Abstract
Abstract. Oxygen-deficient waters in the ocean, generally referred to as oxygen minimum zones (OMZ), are expected to expand as a consequence of global climate change. Poor oxygenation is promoting microbial loss of inorganic nitrogen (N) and increasing release of sediment-bound phosphate (P) into the water column. These intermediate water masses, nutrient-loaded but with an N deficit relative to the canonical N:P Redfield ratio of 16:1, are transported via coastal upwelling into the euphotic zone. To test the impact of nutrient supply and nutrient stoichiometry on production, partitioning and elemental composition of dissolved (DOC, DON, DOP) and particulate (POC, PON, POP) organic matter, three nutrient enrichment experiments were conducted with natural microbial communities in shipboard mesocosms, during research cruises in the tropical waters of the southeast Pacific and the northeast Atlantic. Maximum accumulation of POC and PON was observed under high N supply conditions, indicating that primary production was controlled by N availability. The stoichiometry of microbial biomass was unaffected by nutrient N:P supply during exponential growth under nutrient saturation, while it was highly variable under conditions of nutrient limitation and closely correlated to the N:P supply ratio, although PON:POP of accumulated biomass generally exceeded the supply ratio. Microbial N:P composition was constrained by a general lower limit of 5:1. Channelling of assimilated P into DOP appears to be the mechanism responsible for the consistent offset of cellular stoichiometry relative to inorganic nutrient supply and nutrient drawdown, as DOP build-up was observed to intensify under decreasing N:P supply. Low nutrient N:P conditions in coastal upwelling areas overlying O2-deficient waters seem to represent a net source for DOP, which may stimulate growth of diazotrophic phytoplankton. These results demonstrate that microbial nutrient assimilation and partitioning of organic matter between the particulate and the dissolved phase are controlled by the N:P ratio of upwelled nutrients, implying substantial consequences for nutrient cycling and organic matter pools in the course of decreasing nutrient N:P stoichiometry.
Highlights
Oxygen minimum zones (OMZs) of the tropics and subtropics occur in conjunction with highly productive eastern boundary upwelling systems, e.g. the Peru Current in the eastern tropical South Pacific (ETSP) and the Canary Current in the eastern tropical North Atlantic (ETNA)
The stoichiometry of microbial biomass was unaffected by nutrient N : P supply during exponential growth under nutrient saturation, while it was highly variable under conditions of nutrient limitation and closely correlated to the N : P supply ratio, PON : POP of accumulated biomass generally exceeded the supply ratio
These results demonstrate that microbial nutrient assimilation and partitioning of organic matter between the particulate and the dissolved phase are controlled by the N : P ratio of upwelled nutrients, implying substantial consequences for nutrient cycling and organic matter pools in the course of decreasing nutrient N : P stoichiometry
Summary
Oxygen minimum zones (OMZs) of the tropics and subtropics occur in conjunction with highly productive eastern boundary upwelling systems, e.g. the Peru Current in the eastern tropical South Pacific (ETSP) and the Canary Current in the eastern tropical North Atlantic (ETNA). Large amounts of sinking microalgal biomass enhance consumption of dissolved oxygen (O2) in the mesopelagic zone indirectly via microbial degradation of organic matter (Helly and Levin, 2004). Besides these loss processes, deoxygenation is further promoted by a pronounced stratification of the upper water column off the upwelling centers, impeding ventilation of the O2-depleted intermediate water body (Reid, 1965; Luyten et al, 1983). Franz et al.: Production, partitioning and stoichiometry of organic matter
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