Food web structure in three contrasting estuaries determined using stable isotope (δ¹³C) analysis

Abstract

Food web structure in three contrasting estuaries, the freshwater-deprived Kariega, the freshwater-dominated Great Fish River and the temporarily open/closed Kasouga estuary, along the south-east coast of southern Africa was investigated in summer (14 November) 2000 by employing stable carbon (δ¹³C) isotope analysis. Results of the analysis indicated that there were distinct differences in the primary sources of carbon utilised by zooplankton in the three estuaries. In the Kariega estuary two distinct zooplankton feeding guilds were identified, the first having moderate (δ¹³Cvalues ranging between -16 and -18, the second having more enriched (δ¹³C values varying between -10 and -12. Zooplankters of the first guild utilised carbon derived mainly from the microbenthic algae, whereas those of the second guild appeared to obtain theirs from eelgrass, Zostera capensis, and its associated epiphytes. In the temporarily closed/open Kasouga estuary, zooplankton had isotopic values ranging between -17.8 and -18.1. The most likely source of carbon for the zooplankton in this estuary is the extensive microphytobenthic alga stocks usually recorded in the system. In contrast to the freshwater-deprived estuaries, the depleted isotopic values of the zooplankton samples (-28 to -26) from the Great Fish River estuary suggest that the primary source of carbon consumed there was derived from phytoplankton. Data obtained in this study are in agreement with those of previous studies conducted in the region and highlight the importance of freshwater input in determining the trophodynamics in estuarine systems in southern Africa.


Keywords: estuaries; food webs; isotope analysis


(Afr J Aqua Sci: 2002 27(2): 107-115)