Hurricane Effects on a Shallow Lake Ecosystem and Its Response to a Controlled Manipulation of Water Level

Abstract

In order to reverse the damage to aquatic plant communities caused by multiple years of high water levels in Lake Okeechobee, Florida (U.S.), the Governing Board of the South Florida Water Management District (SFWMD) authorized a "managed recession" to substantially lower the surface elevation of the lake in spring 2000. The operation was intended to achieve lower water levels for at least 8 weeks during the summer growing season, and was predicted to result in a large-scale recovery of submerged vascular plants. We treated this operation as a whole ecosystem experiment, and assessed ecological responses using data from an existing network of water quality and submerged plant monitoring sites. As a result of large-scale discharges of water from the lake, coupled with losses to evaporation and to water supply deliveries to agriculture and other regional users, the lake surface elevation receded by approximately 1 m between April and June. Water depths in shoreline areas that historically supported submerged plant communities declined from near 1.5 m to below 0.5 m. Low water levels persisted for the entire summer. Despite shallow depths, the initial response (in June 2000) of submerged plants was very limited and water remained highly turbid (due at first to abiotic seston and later to phytoplankton blooms). Turbidity decreased in July and the biomass of plants increased. However, submerged plant biomass did not exceed levels observed during summer 1999 (when water depths were greater) until August. Furthermore, a vascular plant-dominated assemblage (Vallisnera, Potamogeton, and Hydrilla) that occurred in 1999 was replaced with a community of nearly 98% Chara spp. (a macro-alga) in 2000. Hence, the lake’s submerged plant community appeared to revert to an earlier successional stage despite what appeared to be better conditions for growth. To explain this unexpected response, we evaluated the impacts that Hurricane Irene may have had on the lake in the previous autumn. In mid-October 1999, this category 1 hurricane passed just to the south of the lake, with wind velocities over the lake surface reaching 90 km h-1 at their peak. Output from a three-dimensional hydrodynamic / sediment transport model indicates that during the storm, current velocities in surface waters of the lake increased from near 5 cm s to as high as 100 cm s. These strong velocities were associated with large-scale uplifting and horizontal transport of fine-grained sediments from the lake bottom. Water quality data collected after the storm confirmed that the hurricane resulted in lake-wide nutrient and suspended solids concentrations far in excess of those previously documented for a 10-year data set. These conditions persisted through the winter months and may have negatively impacted plants that remained in the lake at the end of the 1999 growing season. The results demonstrate that in shallow lakes, unpredictable external forces, such as hurricanes, can play a major role in ecosystem dynamics. In regions where these events are common (e.g., ...

Lake management (muck removal) and hurricane impacts to the trophic state of Lake Tohopekaliga, Florida

“…Similarly, lake average total phosphorus concentration in Lake Okeechobee, Florida (approximate surface area 1,800 km2), increased from 90 µg/L 2 weeks prior to Hurricane Irene (fall 1999) to 220 µg/L (a 144% increase) 2 weeks after the hurricane (Havens et al 2001). These large lake comparisons are much closer to data collected in Lake Tohopekaliga than those with the smaller lakes, suggesting that the hurricanes do impact larger lakes more than smaller lakes.…”

Fertile sediment and ammonium enrichment decrease the growth and biomechanical strength of submersed macrophyte Myriophyllum spicatum in an experiment

“…-enriched water a situation that also occurs in eutrophic lakes. All these results in this study, at least partly, explain the common phenomenon that after rapid floods or strong winds, there are lots of allofragments or uprooting individuals of submersed macrophytes (Cui et al, 2000;Havens et al, 2001), especial in eutrophic zones (Schutten et al, 2005), which results in the disappearance of submersed macrophytes and their difficult restoration in eutrophic lakes (Schutten et al, 1997(Schutten et al, , 2004Schutten & Davy, 2000), such as Potamogeton malaianus and Vallsineria spiralis in Poyang Lake, in Jiangxi province, China (the largest freshwater lake in China, Cui et al, 2000), which declined rapidly after a catastrophic flood in 1998 and has not yet been restoration (Cui et al, 2000;Li et al, 2004), and H. verticillata, Potamogeton illinoinensis, and V. americana in Lake Okeechobee in Florida, USA (an eutrophic lake, Havens et al, 1996), which rapidly disappeared after a hurricane during OctoberNovember 1999 and were replaced by Chara (Havens et al, 2001). …”

Adaptation of submerged macrophytes to both water depth and flood intensity as revealed by their mechanical resistance

“…However, H. verticillata was the least adaptive species in both biomechanical and morphological views. Overall, the differences in the mechanical resistance of SAV in this study, at least partly, explain the common phenomenon that rapid floods eliminate above-ground plant biomass primarily through mechanical damage (Best et al, 2001;Havens et al, 2001), which results in the Fig. 4 Average values of tensile properties (mean ± SE, N = 9) of Myriophyllum spicatum, Hydrilla verticillata, Potamogeton maackianus, Ceratophyllum demersum, and P. malaianus stems in this experiment.…”

“…A rapid water level rise, especially that induced by a summer flood after exceptionally strong rainfall events, generally results in large reductions in biomass and shoot spatial extent, as well as the disappearance of SAV in freshwater lakes (Wallsten & Forsgren, 1989;Gacia & Ballesteros, 1996;Fernández-Aláez et al, 1999;Cui et al, 2000;Havens, 2003;Yang et al, 2004), due to low-light availability (Chambers & Kalff, 1985a;Best et al, 2001) and/or increased shoreline wave energy (Scheffer, 1998;Havens et al, 2001). Light availability significantly affects the maximum biomass and colonization depth of SAV (Keddy, 1983;Chambers & Prepas, 1988), as well as the mechanical resistance of terrestrial plants .…”