Intertidal microalgal production and molluscan herbivory in relation to season and elevation on two rocky shores on the east coast of southern Africa

Abstract

Microalgal production (ungrazed rock) and standing stock (grazed) were measured over a period of 15 months in the lower, middle and upper balanoid zones of two rocky snores in the Transkei region of southern Africa. The mean monthly production of chlorophyll a, which ranged from 0,3 to 2,0 μg·cm⁻². was inversely correlated with tidal elevation. Highest rates of production (3,5–6,0 μg·cm⁻²) were recorded in winter at all tidal levels. The amplitude of seasonal fluctuations decreased with tidal height. At the middle and upper levels standing stock usually exceeded monthly production, whereas the opposite often occurred in the lower balanoid. No temporal or spatial trends in standing stock were evident. The intertidal distribution of grazing gastropods (excluding littorinids) correlated positively with primary production but showed no relationship with standing stock. No clear seasonal trends in grazer abundance were evident. The fact that standing stock exceeds monthly production suggests that grazing pressure is insufficient to balance primary production in the middle and upper balanoid zones on these shores. Grazers do, however, remove most of the primary production in the lower balanoid zone. Possible mechanisms underlying the observed patterns are discussed.