FY 2021

Linking a latitudinal gradient in ocean hydrography and elemental stoichiometry in the eastern Pacific Ocean

Lee, J.A., C.A. Garcia, A.A. Larkin, B.R. Carter, and A.C. Martiny

Global Biogeochem. Cycles, 35(5), e2020GB006622, doi: 10.1029/2020GB006622, View online (2021)

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°N to 69°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.

Plain Language Summary. Compiled observations of particulate organic matter elemental ratios indicate conservation of N and P where nutrients are scarce, and vice versa in nutrient-rich upwelling and polar regions. However, because the compiled datasets vary in methodology, meso-scale trends are unable to be resolved. In the current study, we observe strong gradients in particulate organic matter (POM) C:N:P ratios using consistent methods for a latitudinal transect in the eastern Pacific Ocean. Single environmental factors were unable to predict variation in C:N:P across regions suggesting a more complex regulation. Ratios of C:N and C:P in the South Pacific Subtropical Gyre were unexpectedly high for a subtropical gyre in the southern hemisphere. A single-nutrient model (nitrate or phosphate) produced significant regional biases, leading us to hypothesize multiple-nutrient models as necessary under conditions of severe nutrient stress. In the Southern Ocean, we measured total and small size fractions to estimate significantly lower C:N:P ratios of larger POM. The N:P ratio of large POM are nearest to the N:P ratio of exported organic matter estimated from remineralized nutrients in the subsurface. This analysis will help evaluate the regional importance of temperatures, nutrient availability, and community structure on biogeochemical cycling.