Connectivity and complexity of floodplain habitats govern zooplankton dynamics in a large temperate river system

Abstract

Summary Large river–floodplain systems are characterised by seasonal flow variability. High flows lead to hydrological connection between the main channel and inundated off‐channel lakes, wetlands and floodplains, which provide essential habitats for riverine biota. We tested the following hypotheses: (i) that crustacean zooplankton are more abundant in connected lentic habitats such as riverine lakes and wetlands and have a different community composition compared with the main channel and (ii) that vegetation structure will moderate abundances of crustacean zooplankton in the lower reaches of the Waikato River, New Zealand. Zooplankton densities in main channel inflows and inundated floodplains showed clear seasonal changes, with cladoceran and copepod abundance peaks occurring in the majority of sites 2–3 weeks following the peak spring discharges (coinciding with the retreat of water from floodplains into the main channel). Mean densities of zooplankton were highest in the inflows originating from riverine lakes (10–20 ind. L−1) where rotifers were dominant. We recorded significantly higher abundances of copepods in peat bog and swamp wetland inflows (c. 5 ind. L−1) relative to the main river channel and riverine lake inflows (0.1–1 ind. L−1). Some lake inflows also had high numbers of cladocerans (5–10 ind. L−1). Inundated floodplains displayed heterogeneity in zooplankton community composition in relation to their structural complexity. Flooded forest accommodated higher numbers of copepods (c. 8 ind. L−1) and cladocerans (c. 17 ind. L−1) than flooded grassland, where zooplankton assemblages resembled those in the main river channel and were characterised by dominance of rotifers and low overall zooplankton densities (1–2 ind. L−1). Our results suggest that seasonal flow and flood pulses, which determine the degree of connectivity of the main channel with the floodplain and off‐channel habitats, govern zooplankton densities and community structure in this large temperate river. Furthermore, the structural complexity of floodplain habitats may play an important role in enhancing riverine zooplankton diversity. We postulate that the post‐flood peak of large‐bodied cladocerans and copepods might have historically played an important role in the provision of food for juvenile fish such as migrating Galaxiidae.

The density and biomass of mesozooplankton and ichthyoplankton in the Negro and the Amazon Rivers during the rainy season: the ecological importance of the confluence boundary

“…We also found a higher density of mesozooplankton communities in black water river compared to white water river. The density of zooplankton in tropical large rivers depend largely on the supply from adjacent lentic sources (standing water bodies) connected to the river such as channel and floodplain habitats (Rzoska, 1978; Saunders & Lewis, 1988a; Saunders & Lewis, 1989; Basu & Pick, 1996; Reckendorfer et al, 1999; Górski et al, 2013). The zooplankton sampling period in this study corresponds to the rising water period (March), where rising riverine water starts to wash out ambient zooplankton from associated lentic sources into the rivers (Saunders & Lewis, 1988a; Saunders & Lewis, 1988b; Saunders & Lewis, 1989).…”

Echoes of a flood pulse: short-term effects of record flooding of the Illinois River on floodplain lakes under ecological restoration

“…Nutrient pulses and productivity resulted in higher trophic level responses including a clear bacterioplankton species and abundance shift and higher zooplankton densities. Increased zooplankton densities immediately after the flooding were likely linked to the introduction of riverine species and augmented reproduction of existing backwater zooplankton (Górski et al, 2013). Zooplankton assemblage structure was heavily influenced by riverine inputs, dominated by rotifers in terms of both density and total biomass as reported from other studies within the Illinois River (Schuyler et al, 2009;Sass et al, 2014).…”

“…Subsequently, many studies have examined the basic underpinning that floodplain-river connections are important drivers of biodiversity and have suggested that other factors also contribute to the ecology of large river systems in temperate areas (Tockner et al, 2000;Junk & Wantzen, 2004). However, frequent and high-magnitude floods during the growing season can be associated with many undesirable effects (Jackson & Pringle, 2010), such as high-nutrient loadings (Kreiling et al, 2013), decreased invertebrate diversity (Galir & Palijan, 2012;Górski et al, 2013), dispersal of invasive species, suppressed growth of submersed aquatic vegetation, and limited recruitment of woody vegetation (Sparks, 1995;Robertson et al, 2001;Tockner & Stanford, 2002). Responses of abiotic and biotic factors to different flooding intensities need to be better understood in order to quantify tradeoffs among ecological services associated with benefits of restoring and reconnecting backwater lakes and wetlands, particularly in hydrologically modified river systems (Ward et al, 2001;Tockner & Stanford, 2002).…”

Linking ecological science with management outcomes on New Zealand's longest river

“…Receding floodwaters potentially play significant roles in fuelling productivity in the main river channel through the hatching of dormant zooplankton from flood plain soils following inundation (Catlin, Collier, & Duggan, ; Górski, Collier, Duggan, Taylor, & Hamilton, ). In temperate river systems elsewhere, this input has been hypothesized to be the primary function of inundated flood plains in recruitment of larval fish, rather than direct provision of nursery habitats (Górski et al, ).…”