A new analysis suggests that the swimming of plankton and plankton-eating fish plays a central role in driving local peaks in extreme biological productivity in tropical coral reefs, creating "sweet zones" for abundant fish. Renato Morais of James Cook University in Australia and colleagues made the findings in a study published Nov. 2 in the open journal PLOS Biology.
While some ecosystems are limited by their inherent productivity (e.g., from photosynthesis), previous research has shown that flowing resources like plankton can act as carriers to transfer energy and nutrients from offshore ecosystems to coral reef ecosystems. This transfer of resources between ecosystems is known as spatial assistance, and they enable ecosystems to transcend the limits of their inherent capacity for biological productivity, resulting in richer life. However, the extent to which the movement of plankton and plankton-eating fish contributes to the abundance of tropical marine ecosystems has not been able to figure out.
To help clarify and quantify this role, Morais and colleagues integrated and analyzed a large amount of data on visual fish counts. One dataset covers tropical waters in much of the Indian and Pacific Oceans, while the other fish count data comes from three specific tropical regions that represent the diversity of coral reef ecosystems found in a larger dataset.
The analysis shows that plankton-eating fish do play an important and extensive role as spatially assisted carriers for tropical coral reefs. By preying on offshore plankton, they provide additional resources for coral reef ecosystems, thereby driving the development of local extreme biological productivity – including for their own predators. In these "sweet zones", where plankton-eating fish account for more than 50% of total fish production, one may find conditions there best for harvest fishing.
The researchers note that their findings have particular implications for the future of tropical reef fisheries. Coral reefs continue to degrade and offshore productivity is expected to decline, so the importance of concentrating these dwindling resources for fishermen is likely to increase.
Morais added: "Despite happening in malnourished oceans, how can tropical oceans sustain high yields and intensive coastal fisheries? The spatial assistance of phytoplankton greatly increases the biomass production of local reef fish, creating a 'sweet zone' where fish are concentrated. By harvesting marine productivity, phytoplankton bypass the spatial constraints imposed by local primary productivity and create 'oases' for the mass production of tropical marine life. ”