The Large Marine Ecosystem of the Arafura Sea: What are the physical drivers? (APSF 15-4)

APSF 15-4 | Amount: $ | Project Leader: J Kaempf | Project Period: Jul 2015 - Jul 2018

A project undertaken at the School of Environment, Flinders University, and supervised by Jochen Kaempf

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Study unravels the mystery of phytoplankton blooms in the Arafura Sea

After more than 50 years of first detailed hydrographic observations in the region, a recent investigation by Associate Professor Jochen Kaempf – funded by the Australia & Pacific Science Foundation – is the first that uncovers the physical reasons behind the creation of majestic phytoplankton blooms in the northwestern Arafura Sea.

Each year during the southeast monsoon (June to November) phytoplankton blooms of majestic areal extent (300 by 300 kilometres) are a prominent feature of the northwestern Arafura Sea, as can be easily identified in satellite images of the concentration of chlorophyll-a, i.e. the green pigment of phytoplankton (see graph).

Figure 1. Ocean-color-based chlorophyll-a concentration in the Arafura Sea for August 2014. The ellipse indicates the region in which widespread phytoplankton blooms develop during the southeasterly monsoon. Source of MODIS-Aqua data: Giovanni NASA.

Early investigations (Rochford, 1966) had already established that waters from the upper continental slope of the Banda Sea fertilize the study region with nutrients, but the associated physical drivers remained unknown until now.

The clue to the unusual hydrodynamics in this region is its shallowness (<50 m) and close vicinity to the equator.  In such circumstances, the surface and bottom Ekman layers, which normally develop without interference from each other in regions of greater water depth, overlap and effectively cancel out rotational effects due to the Coriolis force. This feature gives rise to the classical lee effect that, in semi-enclosed and fully enclosed water bodies (e.g. lakes), creates undercurrents running opposite to the wind-induced surface flow and, consequently, upwelling at upwind coasts (Kämpf, 2015a, Ocean Dynamics).

When applied to the Arafura Sea, findings from a process-oriented hydrodynamic modelling study demonstrate that the southeast monsoon creates widespread undercurrents in the Arafura Sea that slowly but persistently pump nutrient-rich sub-surface water from the upper continental slope of the Banda Sea into the region (Kämpf, 2015b, Geophysical Research Letters). Kämpf (2016) confirm and further investigate the details of this upwelling process. Finally, these research findings are also featured in the chapter “Seasonal Wind-driven Coastal Upwelling Systems” in the monograph by Kämpf and Chapman (2016).

Given this new knowledge of physical factors driving phytoplankton dynamics in the Arafura Sea, the scientific community can now start to explore the interannual variability of phytoplankton blooms in the Arafura Sea and possible future changes under the global-warming scenario. The findings, reported here, also contribute to develop management plans for the Large Marine Ecosystem of the Indonesian Seas.

Kämpf, J. (2015a) Interference of wind-driven and pressure gradient-driven flows in shallow homogeneous water bodies. Ocean Dynamics, 65(11) pp. 1399-1410, doi: 10.1007/s10236-015-0882-2
Kämpf, J. (2015b) Undercurrent-driven upwelling in the northwestern Arafura Sea, Geophysical Research Letters, 42, 9362 -9368, doi:10.1002/2015GL06616
Kämpf, J. (2016) On the majestic seasonal upwelling system of the Arafura Sea. Journal of Geophysical Research, 121(2), 1218–1228, doi: 10.1002/2015JC011197.
Kämpf, J., and P. Chapman (2016) Upwelling Systems of the World: A Scientific Journey to the Most Productive Marine Ecosystems. Springer International Publishing, 433 pages.
Rochford, D. J. (1966) Some hydrological features of the eastern Arafura Sea and the Gulf of Carpentaria in August 1964, Australian Journal of Marine and Freshwater Research, 17, 31–60.