Blue carbon habitat suitability assessments in Scotland as part of Carbon Neutral Islands project

The findings of the study were used to develop a ‘ground-truthing’ strategy with several different survey techniques used to collect site specific data related to habitat extent, density, coverage, and the health of these blue carbon habitats.

This gathering of evidence would enable the production of more accurate and verified blue carbon habitat extent maps. These maps, together with evidence gathered on the carbon storage and sequestration values associated with different blue carbon habitats and oceanic sediment, can be used to better understand how much carbon is stored (in carbon sinks) and how much carbon is sequestered (the amount captured per year) by the marine environment surrounding each island.

Understanding the carbon storage of marine environments can help to inform marine policy and spatial planning, to prevent damage to these valuable habitats. Preserving and enhancing these habitats will encourage the sustainable use of marine spaces, protect vulnerable species, and help islands on the road to Net-Zero.

To define the blue carbon boundaries around each island, Adler and Allan established a 3-mile buffer around the coastline of each island (in line with the water framework) or determined the halfway point between two land masses – whichever was less.

We then conducted a detailed examination of available literature (both on a national and international scale) to collate the information available regarding relevant blue carbon ecosystems. This included the distribution and requirements of key species and habitats required for this study, as well as carbon density values and carbon sequestration rates.

Large gaps were identified in the available data which verified spatial extents of blue carbon habitats around the Scottish islands, with the exception of saltmarsh and seafloor sediment which had been extensively mapped. We therefore used what was available – presence point data - to model the probability of occurrence for habitats including seagrass, kelp, maerl, bryozoan thicket, serpulid reef, horse mussels, flame shells and brittlestar. Each species prefers different environmental conditions and can only thrive if the conditions are suitable. We assessed a suite of environmental variables (e.g. light, substrate, temperature, wave exposure, current, and dissolved oxygen concentrations) to provide an estimated probability of presence for each species and highlight areas where there is potential for the species to be introduced in the future.

The predictive model highlighted areas where there is a >90% likelihood that the conditions are suitable for each species. Following this, ground-truthing surveys were designed and targeted in the >90% areas to determine firstly; if the species was indeed present and, if so; in what condition. If the species was not present, other tests were carried out to determine if there were specific factors impacting the ability for that species to thrive – e.g. water quality, pollutants, pH, sediment types, microclimate etc.

Together with the carbon density and sequestration values, these habitat suitability maps make it possible to gain an understanding of the storage and sequestration potential of blue carbon habitats around each island.