The nutrient status of the Sundays River estuary South Africa

Abstract

The nutrient status of the Sundays, an important estuarine system in the eastern Cape, South Africa, is described, together with other physico-chemical parameter and compared with two other local systems, namely the Swartkops and the Kromme. In the Sundays, significant nutrient gradients between the head and mouth of the estuary were found for nitrite, nitrate, phosphate, total phosphorus, silicate and faecal E. coli. Significant correlations were also found between riverflow/rainfall and some headwater nutrients. Highly significant correlations were similarly found between total inorganic nitrogen, total phosphorus and silica and salinity. In each case the relationship was linear indicating a conservative nutrient distribution along the estuary. An overall N:P ratio of 3.2:1 was obtained for the Sundays estuary (compared with 1.12:1 for Swartkops and 0.8:1 for Kromme). N:P ratios calculated separately for the head and mouth regions were 4.52:1 and 2.51:1, respectively, suggesting strong nitrogen enrichment in the headwaters from citrus fertilizer seepage upstream. Mean nitrate values for the Sundays, Swartkops and Kromme estuaries were 0.629, 0.516 and 0.064 mg l −1 respectively. Although phosphate values in the Sundays were double that of the Kromme (0.233 and 0.121 mg l −1 respectively), they were low compared to the Swartkops (1.320 mg l −1) which has phosphate enrichment due to urban sewage and industrial effluent. Faecal, E. coli was concomitantly higher in the Swartkops (640 per 100 ml −1) compared to the pollution free Sundays and Kromme (18 and 26 per 100 ml −1, respectively). Both the Swartkops and the Kromme displayed sink distributions for N and P. Both these estuaries are also rich in macrophytes with open Spartina marshes and intertidal Zostera beds. By contrast, the channel-like Sundays estuary is conservative for N, P and Si and has no saltmarshes. It is suggested therefore that the Sundays contributes nutrients to the adjacent, extremely productive Sundays beach surf zone system, which, despite nutrient recycling still requires large amounts of extrinsic nutrients in order to maintain high productivity.