An assessment of long-term post-restoration water quality trends in a shallow, subtropical, urban hypereutrophic lake

Abstract

City Park Lake is a shallow urban hypereutrophic lake located in Baton Rouge, Louisiana, with a surface area of 0.23 km 2 and a mean depth of 1.2 m. By the late 1970s, the lake had become highly eutrophic and suffered from frequent and severe algal blooms and fish kills. A major restoration effort was undertaken in 1983 that consisted of dredging and the repair of sewage infrastructure. Immediate improvements in water quality were observed following restoration; algal blooms and fish kills were virtually eliminated for nearly a decade. However, large floating mats of filamentous algae periodically occurred during the early 1990s. Results of a water quality sampling program conducted in 2000 and 2001 indicated that phosphorus has once again reached pre-restoration levels, and nitrogen levels have decreased well below those observed during pre-restoration years. Whereas phosphorus-limited conditions predominated in the years preceding the 1983 restoration, results of the 2000–2001 sampling program indicate that the lake has become nitrogen-limited with respect to photosynthetic activity. This trend in nutrient levels has likely influenced the recent predominance of filamentous over unicellular species of algae observed during the last decade. Nearly 4 years of drought-like conditions beginning in 1998 have resulted in an overall increase in the hydraulic retention time of the lake. This condition has resulted in organic staining of the lake waters, or the development of a tea-like color due to the decomposition of organic compounds. This phenomenon has played a major role in inhibiting the sunlight available for filamentous algal growth since 1998 and the absence of filamentous algae during the 2000–2001 sampling program.