Modelling a bottom diurnal boundary layer and its control of massive alga blooms in an estuary

摘要

Harvey Estuary, which lies in the Mediterranean climate of southwest Australia, experienced massive blooms of the blue-green alga Nodular sponginess during the 1980s. Bloom size showed great inter-annual variability which was linked to minimum winter salinity. The blooms were initiated by phosphorus released from sediments due to a thin bottom boundary layer, which formed at night after cessation of a daily seabreeze, and w3as depleted for oxygen by decay of detritus from the winter diatom bloom. Two models of this bottom layer are analyzed. One is a molecular diffusive boundary layer, whose thickness is estimated from the Batchelor length scale for oxygen and turbulent dissipation by the very weak nocturnal tide. The other model envisages bottom stratification due to nocturnal intrusion, and convection, of a thin layer of higher salinity water. Agreement is obtained with the observed inter-annual variability of the size of the Nodularia bloom. Limited 1980s data confirm the existence of a thin anoxic, and highly stratified, bottom layer whose thickness is similar to the present predictions. The blooms seem to have been a direct result of the diurnal cycle in vertical mixing cycle and he small tidal range.