SEA Currents: Robert C. Seamans
S251 Weblog 08 March 2014
18° 49.3’ W x 140° 19.8’ W
Course & Speed
Motorsailing with staysails, course 305 degrees toward the atoll of Hao.
Winds force 2 from ESE, starry skies.
We have now visited six of the some 109 islands making up the country that most Tahitian speakers simply call Te Fenua. Fenua in literal translation means land or ground, the bits of terra firma in this the biggest ocean on the planet. It is a remarkable thing, making a country out of the ocean with just these little slivers of land. The islands themselves play an equally remarkable part in this; their shapes, reflecting their geological history, in turn shape the lives of their human inhabitants in profound ways.
All these islands started as mid ocean volcanoes. Some, like Tahiti, are as recent as mere 800 000 years, which, while perhaps still sounding old, is a mere eye-blink in earth’s history. These young islands are tall, lush and visually dramatic with deep valleys, sheer cliffs and abundance of waterfalls. They are very fertile and support diverse agriculture of fruits and vegetables that nourished the Polynesian inhabitants over the centuries.
Something peculiar happens on the fringes of these islands in the tropical ocean. Corals begin to grow and in time form fringing reefs all around the islands. These reefs grow outward and form broad protected lagoons between the island shores and the crests of the living reefs. While the reef grows with the corals, the original volcanic peak of the island begins to erode and sink in a process that lasts some millions of years. The result of these two countervailing processes is the formation of a low-lying atoll island, a slender ring of exposed coral and sand flanked on one side by the sea, and on the other the lagoon. Agriculture on these islands is hard, securing water at times uncertain, but the fish in the lagoon is plentiful.
A significant part of the life onboard our ship is spent studying how the surrounding ocean shapes these islands, and how they in turn sometimes influence the ocean itself. This interplay is crucial; the reef growth is dependent on the chemistry and temperature of the ocean water. In turn, the islands can act, in concert with ocean currents, as giant mixers, bringing to surface cool and nutrient rich waters from the depths.
We study this interplay by measuring various aspects of the surface ocean, ranging from temperature and pH to nutrients and plankton abundance. Temperature and pH lead a list of concerns for the long-term health of the reefs and hence the islands themselves. Nutrients and plankton are the stuff that feed the fish and other resources essential for the island populations. To gather this data, we stop the ship two times a day to deploy various instruments and plankton nets. These deployments happen around 10:30 am and pm, and this work has become part of the ship’s routine with the students at this stage of the cruise trained to operate almost all of the science gear on board.
What are we finding out? Well, the ocean currents as measured by us with our onboard current meter do indeed help bring about upwelling and mixing in the near-shore waters, and phytoplankton, the plant-analogue primary producers of the sea take ready advantage of these resources; the coastal ecosystems are proving richer even amidst the low productivity of the open ocean waters.
In Mangareva we saw an island in the middle of the transition from a high volcanic island to an atoll. Two lofty peaks still overlook a large lagoon almost twenty five miles across, with smaller remnant mountains breaking the sea surface to form tall green islets dotting the lagoon. The reef encircling the perimeter of the original pre-erosion island is now a far distant band of breaking surf, but it still manages to keep in some of the nutrient rich water running down the mountains in small streams. To take advantage of this lagoon oasis there are well over a hundred pearl farms engaged in the aquaculture of the famed black pearl oyster Pinctada margaritifera, a brilliant example of the intimate interplay of human culture and the shape of their environment.
In the end it is this interplay that makes these islands so interesting to study. Of course we’re all subject to such interactions in our own respective places on the globe, but they can be harder to see in the middle of urban and suburban landscapes of major continental landmasses. Here, this interplay stands plain for us to see. We continue our voyage, learning from both the ocean and the people of these islands lessons about how, for better or for worse, to live with our ocean planet.