The survival and behavioural responses of a near-shore chironomid and oligochaete to declining water levels and sandy substratum drying

Organisms inhabiting shallow near-shore waters experience emersion during water-level fluctuations, which are increasing in frequency and magnitude due to climate change and anthropogenic activities. We experimentally investigated the survival and behaviour of two common shallow-water and psammophilous invertebrates, the chironomid Stictochironomus sticticus and the oligochaete Potamothrix moldaviensis, during sandy substratum drying. We examined horizontal migrations following a decreasing water level in inclined tanks and vertical migrations into substratum. In drying tanks, the substrate was initially submerged and then water was gradually evaporated to expose the surface sediments. In control tanks, the substratum remained submerged. We hypothesised that the organisms would tolerate drying (surviving at least while the substrate remained moist) and/or migrate vertically and/or horizontally under drying conditions. Chironomids were tolerant of drying, with 90% mortality occurring at the sediment water content (LWC90) of 0.3% (dry sand). They exhibited neither horizontal nor vertical migrations in response to drying. Oligochaetes were far less resistant to drying (LWC90 = 13.9%, corresponding to humid sand) and did not exhibit any behavioural adaptations. Thus, our hypothesis was supported with regard to the resistance to drying of chironomids, but not that of oligochaetes. Contrary to our hypothesis, neither species exhibited behavioural adaptations to drying.