117
To date, the annual maximum biomass of Sargassum species in Japan has been
reported to be <1 to >4 kg dry weight/m
2
, and annual production to be 1 to >5 kg
dry weight/m
2
/year, but the amount varies depending on the species (and within the
same species) and habitat. However, the ratio of annual production to maximum
biomass (P/B MAX ratio) has been reported in the range of 1.0–1.9 for all Sargassum
species, and mostly within the narrower range of 1.0–1.5. Therefore, we can only
roughly estimate the annual production by determining the annual maximum biomass in the peak growing season of a Sargassum bed of a target species and multiplying it by the average P/B MAX ratio (Murase 2010).
However, we need to recognize the possibility of underestimating the production
of Sargassum with the productive structure analysis. This method is based on the
premise that newly produced tissues or organs are persistent, at least over the duration of the survey interval, and they accumulate on the upper part of the thallus. If
some or all of that part is subject to destruction through grazing by herbivorous
animals or through wave action, the lost part will not be accounted for in the production estimate (Murase 2010). Also, it is highly likely that, when rapid regeneration
of tissues and organs occurs after any partial loss of thalli, it will be overlooked in
the evaluation. In this case, the true P/B MAX ratio should be larger than the value
obtained in the survey. In past cases, production has been measured on the premise
that excessive grazing by animals or physical damage to thalli does not occur.
However, in recent years, the environment in coastal areas of Japan has been changing, and the grazing effects of herbivorous fish and invertebrates on macroalgal beds
cannot be ignored.
4.3.2.2 Biomass and Characteristics of Sargassum Species Along the
Coast of Japan
Unlike eelgrass beds where Z. marina is the sole constituent in most cases, many
species form Sargassum beds, and there are significant differences in biomass
between the species. Therefore, it is impossible to represent the biomass of
Sargassum beds in some regions with the biomass of a single species. Fortunately,
relatively large amounts of data on the biomass and species composition are available from ecological surveys of Sargassum beds. We collected biomass data of
Sargassum beds gathered from past surveys done during the peak growing season
(mainly, from winter to spring). Some of these data include biomass of macroalgae
other than Sargassum mixed in the beds (mostly small green and red algae), but it
was assumed that these “undergrowth” algae are quantitatively negligible. For the
biomass, 20% of the wet weight was used as the dry weight of Sargassum.
Figure 4.9 shows the biomass distribution of Sargassum beds in the Japan Sea,
Pacific Honshu, Seto Inland Sea, and open Kyushu-Shikoku regions. Data for beds
of both single and mixed species were available. The mean biomass was 1.2 kg dry
weight/m
2
in the Seto Inland Sea, 0.7 kg dry weight/m
2
in the Pacific Honshu region,
and about 1.0 kg dry weight/m
2
in the other two regions. There was a large variation
in the biomass in every region, and it was difficult to identify clear differences
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…
To date, the annual maximum biomass of Sargassum species in Japan has been
reported to be <1 to >4 kg dry weight/m
2
, and annual production to be 1 to >5 kg
dry weight/m
2
/year, but the amount varies depending on the species (and within the
same species) and habitat. However, the ratio of annual production to maximum
biomass (P/B MAX ratio) has been reported in the range of 1.0–1.9 for all Sargassum
species, and mostly within the narrower range of 1.0–1.5. Therefore, we can only
roughly estimate the annual production by determining the annual maximum biomass in the peak growing season of a Sargassum bed of a target species and multiplying it by the average P/B MAX ratio (Murase 2010).
However, we need to recognize the possibility of underestimating the production
of Sargassum with the productive structure analysis. This method is based on the
premise that newly produced tissues or organs are persistent, at least over the duration of the survey interval, and they accumulate on the upper part of the thallus. If
some or all of that part is subject to destruction through grazing by herbivorous
animals or through wave action, the lost part will not be accounted for in the production estimate (Murase 2010). Also, it is highly likely that, when rapid regeneration
of tissues and organs occurs after any partial loss of thalli, it will be overlooked in
the evaluation. In this case, the true P/B MAX ratio should be larger than the value
obtained in the survey. In past cases, production has been measured on the premise
that excessive grazing by animals or physical damage to thalli does not occur.
However, in recent years, the environment in coastal areas of Japan has been changing, and the grazing effects of herbivorous fish and invertebrates on macroalgal beds
cannot be ignored.
4.3.2.2 Biomass and Characteristics of Sargassum Species Along the
Coast of Japan
Unlike eelgrass beds where Z. marina is the sole constituent in most cases, many
species form Sargassum beds, and there are significant differences in biomass
between the species. Therefore, it is impossible to represent the biomass of
Sargassum beds in some regions with the biomass of a single species. Fortunately,
relatively large amounts of data on the biomass and species composition are available from ecological surveys of Sargassum beds. We collected biomass data of
Sargassum beds gathered from past surveys done during the peak growing season
(mainly, from winter to spring). Some of these data include biomass of macroalgae
other than Sargassum mixed in the beds (mostly small green and red algae), but it
was assumed that these “undergrowth” algae are quantitatively negligible. For the
biomass, 20% of the wet weight was used as the dry weight of Sargassum.
Figure 4.9 shows the biomass distribution of Sargassum beds in the Japan Sea,
Pacific Honshu, Seto Inland Sea, and open Kyushu-Shikoku regions. Data for beds
of both single and mixed species were available. The mean biomass was 1.2 kg dry
weight/m
2
in the Seto Inland Sea, 0.7 kg dry weight/m
2
in the Pacific Honshu region,
and about 1.0 kg dry weight/m
2
in the other two regions. There was a large variation
in the biomass in every region, and it was difficult to identify clear differences
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…