State water officials are urging caution around two Merced county waterways this weekend over possible toxic algae blooms that can be harmful to people and pets. In some cases, toxic algae exposure can lead to death.

The Department of Water Resources issued algae advisories on Friday for San Luis Reservoir and O’Neill Forebay. Both popular recreation spots are located off of Highway 152, about 12 miles west of Los Banos.

Exposure to the toxins can occur while swimming or participating in other water activities, and when accidentally swallowing lake water. Dogs and small children are most likely to be affected by toxic algae because of their smaller body size and likelihood of being in the water for longer periods, a notice on the DWR website says.

The website says there is no sure way to tell if an algal bloom is toxic just by looking at it – but there are indicators that a bloom may be harmful. Danger signs include:

• Small blue-green, green, white, or brown particles in the water.
• Streaks in the water that look like spilled paint.
• Mats, scum, or foam at the surface or along the shoreline.
• Fishy odors or a smell similar to gasoline or septic waste.

Even if an area looks clear, it might not be safe. Bloom conditions can change rapidly, and wind and waves may move or concentrate the bloom into different regions of a reservoir, the DWR website warns.

San Luis Reservoir: Caution

The agency has issued a caution advisory for San Luis Reservoir.

DWR says it’s safe to swim in the water, but visitors should stay away from algae and scum in the water. Do not let pets and other animals go into or drink the water or eat any scum on the shore.

Shellfish from the reservoir should not be eaten, the agency said. Fish should be thoroughly cleaned and washed with tap or bottled water before cooking.

O’Neill Forebay: Warning

DWR issued an algae warning for the O’Neill Forebay, which includes the following advisories:

• No swimming.
• Pets and other animals should not enter or drink the water, or go near scum.
• Avoid all cloudy or discolored water.
• Do not eat shellfish from the water.
• Do not use forebay water for drinking or cooking. Boiling or filtering will not make the water safe.
• For fish caught in the forebay, throw away guts and clean fillets with tap or bottled water before cooking.