PHYTOPLANKTON PRODUCTIVITY IN SAGINAW BAY, LAKE HURON - EFFECTS OF ZEBRA MUSSEL (DREISSENA-POLYMORPHA) COLONIZATION

Phytoplankton photosynthesis-irradiance parameters, chlorophyll concen trations, under-water extinction coefficients (kPAR), and surface irra diance were determined at 8-10 sites on 27 occasions in Saginaw Bay fr om spring 1990 through fall 1993 corresponding to a period before and after the establishment of large zebra mussel populations (began in su mmer 1991). Similar measurements, with the exception of the photosynth etic parameter, a had also been made in 1974/75 at eight sites on nine occasions. In inner Saginaw Bay where zebra mussels were primarily fo und, chlorophyll and kPAR values decreased, while the photosynthetic p arameters, P-max and alpha, increased after zebra mussel colonization. At sites in the outer bay where no zebra mussels were found, chloroph yll and kPAR values did not change after zebra mussel colonization, wh ereas photosynthetic parameters increased. Decreases in chlorophyll an d kPAR in the inner bay were related to the zebra mussel, but increase s in photosynthetic parameters in both the inner and outer bay were no t. Areal-integrated and volumetric phytoplankton productivity decrease d by 38% and 37%, respectively, in inner Saginaw Bay after the establi shment of zebra mussels; phytoplankton productivity at outer bay contr ol sites was similar during the same period. Decreased phytoplankton p roductivity in the inner bay was attributable to the large decrease in chlorophyll as increases in underwater irradiance (increased kPAR) an d photosynthetic parameters could not compensate for the chlorophyll e ffect. Increase in underwater irradiance produced a significant increa se in light to the benthic region and contributed to increased benthic primary productivity; ratio of photic zone to station depth increased in inner Saginaw Bay, from 0.6-0.8 before the zebra mussel colonizati on (1974-1990) to 1.1-1.3 after colonization (1992-1993). Overall, pri mary productivity in the inner bay did not exhibit a notable change af ter zebra mussel colonization as decreases in phytoplankton productivi ty were accompanied by increases in benthic primary productivity. Thus , zebra mussels altered inner Saginaw Bay from a pelagic-dominated sys tem to a benthic/pelagic system which will have long-term effects on f ood web structure and productivity at higher trophic levels.