Window on the Lake: Why all the water weeds?
Clear Lake has experienced an interesting population shift the last few years. This has resulted in a ring of water weeds around the rim of the large western arm. The western arm and the other arms are almost like different lakes when noting the weeds and water clarity.
So what”s going on and is the water quality better? Sadly no, the lake is just as loaded with phosphorus as ever. I”ve seen this phenomenon before and it”s no surprise threadfin shad are involved. Clear Lake is ordinarily a rather turbid system with inorganic suspended particulate matter in the winter and phytoplankton (microscopic algae) blooms in the summer. A highly nutrient fueled lake can almost be expected to have runaway populations of one thing or another dominating the system.
When the non-native threadfin shad appeared in the mid 1980s, they exploded into a dominate zooplankton grazing machine with populations filling the whole lake. You couldn”t turn around without seeing one predator species or another taking turns doing well on a diet of threadfin shad. From pelicans to the introduced bass, either populations or body size grew in response. The threadfin grazed down the zooplankton, which ordinarily feed on and kept the phytoplankton populations in check. With the loss of zooplankton for food, many other young fish species also did not fare well (such as hitch) and phytoplankton numbers clouded the water in the summer reducing light penetration into the water column. Since light must reach the bottom to encourage the start of native and non-native water weeds, those emergent plants receded to the very margins of the lake.
But threadfin are an unstable boom or bust species, particularly in introduced environments, and a bust back to smaller populations is inevitable. I saw this in many lakes when threadfin were introduced by Fish and Game in early lake experiments. The bust happened in the 1991 cold snap and with few threadfin to impede them, the zooplankton exploded back with a vengeance in the western arm, clearing the water column of phytoplankton. The sun could now reach the sediments at a new limit of clarity (about 7 feet). Encouraged by sun, weeds (also fueled by phosphorus) could get a start farther from shore. Sunlight reaching large areas of bottom sediments also supported certain species of cyanobacteria (blue-green algae) such as lyngbya, which sequester phosphorus at the sediment-water interface.
As cyanobacteria cells grow and compete for light, they float to the surface where they are at the mercy of the western winds; hence, their green cloudy water appearance in the eastern arms and smelly dead cyanobacteria cakes. Cyanobacteria species will bloom in a highly phosphorus-loaded lake independent of threadfin shad influence.
The problem with cyanobacteria is that they are not grazed heavily by zooplankton or other species and, in fact, are toxic to many species. So both the water weed and cyanobacteria issues for Clear Lake are linked to nutrient loading, with the cyanobacteria blooms a direct product of the phosphorus overloading. Phosphorus limiting factors such as low iron are keeping us from a total cesspool experience. There are data that support additional explanations, but it”s hard not to follow this line of thinking when dealing with threadfin. The interesting note is that the large aquatic weed response has also stabilized exploitation of zooplankton because other fish species protected by the weeds also feed on the threadfin keeping them in check.
All of this again points to certain treatment for Clear Lake”s unsavory conditions; control upland surface runoffs and restore shoreline emergent vegetation and wetland functions. But then, you knew that.
Jim Steele is a Retired Cal Fish and Game scientist, registered professional forester, part-time consultant and a full-time Lake County resident-volunteer.
