Inter-annual Variability of Bottom Hypoxia in Shallow Mediterranean Estuaries

Shallow mediterranean estuaries receive most of their riverflow during winter, and ocean exchange is usually sufficiently weak that estuarine salinities remain depressed below ocean values throughout spring so creating vertical stratification. These are ideal conditions for bottom hypoxia during spring and an analytical model is presented for the time duration of this hypoxia. It shows that spring hypoxia only occurs if the winter salinity lies below a threshold value which depends both on sediment oxygen demand and physical parameters of the basin. The propensity of a shallow mediterranean estuary 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 for water quality in basins that are very shallow and microtidal. For one such basin, Harvey Estuary in south-west Australia, the modelled hypoxia threshold corresponds to the winter threshold salinity for blooms of Nodularia spumigena observed during the 1980s. The model is further supported by taking the annual values of observed organic phosphorus content in those blooms and dividing by the number of days of modelled hypoxia for each year. These calculated values of phosphorus content per day are all within a factor of 2 of the published values for the independently measured daily phosphorus release from sediments in the estuary when subjected to hypoxic conditions.