Pelagic Compartments Research Articles

Water column oxygenation by Posidonia oceanica seagrass meadows in coastal areas: A modelling approach

BackgroundSeagrass meadows are among the most abundant marine coastal ecosystems in the world. The wide variety of species, a worldwide distribution with overall high abundance, and especially their high productivity make them a plausible nature-based blue carbon solution to mitigate atmospheric CO2 levels. In the Mediterranean Basin, the endemic angiosperm Posidonia oceanica plays a remarkable role as a marine habitat provider in shallow waters through its vertical growth and as a carbon sink storing allochthonous carbon and biomass underneath the meadows. ObjectivesHere, we assess the capacity of a pristine meadow to oxygenate the water column in the coastal area of the Balearic Islands through an evaluation of the metabolic rates in the benthic compartment as well as the resulting oxygen concentrations in the pelagic compartment. MethodsGross primary production (GPP), respiration (R), and net community production (NCP) are determined from dissolved oxygen (DO) measurements using two different calculation methods: a model developed for this purpose is used for data obtained from water column sensors and benthic multiparametric sensors, whereas the mass balance of measured DO is used to calculate the metabolic rates inside benthic chambers. ResultsThe meadow at our study site was characterised as a net autotrophic ecosystem throughout the year. Oxygen productivity was significantly higher in the benthic compartment than in the water column and followed clear seasonal patterns, with enhanced productivity during spring. NoveltyThis work shows the key role of a healthy Posidonia oceanica ecosystem as a water column oxygenator by comparing primary production using three different sampling strategies. The potential of the seagrass as climate change mitigator and its importance for the Mediterranean coasts should be considered in future coastal planning strategies.

Spatial and seasonal variability of the carbon isotopic signature of Daphnia and their ephippia in four French lakes: Implications for the study of carbon transfers in lake food webs

Abstract Carbon isotope analysis (δ13C) of Daphnia resting‐eggs (ephippia) in sedimentary records can be used to reconstruct past carbon transfers in pelagic food webs in lakes. However, there may be seasonal variability of cladoceran δ13C and ephippia production that could affect their use as palaeoecological indicators of pelagic carbon transfers. This is particularly likely in stratified lakes where availability of different pelagic carbon sources is seasonal. In addition, there are currently no studies on spatial variability of ephippia δ13C in deep areas of lakes and its implications for sampling strategies in paleolimnological studies. Four French lakes were sampled for over a year to evaluate seasonal variation of the carbon sources consumed by Daphnia using analyses of the Daphnia carbon isotope signature (δ13CDaphnia) and suspended organic matter signature (δ13Cseston). Daphnia ephippia were also collected from surface sediments at maximum depth in the four lakes. Ephippia signatures (δ13Cephippia) were compared to the δ13CDaphnia to evaluate ephippia production periods and the implications for the use of δ13Cephippia in trophic functioning studies. In addition, spatial variability of δ13CDaphnia among the four lakes and the variability of δ13Cephippia in the deep area of one lake were assessed to determine the relevance of a single sampling point in the context of paleolimnological studies. A similar pattern was observed in all four lakes: δ13CDaphnia was close to δ13Cseston during the summer stratification period, but δ13CDaphnia became much lower than δ13Cseston after the autumnal turnover. Ephippia seem to be produced either during the summer stratification period or after autumnal turnover depending on the lake. No spatial variability was observed among the sampling points in δ13CDaphnia in the four lakes, and δ13Cephippia seems to be spatially randomly distributed in the deep zone of the studied lake. In the four lakes, summer δ13CDaphnia values were never below −40‰, reflecting mainly the consumption of phytoplankton, which in turn uses varying amounts of CO2 from respiration (depending on the lake). After autumnal turnover, the δ13CDaphnia values reached values far below −40‰, reflecting the transfer of a non‐negligible part of C‐CH4 for three of the lakes. Seasonal stratification and turnover mechanisms seem to influence the availability of carbon sources in the pelagic compartment of the four lakes. This study shows that the timing of ephippia production affects the information provided by δ13Cephippia due to seasonal differences in the mechanisms (stratification, autumnal turnover) that determine which carbon sources are available in the pelagic compartment. Therefore, using δ13Cephippia to study past pelagic transfers of carbon in stratified lakes may entail uncertainty if used alone and requires multi‐proxy studies. Finally, δ13Cephippia appear to be homogeneously distributed in the deepest area of a medium‐sized, single basin. A single core retrieved from the deepest part of the lake should therefore provide a representative sample of the ephippia produced in a lake.

