Determining Tipping Points and Responses of Macroinvertebrate Traits to Abiotic Factors in Support of River Management

Abstract

Simple Summary Questions on how features of aquatic invertebrates (e.g., size, feeding style, respiration mode) evolve along a gradient of flow velocity, turbidity and elevation are rarely addressed. Furthermore, another outstanding question is what is the environmental condition in which an abrupt change occurs in terms of the compositions and abundance of these organisms' features. To answer the previous questions, we collected macroinvertebrates and measured the flow velocity, turbidity and elevation in the streams of the Guayas basin (Ecuador); subsequently, we analyzed the data. We observed that the abundance of most organisms' features increased with increasing flow velocity, while they decreased with increasing turbidity. Furthermore, there was a substantial drop in the abundance of tegument-respiring invertebrates beyond the turbidity of 5 NTU and an abrupt decline in the diversity of the organisms' features below 22 m above sea level, implying the need to focus water management in these altitudinal regions. Our results suggest that there is a need to implement measures that minimize erosion to alleviate turbidity that affects these organisms. The findings of our study provide a basis to determine critical targets for aquatic ecosystem management. Although the trait concept is increasingly used in research, quantitative relations that can support in determining ecological tipping points and serve as a basis for environmental standards are lacking. This study determines changes in trait abundance along a gradient of flow velocity, turbidity and elevation, and develops trait-response curves, which facilitate the identification of ecological tipping points. Aquatic macroinvertebrates and abiotic conditions were determined at 88 different locations in the streams of the Guayas basin. After trait information collection, a set of trait diversity metrics were calculated. Negative binomial regression and linear regression were applied to relate the abundance of each trait and trait diversity metrics, respectively, to flow velocity, turbidity and elevation. Tipping points for each environmental variable in relation to traits were identified using the segmented regression method. The abundance of most traits increased with increasing velocity, while they decreased with increasing turbidity. The negative binomial regression models revealed that from a flow velocity higher than 0.5 m/s, a substantial increase in abundance occurs for several traits, and this is even more substantially noticed at values higher than 1 m/s. Furthermore, significant tipping points were also identified for elevation, wherein an abrupt decline in trait richness was observed below 22 m a.s.l., implying the need to focus water management in these altitudinal regions. Turbidity is potentially caused by erosion; thus, measures that can reduce or limit erosion within the basin should be implemented. Our findings suggest that measures mitigating the issues related to turbidity and flow velocity may lead to better aquatic ecosystem functioning. This quantitative information related to flow velocity might serve as a good basis to determine ecological flow requirements and illustrates the major impacts that hydropower dams can have in fast-running river systems. These quantitative relations between invertebrate traits and environmental conditions, as well as related tipping points, provide a basis to determine critical targets for aquatic ecosystem management, achieve improved ecosystem functioning and warrant trait diversity.