There are so many interesting and funky places in Florida. Green Spring is like a little emerald in the forest on the northern edge of Lake Monroe. Unlike most springs in Florida, it has high concentrations of sulfur that produce a hard-to-miss aroma and milky green water. It is so small and tucked away in the trees that it is impossible to see on Google Earth, no matter how magnified.
Despite the color, aroma, and almost non-existent dissolved oxygen concentrations, it hosts a huge number of sailfin mollies and mosquitofish, two of the most tolerant and abundant fish in Florida. Both fish have flat heads that allow them to breathe at the air-water interface, or the thin layer of water right at the surface. In many Florida water bodies that either are fed by groundwater or are stagnant, this layer of water has more dissolved oxygen than the water below it.
A female mosquitofish (Gambusia holbrooki) is depicted in the left image and a male sailfin molly (Poecilia latipinna) is depicted in the right image–photos by Missy Gibbs. The mouth of each fish is only slightly below the peak of the dorsal side of the fish, allowing it to cruise around just below the surface of the water.
Danielle Palow was my field assistant for the morning. After a quick seine or two in the springhead area to establish that, yes, in fact, all of those fish that look like mosquitofish and mollies really are mosquitofish and mollies, we sampled the little creek into which Green Spring flows. I have not yet found the source of the creek, but it flows past the spring and into Lake Monroe. The water is somewhat tannic (brown) and, on the day that we sampled, the dissolved oxygen in the creek above the outflow of Green Spring was 8 mg L-1. However, the dissolved oxygen dropped to 3.8 mg L-1 as soon as the spring water, which has an oxygen concentration of 0.3 mg L-1, entered. We measured a more respectable oxygen concentration of 4.9 mg L-1 as the creek approached the lake. To sample fish, we used a seine as a block net across the run. Then I went upstream 50 m or so and tried to scare fish with a dip net so that they would swim downstream into the block net. We repeated this method six times, working our way towards Lake Monroe. It is a very small run (the entire run looks like the photo below), so I do not think that we missed huge numbers of fish. However, we only caught one bluegill (Lepomis macrochirus) and a few mosquitofish with this method. Happily, I did catch a few damselfly larvae and one dragonfly larva and, not as happily, I found a monster Malaysian trumpet snail.