Aquatic macroinvertebrate diversity in an intermittent river network: implications for freshwater biodiversity conservation

Research Context
Aquatic ecosystems, such as rivers, streams and ponds are characterised by varying surface flow conditions. Perennial rivers flow continuously throughout the year, while intermittent rivers experience a predictable or unpredictable loss of surface water (Stubbington et al., 2017). Typically, intermittent rivers/ponds dry in summer months due to low precipitation levels and/or permeable geologies. At a global scale, intermittent rivers cover >50% of river networks and are common across all continents (Skoulikidis et al., 2017). Intermittent rivers and ponds are expected to increase globally due to climatic drying (i.e. less precipitation and increased temperatures; Döll and Schmidt, 2012; Garcia et al., 2017), and increased water abstraction and impoundments. These predicted flow reductions may cause previously perennial rivers to experience intermittent flow (Datry et al., 2014), and perennial ponds to dry.

Effect of flow intermittency on hydrological connectivity and aquatic macroinvertebrate diversity
During flowing phases, rivers have high hydrological connectivity to floodplain waterbodies, including ponds, meander cut-offs, paleochannels and backwater channels. Frequent floodplain inundations cause mixing of water, sediment, nutrients and organisms, which promotes high macroinvertebrate diversity in floodplain waterbodies. These aquatic ecosystems often support high alpha (site), beta (compositional variation between sites) and gamma (regional) diversity (Gergel, 2002). However, due to flow cessation in intermittent rivers during the summer months, many rivers become hydrologically disconnected from their floodplain. Meander cut-offs and paleochannels typically disconnect from the main river and often transition into a series of isolated floodplain ponds that are characterised by a gradient of flow permanence (Hill et al., 2016).

In rivers, flow cessation causes habitat fragmentation and drying, and changes in physico-chemical conditions (Boulton and Lake, 2008). Once flow has ceased, the intermittent section of a river is often characterised by disconnected instream ponds. During flowing phases, macroinvertebrate diversity is typically lower at intermittent compared to perennial sites (Datry et al., 2014). However, these instream ponds may have very high abundances and taxonomic richness of macroinvertebrates. Previous studies have failed to include macroinvertebrate diversity from instream ponds that form during stream drying (Stubbington et al., 2017). Therefore, the aquatic taxonomic richness of intermittent reaches may have been underestimated historically. No studies to our knowledge have examined the contribution, at a landscape-scale, of instream ponds, floodplain ponds and backwater channels to aquatic macroinvertebrate biodiversity, and the changes in aquatic connectivity among multiple freshwater habitats in an intermittent river network. As intermittent freshwaters (i.e. rivers and ponds) are predicted to increase across the globe, information regarding their ecological diversity and functioning is important to the long-term conservation and management of freshwater biodiversity (Hill and Milner, 2018).

This PhD proposal will identify several landscape-scale conservation and management recommendations to ensure the greatest biodiversity possible is supported in an intermittent river network (the intermittent river and the surrounding floodplain waterbodies), focussing on the identification of important flow refugia, the maintenance of connectivity in an intermittent landscape, and habitat complementation (maximising opportunities conservation in a given landscape).

Aims and Objectives
The aim of this PhD research project is to examine the effects of flow cessation on macroinvertebrate diversity and community composition in an intermittent river network. This overall aim is divided into four key areas: biodiversity, refugia, environmental drivers of community composition and the conservation value of freshwater bodies. Each key area is associated with several objectives, which are:
1. Identify the macroinvertebrate diversity and community composition of perennial and intermittent river sites, instream ponds, floodplain ponds and backwater channels.
2. Quantify the contribution of i) instream ponds to aquatic macroinvertebrate diversity within intermittent rivers, and ii) perennial, intermittent river sites, instream ponds, floodplain ponds and backwaters to aquatic macroinvertebrate diversity at a landscape-scale (gamma diversity).
3. Characterise the heterogeneity of macroinvertebrate communities between instream ponds, floodplain ponds and backwater channels.
4. Determine whether instream ponds, floodplain ponds, backwater channels, and leaf packs provide a refuge for aquatic macroinvertebrates during streambed drying.
5. Examine whether spatial proximity (neutral processes) or local environmental variables (niche processes) dominate macroinvertebrate community composition among instream ponds, floodplain ponds and backwater channels.
6. Identify the importance of instream ponds, floodplain ponds and backwater channels to landscape-scale biodiversity conservation and management.