Oceanographers from the University of Hawai'i at Mānoa discovered that microbial communities–from the sunlit surface to extreme depths–in the North Pacific Subtropical Gyre exhibit robust seasonal cycles. The study provides new insight into how high levels of biodiversity are maintained in the open ocean.
"A long-standing question in biological oceanography, which we refer to as the "paradox of the plankton", asks: How can open ocean species diversity be so vast and sustained, in a seemingly homogeneous environment like the open ocean?," said Fuyan Li, lead author of the study and affiliate researcher in the Center for Microbial Oceanography: Research and Education in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST).
The blue, deep waters of the Pacific Ocean have extremely low nutrient concentrations compared to coastal areas that teem with visible life, such as kelp forests off California or coral reefs in Hawai'i.
"Theoretical ecology suggests that one way co-occurring species diversity can be maintained, is if shared resources, such as nutrients, are used at different times of year, thereby minimizing competition," Li shared. "Though seasonal cycles are a fundamental property of many diverse ecosystems, seasonality in the tropics is less pronounced than in temperate or polar ocean habitats."
Tracking microbes through DNA
To determine whether microbial communities at Station ALOHA, a tropical, open ocean research station 60 miles north of O'ahu, Hawai'i, have seasonal cycles, Li and colleagues analyzed microbial DNA in samples collected monthly over eight years. The combination of frequent sampling over a long time period, and high-resolution species identification, allowed the researchers to make these new and unprecedented open ocean observations.
They found that more than 60% of the microbial groups they tracked exhibited seasonal cycling. While these seasonal cycles diminished at depths below 150 meters, surprisingly, they remained measurable in some deep-sea microbial species at depths of nearly two and a half miles.
"Notably, very closely related species or subspecies "bloomed" at different times of the year, similar to seasonal patterns observed in some terrestrial plants and animals," Li said. "Taking turns with respect to nutrient use throughout the year seems to be a key ecological strategy for microbial communities to maintain their diversity."
By sustaining their populations throughout the year, microbial communities consistently supply organic matter and energy to organisms higher in the food web, for example larval fish. In this way, microbes ensure the stability of the marine food web and productivity in waters across the Pacific Ocean.
