Seasonal and spatial patterns in phytoplankton photosynthetic parameters in a tidal freshwater river

The photosynthetic response to irradiance was quantified for phytoplankton from the tidal freshwater Potomac River biweekly to monthly over a period of six years. Samples were collected from two shallow embayments and portions of the deeper river mainstem. Photosynthetic rate was measured in the laboratory at in situ temperature over a range of irradiance levels and photosynthetic parameters were calculated using nonlinear regression. p(max)(B), the maximum photosynthetic rate standardized to chlorophyll a, increased with temperature up to 25 degrees C with a Q(10) of 2.02. Above 25 degrees C, p(max)(B) essentially constant with temperature. Lesser correlation between p(max)(B) and ambient irradiance could be explained by the correlation of irradiance with temperature. alpha, the slope of the P-I curve at low light, was correlated with both ambient irradiance and temperature. Highest alpha values were found in late summer when high temperature and intermediate ambient irradiance were observed. Spring and early summer were characterized by low alpha. Despite low light penetration, I-k and alpha values were indicative of sun limitation possibly due to intermittent high light levels experienced during mixing. I-k showed a clear seasonal trend directly related to days from summer solstice. Spatial patterns were minimal except that I-k was consistently lower in one shallow embayment than in the other two areas. Seasonal patterns in photosynthetic parameters corresponded roughly to changes from a spring diatom population to summer cyanobacterial assemblage.