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−1d−1), high clay (HC), 15 mg SS·L−1·d−1, phosphate enrichment (PHOS), ≥ 250 μg PO4−3P 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 chlorophylla, 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 asAnabaena flos-aquaeandA. circinaliswere 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.