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

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-1 to as high as 100 cm s-1. 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., the tropics and subtropics), consideration should be given to how they might affect long-term lake management programs

Plankton Food Web Responses to Experimental Nutrient Additions in a Subtropical Lake

During two controlled enclosure experiments using water from a subtropical lake, the plankton food web displayed a highly variable response to combined addition of nitrogen and phosphorus. In July, the nutrients stimulated growth of Cylindrospermopsis raciborskii, and the biomass of macrozooplankton and microbial food web components did not increase. In October, the same addition of nutrients stimulated growth of small edible Lyngbya spp., and there were coincident increases in biomass of macrozooplankton and components of the microbial web. Past generalizations that cyanobacteria blooms inhibit growth of other food web components may not always hold true

Large-Scale Mapping and Predictive Modeling of Submerged Aquatic Vegetation in a Shallow Eutrophic Lake

A spatially intensive sampling program was developed for mapping the submerged aquatic vegetation (SAV) over an area of approximately 20,000 ha in a large, shallow lake in Florida, U.S. The sampling program integrates Geographic Information System (GIS) technology with traditional field sampling of SAV and has the capability of producing robust vegetation maps under a wide range of conditions, including high turbidity, variable depth (0 to 2 m), and variable sediment types. Based on sampling carried out in AugustœSeptember 2000, we measured 1,050 to 4,300 ha of vascular SAV species and approximately 14,000 ha of the macroalga Chara spp. The results were similar to those reported in the early 1990s, when the last large-scale SAV sampling occurred. Occurrence of Chara was strongly associated with peat sediments, and maximal depths of occurrence varied between sediment types (mud, sand, rock, and peat). A simple model of Chara occurrence, based only on water depth, had an accuracy of 55%. It predicted occurrence of Chara over large areas where the plant actually was not found. A model based on sediment type and depth had an accuracy of 75% and produced a spatial map very similar to that based on observations. While this approach needs to be validated with independent data in order to test its general utility, we believe it may have application elsewhere. The simple modeling approach could serve as a coarse-scale tool for evaluating effects of water level management on Chara populations

Extreme weather events influence the phytoplankton community structure in a large lowland subtropical lake (Lake Okeechobee, Florida, USA)

We demonstrate a major ecological change in a large lake ecosystem in response to a series of extreme weather events. Phytoplankton community dynamics in subtropical Lake Okeechobee are described from 2000 through early 2008 with emphasis on inter-relationships among phytoplankton populations and associated environmental variables in this large, shallow eutrophic lake. The lake experienced the physical effects of three hurricanes in 2004-2005, which caused massive sediment resuspension, near total elimination of submerged aquatic vegetation, elevated biologically available nutrients and total suspended solids, and lower water transparency. Patterns of long-term co-dominance by nitrogen (N)-fixing cyanobacteria and meroplanktonic diatoms abruptly changed to dominance by only meroplanktonic diatoms. The planktonic cyanobacteria genera Anabaena and Planktolyngbya both decreased approximately an order of magnitude in the post-hurricane period despite large surpluses of bioavailable nutrients. Meroplanktonic diatoms (mostly Aulacoseira spp.) declined approximately 20%, perhaps because of superior competitive ability for light in a polymictic, turbid water column. Canonical Correspondence Analysis (CCA) suggested that reduction in planktonic cyanobacteria after compression of the photic zone and the persistence of meroplanktonic diatoms were related to light utilization traits for the key algal taxa and indicated that pre-existing light limitation and crustacean grazing pressure were intensified in the post-hurricane period. © 2013 Springer Science+Business Media Dordrecht

Deconstructing dogma: Nonoperative management of small bowel obstruction in the virgin abdomen

Management of small bowel obstruction (SBO) has become more conservative, especially in those patients with previous abdominal surgery (PAS). However, surgical dogma continues to recommend operative exploration for SBO with no PAS. With the increased use of computed tomography imaging resulting in more SBO diagnoses, it is important to reevaluate the role of mandatory operative exploration. Gastrografin (GG) administration decreases the need for operative exploration and may be an option for SBO without PAS. We hypothesized that the use of GG for SBO without PAS will be equally effective in reducing the operative exploration rate compared with that for SBO with PAS. A post hoc analysis of prospectively collected data was conducted for patients with SBO from February 2015 through December 2016. Patients younger than 18 years, pregnant patients, and patients with evidence of hypotension, bowel strangulation, peritonitis, closed loop obstruction or pneumatosis intestinalis were excluded. The primary outcome was operative exploration rate for SBO with or without PAS. Rate adjustment was accomplished through multivariate logistic regression. Overall, 601 patients with SBO were included in the study, 500 with PAS and 101 patients without PAS. The two groups were similar except for age, sex, prior abdominal surgery including colon surgery, prior SBO admission, and history of cancer. Multivariate analysis showed that PAS (odds ratio [OR], 0.47; p = 0.03) and the use of GG (OR, 0.11; p < 0.01) were independent predictors of successful nonoperative management, whereas intensive care unit admission (OR, 16.0; p < 0.01) was associated with a higher likelihood of need for operation. The use of GG significantly decreased the need for operation in patients with and without PAS. Patients with and without PAS who received GG had lower rates of operative exploration for SBO compared with those who did not receive GG. Patients with a diagnosis of SBO without PAS should be considered for the nonoperative management approach using GG. Therapeutic, level IV

Nutrient ratios and phytoplankton community structure in the large, shallow, eutrophic, subtropical Lakes Okeechobee (Florida, USA) and Taihu (China)

Analysis of ten- and four-year datasets for the large, shallow, subtropical, and eutrophic Lakes Okeechobee (USA) and Taihu (China), respectively, suggest that resource-ratio explanations for cyanobacteria dominance may not apply to these two lakes. Datasets were examined to identify relationships between nutrient ratios [total nitrogen (TN):total phosphorus (TP) and ammonium (NH4+):oxidized N (NOx)] and phytoplankton community structure (as proportions of cyanobacteria and diatoms to total phytoplankton biomass). Datasets were pooled by sampling month, averaged lake-wide, and analyzed with linear regression. In Okeechobee, the cyanobacteria proportion increased and the diatom proportion decreased with increasing TN:TP. In Taihu, cyanobacteria decreased with increasing TN:TP, but the opposite trend observed for diatoms was marginally significant. Okeechobee cyanobacteria increased and diatoms decreased with increasing NH4+:NOx, but no significant relationships between phytoplankton and NH4+:NOx were observed in Taihu. Both lakes had significant relationships between phytoplankton community structure and total nutrients, but these relationships were the opposite of those expected. Relationships between phytoplankton community structure and water quality parameters from the previous month resulted in improved relationships, suggesting a predictive capability. Statistical analysis of the entire datasets (not pooled) supported these and additional relationships with other parameters, including temperature and water clarity