Tonight I'm going to talk about my backyard. Lake Jackson is a mid-sized lake just north of Tallahassee, Florida. It is a shallow, flat-bottomed lake that is approximately 'L' - shaped, about 8 miles from southern tip of Meginnis Arm all the way to the northeastern end of the lake. The lake has an unusual property of 'disappearing' on a regular basis and then re-appearing. This diary is about the interaction of the disappearance of Lake Jackson with human impacts on the lake.
The bottom of Lake Jackson lies approximately 75 feet above sea level. The lake receives most of its inflow through arms at the southeastern and southwestern corners of the lake as well as drainage in the northeast. There is no outflow on the surface. Water loss is through evaporation or drainage down into the aquifer below.
Like much of Florida, the Tallahassee region lies on top of limestone. Water has dissolved passageways in the limestone over time and now a vast aquifer lies underneath us, consisting of a network of flooded caves and passageways. Two sinkholes connect the lake to the aquifer beneath. During times of extended drought the lake level will drop and the aquifer level will drop beneath it. Debris clogging the sinkholes will collapse as the aquifer drops and the water will suddenly drain out of the lake at a fairly high speed. At the end of this drainage the effect can be very dramatic, as I've been told by those who witnessed the last one. The water spirals down the sinkhole, just like the water leaving a bath tub. Fish and other aquatic animals are often left stranded as the lake bottom is transformed into a muddy plain.
Much of the detail for this introductory system comes from here.
Historically the lake has emptied about once every 25 years, on average. Generally it has begun to refill within a few months of emptying. The last emptying of the lake occurred in 1999. Photos from the Florida Geological Survey show one of the sinkholes and the lake bottom with a stream running across it.
Prior to the 1999 drainage Lake Jackson had been suffering from environmental degradation. The population of Tallahassee as a whole was increasing dramatically and, starting in the 1960s and 70s development had started along the borders of the lake. The development increased storm water runoff, and with no outflow, sediments, nutrients, and toxins such as heavy metals increased in the lake.
The lake had been clear with a sandy bottom. Mud accumulated on the bottom, reducing the ability of bass and sunfish to breed.
Environmentalists saw the drainage as an opportunity to reverse some of this damage. Much of the mud on the bottom of the lake was removed shortly after the drainage. Storm water runoff is now controlled by a facility to the south of Meginnis Arm, preventing the worst of the inflow (although runoff from local developments around the lake is not controlled). Lake Jackson is now considered an Aquatic Preserve.
Another impact of the drainage was the large scale movement of aquatic wildlife from the northern part of the lake into Little Lake Jackson to the west which has remained continuously full of water. Unfortunately a major highway separates the two and the mortality from cars was extreme. A single FSU grad student took it upon himself to try and reduce the carnage and over a decade maintained temporary barriers and raised funds for a permanent barrier, the Lake Jackson Ecopassage which was completed with stimulus funds last year.
Unlike previous events the drought continued and the lake did not refill for nine years. Much of the lake bottom took on the character of a wet prairie although pockets of permanent water remained. Here is a photo of part of the south end of the lake taken August 15, 2008. I took this picture from the deck of what is now our home when we first visited it. It's a pretty crappy picture but hopefully you can see the area of water in the foreground which must sit in a local depression. In the background is a higher area of the lake bottom, then overgrown with terrestrial vegetation.
Just about one week later, Tropical Storm Fay arrived in Tallahassee and dumped over half a meter of rain in just over 24 hours. The lake filled up about 'half-way'. We moved in three months later. Here is our neighborhood dock in October 2008. The end of the dock is still well out of the water and terrestrial vegetation grows down to and into the water.
The view from our house now looks like this. The far side of the lake is now underwater with the dead shrubs emerging from the water.
By the following spring the lake is considerably higher but most of the shoreline vegetation remains grasses.
By the following year (March 2010) water extended completely across the lake with no emergent plant material (this picture was taken while one of our oaks was being 'pruned').
However by last fall, without the lake falling significantly emergent vegetation is visible again.
Instead of returning to its original state of an open water, sandy-bottomed lake Lake Jackson has become quite marshy. A great diversity of aquatic plants began to proliferate along the shoreline and even out into the middle of the lake starting in summer 2010 when this moorhen family was photographed.
Whether it was the prolonged drought, the input of nutrients in the decades prior to 1999, the nutrient input of the decaying terrestrial vegetation, or some combination of the above, something has put Lake Jackson on a different successional pathway than previously. Ecologists have come to appreciate the transient and unstable nature of ecosystems in recent years. Something has tipped the lake into a different pathway.
This is important concept to keep in mind for the decades to come. The changing climate of our planet is going to cause countless such changes around the world.
The current change is fine with me. It reduces the lakes attractiveness to jet skiers and water skiers, making it a more peaceful place. When I first kayaked out in the lake, the water itself seemed fairly lifeless except for the abundant over-wintering coots. Now it is teeming. Here are a few observations from this weekend.
Red-winged Blackbirds now breeding in numbers
Eastern Kingbirds are a common sight
A new arrival in numbers are Purple Gallinules
with their huge feet for walking on the water lilies
Their relatives the moorhens also occur in large numbers.
Ospreys are seen constantly at this time of year
Great Blue Herons have been common since I first laid eyes on the lake
One animal that is surprisingly not seen very often is the alligator. This is a bad picture but it is the only one I have photographed on the lake.
The real 'stars' of the lake are the limpkins (pictured above the fold). These odd birds, relatives of cranes, are specialists on apple snails. The native apple snail had declined here in north Florida and the limpkins had largely disappeared here on the northern edge of their range. However an introduced apple snail has taken hold in the lake and the limpkins are back in numbers.
Here's one on someone's dock
And a pair on someone's front lawn where they were chowing down.