Field experiment on the influences of suspended clay and P on the plankton of a small lake1

Independent and interactive effects of phosphorus and clay loading on pelagic community organization and productivity were tested in a small piedmont lake in North Carolina. Twelve limnocorrals (2‐m diam, 3 m deep) were used for field manipulations. Treatments (in triplicate) were unaltered controls, P loading of 3.3 mg m −2 d −1 , kaolinite clay loading of 100 g m −2 d −1 , and combined clay and P loading. Fertilization with P significantly increased rates of turbidity reduction in comparison to controls (Secchi depth increasing at 7.2 vs. 5.8 cm d −1 ) and also significantly lowered sustained turbidity in treatments under clay loading. Clay loading reduced net community productivity (NCP), Chl a concentrations, and algal cell numbers. Suspended clay also caused a shift in algal community composition; Trachelomonas superba and other flagellates replaced the otherwise dominant blue‐green Spirulina major. Fertilization with P increased NCP and algal densities and favored development of the N 2 ‐fixing blue‐green algae Anabaena spiroides and Anabaena circinalis. Combined P and clay loading produced intermediate values of turbidity, NCP, and Chl a. Simultaneous clay loading eliminated the influence of P fertilization on algal community structure, yielding an assemblage dominated by flagellates. Clay turbidity also caused a shallowing in the daytime distribution of zooplankton.