Publications

2020

Arteaga, Lionel A. et al. “Seasonal Modulation of Phytoplankton Biomass in the Southern Ocean.” Nature Communications 11 (2020): 5364.
Over the last ten years, satellite and geographically constrained in situ observations largely focused on the northern hemisphere have suggested that annual phytoplankton biomass cycles cannot be fully understood from environmental properties controlling phytoplankton division rates (e.g., nutrients and light), as they omit the role of ecological and environmental loss processes (e.g., grazing, viruses, sinking). Here, we use multi-year observations from a very large array of robotic drifting floats in the Southern Ocean to determine key factors governing phytoplankton biomass dynamics over the annual cycle. Our analysis reveals seasonal phytoplankton accumulation (‘blooming’) events occurring during periods of declining modeled division rates, an observation that highlights the importance of loss processes in dictating the evolution of the seasonal cycle in biomass. In the open Southern Ocean, the spring bloom magnitude is found to be greatest in areas with high dissolved iron concentrations, consistent with iron being a well-established primary limiting nutrient in this region. Under ice observations show that biomass starts increasing in early winter, well before sea ice begins to retreat. The average theoretical sensitivity of the Southern Ocean to potential changes in seasonal nutrient and light availability suggests that a 10% change in phytoplankton division rate may be associated with a 50% reduction in mean bloom magnitude and annual primary productivity, assuming simple changes in the seasonal magnitude of phytoplankton division rates. Overall, our results highlight the importance of quantifying and accounting for both division and loss processes when modeling future changes in phytoplankton biomass cycles.
Pahlow, Markus et al. “Optimality-Based Non-Redfield Plankton-Ecosystem Model (OPEM v1.0) in the UVic-ESCM 2.9. Part I: Implementation and Model Behaviour.” Geoscientific Model Development 13 (2020): 4663–4690.

 Uncertainties in projections from Earth system models (ESMs) are associated to a large degree with the imperfect representation of the marine plankton ecosystem, in particular the physiology of primary and secondary producers. Here we describe the implementation of an optimality-based plankton-ecosystem model (OPEM) with variable C:N:P stoichiometry in the University of Victoria ESM (UVic) and the behaviour of two calibrated reference configurations, which differ in the assumed temperature dependence of diazotrophs.

Predicted tracer distributions of oxygen and dissolved inorganic nutrients are similar to those of an earlier fixed-stoichiometry model (Keller et al., 2012). Compared to the classic fixed-stoichiometry model, OPEM is closer to recent satellite-based estimates of net community production (NCP), despite overestimating net primary production (NPP), can better reproduce deep-ocean gradients in the NO3:PO43− ratio, and partially explains observed patterns of particulate C:N:P in the surface ocean. Allowing diazotrophs to grow (but not necessarily fix N2) at similar temperatures as other phytoplankton results in a better representation of surface Chl and NPP in the Arctic and Antarctic Oceans.

Deficiencies of our calibrated OPEM configurations may serve as a magnifying glass for shortcomings in global biogeochemical models and hence guide future model development. The overestimation of NPP at low latitudes indicates the need for improved representations of temperature effects on biotic processes, as well as phytoplankton community composition, which may be represented by locally-varying parameters based on suitable trade-offs. Discrepancies between observed and predicted vertical gradients in particulate C:N:P ratios suggest the need to include preferential P remineralisation, which could also benefit the representation of N2 fixation. While OPEM yields a much improved distribution of surface N* (NO3− 16·PO43− + 2.9 mmol m−3), it still fails to reproduce observed N* in the Arctic, possibly related to a mis-representation of the phytoplankton community there and the lack of benthic denitrification in the model. Coexisting ordinary and diazotrophic phytoplankton can exert strong control on N* in our simulations, which questions the interpretation of N* as reflecting the balance of N2 fixation and denitrification.

