Ecological performance differs between range centre and trailing edge populations of a cold water kelp: implications for estimating net primary productivity

Kelp forests are extensive, widely distributed and highly productive. The vast majority of kelp-derived organic matter enters the marine environment as detritus. Emerging evidence suggests a considerable proportion of this detritus is sequestered in long-term sinks, yet the importance of kelp within local and global carbon cycles remains poorly resolved. Here, we examined how individual performance and site-level Net Primary Productivity (NPP) varied between range-centre and trailing-edge populations of Laminaria digitata in the northeast Atlantic. We examined growth and performance of L. digitata at six UK sites, with three sites nested within both range centre and trailing-edge regions, encompassing a temperature gradient of ~2.5°C. During the peak growth season, range-centre individuals were up to three times heavier and accumulated biomass twice as fast as their trailing-edge counterparts. This was not apparent during the reduced growth season, when populations within both regions had similar biomass profiles. In total, annual NPP estimates were considerably lower for trailing-edge (172 ± 31 g C m^-2 yr^-1) compared to range-centre (325 ± 37 g C m^-2 yr^-1) populations. Our first-order UK estimates of total standing stock and NPP for L. digitata suggest this species makes a significant contribution to coastal carbon cycling. Further work determining the ultimate fate of this organic matter is needed to understand the overall contribution of kelp populations to regional and global carbon cycles. Nevertheless, we highlight the need for large-scale sampling across multiple populations and latitudes to accurately evaluate kelp species’ contributions to coastal carbon cycling.