Modelling seabed sediment physical properties and organic matter content in the Firth of Clyde

Abstract

High-quality quantitative maps of seabed sedimentary physical and geochemical properties have numerous research and conservation applications, including habitat and ecosystem modelling, marine spatial planning, and ecosystem service mapping. However, such maps are lacking for many ecologically and economically important marine areas. Using legacy data supplemented by measurements from recent benthic surveys, modelled hydrodynamic variables, and high-resolution bathymetry, quantitative maps for the top 10 cm of seabed sediment were generated via a combination of statistical and machine-learning techniques for the Firth of Clyde, a semi-enclosed coastal sea on the west coast of Scotland. The maps include sediment fractions of mud, sand, and gravel; whole-sediment median grain size; sediment permeability and porosity; rates of natural seabed abrasion; and sediment particulate organic carbon and nitrogen content. Properties were mapped over an unstructured grid so that very high resolutions were achieved close to the coastlines, where sediments may be expected to be spatially heterogeneous. Overall, the maps reveal extensive areas of very low sediment permeability coupled with low rates of natural seabed disturbance. Moreover, muddy sediments in the inner Firth of Clyde, Inchmarnock Water, and the sea lochs are enriched in organic carbon and nitrogen relative to the sediments of the outer Firth of Clyde. As a demonstration of the value of these maps, the standing stock of organic carbon and nitrogen in the surficial sediments of the Clyde was calculated. The Clyde stores 3.42 and 0.33 million t of organic carbon and nitrogen in the top 10 cm of seabed sediment, respectively, substantially contributing to Scotland's coastal and shelf blue carbon stocks. Data products are available from https://doi.org/10.15129/2003faa2-ee93-4c11-bb16-48485f5f136d (Heath and Pace, 2021).