Food web interactions in a human dominated Mediterranean coastal ecosystem

Mediterranean coastal ecosystems provide various valuable ecosystem goods and services; however, they are vulnerable to ecological degradation due to a dramatic increase in resource use and environmental stress. Disentangling the effects of multiple human interventions on coastal ecosystems requires whole description of food web interactions using quantitative tools. A mass balance Ecopath model has been developed here for Saronikos Gulf, a naturally oligotrophic Mediterranean coastal ecosystem with a long history of human interventions. Our main focus was to describe the structure and functioning of the ecosystem, investigate the trophic interplay among the various compartments of the food web under the impact of mixed multi-gear fisheries, and to quantify resilience related emergent ecosystem properties. To this end, we reviewed a large amount of local and regional biological information which was integrated in 40 functional groups covering all trophic levels, while fishing activities were described with 7 fleets. The model shared characteristics of both productive (e.g., high amount of flows) and oligotrophic systems (e.g., low biomass accumulation) and presented typical features of Mediterranean ecosystem functioning, such as the importance of detritus as an energy source, strong benthic-pelagic coupling and the dominance of the pelagic compartment in terms of total production and consumption. Trophic forcing in the ecosystem of Saronikos Gulf was complex with both top-down and bottom-up drivers being important. Zooplankton was the central nexus between basal resources and higher trophic levels, while top predators such as hake, squids and anglerfish were identified as keystone species presenting a significant overall effect on the food web via direct and indirect trophic interactions. Ecological indicators depicted a moderately complex food-web of a large and immature ecosystem with its strengths in reserve being affected by environmental degradation. Additionally, exploitation indices classified fishing activities in Saronikos Gulf as unsustainable, affecting several target groups, including high trophic level species. However, the morphological and bathymetric complexity of Saronikos Gulf seems to function as a natural ecological reserve for the ecosystem by providing nursery grounds to various species (e.g., hake, small pelagic fishes) and supporting important fish stocks for local fisheries.

Biogeochemical modelling of a tropical coastal area undergoing seasonal upwelling and impacted by untreated submarine outfall

A coupled 3D hydrodynamic-ecological model was applied to the Santa Marta Coastal Area (SMCA, Colombian Caribbean) to provide insights into the role of external stressors (e.g. wastewater outfall and upwelling) on the water quality and benthic – pelagic coupling. The model was calibrated and validated based on benthic metabolic measurements, satellite–derived chlorophyll-a (Chl-a) and sea surface temperature (SST) maps, field and literature water quality data.The model was able to reproduce the complex dynamics and fast transitions of temperature, nutrients, and phytoplankton, including the stratification and mixing periods during the non-upwelling (NUPW) and upwelling (UPW) seasons. Wide and fast changes in the temperatures and the highly flushed environment prevented excess phytoplankton growth and nutrient accumulation in the benthic and pelagic compartments. The model proved to be a reliable research tool to analyze the interactive effects of upwelling and untreated wastewater on the functioning of a tropical bay.

Chemical Characterization and Quantification of Titanium Dioxide Nanoparticles (TiO2-NPs) in Seafood by Single-Particle ICP-MS: Assessment of Dietary Exposure.

