Comparing planktonic heliozoan abundance in deep and shallow basins of Lake Biwa: exploring optimal growth temperature, food resources and habitat use

Abstract

Abstract Heliozoans are ambush protists that inhabit most limnetic systems. However, ecological information about planktonic heliozoans is still limited. In this study, we investigated seasonal changes in planktonic heliozoan abundance in the deep northern and shallow southern basins of Lake Biwa at pelagic stations between August 2020 and December 2021. We compared planktonic heliozoan abundance levels between the two basins and examined the relationship between heliozoan abundance and environmental variables. In both basins, high levels of planktonic heliozoan abundance were detected in autumn, winter and spring. In the south basin, there was a significant positive correlation between planktonic heliozoan abundance and chlorophyll a concentration as well as the abundance of the diatoms Aulacoseira granulata and Aulacoseira ambigua. These results suggest that planktonic heliozoans directly or indirectly consume diatoms and that heliozoan abundance is subjected to bottom-up control. Furthermore, planktonic heliozoan abundance in the south basin was higher than that in the north basin, possibly due to the presence of waterweeds, which can serve as habitat for heliozoans, and the redistribution of planktonic heliozoans through resuspension from lake bottom sediments.