Wetlands occupying river floodplains play a significant role in nutrient cycling and the storage of sediment and water in the landscape. They also support substantial primary production and high biodiversity, which is sustained by intermittent inundation (Junk et al., 1989; Tockner et al., 2000). Following inundation of dry floodplain wetlands nutrients are released from sediments stimulating primary and secondary production (Lake et al., 2006; McInerney et al., 2017). Invertebrates occupy an important role in floodplain wetlands linking aquatic and terrestrial carbon sources and nutrients to higher trophic levels such as fish and waterbirds (Boon & Shiel, 1990; Lindholm & Hessen, 2007). Following the inundation of a wetland invertebrate density and biomass can increase through emergence from egg banks, aerial colonisation and recruitment through breeding (Jenkins & Boulton, 2003; Siziba et al., 2013). High productivity and invertebrate abundance can be achieved within weeks following inundation (Lindholm & Hessen, 2007; Growns et al., 2020a). The link between inundation and increased productivity exists for both microinvertebrates (defined here as animals belonging to the Orders Cladocera and Ostracoda, Subclass Copepoda and Phylum Rotifera) and macroinvertebrates (all other invertebrate aquatic taxa) (Angeler et al., 2000; Baber et al., 2004; James et al., 2008; Chessman & Hardwick, 2014; Balkic et al., 2018).
Biotic and environmental factors influence the succession of different microinvertebrate groups following inundation (Dias et al., 2016). In relatively undisturbed wetlands rotifers generally increase in abundance immediately following inundation but decline in response to increases in the abundance of cladocerans (Burns & Gilbert, 1986; Baranyi et al., 2002). Rotifer declines maybe due to either competition for food resources or direct physical interference of the cladocerans (Gilbert & Slemberger, 1985; Gilbert, 1988; Dias et al., 2016). Lindholm and Hessen (2007) noted a temporal succession of three species of cladocerans following flooding but then rapid declines in each species and suggested overgrazing of phytoplankton was the cause. However, in disturbed wetlands following increases in turbidity or turbulence rotifers can become dominant (Gabaldon et al., 2017; Zhou et al., 2018). A generalised temporal succession does not exist for macroinvertebrate (Batzer, 2013). However, successions have been demonstrated (Moorhead et al., 1998; Jeffries, 2011). It has been suggested that the colonisation traits of different groups is likely to influence the sequence of arriving taxa. For example, diapausing groups such as certain molluscs can colonise quickly while groups that rely on aerial colonisation, such as Odonata, may establish later (Datry et al., 2017; Pires et al., 2019).
Increasing demand for freshwater during the last century has severely affected wetland ecosystems to the point that they represent some of the most seriously degraded environments in the world (Lemly et al., 2000; Vorosmarty et al., 2010; Davidson et al., 2018). Most of the world’s river systems and associated wetlands are now subject to human impacts (Tonkin et al., 2019). Since the mid 20th century, rivers and their wetlands have experienced significant pressure due to changes in hydrology, habitat degradation, and loss of biodiversity (Prieditis, 1999; Masing et al., 2000; Tourenq et al., 2001). The degradation of wetlands requires increasingly sophisticated approaches to the management of rivers that feed these ecosystems (Thompson et al., 2019). An important tool which has emerged has been the delivery and management of environmental flows to restore, protect or enhance environmental outcomes both in-channel and in associated floodplains (Arthington et al., 2018; Whipple & Viers, 2019). While there has been considerable progress in the science underpinning environmental flows there remain substantial gaps in our understanding (Poff & Zimmerman, 2010; Davies et al., 2014). The sharing of limited water resources between anthropogenic users and biodiversity conservation remains a contentious issue (Graham, 2009). It is thus important that the benefits of increased allocation of water to wetland flooding is further documented, so that the benefits can be considered against the potentially associated socioeconomic costs (Chessman & Hardwick, 2014).
In this study we examined the effects of environmental flows on water chemistry and three groups of invertebrates, including benthic and pelagic invertebrates and macroinvertebrates, in wetlands on the Gwydir River system in the north of the Murray-Darling Basin. We hypothesised that wetlands that were inundated for longer periods of time would have altered water chemistry and support a greater richness and abundance of invertebrates, thus altering their assemblage structures. In addition, we also hypothesised that invertebrates would follow clear temporal successional pathways following inundation.