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mats of cladophora at Toft point.

We recently took some friends to Toft Point in Door County and as soon as we got out of the car we were greeted by a smell familiar to all aquatic ecologists--dead algae. Although the forest obscured our view of the beach I knew we would find the cobbles covered with decaying Cladophora.

This branching, green filamentous algae is native to the coastal areas of the Great Lakes where it grows attached to rocks and other hard surfaces. In the 1960s and 70s it became a nuisance when phosphorus levels in the lakes were high. Phosphorus is a plant nutrient that enters waterways due to run-off from agricultural fields, sewage and wastewater discharges. Regulations limiting phosphorus inputs into the lakes resulted in a a decline in phosphorus and Cladophora in the 80s and 90s.

So why is it back? Scientists are not entirely sure. Restrictions on phosphorus have not changed but there may be increased nutrient inputs from farms and urban areas. Its also possible that increased water clarity and water temperature have improved growing conditions for the algae.

Another possibility is more complex and has to do with how phosphorus moves through the food web. Historically the food web in the lake has been pelagic, with phosphorus controlling the growth of phytoplankton (algae) in the open water column. Scientists suspect that the introduction of exotic zebra mussels and quagga mussels (Dreissena polymorpha and Dreissena bugensis) into the lakes have changed how phosphorus cycles throughout the lake. In the past phosphorus was distributed throughout the water column. Zebra mussels filter particles out of the water column and package undigested particles as tiny bundles of waste called "psuedfeces" that fall to the bottom and stay there. By doing this zebra mussels are effectively concentrating nutrients like phosphorus at the bottom of the lake where attached benthic algae can grow with little predation or competition. So we are now seeing excessive growth of this algae that breaks apart in storms and washes up on the beaches to decompose.

For more information on Cladophora see:


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Last updated on March 20, 2015