User:Wittlj/Lake Okeechobee

Limnology

Lead

This shallow lake has been previously studied for its algal blooms. Lake Okeechobee is known for its algal blooms in consequence of increased eutrophication. Algal blooms like this can be harmful to the environment, including the lake's flora and fauna, because of released toxins. The vegetation at Lake Okeechobee is important in maintaining the oxygen in the lake, of which many aquatic biota are reliant on. There are various species of biota in Lake Okeechobee that are interdependent on each other for food, habitat, and other resources. Multiple limnological studies and related research has been conducted at Lake Okeechobee.

Characteristics

Lake Okeechobee is shallow lake, with an average depth of only 3 meters, and has a fetch of 54 km. In total, the lake has a surface area of 1730 km². The lake is normally mixed, but on days with direct sunlight and limited wind, the lake can exhibit diurnal thermal stratification. Although daily thermal stratification is brief, a hypolimnion can from during this time resulting in decreased amounts of dissolved oxygen at the lake bottom. Lake transparency, measured as secchi depth, is found to be inversely correlated with the amount of suspended solids in the lake. Suspended solids varied with season with higher amounts of suspended solids in the winter, and thus less transparency on average, and lower amounts of suspended solids in the summer, leading to more transparency on average. Secchi depths not only varied across seasons, but also by location in the lake. Secchi depths ranges average from about 0.2 - 0.5 meters in the winter, depending on location in the lake, and 0.3 - 0.9 meters in the summer. Secchi depths of 1.7 meters have been recorded, indicating higher transparencies than average for the lake.

Fauna

Lake Okeechobee is home to more than 40 species of native fish, along with introduced species, such as the Mayan cichlid, Cichlasoma urophthalmus. Fish species displaying varying distributions throughout the lake depending on seasonality, site depth, sediments, and turbidity. Yearly fish recruitment was found to be positively correlated with increased water levels, providing more substrate and protection. These fish populations support different wading birds, including various species of egrets, ibises, wood storks, and herons, along with alligator populations. Fish diets in Lake Okeechobee depend on macro-invertebrates and zooplankton, such as calanoids, cyclopoids, and cladoceran. Lake Okeechobee supports over 3,800 different arthopods, including insects and arachnids, along with around 400 species of nematodes.

Flora

Vascular macrophytes are important in the nutrient dynamics of lakes, along with creating micro-habitats for fish and invertebrates, and providing substrate for epiphytes. Macrophytes provide the lake with oxygen through photosynthesis, along with acting as a buffer for eutrophication by uptake of phosphorus. However, with increased eutrophication of lakes along with climate change, trends are showing decreased richness of macrophytes. Because the fauna are so reliant on the macrophytes for habitat, food, and protection from predation, a decrease in macrophyte diversity and abundance has negative consequences on fauna richness. Macrophyte abundance is dependent on many abiotic factors such as water depth, water transparency and light availability, and nutrients, along with influence of biotic factors. Increasing phytoplankton and algal blooms from eutrophication and nutrient abundance can decrease water transparency and light availability to submerged macrophytes, providing one explanation how macrophytes are sensitive to eutrophication. Some submerged macrophytes that have been recorded at Lake Okeechobee include southern naiad (Najas guadelupensis), Illinois pondweed (Potamogeton illlinoensis), vallisneria (Vallisneria americana), and hydrilla (Hydrilla verticillata). Lake Okeechobee is afflicted with the invasive terrestrial plant, torpedograss (Panicum repens).

Eutrophication and Algal Blooms

The concerning levels of total phosphorous (TP) began to be noticed in 1970s, and since then inputs of TP have averaged 516 tons per year. These yearly inputs can vary based on the volume of runoff entering the lake. The years 2005 and 2018 had particularly large volumes of water and TP inputs in relation to hurricanes increasing runoff. Despite limiting TP inputs by decreasing phosphorus use in agriculture, Lake Okeechobee has yet to be reach the aimed target set by the South Florida Water Management District's in the 1980s of reducing the lake's TP by 40 μg/L. Although proposed by the South Florida Water Management District, this initiative of limiting the lake's TP to 40 μg/L was adopted by The Lake Okeechobee Technical Advisory Committee (LOTAC), the United States Environmental Protection Agency (USEPA), and the Florida Department of Environmental Protection (FDEP), but phosphorous inputs have yet to be controlled enough to reach this goal. Concerning estimates of phosphorus assimilation capacity indicates that even if phosphorus inputs were to be stopped, or severely limited, the extensive saturation of the lake would result in it taking years before improved water quality can be observed.

