RESPONSE OF PERIPHYTON COMMUNITIES TO CLAY AND PHOSPHATE LOADING IN A SHALLOW RESERVOIR1

Abstract

ABSTRACTEnclosures in a small piedmont reservoir in the southeastern United States were used to determine the effects of loading by phosphate and two clay sediments on periphyton community structure and production during the summer growing season. The experimental design included replicated controls and the following treatments added at 2‐to 3‐day intervals: phosphate (PHOS), kaolinite (KAOL), montmorillonite (MONT), kaolinite with phosphate (K+P), and montmorillonite with phosphate (M+P). Periphyton were sampled from polyethylene strips of the same material as the enclosure walls, suspended at depths of 0.5 m and 2.0 m to assess treatment effects on shallow (epilimnetic) vs. deep (hypolimnetic) communities. Colonization after 40 days was sparse, dominated in biovolume by blue‐green algae in all but the epilimnion of controls, in which small gelatinous green algae and chrysophyte flagellates were abundant. The biovolume of shallow periphyton after 79 days was lowest in KAOL and MONT, intermediate in controls and PHOS, and greatest in clay + P. Hence, P enrichment alleviated the deleterious effects of clay on periphyton production, and clay + P appeared to act synergistically in stimulating benthic algal growth. Periphyton biovolume was comparable among shallow and deep controls, PHOS, and treatments with the rapidly settling kaolinite. Lowest biovolume occurred at depth in treatments with montmorillonite under extreme light attenuation imposed by the suspended, finely particulate clay. The relative contribution of blue‐green algae to total periphyton production was highest in clay + P treatments and lowest in controls. Heterotrophic dinoflagellates increased in the shallow periphyton under clay loading, and in the deep communities under PHOS and M + P. Periphyton contributed only 20–32% of total mesocosm productivity in controls, PHOS, and KAOL, increasing to about 40% in MONT, and to 72–91% of the total in M + P and K + P. The data indicate that benthic microalgae can represent a major proportion of the primary production in shallow reservoirs under high phosphate and sediment loading.