A changing river: Measuring nutrients fluxes to the South China Sea

Below is a fascinating insight into GT EAS oceanography research, in conjunction with the School of Biology, that’s happening right now in the South China Sea. Many thanks to Dr. Bracco and undergrad Riannon Colton for sharing their experiences with us! We will be posting updates from their research here regularly. Be sure to take a look at Riannon’s blog to read all about the life of an oceanography researcher at sea!

Credit: Dr. Annalisa Bracco

Why Vietnam?

Anthropogenic pressures are threatening Vietnam’s coastal waters despite their vital role in the regional and national economy. Policies and developments in the next ten years will determine the chances to preserve both the Mekong Delta – that is subsiding at an alarming rate – and Vietnam’s relatively pristine coastal ocean. The socio-economical impacts of maintaining current practices in light of the challenges ahead would affect millions of people.

MekongMap

The Mekong River is a critically important resource for millions of people living in Southeast Asia. Its delta and the waters it delivers to the South China Sea are vital to the coastal communities that depend on it.

A few facts

The Mekong Delta is largely used for agricultural production (mostly rice), but aquaculture has been rapidly increasing at the expense of mangrove and hardwood forests. Those and other land use changes are contributing to increased nutrient loading to the Mekong River waters and in turn to the Vietnamese portion of the South China Sea, promoting coastal and offshore eutrophication.

In the near future, additional and larger anthropogenic forcings will profoundly alter the linkages between the Mekong system and the South China Sea, which supplies a critical part of Vietnam’s seafood needs.  Recent and planned construction of dams and reservoirs in the Mekong Basin will fundamentally change the discharges of water, sediment, and nutrients to the ocean. By 2030, the construction of 11 new reservoirs in the lower Mekong basin, together with 62 hydropower dams distributed along the Mekong and its major tributaries in both China and Vietnam will reduce the mean seasonal flow cycle (by up to an order of magnitude) and sediment loading (up to 80%) of the Mekong River.  The diminished freshwater and sediment input will sharply reduce nutrient supply and modify nutrient cycling, altering the biogeochemistry and circulation of the whole basin in ways as yet unexplored.

At this time there are no studies or initiatives to evaluate the connections between regional policy decisions and the future of the South China Sea marine ecosystem. There are limited capacity building initiatives to train marine scientists on the trans-disciplinary research that is so urgently needed to address this major ocean threat.

Our contribution so far

Dr. Joseph Montoya (School of Biology) and Dr. Annalisa Bracco (EAS) have the extraordinary opportunity to conduct two research cruises on the R/V Falkor to the South China Sea, one ongoing and a second one in September of this year through funding –in the form of ship time – from the Schmidt Ocean Institute. Joining them are Riannon Colton (EAS undergrad), Caroline Reddy (Biology, technician) and Ana Clavere Graciette (Biology, postdoc).

Falkor

Scientists boarding the R/V Falkor for the first time in Nha Trang, Vietnam on June 1st 2016. They departed Nha Trang June 3rd and arrived to the first station on the 4th in the early morning. (courtesy of Riannon Colton)

The goal of these research cruises is to provide a critical baseline for understanding the impacts of the changes under way in the river delta and adjacent ocean waters by characterizing physical and biogeochemical conditions of the coastal waters affected by the Mekong River plume through a broad survey during the critical Southwest Monsoon season (June to September).

It is the first effort of this sort on a US research vessel in the past 30 years.

Scientists from Columbia University and the Leibniz-Institute für Ostseeforschung in Rostock, Germany join us, together with Vietnamese colleagues at the Institute of Oceanography in Nha Trang and at the Ho Chi Minh City University of Science.

The Vietnamese coastal waters are particularly rich in fish during the summer season. This is due to the elevated concentrations of nutrients (nitrogen, phosphorous, silica, iron…) within the surface ocean layer where photosynthesis can take place. High chlorophyll levels then cause zooplankton blooms that in turn attract predators (fish) and fishing vessels. Those nutrients are supplied to the upper water column where they are consumed by phytoplankton, through wind-driven upwelling (the seasonal monsoonal winds tend to push the water along the coast towards the north, bringing more water from underneath at the surface) and/or by the Mekong River, whose waters are particularly rich in nitrogen and phosphorous collected while streaming across China and Vietnam.

Today we are sampling our second station, near the Vietnam coast and within the coastal area impacted by wind-driven upwelling. Further south elevated concentrations of chlorophyll are linked less to upwelling and more to the abundance of nutrients associated with the Mekong River outflow.

Satellite Data

Satellite images of SST (sea surface temperature) (left) and chlorophyll (right) on June 4, 2016. Colder SST are associated to wind-driven upwelling, while elevated chlorophyll levels can be seen in upwelling areas and in waters modified by the Mekong River plume. Stations covered so far are indicated in both images by white dots. (Courtesy of Ajit Subramaniam)

Fishermen

Fishermen worried that we could be cutting through their lines on June 5th, 2016 at our second station. (A. Bracco)

Keep checking in to follow along as our students and faculty continue on this important mission to understand how the Mekong River Delta may be changing in the very near future.

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