The Scottish saltmarsh survey (SSS) was a national survey which was completed in 2016 and details the areal extent of NVC classes across all saltmarshes greater than >0.03 km2 in area (Haynes, 2016). Field surveys for the SSS were undertaken between 2010-2012 by Nature Scotland (formerly known as Scottish Natural Heritage) on all saltmarshes > 0.03 km2 in size, with an additional 10 perched and 25 small marshes (<0.003km2) also included in the survey (Haynes, 2016). In recognition that the SSS took place some time ago, a ± 5% error has been applied to the areal extent of each NVC class and to the area of each saltmarsh as a whole, potentially accounting for possible erosion or accretion in the last decade (Ladd et al., 2019). By applying this error term, the uncertainties in marsh extent will be better represented in the calculations and propagated into the final soil OC stock estimates.
The perched saltmarshes are generally found on cliffs where salt spray from the ocean sustains the salt tolerant vegetation (Fig.6). While saltmarsh vegetation is present, these habitats are largely devoid of any significant underlying OM-rich soil and are therefore excluded from this analysis (Haynes, 2016; Porter et al., 2020). The SSS identified 226 saltmarshes under 0.03 km2 across Scotland (Haynes, 2016). In total, 25 of these small saltmarshes (<0.03 km2) were surveyed; these ranged between 0.001km2 and 0.03 km2 in size. To best estimate the OC stored in the soil of the 201 unmapped small saltmarshes, three approaches were taken.
(1) Use of an idealised (i.e. normal) distribution across the 0 - 0.03 km2 range.
(2) Utilise the distribution of the areal extent of the 25 surveyed small saltmarshes.
(3) Assume that all the saltmarshes were 0.03 km2 in size.
The mean, standard deviation, min, max area values were determined from each distribution. While NVC data for the majority of these small area saltmarshes are unavailable, the approach adopted provides a range of estimates that will be useful in constraining the total soil OC stock of all Scottish saltmarshes and avoids an inevitable sampling bias towards Scotland’s larger saltmarshes (>0.03 km2).