SPECIFIC GROWTH-RATES OF HETEROTROPHIC PLANKTON ORGANISMS IN A EUTROPHIC LAKE DURING A SPRING BLOOM

The in situ growth of the dominating pelagic organisms at several trophic levels was investigated during a spring bloom characterized by well-mixed cold water. The study includes primary production and the carbon flow through the nano-, micro- and mesozooplankton populations based on population dynamics and specific growth rates. The phytoplankton biomass and production were totally dominated by small algae <20 mu m, of which similar to 5% were <3 mu m, potentially a food source for the nano- and microzooplankton. The mean carbon-specific primary production was 0.15 day(-1) and was regulated solely by light. The mean volume-based specific growth rate of bacterioplankton was modest, 0.1 day(-1), and probably controlled by the low temperature. The volume-based specific growth rates of heterotrophic nanoflagellates, ciliates, rotifers and copepods were 0.35, 0.13, 0.16 and 0.03 day(-1), respectively. The observed growth of the heterotrophic plankton was generally not food limited, but was controlled by temperature. The stable temperature during the experiment therefore allows a cross-taxonomic comparison of specific growth rates. The b exponent in the allometric relationship (G = aV(b)) between volume-specific growth rate (G) and individual body size (V) was -0.15 +/- 0.03 for all filtrating zooplankton, indicating an in situ scaling not far from the physiological principles originally demonstrated for laboratory populations.