Biogeochemistry

Ocean biomes blended

The ratio of nutrient elements in marine subsurface waters is much the same everywhere, even though biogeochemically distinct ocean biomes exist. A modelling study that includes mixing solves this conundrum. See Article p.550

The creation of organic matter at the ocean's surface by plankton, and its continuous rain to deeper waters, has a profound impact on most of the dissolved components of sea water. The main features of this marine biological pump have been clear for some time, as has its crucial role in slowing the atmospheric accumulation of carbon dioxide — it sequestrates more than 2 × 1015 grams of carbon derived from fossil fuels per year1. Quantitative predictions of how this system will interact with altered climate states are difficult to make, however, because of the broad physiological diversity of marine plankton, the absence of detailed information on the temporal and geographic variations of specific populations, and the lack of numerical tools with which to address the many variables relevant to defining biogeochemical fluxes.

On page 550 of this issue, Weber and Deutsch2 use recent advances in numerical modelling to constrain major nutrient pools in the marine biological pump more effectively than ever before. In this way, they show that mixing between distinct ocean biomes has a significant role in producing marine biogeochemical relationships that were previously attributed to spatially invariant biological processes.

Laboratory and field studies document a wide range of nutrient utilization ratios — the ratios of carbon, nitrogen and phosphorus consumption — in marine phytoplankton, where the variation is driven by the organisms' taxonomic diversity and by responses of different populations of the same species to environmental conditions3,4. This information has coexisted somewhat uncomfortably with the dominant approach to modelling large-scale ocean biogeochemical fluxes, in which biogeochemical relationships among these nutrient elements are defined as constants, known as Redfield ratios5.

There are good reasons for the long-standing use of Redfield ratios by marine biogeochemists, however. The primary one is the remarkable consistency of the relationships between the concentrations of major nutrients in subsurface waters, where most of the respiration of the organic matter originally produced by plankton occurs. These ratios (of carbon to nitrogen, or nitrogen to phosphorus, for example) are much the same as the average ratios found in plankton in surface waters6. Therefore, despite evidence disproving the existence of universal nutrient ratios in plankton, the empirical reliability of the Redfield ratios has made them useful in various applications. The demonstration by Weber and Deutsch2 that horizontal mixing contributes significantly to maintaining the relationships among nutrients in subsurface waters suggests that the observed constancy of these ratios does not depend on a strict spatial uniformity in the vertical fluxes of nutrients, and instead is compatible with a range of surface biogeochemical regimes.

Antarctica in the mix. Weber and Deutsch2 find that mixing of water from distinct biomes in the Southern Ocean around Antarctica, seen here in a satellite image, helps to maintain the global average ratio of nitrogen to phosphorus in subsurface waters. Credit: T. VAN SANT, GEOSPHERE PROJECT/PLANETARY VISIONS/SPL

Weber and Deutsch's modelling work builds not only on relatively recent advances in ocean sampling, but also on advances in numerical modelling methods. As modern oceanographic sampling has pushed farther into remote (and nutrient-rich) regions such as the Southern Ocean, the existence of surface-water areas that have nitrogen/phosphorus (N/P) utilization ratios significantly lower than Redfield's has emerged7,8. Like terrestrial environments, these regions can be characterized as distinct biomes on the basis of temperature, mixing and light conditions9. In contrast to terrestrial systems, however, the fluid nature of marine biomes greatly increases the interactions between them, a feature that looms large in Weber and Deutsch's analysis2.

In addition to the recognition of biogeochemically distinct marine biomes, a second crucial factor in the authors' study is the development of computational tools that allow numerical models of ocean biogeochemistry to rapidly attain a steady state10. The results of these models inevitably reflect how they were initially structured, as well as the concentrations of seawater constituents, such as dissolved organic matter, for which there are no extensive data. Assessing the sensitivity of model results to the use of alternative parameters is a critical but time-consuming task using typical numerical approaches. The fast initialization methods used by Weber and Deutsch enabled them to more extensively evaluate alternative choices of the most relevant factors, resulting in more robust conclusions.

A large-scale numerical model that couples biological and physical aspects of the ocean, such as the one used by the authors2, may be an unusual tool for a detailed biogeographical analysis of ocean plankton. Nevertheless, the authors' investigation of silicate-containing regions of the Southern Ocean leaves little doubt that diatom growth is associated with the low N/P ratio emanating from Antarctic waters. Diatoms — single-celled algae — consume silicate to construct their frustules (hard external layers), but the exact physiological mechanisms correlating their growth to low N/P ratios remain unclear, as does the identity of the subgroups of diatoms that dominate in this correlation. A more complete picture of the functional genes present in marine diatoms is emerging from whole-genome sequencing, and may help to answer these questions. A major implication of Weber and Deutsch's work, however, is that previous models that partitioned observed variations in N/P ratios between nitrogen fixation and denitrification (nitrogen gain or loss in the ocean)11 have neglected a third process — Southern Ocean diatom growth. The effect of this process on ocean N/P ratios in selected water masses will now be an additional factor to take into account when analysing the marine nitrogen cycle.

Challenges lie ahead in applying Weber and Deutsch's approach to lower latitudes, but it is essential to assess the role of circulation-induced averaging of nutrient relationships on a global scale. In the region modelled in the present work2, the dominant mixing pathways are well understood, the productivity and nutrient fluxes are large, and nitrogen fixation and denitrification are minor players. At lower latitudes, few, if any, of these helpful attributes apply. Even so, the authors' demonstration that mixing between ocean biomes contributes significantly to maintaining average subsurface nutrient relationships that can be distinct from the nutritional flux in those biomes is new and exciting. It suggests that future analyses of interactions between climate and ocean biogeochemistry must consider changes in the biogeography of specific marine plankton communities, and not just the intensity of total production. This will help to structure new work in both marine ecology and biogeochemistry.

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Sambrotto, R. Ocean biomes blended. Nature 467, 538–539 (2010). https://doi.org/10.1038/467538a

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