Big impacts from small abstractions: The effects of surface water abstraction on freshwater fish assemblages

Abstract

Abstractions and diversions are prevalent in river networks worldwide; however, specific mechanisms and measures reflecting changes in functional characteristics of aquatic assemblages in response to flow abstraction have not been well established. In particular, the influence of small takes on fish assemblages is poorly understood. Field surveys and stable‐isotope analyses were used to evaluate the impact of differing levels of flow abstraction on fish assemblage structure, and native–non‐native patterns of coexistence, associated with small surface water abstractions in four streams in New Zealand. Study design accounted for longitudinal processes (spatial autocorrelation) to isolate the effects of abstractions on fish assemblages. Reaches with reduced flows downstream of abstraction points had significantly lower fish abundances per metre of stream length, probably because of decreased habitat size, altered interspecific interactions and barriers to movement. The loss of larger fish in reaches with high abstraction resulted in shallower mass–abundance slopes and shorter stable isotope‐derived food‐chain lengths, likely to have been caused by fewer trophic links in the food web. The large fish absent from these reaches were flow‐sensitive introduced salmonids, resulting in higher relative abundances of small‐bodied native fish, probably as a result of predatory and competitive release. Quantification of metrics designed to characterize ecosystem functioning as well as abundance and species composition indicated that small water abstractions can alter both the structure and composition of stream fish assemblages and modify the outcomes of native–non‐native species interactions. Thus, a better understanding of the effects of small abstractions could be used to improve the strategic management of fish in invaded riverscapes.

Could spatial heterogeneity in flow disturbance drive temporal stability of native–invasive species co‐occurrence in riverscapes?

“…Both groups of fish occupy streams spanning the entire range of flood‐driven disturbance we examined, but peak trout abundance generally occurs in more stable streams than galaxiid peak abundance (Jellyman & McIntosh, 2020). Moreover, the preference of large predatory salmonids for more hydrologically stable stream branches, limits the abundance of small galaxiids there, meaning galaxiids are often more abundant in flood‐prone branches (Boddy et al., 2020).…”

Food‐web energy fluxes, energy transfer efficiency, and diversity respond distinctively to pollution and water diversion in rivers

“…structure have been previously documented (e.g. Boddy et al, 2020;Mor et al, 2019). However, there are few studies assessing the combined effects of these two stressors on the organisation of food webs and on ecosystem functioning.…”

“…Water diversion and pollution may also modify organismal body size, a predictor of the position species occupy in the food web (Woodward et al., 2005). Some mechanisms responsible for changes in body size include increases of pollution tolerant taxa (Peralta‐Maraver et al., 2019) or decreases in abundance of large organisms due to water abstraction (Boddy et al., 2020). These changes can modify the slope of the relationship between abundance and body mass (i.e., size spectrum), which can be interpreted as a change in the efficiency of energy transfer along food webs (Woodward et al., 2005).…”

Water diversion and pollution interactively shape freshwater food webs through bottom‐up mechanisms

“…There are a number of studies on the isolated effects of water diversion and water pollution on food web structure (e.g. Boddy et al, 2020;Walters &Post, 2008 andGarcía et al, 2017;Morrissey et al, 2013;Price et al, 2019). There are even a few studies that have addressed the joint effects of nutrient pollution and water diversion (e.g.…”