Fate Of Primary Production Research Articles

Changes in phytoplankton size–structure alter trophic transfer in a temperate, coastal planktonic food web

AbstractMicrozooplankton grazing is an essential parameter to predict the fate of organic matter production in planktonic food webs. To identify predictors of grazing, we leveraged a 6‐yr time series of coastal plankton growth and grazing rates across contrasting environmental conditions. Phytoplankton size–structure and trophic transfer were seasonally consistent with small phytoplankton cell dominance and low trophic transfer in summer, and large cell dominance and higher trophic transfer in winter. Departures from this pattern during two disruptive events revealed a critical link between phytoplankton size–structure and trophic transfer. An unusual summer bloom of large phytoplankton cells yielded high trophic transfer, and an atypical winter dominance of small phytoplankton resulted in seasonally atypical low trophic transfer. Environmental conditions during these events were neither seasonally atypical nor unique. Thus, phytoplankton size–structure rather than environmental conditions held a key‐role driving trophic transfer. Phytoplankton size–structure is easily measurable and could impart predictive power of food‐web structure and the fate of primary production in coastal ecosystems.

Microzooplankton grazing and community composition in a high-productivity marine ecosystem

Abstract Eastern Boundary Upwelling Ecosystems (EBUE’s) are highly productive, and due to their extensive fisheries, of major economic importance. Compared to oligotrophic systems, however, relatively little is known about the fate of primary production in these systems. In order to investigate the role of microzooplankton in grazing primary production in the Canary Current EBUE, grazing experiments were conducted as part of the Meteor M129 expedition. Water for experiments was taken from depths with the highest chlorophyll a concentrations and prescreened to remove mesozooplankton. Microzooplankton could consume between 60% and 95% of primary production, as measured by chlorophyll growth rates. When individual prey groups were examined, however, clear grazing effects were more mixed, with strong effects on nanoautotrophs and heterotrophic bacteria, varied effects on Synechococcus, and few statistically significant grazing effects on Prochlorococcus, pico-eukaryotes or heterotrophic nanoflagellates. The microplankton grazer community was diverse, with 22–33 ciliate and 10–26 dinoflagellate morphotypes. The large size range of the microplankton grazers and their high diversity resulted in a broad collective food niche and considerable niche overlap. This suggests that with sufficient diversity in the grazer community, changes in that community over the course of experiments may have minimal functional effect.

Experimental assessment of the sensitivity of an estuarine phytoplankton fall bloom to acidification and warming

Abstract. We investigated the combined effect of ocean acidification and warming on the dynamics of the phytoplankton fall bloom in the Lower St. Lawrence Estuary (LSLE), Canada. Twelve 2600 L mesocosms were set to initially cover a wide range of pHT (pH on the total proton scale) from 8.0 to 7.2 corresponding to a range of pCO2 from 440 to 2900 µatm, and two temperatures (in situ and +5 ∘C). The 13-day experiment captured the development and decline of a nanophytoplankton bloom dominated by the chain-forming diatom Skeletonema costatum. During the development phase of the bloom, increasing pCO2 influenced neither the magnitude nor the net growth rate of the nanophytoplankton bloom, whereas increasing the temperature by 5 ∘C stimulated the chlorophyll a (Chl a) growth rate and maximal particulate primary production (PP) by 76 % and 63 %, respectively. During the declining phase of the bloom, warming accelerated the loss of diatom cells, paralleled by a gradual decrease in the abundance of photosynthetic picoeukaryotes and a bloom of picocyanobacteria. Increasing pCO2 and warming did not influence the abundance of picoeukaryotes, while picocyanobacteria abundance was reduced by the increase in pCO2 when combined with warming in the latter phase of the experiment. Over the full duration of the experiment, the time-integrated net primary production was not significantly affected by the pCO2 treatments or warming. Overall, our results suggest that warming, rather than acidification, is more likely to alter phytoplankton autumnal bloom development in the LSLE in the decades to come. Future studies examining a broader gradient of temperatures should be conducted over a larger seasonal window in order to better constrain the potential effect of warming on the development of blooms in the LSLE and its impact on the fate of primary production.