The significant increase in the production and variety of nanoparticles (NPs) has led to their release into the environment, especially into the marine environment. Titanium dioxide nanoparticles (TiO2-NPs) are used in different industrial sectors, from the food industry to several consumer and household products. Since the aquatic environment is highly sensitive to contamination by TiO2-NPs, this work aimed to give a preliminary assessment of the contamination of packaged seafood, where the food additive TiO2 (E171) is not to be intentionally added. This allowed providing a chemical characterization and quantification of TiO2-NPs in processed canned fish products belonging to different trophic positions of the pelagic compartment and in canned clam. The new emerging technique called single-particle inductively coupled plasma mass spectrometry (spICP-MS) was applied, which allows the determination of nanoparticle number-based concentration, as well as the dissolved titanium. This study highlights how processed food, where the pigment E171 was not intentionally added, contains TiO2 in its nanoparticle form, as well as dissolved titanium. Processed clam represented the seafood with the highest content of TiO2-NPs. In pelagic fish species, we found progressively higher levels and smaller sizes of TiO2-NPs from smaller to larger fish. Our results highlight the importance of planning the characterization and quantification of TiO2-NPs in food both processed and not, as well as where the pigment E171 is intentionally added and not, as it is not the only source of TiO2-NPs. This result represents a solid step toward being able to estimate the real level of dietary exposure to TiO2-NPs for the general population and the related health risks.

Key prey indicates high resilience on marine soft bottom habitats

The key prey was determined based on the diet and spatial patterns of the Nektonic community in southern Brazil. The proposed tool to discriminate key prey was based on simple probabilistic methods and analytical procedures that integrate freely available software on the web. To avoid using arbitrary criteria in key prey determination it was used an indicator based on an adapted outlier analysis including a run of principal component analysis (PCA) and then the choice of prey that fall out of the 99% concentration ellipse. The results showed three key prey identified at species level: the shrimp Artemesia longinaris in the coastal habitats and euphausid Euphausia similis and anchovy Engraulis anchoita in the continental shelf habitats (warm and cold). The analysis of the diets of the indicator species of three necktonic assemblages showed that all of them had both pelagic and detritus as primary sources of energy. However, in shallow coastal waters prevailed access to benthic food web key prey. In deeper areas, the Warm shelf assemblage accessed more evenly the epifauna, infauna and the pelagic compartments, while the Cold shelf assemblage was more dependent on planktonic production and had a prevalence of pelagic key prey. Is demonstrated the importance of the identification of key prey, since it may indicate greater or lesser stability of predator populations depending on whether they come from compartments with more or less dynamic primary production processes, including climate-related changes that may affect the predator prey interactions. This study confirmed the prediction that demersal nekton has high disturbance recovery capacity, which may mask for decades the growing impact of fishing.

The Hierarchic Treatment of Marine Ecological Information from Spatial Networks of Benthic Platforms.

Measuring biodiversity simultaneously in different locations, at different temporal scales, and over wide spatial scales is of strategic importance for the improvement of our understanding of the functioning of marine ecosystems and for the conservation of their biodiversity. Monitoring networks of cabled observatories, along with other docked autonomous systems (e.g., Remotely Operated Vehicles [ROVs], Autonomous Underwater Vehicles [AUVs], and crawlers), are being conceived and established at a spatial scale capable of tracking energy fluxes across benthic and pelagic compartments, as well as across geographic ecotones. At the same time, optoacoustic imaging is sustaining an unprecedented expansion in marine ecological monitoring, enabling the acquisition of new biological and environmental data at an appropriate spatiotemporal scale. At this stage, one of the main problems for an effective application of these technologies is the processing, storage, and treatment of the acquired complex ecological information. Here, we provide a conceptual overview on the technological developments in the multiparametric generation, storage, and automated hierarchic treatment of biological and environmental information required to capture the spatiotemporal complexity of a marine ecosystem. In doing so, we present a pipeline of ecological data acquisition and processing in different steps and prone to automation. We also give an example of population biomass, community richness and biodiversity data computation (as indicators for ecosystem functionality) with an Internet Operated Vehicle (a mobile crawler). Finally, we discuss the software requirements for that automated data processing at the level of cyber-infrastructures with sensor calibration and control, data banking, and ingestion into large data portals.

