Nutrient Limitation of Summer Phytoplankton Growth in Two Missouri River Reservoirs

Abstract

A survey of phytoplankton standing crop and primary productivity was made in Lewis and Clark Lake and Lake Francis Case during 1968. Gross productivity during the summer (June, July, August) averaged 0.81 gC/m2 ° day in the former reservoir and 0.38 gC/m2 ° day in the latter. Average standing crops of phytoplankton (mm3/liter) during the same period were nearly threefold greater in Lewis and Clark Lake. In 1969 and 1970, natural phytoplankton populations from the two reservoirs were enriched with single additions of nitrogen and phosphorus and incubated in situ. Effects of enrichment were assessed by a comparison of 14C uptake and chlorophyll a concentrations in the enriched and control samples. In Lake Francis Case a pronounced phosphorus deficiency was observed in both years. Phytoplankton in Lewis and Clark Lake also appeared to be phosphate limited during the summer, but relative increases in growth due to additions of phosphorus were limited by the rapid depletion of natural levels of inorganic nitrogen. It is suggested that phosphorus enrichment from the Niobrara River (the only significant tributary between the two reservoirs) is the primary cause of increased phytoplankton production in Lewis and Clark Lake.