Alain Menesguen

Modelling nitrogen, primary production and oxygen in a Mediterranean lagoon. Impact of oysters farming and inputs from the watershed

An ecosystem model based on nitrogen cycling and oxygen has been developed for the Thau lagoon. It takes into account the specific features of this Mediterranean lagoon, a semi-confined system with watershed inputs and oyster farming. The ecosystem model uses currents calculated by a two-dimensional hydrodynamic model and integrated into a box model. This model is compared with a year survey data and used to estimate nitrogen and oxygen fluxes between the different ecosystem compartments. The yearly simulation shows that the ecosystem behavior is driven by meteorological forcing, especially rain which causes watershed inputs. These inputs trigger microphytoplankton growth, which is responsible for new primary production. During dry periods, nitrogen is recycled into the lagoon thanks to oysters excretion, sediment release, microzooplankton excretion and mineralization. Ammonium produced in this way is consumed by a population of pico- and nanophytoplankton causing regenerated primary production. Consequently, the ecosystem remains highly productive in summer even without external inputs. Shellfish farming also plays an important role in the whole lagoon through biodeposition. Driven by biodeposition, sediment release is the major source of nitrogen in the water column and causes oxygen reduction. The oysters contribute to the recycling activity by excretion, which supports the regenerated primary production. They are also involved in oxygen consumption by respiration which can cause local hypoxia. Further improvements are proposed before this model may become a functional environmental model for a lagoon ecosystem.

Resistance of a coastal ecosystem to increasing eutrophic conditions: the Bay of Brest (France), a semi-enclosed zone of Western Europe

Abstract The Bay of Brest is a semi-enclosed coastal ecosystem receiving high nutrients loading from freshwater inputs. In order to analyse the response of phytoplankton stocks to increasing eutrophic conditions, a survey of the annual cycle of hydrographic properties, nutrients and chlorophyll a concentrations, and carbon uptake rates was performed at four stations in 1993. This database has been compared to earlier measurements performed during several comparable surveys within the last 20 years. As compared to the seventies, a doubled nitrate loading is now entering this ecosystem, which is related to increased agricultural activities on the drainage basins, while the geographical origin of the nitrate input has been modified. As a result of these anthropogenic modifications, summer averaged Si/N stoichiometric balance has decreased during the two last decades but, contrary to what has been observed in other coastal ecosystems, phytoplankton stocks have not increased. Several ecological factors have hindered eutrophication: the high hydrodynamic mixing with adjacent marine waters, caused by the macrotidal regime, induces important nutrients losses, temperature and mostly light limit primary production while Si and P high recycling maintain nitrogen limitation in this ecosystem. Conjunction of these non-anthropogenic factors explains the global stability of phytoplankton stocks.

Nitrogen loadings and macroalgal (Ulva sp.) Mass Accumulation in Brittany (France)

Abstract The most common manifestation of eutrophication along the coasts of Brittany (France) is the proliferation of free-floating green macroalgae of the genus Ulva on some beaches during spring and summer. In the most eutrophicated site, the Bay of Saint-Brieuc, a monitoring program has been running since 1986 until 1992 (1987 excepted) to estimate during spring and summer the nitrogen and phosphorus river loadings, the growth rate of macroalgae, their N- and P contents and the annual maximum biomass present on the site. Every year, algae get increasingly N-depleted during the vernal rapid growth phase and stay N-starved (1.5% of dry weight) all summer long. Algal phosphorus content, on the contrary, remains roughly constant at a rather high level (0.4% of dry weight). This physiological argument for nitrogen control on algal growth is confirmed by independent results at the ecosystem level. Year-to-year fluctuations of the observed annual maximum biomass at the site have been successfully correlated ...

