INCREASED ABUNDANCE AND DEPTH OF SUBMERSED MACROPHYTES IN RESPONSE TODECREASED TURBIDITY IN SAGINAW BAY, LAKE HURON

Submersed macrophyte communities and turbidity near shore were measure d from 1991 to 1993 to determine if more light resulting from coloniza tion of zebra mussels (Dreissena polymorpha Pallas) into Saginaw Bay i n 1990 corresponded with changes in macrophyte distribution. Turbidity was sampled along five transects distributed at intervals perpendicul ar to the perimeter of inner Saginaw, Bay to monitor changes in light available to plants in Saginaw Bay. Vegetation was sampled in July alo ng these transects to determine the distribution and composition of th e macrophyte communities each summer. We also measured the maximum dep th of colonization and the area of plant coverage by use of 31 transec ts evenly distributed around Saginaw Bay in August. Turbidity decrease d (P less than or equal to 0.097) at transects in northern littoral re gions from 1991 to 1993 over submersed plant communities and uncoloniz ed sediments, but not in southern littoral regions. The relative abund ance of submersed macrophytes increased (P less than or equal to 0.000 1) at all transects from 1991 to 1993, especially at transects where t urbidity decreased significantly. Maximum depth of colonization (2.0 m ) and the area of macrophyte coverage (101.3 km(2)) increased in Sagin aw Bay, especially in the northwestern littoral region of the bay. Mac rophytic chlorophytes, charophytes, and Vallisneria americana increase d (P less than or equal to 0.003) in relative abundance most at transe cts where turbidity decreased significantly. These results demonstrate that even in a large well-mixed lacustrine environment, zebra mussels have the capacity to reduce turbidity sufficiently to allow submersed macrophytes to expand their distribution and abundance.