Off the coast of southwestern India, seasonal upwelling affects seawater composition and biological activity. Understanding these changes is important for predicting the potential impacts of human activity in the Arabian Sea, a tropical marine ecosystem.
In 2012, Gupta et al. carried out a detailed study over the entire year to track the ocean biogeochemical effects of upwelling west of the coastal city of Kochi, southwestern India. From aboard the research ship FORV Sagar Sampada, they collected seawater samples along an east–west transect across the continental shelf once every 30–45 days for a total of 10 times. Samples were collected at various depths up to 100 meters using a CTD rosette—an instrument that measures the salinity, temperature, and depth of ocean water.
During India’s winter monsoon, seawater above the continental shelf of the southeastern Arabian Sea is relatively warm, oxygen rich, and lacking in nutrients (such as nitrogen, phosphorus, and silica). Upwelling begins between January and March, replacing the warm water with cooler water that is nutrient rich but oxygen poor. Upwelling peaks during the summer monsoon in July–August, when rains lower surface salinity, and it ends by October.
The researchers compared their data to the results of the first time series study of the area, which was performed 5 decades ago. Modern oxygen levels are generally very similar to the previous ones, except that weaker upwelling in 2012 resulted in higher oxygen levels at the peak of the upwelling period. Unlike many coastal regions around the world, the unchanged oxygen levels off Kochi in the last 5 decades, despite its unprecedented urbanization, show little effect of human activities to sustain a healthy coastal ecosystem. These results appear to show that tropical inland aquatic ecosystems like Kochi estuary are probably adapting to increased human waste by consuming most of it before discharging to the coastal sea.
In April of 2012, despite fewer nutrients, the warm, calm, and oxygen-rich waters supported increased chlorophyll concentrations in the water samples, in conjunction with a flourishing population of Trichodesmium erythraeum cyanobacteria, by fixing atmospheric nitrogen, as reported by T. Jabir et al. (International Journal of Oceanography, 2013, doi:10.1155/2013/350594). In July, the summer monsoon winds enriched nutrients through upwelling and promoted mixed phytoplankton growth. Chlorophyll and nutrient levels dropped after the summer monsoon subsided in October, and cyanobacteria blooms reoccurred in October through December.
The scientists also incubated some of their water samples to track plankton growth. An aquatic ecosystem can be net heterotrophic (uses more energy than it creates) or net autotrophic (creates more energy than it uses). The experiments showed that seawater above the inner shelf (closer to Kochi) was net autotrophic during spring of 2012 but switched to net heterotrophy for the summer monsoon. For the whole year, however, the inner shelf was net autotrophic.
Overall, the collected data showed that the biogeochemistry of seawater above the continental shelf off the coast of Kochi undergoes significant changes throughout the year. Unlike in other coastal waters, human activities do not appear to have reduced oxygen levels here, indicating a sustained healthy marine ecosystem. However, oxygen levels are severely depleted below the pycnocline—the temperature and salinity gradient that occurs with ocean depth. Climate change could alter these levels in the future, influencing the area’s biogeochemistry and fishery resources. (Journal of Geophysical Research: Biogeosciences, doi:10.1002/2015JG003163, 2016)
—Sarah Stanley, Freelance Writer
Citation: Stanley, S. (2016), An unprecedented view of biogeochemistry off India’s west coast, Eos, 97, doi:10.1029/2016EO046753. Published on 25 February 2016.