Heterotrophic bacterial production, respiration, and growth efficiency associated with upwelling intensity in the Ulleung Basin, East Sea

We investigated bacterial production (BP) and respiration (BR), as well as the physico-chemical properties of the water column, to elucidate the effect of upwelling on heterotrophic bacterial metabolic activities and growth efficiency (BGE) in July 2012 and May 2013 in the Ulleung Basin (UB), East/Japan Sea. The upwelled conditions were characterized by higher chlorophyll-a (Chl-a) concentrations resulting from the upward shift of the nitracline compared to that of the non-upwelled condition. Analyses of the size fractions of Chl-a and pigment composition revealed that large size phytoplankton (> 20µm), mainly consisting of diatoms, appeared to be the major phytoplankton component. BP and BR were significantly correlated with Chl-a (P < 0.001), but the correlations with temperature were not significant (P > 0.05). These results suggest that bacterial metabolic activities are stimulated by the availability of organic resources enhanced by upwelling in the UB. Further statistical analysis showed that the difference in BP and BGE with variations in upwelling intensity were significant (P = 0.018 for BP, P = 0.035 for BGE), but the difference in BR was not significant (P = 0.321). These results suggest that metabolic energy is partitioned more for BP under a strong upwelling condition, i.e. high nutrient and Chl-a conditions. In contrast, the energy generated via respiration was partitioned more for maintaining metabolism rather than for biomass production under weakly or non-upwelled conditions, i.e. stratified and low Chl-a conditions. Overall, our results suggest that any changes in upwelling intensity would significantly affect the carbon cycle associated with the fate of primary production, and the role of the microbial loop in the UB where changes in the intensity and frequency of upwelling associated with climatic changes are in progress.

Diversity of Magneto-Aerotactic Behaviors and Oxygen Sensing Mechanisms in Cultured Magnetotactic Bacteria

Microorganisms living in gradient environments affect large-scale processes, including the cycling of elements such as carbon, nitrogen or sulfur, the rates and fate of primary production, and the generation of climatically active gases. Aerotaxis is a common adaptation in organisms living in the oxygen gradients of stratified environments. Magnetotactic bacteria are such gradient-inhabiting organisms that have a specific type of aerotaxis that allows them to compete at the oxic-anoxic interface. They biomineralize magnetosomes, intracellular membrane-coated magnetic nanoparticles, that comprise a permanent magnetic dipole that causes the cells to align along magnetic field lines. The magnetic alignment enables them to efficiently migrate toward an optimal oxygen concentration in microaerobic niches. This phenomenon is known as magneto-aerotaxis. Magneto-aerotaxis has only been characterized in a limited number of available cultured strains. In this work, we characterize the magneto-aerotactic behavior of 12 magnetotactic bacteria with various morphologies, phylogenies, physiologies, and flagellar apparatus. We report six different magneto-aerotactic behaviors that can be described as a combination of three distinct mechanisms, including the reported (di-)polar, axial, and a previously undescribed mechanism we named unipolar. We implement a model suggesting that the three magneto-aerotactic mechanisms are related to distinct oxygen sensing mechanisms that regulate the direction of cells’ motility in an oxygen gradient.

Vertical export of particulate organic carbon: Attenuation, composition and loss rates in the northern Barents Sea