Photo taken on July 1st, 2016 during Lake Okeechobee's extensive algal bloom caused by the increased runoff from the weather conditions of the El Nino event.

These inputs of phosphorus provide optimal conditions for harmful algal blooms (HABs). Cyanobacteria (CyanoHABs), which need nitrogen and phosphorus for growth, have the ability to fix atmospheric nitrogen. With this ability along with the high inputs of phosphorous, the shallow nature of the lake providing plenty of sunlight, and cyanobacteria's preference for warm waters, Lake Okeechobee is an optimal environment for a cyanobacteria algal bloom. The presence of various species of cyanobacteria in Lake Okeechobee have been recorded since the 1980s. Cyanobacteria produce various toxins, including microcystin, which is not only harmful to the environment, but humans. In 2016, Lake Okeechobee experienced an extensive cyanobacteria algal bloom that lasted from May to mid-July. During the previous 2015-16 winter, there were relatively high recorded temperatures, and higher than average rates of precipitation and storms in relation to the El Niño event. As mentioned, higher rates of precipitation can lead to greater influxes of runoff which unload more phosphorous into the lake, enabling harmful algal bloom. Along with this algal bloom in 2016, other algal blooms have been found to occur in relation to hurricanes and other climate events leading to increased water flow into the lake.

Research at Lake Okeechobee

Research done by James et al. (2009) aimed to evaluate and compare shallow lakes, including Lake Okeechobee and Lake Taihu in P.R. China, including their light, temperature, and nutrient dynamics. This research provides important knowledge on conditions that influence algal blooms. They found that for both lakes, wind, nutrients, water depth, and water transparency varied seasonally, and this had implications on phytoplankton abundance. Different locations in the lake may have had different limiting factors based on the light and nutrient availability in those locations. At Lake Okeechobee specifically, algal blooms were found to have strong effects during the winter on the western side of the lake.

In the limnological study conducted by Beaver et al. (2013) at Lake Okeechobee, lake phytoplankton composition was examined in response to conditions of anthropogenic inputs, including nutrient inputs, along with natural events, like extreme weather conditions. Lake Okeechobee was a great location for this study because of its long history of agricultural runoff causing algal blooms, along with its location in the Gulf of Mexico making it susceptible to weather events like tropical storms and hurricanes. From 2000-2008, phytoplankton samples were collected using an integrated tube sampler, and weather conditions, including temperature and wind conditions, were recorded. They found that phytoplankton composition transitioned from non-nitrogen fixing cyanobacteria dominating the lake before 2000, to nitrogen fixing cyanobacteria dominating the lake after 2000 and up until 2004 as phosphorus inputs were high and nitrogen was limiting. This time was referred to as the "pre-hurricane" time period, and the period after the 2004-2005 hurricane season was referred to as the "post-hurricane" period. During the post-hurricane period, light became limiting and influenced phytoplankton composition.

Kramer et al. (2018) studied Lake Okeechobee during and after its major 2016 algal bloom that was related to the El Niño event. They collected information on nutrient availability, phytoplankton communities, and the presence of toxins, along with the genetic makeup of the phytoplankton communities and their genetic abilities to produce toxins. Additionally, they conducted nutrient experiments to couple with their findings. They found that cyanobacteria with the ability to do nitrogen fixation were in high abundance during this 2016 algal bloom. During this time, nitrogen was a limiting factor due to the extreme amounts of phosphorus in the freshwater ecosystem. The field experiments conducted with this study found that microcystin, the toxin produced by cyanobacteria, was produced in higher quantities when there was more nitrogen present.

A study conducted by Pei, Zhang, and Mitsch (2020) examined nitrate concentrations, and their respective isotope compositions, in hopes of determining origins of major inflows and outflows of nitrogen into the lake and what their respective contributions are. They found that ammonium based fertilizers and soil nitrogen were the largest contributors to nitrate found in the lake. Manure and precipitation were two other sources of nitrate. These results can aid in monitoring and regulation of nitrogen uses around Okeechobee, and subsequently aid in restoring the lake.