R. Thomas James

N:P ratios, light limitation, and cyanobacterial dominance in a subtropical lake impacted by non-point source nutrient pollution

A long-term (28-year) data set was used to investigate historical changes in concentrations of phosphorus (P), nitrogen (N), N:P ratios, and Secchi disk transparency in a shallow subtropical lake (Lake Okeechobee, Florida, USA). The aim was to evaluate changes in the risk of N2-fixing cyanobacterial blooms, which have infrequently occurred in the lakes pelagic zone. Predictions regarding bloom risk were based on previously published N:P ratio models. Temporal trends in the biomass of cyanobacteria were evaluated using phytoplankton data collected in 1974, 1989-1992, and 1997-2000. Concentrations of pelagic total P increased from near 50 microg l-1 in the mid-1970s to over 100 microg l-1 in the late 1990s. Coincidentally, the total N:P (mass) ratio decreased from 30:1 to below 15:1, and soluble N:P ratio decreased from 15:1 to near 6:1, in the lake water. Published empirical models predict that current conditions favor cyanobacteria. The observations confirm this prediction: cyanobacteria presently account for 50-80% of total phytoplankton biovolume. The historical decrease in TN:TP ratio in the lake can be attributed to a decreased TN:TP ratio in the inflow water and to a decline in the lakes assimilation of P, relative to N. Coincident with these declines in total and soluble N:P ratios, Secchi disk transparency declined from 0.6 m to near 0.3 m, possibly due to increased mineral turbidity in the lake water. Empirical models predict that under the turbid, low irradiance conditions that prevail in this lake, non-heterocystous cyanobacteria should dominate the phytoplankton. Our observations confirmed this prediction: non-N2-fixing taxa (primarily Oscillatoria and Lyngbya spp.) typically dominated the cyanobacteria community during the last decade. The only exception was a year with very low water levels, when heterocystous N2-fixing Anabaena became dominant. In the near-shore regions of this shallow lake, low N:P ratios potentially favor blooms of N2-fixing cyanobacteria, but their occurrence in the pelagic zone is restricted by low irradiance and lack of stable stratification.

Variability, uncertainty, and sensitivity of phosphorus deposition load estimates in South Florida

Atmospheric deposition, a substantial source of phosphorus (P) to the Florida Everglades, has been measured on a weekly basis in South Florida since 1974, but P measurements are highly variable due to random noise in the data. This study applies statistical approaches that calculatethe variability and uncertainty of the P load estimation model based on wet and dry P concentrations and rainfall volume.The average mean and standard deviation of the estimated P deposition rates for 13 sites in South Florida are 41±33 mg P m-2 yr-1. First order analysis of the random and measurement errors in the input variables produces a propagation error estimate in P load calculation. The atmospheric P deposition load shows high spatial and temporal variability with no consistent long-term trends. Because of the random noisy nature of P deposition, estimated P deposition loads have a significant amount of uncertainty no matter what type of collection instrument is used. Thus, duplicate sampling is highly recommended to increase the amount of uncontaminated data.

Comparative analysis of nutrients, chlorophyll and transparency in two large shallow lakes (Lake Taihu, P.R. China and Lake Okeechobee, USA)

This article compares limnological attributes of two of the world’s largest shallow lakes—Lake Okeechobee in Florida, USA and Lake Taihu in P.R. China. Both the systems support an array of ecological and societal values including fish and wildlife habitat, public water supply, flood protection, and recreation. Both have extensive research programs, largely because of concern regarding the lakes’ frequent cyanobacterial blooms. By evaluating these systems together, we compare and contrast properties that can generally advance the understanding and management of large shallow lowland lakes. Because of shallow depth, long fetch, and unconsolidated mud sediments, water chemistry, and transparency in both the lakes are strongly influenced by resuspended sediments that affect light and nutrient conditions. In the central region of both the lakes, where depth is the greatest, evaluation of limiting factors by a trophic state index approach indicates that light most often limits phytoplankton biomass. In contrast, the more sheltered shoreline areas of both the lakes display evidence of nitrogen (N) limitation, which also has been confirmed in nutrient assays conducted in earlier studies. This N limitation most likely is a result of excessive levels of phosphorus (P) that have developed in the lakes due to high external loads over recent decades and the currently high internal P recycling. Comparisons of these lakes show that Lake Taihu has higher N than, similar total phosphorus (TP) and similar light conditions to that of Lake Okeechobee, but less chlorophyll a (CHL). The latter may be as a result of lower winter temperatures in Lake Taihu (around 5°C) compared to Lake Okeechobee (around 15°C), which could reduce phytoplankton growth and abundance through the other seasons of the year. In these systems, the important role of light, temperature, and nutrients in algal bloom dynamics must be considered, especially due to possible adverse and unintended effects that might occur with projects such as sediment removal, and in the long term, in regard to buffering lake responses to external load reduction.