Abstract The fate of primary production (PP) is closely linked to the ecosystem structure in aquatic environments. High pelagic consumption and recycling reduce quantity and quality of vertically exported organic material, while low to moderate pelagic consumption allows more carbon of higher quality to reach benthic communities. To evaluate the driving forces influencing changes in quantity and composition of vertical flux with depth and environmental conditions in ice-covered waters, short-term sediment trap deployments at 6–8 depths from 20 to 200 m were conducted. Eleven stations in the northern Barents Sea were investigated during early, peak and late bloom scenarios in 2003–2005. Vertical particulate organic carbon (POC) export ranged 140–760 mg C m−2 d−1 at 30 m and 58–720 mg C m−2 d−1 at 90 m depth. Strongest vertical carbon flux attenuation was observed during peak bloom scenarios. The pycnocline always defined the depth of the strongest attenuation. The POC export was highly correlated with Chl a flux (r2=0.89) and with a low POC/Chl a ratio, this indicates that fresh material is being exported to depth. A tight coupling between POC export at 90 m and particulate PP (r2=0.61) was observed and suggests that on an average 36% of daily PP was exported as POC. Deep vertical mixing observed in the Polar Front or less stable water masses did, however, enhance the vertical export and loss rates considerably. Annual estimates of vertical POC export to PP, suggests weaker retention, and thus stronger pelagic–benthic coupling in Arctic compared to the Atlantic region of the Barents Sea.

Research Opportunities and Challenges in the Indian Ocean

The Indian Ocean is a dynamically complex and highly variable system, with circulation features and biogeochemical properties that are unusual in many respects. Yet the Indian Ocean (IO) remains one of the most undersampled and least understood of the world's ocean basins. In this article, we define several outstanding research questions that need to be addressed in the IO related to ocean currents and variability, the controls and fate of primary production, global change and anthropogenic impacts, and the role of higher trophic levels in ecological processes and biogeochemical cycles. We also discuss a unique opportunity that has recently arisen for staging research in the IO.

Accounting for grazing dynamics in nitrogen‐phytoplankton‐zooplankton (NPZ) models

Nitrogen‐phytoplankton‐zooplankton (NPZ)‐type models are widely used to explore the dynamics of marine planktonic ecosystems. Within these models, grazing by zooplankton on phytoplankton that are subjected to varying degrees of nitrogen limitation is described using N‐based grazing kinetics together with fixed N assimilation efficiency. There is no empirical evidence for zooplankton displaying such behavior; on the contrary, there is evidence for a decline in zooplankton growth rates on consumption of N‐impoverished prey, with decreased assimilation efficiencies coupled with decreased ingestion rates of nutrient‐limited (i.e., poor quality, low N: C) prey. Unwittingly, then, traditional NPZ models make unjustified assumptions concerning changes in predator‐prey interactions on consumption of low‐quality prey. We explore the effects of this flawed description, also asking why NPZ models can still give reasonable descriptions of field data. Our conclusion is that one flaw may be countered by another, namely, by an inadequate description of nongrazing phytoplankton losses. In nature, these nongrazing losses are enhanced within nutrient‐depleted phytoplankton populations. In models, a decline in grazing losses on nutrient‐deprived phytoplankton is, de facto, compensated for by enhanced nongrazing losses. While the fit of the revised model to the data is not dissimilar to that of the original model (with its inappropriate descriptions of grazing and nongrazing phytoplankton mortality), the fate of primary production is very different. With the biologically more acceptable description, more material flows through the detrital compartment, with important implications for trophic dynamics. Care must be taken not to oversimplify descriptions of biology in models, as these may directly or indirectly mask the simulation of other important ecological processes.

Influence of ciliated protozoa and heterotrophic nanoflagellates on the fate of primary production in the northeast Atlantic Ocean

