On the relationships between primary, net community, and export production in subtropical gyres

Abstract

It has been proposed that net primary production (NPP), net community production (NCP), particulate organic carbon export (Phi(POC)) and the relationships among them are governed by local environmental conditions that favor either a microbially dominated assemblage leading to a regeneration loop (low ratio of (Phi(POC) to NPP) or a system dominated by large plankton with export pathway characteristics (high ratio of (Dpoc to NPP). We analyze more than 10 years of data from two subtropical time-series stations (Hawaii Ocean Times-series (HOT) in the North Pacific, and Bermuda Atlantic Time-Series (BATS) in the North Atlantic) in order to investigate this regeneration loop versus export pathway hypothesis and in particular to test the idea that the switch between the two is controlled by enhanced input of nutrients. In the decadal long-term mean, the relationships between NPP, (Dpoc and NCP, which we take here as a proxy for export production, reveal export pathway characteristics at BATS, while HOT is dominated by the regeneration loop. This difference is consistent with the stronger seasonal forcing at BATS and the resulting higher new nutrient input. However, these characteristics are only valid for parts of the year. Especially at BATS, the export pathway exists only in spring and the system reverts to a regeneration loop in summer and fall. This is consistent with our hypothesis given the strong summer-time stratification and the resulting low levels of new nutrient input. On interannual time-scales, we find little evidence for statistically significant alterations of the long-term mean characteristics, a finding we ascribe to a combination of limited magnitude of forcing, length of the data records, and possibly an inherent lack of predictability. A comparison of our results for the ratio between NCP and NPP (e-ratio) and the ratio between Phi(POC) and NPP (pe-ratio) with those predicted by the models of Laws et al. [Temperature effects on export production in the open ocean. Global Biogeochemical Cycles 14(4), 1231-1246] and Dunne et al. [Empirical and mechanistic models for particle export ratio. Global Biogeochemical Cycles 19, GB40261 respectively, show reasonable agreement for the long-term mean, but these models fail to capture the observed interannual variability in these ratios. (c) 2006 Elsevier Ltd. All rights reserved.