NUTRIENT CHANGES IN SAGINAW BAY, LAKE HURON, AFTER THE ESTABLISHMENT OF THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA)

Concentrations of particulate and dissolved nutrients in Saginaw Bay, Lake Huron, were examined relative to zebra mussel colonization which occurred summer 1992. The magnitude and spatial pattern of changes ind icate that mussels had a significant impact on nutrients in Saginaw Ba y. Annual means for total suspended solids, particulate organic carbon , particulate phosphorus, and particulate silica in the inner bay were significantly lower in 1992 and 1993 (post-zebra mussel) than in 1991 (pre-zebra mussel). Annual means decreased from 11.5 mg L(-1), 1.45 m g C L(-1) (121 mu M), 20.4 mu g P L(-1)(0.66 mu M), and 1.52 mg SiO2 L (-1) (24 mu M) respectively in 1991 to 4.4 mg L(-1), 0.79 mg C L(-1) ( 66 mu M), 11.2 mu g P L(-1) (0.36 mu M), and 0.77 mg SiO2 L(-1) (12 mu M) in 1993. In contrast, there were no significant differences among years for these parameters at control stations, which were located in the outer bay and had no known populations of mussels. Annual means fo r nitrate, ammonium, and silica were significantly higher in the inner bay in 1992 than in 1992, but not significantly different in 1993. Me ans increased from 0.39 mg N L(-1), 21.0 mu g N L(-1), and 1.11 mg SiO 2, L(-1) respectively in 1991 to 0.47 mg NL(-1) 30.9 mu g N L(-1), and 1.71 mg SiO2 L(-1) in 1992. No significant differences were observed for these parameters in the control 2 group. Differences between 1992 and 1993 may reflect differences in the amount of runoff and circulati on between Saginaw Bay and Lake Huron. A phosphorus budget indicated t hat zebra mussels were a significant sink for phosphorus. Mussels from the inner bay accumulated 108, 682, and 52 t respectively in 1991, 19 92, and 1993. Comparatively, the annual pool of phosphorus in the wate r column of the inner bay decreased from a pre-zebra mussel (1979-1980 ) average of 712 t to 421 and 382 t in 1992 and 1993 respectively.