Heterotrophic nanoflagellates and ciliates and their herbivorous activity were studied within the framework of the Programme Océan Multidisciplinaire Méso Echelle (POMME) in the northeastern Atlantic between 16°–22°W and 38°–45°N during winter, spring, and late summer/autumn 2001. Ciliate ingestion rates of Synechococcus and eukaryotic algae were measured using fluorescently labeled prey. Heterotrophic nanoflagellate ingestion rates of Synechococcus and Prochlorococcus were also estimated. Heterotrophic nanoflagellate and ciliate standing stock within the surface layer (0–100 m) showed seasonal variation, with maximal values in spring (866 mg C m−2 and 637 mg C m−2, respectively). Oligotrichs dominated the ciliate assemblages, except at one site visited during spring, where a tintinnid bloom was observed. Ingestion of photosynthetic cells less than 10 μm in size was positively correlated (r = 0.7, p &lt; 0.05, n = 12) with primary production and accounted for 2–94% of this. Phytoplankton consumption reflected differences in the evolution of the phytoplankton bloom and in the structure of the microbial food web, both associated with the strong mesoscale hydrodynamic variability of the study area. In that context it is worthy to note that when tintinnids reached high abundances locally (1260 cells L−1), their impact as phytoplankton grazers was important and reached 69% of primary production. Generally, heterotrophic nanoflagellates and ciliates were relatively more important in determining the fate of phytogenic carbon during spring. Another interesting feature of primary production consumption was that during the autumn, when Prochlorococcus dominated the phytoplankton community, the protozoan grazing activity was ineffective in regulating the fate of primary producers.

Annual microplankton cycles in Villefranche Bay, Ligurian Sea, NW Mediterranean

Abundance and composition of microplankton were studied over a period of 2 years at two depths in Villefranche Bay (Ligurian Sea, NW Meditrranean Sea). Diatoms dominated the microplankton in late spring and autumn, whereas dinoflagellates composed the major part of the microplankton in summer. The silicoflagellate Dictyocha, fibula and the diatom Thalassionema frauenfeldii dominated in winter. Ciliates showed low variability throughout the year with the lowest abundance in February and an increase which coincided with the diatom maxima during autumn in both years. In 1998, the spring bloom (in May) was mainly composed of dinoflagellates near the surface and of diatoms in deeper layers. Subsurface diatom maxima were observed in August-September and November. In 1999, diatoms peaked in May both at the surface and at the depth of 50 m. They showed a strong maximum in October. Dinoflagellates and tintinnids showed maxima in early November. Comparisons with previous studies reveal that (i) changes in species composition have not been significant, (ii) the silicoflagellate's abundance is lower during the present study, (iii) the sequential spring bloom is composed of a pico-nanoplankton bloom in March and microphytoplankton in May, whereas in other western Mediterranean areas the spring microphytoplankton bloom is reported in February and March, (iv) high water transport through the Corsica channel coinciding with low or negative winter values of North Atlantic Oscillation (NAO) index are associated with the anomalous strong development of the spring diatom blooms in the Bay of Villefranche, whereas the usual trend is the lack of or weak development of the spring diatom bloom. This feature may determine the nature and the fate of primary production and the interannual variability in the relative importance of the microbial food web versus the microbial loop.

Effects of virus infection on respiration rates of marine phytoplankton and microplankton communities

The possible influence of viral infection on respiration rates in marine microbial pelagic communities was assessed by means of 3 experiments on respiration rate with viral concentrate addi- tion on single-species cultures of Mantoniella sp. and Micromonas pusilla and another 3 on natural microplankton communities (organisms <200 µm) from the Kattegat Sea (Astol) and the Baltic Sea. Coastal surface seawater samples were taken during cruises of the RVs 'Ancylus' and 'Argos' during winter and spring 2000. Approximately 50 to 70 l of seawater were concentrated by ultrafiltration. The experiments were started by adding a viral particle concentrate to a container with algae or a natural microplankton community; a control container was kept free of the viral concentrate addition. Oxygen concentration determinations were carried out on each treatment and control to measure respiration rates throughout the incubation period. The in vivo chlorophyll a fluorescence was also monitored as an indication of algal infection. The rates of respiration indicated that the addition of the viral particle concentrate affected the respective metabolisms of the Mantoniella sp. and Micromonas pusilla cultures as well as natural microplankton communities. Viral infection decreased the Man- toniella sp. respiration rate (by 96%) and increased the Micromonas pusilla respiration rate (by 235%). Hence, if our results can be extrapolated to nature, then, at least in a bloom situation, the fate of primary production and carbon fluxes could be strongly modulated by viral infection. The addition of a viral particle concentrate to the microplankton community generated complex responses in terms of respiration rates, which increased (by 84%) or remained similar to the controls. Our results suggest that viral infection of microplanktonic organisms could be one of the factors significantly modifying pelagic carbon fluxes.

