Nutrient Utilization and Primary Production during Low–Flow Conditions in the Cahaba River, Alabama

Measurements of nutrients in July 1999, at nine stations along a 20-mile segment of the Cahaba River, Alabama, showed uptake fluxes for nitrate and ortho-phosphate on the order of 0.2 mg/l/mile and 0.044 mg/l/mile respectively. Nutrient uptake was paralleled by concomitant changes in productivity along the stretch of river studied immediately upstream and downstream of a municipal wastewater facility. Gross community primary production ranged from 0.4 — 1.1 grams-C/M3/day and gross community respiration ranged 0.3 — 1.3 grams-C/M3/day. Productivity to respiration (P:R) ratios greater than one (indicating an autotrophic system) were more common at the downstream stations and were more frequently less than one (indicating heterotrophic systems) at the midsegment stations. Comparison of net community primary production and net planktonic primary production (as measured by light-dark bottles) showed the periphyton or benthic algal components to be dominant primary producers in this river segment. Evidence for nutrient limitation of primary production by comparison of dissolved inorganic nutrient ratios was not demonstrated. Concentrations of both inorganic nitrogen and phosphorus species were well above levels expected to show limitation to either phytoplankton or periphyton within the segment, but elevated alkaline phosphatase levels in periphyton samples were suggestive of phosphorus limitation in the downstream portions of the study area. Suggestions are presented for subsequent eutrophication analyses to developing site-specific models of nutrient/production relationships for development of Total Maximum Daily Loads (TMDL's).