A synthesis of floodplain aquatic ecosystem metabolism and carbon flux using causal criteria analysis

Abstract

The transformation of solar energy into organic matter by autotrophs (gross primary production [GPP]) and the use of that energy by autotrophs and heterotrophs (ecosystem respiration [ER]) describe the total energy available to support food webs. Rates of GPP and ER vary with temperature, light, hydrology, nutrients, and organic matter supply and quality yet despite their obvious importance, spatiotemporal variation of metabolic patterns among floodplain habitats, and their relationship to inundation dynamics remain unclear. We set out to review the peerreviewed literature surrounding the influence of the magnitude, frequency, and duration of floodplain inundation on aquatic ecosystem metabolism and carbon flux by rigorously testing a suite of cause-effect hypotheses using a causal criteria analysis. Causal criteria analysis is a literature synthesis approach developed to address a lack of experimental data and subsequent weak inference of causal relationships. We found support for 3 of the 14 hypotheses we tested relating to putative causal relationships: (1) large floods transfer more carbon from floodplains to the river channel than small floods via the increase in inundation area leading to more overall leaching of floodplain litter, (2) in high turbidity floodplain habitats rates of GPP are reduced by restrictions to photic depth, and (3) a positive correlation between nutrients and GPP-generally GPP in floodplain wetlands increases with nutrient levels. We obtained inconsistent evidence for a causal relationship between macrophytes and aquatic GPP, with studies reporting both a negative influence from decreased light caused by macrophyte shading and a positive influence from structural support provided by macrophytes for periphyton growth. For the remaining 10 hypotheses, there was insufficient evidence to support causal relationships, including for any hypotheses relating to frequency or duration of floodplain inundation. Our results emphasize that despite an apparent wealth of metabolic studies in riverine ecosystems, floodplain metabolic dynamics remain poorly studied, likely due to less investment and increased difficulty compared to lotic waters. The review also highlighted aspects of floodplain aquatic ecosystem metabolism for which there are significant knowledge gaps in the literature, in particular metabolic responses to inundation frequency and duration. Our results call attention to the importance of site specificity and temporal changes when predicting putative cause-effect relationships between floodplain inundation and metabolic patterns.

Seeing the floods through the trees: Using adaptive shortwave infrared thresholds to map inundation under wooded wetlands

“…The SWIM‐UV method provides a complement to proven open water classifiers such as WOfS, and in combination they enable the determination of overall inundated extent, and when combined with hydrograph data can be used to calculate timing and duration of inundation events. This information is critical to estimating floodplain productivity (McInerney et al, 2023; Wohl & Knox, 2022) and for river management, such as estimating blackwater or hypoxia risks arising from inundated organic matter on floodplains (Gibbs et al, 2022). Inundation plays an essential role in maintaining and provisioning key habitat for a range of biota, such as macro‐invertebrates (Benke, 2001; Jenkins & Boulton, 2003), for fish habitat and recruitment (Gibbs et al, 2023; Gorski et al, 2011).…”

“…As an additional assessment of SWIM-UV's extent, output from a MIKE FLOOD hydrodynamic flood model for the Chowilla floodplain (FP) was The SWIM-UV method provides a complement to proven open water classifiers such as WOfS, and in combination they enable the determination of overall inundated extent, and when combined with hydrograph data can be used to calculate timing and duration of inundation events. This information is critical to estimating floodplain productivity (McInerney et al, 2023;Wohl & Knox, 2022) and for river management, such as estimating blackwater or hypoxia risks arising from inundated organic matter on floodplains (Gibbs et al, 2022).…”

Incorporating the riverscape into models of river–floodplain function

“…Ecosystem expectations and roles of large riverscapes, including expected patterns of carbon fixation, organic matter processing, biomass production, metabolism, nutrient transformation, and gas exchange are still unknown relative to those in lakes, small streams, and wetlands (Palmer and Ruhi, 2019;Battin et al, 2023;McInerney et al, 2023). Linking physical processes, organisms, and ecosystem responses should improve the development of effective indicators (McInerney et al, 2023) as well as inform researchers about how riverscapes contribute to global processes such as climate change and respond to local conditions. Using direct estimates of wholeecosystem secondary production as a directly comparable indicator of riverscape function is another important approach (Cross et al, 2013).…”