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Biological oceanography

Productivity Comes In Waves

Li, D., Chou, W. C., Shih, Y. Y., Chen, G. Y., Chang, Y., Chow, C., Lin, T.Y., & Hung, C. C. (2018). Elevated particulate organic carbon export flux induced by internal waves in the oligotrophic northern South China Sea. Scientific reports8(1), 2042.

What is an Internal Wave?

Think about all of the waves you can see. Waves break along the shore of a beach or against a rocky cliff abutting the sea. We can generate waves by dropping a pebble into the water or a penny into a pond. All of these are surface waves; However waves can also occur deep within the ocean, which are called internal waves. Internal waves are very similar to the waves we see crashing on the beach. Yet, waves on the surface tend to move more quickly than waves in the ocean. 

Internal waves are important in the ocean because they can transport deeper water that has much more nutrients in it up into the sunlit waters. If you think about how waves at the surface move water up and down, internal waves operate similarly. Water that is originally deeper is typically colder and contains more nutrients. If a wave passes by, it can push this colder, nutrient rich water up closer to the surface. Likewise, the warmer water that was originally closer to the surface can be pushed down. This causes the water to mix! Phytoplankton need both nutrients and sunlight to survive, grow, and collectively produce half of the worlds oxygen. When phytoplankton gain access to these nutrient rich waters, there can be blooms. Not only is this important for oxygen production, it’s important because phytoplankton are the base of the ocean food chain. Also when they die and sink or are eaten and excreted by other marine organisms, this material can become a carbon sink through the biological pump. With drastically increasing carbon dioxide concentrations in the air, the ocean acting as a major carbon sink is critical to the Earth system. 

This important linkage between the internal waves and the ocean carbon storage led Li and co-authors to collect data on how the presence of such waves influences phytoplankton activity in the South China Sea. The South China Sea is a great location to test this theory because it has some of the largest internal waves.

The Study

Figure 1. This is a NASA Worldview satellite image of the South China Sea on August 4, 2016. K1 and K2 are the study sites where instruments were deployed. Arrows point to the locations of the internal waves. Image credit: Li et al., 2018.

Traditionally, it is very challenging to identify internal waves from a ship. However, savvy ocean tech allowed the researchers to observe the waves using CTD sensors (CTD stands for conductivity, temperature and depth). CTDs are one of the quintessential physical oceanographic measurements. Conductivity is related to how salty the water is. The depth measurements are based on pressure readings and allowed the researches to see the movement of the sensors from the internal waves. As the waves passed, the sensor would move up and down along with it, much like a surfer paddling out to sea bobbing with the waves. Researchers also checked for the presence of the waves using satellite imagery collected by NASA, as shown in figure 1. 

The CTD was deployed on a drifting sediment trap. Drifting sediment traps are large cylindrical bottles attached to a platform that move along with the flowing water. These traps sit well below the surface to catch sinking carbon containing material (what was once phytoplankton or other now dead organisms and their fecal material) (figure 2). However, being tethered to a buoy at the surface allows for real-time position reporting. When the trap is recovered, the sample is then analyzed in the lab for the amount of carbon.

Figure 2. Different types of sediment traps to compute particulate organic sinking rates. Image Credit: NOC/V.Byfield. Source: http://www.rapid.ac.uk/abc/bg/bcp.php

Ultimately the goal of the scientists was to see if there is a relationship between changes in carbon sinking to internal waves. Since they predicted flourishing phytoplankton activity is due to the transport of nutrients by waves, they also measured the amount of nutrients present at each of the sampling stations.

Results

In regards to the CTD data, researchers found that there were large changes in temperature consistent with the presence of internal waves at both stations, which was confirmed with the satellite imagery. They also found changes in nitrate, a type of nutrient, and the particulate organic carbon concentrations as internal waves passed. The nitrate nearly doubled after the internal wave passed station 1. 

Comparing this study’s results to nearby areas without internal waves showed that particulate organic carbon sinking rates were higher in the presence of internal waves. This, in conjunction with the increasing nutrient concentrations, supported the original prediction. Nutrient rich water brought in by the waves could spark phytoplankton productivity causing the area to also have higher rates of sinking organic matter. However, this study was conducted on short deployments and would not be able to account for or explain if there is a time lag from when the waves occur to when the material produced from this event reaches the point at which it becomes dead sinking material.

Implications

Overall, researchers found that productivity comes in (internal) waves, for the phytoplankton. I like to think that my productivity comes in waves, too!  Through looking at the response of ocean biology to physics, the researchers observed that generally speaking areas where internal waves were present also had more sinking carbon particles. This is important because scientists are currently trying to better understand how the ocean acts as a carbon sink, which is critical to to global climate. Being able to relate the presence of internal waves to more sinking carbon in one location could help future research test this relationship in other areas.

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