Summary 1. A 2 × 2 factorial design was employed to look at the influence of two levels of phosphorus (P; high and low) and two levels of light (high and low) and their interactions, on phytoplankton abundance, elemental tissue composition and community structure in two seasons (April and June) of 2005. 2. A novel feature of the experiment was the creation of high and low P levels by manipulating sediment core conditions in the laboratory. Sediment cores were incubated with their associated overlaying water column from four different sites in Mona Lake, Michigan, under anaerobic or aerobic conditions, respectively. 3. After 24 days, the water overlaying the sediment cores was collected and used as growth media for phytoplankton collected from Mona Lake. Phytoplankton communities were grown in the laboratory in the high or low P water, and subjected to high (250 μmol m−2 s−1) or low (10 μmol m−2 s−1) light for 9 (April) or 14 (June) days. 4. In the April experiment, high P treatments resulted in significantly higher chlorophyll‐a concentrations, significantly lower C : P ratios from two of the four sites, and greater dominance by Scenedesmus at all sites relative to low P treatments. High light generally led to higher chlorophyll‐a concentrations, higher C : P ratios and greater Scenedesmus and Fragilaria biovolume at all sites. A significant interaction was measured between P and light for chlorophyll‐a and Scenedesmus biovolume, suggesting the influence of P was more apparent at high light than at low light levels. 5. In the June experiment, high P increased ash‐free dry mass (AFDM), lowered C : P ratios and resulted in increased Pediastrum biovolume. High light levels led to greater chlorophyll‐a concentrations, AFDM and C : P ratios, as well as increased biovolumes of Scendesmus, Pediastrum and Fragilaria. A significant interaction was found between P and light for all three taxa, as the positive influence of P was more pronounced at high light levels. 6. The results of our study demonstrate that sediment‐derived P stimulates phytoplankton growth, but that its effect on phytoplankton dynamics is modulated by other factors, such as light.