Influence of Sediment and Phosphorus Loading on Phytoplankton Communities in an Urban Piedmont Reservoir

Abstract

ABSTRACT The response of phytoplankton communities to an imposed gradient of sediment loading under low versus high phosphate enrichment was assessed with enclosures (2-m-diameter polyethylene columns, open to the surface and the sediment) in a small piedmont reservoir within an urbanized watershed. The experimental design included replicated controls [CON, with ambient suspended solids (SS) and phosphorus (P)] and treatments low clay (LC), a natural hydrated sediment mix, sufficient clay initially added to effect an average daily concentration of 5 mg SS L−1 d−1), high clay (HC), 15 mg SS·L−1·d−1, phosphate enrichment (PHOS), ≥ 250 μg PO4 −3 P L−1); LC+P, and HC+P, imposed for ca. 10 weeks in the summer growing season. Clay additions without P led to decreased phytoplankton production (as chlorophyll a, chla) relative to controls, with colonial blue-greens and colonial greens increasing in LC, and HC dominated by filamentous blue-greens and mixotrophic flagellates. Significant increases in chla occurred in the epilimnion and hypolimnion of the P treatment, and in the epilimnion of LC+P, and HC+P. Low variability in community response occurred in treatments with sediment loading alone, whereas the P additions imposed high variability among replicates within treatments considering both total algal production and the timing of blooms. Overall, P enrichment stimulated nuisance algal blooms whether alone or with low sediment inputs, and increased P mitigated the adverse effects of high sediment loading in supporting blue-green (cyanobacteria) blooms. Undesirable algae such as Anabaena flos-aquae and A. circinalis were able to maintain populations under high sediment loading, and may serve as an innoculum for development of noxious blooms when shallow, turbid systems experienced high P enrichment.