Nutrient budgets and carbon export in the subtropical North Atlantic gyre

Susanne Neuer, Arizona State University, susanne.neuer@asu.edu (Presenting)
Peer Helmke, Arizona State University, peer.helmke@asu.edu
Andres Cianca, Instituto Canario de Ciencias Marinas, andres@iccm.rcanaria.es

Long term time series data offer the opportunity to test hypotheses regarding our current paradigms in ocean biogeochemistry. In his project we investigated the cause of the observed difference in export production between the two subtropical North Atlantic gyre time series stations, BATS and ESTOC, situated at about the same latitude but on opposite sides of the gyre. The two stations exhibit similar seasonality in phytoplankton biomass and productivity but export production is higher at BATS by a factor 2-3 (when measured with 500 m moored traps) to 3-4 times (when measured with surface tethered traps). We investigated in this project if A) new nutrient availability was higher at BATS compared to ESTOC, and B) if other factors could cause the higher export production, such as differences in particle ballasting and/or species composition and decay rate of organic carbon.



Cianca et al. (JGR Oceans, 2007) found that nutrient availability at BATS is about 1.4 times higher compared to ESTOC, mainly due to a greater eddy-induced nutrient input. This larger supply of new nutrients could partially explain the higher export production at BATS at the 500 m reference depth. In addition, nitrogen fixation might be an additional nutrient source at BATS (albeit absolute magnitude remains unresolved) but not at ESTOC, which aids in closing the budget. Additionally, when comparing surface tethered traps at ESTOC and BATS we found that indeed the decay rate was higher (more negative) in the upper 500 m at BATS compared to ESTOC and that a difference in community composition is likely, i.e., more “free” i.e. labile or un-ballasted POC is sinking at BATS compared to ESTOC. This may be due to differences in community composition and contribution of non ballasted phytoplankton groups contributing relatively more to downward flux at BATS compared to ESTOC.