Managed floodplain inundation maintains ecological function in lowland rivers

https://doi.org/10.1016/j.scitotenv.2020.138469Get rights and content

Highlights

  • Chemical and biological responses to flood-return waters were examined.

  • Carbon and nutrients from managed floodwaters stimulated secondary production.

  • Terrestrial carbon fuelled the riverine food web.

  • Managed flows promote clear ecological benefits to rivers.

Abstract

Managed environmental flows are one mechanism by which managers may restore carbon dynamics, diversity and ecological function of rivers affected by anthropogenic activities. Empirical studies that quantify such interactions in detail are few, so we measured the amounts of dissolved organic carbon, nutrients, algae and invertebrates in the main river channel following a managed environmental flow that inundated an adjacent floodplain forest. Dissolved organic carbon (DOC), seston carbon, total nitrogen (TN), and chlorophyll-a (chl-a) concentrations were greatly increased downstream. The net yield of DOC, seston carbon, TN and chl-a from the floodplain peaked at approximately 100, 50, 5 and 0.1 t d−1, respectively during the major flow event. Total phosphorus mobilisation peaked at approximately 0.4 t d−1. Stable isotope analysis showed that allochthonously-derived carbon was rapidly incorporated into biofilm and grazing macroinvertebrates, persisting in riverine food webs for up to four months following the flood. During a subsequent smaller flow event, the floodplain either generated no further carbon or nutrients, or was a sink for carbon and nutrients. Our results provide empirical support for the River Wave Concept and show that allowing floodplain water to return to the river downstream of forests is important for maintaining ecological function within the river channel.

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

References (71)

  • C. Baranyi et al.

    Zooplankton biomass and community structure in a Danube river floodplain system: effects of hydrology

    Freshw. Biol.

    (2002)
  • L.J. Baumgartner et al.

    Using flow guilds of freshwater fish in an adaptive management framework to simplify environmental flow delivery for semi-arid riverine systems

    Fish Fish.

    (2014)
  • P.I. Boon et al.

    Consumption of cyanobacteria by freshwater zooplankton: implications for the success of ‘top-down’ control of cyanobacterial blooms in Australia

    Mar. Freshw. Res.

    (1994)
  • M.T. Brett et al.

    How important are terrestrial organic carbon inputs for secondary production in freshwater ecosystems?

    Freshw. Biol.

    (2017)
  • B.K. Bum et al.

    Factors regulating phytoplankton and zooplankton biomass in temperate rivers

    Limnol. Oceanogr.

    (1996)
  • S.E. Bunn et al.

    Basic principles and ecological consequences of altered flow regimes for aquatic biodiversity

    Environ. Manag.

    (2002)
  • J.J. Cole et al.

    Plumbing the global carbon cycle: integrating inland waters into the terrestrial carbon budget

    Ecosystems

    (2007)
  • R.A. Cook et al.

    River metabolism and carbon dynamics in response to flooding in a lowland river

    Mar. Freshw. Res.

    (2015)
  • K.W. Cummins

    Trophic relations of aquatic insects

    Annu. Rev. Entomol.

    (1973)
  • A.I. van Dijk et al.

    The Millennium Drought in southeast Australia (2001–2009): natural and human causes and implications for water resources, ecosystems, economy, and society

    Water Resour. Res.

    (2013)
  • W.K. Dodds et al.

    Quality and quantity of suspended particles in rivers: continent-scale patterns in the United States

    Environ. Manag.

    (2004)
  • D. Dudgeon et al.

    Freshwater biodiversity: importance, threats, status and conservation challenges

    Biol. Rev.

    (2006)
  • Ecological_Associates et al.

    Environmental Water Delivery: Yarrawonga to Tocumwal and Barmah-Millewa

    (2011)
  • R. France et al.

    d13 variability of benthic algae: effects of water colour via modulation by stream current

    Freshw. Biol.

    (1998)
  • D.J. Furst et al.

    Floodplain connectivity facilitates significant export of zooplankton to the main River Murray channel during a flood event

    Inland Waters

    (2014)
  • GBCMA

    The Living Murray Barmah-Millewa Forest Icon Site Annual Synthesis Report 2011–2012

    (2013)
  • D.P. Giling et al.

    How might cross-system subsidies in riverine networks be affected by altered flow variability?

    Ecosystems

    (2015)
  • W.L. Hadwen et al.

    Longitudinal trends in river functioning: patterns of nutrient and carbon processing in three Australian rivers

    River Res. Appl.

    (2010)
  • S.K. Hamilton et al.

    Causes of seasonality in the chemistry of a lake on the Orinoco River floodplain, Venezuela

    Limnol. Oceanogr.

    (1987)
  • J.H. Hawking et al.

    Colour Guide to Invertebrates of Australian Inland Waters

    (1979)
  • T. Hein et al.

    Hydrology as a major factor determining plankton developement in two floodplain segments and the river Danube, Austria

    Archiv fuer Hydrobiologie Supplement band

    (1999)
  • M.J. Hinton et al.

    Sources and flowpaths of dissolved organic carbon during storms in two forested watersheds of the Precambrian shield

    Biogeochemistry

    (1998)
  • S. Hladyz et al.

    Influence of substratum on the variability of benthic biofilm stable isotope signatures: implications for energy flow to a primary consumer

    Hydrobiologia

    (2011)
  • S. Hladyz et al.

    Temporal variations in organic carbon utilization by consumers in a lowland river

    River Res. Appl.

    (2012)
  • P. Humphries et al.

    The river wave concept: integrating river ecosystem models

    BioScience

    (2014)
  • Cited by (0)

    View full text