Sandy sediments as active biogeochemical reactors: compound cycling in the fast lane

Abstract

The concept of assessing benthic biogeochemical standing stocks as an ecologically relevant parameter has been challenged by one of a dynamic nature: in sands, low standing stocks may mean low carbon burial efficiency due to rapid turnover aided by advective interfacial flows. This concept suggests that a large, diverse and very adaptable population of microbes is present in sands, and that these have a previously unforeseen biogeochemical importance. This view has profound consequences for the scientific outlook on the ecological role of permeable sediments and on the methodological strategies used in the study of coastal ecosystems. Based on a review of the current literature and results gathered at a coastal setting, progress within this new paradigm is examined and underlying questions are speculated upon. The evidence so far shows that, in permeable sediments and at timescales of seconds to a few hours, the dynamics of advective flow to a large extent control microbial diversity, the rates of microbial processes, the size of organic and inorganic pools, and even their respective changes. The importance of obtaining further information on the microbial diversity, the structure of different communities present in sands and their link to biogeochemical function arises from recent field studies. What is also clear from the available evidence is that only a combination of different techniques and approaches, some of which are under development on the fringes of previously almost water-tight research areas, will further understanding of the important functional role of microbial populations in benthic ecosystems.

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

“…Indeed, even a low frequency of disturbance can cause significant changes to the biotic and abiotic components of the system and it is possible that intensive fishing disturbance can cause a reduction of [NO 3 -N] that relates to over-mobilisation of sediment, increased microbial activity, and net loss of N from the sediments (Bertics et al 2010, Laverock et al 2011). Whilst it is clear that non-cohesive sediment communities are vulnerable to changes in the frequency of fishing, even though the fishing pressure is relatively low, near-bed current flows and the permeability of the sediment profile mean that organic carbon cycling is already rapid (Rocha 2008), making it difficult to assess how nutrient sediment-water exchange is affected as communities are modified. Cohesive sediments by contrast, are dominated by diffusive processes, where the remineralisation of organic matter is largely driven by the reduction of solutes (Kitidis et al 2017).…”

“…However, non-cohesive sediments also play a major role in the turnover of particulate organic matter, despite comparatively lower levels of living biomass, because of the influence of advective processes (Rao et al 2007(Rao et al , 2008. These mechanistic differences mean that in cohesive sediments the remineralisation of organic carbon and generation of macronutrients is largely driven by the reduction of solutes, such as nitrate and sulphate, whilst in non-cohesive sediments remineralisation processes reflect the extent of advective porewater flows (Rocha 2008). However, sediment or habitat type can be a poor predictor of biogeochemical performance (Dernie et al 2003) because the active redistribution of particles and fluids by infaunal macro-invertebrates disproportionately influences benthic fluxes and total benthic metabolism (Banta et al 1999;Mermillod-Blondin et al 2004).…”

Marine Macrophyte Wrack Inputs and Dissolved Nutrients in Beach Sands

“…Nutrient cycling in coarse permeable sediments, including intertidal and continental shelf sands, has received considerably less attention (Rocha 2008). The assumption that the relatively low organic content generally present in these sediments (one to two orders of magnitude lower) is correlated with low biogeochemical activity, however, has been challenged by a number of recent studies (e.g., Anschutz et al 2009;Boudreau et al 2001;Huettel and Rusch 2000;Jahnke et al 2005;Rocha 2008;Rusch et al 2006), suggesting that this may represent an important oversight for nutrient dynamics of coastal and continental shelf ecosystems.…”

Where Carbon Goes When Water Flows: Carbon Cycling across the Aquatic Continuum

“…High mineralization rates in sediments can cause a strong benthic-pelagic coupling of nutrient cycles Gibbes et al, 2008;Røy et al, 2008). There is also accumulating evidence that reactive marine-derived OM is rapidly degraded in shallow permeable sediments Ehrenhauss et al, 2004;Santos et al, 2009;Avery et al, 2012), which has led to their characterization as efficient biogeochemical filters for bioavailable compounds from coastal seawater (Rocha, 2008;Anschutz et al, 2009). However, submarine groundwater discharge from tidal flats and beaches to the coastal ocean also transports terrestrial DOM, a part of which may be relatively unreactive (Dittmar et al, 2012;Seidel et al, 2014Seidel et al, , 2015bCouturier et al, 2016).…”