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.

Hurricane effects on a shallow lake ecosystem, Lake Okeechobee, Florida (USA)

This unique case study of Lake Okeechobee - a large, shallow and culturally eutrophic lake in south Florida - documents the effects of hurricanes on its water quality, sediment, phytoplankton and submersed aquatic vegetation (SAV). Three hurricanes (Frances and Jeanne in 2004 and Wilma in 2005) that swept directly over the lake led to a number of expected changes throughout the system: 1) high winds produced large seiches, strong waves and currents that redistributed bottom sediments and uprooted SAV and emergent macrophytes; 2) sediment disturbance resulted in increased suspended solids and nutrients in the water column, reduced Secchi transparency and affected SAV recovery, phytoplankton biomass and phytoplankton species dominance; and 3) heavy rainfall increased flows, nutrient loads and lake water levels. Changes in suspended solids, most nutrient concentrations, phytoplankton and SAV persisted for two years after the hurricanes. This persistence was attributed to unconsolidated surface sediment that increased in thickness because of the storms and was more easily resuspended during subsequent wind events. Drought conditions and low lake levels in the second year after the hurricanes led to some recovery of SAV, primarily in the form of the non-vascular musk grass (Chara spp.). The absence of high-intensity hurricanes in the near future should aid in SAV recovery and return the nearshore region to a macrophyte-dominated clear-water state. Our results demonstrate the importance of sediment disturbance and water levels in shallow lakes that are vulnerable to extreme weather events.

Three-dimensional Water Quality and SAV Modeling of a Large Shallow Lake

ABSTRACT This study presents the Lake Okeechobee Environment Model (LOEM), a three-dimensional (3D) hydrodynamic, sediment, water quality, and submerged aquatic vegetation (SAV) model of Lake Okeechobee, Florida. The LOEM is developed under the framework of the EFDC model (Hamrick 1992). Lake Okeechobee is the largest subtropical lake in North America (1,730 km2), it is very shallow (mean depth 2.7 m), and it includes a littoral habitat that makes up 20% of its area. The LOEM is calibrated, verified, and validated to 3 years of water quality and SAV data. The water quality results are consistent with observed data from 25 locations in the lake. The model is capable of reproducing key water quality characteristics of the lake without having to resort to extensive, site-specific parameter manipulations. The SAV model is calibrated using measured SAV data in the lake. The SAV model is capable of representing the spatial and temporal variations of SAV variations in the lake well. The LOEM is applied to study water quality and SAV processes in the lake. The model results are consistent with observed data indicating that algal growth in the lake is primarily nitrogen limited in the summer and nitrogen and light co-limited in the winter. Lower water elevation generally leads to larger SAV area. SAV can have positive impact on the lake water quality by reducing algae concentration. The calibrated, verified, and validated LOEM model serves as a useful tool to support lake management.

Phosphorus dynamics at multiple time scales in the pelagic zone of a large shallow lake in Florida, USA

Phosphorus (P) dynamics in large shallow lakes are greatly influenced by physical processes such as wind-driven sediment resuspension, at times scales from hours to years. Results from long-term (30 year) research on Lake Okeechobee, Florida (area 1,730 km2, mean depth 2.7 m) illustrate key features of these P dynamics. Variations in wind velocity result in changes in water column transparency, suspended solids, and total P (TP). In summer there are diurnal changes in TP associated with afternoon winds, and in winter, when strong winds occur for multiple days, monthly average TP remains high compared to summer. The magnitude of daily and seasonal TP changes can exceed 100 μg l−1. Hurricanes and tropical storms also cause extreme changes in TP that are superimposed on seasonal dynamics. When a hurricane passed 80 km south of the lake in October 1999, mean pelagic TP increased from 88 to 222 μg l−1. During large resuspension events, light attenuation is substantially increased, and this influences the biomass and spatial extent of submerged plants, as well as water column TP. In Lake Okeechobee, TP concentrations typically are ∼20 μg l−1 when submerged plants are dense, and soluble reactive P concentrations are reduced below detection, perhaps by the periphyton and plant uptake and by precipitation with calcium at high pH. In contrast, TP exceeds 50 μg l−1 when submerged plants and periphyton are absent due to prolonged deep water, and phytoplankton biomass and algal bloom frequency both are increased. In Lake Okeechobee and other large shallow lakes, complex models that explicitly consider wind-wave energy, hydrodynamics, and sediment resuspension, transport, and key biological processes are needed to accurately predict how lake water TP will respond to different management options.

