Effects of temperature and current discharge on the concentration and photosynthetic activity of the phytoplankton in the upper Mississippi River

SUMMARY. Temperature and current discharge regulated phytoplanktonic concentration, chlorophyll‐a concentration, the light‐saturated rate of photosynthesis (P max ), and photosynthetic capacity (P cap ) in the Mississippi River at Prairie Island, Minnesota. The chlorophyll‐a maximum was 48 mg m −3 in 1975, a wet year with a high current discharge, and 190 mg m −3 in 1976, a relatively dry year. The highest values of P max were 0.37 (mgO 2 I −1 h −1 ) in 1975 and 1.60 in 1976. P cap varied from 3 to 21 (gO 2 per g chlorophyll‐a h −1 ) both years, and its value was highly correlated with temperature. The temperature optimum shifted from 16°C for P cap in the spring, to greater than 28°C in the summer. Multiple regression analysis indicated a second‐order relationship of P cap in the spring to temperature. Other independent variables explained only negligible variation of P cap .