BIOGEOCHEMICAL ASPECTS OF BOTTOM ANOXIA IN A MEDITERRANEAN LAGOON (THAU, FRANCE)

Physical and chemical characteristics (temperature, salinity, dissolve d oxygen), nutrients [dissolved inorganic nitrogen (DIN), soluble reac tive phosphorus (SRP) and silicate], dissolved organic nitrogen (DON) and phosphorus and particulate matter [particulate organic carbon (POC ) and nitrogen (PON) and chlorophyll a] were measured at a station loc ated in the deepest part of the Thau lagoon (8.5 m), France, during a 10 d period of bottom anoxia in summer 1994. The upper 8 cm of sedimen t were also analyzed for ammonium (NH4+) and SRP concentrations in the porewater. The study period was characterized by mean wind speed unde r 4 m s(-1) which induced an increase of surface temperature from 18 t o 29 degrees C with the formation of a thermocline. The correlation (p < 10(-4)) between the wind speed averaged over the previous day and t he difference between bottom and surface temperatures showed that the wind constituted the main vector of vertical mixing. The lack of wind led to dissolved oxygen depletion in the bottom 2 m and to a strong po rosity increase in the upper 10 cm of sediment (80 to >95%). Anoxic co nditions increased NH4+ and SRP concentrations in porewater from 231 /- 89 to 1305 +/- 305 (+/- SD) mu M and from 6.6 +/- 0.8 to 108 +/- 43 mu M respectively. The potential increase of NH4+ concentrations in p orewater estimated from the mineralization of the microphytobenthos ex plained 30% of the measured increase in the upper 8 cm of porewater. T he study period was characterized by an increase in NH4+ SRP and disso lved Si concentrations in the bottom water column (maxima respectively 24.2, 4.9 and 57 mu M). Linear regressions between nutrient concentra tions in the water column and temperature revealed a strong enhancemen t of bottom fluxes during anoxia. Nitrate + nitrite (NO3- + NO2-) were absent in anoxic waters and remained below 0.5 mu M in oxic waters. M onthly concentrations of SRP in the water column of the Thau lagoon fr om 1970 to 1994 revealed anoxia events by summer peak values correspon ding to strong bottom anoxia. Nevertheless, the general trend was a de crease due to the gradual control of eutrophication which should lead to the disappearance of bottom anoxia within the next decade. A fracti on of the nutrients of benthic origin was transformed into planktonic particulate matter via primary production which increased the concentr ations of chi a from about 1 to 15 mu g l(-1). Concentrations of DON a nd PON in the water column were significantly correlated, suggesting t hat DON compounds were released by the pelagic food web but not direct ly by the sediment. The increase of total nitrogen (DIN + DON + PON), considering the water column as a closed box during 10 d of winds unde r 5 m s(-1), was estimated at 1.7 mu mol N l(-1) d(-1) and would corre spond to a benthic flux of 600 mu mol N m(-1) h(-1) for 10 d.