Ⅰ. HIGHLIGHTS OF THE YEAR
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Variability in the net ecosystem productivity (NEP) of seaweed farms†
Y. Sato,∗1,∗2 G. N. Nishihara,∗3 A. Tanaka,∗4 and D. F. C. Belleza∗3The critical role of vegetated ecosystems in carbon sequestration has attracted substantial interest across various disciplines. With growing evidence of the potential of macroalgae ecosystems to capture carbon, there
is burgeoning interest in applying newfound knowledge ofcarbon capture rates to better understand this potential
for carbon sequestration. Seaweed farms are expected toplay a signiﬁcant role in carbon capture; advocates forexpanding 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 their potential as exporters of carbon is under debate. All in all, we present
our investigation on three seaweed farms, two Undaria
pinnatiﬁda (Wakame) farms in northern Japan (Matsushima Bay and Hirota Bay) and one Cladosiphon okamuranus (Mozuku) farm in southern Japan (Bise Point).
In addition, data were collected from two natural seaweed ecosystems (Arikawa Bay and Omura Bay) and one
degraded seaweed ecosystem (Tainoura Bay) in Nagasaki
Prefecture. We examined the frequency of autotrophic
days and compared potential carbon capture rates fromthese six geographic locations across Japan.
We estimated potential carbon capture rates by calculating the net ecosystem productivity (NEP) from continuous recordings of dissolved oxygen concentrations
under natural environmental conditions. Water temperature and dissolved oxygen concentrations were recordedwith dataloggers at a rate of one sample every ten minutes. In the natural and degraded ecosystems, one datalogger was placed directly above the sediment, another
50 cm above the sediment, and a third 50 cm below thewater surface (i.e., a total of three instruments and amean depth of 3 m for Omura Bay, 5 m for Arikawa
Bay, and 8 m for Tainoura Bay). The dataloggers at
the seaweed aquaculture farms were placed on the cultivation ropes and 1 m (Bise Point) and 2 m (HirotaBay and Matsushima Bay) below the cultivation ropes(i.e., a total of two instruments). The instruments wereplaced so that the majority of the biomass of the seaweed
was between the data loggers. Recordings were carriedout for four to twenty days before the instruments wereretrieved for maintenance and data oﬄoading. Calculations of productivity were based on the ensemble meanof the dissolved oxygen time series recorded by the dataloggers to account for vertical heterogeneity. The grossecosystem production (GEP) rates and ecosystem respiration (ER) rates were estimated from the diurnal ﬂuc†

∗1
∗2
∗3
∗4

Condensed from the article in Front. Mar. Sci. 9, 861932 (2022)
RIKEN Nishina CenterRiken Food Co., Ltd.
Organization of Marine Science Technology, Nagasaki University
Faculty of Science, University of Ryukyus

Fig. 1. Observed (colored points) and conditional mean of
the net ecosystem production (black symbols) rates ofthe study sites. The black horizontal lines indicate the95% highest density credible interval of the predicted values. Hirota Bay and Matsushima Bay are Wakame farms,
Bise Point is Mozuku farm, Arikawa Bay and Omura Bayare sites of the natural Sargassum spp. ecosystem, andTainoura Bay is a site where a seaweed ecosystem has degraded. The net ecosystem production in O2 and C are
calculated assuming a 1 to 1 molar ratio.

tuations of the dissolved oxygen concentration with theopen-water method (e.g., Hinode et al., 2020). Brieﬂy,
the NEP rate can be deﬁned as NEP = GEP + ER.
The NEP rates for the natural ecosystems in Arikawa
Bay and Omura Bay were equivalent to 0.043 and0.054 [g C m−2 d−1 ] m−1 , respectively (Fig. 1). Consering for the degraded ecosystem in Tainoura Bay, it
was −0.01 [g C m−2 d−1 ] m−1 . We noticed that theWakame farm in Matsushima Bay experiences autotrophy more often than natural ecosystems, although autotrophy of the Wakame farm in Hirota Bay and Mozuku
farm at Bise Point was less frequently observed. Nevertheless, up to 14.1 g C m−2 (0.110 g C m−2 d−1 ) wascaptured by the production of Wakame and 3.6 g C m−2(0.034 g C m−2 d−1 ) was captured by Mozuku, and thetotal yield of carbon captured during 2021 productionseason for these farms was 43,385 kg C. Seaweed farms
are still highly valuable for their contribution to thelivelihood of coastal communities and have lesser detrimental impact on the environment than other forms ofaquaculture (e.g., Visch et al., 2020). We believe thatthe potential for carbon capture will not be homogenous
across species and location, and that more studies oncarbon ﬂux at organismal and ecosystem scales remain
to be conducted.
References
1) K. Hinode et al., Phycol. Res. 68, 298 (2020).
2) W. Visch et al., J. Appl. Phycol. 32, 3199 (2020).

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