NHDES posts beach advisories when sample analyses result in bacteria levels above the state standard, indicating the possible presence of disease-causing organisms, or a toxic cyanobacteria scum. These advisories are recommendations to the public to avoid water contact of any kind and activities at the beach until further analyses reveal safe conditions.
When is a Beach Advisory posted?
There are many factors that can create E.coli levels to be high. In summer it can be a combination of a lot of people using a particular beach and Mother Nature can contribute. For instance swans and ducks are very big polluters in the water and especially on beaches themselves.
For a beach at a freshwater lake, an advisory is posted if 2 or more samples collected exceed the state standard of 88 counts of E. coli per 100 milliliters (ml) of water OR when one sample exceeds 158 counts of E. coli per 100 ml of water.
An advisory is posted if a potential toxin-producing cyanobacterial scum is present at any beach with greater than 50 percent cell dominance of the samples total cell count OR the cyanobacteria cell count is greater than 70,000 cells per ml of water. Cyanobacteria, also know as "blue-green algae," form mats on the surface of water and can produce toxins that are harmful to humans and dogs. Blooms of cyanobacteria typically occur in slow-moving or still water, and weak streams, when the water is warm, gets plenty of sunlight, and is rich in nutrients like and nitrogen and phosphorous.
At a coastal beaches, an advisory is posted if sample results exceed 104 counts of Enterococci per 100 ml of water.
It is just nasty stuff so if you see it you should call NH DES.