Consequences of the experimental removal of Sabella spallanzanii (Gmelin, 1791) from the fouling assemblage of a Mediterranean harbour

Filter feeding invertebrates are a relevant component of fouling assemblages with a pivotal role in ecological processes, since they improve water quality, enhance habitat heterogeneity and transfer organic matter from the water column to the benthos. They modulate the availability of resources to other species, with effects on the density and behavior of the surrounding macrofauna. The fanworm Sabella spallanzanii, one of the largest and most abundant Mediterranean filter feeders, provides a shelter for predation and a secondary substrate for algae and settlement for sessile invertebrates. We tested its role in driving the structure of fouling assemblages, through a removal experiment.The experiment was one-year-long, with four sampling times. The effect of the removal on the fouling community was marginal in terms of species richness and evenness, while the biomass showed important differences, with a constant increase over time with higher values in the samples containing S. spallanzanii. At the end of observations, the biomass reached the value of 3917 g DW m-2 in controls and 2073 g DW m-2 in treatments. The empty space left by fanworms was not used by other species with similar biomasses. It is possible that the functioning of fouling communities may, in the event of loss of species, fluctuate in terms of biomass mobilization to different compartments, either towards the pelagic compartment or to the detritus chain. In systems with reduced water turnover, this by-pass can have important consequences in terms of stability and ecological balance.

Coupling carbon metabolism and dissolved organic carbon fluxes in benthic and pelagic coastal communities

The knowledge of the production and carbon transference dynamics between adjacent communities is essential to gain a better understanding on the role of coastal areas in the global carbon cycle. Here, we assess the carbon metabolism (through dissolved oxygen –DO– concentration) and the net dissolved organic carbon (DOC) fluxes for benthic (dominated by the seagrass Cymodocea nodosa, the rhizophytic green algae Caulerpa prolifera and unvegetated sediments) and pelagic plankton communities co–habiting the same shallow coastal area in Cadiz Bay, southern Spain. Both the seagrass and macroalgae meadows were highly autotrophic, but the carbon metabolism of C. prolifera community shifted seasonally from net autotrophic to net heterotrophic. Unvegetated benthic communities were slightly heterotrophic throughout the year, while plankton community was net autotrophic. This study reveals how the different components of the ecosystem (i.e. macrophyte, sediment or plankton community) may have a seasonal variability in the contribution to the NCP in shallow coastal areas. Moreover, our results suggest that pelagic communities can be essential to maintain autotrophy of coastal system during periods of low benthic productivity. Regarding DOC, benthic communities were net DOC producers, undergoing a marked seasonality with maximum net DOC production during summer. This indicated an extra carbon assimilation not usually contemplated in productivity studies, which entails an underestimation of gross production in these communities. A strong relationship between net DOC flux and net community production (NCP) was found for the three benthic communities. The plankton community was a net DOC consumer indicating a strong coupling between the benthic and pelagic compartments in the bay. When up–scaling our results to the whole bay, the NCP estimated for the benthos was 8800 ± 3770 Tons C y−1, and the net DOC flux was 2465 ± 830 Tons C y−1, which were mainly due to vegetated communities, indicating the importance of coastal vegetated benthic ones in the carbon metabolism and DOC fluxes.

Green Tides: New Consequences of the Eutrophication of Natural Waters (Invited Review)