2019

Arteaga, Lionel A. et al. “Nutrient Controls on Export Production in the Southern Ocean.” Global Biogeochemical Cycles 33.8 (2019): 942–956.
We use observations from novel biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling program to estimate annual net community production (ANCP; associated with carbon export) from the seasonal drawdown of mesopelagic oxygen and surface nitrate in the Southern Ocean. Our estimates agree with previous observations in showing an increase in ANCP in the vicinity of the polar front (∼3 mol C m−2 y−1), compared to lower rates in the subtropical zone (≤ 1 mol C m−2 y−1) and the seasonal ice zone (<2 mol C m−2 y−1). Paradoxically, the increase in ANCP south of the subtropical front is associated with elevated surface nitrate and silicate concentrations, but decreasing surface iron. We hypothesize that iron limitation promotes silicification in diatoms, which is evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic polar front. High diatom silicification increases the ballasting effect of particulate organic carbon and overall ANCP in this region. A model-based assessment of our methods shows a good agreement between ANCP estimates based on oxygen and nitrate drawdown and the modeled downward organic carbon flux at 100 m. This agreement supports the presumption that net biological consumption is the dominant process affecting the drawdown of these chemical tracers and that, given sufficient data, ANCP can be inferred from observations of oxygen and/or nitrate drawdown in the Southern Ocean.

2018

Arteaga, Lionel et al. “Assessment of Export Efficiency Equations in the Southern Ocean Applied to Satellite-Based Net Primary Production.” Journal of Geophysical Research: Oceans 123 (2018): 2345–2364.

Carbon export efficiency (e-ratio) is defined as the fraction of organic carbon fixed through net primary production (NPP) that is exported out of the surface productive layer of the ocean. Recent observa- tions for the Southern Ocean suggest a negative e-ratio versus NPP relationship, and a reduced dependency of export efficiency on temperature, different than in the global domain. In this study, we complement information from a passive satellite sensor with novel space-based lidar observations of ocean particulate backscattering to infer NPP over the entire annual cycle, and estimate Southern Ocean export rates from five different empirical models of export efficiency. Inferred Southern Ocean NPP falls within the range of previous studies, with a mean estimate of 15.8 (6 3.9) Pg C yr21 for the region south of 30S during the 2005–2016 period. We find that an export efficiency model that accounts for silica(Si)-ballasting, which is constrained by observations with a negative e-ratio versus NPP relationship, shows the best agreement with in situ-based estimates of annual net community production (annual export of 2.7 6 0.6 Pg C yr21south of 30S). By contrast, models based on the analysis of global observations with a positive e-ratio ver- sus NPP relationship predict annually integrated export rates that are 33% higher than the Si-dependent model. Our results suggest that accounting for Si-induced ballasting is important for the estimation of car- bon export in the Southern Ocean.

2016

Arteaga, Lionel, Markus Pahlow, and Andreas Oschlies. “Modeled Chl:C Ratio and Derived Estimates of Phytoplankton Carbon Biomass and Its Contribution to Total Particulate Organic Carbon in the Global Surface Ocean.” Global Biogeochemical Cycles 30 (2016): 1791–1810.

Chlorophyll (Chl) is a distinctive component of autotrophic organisms, often used as an indicator of phytoplankton biomass in the ocean. However, assessment of phytoplankton biomass from Chl relies on the accurate estimation of the Chl:carbon(C) ratio. Here we present global patterns of Chl:C ratios in the surface ocean obtained from a phytoplankton growth model that accounts for the optimal acclimation of phytoplankton to ambient nutrient, light, and temperature conditions. The model agrees largely with observed/expected global patterns of Chl:C. Combining our Chl:C estimates with satellite Chl and particulate organic carbon (POC), we infer phytoplankton C concentration in the surface ocean and its contribution to the total POC pool. Our results suggest that the portion of POC corresponding to living phytoplankton is higher in subtropical latitudes and less productive regions (∼30–70%) and decreases to ∼10–30% toward high latitudes and productive regions. An important caveat of our model is the lack of iron limiting effects on phytoplankton physiology. Comparison of our predicted phytoplankton biomass with an independent estimate of total POC reveals a positive correlation between nitrate concentrations and nonphotosynthetic POC in the surface ocean. This correlation disappears when a constant Chl:C is applied. Our analysis is not constrained by assumptions of constant Chl:C or phytoplankton:POC ratio, providing
a novel independent analysis of phytoplankton biomass in the surface ocean. These results highlight the importance of accounting for the variability in Chl:C and its application in distinguishing the autotrophic and heterotrophic components in the assemblage of the marine plankton ecosystem.

