Managed floodplain inundation maintains ecological function in lowland rivers
Graphical abstract
Introduction
Flow modification is one of the major threats to freshwater biodiversity (Dudgeon et al., 2006). Impoundments in rivers form physical barriers that capture water during high flows and reduce peaks, ultimately generating steady patterns in discharge throughout the hydrograph. Modified flows reduce lateral and longitudinal connectivity and can even lead to reversal in timing of high flow events (high summer irrigation flows replacing low natural summer flows) (Bunn and Arthington, 2002). However, parts of the hydrograph can be reinstated by adding managed flow events on top of naturally occurring events, thereby extending the flood magnitude and duration of floodplain inundation.
Floods mobilise large amounts of terrestrially-derived material on floodplains and transport it to the river channel (Junk et al., 1989; Tockner et al., 1999; Giling et al., 2015; Nielsen et al., 2016). Global estimates suggest that terrestrial sources contribute up to 1.9 × 1012 Tonnes C y−1 to inland waters and of this, at least 40% supports active transformation processes within aquatic systems (Cole et al., 2007). Studies carried out on rivers when flows are constrained within the main channel demonstrate that phytoplankton are the dominant basal resource in lowland rivers (Thorp and Delong, 2002; Oliver and Merrick, 2006; Hadwen et al., 2010; Roach, 2013; Brett et al., 2017). The principal role of algae in food webs has been supported by studies that showed algae can be the primary source of carbon in riverine food webs, even when floodplain inundation occurs (Lewis et al., 2001). However, emerging evidence suggests carbon and nutrients derived from floodplains can be important for fuelling aquatic food webs (Reid et al., 2008; Zeug and Winemiller, 2008b; Hladyz et al., 2012; McInerney et al., 2017). Riverine biofilms have been shown to incorporate dissolved organic carbon (DOC) derived from floodplains, with δ13C values depleted by between 4 and 7‰ following a flood pulse, indicating a shift from algal to terrestrial carbon assimilation (Hladyz et al., 2011a; Cook et al., 2015). Evidence of terrestrial carbon subsidies to aquatic food webs has also been demonstrated by increased production of zooplankton (Mitrovic et al., 2014). Floodplain inundation can stimulate the exchange of nutrients, primary (algae) (Hamilton and Lewis, 1987; Hein et al., 1999; Tockner et al., 1999; Nielsen et al., 2016) and secondary (zooplankton and macroinvertebrates) production (McInerney et al., 2017), and it is recognised that that highest secondary production often occurs during (or after) periods of connectivity between the river channel and floodplain (Saunders and Lewis, 1988; Saunders and Lewis, 1989; Ning et al., 2013; Furst et al., 2014; Nielsen et al., 2016).
Given the evidence that has emerged on the potential value of floodwaters to rivers, water managers are now using managed environmental flows (eFlows) as a mechanism to manage and derive ecological benefits to river channels (Arthington et al., 2006; Poff et al., 2010). Despite the increasing evidence of terrestrial carbon subsidies to food webs, there are still relatively few studies and empirical data that have examined riverine responses to floodplain manipulation events. Thus, our knowledge regarding the outcomes of such watering, the extent to which carbon can be mobilised and its contribution to riverine food webs remains contradictory (Zeug and Winemiller, 2008a; Baldwin et al., 2016).
The Millennium Drought in south-eastern Australia (2001 to 2009) significantly reduced rainfall and river flows across south-eastern Australia (van Dijk et al., 2013). During this period Barmah-Millewa Forest (BMF) received no significant flooding. In 2010, the drought broke and major flooding of BMF occurred, with the initial flood pulse mobilising significant quantities of carbon (Nielsen et al., 2016), resulting in a widespread hypoxic blackwater event (Whitworth et al., 2013) and adverse impacts on native fish and macrocrustaceans (King et al., 2012). Throughout 2011–2012, several natural flood events occurred initiating waterbird breeding events. To sustain water on the floodplain to allow these breeding events to go to completion, environmental water was used to augment these floods and floodwaters persisted on the floodplain (GBCMA, 2013).
Given the uncertainties surrounding the value of terrestrial carbon subsidies to instream food webs, the purpose of our work was to examine the extent that carbon, nutrients and zooplankton could be derived from a natural flood and environmental watering. We also examined how riverine biofilms and macroinvertebrates associated with biofilms and the littoral zone responded to floodplain inundation events. We hypothesised that large amounts of terrestrial carbon, nutrients and zooplankton would be exported from the floodplain to the river channel and that microbes within the biofilms would consume terrestrial carbon. We also predicted that consumers associated with biofilms would show a similar response, and their stable isotope carbon ratios would reflect those of the terrestrial carbon.
Section snippets
Sites and sampling design
Barmah-Millewa Forest is a large floodplain forest situated on the Murray River in south-eastern Australia, which floods via natural events and managed environmental water allocations, with floodwaters draining back into the river channel downstream of the forest (Fig. 1). A detailed description of the forest has been provided previously (Cook et al., 2015). Importantly, the combined capacity of the Murry and Edward River channels within the BMF is ~11,000 ML day−1 (as measured at Tocumwal).
Carbon sources
Concentration of DOC varied at different times and sites (PERMANOVA P<0.05) (Table 1, Fig. 2a). Pairwise comparisons indicate that, in general, the DOC concentrations in the water column at Barmah and 4 Post Reserve were greater than at our upstream site (Tocumwal) (P<0.05), but that there was no difference between Barmah and 4 Post Reserve (P=0.121). The concentration of DOC increased at Barmah and 4 Post in response to flow events that exceeded the channel capacity of BMF, most notably during
Discussion
We set out to investigate the importance of terrestrial carbon subsidies to instream food webs. We hypothesised that carbon, nutrients and zooplankton would be exported from the floodplain and that microbes within biofilms would consume floodplain-derived carbon. We further hypothesised that consumers within biofilms would reflect a stable isotope carbon value similar to terrestrial carbon. We found clear evidence that flood-return waters from a forested floodplain led to increased biomass of
CRediT authorship contribution statement
Gavin N. Rees: Conceptualization, Methodology, Funding acquisition, Formal analysis, Writing - original draft. Robert A. Cook: Conceptualization, Methodology, Funding acquisition, Data curation, Formal analysis, Writing - review & editing. Nathan S.P. Ning: Conceptualization, Methodology, Writing - review & editing. Paul J. McInerney: Writing - review & editing. Rochelle T. Petrie: Conceptualization, Methodology, Data curation, Formal analysis. Daryl L. Nielsen: Funding acquisition, Formal
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded jointly by the Murray–Darling Basin Authority and the Australian Department of Environment's National Environment Research Program. We thank Dr. Rick Stoffels for helpful discussion with analyses, John Pengelly for determining the nutrient, DOC and chlorophyll-a concentrations; Douglas Ford and Greg Skrzypek of the West Australian Biogeochemistry Centre for stable isotope analysis, and Keith Ward of the Goulburn–Broken Catchment Authority for his valuable input into this
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