Abstract
Abstract. Nearshore zones experience increased sedimentation due to coastal development and enhanced loads of fine terrestrial sediment (hereafter, TS) in river waters. Deposition of TS can alter seabed biogeochemical processes but the effects on benthic ecosystem functioning are unknown. The results of a past experiment with defaunated, intertidal sediment suggest that a decrease in the oxygenation of this sediment by a thin (mm) TS deposit causes substrate rejection (refusal to bury) by post-settlement juvenile recruits of the tellinid bivalve Macomona liliana. We further examined this behaviour, asking if such deposits negatively affect burial when applied to intertidal sediment that is oxygenated by bioturbation (C) or depleted of dead and living organic matter (D). We observed recruits on the surface of four treatments: C, D, and the same sediments to which we added a 1.7–1.9 mm layer of TS (CTS, DTS). The TS deposit decreased the oxygenation and the pH of the underlying intertidal sediment (CTS) confirming previous results, but significantly increased but not decreased the probability of burial, irrespectively of treatment. Juveniles more likely buried into C than into D. The mechanism that caused previously observed substrate rejection by post-settlement juvenile M. liliana remains unclear but our results suggest that contact of the recruits with the TS deposit does not cause substrate rejection. We now hypothesise that conditioning of sediment by bioturbation can mediate negative effects of TS deposits on the recruits' burial behaviour.
Highlights
Changes in land use, a rising sea level, and extreme rainfall events increase the supply of terrestrial sediment to coastal habitats, either as suspended particles via waterways or directly from landslides (Milliman and Meade, 1983; Thrush et al, 2004)
Cummings et al (2009) demonstrated that millimetre-thin deposits of TS over defaunated intertidal sediment negatively affect the burial of post-settlement juvenile Macomona liliana and suggested that the juveniles are responding to reduced solutes diffusing from the deposit-underlying sediment but not to contact with the TS deposit
We deposited TS over intact, bioturbated sediment and sediment from which we had removed organic matter by combustion and failed to demonstrate a negative effect of TS on juvenile burial. This result supports evidence from Cummings et al (2009) that rejection of a TS-covered sediment by the juveniles is not due to their contact with the deposit. It raises the question of the roles of the resident macrofauna and sediment organic matter content in the juveniles’ response to TS deposition – an interesting challenge for future experiments
Summary
A rising sea level, and extreme rainfall events increase the supply of terrestrial sediment (hereafter, TS) to coastal habitats, either as suspended particles via waterways or directly from landslides (Milliman and Meade, 1983; Thrush et al, 2004). The suspended TS eventually settles, forming a deposit on the surface of the softsediment seafloor This deposit – until reworked by benthic fauna or suspended by flow – alters functions of the sedimentary ecosystem. It affects benthic organic carbon decomposition and production by impeding the sediment–seawater exchange of reactive solutes, altering the behaviour of benthic species and associated solute reaction dynamics or, depending on the scale of the deposit, the composition of the benthic species assemblage (Thrush et al, 2003; Cummings et al, 2009; Woodin et al, 2012). The effect of thin TS deposits on the exchange of reactive solutes will be a function of macrofaunal activity, which is influenced by how the deposit formed Compare these two scenarios: suspended TS may form a millimetre-thick deposit on the surface of coastal sediment already colonised by burrowing
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