Effect of epiphytes on eelgrass photosynthesis

It is suggested that macrophytes can limit the epiphytic stands by excreting algal antibiotics or by keeping a high replacement rate of photosynthetic tissues as illustrated by eelgrass in Vellerup Vig.

Abstract

The effect of epiphytes on eelgrass photosynthesis was measured at varying light intensities and HCO3− concentrations by means of the 14C-technique. Eelgrass was collected in Vellerup Vig, Denmark during October and November 1975. The epiphytes, mainly diatoms of the species Cocconeis scutellum Ehr., formed a crust several layers thick on the older leaves. The epiphytes reduced the photosynthetic rate of the leaves by acting both as a barrier to carbon uptake and by reducing light intensity. At optimal light intensity, the reduction was about 45% at 0.2 meq. HCO3−1−1 and it gradually decreased to nearly zero at 2.55 meq.1−1. At varying light intensity and a HCO3− concentration of 1.7 meq.1−1, corresponding to Vellerup Vig water, both effects of the epiphytes were seen. Above 7.2 mW cm−2, they caused a constant reduction of photosynthesis due to carbon deficiency. Below 7.2 mW cm−2, the reduction increased linearly to about 58% at 0.44 mW cm−2 corresponding to the increasing importance of shading from the epiphytes. Influence of epiphytic populations on photosynthesis and survival of aquatic macrophytes is discussed. It is suggested that macrophytes can limit the epiphytic stands by excreting algal antibiotics or by keeping a high replacement rate of photosynthetic tissues as illustrated by eelgrass in Vellerup Vig.