Nitrogen in Insects: Implications for Trophic Complexity and Species Diversification

Disparities in nutrient content (nitrogen and phosphorus) between herbivores and their plant resources have lately proven to have major consequences for herbivore success, consumer-driven nutrient cycling, and the fate of primary production in ecosystems. Here we extend these findings by examining patterns of nutrient content between animals at higher trophic levels, specifically between insect herbivores and predators. Using a recently compiled database on insect nutrient content, we found that predators exhibit on average 15% greater nitrogen content than herbivores. This difference persists after accounting for variation from phylogeny and allometry. Among herbivorous insects, we also found evidence that recently derived lineages (e.g., herbivorous Diptera and Lepidoptera) have, on a relative basis, 15%-25% less body nitrogen than more ancient herbivore lineages (e.g., herbivorous Orthoptera and Hemiptera). We elaborate several testable hypotheses for the origin of differences in nitrogen content between trophic levels and among phylogenetic lineages. For example, interspecific variation in insect nitrogen content may be directly traceable to differences in dietary nitrogen (including dilution by gut contents), selected for directly in response to the differential scarcity of dietary nitrogen, or an indirect consequence of adaptation to different feeding habits. From some functional perspectives, the magnitude rather than the source of the interspecific differences in nitrogen content may be most critical. We conclude by discussing the implications of the observed patterns for both the trophic complexity of food webs and the evolutionary radiation of herbivorous insects.

Nitrogen in Insects: Implications for Trophic Complexity and Species Diversification

Disparities in nutrient content (nitrogen and phosphorus) between herbivores and their plant resources have lately proven to have major consequences for herbivore success, consumer‐driven nutrient cycling, and the fate of primary production in ecosystems. Here we extend these findings by examining patterns of nutrient content between animals at higher trophic levels, specifically between insect herbivores and predators. Using a recently compiled database on insect nutrient content, we found that predators exhibit on average 15% greater nitrogen content than herbivores. This difference persists after accounting for variation from phylogeny and allometry. Among herbivorous insects, we also found evidence that recently derived lineages (e.g., herbivorous Diptera and Lepidoptera) have, on a relative basis, 15%–25% less body nitrogen than more ancient herbivore lineages (e.g., herbivorous Orthoptera and Hemiptera). We elaborate several testable hypotheses for the origin of differences in nitrogen content between trophic levels and among phylogenetic lineages. For example, interspecific variation in insect nitrogen content may be directly traceable to differences in dietary nitrogen (including dilution by gut contents), selected for directly in response to the differential scarcity of dietary nitrogen, or an indirect consequence of adaptation to different feeding habits. From some functional perspectives, the magnitude rather than the source of the interspecific differences in nitrogen content may be most critical. We conclude by discussing the implications of the observed patterns for both the trophic complexity of food webs and the evolutionary radiation of herbivorous insects.

Primary production and biomass of size-fractionated phytoplankton off Antofagasta, Chile (23–24°S) during pre-El Niño and El Niño 1997

