Bivalve-enhanced nitrogen removal from coastal estuaries

Abstract

Interest in use of bivalves to remediate estuarine eutrophication has increased in recent years. High variation among data sets, substantial focus on particle removal, and insufficient links to anthropogenic nitrogen (N) sources encouraged this empirical examination of N removal by bivalves from estuaries receiving different N loads. We determined the capacity of the oyster Crassostrea virginica to remove N by comparing N assimilated into tissues with anthropogenic N from land or available in phytoplankton. Oyster growth yielded 0.2-0.4 g N in tissues and depended on estuary-specific conditions. d 15 N values confirmed that N in oyster tissues derived from local anthropogenic sources. At representative restoration and aquaculture conditions (≤400 oysters·m -2 at 0.5%-1.0% of estuary area), estimated N removal was ≤15% of landderived loads and <1% of phytoplankton N. N removal via biogeochemical processes was negligible during grow-out, but became important after oysters attained harvestable size. This study explicitly demonstrates that oysters assimilated landderived N, but suggests that bivalve bioremediation consider trade-offs between intensity of planting, ecological effects, and available space.Résumé : Le recours aux bivalves pour contrer l'eutrophisation estuarienne suscite un intérêt croissant ces dernières années. La variabilité élevée d'un ensemble de données à l'autre, un accent important sur le retrait de particules et l'insuffisance de liens établis vers des sources anthropiques d'azote (N) ont motivé l'examen empirique du retrait de l'azote par des bivalves d'estuaires qui reçoivent différentes charges d'azote. Nous avons déterminé la capacité de l'huître, Crassostrea virginica, de retirer l'azote en comparant le N assimilé dans leurs tissus au N anthropique de source terrestre ou disponible dans le phytoplancton. La croissance d'une huître produit de 0,2 g à 0,4 g de N dans les tissus, selon les conditions propres à l'estuaire. Les valeurs de d 15 N ont confirmé que le N dans les tissus d'huîtres provient de sources anthropiques locales. Dans des conditions de restauration et d'aquaculture représentatives (≤400 huîtres·m -2 sur de 0,5 % à 1,0 % de la superficie de l'estuaire), le taux de retrait estimé de l'azote était de ≤15 % des charges de sources terrestres et de <1 % du N provenant du phytoplancton. Le retrait du N par des processus biogéochimiques s'est avéré négligeable durant le grossissement, mais devenait important après que les huîtres aient atteint une taille commerciale. L'étude démontre explicitement que les huîtres ont assimilé du N de source terrestre, mais suggère que la biorestauration à l'aide de bivalves devrait tenir compte des compromis entre l'intensité de l'ensemencement, les effets écologiques et l'espace disponible.[Traduit par la Rédaction]

Denitrification and nutrient assimilation on a restored oyster reef

“…Previous work on wild oysters provides estimates of total nitrogen and phosphorus content that are 5 and 97% greater, respectively, than the values in the present study despite smaller shell heights (Newell 2004). Estimates of nitrogen in the soft tissue of oysters with a shell height of 76.2 mm given by Carmichael et al (2012) range from 16% lower than our values to 60% higher. Some of the variation in estimates of nutrient content per oyster in these studies stems from differences in the percent nitrogen and phosphorus estimates used in calculations, but most results from the differences in dry weights of oyster tissue and/or shell between studies.…”

“…Average standing stock biomass, assimilated nitrogen and assimilated phosphorus at the restored site were 2 to 3 orders of magnitude higher than at the control site. The percentage of nitrogen in oyster soft tissues in the present study were 8 to 32% higher than those previously reported (Newell 2004, Higgins et al 2011, Carmichael et al 2012, while the percentages of phosphorus were 52 to 58% higher (Newell 2004, Higgins et al 2011. Percentages of nitrogen and phosphorus in oyster shell in the present study were 30 and 60% lower, respectively, than previously reported for wild oysters (Newell 2004), but fell within the range of values reported for oyster aquaculture (Higgins et al 2011).…”

“…Regardless of how long nutrients are retained within reef organisms and the structures they create, the δ 15 N signatures of benthic macrofauna commonly increase with increasing anthropogenic nitrogen load, confirming that they assimilate anthropogenically derived nitrogen from their food sources (e.g. Martinetto et al 2006, Carmichael et al 2012. To the extent that this nitrogen flows through benthic and pelagic food webs without being remineralized, it will reduce the pool of nitrogen available for phytoplankton growth.…”

“…Prior studies of oyster assimilation have used selected strains and/or grown oysters under environmental conditions that differ from natural or restored oyster reefs (Higgins et al 2011, Carmichael et al 2012. Prior studies documenting the macrofaunal community found on restored subtidal oyster reefs (e.g.…”

Potential impacts of the 2010 Deepwater Horizon oil spill on subtidal oysters in the Gulf of Mexico

“…For instance, oysters enhance the recruitment and growth of economically valuable and ecologically important finfish and crustaceans, thereby augmenting the productivity of these species (Coen et al 1999, Breitburg et al 2000, Harding & Mann 2001, Soniat et al 2004, Grabowski et al 2005, Tolley & Volety 2005. Oyster reefs concentrate bottom deposits of feces that promote bacterially mediated denitrification, thereby counteracting anthropogenic nitrogen loading (Newell et al 2002, Piehler & Smyth 2011, Carmichael et al 2012, Kellogg et al 2013, Smyth et al 2013. When oyster reefs filter the water and enhance light penetration, they promote other valuable estuarine habitats such as submerged aquatic vegetation (Newell 1988, Everett et al 1995, Newell & Koch 2004, Carroll et al 2008, Wall et al 2008).…”