An index to track the ecological effects of drought development and recovery on riverine invertebrate communities
Introduction
Droughts have an important role in shaping lotic ecosystems (Extence, 1981, Humphries and Baldwin, 2003, Lake, 2011, Lu et al., 2016; Piniewski et al., in press). A potential increase in extreme events associated with our changing climate suggests the frequency of droughts is likely to increase in many areas of the globe (Dai, 2011, Prudhomme et al., 2014). Some recent studies indicate that the magnitude and frequency of short duration drought events (<18 months) will increase in the future in tandem with rises in flood frequency (Ledger and Milner, 2015, Watts and Anderson, 2013). While climate change is expected to intensify drought in many regions, its short and long-term ecological effects are poorly understood (Bogan et al., 2014).
Drought is a natural disturbance in rivers that influences community structure and functioning (Lu et al., 2016), altering species composition, abundance and richness (Atkinson et al., 2014) and favouring specialist species (Mainstone 1999). The impact of drought on ecological communities depends both on its duration and intensity, as well as antecedent conditions (Bogan et al., 2015, Chessman, 2015, Stubbington et al., 2014). Lake (2003) distinguishes between regular seasonal and predictable droughts, as in Mediterranean regions, from supra-seasonal droughts which are usually unpredictable in nature and are associated with longer periods of drying across multiple seasons. Supra-seasonal droughts normally include one or more seasons typically associated with higher river flows. The distinction between different types of drought is important since the biota within rivers which experience regular seasonal channel drying typically display adaptions to such conditions (Boulton, 2003, Bogan et al., 2015), whereas unpredictable supra-seasonal droughts have the potential to result in greater ecological effects due to their protracted nature (e.g. Wood and Armitage, 2004). In addition, the antecedent conditions and timing of supra-seasonal droughts are important controls on the community effects (Dewson et al., 2007, Lake, 2011). The effects of a drought on river macroinvertebrate communities will vary according to the river type, in particular whether it is a groundwater or surface water-dominated river, the pattern of drying and degree of physical modification (see Lake, 2011 for review). More physically diverse river reaches, including those with marginal habitats or with variable water depth and flow-velocity, provide habitat heterogeneity to support a wider range of taxa. This physical heterogeneity is widely considered to result in populations and communities which are more resilient to extreme hydrological events by the provision of refugia which facilitate rapid recovery following disturbances (Townsend and Hildrew, 1994, Dunbar et al., 2010a, Dunbar et al., 2010b, Chester and Robson, 2011).
Drought disturbances typically exhibit a ramp pattern with the magnitude of effects growing with increasing duration of the event. Conditions during a drought may fluctuate, however, with brief rainfall events providing occasional inputs of water, but the magnitude of the drought steadily increases over time (ramps up) and often affects progressively greater spatial scales (Lake, 2000, Parry et al., 2017). The response of the aquatic stages of lotic communities to drought is punctuated by significant step changes, as thresholds between critical water levels are crossed (Boulton, 2003, Boulton and Lake, 2008). The step-like nature of these changes, as thresholds are exceeded, result from the abrupt loss or fragmentation of habitats (e.g. riffle areas), alteration in physico-chemical conditions and the loss of lateral, longitudinal and/or vertical connectivity (Boulton, 2003, Boulton and Lake, 2008). The ability of biota to withstand a disturbance (resistance) and their subsequent capacity to re-colonise (resilience) reflect the availability of refugia in the channel and wider catchment (Lake, 2000). Species and communities which possess strategies to survive low-flows, lentic conditions and drying, or are highly mobile, may be able to recolonize and recover rapidly after the cessation of drought conditions. The time taken to re-colonise, however, is typically taxon-specific and reflects the timing, intensity, and duration of individual drought events (Boulton, 2003, Boulton and Lake, 2008).
