Christian Grenz

Spatial variability in oxygen and nutrient fluxes at the sediment-water interface on the continental shelf in the Gulf of Lions (NW Mediterranean)

Abstract The spatial variability of oxygen and dissolved nutrient exchanges across the sediment-water interface was studied on the continental shelf in the Gulf of Lions (NW Mediterranean Sea). Replicate sediment cores were sampled at nine stations (64–162 m depth) along two lines parallel to the coast during two cruises in March and June 1998. Sediment-water exchanges were measured using the core incubation technique. Surficial sediments, bottom water and interstitial water characteristics were also described. Fluxes of oxygen (3.72–8.83 mmol m–2 d–1), nitrate (0.026–0.283 mmol m–2 d–1), ammonium (–0.022 to 0.204 mmol m–2 d–1), nitrite (–0.034 to 0.002 mmol m–2 d–1), phosphate (–0.007 to 0.029 mmol m–2 d–1) and silicate (0.504–1.656 mmol m–2 d–1) were generally quite low. This has to be related to the oligotrophy of the Mediterranean Sea. Fluxes showed a weak spatial variability, and a significant correlation could be established between oxygen fluxes and the organic carbon content of surficial sediments. A general increase in ammonium, nitrate and phosphate release was also observed towards the coast and the mouth of the Rhone River. Nitrite uptake and silicate release showed high variability between cruises, and the change in silicate fluxes depended mainly on the location on the eastern or western part of the continental shelf. Over the whole continental shelf, calculated sediment mineralization rate represents 342 kt a–1 of organic carbon. The annual release from the sediments approximates to 14.1 kt dissolved inorganic nitrogen, 2.9 kt P, and 165 kt dissolved silica, which represent, respectively, an amount close to 5%, 7% and 28% of the nutrient requirements for primary production. When compared to nutrient inputs from the Rhone River, sediments appear to play a significant role in the biogeochemical cycles of the Gulf of Lions system, mainly for inorganic phosphorus and dissolved silica.

Impact of wind and freshwater inputs on phytoplankton biomass in the coral reef lagoon of New Caledonia during the summer cyclonic period: a coupled three-dimensional biogeochemical modeling approach

A coupled three-dimensional physical-biological model was developed in order to simulate the ecological functioning and potential impacts of land-derived inputs in the southwest lagoon of New Caledonia. This model considered pelagic biogeochemical cycling of organic matter, taking into account advection and diffusion processes driven mainly by local wind fields and freshwater discharges. Modeled phytoplankton dynamics were strongly correlated with both freshwater nutrient inputs and wind-driven hydrodynamic processes, the latter resulting in a large input of oceanic water from the southeast part of the lagoon under trade wind conditions. In situ data obtained during the summer (January 1998) under trade wind conditions supported predicted concentration gradients along several coast to reef transects and provided a validation of the coupled physical-biogeochemical model. An additional sensitivity analysis showed that the alteration of the biogeochemical parameters did not strongly affect the results of the model. Freshwater inputs of nutrients were simulated using a realistic scenario corresponding to the summer rainy season of 1997–1998 in New Caledonia. Despite occasional flooding events from the main rivers considered in these simulations, no significant meso-scale phytoplankton bloom was identified. Hydrodynamically driven dispersion and rapid uptake of nutrients by phytoplankton were sufficient to spatially constrain the impact of river inputs and maintain oligotrophic conditions. The fine spatial grid of our three-dimensional model demonstrated that eutrophication in the southwest lagoon of New Caledonia is confined to the most restricted coastal embayments, while most of the lagoon experiences sustained oligotrophic conditions.

The contribution of Mytilus sp. in radionuclide transfer between water column and sediments in the estuarine and delta systems of the Rhône river

The fate of three radionuclides (3H, 137Cs, 106Ru) has been studied in the spreading area of the Rhone river and the nearby Gulf of Fos. The role of filter-feeders such as Mytilus sp., in this fate was investigated by periodically sampling the water column, molluscs and underlying sediments during 1986 and 1987. Results show that radionuclides may be a valuable tracer for Rhone river effluents entering the coastal area either in sediments or in the flesh of filter feeders. For example, tritium levels in the organic matter of superficial sediment decreases from the river mouth (3113 Bq l−13H of combustion water) to more offshore areas (219 Bq l−13H). This phenomenon is related to the settling characteristics of suspended matter in such areas. In areas with high biological activity, the role of filter feeders seems to dominate the transfer of radionuclides from the water column to the bottom, due to concentration of these elements in biodeposits. Deposition rates ranged from 13–50 Bq m−2 d−1 137Cs for September and May respectively. This transfer undergoes temporal fluctuations correlated with seasonal variations of the main hydrobiological parameters.