Studies of the silicon cycle in the Atlantic Ocean have suggested that subtropical gyres account for 13% of global marine silica production, but more recent data from the North Pacific Subtropical Gyre (NPSG) show much higher rates of silica production that imply an increased contribution of subtropical gyres. Preliminary results also reveal that the seasonal silica production cycles of the two gyres are several months out of phase with each other. Unlike the Sargasso Sea where diatoms bloom regularly in spring in response to winter convective overturn, permanent stratification prevents spring diatom blooms events in the NPSG. Annual diatom blooms do occur in the NPSG, but enigmatically they occur during summer when stratification is strongest and nutrient concentrations are at their seasonal minimum. These enigmatic summer blooms contribute significantly to carbon and nitrogen export in the NPSG and they likely dominate the annual silicon cycle.
In this project, a researcher from the University of California at Santa Barbara will investigate how seasonal changes in the intensity of Si limitation and silica dissolution regulate the annual cycle of silica production in the NPSG and contribute to its unique seasonality. His hypothesis is that high rates of net silica production during summer blooms are driven mainly by the extraordinarily efficient silicic acid uptake systems of bloom diatom species present in the NPSG that allow them to overcome Si limitation and exploit low concentrations of silicic acid. Time series of rate measurements will be made in collaboration with the HOT program to define the annual silicon cycle at station ALOHA. The project will also collaborate with the new "Center for Microbial Oceanography: Research and Education" (CMORE) Science and Technology Center at the University of Hawaii to study summer blooms.
In terms of broader impacts, the proposed project will expand our understanding of the mechanisms controlling the contribution of diatoms to elemental cycling in open ocean ecosystems. The proposed research on Si cycling combined ongoing studies of C, N and P cycling at station ALOHA will allow, for the first time, an opportunity for a coordinated analysis the cycling of all four of these elements simultaneously in an oligotrophic gyre. The project will also develop new methods to quantify 32Si from silica production rate experiments. Both graduate students and undergraduate students will be trained and the PI will work with UCSB's Marine Science Institute (MSI) to develop K-12 educational materials related to the biological carbon pump and the importance of diatoms in marine ecosystems through MSI's "Oceans to Classroom" program.
