Regulation of Redfield ratios in the deep ocean

Abstract

Biotic regulation of the environment at global scales has been debated for several decades.An example is the similarity between deep-ocean and phytoplankton mean N:P ratios. N and P cycles are heavily altered by human activities, mainly through an increase in nutrient supply to the upper ocean. As phytoplankton only access nutrients in the upper ocean, it is critical to understand (1) to what extent phytoplankton are able to regulate N and P concentrations as well as their ratio in the deep, inaccessible layer and (2) what mechanisms control the value of the deepwater N:P ratio and the efficiency of its biotic regulation. With a model of N and P cycles in the global ocean separated in two layers, we show that the value of the deepwater N:P ratio is determined by nonfixer's N:P ratio, recycling, and denitrification. Our model predicts that although phytoplankton cannot efficiently regulate deep nutrient pools, they can maintain nearly constant ratios between nutrients because compensatory dynamics between nonfixers and nitrogen fixers allows a control of deepwater chemistry through nutrient recycling. This mechanism could explain the near constancy of the deepwater N:P ratio, in agreement with Redfield's (1934Redfield's ( , 1958 classical hypothesis. Surprisingly, N:P ratio of phytoplankton does not affect their ability to regulate the deepwater N:P ratio. Our model suggests that increased water column stratification as a result of global climate change may decrease the stability of the N:P ratio in the deep ocean over long temporal and spatial scales.

Biotic regulation of non-limiting nutrient pools and coupling of biogeochemical cycles

“…We extend a previous model of resource regulation with resource access limitation (Auguères and Loreau, 2015b) to the biogeochemical cycles of two nutrients. In this model, nutrients occur in two pools, one that is accessible to autotrophs, and the other that is inaccessible to them, because of either physical or chemical barriers.…”

“…To differentiate the characteristics of the two nutrient cycles, we add a subscript corresponding to the nutrient considered (i.e. 1 or 2) to all the variables and parameters described in Auguères and Loreau (2015b). Model parameters are described in Table 1.…”

“…Previous theoretical studies on biotic regulation of biogeochemical cycles have focused on regulation of the concentration of a single limiting nutrient (Auguères and Loreau, 2015b), or of the concentrations of two nutrients that limit the growth of two functional groups of organisms (e.g. Tyrrell, 1999;Lenton and Watson, 2000;Auguères and Loreau, 2015b).…”

“…Tyrrell, 1999;Lenton and Watson, 2000;Auguères and Loreau, 2015b). The ability of autotrophs to regulate nutrient concentrations in their environment, however, should be different for limiting and non-limiting nutrients.…”

“…Contrary to models applied to Redfield ratios in the ocean (e.g. Tyrrell, 1999;Lenton and Watson, 2000;Auguères and Loreau, 2015b), our model describes the dynamics of a single population of autotrophs. Both inorganic nutrients -one of which is limiting and the other is non-limiting for the growth of autotrophs -occur in two pools, one accessible and the other inaccessible to autotrophs.…”