Most of you have seen green algae blooms in parts of the lake in recent weeks. A few of these have been tested in both the first and second basins and some were found to contain cyanobacteria toxins based on rapid tests by the LBA. As noted earlier in the season, cyanotoxins are toxic primarily if ingested so small children and dogs are especially susceptible and should be kept out of the lake whenever any green is visible. Toxins can also be released later as the cyanobacteria die off. Some swimmers have experienced skin rashes and eye irritation.
This algae bloom phenomenon has occurred in previous years around this time but to a lesser extent. The likely cause is accumulation of the critical nutrient phosphorus in the sediment at the bottom of the lake. Phosphorus can come from landscape fertilizer runoff, especially where there is no vegetative buffer along the shoreline, from leaves and debris allowed to wash in, from failing septic systems, from pet waste, and from some dish washing detergents.
Phosphorus is absorbed into aquatic weeds and much of it gets bound to sediment at the bottom. During summer months the upper layers of water warm up but the bottom remains cool and its greater density prevents it from mixing with upper layers. In addition, dissolved oxygen is consumed near the bottom by organic decay and no mixing. The very low oxygen condition causes release of bound phosphorus which still remains in the cold, dark water layers at the bottom. In the fall, with cooling of the upper layers of the lake, the water begins mixing and soon becomes uniform, with the phosphorus and other nutrients coming up to the warmer sunlit surface–ideal conditions for algae to thrive. With the greater rainfall this summer there may have been greater runoff of nutrients.
There is no good short-term solution to this problem. It requires sustained watershed management by all of us to prevent additional phosphorus from entering the lake. If this can be done, the phosphorus available from the sediment will eventually be reduced and so will algae growth.