Most coral reef scientists study charismatic organisms, such as corals and fish, while very few scientists focus on plankton. Since coral reefs have an abundance of beautiful and colorful creatures, small and inconspicuous plankton may be less attractive to many coral reef scientists. Of the few existing coral reef plankton studies, most of them are either bacterioplankton or phytoplankton, which are “relatively” easy to handle. There have been far fewer studies of zooplankton, although they are one of the important key players in the lower trophic levels in coral reefs.
“Zooplankton is recognized as a “black box” among many scientists working on coral reefs,” says Ryota, a visiting scientist in the Smith lab. In fact, there was only one zooplankton ecology paper among more than 2,000 papers presented at the International Coral Reef Symposium (ICRS), or the so-called “Olympic games of coral reefs,” in Hawaii last year (June 2016).
However, zooplankton play a significant role in trophodynamics, serving as one of the important food sources to various reef fish and benthic planktivorores, including corals. “Approximately half of the benthic animals on coral reefs are filter feeders, or particle feeders, which feed on zooplankton and particulate organic matter,” says Ryota, “Even larvae and juveniles of many species of reef fishes grow by feeding specifically on zooplankton.”
Ryota and his team summarized the biomass and production of coral reef zooplankton in the past five decades, and focused on the paradox of reef zooplankton trophic structure, where there is abundant zooplankton biomass yet lower phytoplankton biomass in coral reef environments. This paradox has been discussed in previous studies, but they all are fragmentary and rather qualitative. Thus, comprehensive and quantitative studies were required to elucidate planktonic trophic structure in coral reef ecosystems.
The study team comprehensively investigated the biomass and production from bacteria, phytoplankton, protozoan (nano- and microzooplankton) and mesozooplankton in the coral reef waters of Okinawa, Japan, and compared the production from both the grazing phytoplankton food web and the microbial food web to that of mesozoopankton. They clarified that the grazing food web (phytoplankton), alone, did not satisfy the requirement of mesozooplankton throughout the year. Alternatively, the contribution of the microbial food web was important. Yet, the combined production from both food webs did not satisfy the mesozooplankton requirement in some seasons, emphasizing the importance of detritus in this food-limiting system.
“Detailed investigation on the origin and production of detritus will open the door for better understanding of pelagic trophodynamics in coral reef ecosystems,” Ryota said.