Islands Help Create Phytoplankton Oases In Oceanic Deserts

The productive waters surrounding Palmyra Atoll. Zafer Kizilkaya
Janet Fang 17 Feb 2016, 02:21

In nutrient impoverished parts of the ocean, islands can sustain marine ecosystems by enhancing plankton blooms that ultimately help increase food resources, according to new work published in Nature Communications this week. 

Phytoplankton consist of photosynthesizing microorganisms, and they’re an essential source of energy in marine ecosystems. Across the central and western Pacific, islands and ring-shaped coral reefs (called atolls) that are exposed to high levels of nearshore phytoplankton support higher fish biomass and a greater abundance of reef-building organisms. But why? Decades-old observations suggested that phytoplankton biomass surrounding island reefs can sometimes be anomalously high compared to the surrounding ocean thanks to the so-called “Island Mass Effect.” But whether or not the effect is a pervasive phenomenon remained a mystery. 

To examine the extent of the Island Mass Effect across the Pacific Ocean, a team led by Jamison Gove from the Pacific Islands Fisheries Science Center studied 35 coral reef islands and atolls, including the northwestern and main Hawaiian Islands, Phoenix Islands, Line Islands, and American Samoa. The team used satellite imagery and ship-based surveys to measure chlorophyll-a, a proxy for phytoplankton biomass.

Turns out, the Island Mass Effect is a near-ubiquitous feature in 91 percent of the coral reef ecosystems they studied. These islands and atolls show a long-term enhancement in nearshore chlorophyll-a, and the main drivers of this enhancement are island size and type, the slope of the sea floor, the area of the reef, and local human impacts (including nutrient input). 

Their findings also show that the Island Mass Effect could increase phytoplankton biomass by up to 85.6 percent compared to normal oceanic conditions elsewhere – providing energy sources and promoting the growth of coral reef communities and fishing stocks. Even the inter-island migratory patterns of marine apex predators (such as tiger sharks) seem to be driven by phytoplankton biomass variations.
 
However, phytoplankton boons could also potentially drive toxic algal blooms – which are deadly to fish, seabirds, and marine mammals – as well as “fleshy” algal growth that doesn’t contribute to reef-building. 

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