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Biogeochemistry

Warmer Waters Will Trap Nutrients Down Deep

Article: Moore et al., Sustained climate warming drives declining marine biological productivity, Science, 359, doi: 10.1126/science.aao6379, 2018

A Grim Future

The year is 2300. It is too warm and there isn’t enough food, and it is getting harder and harder to survive in the area your ancestors inhabited the last 200+ years. You have tried to adapt to the changing climate but your efforts are not enough. You have heard a rumor that there is ample food on the other side of the globe but the owners cannot share their bounty. This may sound like a young adult dystopian novel, but in reality it is a probable future for phytoplankton in the ocean thanks to a myriad of changes in our ocean-atmosphere system due to global warming.

Fact or Fiction?

Researchers are starting to think this fictional-seeming dystopia is in fact a likely scenario in our warming world. While climate models are traditionally run through 2100 because this is considered a tipping point for our climate, a recent modelling study was run until 2300 to see what would happen in the ocean.  These researchers argued that stopping models at 2100 may miss large shifts in biogeochemistry that occur later on as the cumulative effects of climate change take place. They found that the amount of photosynthesis by phytoplankton, referred to as primary production, will increase in the Southern Ocean and lead to a drawdown of nutrients to deeper waters. But a changing circulation pattern will prevent the resultant nutrients from being transported around the globe. These nutrients will be trapped in deep water unable to reach the surface.

 Our Current Ocean

The Southern Ocean is a key player in the transport of nutrients to waters around the world. Antarctic intermediate waters transfer nutrient-rich water northward below the surface, providing nutrients to the low-latitude seas.  The Southern Ocean takes up a significant global portion of the heat and CO2 driving it into deep waters. This deep water is moved throughout the oceans, and through upwelling brings nutrient-rich water to the surface. There is also the surface movement of nutrients from the Southern Ocean northwards towards the tropics.

A changed ocean system

If current warming rates continue, the future global ocean system will be very different from its present state.  There will be greater stratification between the nutrient-rich deep water and the surface waters, which will slow down primary production. This will in part happen because the nutrients will be trapped in the mid-level waters near the Southern Ocean. Rising air and ocean temperatures will cause global wind patterns to change. The nutrient-rich water will be stuck around the Southern Ocean and sink into deep water. What will differ from the current ocean is that the deep water will be trapped at depth. The stratification will prevent this nutrient-rich water from reaching the surface inhibiting primary production. Figure 1 shows the difference in temperature between the 1990s and the year 2290. The areas of red mean that the temperature will increase compared to the 90s (by up to 10oC). The temperature change is the most drastic around the poles. Because of this temperature increase, there will be minimal exchange between the deep water and the surface.

Figure 1: A global map showing difference in surface wind and ocean temperature (color bar in Celsius). This difference represents the change between the predicted 2290 ocean and 1990s ocean.

Other model parameters confirmed that the nutrients needed for primary production would be trapped at depth. Figure 2 shows the difference in phosphate between 1990s and the year 2290 throughout the ocean (as a transect). An increase in phosphate is predicted below 1000m depth throughout the ocean basin. However, with the exception of a plume of phosphate just below the surface near Antarctica, it’s projected that phosphate will be depleted throughout most of the surface ocean.

Figure 2: A transect from 60N to 60S showing the difference in phosphate concentration throughout the ocean between 2290 and the 1990s.

The Big Picture

The researchers in this study were not alone in their assessment. A collection of climate models all converged on the same picture of the 2300 ocean: areas of nutrient abundance and scarcity, with limited exchange between the two.  What does this mean for ocean life? As it becomes harder for phytoplankton to grow and thrive, the entire food chain will be impacted. Smaller plankton will thrive in the warm, nutrient replete waters compared to larger plankton. The changes in phytoplankton community structure will reverberate to higher trophic levels. If the species of phytoplankton Antarctic krill rely on cannot survive in the warmer, nutrient-depleted waters, krill may go extinct. Krill are very important to the Southern Ocean food web. On a global scale, net primary production will decline by 15%, but there will be larger relative reductions in the North Atlantic, western Pacific, and southern Indian oceans. The zooplankton productivity will follow a similar pattern, also causing a chain reaction up the food web. This study found that higher trophic levels could decline by 20% globally and 60% in the North Atlantic! There’s no denying that the changes we’ve been imposing upon our climate will have lasting effects for hundreds of years. The ocean nutrient system will equilibrate as the earth cools and sea ice comes back but not before drastic changes to the system we know now.

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