Linking a Latitudinal Gradient in Ocean Hydrography and Elemental Stoichiometry in the Eastern Pacific Ocean

A past global synthesis of marine particulate organic matter (POM) suggested latitudinal variation in the ratio of surface carbon (C): nitrogen (N): phosphorus (P). However, this synthesis relied on compiled datasets that may have biased the observed pattern. To demonstrate latitudinal shifts in surface C:N:P, we combined hydrographic and POM observations from 28 degrees N to 69 degrees S in the eastern Pacific Ocean (GO-SHIP line P18). Both POM concentrations and ratios displayed distinct biome-associated changes. Surface POM concentrations were relatively low in the North Pacific subtropical gyre, increased through the Equatorial Pacific, were lowest in the South Pacific subtropical gyre, and increased through the Southern Ocean. Stoichiometric elemental ratios were systematically above Redfield proportions in warmer regions. However, C:P and N:P gradually decreased across the Southern Ocean despite an abundance of macro-nutrients. Here, a size-fraction analysis of POM linked increases in the proportion of large plankton to declining ratios. Subsurface N* values support the hypothesis that accumulated remineralization products of low C:P and N:P exported POM helps maintain the Redfield Ratio of deep nutrients. We finally evaluated stoichiometric models against observations to assess predictive accuracy. We attributed the failure of all models to their inability to capture shifts in the specific nature of nutrient limitation. Our results point to more complex linkages between multinutrient limitation and cellular resource allocation than currently parameterized in models. These results suggest a greater importance of understanding the interaction between the type of nutrient limitation and plankton diversity for predicting the global variation in surface C:N:P.

Automated summary

The study found distinct latitudinal variations in surface particulate organic matter (POM) concentrations and ratios in different ecosystems of the eastern Pacific Ocean, with the lowest POM concentrations in the South Pacific subtropical gyre and decreasing C:P and N:P ratios across the Southern Ocean. The results highlight the complex linkages between multinutrient limitation, cellular resource allocation, and plankton diversity in predicting global variations in surface C:N:P.