Modelling eutrophication events in a coastal ecosystem. Sensitivity analysis

To study the eutrophication of Vilaine bay, we developed a three-dimensional hydrodynamic model and a biological model simulating phosphorus and nitrogen cycles and dissolved oxygen. To couple these models, advective currents, calculated by the hydrodynamic model, were averaged over time and space. The resulting ecological box model simulates seasonal evolution of nutrients, phytoplankton and oxygen reasonably well and reproduces the bays spatial heterogeneity. Following the calibration step, the models behaviour was analysed by means of various sensitivity studies, which led to several conclusions about eutrophication of the bay. In winter and spring, the Vilaine bay behaves like an open system enriched by the Loire plume. During the rest of the year, the bay can be considered as a closed system where the river Vilaine is the primary source of nutrients. Oxygen depletion is mostly due to consumption in the water column (plankton community respiration and remineralization). Reduction of nitrogen inputs from the river are more effective in reducing phytoplankton biomass and hypoxia than reduction in phosphorus loadings. Better knowledge of the benthic stocks of nutrients and processes controlling them is required to confirm the previous results. Spatial and temporal scales used in the model allow the simulation of seasonal trends but cannot reproduce episodic events.

Modelling seasonal dynamics of biomasses and nitrogen contents in a seagrass meadow (Zostera noltii Hornem.): application to the Thau lagoon (French Mediterranean coast)

Abstract Anumerical deterministic model for a seagrass ecosystem (Zostera noltii meadows) has been developed for the Thau lagoon. It involves both above- and belowground seagrass biomasses, nitrogen quotas and epiphytes. Driving variables are light intensity, wind speed, rain data and water temperature. This seagrass model has been coupled to another biological model in order to simulate the relative contributions of each primary producer to: (i) the total ecosystem production, (ii) the impact on inorganic nitrogen and (iii) the fluxes towards the detritus compartment. As a first step in the modelling of seagrass beds in the Thau lagoon, the model has a vertical structure based on four boxes (a water box on top of three sediment boxes) and the horizontal variability is neglected until now. This simple box structure is nevertheless representative for the shallow depth Z. noltii meadows, spread over large areas at the lagoon periphery. After calibration, simulation results have been compared with in situ measurements and have shown that the model is able to reproduce the general pattern of biomasses and nitrogen contents seasonal dynamics. Moreover, results show that, in such shallow ecosystems, seagrasses remain the most productive compartment when compared with epiphytes or phytoplankton productions, and that seagrasses, probably due to their ability in taking nutrients in the sediment, have a lower impact on nutrient concentration in the water column than the phytoplankton. Furthermore, in spite of active mechanisms of internal nitrogen redistribution and reclamation, the occurrence of a nitrogen limitation of the seagrass growth during summer, already mentioned in the literature, have also been pointed out by the model. Finally, simulations seems to point out that epiphytes and phytoplankton could compete for nitrogen in the water column, while a competition for light resources seems to be more likely between epiphytes and seagrasses.

Biogeochemical modelling in the Bay of Seine (France): an improvement by introducing phosphorus in nutrient cycles

As part of the French National Programme for Coastal Oceanography, this paper focuses on improvement of biogeochemical modelling in the: Bay of Seine (Eastern Channel), by introducing phosphorus in nutrient cycles. The Bay of Seine receives the Seine river, which exhibits very high nutrient concentrations, and this coastal zone constitutes a typical case of eutrophication in a river plume area. In terms of analyses, sequential extraction and analysis of sedimentary phosphorus were used in order, to measure various forms of particulate phosphorus in suspended matter and sediment (calcium-bound phosphate, Fe/Al-bound phosphate, exchangeable phosphate and organic phosphorus). In the modelling approach, the Bay is divided into 42 boxes and a two-layer, vertical thermohaline model is linked with the horizontal circulation scheme to take vertical stratification into account. The previous biological sub-model, with only nitrogen and silicon as nutrients, was improved by taking account of (a) various forms of bioavailable phosphorus (dissolved phosphate, exchangeable phosphate and organic phosphorus) and (b) experimental parameters, which govern the adsorption/desorption of phosphate at the solid-water interface. The ecological model accurately reproduces dissolved nutrient behaviour in the river plume, and concentrations of diatoms, flagellates and zooplankton are consistent with observed values. Simulated exchangeable phosphate and organic phosphorus in suspended matter are in the range of measured concentrations, whereas these particulate variables are simulated with less accuracy in the sediment. After evaluating the respective roles of nitrogen, phosphorus and silicon as limiting factors for phytoplanktonic growth, the model was used to estimate consequences of reduced phosphorus or nitrogen input for phytoplanktonic production in the Bay of Seine.

