North Pond is currently experiencing an algal bloom, per the definition of the Maine Department of Environmental Protection (DEP). The most recent Secchi disk transparency, measured on 7/21/20, was 1 m (~3 ft). Anything less than 2 m (~7 ft) is considered to be a bloom, and anything less than 1 m can potentially be a harmful algal bloom (HAB). The North Pond Association and the 7 Lakes Alliance collected algae samples and confirmed that this is a cyanobacteria (blue-green algae) bloom, mostly consisting of Dolichospermum (formerly Anabaena). Cyanobacteria can produce toxins which can irritate skin and potentially be fatal to pets when ingested (although there have been no reported incidents of this in Maine). For humans, long term exposure (over the course of years) to the most common toxin observed in our lakes (microcystin) can lead to health impacts.
Just because there is a bloom, it does not mean that toxins are being produced; data from the DEP indicates that toxin production typically occurs later in the season and generally does not reach levels where recreation on the lake is dangerous (e.g. East Pond never reached the EPA’s limit for recreation before the alum treatment, but it did exceed drinking water limits). Nonetheless, the 7 Lakes Alliance is exploring options for quick microcystin testing so that we can track microcystin from week to week for informational purposes.
Hot weather exacerbates the bloom, as cyanobacteria thrive in warmer water, so the bloom will likely continue to worsen. It is recommended by the DEP to limit time swimming in the water (particularly children, who tend to ingest more lake water than adults), rinse off thoroughly when you leave the lake, and keep pets away, particularly from surface scums. Lake residents who use the water for bathing should take short showers. Toxins are chemical compounds so boiling water is not effective at removing them.
Blooms occur because there is too much phosphorus (which is fuel for algae) in the lake. Once a lake starts blooming i