During a major study focusing on the fate of primary production through the food web in an upwelling system located in the Humboldt Current System, Antofagasta, Chile (23–24°S), we examined the spatial variation of phytoplankton size structure. One of the main objectives was to estimate primary production ( 14C) and chlorophyll- a biomass for three phytoplankton size fractions; pico-, nano- and micro-phytoplankton, at coastal (<10 nmi) and oceanic (20–200 nmi) stations during two research cruises, January (pre-El Niño) and July 1997 (El Niño). During January, primary production ranged from 0.5 to 9.5 mg C m −3 h −1 (mean=3.1 mg m −3 h −1). Close inshore, pico- and nanoplankton size fractions accounted for 79% of total primary production and 63% of total chlorophyll- a, while these fractions accounted for 88% of total production and 70% of total chlorophyll- a in offshore oceanic waters. The micro-phytoplankton fraction (>23 μm) increased its relative contribution to primary production from oceanic (12%) to coastal (21%) stations and was related to an upwelling process which was restricted to a narrow inshore band (<15 nmi). During July (El Niño conditions), primary productivity ranged from 0.39 to 10.7 mg C m −3 h −1 (mean=3.4 mg C m −3 h −1). The pico- and nanoplankton fractions dominated both oceanic and coastal stations and both fractions accounted for 67% of the primary production and chlorophyll- a biomass. We suggest that an oceanographic anomaly observed during July (1997 El Niño), associated with the intrusion of warmer oceanic waters to the coastal border, reduced the upwelling of nutrient-rich waters in the upper 200 m to a narrow coastal band (<6 nmi) leading to a higher dominance of pico- and nanoplankton populations in inshore waters. These observations show that pico- and nanoplankton size fractions make a significant contribution to the production and biomass, and may thus represent an alternative energy flow pathway within this upwelling area.

Impact of microzooplankton on the progression and fate of the spring bloom in fjords of northern Norway

The objective of the study was to determine the role of microzooplankton in the fate of primary production and progression of the spring phytoplankton bloom in high latitude fjords. The 3 fjords, Balsfjord, Malangen and Ullsfjord (Norway), varied in oceanic influence and in the rate of development of the spring bloom. The abundance of Phaeocystis pouchetii was relatively low in the spring of 1997 compared to previous years, and diatoms dominated the biomass of the phytoplankton assemblage in all 3 fjords. The mean biomass of microzooplankton in the top 20 m averaged 72, 66 and 80 mg C m -3 and values integrated to 170 m averaged 4560, 4450 and 6820 mg C m -2 in Balsfjord, Malangen and Ullsfjord, respectively. The composition of the microzooplankton was consistent among the fjords and over time, with the proportion of biomass split evenly between nanoflagellates, dinoflagellates and ciliates. Grazing rates of the microzooplankton community were measured with the dilution technique. The impact of microzooplankton grazing was similar among the fjords, accounting for on average 68, 63 and 55% of the production of the < 200 µm phytoplankton in Bals- fjord, Malangen and Ullsfjord, respectively. When integrated to 20 m, based on a microzooplankton biomass-specific ingestion rate, microzooplankton grazing accounted for an estimated 12 to 26% of the gross primary production. However, this does not account for the carbon requirements of a sub- stantial proportion of the microzooplankton that occurred below 20 m. Clearance rates by ciliates of nanophytoplankton cells of a similar size to the single cells of P. pouchetii were determined from the uptake rates of fluorescently labelled algae (FLA). Generally, the taxa of ciliates that were found to ingest FLA accounted for ♢ 50% of the abundance of the ciliate population in each fjord. Taxon-spe- cific ciliate clearance rates of FLA in surface waters varied with ciliate size from 5.6 to 1.3 µl cell -1 h -1 . The FLA-consuming ciliate population cleared a total of 27 〈 10 3 to 141 〈 10 3 µl l -1 d -1 in surface waters. The ingestion rates of the FLA-consuming ciliates were equivalent to between 11 and 29% of the total microzooplankton consumption. It is possible that the grazing pressure exerted by the micro- zooplankton on single cells and small colonies was high enough to decrease the overall competitive- ness of the P. pouchetii populations and contribute to their low abundance.

