TULSA (AP) — As a major holiday weekend approaches, the Grand River Dam Authority is discouraging swimming in Grand Lake because of the presence of blue-green algae. GRDA spokesman Justin Alberty says the agency is “strongly discouraging” swimming or other activities that would bring persons into contact with the water. Alberty says the agency issued the advisory because of the rapidly changing conditions of the blue-green algae levels in areas of the lake. The agency's board of directors will have an emergency meeting in Tulsa on Friday afternoon to discuss the situation. Blue-green algae are microscopic organisms that are naturally present in lakes and streams, usually in low numbers. Most blue-green algae aren't toxic, but toxins harmful to humans and animals can be produced in some algae blooms.