Growth and losses of phytoplankton studied with a new dialysis chamber technique along the river Spree

Abstract

The development of the main phytoplankton taxa was studied in a water parcel flowing along a 28-km stretch of the lower Spree as well as in simultaneously incubated dialysis chambers. The chambers were continuously moved through the water column in order to provide the enclosed algae with light of the same mean intensity and of comparable frequency of fluctuations as experienced by the phytoplankton transported downstream. The difference between the rate of population increase along the river and the growth rate in dialysis chambers equals the sum of all losses prevented inside the chamber but occurring in the river. Centric diatoms dominated phytoplankton biomass for the growing season. They grew in their water parcel during downstream flow in spring (+0.13 ± 0.25 d -1 ) but declined during summer (-0.79 ± 0.44 d -1 , each average ± standard deviation). Green algae showed the highest rates of decline in spring but maintained their abundance most successfully along the river in summer. The other algal groups declined throughout the season with a tendency to stronger reductions in summer. The growth rate (= rate of cell replication of caged phytoplankton minus losses inside the chambers) of centric diatoms was higher in spring (0.57 ± 0.14d -1 ) than during summer (0.25 ± 0.13 d -1 ). All the other algal groups grew faster in dialysis chambers in summer than in spring. The mean calculated loss rate of total phytoplankton increased from 0.44 ± 0.25 d -1 in spring to 1.05 ± 0.32 d -1 during summer. Only the chlorophytes maintained losses at a low level (0.41 ± 0.24d -1 ) even in summer. Different intensity of sedimentation and grazing of benthic filter-feeders may explain longitudinal development of phytoplankton as well as seasonal shifts between diatoms and chlorophytes.