Detrital carbon production and export in high latitude kelp forests

Abstract

The production and fate of seaweed detritus is a major unknown in the global C-budget. Knowing the quantity of detritus produced, the form it takes (size) and its timing of delivery are key to understanding its role as a resource subsidy to secondary production and/or its potential contribution to C-sequestration. We quantified the production and release of detritus from 10 Laminaria hyperborea sites in northern Norway (69.6ºN). Kelp biomass averaged 770 ± 100 g C m-2 while net production reached 499 ± 50 g C m-2 y-1 , with most taking place in spring when new blades were formed. Production of biomass was balanced by a similar formation of detritus (478 ± 41 g C m-2 y-1), and both were unrelated to wave exposure when compared across sites. Distal blade erosion accounted for 23% of the total detritus production and was highest during autumn and winter, while dislodgment of whole individuals and/or whole blades corresponded to 24% of the detritus production. Detachment of old blades constituted the largest source of kelp detritus, accounting for >50% of the total detrital production. Almost 80% of the detritus from L. hyperborea was thus in the form of whole plants or blades and > 60% of that was delivered as a large pulse within 1-2 months in spring. The discrete nature of the delivery suggests that the detritus cannot be retained and consumed locally, and that some is exported to adjacent deep areas where it may subsidize secondary production or become buried into deep marine sediments as blue carbon.

Marine heatwaves and the collapse of marginal North Atlantic kelp forests

“…For all recovered kelp, divers measured the blade length and the distance from the meristem to each of the punched holes. Growth was calculated as the increase in distance from the 5 cm hole to the meristem between sampling times (for tattered blades the 10 cm hole was used) 67 . Erosion was calculated as the difference between the blade length between the two sampling times, accounting for any growth at the basal end.…”

Macroalgal metabolism and lateral carbon flows can create significant carbon sinks

“…Macroalgal beds are considered to be potential carbon-donor sites in the context of blue carbon sequestration (Krause-Jensen et al, 2018). The release and subsequent export of particulate macroalgal carbon (e.g., entire thalli and fragments) via physical processes would contribute to CO 2 sequestration (Krause-Jensen and Filbee-Dexter et al, 2018;Pessarrodona et al, 2018;Kokubu et al, 2019;Pedersen et al, 2020;Fig. 2).…”

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

“…This is likely driven by differences in stipe morphology with L. hyperborea possessing a much larger, heavier and more rigid stipe compared to L. digitata. Annual estimates of NPP ranged from 135 to 402 g C m −2 year −1 , with a mean of 262 g C m −2 year −1 , which is again markedly lower than estimates for L. hyperborea in the NE Atlantic (Kain, 1971;Sheppard et al 1978;Pedersen et al 2020) and for many other macroalgal stands globally (Krause-Jensen and Duarte, 2016). In temperate regions of the NE Atlantic, L. digitata is restricted to a narrow band within low intertidal/ shallow subtidal habitats, as it is competitively inferior to L. hyperborea in most subtidal environments (Hawkins and Harkin, 1985).…”

“…However, this is considerably lower than standing stock estimates for many other kelp populations, including the subtidal congeneric species, Laminaria hyperborea, in the NE Atlantic (e.g. 721 g C m −2 , Smale et al 2016; 770 g C m −2 , Pedersen et al 2020). This is likely driven by differences in stipe morphology with L. hyperborea possessing a much larger, heavier and more rigid stipe compared to L. digitata.…”

“…Rocky coastlines in the northeast (NE) Atlantic region are dominated by kelp forests, which are thought to make a substantial contribution to coastal primary production (Jupp and Drew 1974;Kain 1979;Pessarrodona et al 2018;Pedersen et al 2020). However, the current state of knowledge on the dynamics of NE Atlantic kelp forests is limited, as they have been chronically understudied compared to some other temperate regions .…”