Fate of primary production in Posidonia oceanica meadows of the Mediterranean

Primary production of Posidonia oceanica meadows was estimated at two sites in the Mediterranean basin (Marseilles, France and Ischia, Italy), in the 10 to 20 metre depth range. At the five stations studied, foliar production (blade + sheath) ranged from 67.7 to 146.6 g C m −2 yr −1 and rhizome production from 8.2 to 18.0 g C m −2 yr −1. Based on litter input (rhythm of fall of leaves), stocks (biomass) and kinetics of litter decomposition, the rate of export of this necromass was estimated. The rate of export was strongly influenced by depth and hydrodynamic forces which ranged from 51 to 68% at Marseilles (stations exposed to strong currents) and from 37 to 49% at Ischia. Exclosures at Marseilles were used to assess the proportion of the foliar production consumed by macro-herbivores in this area. The values recorded ranged from 24.8 to 35.3 C m −2 yr −1, of which 44% is absorbed. On the basis of all these results, the fate of stocks and the flux of elements resulting from primary production were determined. For the Marseilles stations, carbon followed four major pathways: 10% was consumed by herbivores, 23% was decomposed in situ as the result of detritivore action, 35% was exported in the form of litter and 32% was stored in the matte (sheaths and rhizomes). For Ischia, the values were 3%, 34%, 27% and 36%, respectively. Extrapolation for the whole Mediterranean gave an estimated total production of P. oceanica meadows of 3.5 million tons of carbon, of which 2.4 million tons are lost to the community through export or storage in the mat.

97/00492 Mesocosm tracer studies. 2. The fate of primary production and the role of consumers in the pelagic carbon cycle of a mesotrophic lake

97/00492 Mesocosm tracer studies. 2. The fate of primary production and the role of consumers in the pelagic carbon cycle of a mesotrophic lake

Mesocosm tracer studies. 2. The fate of primary production and the role of consumers in the pelagic carbon cycle of a mesotrophic lake

The fate of primary production in the pelagic food web in mesotrophic Svinsjøen, southeastern Norway, was studied in a 10‐d mesocosm experiment with 14C addition. Specific assimilation rates for all the main compartments in the system were estimated by applying tracer models and further combined to construct a balanced C budget by means of mathematical inverse methods. The specific carbon assimilation rates for the phytoplankton and bacteria were 0.24 and 0.49 d−1, respectively, and 70% of the bacterial production was fueled by phytoplankton exudates. The C budget shows that roughly half of the gross primary production was lost as exudates and 10% was respired. Only 20% of phytoplankton net production was assimilated by consumers. The rest of the net primary production was lost as dissolved and particulate detritus or by sedimentation. Of the net bacterial production, a third accumulated as bacterial biomass, a third was assimilated by bacterivores, and a third was lost as detritus. The coupling between the microbial loop and the classical grazer food web was insignificant, probably due to low Daphnia biomass. The microbial food web thus acted as a sink rather than as a source of C to the higher consumers. Altogether, &lt;2% of gross primary production was incorporated into consumer biomass.

Suspension-Feeding Bivalves and the Fate of Primary Production: An Estuarine Model Applied to Chesapeake Bay

A probabilistic mathematical model of bivalve suspension-feeding in estuaries is based on bivalve abundance, filtering capacities, and water mixing parameters. We applied the model to five regions of the upper Chesapeake Bay, ranging from shallow tidal fresh habitats to deep mesohaline habitats, for the years 1985 to 1987. Model results indicated that existing suspension-feeding bivalves could consume more than 50% of annual primary production in shallow freshwater and oligohaline reaches of the upper Chesapeake Bay and Potomac River. In deep mesohaline portions of the Chesapeake Bay and Potomac River, suspension-feeding bivalves could consume only 10% of primary production. Independent estimates of benthic carbon demand based on benthic production supported the model predictions. Hydrodynamics of large estuaries restrict the potential of benthic suspension-feeders to crop phytoplankton production because the width and depth of these estuaries limit transport of pelagic waters to the littoral flanks of the estuaries where benthic suspension-feeders can be abundant. Benthic suspension-feeders are dominant consumers in shallow segments of the Chesapeake Bay system, but are suppressed in deeper segments. The suppression is below that set by hydrodynamic limits, and may be due to periodic hypoxia or other factors. Our results suggest that the proposed use of suspension-feeding bivalves to improve water quality of large estuaries will be limited by the depth and width of the estuary, unless the bivalves are suspended in the water column by artificial means.

Mixing, stratification and the fate of primary production in an oligotrophic multibasin lake system (Québec, Canada)

Mixing, stratification and the fate of primary production in an oligotrophic multibasin lake system (Québec, Canada)