In recent decades, alongside the comparatively well-studied bloom caused by phytoplankton, a bloom of marine and fresh waters caused by littoral benthic macroalgae of three genera—Ulva, Cladophora, and Spirogyra—have become a global phenomenon. In the present review, an attempt is made to gain an understanding of why it is these taxa of green filamentous algae that start to grow rapidly in the spring in many water bodies and streams, including oligotrophic waters, and then float up from the bottom, forming floating mats (metaphyton); then their decaying masses are washed ashore and cause substantial ecological and economical losses. Peculiar and common ecological and physiological features of Ulva, Cladophora, and Spirogyra favorable for the formation of green tides are considered. Although eutrophication (the supply of nitrogen and phosphorus from agricultural lands, industrial and domestic wastewaters, and aquaculture) is the evident cause of the increase in algal biomass, it is suggested that the location of external fluxes of inorganic nutrients (surface runoff or groundwater discharge), as well as the biogenic redirection of internal fluxes of nitrogen and phosphorus from pelagial to littoral (benthification), play a key role in the formation of green tides. Measures for controlling green tides are discussed. The necessity for detailed studies of the metaphytonic form of vegetation of benthic macroalgae is emphasized. Obviously, a revision of the present concept of oligotrophic/eutrophic waters which considers only the pelagic compartments of aquatic ecosystems is required.

Optimization of sampling, cell collection and counting for the monitoring of benthic harmful algal blooms: Application to Ostreopsis spp. blooms in the Mediterranean Sea

Toxic blooms of Ostreopsis spp. are greatly challenging to monitor due to the complexity and variability of cell repartition among benthic and pelagic compartments. This results in marked differences in employed methodologies for the survey of their dynamics and hampers the definition of the associated toxic risk. The present study aims at testing and improving common methodologies used for sampling, processing and counting of field samples. It contributes to the identification of the most suitable strategies for the monitoring and mitigation of Ostreopsis blooms in coastal waters. For a sampling based on the collection of macrophytes, the role of the fixative addition and of agitation steps in the efficiency of epiphytic cell collection was defined. For planktonic estimations, the influence of the volume used for concentrating water samples was characterized as a function of Ostreopsis cell abundance. The deployment of artificial substrates was tested and confirmed strong advantages of this new sampling methodology, including an integration of a part of the spatial and temporal variability of the cell distribution.

Intra-annual variation in planktonic ciliate species composition (Protista: Ciliophora) in different strata in a shallow floodplain lake

Abstract Aim: We aimed to evaluate the variation in planktonic ciliate species composition in different strata of the Guaraná Lake, encompassing high and low water periods, at the Upper Paraná River floodplain. Methods Samplings were collected monthly between March 2007 and February 2008, from the epilimnion, metalimnion and hypolimnion. Ciliates samples were filtered using a plankton net of 10µm mesh size and identified in vivo under an optical microscope. Results Among 112 species identified, 13 were found exclusively during the high water periods and 39 during the low water period. Results of nonparametric extrapolation indices evidenced that the observed richness represented between 70% and 90% of the estimated richness. Regarding the variation in species composition, Beta1 index showed that the alteration in composition between strata during the low water period was slightly greater than that registered during high waters. Cluster analysis evidenced a higher dissimilarity in ciliate species composition between periods than among the different strata. The greatest variation in species composition was verified during the distinct hydrological periods, whereas no significant differences were observed for the different strata analysed. Conclusions We found that in the pelagic compartment, ciliate species composition changed significantly between hydrological periods, and a higher similarity in species composition among strata was observed during the high water period. Therefore, alterations in the vertical distribution seem to be related to the homogenizing effect of the floods in the water column stability.

Multi-contamination (heavy metals, polychlorinated biphenyls and polycyclic aromatic hydrocarbons) of littoral sediments and the associated ecological risk assessment in a large lake in France (Lake Bourget)

