Biogeochemical aspects of bottom anoxia in a Mediterranean lagoon (Thau, France)

Abstract

Physical and chemical characteristics (temperature, salinity, dissolved oxygen), nutrients [dissolved inorganic nitrogen (DIN), soluble reactive 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 located 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 sediment were also analyzed for ammonium (NH,') and SRP concentrations in the pore.water. The study period was characterized by mean wlnd speed under 4 m S-' which induced an increase of surface temperature from 18 to 2g°C with the formation of a thermocline. The correlation (p < 10-4) between the wind speed averaged over the previous day and the difference between bottom and surface temperatures showed that the wind conshtuted the main vector of vertical mixing. The lack of wind led to dissolved oxygen depletion in the bottom 2 m and to a strong porosity increase in the upper 10 cm of sediment (80 to >95%). Anoxic conditions increased NH,+ and SRP concentrations in porewater from 231 + 89 to 1305 + 305 (+ SD) ).]M and from 6.6 + 0.8 to 108 t 43 PM respectively. The potential increase of NH,' concentrations in porewater estimated from the mineralization of the microphytobenthos explained 30% of the measured increase in the upper 8 cm of porewater. The study period was characterized by an increase in NH4+, SRP and dissolved Si concentratlons in the bottom water column (maxima respectively 24.2, 4.9 and 57 PM). Linear regressions between nutrient concentrations in the water column and temperature revealed a strong enhancement of bottom fluxes during anoxia. Nitrate + nitrite (No3-+ NO2-) were absent in anoxic waters and remained below 0.5 pM in oxic waters. Monthly concentrations of SRP in the water column of the Thau lagoon from 1970 to 1994 revealed anoxia events by summer peak values corresponding to strong bottom anoxia. Nevertheless, the general trend was a decrease due to the gradual control of eutrophication which should lead to the disappearance of bottom anoxia within the next decade. A fraction of the nutrients of benthic origin was transformed into planktonic particulate matter vla prlmary production whlch increased the concentrations of chl a from about 1 to 15 pg I-'. Concentrations of DON and PON in the water column were significantly correlated, suggest~ng that DON compounds were released by the pelagic food web but not directly by the sediment. The increase of total nitrogen (DIN + DON + PON), considering the water column as a closed box during 10 d of winds under 5 m S-', was estimated at 1.7 pm01 N 1-' d-' and would correspond to a benthic flux of 600 pm01 N m-2 h-' for 10 d.

Influence of shellfish farming activities on the biogeochemical composition of the water column in Thau lagoon

“…The grazing of heterotrophic communities on phytoplankton (Lam Hoai et al 1997) should explain several peak values of POC concentrations, and particularly those following peaks in chl a concentrations. The relationship between PON and POC concentrations at ZA is similar to that reported from data collected in summer 1994 at the same station (Souchu et al 1998). The slope is close to the Redfield C:N ratios in plankton, and the intercept, interpreted as the non-degradable fraction Increasing chl a concentrations reported in January (Fig.…”

“…Consequently, DIN concentrations reach their minimum values in summer under the pressure of primary producers. In contrast, benthic fluxes of SRP are enhanced during summer and tend to accumulate in the water column (Souchu et al 1998). The regenerated primary production is then controlled by endogenous DIN fluxes at the watersediment interface (Mazouni et al 1998a) and in the water itself through the microbial loop (Gasc 1997).…”

“…In Thau lagoon, the oxygen demand by sediments is the major biological process influencing the oxygen cycle (Mazouni et al 1996). Souchu et al (1998) showed that bottom anoxia can develop in the deepest parts of the lagoon in summer, and outside (RQ) as well as inside (ZA) shellfish farming areas. Consequently, biodeposition cannot be considered as responsible for excessive oxygen consumption.…”

Lagoon pollution and nutritive compounds change in urban tropical area

“…Indeed, ammonification reactions performed from organic matter lead to a production of NH 4 + /NH 3 . The dissimilatory reactions from NO 3 -leading to NO 2 -coupled by reduction to nitrate ammonification triggering the NH 4 + / NH 3 production (Castel et al, 1996;Souchu et al, 1998). We could not confirm the extent of nitrification activity in 14 C-tracing method since this test has been achieved (Brion and Billen, 1998).…”

Spectral measurements of underwater downwelling radiance of inland water bodies

“…Regarding the water quality, nutrient inputs to the lake are highly correlated with the phases of flooding. Phosphate concentrations in the lagoon are minimal and close to concentrations of marine waters in winter and maximum in summer due to the sediment flux (Souchu et al, 1998).…”