McHENRY – Petersen Park Beach, 4311 Lakewood Road, reopened Wednesday after being closed on and off this month because of an algae bloom caused by high temperatures and a lack of rain.

In the spring, the area was treated with herbicides to deal with Eurasian milfoil, an aggressive, non-native species that chokes out native plants, McHenry Parks and Recreation Director Pete Merkel said previously. The treatment caused the weeds to die, decompose and wash ashore.

Blue-green algae fed off the nutrient-enriched water and weeds to create the bloom. The situation was worsened by record-high temperatures and the lack of rain.