Field scale evaluation of seaweed aquaculture as a nutrient bioextraction strategy in Long Island Sound and the Bronx River Estuary

Nutrient bioextraction using Gracilaria tikvahiae McLachlan was tested at two sites: one off Fairfield, CT (LIS), and the other at the mouth of the Bronx River Estuary (BRE), during the summer and fall of 2011 and 2012. The estimates of nitrogen (N) removal by Gracilaria over a 90-day growing season were up to 28 and 94 kg N ha^-1 at the LIS and BRE sites, respectively. In July 2012, Gracilaria grew up to 16.5% day^-1 at BRE and 4.8% day^-1 at the LIS site. Tissue N contents at the same periods were 3.7% (BRE) and 1.5% (LIS), respectively. These results demonstrate rapid assimilation of nutrients fueling the growth of new Gracilaria tissue at the BRE site, while nutrients appeared to limit growth at the LIS site during the summer months. The estimated C removal by Gracilaria at the BRE and LIS sites were up to 300 kg ha^-1 (LIS) and 727 kg ha^-1 (BRE), respectively. The potential economic values of N and C sequestration for the period examined in this study were as high as $311 (LIS) and $940 ha^-1 (BRE) for N, and $5.51 (LIS) and $13.32 ha^-1 (BRE) for C if seaweed aquaculture would be included in Connecticut's Nitrogen Trading Program. This represents a potential additional economic incentive for seaweed growers, beyond the direct value of seaweed products. The findings in this study showed that seaweed (Gracilaria) aquaculture can be a useful technique for nutrient bioextraction in urbanized coastal waters, such as the estuaries of New York City (BRE) and Long Island Sound.