Can mussels control the plankton in rivers?—a planktological approach applying aLagrangian sampling strategy

We studied plankton dynamics using a Lagrangian sampling strategy in a 21‐km‐long reach of the river Spree (Germany), located downstream of a shallow lake. The lake was productive and had a high output of plankton into the river. The zooplankton community was dominated by rotifers whose densities reached 130–4,500 ind liter −1 in the lake outlet. The phytoplankton community was dominated by centric diatoms. Downstream, densities of all zooplanktonic taxa decreased exponentially with coefficients of determination of up to R 2 = 0.99. Relative abundances stayed almost unchanged in dominant species, and coefficients of variation did not exceed 15% despite a total reduction of individuals to 3‐30% within the river reach. Chlorophyll a and oxygen contents decreased exponentially, whereas Secchi depths and inorganic nutrient concentrations increased. Population growth rates in zooplankton and phytoplankton were always negative and positively correlated to flow velocity. Ratios of the total number of rotifer eggs to the total number of rotifer females were constant. In Keratella cochlearis and Synchaeta oblonga , the dominant rotifer species, birth rates remained high during downstream transport. Therefore, the negative growth rates in the river (≈−1.5 d −1 ) were caused by an abrupt increase in death rates to levels of about 1.9 d −1 at the lake‐river transition. This shift was induced by an increase in predation rate caused by the filtration activity of unionid bivalves that were abundant in the river reach.