Relationship between Large Scale Environmental Flow Violations and Freshwater Biodiversity

Globally, freshwater ecosystems are in decline due to human actions, making maintenance of environmental flow (EF) in river networks critically important from local to global scales. Yet, our knowledge of the relationship between EF requirement and ecosystem response at different spatial and temporal scales is quite limited. It is clear that there is a vast difference between the scales at which these processes are understood and the scales at which policies are set. Therefore, this study intends to evaluate the relationship between EF violation and freshwater biodiversity at two aggregated spatial scales; viz. 1) globally-aggregated scales and 2) freshwater ecoregions, and test the prevailing assumption of scalability of this relationship. Four EF violation indices (severity, frequency, probability to shift to violated state, and probability to stay violated) and nine independent freshwater biodiversity indicators (calculated from observed biota data except one empirically derived from streamflow deviation) were used to test the EF-biodiversity relationship. In order to reduce the uncertainty in EF violation estimation, an ensemble of four Global Hydrological Models, four Global Circulation Models and five EF calculation methods were used in this study. The results from this study indicate no statistically significant negative relationship between EF violation and freshwater biodiversity at global or ecoregion level (except for streamflow derived bio indicator). While our results thus suggest that just the quantity of water in the streams may not be a main determinant of freshwater biodiversity at large spatial scale, they do not preclude the existence of relationships with more holistic EF methods (e.g. including water temperature, water quality, intermittency, connectivity etc.) or with other biodiversity data or metrics Keywords: Environmental flow violation, freshwater biodiversity, global scale, freshwater ecoregion