2015

Arteaga, Lionel, Markus Pahlow, and Andreas Oschlies. “Global Monthly Sea Surface Nitrate Fields Estimated from Remotely Sensed Sea Surface Temperature, Chlorophyll, and Modeled Mixed Layer Depth.” Geophysical Research Letters 42 (2015): 1130–1138.
Information about oceanic nitrate is crucial for making inferences about marine biological production and the efficiency of the biological carbon pump. While there are no optical properties that allow direct estimation of inorganic nitrogen, its correlation with other biogeochemical variables may permit its inference from satellite data. Here we report a new method for estimating monthly mean surface nitrate concentrations employing local multiple linear regressions on a global 1◦ by 1◦ resolution grid, using satellite-derived sea surface temperature, chlorophyll, and modeled mixed layer depth. Our method is able to reproduce the interannual variability of independent in situ nitrate observations at the Bermuda Atlantic Time Series, the Hawaii Ocean Time series, the California coast, and the southern New Zealand region. Our new method is shown to be more accurate than previous algorithms and thus can provide improved information on temporal and spatial nutrient variations beyond the climatological mean at regional and global scales.

2014

Arteaga, Lionel, Markus Pahlow, and Andreas Oschlies. “Global Patterns of Phytoplankton Nutrient and Light Colimitation Inferred from an Optimality-Based Model.” Global Biogeochemical Cycles 28 (2014): 648–661.

The widely used concept of constant ”Redfield” phytoplankton stoichiometry is often applied for estimating which nutrient limits phytoplankton growth in the surface ocean. Culture experiments, in contrast, show strong relations between growth conditions and cellular stoichiometry with often substantial deviations from Redfield stoichiometry. Here we investigate to what extent both views agree by analyzing remote sensing and in situ data with an optimality-based model of nondiazotrophic phytoplankton growth in order to infer seasonally varying patterns of colimitation by light, nitrogen (N), and phosphorus (P) in the global ocean. Our combined model-data analysis suggests strong N and N-P colimitation in the tropical ocean, seasonal light, and N-P colimitation in the Northern Hemisphere, and strong light limitation only during winter in the Southern Ocean. The eastern equatorial Pacific appears as the only ocean area that is essentially not limited by N, P, or light. Even though our optimality-based approach specifically accounts for flexible stoichiometry, inferred patterns of N and P limitation are to some extent consistent with those obtained from an analysis of surface inorganic nutrients with respect to the Redfield N:P ratio. Iron is not part of our analysis, implying that we cannot accurately predict N cell quotas in high-nutrient, low-chlorophyll regions. Elsewhere, we do not expect a major effect of iron on the relative distribution of N, P, and light colimitation areas. The relative importance of N, P, and light in limiting phytoplankton growth diagnosed here by combining observations and an optimal growth model provides a useful constraint for models used to predict future marine biological production under changing environmental conditions.

2013

Kalvelage, Tim et al. “Nitrogen Cycling Driven by Organic Matter Export in the South Pacific Oxygen Minimum Zone.” Nature Geoscience 6 (2013): 228–234.

Oxygen minimum zones are expanding globally, and at present account for around 20–40% of oceanic nitrogen loss. Heterotrophic denitrification and anammox—anaerobic ammonium oxidation with nitrite—are responsible for most nitrogen loss in these low-oxygen waters. Anammox is particularly significant in the eastern tropical South Pacific, one of the largest oxygen minimum zones globally. However, the factors that regulate anammox-driven nitrogen loss have remained unclear. Here, we present a comprehensive nitrogen budget for the eastern tropical South Pacific oxygen minimum zone, using measurements of nutrient concentrations, experimentally determined rates of nitrogen transformation and a numerical model of export production. Anammox was the dominant mode of nitrogen loss at the time of sampling. Rates of anammox, and related nitrogen transformations, were greatest in the productive shelf waters, and tailed off with distance from the coast. Within the shelf region, anammox activity peaked in both upper and bottom waters. Overall, rates of nitrogen transformation, including anammox, were strongly correlated with the export of organic matter. We suggest that the sinking of organic matter, and thus the release of ammonium into the water column, together with benthic ammonium release, fuel nitrogen loss from oxygen minimum zones.