Outcomes of extreme water levels on water quality of offshore and nearshore regions in large shallow subtropical lake

Abstract

The effects of extreme water levels on water quality were examined for shallow nearshore vs. deeper offshore regions of Lake Okeechobee, a large subtropical lake in Florida USA. Water quality attributes were compared using mid-summer (June-August) observations for the highest and lowest five years of water levels since 1988. Differences in chlorophyll-a (Chl-a), chloride (Cl), color, nutrients, total nitrogen to total phosphorus ratio (TN: TP), Secchi Disk depth, transparency ratio (defined as the ratio of Secchi Disk depth to total depth), and total suspended solids (TSS) were examined to test hypotheses of interactions within and between the two regions under high and low water levels. The offshore region had consistently higher nutrient and TSS concentrations and lower Secchi Disk depth and transparency ratio. At high water levels within the offshore region, Cl, color, TSS, TP and TN were lower and Secchi Disk depth and color were higher. In contrast, at high water levels within the nearshore region, Chl-a, TP, TN, dissolved inorganic N and TSS concentrations were higher, and Cl, color, and transparency ratio were lower. Submerged aquatic vegetation, epiphyton, and benthic algae grow and compete for nutrients with phytoplankton in the nearshore region, but are absent from the deeper offshore region. The contrasting responses of the two lake regions indicate that water-level management actions taken to reduce eutrophication symptoms (e. g., algal blooms) in one region may not work in the other. In the case of Lake Okeechobee, the nearshore region supports most of the lake's ecological and societal values, so low water levels have a net benefit to ecosystem functions. In other lakes this may not be the case, highlighting the importance of understanding spatial variation of phytoplankton responses to water level in large shallow lakes.