Abstract
Rates and drivers of primary productivity are well understood for many terrestrial ecosystems, but remain poorly resolved for many marine ecosystems, particularly those within in coastal benthic environments. We quantified net primary productivity (NPP) using two methods as well as carbon standing stock within kelp forests (Laminaria hyperborea) at multiple subtidal habitats in the United Kingdom (UK). Study sites spanned 9° in latitude and encompassed a gradient in average temperature of ~ 2.5 °C. In addition to temperature, we measured other factors (e.g. light intensity, water motion, nutrients, sea urchin density) that may influence productivity. Although estimates of NPP were highly variable between sites, ranging from 166 to 738 g C m-2 yr-1, our study-wide average of 340 g C m-2 yr-1 indicated that L. hyperborea forests are highly productive. We observed clear differences between NPP and carbon standing stock between our cold northernmost sites and our warm southernmost sites, with NPP and standing stock being around 1.5 and 2.5 times greater in the northern sites, respectively. Ocean temperature was identified as a likely driver of productivity, with reduced NPP and standing stock observed in warmer waters. Light availability was also strongly linked with carbon accumulation and storage, with increased light levels positively correlated with NPP and standing stock. Across its geographical range, total NPP from L. hyperborea is estimated at ~ 7.61 Tg C yr-1. This biomass production is likely to be important for local food webs, as a trophic subsidy to distant habitats and for inshore carbon cycling and (potentially) carbon sequestration. However, given the strong links with temperature, continued ocean warming in the northeast Atlantic may reduce primary productivity of this foundation species, as optimal temperatures for growth and performance are surpassed.
Highlights
Primary productivity underpins most food webs and ecosystems on Earth, and as such, understanding rates, trends and drivers of primary production by autotrophs is a fundamental goal of ecology[1,2]
Despite the critical importance of L. hyperborea and the kelp forest habitats it creates in the northeast Atlantic, several important knowledge gaps pertaining to its biology and ecology still persist, including reliable estimates of net primary productivity (NPP) from multiple populations and across large spatial scales
Variation in NPP related to lamina extension was largely driven by differences in actual biomass accumulation rather than differences in elongation rate or plant density (Fig. S1)
Summary
Primary productivity underpins most food webs and ecosystems on Earth, and as such, understanding rates, trends and drivers of primary production by autotrophs is a fundamental goal of ecology[1,2]. Despite the critical importance of L. hyperborea and the kelp forest habitats it creates in the northeast Atlantic, several important knowledge gaps pertaining to its biology and ecology still persist, including reliable estimates of NPP from multiple populations and across large spatial scales It is clear, that L. hyperborea is a ‘season anticipator’ (sensu23) and exhibits peak growth rates in late winter through spring, when ambient nutrient levels are highest and not limiting[24,25], and that the species has a cool-temperate distribution, ranging from the Iberian Peninsula polewards to northern Norway and Iceland[16,26]. We quantified the standing stock of carbon held within
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