Abstract
Abstract. The very clear waters of the South Pacific Gyre likely constitute an end-member of oligotrophic conditions which remain essentially unknown with respect to its impact on carbon fixation and exportation. We describe a non-intrusive bio-optical method to quantify the various terms of a production budget (Gross community production, community losses, net community production) in this area. This method is based on the analysis of the diel cycle in Particulate Organic Carbon (POC), derived from high frequency measurements of the particle attenuation coefficient cp. We report very high integrated rates of Gross Community Production within the euphotic layer (average of 846±484 mg C m−2 d−1 for 17 stations) that are far above any rates determined using incubation techniques for such areas. Furthermore we show that the daily production of POC is essentially balanced by the losses so that the system cannot be considered as net heterotrophic. Our results thus agree well with geochemical methods, but not with incubation studies based on oxygen methods. We stress to the important role of deep layers, below the euphotic layer, in contributing to carbon fixation when incident irradiance at the ocean surface is high (absence of cloud coverage). These deep layers, not considered up to know, might fuel part of the heterotrophic processes in the upper layer, including through dissolved organic carbon. We further demonstrate that, in these extremely clear and stratified waters, integrated gross community production is proportional to the POC content and surface irradiance via an efficiency index ψ GCP*, the water column cross section for Gross Community Production. We finally discuss our results in the context of the role of oligotrophic gyre in the global carbon budget and of the possibility of using optical proxies from space for the development of growth community rather than primary production global models.
Highlights
Subtropical gyres are vast oceanic oligotrophic biomes representing 60% of the global ocean. Their metabolic state and, their role in the global oceanic carbon cycle is still being debated which finds its roots in early discussions about the magnitude of carbon fixation in these systems (Jenkins, 1982; Lewis et al, 1986; Jenkins, 1988)
Based on in vitro biological techniques performed in the euphotic zone and having characteristic time scales of ∼1 day, it is generally concluded that the ocean is in a net heterotrophic state: rates of carbon fixation are low and the whole community respiration rate (R) exceeds the gross primary production (GPP), so that the so-called Net Community Production (NCP=GPP-R) is always negative
In the vicinity of Marquesas islands and associated with the Peru Chile Current, high cp values are recorded in the upper layers
Summary
Subtropical gyres are vast oceanic oligotrophic biomes representing 60% of the global ocean Their metabolic state (magnitude of carbon fixation vs respiration rates) and, their role in the global oceanic carbon cycle is still being debated (del Giorgio et al, 1997; Williams, 1998) which finds its roots in early discussions about the magnitude of carbon fixation in these systems (Jenkins, 1982; Lewis et al, 1986; Jenkins, 1988). Studies based on geochemical tracer distributions with larger relevant space/time scales support net production and export rates one order of magnitude higher (Jenkins, 1982; Jenkins, 1988; Lewis et al, 1986; Hansell et al, 2004). Modelling studies conclude that mesoscale activity and the so-called eddy-pumping processes cannot explain the discrepancies between both approaches
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