Biological oceanography

The Missing Mechanism: How Ocean Fronts Impact Sinking Organic Matter

Source: Stukel, M. R.; Aluwihare, L. I.; Barbeau, K. A.; Chekalyuk, A. M.; Goericke, R.; Miller, A. J.; Ohman, M. D.; Ruacho, A.; Song, H.; Stephens, B. M.; Landry, M. R., Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction, Proceedings of the National Academy of Sciences 2017, 114 (6), 1252-1257. DOI: 10.1073/pnas.1609435114


The surface waters constitute what comes to mind for most people when they think of the ocean: a shimmering blue expanse rippling with the wind. But behind that postcard image is a complex and important system within the greater carbon cycle of the ocean. The upper 250 feet of the ocean, referred to as the euphotic zone, is the depth reached by sunlight. In the euphotic zone, microscopic marine plants called phytoplankton convert carbon dioxide (CO2) into sugars and other organic molecules (Figure 1).


Figure 1. Carbon cycling by plankton and bacteria in the ocean. In the euphotic zone, (the depths reached by sunlight), phytoplankton take up CO2 and convert it into organic carbon-based molecules. Particulate organic carbon (POC), refers to dead phytoplankton and other organic matter sinking out of the surface ocean. Respiration is the conversion of organic matter into CO2. Reprinted by permission from Macmillan Publishers Ltd: Nature Geoscience 6 (9): 718-724 copyright (2013).


These organic molecules can be transported beneath the surface by a variety of mechanisms, referred to as export production. Exactly how much carbon is exported is not well understood, with estimates ranging between 5 and 21 petagrams (21 billion tons) of carbon per year. A recent study aims to narrow this range by investigating the role of a dynamic phenomenon in the water called ocean fronts in exporting carbon out of the surface waters.


Production in the ocean

Phytoplankton are called the primary producers in the ocean because they are at the base of the food chain. They take resources unusable to most species, including CO2 and sunlight, and convert them into sugars and other compounds that all organisms consume to stay alive. These compounds are organic molecules, which are long chains of carbon atoms. When phytoplankton die or get eaten by larger plankton that produce fecal pellets, the organic molecules they produced leave the surface ocean through sinking and water movement. This departure of organic molecules is called export production and is measured in terms of mass of carbon exiting a region. Currently the magnitude of export production in the surface ocean is not well measured. Accurate estimates of the magnitude of each mechanism involved in the oceanic carbon cycle are crucial for biologists and climate scientists alike, who rely on these numbers to make predictions about the future state of the oceans and atmosphere.

Coastal ecosystems are significant players in the oceanic carbon cycle because they are regions in which a lot of primary and export production occurs. In this study, Stukel et al. analyzed data from 2012 in a region inshore of the California Current, which is an area of frequent fronts (Figure 2).


Figure 2. Map of region studied off the California coast, just inshore of the California Current. Box indicates sampling region and lines indicate path of measurements made by drifting instruments. Adapted from Stukel et al. Copyright (2017) National Academy of Sciences.


What is an ocean front?

A front in the ocean is the encounter of two distinct water masses. Because fronts are complex and highly variable in time and space, no estimate of export in ocean fronts had previously been made. Mesoscale describes the size of the ocean front. It’s a vague word, but here means “mid-scale;” large compared to small ripples in the water, and small compared to the global ocean circulation. Using a combination of anchored and drifting sensors, the authors studied the two main mechanisms of carbon export in mesoscale fronts: gravitational flux (particles sinking) and subduction (water containing organic matter being forced downward).


Mechanisms for increased export

Focusing first on the gravitational flux of organic matter, the authors found that particle export in frontal regions is over twice that in non-frontal regions. But the measured increase in gravitational flux was not alone enough to account for the enhanced export in the fronts. The remaining export was attributed to subduction, which was roughly two-thirds the magnitude of carbon export via gravitational flux. Subduction is a characteristic feature of mesoscale fronts, which create interspersed areas of water being forced up towards the surface or down deeper into the ocean. The combined contribution of gravitational flux and subduction to total export production was estimated to be 530 mg Cm-2day-1.


What makes things sink?

The next step for Stukel et al. was to identify why gravitational flux was enhanced in these frontal regions. They focused on two main mechanisms that could cause increased export production in the fronts: more fecal pellet production by mesozooplankton and diatoms consuming more silicon.

Zooplankton are a type of plankton that eat photosynthetic plankton, and mesozooplankton are simply zooplankton ranging in size from 0.2 to 20 millimeters. Stukel et al. noted that there was a hike in the mesozooplankton population in the region compared to non-frontal conditions in the same area. This population increase would result in more mesozooplankton fecal pellets coming out of the surface ocean.

In addition to the increase in fecal pellets, the researchers noted that the diatoms – a type of photosynthetic phytoplankton – were using more silicon than in non-frontal regions. Diatoms require the nutrients nitrogen, silicon, and iron to grow. It has been observed that when diatoms do not have enough iron, they take up a higher ratio of silicon to nitrogen. Because silicon is heavier than nitrogen, these diatoms become heavier. The authors found that these frontal regions had limited iron, and thus the diatoms had a higher silicon to nitrogen ratio. These heavier diatoms sink more rapidly when they die. In addition, zooplankton eat these diatoms and produce heavier fecal pellets, which also sink faster.


Estimates of enhancement

The researchers concluded that higher fecal pellet production and increased silicon uptake by phytoplankton are likely common in ecosystems at the eastern boundary of ocean basins like the California Current. They estimated that in the California Current ecosystem (CCE), about 8% of the water is within 15 kilometers of a front. Assuming these fronts cause a twofold enhancement of export production, then over 14% of total sinking of particles in the CCE occurs near these fronts. Eastern boundary ecosystems are important for the global total oceanic production, and these estimates suggest that mesoscale fronts play a significant role in carbon export into the ocean interior.


Looking ahead in a warming world

One of the main drivers of mesoscale fronts is the wind. As the planet warms due to global warming, the contrast between land and ocean temperatures is expected to increase. This heightened contrast will result in stronger winds that are favorable to front development, which means the frequency of mesoscale fronts is likely to increase in the future. It follows that export production occurring in mescoscale fronts will likely play an even larger role in the global carbon cycle. Measuring the current particle export in mesoscale fronts is one piece of the greater puzzle of understanding the global carbon cycle and how it might change in the future.


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