Abstract
Abstract. The fate of the organic matter (OM) produced by marine life controls the major biogeochemical cycles of the Earth's system. The OM produced through photosynthesis is either preserved, exported towards sediments or degraded through remineralisation in the water column. The productive eastern boundary upwelling systems (EBUSs) associated with oxygen minimum zones (OMZs) would be expected to foster OM preservation due to low O2 conditions. But their intense and diverse microbial activity should enhance OM degradation. To investigate this contradiction, sediment traps were deployed near the oxycline and in the OMZ core on an instrumented moored line off Peru. Data provided high-temporal-resolution O2 series characterising two seasonal steady states at the upper trap: suboxic ([O2]<25 µmol kg−1) and hypoxic–oxic (15<[O2]<160 µmol kg−1) in austral summer and winter–spring, respectively. The OMZ vertical transfer efficiency of particulate organic carbon (POC) between traps (Teff) can be classified into three main ranges (high, intermediate, low). These different Teff ranges suggest that both predominant preservation (high Teff>50 %) and remineralisation (intermediate Teff 20 < 50 % or low Teff<6 %) configurations can occur. An efficient OMZ vertical transfer (Teff>50 %) has been reported in summer and winter associated with extreme limitation in O2 concentrations or OM quantity for OM degradation. However, higher levels of O2 or OM, or less refractory OM, at the oxycline, even in a co-limitation context, can decrease the OMZ transfer efficiency to below 50 %. This is especially true in summer during intraseasonal wind-driven oxygenation events. In late winter and early spring, high oxygenation conditions together with high fluxes of sinking particles trigger a shutdown of the OMZ transfer (Teff<6 %). Transfer efficiency of chemical elements composing the majority of the flux (nitrogen, phosphorus, silica, calcium carbonate) follows the same trend as for carbon, with the lowest transfer level being in late winter and early spring. Regarding particulate isotopes, vertical transfer of δ15N suggests a complex pattern of 15N impoverishment or enrichment according to Teff modulation. This sensitivity of OM to O2 fluctuations and particle concentration calls for further investigation into OM and O2-driven remineralisation processes. This should include consideration of the intermittent behaviour of OMZ towards OM demonstrated in past studies and climate projections.
Highlights
Eastern boundary upwelling systems (EBUSs) are generally known to be highly productive (Chavez and Messié, 2009), associated with significant primary production (479 to 1213 gC m−2 yr−1) and elevated concentrations of chlorophyll a (1.5 to 4.3 mg m−3)
Transfer efficiency (Teff) presents variations that can be classified into three main characteristic ranges: high with 50 < transfer efficiency (Teff) < 75 % associated with preservation capacity, intermediate with 20 < Teff < 50 % or low with Teff < 6 %, associated with remineralisation capacity
The high particulate organic carbon (POC) flux (> 80 mgC m−2 d−1) end of winter–early spring can provide enough substrates to sustain the anabolic requirement of the microbial activity and shut down the vertical transfer (Teff < 6 %)
Summary
Eastern boundary upwelling systems (EBUSs) are generally known to be highly productive (Chavez and Messié, 2009), associated with significant primary production (479 to 1213 gC m−2 yr−1) and elevated concentrations of chlorophyll a (1.5 to 4.3 mg m−3). The intense and diverse microbial activity (Devol, 1978; Lipschultz et al, 1990; Azam et al, 1994; Ramaiah et al, 1996; Lam et al, 2009; Stewart et al, 2012; Roullier et al, 2014) may induce efficient remineralisation and/or respiration This may be the case in the more oxygenated, warmer upper OMZ layer associated with the oxycline, leading to substantial OM recycling. It is important to explore the detailed O2 feedback effect on particles
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