Internal Nutrient Loads from Sediments in a Shallow, Subtropical Lake

Abstract Fluxes of dissolved inorganic nitrogen, DIN, (as ammonia, NH4-N) and phosphorus (as dissolved reactive phosphorus, DRP) from the sediments to the water column of Lake Okeechobee were determined from two separate techniques: increases in nutrient concentration in the water column above intact cores and concentration gradients determined with pore water equilibrators. These fluxes were estimated from different sediment types within the lake (sand, peat, mud) and at two major inflows (Kissimmee River and Taylor Creek). DRP release from peat sediments was highest. Measurements in other sediments were not significantly different from each other. DRP flux to the lake was estimated as 326 Mt·yr−1 in 1989 and had increased to 472 Mt·yr−1 in 1999. Because of measurement variation, this increase was not statistically different. These estimates of internal DRP loads are greater than estimated external surface DRP loads that averaged 316 Mt·yr−1 from 1979–1988 and 258 Mt·yr−1 from 1989−1999. DIN flux was highest near Taylor Creek. There was no consistent pattern between sediment type and DIN flux. Internal loads of DIN were estimated in 1999 as 4,500 Mt·yr−1, which is greater than the external surface loads of DIN estimated for the period of 1989–1998, of 896 Mt·yr−1. Sediment oxygen demand measured in the cores was strongly correlated to DRP and DIN flux, indicating that these fluxes are largely a result of mineralization of organic material. This study indicates that sediment diffusive fluxes are a significant source of DRP and DIN to the lake water column.

Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake

Abstract Long-term phosphorus (P) loading to lakes has resulted in accumulation of P in sediments. Internal nutrient loading from sediments of shallow lakes such as Lake Okeechobee, Florida, has become a major concern in restoration programs. The objectives of this study were to determine (1) the potential impact of dredging on dissolved reactive P (DRP) flux out of sediments and (2) the equilibrium P concentration (EPCw) of post-dredge sediments. Intact sediment cores from one location representing P-laden mud sediments of the lake were obtained. Four simulated dredging treatments were implemented: control (no dredging-current conditions); top 30 cm; 45 cm; and 55 cm sediment removal. Phosphorus release/retention characteristics of sediments were determined at water-column DRP concentrations of 0, 0.016, 0.032, 0.064, and 0.128 mg/L. The water column in each core was replaced at approximately 60-day intervals, for a period of 1.2 years, with fresh lake water spiked with respective P concentrations. Significant decreases in water column DRP were observed only in sediment cores with 0–30 cm dredging. At ambient water column DRP levels, the P fluxes during the first 32 days were 0.4, 0.1, 0.4 and 0.2 mg P/m2/day for the 0, 30, 45, and 55 cm dredging treatments, respectively, and accounted for 11–38% of total P released during the 431 day study. At the end of the 1.2-year study, estimated EPCw were on the order of 0.033, 0.008, 0.022, and 0.037 mg P/L for 0, 30, 45 and 55 cm dredging treatments, respectively. Dredging the top 55 cm sediments would result in the removal of approximately 123 g P/m2, as compared to 80 and 108 g P/m2 for 30 and 45 cm dredging, respectively. Laboratory experiments suggest that dredging can reduce internal P loading. However, further evaluation is needed to determine the extent to which the controlled laboratory experiments can be used to predict fluxes in the lake under natural conditions, and the long-term sustainability of improving water quality by dredging.

Long-Term Changes in the Sediment Chemistry of a Large Shallow Subtropical Lake

ABSTRACT Nitrogen (N) and phosphorus (P) content and selected physico-chemical properties of Lake Okeechobee sediments were measured in 1988 and 1998. Based on these measurements, sediments were classified as mud, littoral, peat, sand or rock. Although some minor redistribution has occurred over the previous decade, mud sediments of Lake Okeechobee essentially occupy the same total area. Total surface sediment N and P showed little overall change in the ten-year period. However, lake-wide spatial patterns indicate some localized decreases of total P content in the littoral and northern regions of the lake. Porewater dissolved reactive P showed significant increases at most sites, suggesting that the surface sediments are losing their ability to absorb soluble P. Nutrient management practices in the drainage basin did not lead to decreased levels of N and P in the sediments of the lake.