Pinto Lake is a 120 acre recreational lake in Watsonville, CA. The lake develops massive algal blooms every later summer and fall and algal toxin levels typically exceed the State health criteria. As a result, the lake is classified as “impaired” and is closed for contact recreation during the bloom periods. Of particular interest, the death of 31 endangered southern sea otters have been traced to algal toxins which have accumulated in shellfish eaten by the otters. Pinto Lake is the likely source of the toxins in the shellfish and the cause of the otter deaths. Excessive amounts of the nutrient phosphorus is the main cause of the toxic algal blooms. Internal phosphorus loading (leaching from the lakebed sediments) and watershed runoff both contribute phosphorus to Pinto, but a study in 2011 showed that the majority (85%) was coming from the lakebed.
Samples from the bottom of the lake confirmed that phosphorus was very high in the sediments and available to be released into the overlying water column. HAB Aquatic Solutions will be conducting a buffered alum application over a ten day period in early April. The application produces a “floc” that settles to the bottom of the lake. The floc has sites where phosphorus in the sediments become chemically bound as it leaches from the bottom. The floc effectively intercepts and binds the phosphorus, which makes it unavailable for the algae to use for growth. The goals of the project are to dramatically reduce the internal loading of phosphorus from the sediments, lower the amount of phosphorus available to algae in the water, reduce the amount of algae and associated toxins and remove the recreational restrictions at the lake.