Storms bring ocean nutrients to light

Ships and ocean-observing robots have been used to quantify the amount of nutrients that a storm brings up from the Stygian ocean depths to the sunlit surface — a first step in assessing how storms affect oceanic biomass production.

Seawater absorbs most sunlight within 100 metres of the ocean's surface, where available inorganic nutrients are rapidly assimilated by photosynthesizing plankton. As these cells die or are excreted in a consumer's waste, they sink into the dark ocean interior, where microbial decay returns their nutrients to the dissolved inorganic form. The physical processes that lift the resulting nutrient-rich water from the dark interior to the sunlit upper ocean are responsible for sustaining nearly all marine life. Writing in Global Biogeochemical Cycles, Rumyantseva et al.¹ quantify how a mid-latitude storm and its after-effects drive upward pulses of nutrients, thereby advancing our understanding of the processes that nourish plankton at the base of the marine food chain.

Illuminating the processes that bring nutrients into the light has been a preoccupation of oceanographers for generations², a pursuit made more urgent by predictions (see refs 3 and 4, for example) that global warming may suppress the upward nutrient supply, with deleterious effects for many plankton. Such predictions are premised on a causal chain linking the warming of near-surface waters to a strengthening of the density gradient between the surface and interior ocean, which, in turn, increases the amount of energy required to bring subsurface nutrients upwards. However, the future of the physical drivers that provide this energy is rarely considered.

The turbulence caused by stormy seas is thought to be one mechanism for mixing nutrients upwards. This turbulent mixing is rarely documented because of the challenges of conducting ship-based seawater sampling in high winds. Therein lies the novelty of Rumyantseva and co-authors' study, which reports nutrient concentrations, the intensity of turbulent mixing and the velocities of ocean currents before, during and after the passage of a North Atlantic storm. They solved the sampling problem in part by using torpedo-shaped robots called gliders⁵, which continuously measured water-column properties regardless of the weather.

The authors report an approximately tenfold increase in surface nutrient concentrations during the storm, followed by two short-lived bursts of nutrients after the storm. They also observed that the concentration of chlorophyll, the pigment responsible for most photosynthesis, rose by about 50% near the surface between the last day of the storm and several days later.

The study provides insight into how storms can produce upward nutrient pulses, even after they cease. Wind blowing over the ocean during a storm sets its surface layer in motion, but the interior ocean not far below is relatively unperturbed. This creates a sharp change in the ocean-current velocity (that is, high shear) at the interface between the surface and the interior that leads to instabilities in the circulation and the development of turbulence (Fig. 1a). Such turbulence can mix nutrients into the sunlit region, and, in the case of the observed storm, was responsible for the rapid increase in surface nutrient concentrations.

Figure 1: Uplift of nutrients in the ocean during and after a storm.

a, During a storm, the current in the upper ocean accelerates in response to the wind passing across it, whereas the interior ocean not far below is relatively unperturbed. This sets up a highly sheared current at the interface between the upper and the interior ocean, which is susceptible to the development of turbulence. Turbulent mixing (white arrows) thus brings nutrients from the interior to the surface. b, After the storm, the wind gradually weakens, and the upper ocean rocks back and forth while the planet rotates underneath. The current therefore traces a circular path relative to a fixed point on Earth — this is known as an inertial oscillation. When the wind and upper-ocean current align, strong turbulent mixing again brings nutrients to the surface. Rumyantseva et al.¹ have measured the amount of nutrient brought to the surface during and after a storm in the North Atlantic Ocean.

After storm winds relent, the surface waters oscillate like a pendulum, rocking back and forth while the planet rotates underneath. The circular path traced by the water relative to a fixed point on Earth is called an inertial oscillation. The shear spikes again when the wind aligns with the ocean surface current (Fig. 1b) — which occurred approximately twice a day for a steady wind over the inertial oscillation observed by Rumyantseva and colleagues. The authors report that these periods of alignment coincided with an upward delivery of nutrients at a rate 25 times faster than background. However, given that these bursts of intense turbulent mixing lasted only about an hour each, their total contribution to the nutrient supply was smaller than that delivered during the storm.

The authors suggest that the net effect of non-winter storms might locally contribute up to 30% as much nutrient as is supplied by wintertime convective mixing, the mechanism that sustains the annual algal blooming of the North Atlantic. But measurements of a single event cannot resolve how storms influence biology on the scale of ocean basins. A study⁶ in the subtropical North Atlantic showed that cyclones drive only a minor increase in the mean amount of chlorophyll during the hurricane season, because their direct influence is felt by a small fraction of the region. This interpretation was recently questioned, however, by another study⁷ that found an association between year-to-year variability of chlorophyll concentration and total cyclone energy during the hurricane season. It therefore remains unclear whether storms are major drivers of such variability on large scales.

Moreover, neither the current paper nor the subtropical studies^6,7 extended conclusions from chlorophyll concentrations to rates of biological productivity — which is a more relevant metric for understanding the transfer of energy up the food chain, but more challenging to measure. There is thus still much to debate about the magnitude of the large-scale biological response to storms.

Future changes in nutrient supply to the ocean surface will depend on both the energy required to bring nutrients to the surface and that available from winds and other physical forcing⁸. Climate models widely agree that future warming will strengthen the ocean's vertical density gradient and increase the energy required to mix nutrients upwards⁴. The future of storms is less clear. Climate models generally predict a poleward shift of storm tracks and intensified storms in the Southern Hemisphere, whereas corresponding predictions for the Northern Hemisphere — and particularly the North Atlantic — are highly uncertain⁹.

Attempts to predict the biological influence of storms are further hampered by the inadequate representation of inertial oscillations in climate models, a gap that researchers are now working to fill¹⁰. Such improved numerical representations of how storms influence ocean mixing, and a stronger handle on the large-scale influence of storms on present-day biology, will give us a clearer vision of what the future may hold for marine plankton and the organisms that depend on them.¹