Recruitment of Heliozoa, rhizopods and rotifers from the sediments of an extremely acidic lake during spring and early summer
The goal of this study was to investigate the recruitment of zooplankton from the littoral sediment of Lake 111, an acidic lake in north-east Germany, in April (spring) and June (early summer), and its role in coupling the benthos and the pelagic. Maximum heliozoan and rhizopod recruitment occurred in early summer from sediment cores incubated at ambient water temperatures (20°C). Conversely, recruitment of the rotifer Cephalodella sp. was highest in spring at ambient spring temperatures of 12°C. A combination of passive and active recruitment processes is likely responsible. The seasonal abiotic and biotic sediment characteristics were relatively constant and therefore not likely responsible for the observed temporal recruitment pattern. The sediment water and carbon content ranged from 20 to 50% (mean = 29 ± 6% standard deviation) and 2–12% (mean = 5 ± 2% standard deviation), respectively. Similarly, there was little variation in the chlorophyll-a (mean = 0.2 ± 0.2 µg Chl-a g−1 dry weight ≡ 6.1 ± 3.9 mg Chl-a m−2). The in situ sediment bacterial density (0.82 × 109 ± 0.26 × 109 g−1 dry weight ≡ 1.01 × 109 ± 0.34 × 109 cells cm−3) was high. In contrast, the abundance of zoobenthos and their resting stages was low (< 25 individuals cm−3, and mean of 90 ± 75 cysts cm−3, respectively), with no temporal pattern being observed. Temperature was the only abiotic factor influencing recruitment.
This study suggests that, even in relatively young, chemically extreme lakes, the benthos can play an important role in whole lake microbial processes and zooplankton community composition. Such benthic repositories of resting stages potentially provide protection against adverse environmental changes.