The lake littoral sediment is exposed to a large array of contaminants that can exhibit significant spatial variability and challenge our ability to assess contamination at lake scale. In this study, littoral sediment contamination was characterized among ten different sites in a large peri-alpine lake (Lake Bourget) regarding three groups of contaminants: 6 heavy metals, 15 polycyclic aromatic hydrocarbons and 7 polychlorinated biphenyls. The contamination profiles significantly varied among sites and differed from those previously reported for the deepest zone of the lake. An integrative approach including chemical and biological analyses was conducted to relate site contamination to ecological risk. The chemical approach consisted in mean PEC quotient calculation (average of the ratios of the contaminants concentration to their corresponding Probable Effect Concentration values) and revealed a low and heterogeneous toxicity of the contaminant mixture along the littoral. Biological analysis including both laboratory (microcosm assays) and in situ (Acetylcholine Esterase (AChE) and Glutathione S-Transferase (GST) activity measurements) experiments highlighted significant differences among sites both in the field and in laboratory assays suggesting a spatial variation of the biota response to contamination. Linear regressions were performed between mean PEC quotients and biological results to assess whether littoral ecological risk was explained by the contamination profiles. The results highly depended on the study benthic or pelagic compartment. Regarding autochthonous Corbicula fluminea, no significant relationship between mean PEC quotients and biomarker activity was found while a significant increase in AChE was observed on autochthonous chironomids, suggesting different stress among benthic organisms. Both AChE and GST in caged pelagic Daphnia magna showed a significant positive relationship with mean PEC quotients. This study underlines the importance of accounting for spatial variations in lake littoral sediment contamination and the need for performing an integrative approach coupling chemical, field and laboratory analyses to assess the ecological risk.

Climate-driven changes in functional biogeography of Arctic marine fish communities.

Climate change triggers poleward shifts in species distribution leading to changes in biogeography. In the marine environment, fish respond quickly to warming, causing community-wide reorganizations, which result in profound changes in ecosystem functioning. Functional biogeography provides a framework to address how ecosystem functioning may be affected by climate change over large spatial scales. However, there are few studies on functional biogeography in the marine environment, and none in the Arctic, where climate-driven changes are most rapid and extensive. We investigated the impact of climate warming on the functional biogeography of the Barents Sea, which is characterized by a sharp zoogeographic divide separating boreal from Arctic species. Our unique dataset covered 52 fish species, 15 functional traits, and 3,660 stations sampled during the recent warming period. We found that the functional traits characterizing Arctic fish communities, mainly composed of small-sized bottom-dwelling benthivores, are being rapidly replaced by traits of incoming boreal species, particularly the larger, longer lived, and more piscivorous species. The changes in functional traits detected in the Arctic can be predicted based on the characteristics of species expected to undergo quick poleward shifts in response to warming. These are the large, generalist, motile species, such as cod and haddock. We show how functional biogeography can provide important insights into the relationship between species composition, diversity, ecosystem functioning, and environmental drivers. This represents invaluable knowledge in a period when communities and ecosystems experience rapid climate-driven changes across biogeographical regions.

Nutrient regeneration in the water column and at the sediment–water interface in pearl oyster culture (Pinctada margaritifera) in a deep atoll lagoon (Ahe, French Polynesia)

The aim of this work was to provide a first estimation of the overall contribution of pearl oyster culture to nutrient regeneration in a deep atoll lagoon. Nutrient release by pearl oyster culture in the water column and nutrient fluxes at the sediment–water interface were compared in two contrasted conditions (i.e. under the influence or not of pearl oyster farming) in the Ahe atoll (French Polynesia). Nitrogen flux intensity was higher in the water column than at the benthic interface. Nitrogen was released at a rate of 31.36 μmol h−1 m−2 in the water column and 12.05 μmol h−1 m−2 at the sediment–water interface. Average phosphorus flux was 2.85 μmol h−1 m−2 at the sediment–water interface and 2.16 μmol h−1 m−2 in the water column. In this deep lagoon, pearl oyster culture exerted more influence in the pelagic compartment than at the benthic interface where flux rate seemed not to be influenced by the presence of pearl oyster culture. These results demonstrate that it is essential to study these two interfaces in concert when assessing the impact of suspended shellfish farming on nutrient dynamics. Overall, the impact of pearl oyster culture may stimulate phytoplankton growth near cultivation areas through the rapid recycling of inorganic nutrients.

