Production, partitioning and stoichiometry of organic matter under variable nutrient supply during mesocosm experiments in the tropical Pacific and Atlantic Ocean

[摘要] Oxygen-deficient waters in the ocean, generally referred to as oxygenminimum zones (OMZ), are expected to expand as a consequence of globalclimate change. Poor oxygenation is promoting microbial loss of inorganicnitrogen (N) and increasing release of sediment-bound phosphate (P) into thewater column. These intermediate water masses, nutrient-loaded but with an Ndeficit relative to the canonical N:P Redfield ratio of 16:1, aretransported via coastal upwelling into the euphotic zone. To test the impactof nutrient supply and nutrient stoichiometry on production, partitioningand elemental composition of dissolved (DOC, DON, DOP) and particulate (POC,PON, POP) organic matter, three nutrient enrichment experiments wereconducted with natural microbial communities in shipboard mesocosms, duringresearch cruises in the tropical waters of the southeast Pacific and thenortheast Atlantic. Maximum accumulation of POC and PON was observed underhigh N supply conditions, indicating that primary production was controlledby N availability. The stoichiometry of microbial biomass was unaffected bynutrient N:P supply during exponential growth under nutrient saturation,while it was highly variable under conditions of nutrient limitation andclosely correlated to the N:P supply ratio, although PON:POP of accumulatedbiomass generally exceeded the supply ratio. Microbial N:P composition wasconstrained by a general lower limit of 5:1. Channelling of assimilated Pinto DOP appears to be the mechanism responsible for the consistent offsetof cellular stoichiometry relative to inorganic nutrient supply and nutrientdrawdown, as DOP build-up was observed to intensify under decreasing N:Psupply. Low nutrient N:P conditions in coastal upwelling areas overlyingO₂-deficient waters seem to represent a net source for DOP, which maystimulate growth of diazotrophic phytoplankton. These results demonstratethat microbial nutrient assimilation and partitioning of organic matterbetween the particulate and the dissolved phase are controlled by the N:Pratio of upwelled nutrients, implying substantial consequences for nutrientcycling and organic matter pools in the course of decreasing nutrient N:Pstoichiometry.

[学科分类] 地球化学与岩石