Abstract
Assessing the status and distribution of fish populations in rivers is essential for management and conservation efforts in these dynamic habitats and currently, environmental DNA (eDNA) is established as an alternative and/or complementary approach to the traditional monitoring of fish species. In lotic systems, a sound understanding of hydrological conditions and their influence on the local target DNA detection probability and quantity is key for the interpretation of eDNA-based results. However, the effect of seasonal and diurnal changes in discharge and the comparability of semi-quantitative results between species remain hardly addressed. We conducted a cage experiment with four fish species in a glacier-fed, fish-free river in Tyrol (Austria) during summer, fall, and winter discharge (i.e. 25-fold increase from winter to summer). Per season, water samples were obtained on three consecutive days at 13 locations downstream of the cages including lateral sampling at 1-2 m distance across the wetted width. Fish eDNA was quantified by species-specific endpoint PCR followed by capillary electrophoresis. Close to the cages, lateral eDNA distribution was heterogenous, mirrored cage placement within the stream, and showed the diluting effect of increased discharge. Additionally, the eDNA signals were significantly lower for fish species of larger individual size at comparable per-species biomass. For further downstream distances with laterally homogenous eDNA distribution, the signals decreased significantly with increasing distance independent of longitudinal discharge changes. This study exemplifies the importance of prevailing hydrological conditions for the interpretation of eDNA-based data across seasons. To control for heterogenous eDNA distribution and enable comparisons over time, sampling schemes in lotic habitats need to incorporate hydrological conditions and species biology.
Competing Interest Statement
MT is the co-founder of Sinsoma GmbH, a for profit company dedicated to DNA analyses in environmental studies; DK is employed by Sinsoma. CM and RS are co-founders of the ARGE Limnologie GesmbH, a for profit consultancy specialized in aquatic ecology.