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Biochemistry

Eat Organic at Your Local Gyre Margin

Paper: Letscher, Robert T., et al. 2016. Nutrient budgets in the subtropical ocean gyres dominated by lateral transport. Nature Geoscience, v.9: 815–819

If you were a marine organism looking for some grub, where could you find something nutritious? Nutrients in the ocean accumulate in the bodies of living things, which tend to sink to deeper waters when they die. Thus, surface-level ocean waters tend to become depleted of nutrients very quickly, and nutrients must somehow be brought up to replace the lost nutrients at the surface to be useful to the plankton living there.

One way to do this is by upwelling: the wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface. The nutrient-rich water replaces the warmer, nutrient-depleted surface water, and stimulates the growth and reproduction of primary producers such as phytoplankton.

Figure 1: There are five major ocean-wide gyres — the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres. Credit: wikemedia/NOAA.

Figure 1: There are five major ocean-wide gyres — the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres. Credit: wikimedia/NOAA.

Vertical processes like upwelling are thought to be the dominant process for bringing nutrients to nutrient-depleted areas of the surface ocean. However, studies have shown that estimated vertical nutrient fluxes are insufficient to explain observed net productivity in the subtropical ocean gyres. In fact, in the North Atlantic nutrient budget, it has been shown that lateral transport of nutrients helps supply enough nutrients to the surface. However, the North Atlantic is unique in its high iron (Fe) supply, which is generally a limiting nutrient (a nutrient that is more rare, and thus is a factor in limiting plankton growth). The dominant nutrient supply pathways for the other gyres, which vary in limiting nutrient status, require further examination. Thus, the importance of lateral transport for other gyres (Figure 1), has not been demonstrated. The relative importance of the supply mechanisms (lateral vs. vertical), the chemical forms (organic vs. inorganic), and their nutrient availability (nitrogen vs. phosphorus) on subtropical ocean biogeochemistry and ecosystem dynamics is unknown.

Finding pathways of nutrient transport is critical to our understanding of biogeochemical cycles of common nutrients such as nitrogen (N), phosphorus (P), and carbon (C), as well as their feedbacks with the climate system. The biological pump (the sinking of nutrients in the ocean) sequesters C in the deep ocean, while the loss of nutrients in exported organic matter slows biological production.  In the subtropical oceans, nutrient estimate models using only vertical mixing are typically unable to explain the nutrient content at the surface. As such, some researchers have begun looking at lateral transport as an important nutrient source for the subtropical gyres, and thus for the overarching carbon cycle in the ocean.

Researchers at the University of California at Irvine used models to investigate just how important lateral transport of nutrients is outside the North Atlantic, and what roles inorganic vs. organic nutrients have. They did this by using an ocean circulation model and a biogeochemical model. Differing scenarios with limiting nutrients such as N, P, and FE served to simulate 5 different subtropical gyres around the planet.

Using the North Atlantic gyre as a baseline, they began with looking at theoretical and observed estimates of nutrient transport for their simulated gyres of the other 4 oceanic gyres. Previous estimates suggest that lateral exchange of inorganic nutrients (without C), including dissolved inorganic nitrogen (DIN) and phosphorus (DIP) at the gyre boundaries contributes to replenishing surface layer nutrients lost to sinking export. Lateral transport of organic nutrients (with C), including dissolved organic nitrogen (DON) and phosphorus (DOP), is also believed to contribute significantly in the North Atlantic gyre, with an especially large role for the lateral supply of DOP.

Figure 2: Lateral supply and uptake of organic nutrients (DOP and DON). Credit: Letscher, et al. 2016.

Figure 2: Lateral supply and uptake of inorganic nutrients (DIN and DIP) in the 5 gyres. Credit: Letscher, et al. 2016.

 

Figure 3: Lateral supply and uptake of organice nutrients (DON and DOP) in the 5 gyres. Credit: Letscher, et al. 2016.

Figure 3: Lateral supply and uptake of organic nutrients (DON and DOP) in the 5 gyres. Notice the greater magnitude of nutrient uptake than inorganic nutrients in Figure 2. Credit: Letscher, et al. 2016.

Through their models and calculations, the researchers found that the uptake of laterally supplied N and P is similar in magnitude to the vertical exchanges, supplying 24–36% of new N and 44–67% of new P. Uptake of laterally-supplied inorganic nutrients is relatively rapid (~1.5–3 years), with most external inorganic nutrients consumed near modeled gyre margins (Figure 2). Lateral transport of organic nutrients magnitudes are similar to the supply of inorganic; however, the spatial pattern of organic nutrient uptake is more complicated. Due to the longer effective lifetimes of organic nutrients in the surface ocean (~4–8 years) compared to the more rapidly assimilated inorganic forms, supply of external organic nutrients penetrates farther into gyres than inorganic do (Figure 3).

Overall, whether organic or inorganic, the study confirms the importance of shallow lateral nutrient transport across the margins of all 5 oceanic gyres for sustaining subtropical ocean productivity. Organic nutrients do seem to penetrate farther beyond gyre margins as well. These inputs are essential to understand in order to model the biological pump and the carbon cycle in the ocean. Additionally, as the Earth’s waters warm, a more thermally stratified upper ocean will make it more difficult for nutrients to be mixed up to the surface layer, which could increase the importance of lateral transport. Such a shift could cause large-scale changes to ocean biology, including shifts in plankton biogeography and migration of marine organisms that feed on them. Perhaps as the warming oceans become more stratified, we will see less primary production along the coasts where upwelling is prevalent, and lateral transport of nutrients will serve to keep feeding our coastal foodchains.

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