Friday, July 26, 2013

What’s in a lake? Exploring Florida’s diverse lakes: Part 1

Not all lakes are created equal

Florida lakes can be quite different in shape and water quality than their northern counterparts.  Limnologists (i.e., scientists that study lakes and reservoirs) group lake types together based on ecologically-relevant criteria known as ‘trophic state’.  Some of the most useful determinants in lake quality (ultimately how people use and evaluate their lakes) are the amounts of nutrients, particularly phosphorus and nitrogen, in the water column of a lake. Nutrient concentrations are often the limiting factors regarding lake productivity.  These ‘limiting nutrients’ are required by plants and phytoplankton to grow. The growth and proliferation of these primary producers enhance fish and wildlife populations in lakes.

Different lake types and trophic states have different plant and animal communities which can affect perception and recreational use(s) by humans

Before we drive into a discussion on the diversity of Florida lakes, it will be useful to give a background on trophic states and how some hydrogeological processes influence lake trophic states.

Trophic state indices classify lakes as oligotrophic (clear, blue water, nutrient poor), mesotrophic (clear, blue water, intermediate nutrients), eutrophic (murky, dark water, nutrient rich) or hypereutrophic (green water, extremely nutrient rich).  Oligotrophic and mesotrophic lakes are the most common trophic states for lakes in the northern United States, Canada, and Europe.  Historically, lakes in these geographic regions are well-studied (e.g., limnology largely developed at major universities in the northeastern United States and northern Europe). The water quality in these lakes has formed a cultural backbone of recreational usage expectations in European and early American settlement, which persists today.  This has contributed to a stereotype that ‘good and healthy’ lakes are clear-blue lakes, which are highly desirable for recreation (e.g., swimming and boating) and/or lake-front property real estate.  But what constitutes ‘good and healthy’ is not ubiquitous everywhere as the ‘health’ of our lakes often depends on how the lake is formed and whether ‘abnormal’ problems (such as pollution) are occurring.

Lake Origins

How lakes originate often determines the lake’s trophic state.  Differences between Florida and northern lakes can often be attributed to hydrogeological differences in lake formations.  For example, most northern lakes formed via glacial processes; as glacial sheets retreated northwards across the continents they gouged out large crevasses and depressions along the continent.  Ultimately, these carved-out depressions filled with glacial water and became deep, cold-water lakes similar to those found in Canada and the northern United States (e.g., Great Lakes).

The glacial retreat northwards carved out deep basins into the North American bedrock that ultimately became clear-water, oligotrophic lakes

Northern Lakes

The processes that created northern lakes contribute to their typically nutrient-poor and clear-water conditions.  Northern glacial lakes are carved into impermeable bedrock creating an effective seal that restricts nutrient flow into and from the water via the lateral movement of nearby groundwater and other external sources.  This can end up limiting lake production of plants and phytoplankton.  The depths of these lakes also contribute to their oligotrophic classification.  For example, many lakes in the north reach depths of 15-30 m which makes sediment resuspension unlikely. Sediment resususpension is known by limnologists to affect the amount of nutrients, particularly phosphorus, in a lake.  Many of these northern lakes are left with a small rim of aquatic plants on the lake edge leaving the middle of the lake open.  Thus, most northern lakes in their natural state are classified as oligotrophic or mesotrophic and are used for a mixture of recreational boating, swimming, and some fishing.  Eutrophication via farmland runoff and point-source pollutants can increase nutrients in many of these systems (e.g., Lake Erie) and make once nutrient-poor systems into nutrient-rich systems.  Fortunately, some of these changes can be reversed with effective management.  This has left many people with a perception that eutrophic lakes are polluted lakes, and thus eutrophic lakes are ‘unhealthy’.

Lakes are perceived differently by different people.  Here we see a clear-water lake lined with shoreline vegetation but little aquatic plants in the water itself.  Some folks could see this as a good lake for swimming and boating while others might see this as relatively poor fish habitat.

This concludes Part 1 of our entry comparing the diversity of lakes.  Next week we will come back with Part 2 which describes how lakes in Florida  can be quite different from their northern counterparts leading to different trophic states and recreational uses.  The above entry was coauthored by Kyle Wilson and Chris Anderson (from UF and FWC)

1 comment:

  1. Great article. This information is useful to a Florida native.