There is a need to understand the ecological effects of high magnitude supra-seasonal drought events in order to anticipate the effects of climate change and help to balance the need for anthropogenic water supply, whilst maintaining the ecological integrity of river habitats (Wilby et al., 2010). There is also a growing recognition for the need for more robust and defensible data to address multiple issues related to water resources and environmental legislation, such as management of protected species and habitats and maintenance of ecological standards for healthy ecosystems enshrined in the European Community Habitats Directive and Water Framework Directive (WFD) (Acreman and Ferguson, 2010). To make use of these data we need tools and techniques to ascertain the influence of different environmental pressures. The need to assess the ecological effects of variations in river flow led to the development of the macroinvertebrate index: Lotic-invertebrate Index for Flow Evaluation (LIFE; Extence et al., 1999). LIFE uses recognized flow associations to weight invertebrate groups according to their preference for flow velocity. Existing biological indices, such as LIFE in the UK, and others developed in Canada (Armanini et al., 2012), Australia (Rose et al., 2008) and New Zealand (Caruso, 2002), have been correlated with historic hydrological conditions and hydraulic parameters (Extence et al., 1999, Monk et al., 2008, Dunbar et al., 2010a, Dunbar et al., 2010b) with some degree of success. The relationship between the LIFE index and flow volume (discharge) breaks down, however, under extreme low flow conditions (Monk et al., 2006) possibly reflecting the ramp disturbance and threshold-crossing nature of drought pressures.
To address this deficiency, this study aimed to develop a new macroinvertebrate community-based metric, the Drought Effect of Habitat Loss on Invertebrates (DEHLI) index. This paper aims to outline the process of DEHLI calculation and to test its utility by using data from two case studies (involving monthly and annual sampling, respectively) and by undertaking a modelling exercise to test the drought response of both the DEHLI and LIFE indices to a hypothetical multi-year drought, calibrated to actual data from 114 samples. The index is based upon the concept outlined in Boulton and Lake (2008) linking the steps of the ramp disturbance with the sequential loss of aquatic invertebrates to changing abiotic and biotic conditions. It has initially been designed to be derived using data from the Environment Agencies of the United Kingdom, but could be readily adapted for use in any country or global region.
Section snippets
Index structure
The primary theoretical element of the ramp disturbance model of drought (sensu Lake, 2000) is the sequence of changes in hydrological connectivity and wetted habitat (Boulton, 2003, Boulton and Lake, 2008) as the drought progresses (see Fig. 1). The gradual intensifying of drought conditions will initially lead to a reduction in river flow (volume, depth and in some instances, velocity) severing lateral connectivity to marginal riparian habitat (2) and subsequently longitudinal connectivity
River Tham (Glen Brook), Park House Farm, Lincolnshire
The results indicate that DEHLI scores responded to drought and its development in a more sensitive manner than the LIFE index (Fig. 4 and Table 3). From May 2008 and July 2008 there was a marked reduction in surface water flow to the point where much of the river bed was dry and exposed (Fig. 4B and C). LIFE scores, however, did not appear to respond to this, with scores remaining relatively stable until October 2008. In contrast, DEHLI tracked the severity of low flow conditions, with scores
Discussion
The principle which underpins DEHLI is that the effect of drought on aquatic invertebrates follows a ramp disturbance and that this gradual change, punctuated by abrupt losses of habitat and concomitant loss of taxa, is a valuable model to help guide the management of freshwater ecosystems and the resources they support (Boulton, 2003, Boulton and Lake, 2008). It is widely documented that aquatic communities recover, even after the loss of surface water due to drought. Furthermore, it should be
Acknowledgements
The authors would like to thank all of the Environment Agency staff involved in collecting and analysing data from the Tham site and Chilterns sites. We are also grateful for the continuing help and support of our colleagues in commenting on drafts and ideas and assisting with data analysis. For the EA, this includes Darren Finnie, Emma Holden, Holly Longstaff, Alex Pickwell, Sian Ratcliffe, Jake Reeds and Nina Birkby. We also thank Andrew Boulton for helpful comments on the draft and Andrew
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