Hydrodynamic prevention of eutrophication in the Bay of Brest (France), a modelling approach

Abstract The Bay of Brest is a semi-enclosed coastal ecosystem where primary production is nutrient-limited, even if huge nutrients loading from tributaries are present. The most striking feature of the bay is the semi-diurnal tidal influence, resulting in large water exchange with the continental shelf. A historical study of the available data has shown the steadiness of this ecosystem during the two last decades in spite of increasing eutrophic conditions. This study has focused on hydrodynamic exchange which is one of the factors supposed to explain the resistance of this ecosystem to eutrophication: this stirring hinders the formation of a persistent upper mixed layer where phytoplankton would be in contact with nutrient-rich brackish waters and available light. Moreover, horizontal tidal currents lead to huge exchanges with the Iroise Sea and, then, to big losses of nutrients and living matter. To study this hydrodynamic influence thoroughly, a physical/biological model of this bay has been developed. This box model, based on the horizontal tidal circulation, has been developed thanks to “ELISE”, an ecological modelling software and, then, tuned and validated on two data sets corresponding with the years 1977 and 1993. The model has allowed us to quantify the influence of hydrodynamics, climatic conditions and biological factors on biogeochemical processes in this ecosystem. It contributes to explain the good resistance of the Bay of Brest ecosystem to eutrophic conditions; both the hydrodynamic properties of this bay and the grazing pressure have prevented it from disturbances caused by high nitrogen loading from the watersheds and explain the steadiness of phytoplankton stocks in spite of increased loading. So, these results allow us to say that, even if nitrogen inputs increase continues, phytoplankton stocks will not increase in significant proportions. Nevertheless, changes in the phytoplanktonic populations may occur if such an enrichment continues.

A steady-state model of PCB bioaccumulation in the sea bass (Dicentrarchus labrax) food web from the Seine estuary, France

Polychlorinated biphenyls (PCBs) were measured in six compartments of the sea bass (Dicentrarchus labrax) food web from the Seine estuary. Similar PCB patterns were observed in all samples. The main components possess the −245, 234, −2345 chlorine substitution Contamination increases with the trophic level. The highest concentrations were found in sea bass amounting 300 ng g−1 (d.w.) in the oldest individuals. A six-compartment steady-state food web model is proposed for the food web leading to the sea bass. Several exposure pathways are considered in the description of the accumulation by animals: the ingestion of particulate contaminants associated with either phytoplankton and detritus, and the respiratory uptake of truly dissolved contaminants in the overlaying, water. The application of this model indicates that feeding is the principal route of contamination, especially for PCBs which have more than four chlorine atoms and that feeding preference, phytoplankton lipid fraction, and organic carbon, content of detritus are parameters which mostly determine the bioaccumulation in this food web.

Modelling the eutrophication process in a River Plume: The Seine Case Study (France)

Abstract As part of a French National Program aiming at better understanding and modelling the coastal eutrophication process, the Bay of Seine (Eastern Channel) has been chosen as a typical case of phytoplanktonic eutrophication in a river plume. Flow rates and nutrient concentrations in the river Seine show a continuous increase of N loadings during the last decade, whereas Si loadings are stationary and P loadings decrease, due to a reduction of industrial discharges. Seasonal cruises made in 1978 and 1992 showed the nutrient enrichment of the eastern part of the Bay and the high chlorophyll content of the plume area, especially in the surface layer. In order to quantify the respective roles of horizontal advection, vertical stratification, meteorologically-induced versus man-induced variations of nutrient loadings in the global eutrophication process, the mathematical modelling approach was applied. Based on a one nautical mile mesh size map of computed lagrangian tidal residual currents, a box-model ...

Modelling coastal eutrophication: the case of French Ulva mass blooms

During the last decade, the excessive proliferation of Ulva sp. (Chlorophyceae) during spring and summer has been detrimental to recreational use of several beaches of Brittany (France). These worldwide mass blooms are well known in eutrophicated lagoons (e.g. Venice, Tunis), but are paradoxical along open beaches with a large tidal range and low terrestrial nutrient inputs (...)