (CO2aq) via effects on d 13 Co f CO 2aq and photosynthetic fractionation. In contrast, two other taxa (the cyanobacterium Nostoc pruniforme and the red alga Lemanea sp.) showed little variation in d 13 C or fractionation in response to varied inorganic carbon availability because of their distinct modes of inorganic carbon acquisition. Although variation in algal d 13 C might complicate use of d 13 C to resolve consumer diet sources under some circumstances, better understanding of such variation should improve the use of d 13 C techniques in aquatic food web studies.

Bacterial metabolism and growth efficiency in lakes: The importance of phosphorus availability

Abstract: We investigated bacterial responses to variations in dissolved organic carbon (DOC) and nutrient availability by a comparative analysis of bacterial metabolism in lakes ranging from oligotrophic to eutrophic. Bacterial growth, respiration, and growth efficiency were quantified in lake water dilution cultures performed in 20 lakes located in eastern Quebec, Canada, which varied with respect to both DOC and nutrient concentrations. Intrinsic growth rates of the bacteria ranged from 0.1 to 1.4 d 21 , bacterial cell-specific respiration rates ranged from 0.4 to 7.2 fg C cell 21 h 21 , and growth efficiencies ranged from 6.7% to 51.6%. These variations were unrelated to bulk DOC concentrations. Instead, growth rate and efficiency were positively related to total phosphorus concentrations. Specific respiration rate, on the other hand, decreased with increasing phosphorus concentrations, and the magnitude of respiration, on a per-cell basis, strongly influenced observed growth efficiencies. In a series of substrate enrichment experiments, additions of glucose alone failed to stimulate a response in growth rate, mean cell biovolume, or the potential biomass yield in dilution cultures, but all responded positively to phosphorus additions. Our results show that bacterial metabolism and the fate of DOC input to lake microbial communities are strongly dependent on phosphorus availability, rather than total carbon availability. Extreme oligotrophy appears to place high respiratory demands on the bacterioplankton, resulting in very low bacterial growth efficiencies and consequently greater DOC flow to CO 2 than to biomass available for transfer to higher trophic levels. The organic carbon fueling bacterial metabolism in lakes originates from either in situ primary production (autochthonous carbon) or from terrestrial production that is carried into the lake from its watershed (allochthonous carbon). The relative importance of these two sources of carbon is highly variable among lakes. While autochthonous carbon tends to predominate in very eutrophic lakes, allochthonous carbon loading often greatly exceeds in situ primary production in oligotrophic lakes (Wetzel 1992). Input of allochthonous carbon thus represents a potentially large subsidy to the metabolism of lake communities. Several lines of evidence argue that the total respiratory breakdown of organic matter exceeds that produced by in situ primary production in most lakes (e.g., del Giorgio and Peters 1994; Cole et al. 2000). This metabolic imbalance (net heterotrophy; total respiration . gross primary production) implies that allochthonous carbon must support a portion of the lake’s total respiration. Indeed, it has recently been shown that, based on epilimnetic dissolved oxygen mea1

Ascending marine particles: Significance of transparent exopolymer particles (TEP) in the upper ocean

Abstract: The high abundance of transparent exopolymer particles (TEP) in marine and freshwater greatly affects particle dynamics. TEP act as glue for colliding particles and form the matrix in aggregates, thereby altering aggregation dynamics. We studied the sinking behavior of freshly produced, particle-free TEP and of aggregates composed of TEP and latex spheres in a laboratory using water collected from Santa Barbara Channel, California. Particle-free TEP ascend and accumulate in the surface layer of a settling column at an average velocity of 1.6 x 10−4 cm s−1. The estimated density of TEP ranges from 0.70 to 0.84 g cm−3. TEP also transported latex spheres of 45.6 and 1.82 µm in diameter and a density of 1.05 g cm−3 to the surface layer. We describe a simple model illustrating the role of TEP for the vertical transport of solid particles. The densities and relative proportions of TEP, solid particles, and interstitial water within an aggregate determine its sinking or ascending velocity. High ratios of TEP to solid particles retard the sinking of aggregates, prolonging their residence time in the surface ocean. Our results demonstrate that TEP can provide a vehicle for the upward flux of biological and chemical components in the marine environment, including bacteria, phytoplankton, carbon, and reactive trace elements.

