Swimmers and pet owners are encouraged to use caution while recreating at two North Iowa lakes this weekend due to E. coli and microcystin levels.

The Iowa Department of Natural Resources’ weekly sampling at McIntosh Woods State Park in Ventura, Clear Lake State Park and Beed’s Lake State Park in Hampton exceeded the one-time sample maximum of 235 E. coli per 100 milliliters of water with a reading of 2,600, 360 and 550, respectively, Tuesday.

McIntosh Woods and Beed’s Lake also surpassed the state’s threshold for microcystin of 20 micrograms per liter of water with a reading of 24.525 and 53.99, respectively.

Swimming is not recommended at McIntosh Woods and Clear Lake State Park beaches on Clear Lake as well as Beeds Lake Beach, the DNR’s water quality monitoring site says.

High levels of bacteria are attributed to fecal contamination of beach water from improperly constructed and operated septic systems and sewage treatment plants, manure spills and storm water runoff from land with wildlife and pet droppings.

According to the DNR, surface runoff after a heavy rainfall may transport high levels of fecal bacteria to the water at the beach. It also increases the sediment in the water causing it to be murky. Sunlight destroys the bacteria and improves the water quality.

The swimming advisories are among eight, including E. coli- and microcystin-related, issued throughout Iowa after this week’s sampling. Others include beaches at Backbone, Green Valley and Lake of Three Fires state parks.

The advisories don’t mean the beaches are closed, but anyone swimming at the locations should take extra precautions, including showering shortly after swimming and avoiding ingestion of lake water.

A variety of diarrheal diseases and skin, ear and respiratory infections are associated with swimming in contaminated water, the DNR states.

The DNR conducts weekly monitoring of 39 state park beaches for E. coli and microcystin the week prior to Memorial Day through Labor Day.

Photos: Big fish caught in North Iowa

Reach Reporter Ashley Stewart at 641-421-0533. Follow her on Twitter at GGastewart.