Patterns of spatial and temporal variation in phytoplankton chlorophyll a in the River Trent and its tributaries

The variation in phytoplankton chlorophyll a was measured regularly on the River Trent, UK, and its tributaries between April 1995 and July 1997. During the spring maximum the concentration of chlorophyll a increased with distance down the river, consistent with net in situ growth. The apparent rate of net increase in spring ranged from 0.004 to 0.044 km−1, which is calculated from water velocity estimates to be equivalent to a specific growth rate of between 0.04 and 0.57 day−1. During late summer, however, the concentration of chlorophyll a declined down river, the apparent rate of decline being between 0.001 and 0.032 km−1, equivalent to a specific growth rate of between −0.01 and −0.78 day−1. At the lower tidal limit, the seasonal chlorophyll a concentration ranged between an annual minimum of 1.4–1.7 μg l−1 in December and January and an annual maximum of 121–162 μg l−1 in May or June. The chlorophyll a maximum was short lived and the concentration decreased to 6–18 μg l−1 in August–October. The chlorophyll a concentration in four major tributaries, the Tame, Dove, Derwent and Devon, was low in comparison with the main river. In another tributary, the Soar, chlorophyll a concentrations were relatively high and reached an annual maximum of 74 μg l−1 in 1995 and 70 μg l−1 in 1997. Significant differences in chlorophyll a concentration across the main river were detected on one of the three occasions studied. The possible influence of physical, chemical and biological factors in controlling the concentration of phytoplankton chlorophyll a is discussed.