Slow day at Lake Shelby

Had a rough morning at Lake Shelby today. The lake and especially the creek are covered with mats of string algae to the extent that the algae completely covered the creek at many spots. One of the other boats said the park folks had poisoned the algae that week, and some of the mats did look like they were turning brown. I saw three other boats and we all left by 10:30.

Question: although poisoning the string algae would make the lake prettier and easier for paddle-boats, wouldn't poisoning it risk hurting the fish by depleting oxygen and making the water acidic?