Seasonal variations in the vertical distribution of benthic microalgae in the upper sediment of the Mdloti Estuary, South Africa

Abstract

A 12-month survey investigated seasonal variations in the vertical distribution of benthic microalgal biomass in relation to major factors potentially controlling their dynamics in the subtropical temporarily open Mdloti Estuary, KwaZulu Natal, South Africa. Benthic microalgal chla concentrations in the upper 5 cm of sediment ranged from 259 mg m (-2) to 977 mg m (-2) during the closed phase, and from 6.7 mg m (-2) to 305 mg m (-2) during the open phase. Significant differences were obtained between sampling sites and among seasons. A distinctive trend of increasing chla biomass in the deeper layers of the sediment was noted from the upper reaches (head) to the lower reaches (mouth) of the estuary. Chla biomass was virtually homogeneously distributed throughout the upper 5 cm of sediment in the lower reaches. Results suggest that benthic microalgal cells not only exhibited diel rhythms of vertical migration but were actively resuspended into the water column of the Mdloti Estuary. While a decrease in salinity, temperature and Kd favoured an increase in benthic microalgal chla biomass in the upper 5 cm sediment, nutrients in the overlying water had no significant impact on the biomass. Results further indicate a physiologically healthy community at all sites during the open phase, as compared to the closed phase of the estuary. During the closed phase, the pheopigment to chla ratio indicated an actively growing community in the upper reaches and a "stressed" community in the middle and lower reaches. Thus, the large amounts of "buried" benthic microalgal chla biomass found in the upper 5 cm of sediment may represent an important stock of potential primary producers in temporarily open estuarine systems like the Mdloti.