δ 15 N as a tool to demonstrate the contribution of fish-waste-derived nitrogen to an Ulva bloom in Saldanha Bay, South Africa

Abstract

This study uses stable isotope ratios of nitrogen (δ15N) to test the hypothesis that a bloom of the green seaweed Ulva lactuca, which occurred in Saldanha Bay, South Africa, in summer 1993/94 was linked to an adjacent discharge of nitrogen from pelagic fish processing waste. It is suggested that only two significant sources of new nitrogen were available to the Ulva: the natural nitrate flux from coastal upwelling and the fish factory nitrogen effluent. A significant difference (1.9‰) was found in the mean δ15N values between Ulva samples from a control site at Langebaan Lagoon (8.9‰) and those from Saldanha Bay (10.8‰). The latter value, which is relatively enriched in 15N, is consistent with the view that the nitrogen taken up in Saldanha Bay originated from a trophic position corresponding to pelagic fish in the southern Benguela system. The δ15N values from Ulva at the control site are consistent with nitrogen originating from the natural oceanic nitrate pool. It is suggested that this stable isotope method is useful in linking the causes and effects of eutrophication.