An automated modular heating solution for experimental flow-through stream mesocosm systems

Water temperature is a key environmental variable in stream ecosystems determining species distribution ranges, community composition, and ecological processes. In addition to global warming, direct anthropogenic impacts, for example through the influx of power plant cooling water or due to sun exposure after the removal of riparian vegetation, result in elevated water temperatures. However, temperature effects in stream ecosystems have mostly been tested in recirculating experimental systems, which can neither capture diurnal and seasonal variability in other environmental variables nor allow for entrainment of stream organisms. In contrast, open flow-through systems, which are constantly supplied with stream water, offer a more realistic setting for stream ecological experiments, yet are difficult to implement. Here, we outline a heating module for the purpose of differential temperature regulation in a flow-through mesocosm system by automatic control of warm water supply. We validated the functionality of the module in indoor trials as well as in an outdoor ExStream experimental mesocosm system. Furthermore, we tested the implications of different warm water temperatures for the survival of invertebrates drifting through the heating module to derive recommendations for the maximum warm water temperature for mixing with the natural water inflow. The module allows for controlled open flow-through experiments in the field and the key components are flexible and scalable. Therefore, the module can be easily integrated into existing experimental flow-through setups.

Automated summary

Water temperature is a crucial factor in stream ecosystems, impacting species distribution, community composition, and ecological processes. Besides global warming, direct human impacts, like power plant cooling water and removal of riparian vegetation exposing the stream to the sun, also contribute to elevated water temperatures. However, most temperature studies in stream ecosystems have only been conducted in recirculating experimental systems, which fail to capture diurnal and seasonal variations in other environmental factors and do not allow for the entrainment of stream organisms. This research presents a heating module that allows for differential temperature regulation in a flow-through mesocosm system, providing a more realistic environment for stream ecological experiments.