Nutrient cycling variability in seagrass meadows and unvegetated tidal flats

Estuarine soft sediment habitats play important roles in recycling and removing excess nutrients from coastal ecosystems. However, our understanding of what regulates these processes remains limited, particularly in oligotrophic systems experiencing increased loads of land-derived nutrients and sediments. In intertidal seagrass and adjacent unvegetated habitats, we quantified in situ solute fluxes (under light and dark conditions) and denitrification rates (dark only) in an oligotrophic temperate estuary. Nine sites with adjacent habitats, spanning a sediment mud gradient of 5-33%, were sampled during austral summer, alongside 2 unvegetated mudflat sites (39-49% mud) that extended the unvegetated sediment gradient. To see whether differences in solute fluxes between habitats and light availability persisted temporally, 1 site was sampled seasonally for 2 yr. Benthic primary production was a key regulator of nutrient cycling both spatially and seasonally. In all habitats, ammonium efflux was reduced in light conditions compared to dark. Ammonium effluxes were consistently lower in seagrass compared to adjacent unvegetated habitats (by approximately half), likely owing to a greater photosynthetic biomass. Under dark conditions, in the seagrass, more of the total inorganic nitrogen released from the sediment was di-nitrogen gas (N 2 ) relative to the unvegetated flats, as indicated by a 1.3 times higher denitrification efficiency. Sediment mud content was negatively correlated with denitrification. Results emphasise how a reduction in seagrass extent will reduce nutrient removal capacity in oligotrophic estuaries, particularly in regions of increased sedimentation. This loss could impact the resilience of oligotrophic estuaries to nutrient loading, accelerating shifts towards eutrophic states.