Managed floodplain inundation maintains ecological function in lowland 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.

Keywords: Biofilm; Environmental flows; Floodplains; Macroinvertebrates; Riverine food webs; Stable isotopes.