Multiple agricultural stressors and stream benthic algae: the effects of fine sediment particle size and flow velocity reduction on community composition and biological traits

Abstract

There are many challenges facing freshwater systems that originate from urban development, climate change and agriculture. Agricultural intensification is one of the leading causes of freshwater biodiversity decline worldwide. Effective management of the ecological consequences of agricultural intensification requires understanding the effects of multiple stressors operating in agricultural streams. This is because interactions, especially synergisms and antagonisms, can lead to situations where management actions may not lead to a proportional recovery of impacted streams. Previous research has shown that fine sediment and water abstraction are pervasive stressors that can interact and strongly affect stream communities. However, the individual and interactive effects of different grain sizes of deposited fine sediment and abstraction-induced flow velocity reduction have yet to be investigated.

To study the effects and interactions of fine sediment particle size and flow velocity reduction, I performed an experiment in 60 outdoor stream mesocosms. This experiment addressed the above knowledge gap for benthic algal population, community and biological trait responses. I used four fine sediment treatments (no sediment added versus three different grain sizes: 0-0.125mm, 0.125-0.250mm, 1-2mm) combined with three flow velocities simulating increasing levels of water abstraction (fast, medium, slow). Biological response variables included four algal community-level metrics, absolute abundances of 15 common taxa (Chapter 2) and relative abundances of 25 algal trait categories (Chapter 3).

Algal community metrics and common taxa (Chapter 2) showed 16 significant responses to sediment addition, three significant responses to flow velocity reduction and 11 non-additive interactions between the two stressors (interaction frequency 58% of all community/taxon metrics). The majority of these responses to sediment addition and flow velocity reduction were positive. There were also some negative responses, namely the blue-green alga Phormidium spp. and the diatom Gomphonema parvulum. The three flow velocity reduction responses were all negative. Algal traits (Chapter 3) showed 14 significant responses to sediment, two responses to flow velocity and 12 interactions (48% of trait variables). Sediment addition caused seven positive responses and eight negative responses. The two flow reduction responses were both positive.

These findings allow several interesting conclusions. Firstly, added fine sediment with different particle sizes was a more pervasive stressor than flow velocity reduction. Secondly, fine sediment and flow velocity reduction interacted frequently to produce complex response patterns that could not be predicted based on the single-stressor effects involved. This point was illustrated by algal trait interactions that often overrode the main stressor effects, especially for flow velocity reduction. Thirdly, facilitation (an interaction that benefits at least one species and harms neither) was probably more prevalent with the addition of different-sized fine sediment rather than when flow velocity was reduced. My findings also highlight the considerable potential of using biological algal traits for detecting both simple and complex multiple-stressor effects. My main conclusion for freshwater managers is that fine sediment, regardless of its particle size, was a very pervasive stressor in my study, and that consequently efforts should prioritize the reduction of fine sediment inputs to the stream environment. Furthermore, care should be taken when implementing management strategies in situations where multiple stressors operate simultaneously because of the numerous interactions observed.