IMPACT OF MACROALGAL DREDGING ON DYSTROPHIC CRISES AND PHOTOTROPHIC BACTERIAL BLOOMS (RED WATERS) IN A BRACKISH COASTAL LAGOON

The Prevost lagoon (Mediterranean coast, France), was subject to annua l dystrophic crises caused by the biodegradation of opportunistic macr oalgae (Ulva lactuca) in the past. These crises result in anoxic water s with subsequent blooms of Purple Sulphur Bacteria (red waters) which , by oxidizing sulphide, contribute to the reestablishment of oxic con ditions in the water column. Mechanical dredging of the macroalgal bio mass has been carried out in the lagoon since 1991 with the aim of pre venting the ecological and economic disturbances caused by such crises . Dredging began just before the phototrophic bloom when the water was already hypoxic (O-2 = 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 sedime nt surface. The dredging prevented red water formation and drastically modified both phototrophic community structure and activity and bioge ochemical 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 bacteri al community, whose numbers decreased by one order of magnitude (from 2 x 10(6) to 3.9 x 10(5) CFU.mL(-1)). The phototrophic community was n o longer effective in reoxidizing the reduced sulphur compounds remain ing its the sediments, as shown by a drastic sulphate depletion in the superficial sediment layers. Moreover, the increase in the specific b acteriochlorophyll a concentration of the phototrophic purple bacteria and the rapid development of Green Sulphur Bacteria (Prosthecochloris -like microorganisms) indicated that the phototrophic community was gr owing under severe light-limiting conditions due to the resuspension o f 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, hypo xic conditions (O-2 = 4.7 to 4.8 mg . L-1) and low sulphide concentrat ions (H2S = 0.4 to 0.7 mg . L-1) were detected in the water column unt il September. The ecological balance in the lagoon was reestablished o nly in October, whereas, in previous years it had been restored in Aug ust. (C) Elsevier, Paris.