Abstract
Benthic processes in coastal marine environments can enhance the natural removal of bioreactive nitrogen through denitrification (DNF), a valuable ecosystem service as nutrient over-enrichment intensifies globally. Enhancing ecosystem services is an important justification for restoring coastal ecosystems, and while it is known that epifaunal bivalves (oysters or mussels) are capable of influencing nitrogen cycling, empirical measurement of the role of particular species across a range of environmental conditions is missing. Bivalves within shellfish beds are not uniformly distributed and thus clumps and empty patches within restored beds may differentially impact DNF given the importance of local biogeochemistry. This study reports DNF and respiration rates in 4 restored beds that vary in sediment composition, while identifying which ecological and biogeochemical factors best explain the observed variability in measured fluxes. We deployed benthic chambers in sediments with and without mussels at these sites, and measured net N2 fluxes through membrane inlet mass spectroscopy. Sediment organic matter proved to be the most significant predictor of DNF rates in regression tree and random forest models, suggesting that biodeposition by green-lipped mussels enhances nitrogen removal at these sites and that these effects occur across beds despite differences in mussel density. Greatest DNF rates corresponded to lower sediment chlorophyll a concentrations and higher nitrate/nitrite effluxes measured within chambers. Determining the influence of subtidal mussel restoration on significant sediment processes informs future restoration efforts aiming to maximise this nitrogen removal service, while providing insights on underrepresented oligotrophic systems of the southern hemisphere.
Highlights
Nitrogen loading is commonly associated with eutrophication and the degradation of coastal environments, as nitrogen typically limits algal growth and primary production (Howarth & Marino 2006), but in excess, can lead to toxic algal blooms (Paerl 1997, Anderson et al 2002) and hypoxic conditions (Diaz & Rosenberg 2008)
Moderate sediment organic matter (SOM) was observed at all locations, with mean SOM ranging from 2.3 ± 0.02% at Lagoon Bay (LB) to 3.4 ± 0.36% at PP
The total number of species identified per core increased from 10 ± 0.9 and 9.5 ± 1.2 at the muddier sites LB and PP to 14.5 ± 1.2 and 14.5 ± 1.0 species at the sandier sites MR and MK, respectively, indicating an increase in species richness associated with sandier sites
Summary
Nitrogen loading is commonly associated with eutrophication and the degradation of coastal environments, as nitrogen typically limits algal growth and primary production (Howarth & Marino 2006), but in excess, can lead to toxic algal blooms (Paerl 1997, Anderson et al 2002) and hypoxic conditions (Diaz & Rosenberg 2008). As humans continue to generate bioreactive nitrogenous compounds (e.g. ammonium and nitrate available for biological uptake) and fail to control excessive nitrogen inputs to ecological systems, the capacity of coastal environments to remove excess nitrogen becomes of even greater value to humans. This process is currently not well understood. It is estimated that estuaries and coastal shelf regions currently remove 24 Tg of reactive nitrogen each year (Galloway et al 2004), making DNF a valuable service provided by coastal soft sediments
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