Water quality in four reservoirs of the metropolitan region of São Paulo, Brazil

Abstract Aim: This paper describes some limnological variables of four reservoirs with different trophic status (Billings, Guarapiranga, Jundiaí and Paiva Castro) of the Metropolitan Region of São Paulo, Brazil, aiming to characterize the water quality in these reservoirs. Methods Water sampling occurred between October 2011 and September 2012, on subsurface, at the pelagic compartment. Physical and chemical characteristics of water were evaluated, and the Trophic State Index (TSI) was calculated. Multivariate Principal Components Analysis (PCA) was used to ordinate sampling sites and periods in relation to environmental data. Results Two reservoirs were classified as eutrophic (Billings and Guarapiranga), one as mesotrophic (Jundiaí) and one as oligotrophic (Paiva Castro). The highest concentrations of total phosphorus, nitrogen and chlorophyll a were recorded in the eutrophic reservoirs. The lowest values of dissolved oxygen concentration were registered at Guarapiranga reservoir. Residence time in Paiva Castro reservoir was very low (one day); on the other hand, the highest value was recorded in Billings reservoir: 428 days.

Research on self-purification capacity of Lake Taihu.

An effective measure to cope with eutrophication of lakes is to remove nutrients that can cause algal blooming by taking advantage of natural water purification processes. Here, the term "purification" is defined, in a wide sense, as the potential role of a water body to contribute to the reduction of pollutants and thus controlling eutrophication. Also regarded as a kind of ecological regulating services, biological purification involves various processes concerning seasonal nutrient fixation, such as uptake by aquatic macrophyte, biofouling onto foliage substrates, feeding by organisms in higher trophic level, and eternal loss or removal of substance from the water. In order to evaluate the water purification ability, a numerical lake ecosystem model (EcoTaihu) was developed and applied to Lakes Taihu. The model includes the biological interactions between pelagic compartments (phytoplankton and zooplankton, detritus, dissolved organic matter, fish, and nutrients). Under dynamic forcing of meteorological and hydrological parameters, the model was run over years to evaluate the annual nutrient cycles and purification functions. The reproducibility of the model was validated for water body by comparison with the field data from the water quality monitoring campaign. Numerical results revealed that self-purification capacity of nitrogen of Lake Taihu in years 2006, 2008, and 2010 is 4.00 × 10(4), 4.27 × 10(4), and 4.11 × 10(4)ton, respectively, whereas self-purification capacity of phosphorus of Lake Taihu in years 2006, 2008, and 2010 is 1.56 × 10(3), 1.80 × 10(3), and 1.71 × 10(3)ton, respectively.

Source and trophic transfer of mercury in plankton from an ultraoligotrophic lacustrine system (Lake Nahuel Huapi, North Patagonia).

The incorporation and trophic transfer of total and methyl mercury (THg, MeHg) were examined in three size classes of plankton (10-53, 53-200, and >200 μm size range) and a small planktivorous fish, Galaxias maculatus, from the large multi-branched Lake Nahuel Huapi (North Patagonia, Argentina). Three sites representing a large range of lake benthic-pelagic structures (based on depth and shoreline characteristics) and precipitation regimes were sampled. Nitrogen and carbon stable isotopes (δ(15)N, δ(13)C) were analyzed to assess Hg trophodynamics. Selenium concentrations were determined together with THg in order to consider its potential effect on Hg trophodynamics. High THg concentrations (0.1-255 µg g(-1) dry weight (DW)) were measured in plankton, largely in inorganic form (MeHg: 3-29 ng g(-1) DW, 0.02-7% of THg, in the two larger size classes). A trend of increasing THg concentrations, varying in two to three orders of magnitude, with decreasing plankton size was associated with precipitation measured prior to each sampling event. Passive adsorption of dissolved Hg(2+) from wet deposition and runoff is considered to be the principal Hg uptake mechanism at the base of the pelagic food web. Despite the initially high THg uptake in the smaller plankton classes, the transfer to G. maculatus, and consequently to the entire food web, is likely limited due to low proportion of MeHg to THg in plankton. Furthermore, evidence of G. maculatus with benthic feeding habits having higher impact on MeHg trophic transfer compared to the same species with more pelagic (e.g., zooplankton) feeding habits, was observed. Although there is a high THg uptake in plankton, limited amounts are incorporated in the entire food web from the pelagic compartment.