Stream denitrification and total nitrate uptake rates measured using a field 15N tracer addition approach

Abstract: We measured denitrification and total nitrate uptake rates in a small stream (East Fork of Walker Branch in eastern Tennessee) using a new field 15 N tracer addition and modeling approach that quantifies these rates for entire stream reaches. The field experiment consisted of an 8-h addition of 99 atom% K 15 NO3 and a conservative solute tracer. Two 15 N tracer addition experiments were performed on consecutive days, the first under ambient NO concentra2 3 tions (23 m gNL 21 ) and the second with a NO addition of approximately 500 m gNL 21 . We fit first-order NO 2 2 3 3 uptake and two-box denitrification models to the longitudinal measurements of tracer 15

Interactions among dissolved organic carbon, microbial processes, and community structure in the mesopelagic zone of the northwestern Sargasso Sea

Abstract: At the Bermuda Atlantic Time-Series Study (BATS) site, the field observations of dissolved organic carbon (DOC) dynamics indicate that seasonally produced "semilabile" DOC is resistant to rapid microbial degradation in the surface waters but available for microbial remineralization once it is delivered into the mesopelagic zone after convective overtum. In this study, we employed an experimental simulation of convective overtum events to determine whether the remineralization of semilabile DOC would occur in a controlled laboratory setting. Seawater culture experiments were conducted in which surface ('10 m) and mesopelagic (250 m) 0.2-pLm filtrates were inoculated with unfiltered water from 10 and 250 m in an assortment of combinations to simulate various mixtures of nutrients, DOC quantity and quality, and microbial assemblages. Results indicate that (I) microbial inocula from the upper euphotic zone were incapable of remineralizing the seasonally accumulated semilabile DOC (j.mol C L-' resolution) on the timescales of the incubations; (2) the utilization of semilabile DOC was greatest when the inoculum source was from 250 m and the filtrate source was from the upper 10 m; and (3) the decrease in bacterioplankton diversity, estimated with the Shannon-Wiener diversity index, was greater in treatments in which inoculum from 250 m was mixed with filtrate from 10 m than in treatments in which the surface inoculum was mixed with the surface filtrate. Our findings are that a portion of the surface semilabile DOC can be metabolized by microorganisms in a laboratory setting and that mesopelagic nutrients alone are insufficient to stimulate DOC drawdown > 1.3 p.mol L-'. Transformations of microbial community structure were associated with the drawdown of surface DOC in simulated mixing events and suggest that microbial community structure is a factor in surface-layer DOC dynamics.

Small-scale spatial and temporal variability in coastal benthic O2 dynamics: Effects of fauna activity

Abstract: In situ measurements in a shallow water sediment were performed using three different modules—a microprofiling unit, a transparent benthic chamber, and a planar optode periscope. The combined data set revealed an extremely patchy and variable benthic O2 distribution primarily due to temporal variations in fauna activity and photosynthesis. A distinct diel pattern in the fauna activity, dominated by Hediste diversicolor, resulted in strongly elevated O2 uptake rates of ;5.3 mmol m 22 h 21 at the onset of darkness. The activity gradually diminished during the night, and the O2 uptake decreased to less than half the maximum rate just before sunrise. The volume of oxic sediment around burrow structures was influenced by changing environmental conditions (benthic photosynthesis and fauna activity) but grossly exceeded that below the primary sediment surface. The volume specific respiration rate around burrows was more than seven times higher than the equivalent value at the sediment surface. A budget of the O2 consumption revealed that the O2 uptake through the burrow walls just after sunset accounted for the major part of the total O2 uptake on a diel scale. The study demonstrates that light-driven variations in fauna activity can have great effects on the total benthic O 2 consumption rate with large implications for estimated benthic mineralization rates.

Composition and temporal variability of picoeukaryote communities at a coastal site of the English Channel from 18S rDNA sequences

