Nutrient dynamics and primary production in the eutrophic Berre Lagoon (Mediterranean, France)

Abstract

1 - The brackish Berre Lagoon is one of the largest Mediterranean coastal lagoons (155 km²). Since 1966, it has been influenced by a large freshwater discharge from a hydroelectric power plant, which has led to strong changes in the ecosystem structure and functioning. 2 - During 2005 and 2006, we monitored physico-chemical variables, primary production, nitrogen assimilation and regeneration in the water column using the dual-isotopic ¹⁵N/¹³C technique. 3 - Significant seasonal variations were observed for salinity, however summer values were similar to those observed in the lagoon before the power plant opened (30). Nitrate concentrations (NO3^-) varied, yielding very low values in the summer to values of 220 µg N-NO3^-.1^-1 in winter. Ammonium (NH4^-) and phosphate (PO4^3-) were observed in significant concentrations throughout the year (>7 µg N-NH4⁺.l^-1 and 3 µg P-PO4^3-.1^-1 respectively). Total chlorophyll increased slightly from February 2005 to June 2006, never exceeding 17 µg.1^-1 until summer 2006, which was marked by a large accumulation of biomass (> 30 µg.1^-1) maintained until the end of the year. 4 - Mean primary production rates ranged from 50 to 1 600 µgC.1^-1.d^-1, giving a total annual production of 507 gC.m-2.yr-1 in 2005 and 742 gC.m^-2.yr^-1 in 2006. This production is based on the assimilation of 15 000 and 24 000 tons of dissolved inorganic nitrogen (DIN) in 2005 and 2006 respectively (uptake rates ranging from 3 to 77 µg N-NH4^-+.1^-1.d^-1 and from 1 to 63 µg N-NO3^-.1^-1.d^-1). Regeneration processes were very active: 15 to 85 µg N-NH4⁺.1^-1.d^-1 were regenerated in the water column and sustained at least 60% of ammonium uptake. 5 - 95% of the measured annual primary production was regenerated production. These budgets need to 5 - be analysed with caution as biological processes are characterized by high interannual and seasonal variabilities. Regeneration processes in the water column certainly fed by other processes (acting in the sediments) are the basis of eutrophication maintenance in the Berre Lagoon.