What’s in a lake? Exploring Florida’s diverse lakes: Part 2

Last time, we detailed how different hydrogeological processes influence the trophic state of some lakes.  The processes that created northern lakes also contributed to their clear bluewaters and in part contributed to a backbone (and stereotype) in how people perceive and use their lakes.

In this entry, Chris and I help describe the processes that affect Florida lakes and what makes them different, both ecologically and in human perception, compared with northern lakes.

The shoreline of Florida has varied considerably during Glacial and Interglacial periods.  In some eras, Florida became flooded and was considered a shallow sea.

Florida Lake Formations

Most Florida lakes have originated from much different hydrogeological processes than Northern lakes.  During the interglacial periods between recent Ice Ages (~10,000 years ago), Florida was the nutrient-rich, sandy-bottom of a shallow sea.  As such, much of Florida lies atop a foundation of limestone, phosphorus, sand, peat, or other porous materials that allow nutrients (e.g., phosphorus and nitrogen) to readily collect in lakes.  This is quite unlike the impermeable rock-basin carved by glaciers in northern lakes.  Sand and limestone are easily dissolved by percolating rainwater and the dissolution of these materials forms basins in which water collects.  Unlike deeper glacial lakes, many of these ‘solution lakes’ are shallow (most <3m) with elevated nutrient concentrations that allows for abundant aquatic plant growth in these lakes. Sediment resuspension, which contributes to elevated nutrient concentrations, occurs more frequently and intensely in shallower lakes compared to deeper lakes.  Combined with shallow basins, elevated nutrient levels, plants and phytoplankton in unstained solution lakes can grow all the way through the water column, and sometimes in the middle of the lake.

This map highlights that the geologic foundation in Florida is covered in nutrient rich phosphate (in pink) and peat (in red) as well as porous sand (all shades of brown) contributing to eutrophic Florida lakes.

Even More Florida Lakes

Florida is home to other lake types than just eutrophic, solution lakes.  Other lakes have formed from different processes including: tectonic (Lake Okeechobee is considered a ‘new-land’ lake created by receding sea levels), manmade lakes called reservoirs (e.g., Rodman Reservoir or Lake Seminole), river-made lakes such as those found along the St. John’s river (e.g., Lake George), and coastal dune lakes (lakes created by coastal changes from oceanic currents and tides), such as those along the shores of the Gulf of Mexico in Florida’s panhandle.  Lakes can also exist due to a combination of processes, such as a solution basin that is filled in with floodwaters from a nearby river.

Side by side comparision of an oligotrophic (left) and eutrophic (right) lakes.  The 'pea-soup' green on the right highlights how nutrient rich and productive the lake is, but this is not necessarily 'unhealthy' as some people may assume.

Effects of Lake Diversity

Due to the diversity in lake types and in Florida’s geologic foundation (e.g., phosphorus beds, limestone, peatland), Florida lakes are represented by a wide range of trophic states.  This includes a range of phosphorus, nitrogen, chlorophyll-a (a measure of a lake’s photosynthetic production), color, among others.  With regards to color, some lakes in Florida have relatively clear-water that is stained a reddish-brown tint by tannic acids, a byproduct of decomposed woody trees, such as cypress.  Others are clear-water stained somewhat black by humic acid, a result of nearby wetlands and peatlands.  Differences in color due to these dissolved compounds can create a sort of shading effect (creating differences in the ability for plants and phytoplankton to photosynthesize), and potentially change the lake’s thermal structure.  This creates differences in the habitats available for plants and animals between stained lakes and unstained lakes.  Florida’s lakes can exhibit a range of color from one waterbody to another (from 0 to 400 platinum-cobalt units).  Two nearby lakes can have drastically different trophic states, and thus plant and animal communities, which results in different recreational use(s) if one is stained dark by tannic acid and the other is not.  The wide range of water quality in Florida lakes creates a wide diversity in lake types and recreational uses.

Why Can’t All Florida Lakes be Oligotrophic?

Many of Florida’s lakes are eutrophic due to the geologic formations of their lake basins.  Plants and phytoplankton have easy access to nutrients due to the phosphorus-rich soils in Florida’s foundation, and the shallow basins ensure that nutrients remain accessible for production (e.g., sediment resuspension).  In deeper lakes (such as those in the north), nutrients from decomposing plants and animals can descend to the lake-bottom remaining there over time effectively removing nutrients from the lake due to the lack of sediment resuspension.  Many of Florida’s lakes are darker waters with high levels of nutrients and thus abundant populations of fish and wildlife. The growing season is long relative to northern lakes, which allows for increased annual biological productivity.  In comparison to northern lakes, Florida lakes can look “dirty” and “impaired”, but they are often in their natural state.
 Considering these differences, we see that some Florida lakes cannot be like their northern counterparts.  Florida has a diversity of lake types and trophic states that Floridians may not be aware of.  Here in Florida, we have a little bit greener waters with more plants, as such we get highly abundant fish and wildlife populations on these lakes.  Lake-users may be disappointed by these differences as they might wish to swim or boat more often like they would on northern lakes.  This doesn’t mean that these lakes are unhealthy.  We must always consider the context of how these complex ecological systems form and ultimately function when evaluating how we would like to use them and whether they are considered ‘healthy’ or ‘unhealthy’.

The following entry was coauthored by Kyle Wilson (UF) and Chris Anderson (UF and FWC)