Phytoplankton productivity in Saginaw Bay, Lake Huron: Effects of zebra mussel (Dreissena polymorpha) colonization

Phytoplankton photosynthesis-irradiance parameters, chlorophyll concentrations, under-water extinction coefficients (kPAR), and surface irradiance were determined at 8-10 sites on 27 occasions in Saginaw Bay from spring 1990 through fall 1993 corresponding to a period before and after the establishment of large zebra mussel populations (began in summer 1991). Similar measurements, with the exception of the photosynthetic parameter, a had also been made in 1974/75 at eight sites on nine occasions. In inner Saginaw Bay where zebra mussels were primarily found, chlorophyll and kPAR values decreased, while the photosynthetic parameters, P-max and alpha, increased after zebra mussel colonization. At sites in the outer bay where no zebra mussels were found, chlorophyll and kPAR values did not change after zebra mussel colonization, whereas photosynthetic parameters increased. Decreases in chlorophyll and kPAR in the inner bay were related to the zebra mussel, but increases in photosynthetic parameters in both the inner and outer bay were not. Areal-integrated and volumetric phytoplankton productivity decreased by 38% and 37%, respectively, in inner Saginaw Bay after the establishment of zebra mussels; phytoplankton productivity at outer bay control sites was similar during the same period. Decreased phytoplankton productivity in the inner bay was attributable to the large decrease in chlorophyll as increases in underwater irradiance (increased kPAR) and photosynthetic parameters could not compensate for the chlorophyll effect. Increase in underwater irradiance produced a significant increase 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 colonization (1974-1990) to 1.1-1.3 after colonization (1992-1993). Overall, primary productivity in the inner bay did not exhibit a notable change after zebra mussel colonization as decreases in phytoplankton productivity were accompanied by increases in benthic primary productivity. Thus, zebra mussels altered inner Saginaw Bay from a pelagic-dominated system to a benthic/pelagic system which will have long-term effects on food web structure and productivity at higher trophic levels.