Structure and dynamics of food webs in the water column on shelf and slope grounds of the western Mediterranean

Benthic–pelagic coupling is an important process connecting species throughout the water column, particularly, in deep-sea systems where faunal assemblages can be dense if indirectly sustained by production from the above. Through stable isotope analyses, this study explored the sources of production, trophic structure, and bentho-pelagic coupling in two locations with contrasting oceanographic conditions from the western Mediterranean, in the Balearic (BsB) and the Algerian (AsB) sub-basins. The samples of 89 dominant species (23 decapods, 19 cephalopods, 33 fishes, among the other taxa), inhabiting the hyperbenthic and pelagic domains, from the shelf break (250m), upper slope (650m), and middle slope (850m) were analyzed. Results suggested long food webs of approximately four trophic levels (TrLs) that were sustained by planktonic source material in shallower waters and degraded particulate organic matter of planktonic origin in deeper waters. Most of the collected species (70%) occupied intermediate trophic positions between the 3rd and 4th TrLs. The species δ15N and δ13C values exhibited a broad range, consistent with the high diversity that might be attributed to the oligotrophic conditions. As the depth increased, stronger segregation occurred between the trophic groups, and spatial differences were found among consumers of the two locations. Species in the AsB always had consistently higher δ15N values than in the BsB, which could possibly be attributed to the basal δ15N that was present through the food web. Despite the contrasting basin characteristics, a similarly close bentho-pelagic coupling pattern was observed at both locations, except at the deepest ground, especially at the AsB, where the mean δ13C values from the hyperbenthic and pelagic compartments were more distant. This could be related to the higher degree of reworking of organic matter in the AsB. Overall, these findings suggested the need for a depth-stratified approach to analyze deep-sea food webs in the study site in future studies.

Bottom‐up effects of lake sediment on pelagic food‐web compartments: a mesocosm study

Summary Sediment plays a key role in organic matter (OM) and internal nutrient cycling in lakes. The role of sediment as a source of OM and its potential bottom‐up effects on the pelagic food web have rarely been studied. Particularly, the influence of the biochemical composition of sediment OM on pelagic compartments remains largely unknown. During a 5‐month experiment, we studied the influence of two different sediments added at the bottom of large replicated mesocosms on the biomass of seston and zooplankton, and their elemental and lipid compositions. The influence of sediment treatments on sedimentation rates, elemental and biochemical compositions and potential biodegradability of recently sedimented OM (c. 1 week) was also examined. The two added sediments (S1 and S2) presented contrasting elemental and biochemical compositions and potential biodegradabilities. According to their contents of organic carbon, nitrogen, proteins, sugars and polyunsaturated fatty acids, S2 appeared to be much more biodegradable than S1. Therefore, the S2 sediment was expected to release more nutrients and OM to the water column than S1, leading to changes in communities, stoichiometry and lipid compositions of pelagic compartments. Probably due to its very poor content of labile compounds, the presence of S1 at the bottom of the mesocosms did not induce changes in the biomass of seston and zooplankton. Only few changes in the stoichiometry of these compartments were observed. In contrast, S2 sediment released more phosphorus and dissolved OM into the water column than S1. As a result, the S2 treatment induced an increase in seston biomass and therefore in zooplankton biomass via herbivory. None of the sediment treatments affected the lipid composition of seston and zooplankton. Moreover, neither S1 nor S2 induced changes in the sedimentation rates, elemental and lipid compositions, and potential biodegradability of recent sediments. Our mesocosm experiment suggests that differences in the quality of lake sediments lead to moderate changes in the pelagic communities in the absence of planktivorous or omnivorous fish. Our results could partly explain the efficiency of biomanipulation for improving water quality of eutrophic lakes despite potential nutrient release from sediment.