Abstract
Human activities alter biodiversity, influencing bottom-up and top-down control on food webs which can affect ecosystem functioning. In marine ecosystems, large bivalves play a critical role in benthic–pelagic coupling including nutrient cycling; however, their influence on the uptake of detrital organic matter by benthic communities is less understood. In a replicated factorial field experiment, we examined how the presence or absence (overharvesting scenario) of a large suspension-feeding clam on an intertidal sandflat and the addition of isotopically enriched macroalgal (Ulva sp.) detritus (eutrophication scenario) influenced infaunal biodiversity, and how changes in trophic interactions influenced key ecosystem functions (nutrient cycling and benthic metabolism and primary production). Both clams and Ulva increased community metabolism, but only clams had an effect on nutrient regeneration. We used the 13C- and 15N-enriched Ulva to quantify the effect of clams on detritus uptake in fauna and recovery in sediment. Due to their large biomass, nitrogen incorporation by clams constituted one-third of the infaunal community uptake after 14 days. Clam uptake likely resulted from ingestion of resuspended microphytobentos which had utilized 15N leaking out from decomposing Ulva. In plots without Ulva addition, the effect of clams on the overall resource utilization by the benthic community using natural abundance isotope niche metrics were tested. In plots without clams, the isotope niche of the community was reduced, and less carbon of pelagic origin was channelled into the infaunal food web. Our results imply that the loss of clams changes trophic pathways and reduces community uptake of macroalgal detritus, potentially exacerbating eutrophication.Graphic
Highlights
Anthropogenic activities reduce biodiversity and alter species composition, influencing both bottomup and top-down control on food webs with potentially large effects on ecosystem functioning and the ecosystem services provided to humans (Duffy and others 2007; Cardinale and others 2012)
Apart from clams, 36 macro-infaunal species or taxa were found (n = 96 cores, excluding ostracods and nematodes); the community was dominated in both abundance and biomass by three species of amphipods and one maldanid head-down depositfeeding polychaete, M. stewartensis
The fact that other species cannot compensate for the loss of A. stutchburyi during the timeframe of this study demonstrates its role as a key species in estuarine ecosystems; its loss will result in reduced resource utilization by the benthic community and in reduced trophic transfer of detrital subsidies from macroalgae
Summary
Anthropogenic activities reduce biodiversity and alter species composition, influencing both bottomup and top-down control on food webs with potentially large effects on ecosystem functioning and the ecosystem services provided to humans (Duffy and others 2007; Cardinale and others 2012). In understanding how the environmental setting modulates relationships (for example, Hiddink and others 2009) and the underlying mechanisms (Karlson and others 2010, 2016). Such understanding is necessary for better predictions of how changing biodiversity will affect functioning to inform management strategies on a regional level. The role of biodiversity and in particular large bivalves for cumulative processes (for example, nutrient fluxes which can be measured in situ in aquatic habitats) has been extensively studied (for example, Norkko and others 2013; Jones and others 2011), the direct and indirect effects of biodiversity on the fate of detritus (faunal uptake and subsequently trophic transfer) and associated changes in sediment biogeochemistry are poorly understood. Uptake of detritus is a fundamental ecosystem process where benthic infauna converts dead organic material to secondary production, which is available for higher trophic levels (Snelgrove and others 1997)
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