The Role of Boreal Seagrass Meadows in the Coastal Filter

By removing nutrients from the water, coastal ecosystems serve as a filter between land and the open sea. Seagrasses contribute to the coastal filter by trapping and absorbing nutrients. Understanding the processes and environmental conditions underpinning the variability in nutrient retention among and within seagrass meadows is important to evaluate their role in the coastal filter across geographic regions, especially in less studied regions. This study evaluates the role of eelgrass (Zostera marina) meadows in the coastal filter in boreal Newfoundland, Canada, and identifies environmental traits driving variability in nutrient fluxes. We measured carbon (Corg) and nitrogen (N) proportions and stable isotopic composition in the surface sediment (top 5 cm) of three eelgrass meadows. Sediment cores were collected from different locations (i.e., inside, edge, outside) relative to each meadow. Sediment %N (0.22%), %Corg (2.82%), Corg stock (11.1 Mg Corg ha-1), and N stock (0.91 Mg N ha-1) were elevated in our study sites; however, nutrient content was not consistently higher inside the meadow than at the edge or outside. Variability in nutrient retention was best explained by a negative relationship with sediment bulk density. Additionally, differences in carbon isotopic (delta 13Corg) enrichment between eelgrass tissue (-11.6 parts per thousand) and sediment (-22.1 parts per thousand) within sites indicated that sediment nutrients were predominantly derived from allochthonous marine sources, where variability was best explained by salinity. This study improves the understanding of the role of eelgrass to nutrient cycles in boreal coastal systems and the potential of eelgrass as a blue carbon ecosystem. Seagrasses trap incoming nutrients, such as carbon and nitrogen in the coastal zone, serving as a coastal filter. This function increases habitat productivity and results in further benefits to human well-being (e.g., carbon storage). However, morphometric traits and the surrounding hydrographic environment differ among meadows, resulting in variable coastal filter efficiencies. Understanding the habitat traits driving variability in coastal filter function is important to understand the role of seagrass in nutrient retention across different regions. We present estimates of carbon and nitrogen retention in the surface sediment across three eelgrass meadows in boreal Newfoundland, Canada; a system where the role of eelgrass as a coastal filter has never been assessed. Variability in nutrient retention at both the meadow and bay scales was best explained by a negative relationship with sediment bulk density. Additionally, carbon retained in the surface sediment of the eelgrass meadows was predominantly derived from marine, non-eelgrass sources, where variability in nutrient sources was best explained by salinity. By quantifying carbon and nitrogen cycles in a new region and describing habitat mechanisms underlying the variability in nutrient retention, our findings will benefit future nitrogen burial and blue carbon estimation in the coastal zone. Understanding eelgrass traits that enhance coastal filter function across spatial scales exposes the potential of eelgrass to support ecosystem servicesEelgrass meadow sediments in boreal Canada are rich in carbon and nitrogen, where carbon is predominantly derived from marine allochthonous sourcesIn systems where terrestrial carbon inputs are low, coastal filter functioning is enhanced in eelgrass meadows with low sediment bulk density