Comparison of Productivity, Plankton Types and Carbon Export Mechanisms in two Different Regimes of Subtropical North Atlantic: a Modeling Study

Improved structure and mechanisms of carbon export and sequestration within marine ecosystem models is vital to better understand and predict changes in the global carbon cycle. We have implemented a 1D lower trophic ecosystem model at long-term time-series stations (BATS and ESTOC) in the North Atlantic for the years 1996-2000. We have investigated the dynamics of the productivity and carbon export, and mechanisms regulating them. Our simulations agree with the previous observations that show similar productivity levels at both sites (126.48 gC/m2/y and 92.76 gC/m2/y at BATS and ESTOC respectively). Results indicate that intense vertical mixing at BATS maintains similar growth despite lesser nutrient availability compared to ESTOC. Results showed a similar community structure with cyanobacteria and picoeukaryotes dominating the phytoplankton biomass and supporting production at both sites. Simulations showed more diatom biomass at ESTOC with ~6% of total phytoplankton biomass vs ~2% at BATS. Zooplankton input to detritus through mortality and unassimilated grazing dominates the carbon export at both sites (~46%) followed by picoeukaryotes (~18%). Our reference simulations are successful in reproducing the carbon export at BATS (model: 6.51 gC/m2/y at 200 m), however failing to represent the 3-5 fold lower export at ESTOC (model: 4.89 gC/m2/y). Our sensitivity analyses suggest that further advancement of recycling algoritms is necessary to capture lower e-ratios at ESTOC. Variable detrital sinking rates at depth and zooplankton diel vertical migration (DVM) additions to the algorithm were applied and the DVM application increased the carbon export by 2-fold. DVM increased export rates at depths ~200-300 m due to active transport, thus this increase should be balanced by variable sinking rates, or increased recycling. We have introduced enhanced recycling by increased excretion of zooplankton and observed improvements in decreasing export near the surface at ESTOC.