Abstract: We analyzed picoeukaryote assemblages at a French coastal site of the English Channel by sequencing cloned eukaryotic 18S rRNA genes in eight genetic libraries constructed from environmental samples (seven coastal, one estuarine) collected at different periods of the year. Eight hundred clones were examined by amplified restriction fragment length polymorphism (RFLP) using the restriction enzyme HaeIII. The coverage value calculated from the relative distribution of RFLP types was low, indicating that the library diversity was not entirely recovered. A variable region of the rRNA gene was partially sequenced (550 bases) and analyzed for 397 clones. Thirty-two clones were affiliated with metazoans. Of the remaining clones, 132 were affiliated to algal classes (especially Prasinophyceae, Cryptophyceae, Dinophyceae, and Prymnesiophyceae) and 107 to known heterotrophic groups (Cercozoa, choanoflagellates, stramenopiles, and ciliates). One hundred three sequences fell into uncultivated groups of stramenopiles (43 clones) and alveolates (60 clones). We also found two potentially novel eukaryotic lineages, represented by 9 and 14 clones, respectively, not belonging to any known eukaryotic group. The overall composition of the picoeukaryote community remained fairly stable at the class/division level except during the early summer diatom bloom, when groups such as the Cryptophyceae and the ciliates completely disappeared. However, at a finer taxonomic level (corresponding to 98% sequence identity), the majority of the operational taxonomic units (OTU) were only observed once.

Seasonal variation of the δC and δN of particulate and dissolved carbon and nitrogen in Lake Lugano: Constraints on biogeochemical cycling in a eutrophic lake

Abstract: We determined the d 13 C and d 15 N of water-column particulate organic matter (POM), dissolved inorganic carbon, and nitrate, together with water chemistry and phytoplankton biomass and species composition every month in eutrophic Lake Lugano. As primary productivity increased during spring, the d 13 C of photic-zone POM increased from 234‰ to 224‰. This 13 C enrichment reflects decreasing C-isotope fractionation between organic and inorganic carbon pools in response to decreasing surface water [CO2(aq)]. Variations in the d 15 N of surface-water POM (12‰ to 18‰) collected during the productive period were attributed to isotope effects associated with nitrate uptake, nitrogen fixation, and mixing of different organic matter sources. The apparent N-isotope enrichment («) associated with nitrate assimilation varied with «52 1.0‰ 6 0.9 for diatoms and «52 3.4‰ 6 0.4 for green algae. The mechanisms controlling the N-isotopic composition of surface-water nitrate include the combined processes of nitrate assimilation, nitrification, mixing of water masses, and external nitrate loading. There was no consistent relation between the d 15 N of POM, the d 15 N of nitrate, and the nitrate concentration in surface waters. Low d 13 C( 260‰) of POM from the benthic nepheloid layer (BNL) indicated that as much as 80% of the POM in the BNL consisted of methanotrophic bacteria. The d 13 C and d 15 N of near-bottom POM allowed tracing a ‘‘bacteriocline’’, which rose with the expanding volume of bottom-water suboxia. Associated with the development of anaerobic conditions and coupled to decreasing [NO ], the d 15 N of bottom-water nitrate progressively increased 2 3 from 18‰ to 127‰, indicating active denitrification in the hypolimnion. This study demonstrates the potential of natural-abundance level measurements of stable carbon and nitrogen isotopes to trace biogeochemical processes in lakes. However, the d 15 N of POM in eutrophic lakes is of limited use as an indicator of the degree of nitrate utilization and trophic state.

Why the limiting nutrient differs between temperate coastal seas and freshwater lakes: A matter of salt

Abstract: Comparison of bottom-water chemistry in the marine‐limnic habitat gradient shows greater phosphorus availability in marine waters, primarily because of enhanced iron sequestration by sulfide. In the oxidative hydrolysis of iron and the concomitant precipitation of phosphate, a minimum of two iron atoms are needed to precipitate one phosphate molecule (Fe : P 5 2). However, dissolved Fe : P , 2 predominates in anoxic marine waters, therefore leaving some phosphate in solution after oxygenation because of a shortage of dissolved iron for phosphate coprecipitation by iron oxyhydroxide. In contrast, anoxic bottom waters in most freshwater lakes show Fe : P . 2, allowing almost complete phosphate removal on oxygenation. This difference is a consequence of the high sulfate content of sea salt, and a main reason why nitrogen normally limits net primary production in temperate coastal waters, in contrast to the predominant phosphorus limitation of near-neutral lakes.

Viral release of iron and its bioavailability to marine plankton

Abstract: The biological availability of Fe has been demonstrated to strongly influence both primary and secondary production in pelagic as well as coastal upwelling high-nutrient low-chlorophyll (HNLC) regimes. Although nearly all of the dissolved Fe in marine surface waters is thought to be complexed by organic ligands, the character and origin of these Fe-organic complexes remains a mystery. Here we report that the activity of naturally occurring viral populations in an HNLC coastal upwelling system can regenerate sufficient concentrations of dissolved Fe to support the growth of the native phytoplankton community. When combined with studies that have demonstrated that Fe in virus-mediated lysates of heterotrophic bacteria and cyanobacteria is highly bioavailable to model marine plankton, our data demonstrate that viral activity in this marine system (and potentially others) is critical to the recycling of organically complexed Fe that supports as much as 90% of primary production in HNLC surface waters.

