Islands play a key role in sustaining marine ecosystems in the Pacific Ocean, according to a study published in Nature Communications this week. The findings indicate that island presence enhances the amount of phytoplankton - photosynthesizing microscopic organisms that inhabit the upper layers of the oceans. This in turn enables diverse marine ecosystems to thrive in waters that would otherwise be nutrient impoverished.
Phytoplankton can be considered the ‘grasslands’ of the oceans - they are the fundamental building block of marine ecosystems and key to the sustainability of global fisheries. A small number of studies previously noted that phytoplankton biomass surrounding some islands can be anomalously high relative to the surrounding ocean; yet the prevalence of this so-called ‘island mass effect’ (IME) has remained relatively unknown, until now.
Jamison Gove and colleagues study 35 coral reef islands, using satellite imagery and ship-based surveys to assess the extent of the IME across the entire Pacific Ocean basin. They show that 91% of island and atoll-reef ecosystems exhibit long-term enhancement in chlorophyll-a (a measure of phytoplankton biomass), with statistical analyses indicating that island size and type, sea-floor slope, reef area, and human habitation are the primary drivers of this enhancement. Overall, the authors found that the IME could enhance phytoplankton biomass by up to 86% compared to normal ocean conditions, thereby providing basal energy sources for higher trophic levels.
Although increased phytoplankton biomass can be a boon to marine ecosystems, promoting the growth of coral reef communities and fishing stocks, it can also potentially drive toxic algal blooms and fleshy (non reef-building) algal growth, which may be deadly to fish and seabirds. In addition, potential future changes in ocean currents may carry additional nutrients to these reef areas in the Pacific Ocean, rendering the continued study of these ecosystems necessary, caution the authors.