CAN MUSSELS CONTROL THE PLANKTON IN RIVERS - A PLANKTOLOGICAL APPROACH APPLYING A LAGRANGIAN 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 plank ton into the river. The zooplankton community was dominated by rotifer s whose densities reached 130-4,500 ind liter(-1) in the lake outlet. The phytoplankton community was dominated by centric diatoms. Downstre am, densities of all zooplanktonic taxa decreased exponentially with c oefficients of determination of up to R-2 = 0.99. Relative abundances stayed almost unchanged in dominant species, and coefficients of varia tion did not exceed 15% despite a total reduction of individuals to 3- 30% within the river reach. Chlorophyll a and oxygen contents decrease d exponentially, whereas Secchi depths and inorganic nutrient concentr ations increased. Population growth rates in zooplankton and phytoplan kton were always negative and positively correlated to flow velocity. Ratios of the total number of rotifer eggs to the total number of roti fer females were constant. In Keratella cochlearis and Synchaeta oblon ga, the dominant rotifer species, birth rates remained high during dow nstream transport. Therefore, the negative growth rates in the river ( approximate to-1.5 d(-1)) were caused by an abrupt increase in death r ates to levels of about 1.9 d(-1) at the lake-river transition. This s hift was induced by an increase in predation rate caused by the filtra tion activity of unionid bivalves that were abundant in the river reac h.