Anaerobic N2 production in Arctic sea ice

Abstract: We quantified anaerobic N 2 production through bacterial denitrification and anaerobic NH oxidation (anammox) 1 4 in first-year ice from Young Sound (748N) and in an ice floe off Northeast Greenland (79 8N). Bacterial denitrification activity (100‐300 nmol N L 21 sea ice d 21 ) occurred in the lower 0.5 m of the sea ice, which had high concentrations of NO , NH , and dissolved organic carbon (DOC). Despite sea-ice algal production in the lower sea-ice layers, 21 34 heterotrophic activity resulted in a net O 2 consumption of 13 mmol O2 L 21 sea ice d 21 in the lower 0.5-m ice layers. Together with melting of deoxygenated ice crystals, this led to anoxic conditions in the brine system favoring conditions for anaerobic NO reduction. Numbers of anaerobic NO -reducing bacteria in the same ice layers were 22 33 high (1.1 3 10 5 cells ml 21

Carbon dioxide degassing and inorganic carbon export from a marsh‐dominated estuary (the Duplin River): A marsh CO2 pump

Abstract: We investigated the seasonal changes and the interactions of the CO2 system in a marsh-dominated estuary, the Duplin River, on Sapelo Island, Georgia. Surface water pCO2, total alkalinity (TA), and total dissolved inorganic carbon (DIC) showed a pronounced seasonal progression in this system. The estuary released 256‐306 g C m 22 (of water) yr 21 of CO2 to the atmosphere and exported 109 g C m 22 (of water) yr 21 of DIC to adjacent coastal waters. There was a clear seasonal pattern in both fluxes with spring minima, and late summer and fall maxima. Release of inorganic carbon as a respiratory product from surrounding salt marshes is the primary process supporting these two carbon export terms. Concurrently, TA was exported from the Duplin River mainly in summer and fall as a result of anaerobic respiration in marshes. Based on data from the upper Duplin River, export of inorganic carbon from salt marshes was 156 g C m 22 (of marsh) yr 21 . Extrapolation of this value to all southeastern U.S. salt marshes indicates that they cumulatively export 0.7 3 10 12 g inorganic carbon annually to coastal waters. This export rivals that of riverine inorganic carbon flux. We propose that the CO 2 fixation of marsh grasses and the subsequent export of inorganic and organic carbon is one major mechanism that causes the marsh-influenced nearshore and offshore waters in the southeastern U.S. to be annual net sources of atmospheric CO2. We also suggest that this process provides an efficient and unique means for ocean carbon sequestration of atmospheric CO 2.

Dissolution mortality of juvenile bivalves in coastal marine deposits

Abstract: We evaluate experimentally the effect of carbonate saturation state at the sediment-water interface (SWI) on survivorship of various size classes of the juvenile bivalve Mercenaria mercenaria. Populations of 0.2-mm, 0.3-mm, 1-mm, and 2-mm M. mercenaria were introduced to sediments realistically undersaturated (experimental, saturation state with respect to aragonite = Ώaragonite = IMP/K′sp = ~0.3) and saturated (control, Ώaragonite = ~1.5) with respect to aragonite in order to evaluate the impact of saturation state and dissolution on survivorship. Linear regression analysis was used to examine mortality within each treatment over time and show significant mortality for each size class in experimental-undersaturated treatments only (P < 0.05). Mortality rates in experimental undersaturated sediments were -11.8, -4.8, -1.9, and -1.1% d−1 for the 0.2-, 0.3-, 1.0-, and 2.0-mm bivalves, respectively. Analysis of covariance (ANCOVA) was used to examine differences in mortality between treatments over time and show significantly different mortality rates for the 0.2-, 0.3-, and 1-mm individuals (P < 0.05). Dissolution may represent a previously unrecognized yet significant source of mortality for "just-set" juvenile bivalves, particularly the very small individuals that have been largely ignored in recruitment studies to date. Dissolution-induced mortality may help explain the exponential losses of juvenile bivalves following their transition from the pelagic larval phase to the benthic juvenile phase.