The effects of prolonged aquatic macrophyte senescence on the biology of the dominant fish species in a southern African coastal lake

The littoral zone of Swartvlei, an estuarine lake on the southern Cape coast, was characterized by extensive beds of submerged aquatic macrophytes. May 1979 marked the beginning of a regression phase which lasted more than three years. The senescence of the Potamogeton pectinatus and charophyte beds resulted in a 60% decline in primary production, a 74% slump in littoral invertebrate biomass and a 54% decline in the abundance of the fishes Monodactylus falciformis (Lacepede) and Rhabdosargus holubi (Steindachner) associated with the macrophytes. Gravimetric and calorific analyses of their food revealed that invertebrates and filamentous algae from the littoral zone were of major importance. The two fish species consumed approximately 2% of the primary production during the Potamogeton canopy phase and 3% during the Potamogeton senescent phase. Invertebrate consumption declined from 33 mg m^?2 day^?1 during the canopy phase to 8 mg m^?2 day^?1 during the senescent phase. The disappearance of the Potamogeton and charophyte beds also resulted in a decrease in the condition of both species but the effect was indirect and related mainly to the collapse of invertebrate stocks associated with the plants.