Recently, Kim et al. (2014a, 2015a) cultivated Gracilaria tikvahiae and
Saccharina latissima in open water nutrient bioextraction farms in Long Island
Sound (LIS) and the Bronx River Estuary (BRE), and evaluated the nutrient
bioextraction performance in urbanized estuaries. They estimated that the biomass
yields of G. tikvahiae and S. latissima could be up to 21 and 62 metric tons fresh
weight per hectare, respectively. The potential nitrogen removal could exceed
320 kg N ha
−1 year
−1 from LIS (280 kg ha
−1 from Saccharina and 40 kg ha
−1
from Gracilaria) and 430 kg N ha
−1 year
−1 from the BRE site (280 kg ha
−1 from
Saccharina and 150 kg ha
−1 from Gracilaria).
Seaweed is also an important CO 2 sink and the duration of net CO 2 removal can
be extended if the biomass is used in environmentally friendly ways (Chung et al.
2013). Kim et al. (2014a, 2015a) estimated that Gracilaria and Saccharina could
sequester up to 300 (LIS) and 727 kg C ha
−1 (BRE), and 1800 (LIS) and
1350 kg C ha
−1 (BRE), respectively. Together, over 2000 kg C ha
−1 could be
removed by alternating these two species at the seaweed farm sites. An economic
value of nutrient bioextraction was estimated. Considering the most recent nutrient
credit values in the USA for these two elements (US $12.37 kg
−1 N, US $6.00–US
$60.00 mt
−1 C (as CO 2 ), Stephenson and Shabman 2011; CDP 2013; CT DEEP
2014; Tedesco et al. 2014), the potential economic values of C and N removal could
exceed $3000 ha
−1
, which could be additional income for seaweed growers beyond
the value of seaweed products, if seaweed aquaculture is incorporated in the
Connecticut Nitrogen Credit Trading Program and a carbon-pricing scheme (CDP
2013).
Considering the global seaweed production and tissue carbon and nitrogen
contents for each species, total extractive nitrogen and carbon by seaweed aquaculture can be estimated. Considering a 10:1 DW:FW ratio and average values of
nitrogen (Pyropia/Porphyra: 5.5%, Gracilaria: 3.0%, Kappaphycus/Eucheuma:
1.7%, kelp: 2.0% and Sargassum: 4.1%) and carbon (Pyropia/Porphyra: 38%,
Gracilaria: 28%, Kappaphycus/Eucheuma: 29%, kelp: 30% and Sargassum: 34%),
the total nitrogen and carbon removal by these five major aquaculture groups is
approximately 54 million t of nitrogen per year and 700 million t of carbon per
year (equivalent to 2600 million t of CO 2 ), respectively (Asare and Harlin 1983;
Gerard 1997; Schaffelke and Klumpp 1998; Gevaert et al. 2001; Schaffelke 2001;
Chung et al. 2002; Rawson et al. 2002; Sahoo and Ohno 2003; Dean and Hurd
2007; Kim et al. 2007, 2014a, 2015a; Buschmann et al. 2008; Abreu et al. 2009;
Robertson-Andersson et al. 2009; Levine and Sahoo 2010; Broch et al. 2013). This
is, in fact, a significant amount of carbon and nitrogen removal. In 2013, global
nitrogen discharge to coastal waters and the open ocean via leaching and riverine
transport was estimated to be up to 70 billion t (Fowler et al. 2013). During the
same period, carbon emissions due to fossil fuel use (and cement production) was
9.9 billion t. Seaweed aquaculture during the same period has removed approximately 0.13% of nitrogen discharge from leaching and riverine transport and 6.6%
of carbon emission via fossil fuel use.
The extractive sequestration of nutrients by seaweeds provides ecosystem services that need to be recognized and valued appropriately (Chopin 2014). Much has
56
B.H. Buck et al.
Saccharina latissima in open water nutrient bioextraction farms in Long Island
Sound (LIS) and the Bronx River Estuary (BRE), and evaluated the nutrient
bioextraction performance in urbanized estuaries. They estimated that the biomass
yields of G. tikvahiae and S. latissima could be up to 21 and 62 metric tons fresh
weight per hectare, respectively. The potential nitrogen removal could exceed
320 kg N ha
−1 year
−1 from LIS (280 kg ha
−1 from Saccharina and 40 kg ha
−1
from Gracilaria) and 430 kg N ha
−1 year
−1 from the BRE site (280 kg ha
−1 from
Saccharina and 150 kg ha
−1 from Gracilaria).
Seaweed is also an important CO 2 sink and the duration of net CO 2 removal can
be extended if the biomass is used in environmentally friendly ways (Chung et al.
2013). Kim et al. (2014a, 2015a) estimated that Gracilaria and Saccharina could
sequester up to 300 (LIS) and 727 kg C ha
−1 (BRE), and 1800 (LIS) and
1350 kg C ha
−1 (BRE), respectively. Together, over 2000 kg C ha
−1 could be
removed by alternating these two species at the seaweed farm sites. An economic
value of nutrient bioextraction was estimated. Considering the most recent nutrient
credit values in the USA for these two elements (US $12.37 kg
−1 N, US $6.00–US
$60.00 mt
−1 C (as CO 2 ), Stephenson and Shabman 2011; CDP 2013; CT DEEP
2014; Tedesco et al. 2014), the potential economic values of C and N removal could
exceed $3000 ha
−1
, which could be additional income for seaweed growers beyond
the value of seaweed products, if seaweed aquaculture is incorporated in the
Connecticut Nitrogen Credit Trading Program and a carbon-pricing scheme (CDP
2013).
Considering the global seaweed production and tissue carbon and nitrogen
contents for each species, total extractive nitrogen and carbon by seaweed aquaculture can be estimated. Considering a 10:1 DW:FW ratio and average values of
nitrogen (Pyropia/Porphyra: 5.5%, Gracilaria: 3.0%, Kappaphycus/Eucheuma:
1.7%, kelp: 2.0% and Sargassum: 4.1%) and carbon (Pyropia/Porphyra: 38%,
Gracilaria: 28%, Kappaphycus/Eucheuma: 29%, kelp: 30% and Sargassum: 34%),
the total nitrogen and carbon removal by these five major aquaculture groups is
approximately 54 million t of nitrogen per year and 700 million t of carbon per
year (equivalent to 2600 million t of CO 2 ), respectively (Asare and Harlin 1983;
Gerard 1997; Schaffelke and Klumpp 1998; Gevaert et al. 2001; Schaffelke 2001;
Chung et al. 2002; Rawson et al. 2002; Sahoo and Ohno 2003; Dean and Hurd
2007; Kim et al. 2007, 2014a, 2015a; Buschmann et al. 2008; Abreu et al. 2009;
Robertson-Andersson et al. 2009; Levine and Sahoo 2010; Broch et al. 2013). This
is, in fact, a significant amount of carbon and nitrogen removal. In 2013, global
nitrogen discharge to coastal waters and the open ocean via leaching and riverine
transport was estimated to be up to 70 billion t (Fowler et al. 2013). During the
same period, carbon emissions due to fossil fuel use (and cement production) was
9.9 billion t. Seaweed aquaculture during the same period has removed approximately 0.13% of nitrogen discharge from leaching and riverine transport and 6.6%
of carbon emission via fossil fuel use.
The extractive sequestration of nutrients by seaweeds provides ecosystem services that need to be recognized and valued appropriately (Chopin 2014). Much has
56
B.H. Buck et al.