Impact of urban runoff on phosphorus, nitrogen, and dissolved oxygen in a shallow subtropical lake

Abstract

This research assessed the current conditions of water quality in a shallow subtropical lake, influenced by a highly developed urban environment. Specifically, the research focused on the dynamics of phosphorus, nitrogen, and dissolved oxygen, as well as the effects of stormwater runoff on lake water quality. Furthermore, the research quantified gross primary production, net primary production, and respiration in order to discern seasonal variation in lake metabolism. A water quality monitoring platform with multi-parameter probes was deployed in the center of University Lake on the Louisiana State University campus. A series of lake water quality parameters including dissolved oxygen (DO) concentration, temperature, pH, specific conductivity, and cyanobacteria were recorded continuously at a 15-min interval from July 2008 to October 2009. In addition, water samples were collected monthly as well as after major rain events during the study period to determine changes in total phosphorus (TP), nitrate (NO3-), nitrite (NO2-), and total Kjeldahl nitrogen (TKN) concentrations. Results show a monthly average concentration of 0.286 mg/L TP (min – max: 0.167 - 0.621 mg/L), 0.053 mg/L NO3-N (below detection – 0.24 mg/L), 0.045 mg/L NO2-N (below detection – 0.012 mg/L), and 2.5 mg/L TKN (below detection – 5.12 mg/L). Mean storm event loading of 28.1 kg (7.5 – 47.8 kg) caused an immediate increase in total phosphorus within the water column by 14.1%. This resulted in a mean TP lake concentration of 0.383 mg/L. A similar trend was observed in a lesser degree in NO3-N and TKN. Phosphorus loads in the lake were correlated with runoff volume (r2 = 0.71), suggesting runoff volume is the most important factor effecting lake TP concentration after a storm event. Analysis of metabolism rates found a mean annual gross primary productivity value of 4.41 g O2/m2/day, a mean annual net primary production value of 2.13 g O2/m2/day, and a mean annual respiration value of 5.90g O2/m2/day. Annually, 1610 g O2/m2 were produced while the annual sum of respiration was 2150 g O2/m2. Respiration rates were mostly equal to or greater than productivity rates throughout the year, indicating that this shallow subtropical urban lake was net heterotrophic throughout most of the year.