The nutrient status of the Kromme River estuary, South Africa

Abstract

The Kromme is a small estuary in rural eastern Cape, South Africa with a relatively unpopulated and undisturbed catchment. Physical and chemical variables including the major nutrients and E. coli were examined. It was found to be a well oxygenated, well mixed estuary with some stratification in its upper reaches. Nutrient levels were generally low with nitrate values higher in winter than during summer. While no overt seasonality was detected for phosphorus, levels were comparatively high. Faecal E. coli levels were low with a mean count of 26 per 100 ml for the estuary. Along the Kromme, nutrients were highest at the confluence of the Geelhoutboom tributary suggesting it was a major nutrient contributor. At the head of the Kromme, silicate and E. coli were significantly linked to riverflow, while nitrate was correlated with rainfall. In the Geelhoutboom, only nitrate was correlated with riverflow and rainfall. When nutrient gradients (total inorganic nitrogen, total phosphorus and silicate) were correlated against salinity, significant relationships were observed for nitrogen and silicate (Kromme and Geelhoutboom) and for phosphorus along the Geelhoutboom, but not the Kromme, reflecting differences in sub-catchments. An overall N:P atomic ratio of 0.89:1 was found for the system, with more nitrogen relative to phosphorus at the head of the Kromme and vice versa for the Geelhoutboom and mouth. The Kromme is low in nitrogen, conservative in silicate and low in phosphorus while the Geelhoutboom supplies nitrogen, phosphorus and silicate to the system. Although there may be nutrient absorption by sediments in the upper estuary, and by macrophytes in the lower estuary, the Kromme may be classified as a non-conservative mesotrophic marine-dominated estuary which probably exhibits some nutrient recycling.