Nutrient retention and transformation in relation to hydraulic flushing rate in a small impoundment

Abstract

SUMMARY. The fluxes of water, chloride, silicale and various forms of nitrogen and phosphorus into and out of a 3925 ha impoundment (Lake Taiquin) in north Florida were examined over a 3 year period. Annual hydraulic flushing rates for the lake varied by a factor of more than 2 during the study period and provided an opportunity to examine the effect of flushing rate on nutrient cycling within the lake. The results support the hypothesis that nutrient retention and transformation in lakes and impoundments with high flushing rates (> 1 year‐1) are appreciably influenced by annual variations in flushing rates. Higher fractions of input silicate and total phosphorus were retained by Lake Taiquin during the years with average flushing rates (8 year‐1) than during a year with abnormally high flushing rate (16 year‐1). Nitrogen was not appreciably retained during any of the three study years, and thus N: P ratios in the lake outflow were higher than in the inflow.Phytoplankton productivity was appreciably higher (70%) during the year with the higher flushing rate, apparently in response to higher reactive phosphorus concentrations in the lake and perhaps to a higher standing crop of phytoplankton during that year. Reactive phosphorus and dissolved inorganic nitrogen inputs could account for only about 30% and 25%, respectively, of the phosphorus and nitrogen required to support observed photosynthetic rates in all three study years. Recycling of nutrients apparently accounted for most of the remainder, although nitrogen fixation could not be ruled out as a factor in the nitrogen budget.