Impact of macroalgal dredging on dystrophic crises and phototrophic bacterial blooms (red waters) in a brackish coastal lagoon
Résumé
The Prevost lagoon (Mediterranean coast, France), was subject to annual dystrophic crises caused by the biodegradation of opportunistic macroalgae (Ulva lactuca) in the past. These crises result in anoxic waters with subsequent blooms of Purple Sulphur Bacteria (red waters) which, by oxidizing sulphide, contribute to the reestablishment of oxic conditions in the water column. Mechanical dredging of the macroalgal biomass has been carried out in the lagoon since 1991 with the aim of preventing the ecological and economic disturbances caused by such crises. Dredging began just before the phototrophic bloom when the water was already hypoxic (O2 = 0.7 mg·L-1) and contained sulphilde (H2S = 7.3 mg·L-1) and purple patches of phototrophic bacteria (Thiocapsa sp.) that were beginning to develop on decaying macroalgae at the sediment surface. The dredging prevented red water formation and drastically modified both phototrophic community structure and activity and biogeochemical sulphur cycling. The dredging permitted the reestablishment of oxic conditions for a short period only (1-13 August). Resuspension of the superficial sediment layers disturbed the phototrophic bacterial community, whose numbers decreased by one order of magnitude (from 2 x 106 to 3.9 x 105 CFU.mL-1). The phototrophic community was no longer effective in reoxidizing the reduced sulphur compounds remaining in the sediments, as shown by a drastic sulphate depletion in the superficial sediment layers. Moreover, the increase in the specific bacteriochlorophyll a concentration of the phototrophic purple bacteria and the rapid development of Green Sulphur Bacteria (Prosthecochloris-like microorganisms) indicated that the phototrophic community was growing under severe light-limiting conditions due to the resuspension of sediment particles in the water. These conditions did not allow the phototrophic bacterial community to efficiently reoxidize the reduced sulphur compounds originating from the sediments. In consequence, hypoxic conditions (O2 = 4.7 to 4.8 mg·L-1) and low sulphide concentrations (H2S = 0.4 to 0.7 mg·L-1) were detected in the water column until September. The ecological balance in the lagoon was reestablished only in October, whereas, in previous years it had been restored in August.