Variability in the Net Ecosystem Productivity (NEP) of Seaweed Farms

Abstract

The important role of vegetated ecosystems in the sequestration of carbon has gained strong interest across a wide variety of disciplines. With evidence growing of the potential for macroalgae ecosystems to capture carbon, there is burgeoning interest in applying newfound knowledge of carbon capture rates to better understand the potential for carbon sequestration. Seaweed farms are expected to play a significant role in carbon capture; advocates for the expansion of seaweed farms are increasing in many countries. In general, seaweed farms are expected to be highly productive, although whether they are autotrophic or heterotrophic ecosystems and hence potential exporters of carbon, is under debate. Therefore, we present our investigation of three seaweed farms, two in northern Japan and one in southern Japan. We examine the frequency of autotrophic days and compare potential rates of carbon capture of the seaweed farms with two natural macroalgae ecosystems and one degraded site. We estimated potential carbon capture rates by calculating the net ecosystem productivity from continuous recordings of dissolved oxygen concentrations under natural environmental conditions. The net ecosystem production rates for the natural ecosystems in Arikawa Bay and Omura Bay were equivalent to 0.043 and 0.054 [g C m-2 d-1] m-1, respectively. Whereas, for the degraded ecosystem in Tainoura Bay, it was -0.01 [g C m-2 d-1] m-1. We reveal that the Undaria pinnatifida farm in Matsushima Bay experience autotrophy more often than natural ecosystems, although for seaweed farms producing U. pinnatifida in Hirota Bay and Cladospihon okamuranus at Bise Point, autotrophy was less frequently observed. Nevertheless, up to 14.1 g C m-2 (0.110 g C m-2 d-1) was captured by the production of U. pinnatifida and 3.6 g C m-2 (0.034 g C m-2 d-1) was captured by C. okamuranus, and the total yield of carbon captured during 2021 production season for these farms was 43,385 kg C.

Nitrogen and Carbon Removal Capacity by Farmed Kelp Alaria marginata and Saccharina latissima Varies by Species

“…This is necessary because the margin between kelp farms serving as nutrient sinks versus nutrient sources is thin. In a study monitoring carbon flux in three Japanese kelp farms [36], results as to whether a kelp farm is a sink or source for carbon were mixed and suggested that it depends on how long the condition of site allows the kelp to live in an autotrophic versus heterotrophic state. In our study, the low-nitrogen condition of the sites likely shortens the time in which those farms serve as sinks, and we propose that the high-nitrogen demand of A. marginata can further shorten that window.…”

Stipe of the kelp Eisenia bicyclis as a long-lasting carbon sink

“…In recent years, seaweeds including macroalgae have become a global focus, particularly because of their inclusion within coastal plant communities (Duarte et al 2013) and the role they play in marine carbon storage and sequestration (Krause-Jensen and Duarte 2016). Seaweeds store and increase coastal carbon capacity (Sato et al 2022), adding to the coastal assimilation of carbon (Muraoka 2004;Sondak et al 2017;Ould and Caldwell 2022). Due to their e ciency as primary producers (Mann 1973; Pessarrodona et al 2022), seaweed carbon assimilation accounts for 70% of coastal assimilation despite the relatively small global area of coastal habitats (Nellemann et al 2009).…”

Density regulation of aquaculture production and its effects on commercial profit and quality as food in the cosmopolitan edible seaweed Undaria pinnatifida

“…Large brown macroalgae, including kelp (Order Laminariales) and fucoids (Order Fucales), are dominant taxa in subpolar and temperate reef ecosystems and are known to be highly productive (Steneck et al, 2002). Moreover, increasing evidence suggests their potential for use in carbon sequestration and therefore contribution to mitigating climate change (Fillbee-Dexter and Wernberg, 2020;Watanabe et al, 2020;Sato et al, 2022). However, regional kelp forests dominated by perennial species are declining under ocean warming in temperate areas (Smale, 2020).…”