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Biological oceanography

Turn Up The Heat, Turn Down the Productivity

Moore, J.K., Fu, Weiwei., Primeau, F., Britten, G.L., Lindsay, K., Long, M., Doney, S.C., Mahowald, N., Hoffman, F. & Randerson, J.T. (2018). Sustained climate warming drives declining marine biological productivity. Science, 359,  1139-1143. DOI: 10.1126/science.aao6379

Background: Global Climate Models Predict Decreased Biological Production

Global climate models predict a decline in ocean net primary production. This is the difference between the amount of carbon dioxide that phytoplankton take in to photosynthesize and the amount that is released by the phytoplankton during metabolism. This process is important because it is the source of half of the oxygen we breathe and the basis for the oceanic biological pump. The biological pump is one mechanism the ocean has for taking in and storing carbon dioxide humans put into the atmosphere (figure 1). It operates as phytoplankton, tiny ocean plants, use sunlight, nutrients and carbon dioxide to photosynthesize converting carbon dioxide into organic matter. As phytoplankton die or are eaten and eventually excreted by zooplankton, they can sink into the deep ocean. Once this carbon containing material reaches the deep ocean it can be stored there for long periods of time.

Figure 1. Schematic of how carbon dioxide from the atmosphere dissolves in the ocean and eventually is transported and stored in the deep ocean sediments via the biological pump. Image credit: Roman J, McCarthy JJ (2010). Wikipedia Commons. https://commons.wikimedia.org/wiki/File:WhalePump.jpg.

 

Despite global climate model predictions that net primary production will decrease over time, it can vary from one region to another. For example, the Southern Ocean is expected to have an increase in productivity. Even though productivity in the Southern Ocean is expected to increase, the total amount of productivity decrease in other regions will outpace the increase in the Southern Ocean, so overall production will weaken.

You may wonder how one region can have more productive waters relative to others. This study focuses on one mechanism that controls productivity called nutrient trapping. Nutrient trapping occurs when the biological pump transports carbon and nutrient containing material downward. As this material decomposes nutrients are released. If the sinking of this material occurs faster than the ocean can physically move the nutrients back up, through a process called advection, the nutrients will be trapped. This means nutrient concentrations will be high at the location of sinking and lower in surrounding waters. In context, higher biological productivity in the Southern Ocean traps the nutrients there and transports less nutrients to northern areas causing decreased productivity in the north.

Methods: The Model

This study used a model called the Community Earth System Model, that allows for global climate change predictions. In their scenario, they predict carbon dioxide concentrations to increase to 1960 parts per million by year 2250, at which point the carbon dioxide levels would remain stable. This means that they expect humans will continue to emit carbon dioxide for years to come, as current levels are around 410 parts per million and continuously increasing (figure 2).

Figure 2. This is what is known as the Keeling Curve. It shows atmospheric carbon dioxide levels from 1958 to 2018. Image credit: Scrippsnews. https://commons.wikimedia.org/wiki/File:KeelingCurve-5-26-18.jpg

Results: What Causes the Decline in Ocean Biologic Production?

There are three distinct processes the authors observed in model output as a result of long-term global warming. The first process is nutrient trapping (see above) in the Southern Ocean. Nutrient trapping reduces the amount of nutrients available for northern waters. The second is increased stratification. Increased stratification is the separation, or layering, of water masses due to differing properties like temperature and saltiness. This reduces the exchange of water from one layer to another, meaning it is harder to move nutrients from the deep ocean to the surface where the nutrients would be used in production. The third mechanism is a reduction in the generation of water rich in nutrients in the deep ocean, meaning there is less of a source of nutrient-rich water.

There are many changes within the Earth system that occur leading up to the nutrient trapping. The westerly winds in the Southern Hemisphere become stronger and shift further south towards the pole. Wind helps drive ocean currents and, in this case, the strengthening of the wind pulls more water away from the coast of Antarctica. To counteract the water moving away from the coast, upwelling occurs along the coast where deep water moves upward to replace the water that left. Because this process becomes stronger in the model, areas that currently have water flowing downwards are predicted to shift to becoming areas of upwelling. Upwelling tends to encourage production because it can introduce nutrients to phytoplankton allowing them to grow more readily. In addition to upwelling, the models show surface waters warming by 6 degrees C and the melting of sea ice. Melting sea ice allows phytoplankton to receive 245% more light, providing peak growing conditions for phytoplankton in the Southern Ocean. This enables phytoplankton to grow faster, with a 52% increase in growth rate, which in turn, increases biological production.

Unfortunately the increase in biological production in the Southern Ocean prevents production in northern waters due to nutrient stress outside of the Southern Ocean. The nutrient stress becomes so strong that north of the Southern Ocean declines in productivity resulted in global productivity decrease by 15% by year 2300. This is shown in figure 3. Early on, there is a lot of particulate organic carbon (POC) flux north of the Southern Ocean around the equator (the very red stripe at 0 deg N) and very low flux along the coast of Antarctica (blue/green areas in the south). However, as time passes, the regime shifts to having much more flux in the Southern Ocean and less in the North (best seen in the upper right image where there is a large orange and red band around the southern coast). Similar trends are shown in the bottom panels which show phosphate concentrations. Phosphate is a major nutrient used in photosynthesis. The left shows how in 1990 phosphate was well distributed and largely present north of the Southern Ocean, as well as in the Southern Ocean. However, over time, concentrations decrease in the North and phosphate increases are localized in the Southern Ocean.

 

Figure 3. Global particulate organic carbon (POC) flux (a) and phosphate concentrations (b) in the 1990s, as well as changes in POC flux and phosphate concentrations in the 2090s and 2290s to show how global warming impacts carbon export and nutrient distributions. Image credit: Moore et al. (2018).

Broader Impact

Overall, this paper shows a worst-case scenario for climate change. The changes observed in this model the authors posit to be long lasting. They suggest that the lack of surface ocean nutrients observed in the model at year 2300 would not recover and reverse until the climate cooled again and sea ice returned. Sea ice is the tipping point and when once it melts causing a global shift in distribution of nutrients and decline in biological production, it could take thousands of years to return nutrients to the upper ocean and restore normalcy.

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