Island Mass Effect
Have you ever paddled down a river in a canoe, walked the edges of a forested stream, or been caught on a street corner in the rain waiting for the light to turn? If so, then you will have seen (though may not have appreciated) the complex pattern of water flow created by a stray boulder, a rock, or a pile of leaves. Each of these obstructions causes currents or eddies. On a much larger scale, a similar phenomenon can be observed as oceanic currents are impeded by islands. Often, upwelling of deep, nutrient-rich water accompanies complex currents in an island’s wake. These upwelling events may support an increase in biological productivity, from primary producers and consumers to higher trophic levels. The observation of increased biological productivity associated with islands has been referred to as the “island mass effect.”
Data from C-190 Thomas Evans and William Rich
Students at SEA have studied this phenomenon near several Caribbean and Pacific islands. Using a hull-mounted, acoustic doppler current profiler, or ADCP (the only one of its kind on a sailing vessel), students are able to characterize complex current patterns down to depths of 600m.SEA’s vessels are also equipped with a clean seawater flow-through system that monitors temperature, salinity, and fluorescence (an estimate of productivity) in real time. This instrumentation allows students to observe small-scale upwelling events.
On a recent SEA trip, students clearly identified patterns of eddy formation and upwelling in the wake of several Caribbean islands, including Islas los Roques off the coast of Venezuela. In the same area, students used data from neuston tows to identify an island mass effect as chlorophyll-a concentrations, zooplankton biomass, and number of fish larvae all increased in comparison with other Caribbean stations.