Organism-Sediment Interactions Govern Post-Hypoxia Recovery of Ecosystem Functioning

Abstract

Hypoxia represents one of the major causes of biodiversity and ecosystem functioning loss for coastal waters. Since eutrophication-induced hypoxic events are becoming increasingly frequent and intense, understanding the response of ecosystems to hypoxia is of primary importance to understand and predict the stability of ecosystem functioning. Such ecological stability may greatly depend on the recovery patterns of communities and the return time of the system properties associated to these patterns. Here, we have examined how the reassembly of a benthic community contributed to the recovery of ecosystem functioning following experimentally-induced hypoxia in a tidal flat. We demonstrate that organism-sediment interactions that depend on organism size and relate to mobility traits and sediment reworking capacities are generally more important than recovering species richness to set the return time of the measured sediment processes and properties. Specifically, increasing macrofauna bioturbation potential during community reassembly significantly contributed to the recovery of sediment processes and properties such as denitrification, bedload sediment transport, primary production and deep pore water ammonium concentration. Such bioturbation potential was due to the replacement of the small-sized organisms that recolonised at early stages by large-sized bioturbating organisms, which had a disproportionately stronger influence on sediment. This study suggests that the complete recovery of organism-sediment interactions is a necessary condition for ecosystem functioning recovery, and that such process requires long periods after disturbance due to the slow growth of juveniles into adult stages involved in these interactions. Consequently, repeated episodes of disturbance at intervals smaller than the time needed for the system to fully recover organism-sediment interactions may greatly impair the resilience of ecosystem functioning.

Mediation of macronutrients and carbon by post-disturbance shelf sea sediment communities

“…Here, we investigate whether post-disturbance changes in epifaunal and infaunal assemblage structure result in associated changes in organic carbon and nutrient cycling along gradients of chronic fishing activity in fishing grounds that contrast in sediment type. Our a priori assumption was that benthic macrofaunal communities would be restructured by chronic physical disturbance and that the adjusted post-disturbance community would persist long after the perturbation event (van Colen et al 2012). Consequently, the relative role of these surviving communities in moderating organic carbon and macronutrient dynamics will be context specific and relate to the introduction, removal or rebalancing of traits that directly influence the processes that govern remineralisation, advection, resuspension and burial within environmental context (Godbold and Solan 2009).…”

Functional trait responses to sediment deposition reduce macrofauna-mediated ecosystem functioning in an estuarine mudflat

“…A higher biodiversity can convey a higher resilience and a more efficient functioning of ecosystems in terms of, for example, nutrient cycling and primary productivity (Cardinale et al, 2012;Hooper et al, 2005). Since biodiversity-mediated ecosystem functioning depends on the functional identities of the species present in the community and their densities (Braeckman et al, 2010;Van Colen et al, 2013), functional community descriptors often predict functioning better than taxonomic diversity (Wong and Dowd, 2015). Functional traits, e.g.…”

“…Natural sedimentation is caused by surface runoff from the catchment area or by tidal movements; the former can be intensified by land use change . Furthermore, dredging and dumping activities also contribute to sediment deposition, either directly or by creating sediment plumes that subsequently settle down on the seabed (Van Lancker and Baeye, 2015). Such deposition events are expected to alter the productivity of coastal soft-sediment habitats via direct and indirect mechanisms that affect biogeochemical cycling.…”

“…bioturbation) and the assisted transfer of solutes through the sediment (i.e. bio-irrigation) (Braeckman et al, 2010(Braeckman et al, , 2014Van Colen et al, 2012;Thrush et al, 2006). Though both processes are interrelated and sometimes grouped under the umbrella term "bioturbation" (Kristensen et al, 2012), we opted to use them as separate concepts, in order to clearly distinguish between particle reworking and solute transfer.…”

“…We hypothesize that sediment deposition reduces oxygen availability to the community underneath, consequently affecting the survival of the macrobenthos and inducing escaping behaviour (Riedel et al, 2008;Villnäs et al, 2012). This may influence biogeochemical cycling by affecting bioturbation or bio-irrigation (Van Colen et al, 2012;Renz and Forster, 2014).…”