Inter-annual variability of bottom hypoxia in shallow Mediterranean estuaries

Shallow mediterranean estuaries receive most of their riverflow during wint er, and ocean exchange is usually sufficiently weak that estuarine saliniti es remain depressed below ocean values throughout spring so creating vertic al stratification. These are ideal conditions for bottom hypoxia during spr ing and an analytical model is presented for the time duration of this hypo xia. It shows that spring hypoxia only occurs if the winter salinity lies b elow a threshold value which depends both on sediment oxygen demand and phy sical parameters of the basin. The propensity of a shallow mediterranean es tuary to become hypoxic is related not only to vertical mixing, but depends strongly on the vertical shear of currents and this is a critical factor f or water quality in basins that are very shallow and microtidal. For one su ch basin, Harvey Estuary in south-west Australia, the modelled hypoxia thre shold corresponds to the winter threshold salinity for blooms of Nodularia spumigena observed during the 1980s. The model is further supported by taki ng the annual values of observed organic phosphorus content in those blooms and dividing by the number of days of modelled hypoxia for each year. Thes e calculated values of phosphorus content per day are all within a factor o f 2 of the published values for the independently measured daily phosphorus release from sediments in the estuary when subjected to hypoxic conditions . (C) 2001 Academic Press.