RAPID - Assessing the Response of the Seychelles-Chagos Thermocline Ridge Ecosystem to an Indian Ocean Dipole Event

Lead PI: Dr. Joaquim I Goes

Unit Affiliation: Biology and Paleo Environment, Lamont-Doherty Earth Observatory (LDEO)

March 2020 - February 2023
Indian Ocean
Project Type: Research

DESCRIPTION: Phytoplankton constitute the base of the marine food web. Their diversity, productivity and abundance have a huge influence on fisheries. For example, seasonal increases in phytoplankton biomass caused by the surge of deep ocean nutrient rich waters into the upper sunlit layers in the Equatorial Indian Ocean lead to an increase in tuna stocks. However, non-periodic increases of sea surface temperatures in the Seychelles-Chagos Thermocline Ridge region in the eastern Equatorial Indian Ocean, a phenomenon known as the Indian Ocean Dipole, weakens deep ocean water surge, which seems to alter phytoplankton community structure and to reduce phytoplankton's productivity to the detriment of tuna stocks. Yet, the impact from such unpredictable events is not fully understood. This project investigates the impacts of an ongoing Indian Ocean Dipole, one of the most severe on record, on phytoplankton. The broader impacts of the project relate to its value to inform fisheries management plans in that area. Data derived from this study is valuable to understand and predict wider changes in the food web in a region that is coming under increasing pressures of global warming. All the data is made publicly available.

The research team is working with South Korean collaborators to understand how changes in phytoplankton biomass and in deep chlorophyll maxima may be linked to changes in phytoplankton productivity and growth rates due nutrient and/or iron limitation as a result of the weakening of deep-water upwelling. The team is also investigating the impact of the dipole event on cell size of dominating phytoplankton populations. Finally, they are testing if the phytoplankton community rely primarily on recycled as opposed to new nitrogenous nutrients for photosynthesis and growth during dipole events. The hypotheses are tested through extensive biological, hydro-chemical and biogeochemical measurements that include: 1) microscopic, Flow Cytometry, FlowCAM and HPLC pigment based analysis of phytoplankton biomass, community composition and size structure and Fast Repetition Rate based measurements of phytoplankton photosynthetic competency along a cruise track and different depths in the water column, and 2) on-deck incubation based photosynthesis and nutrient uptake experiments. In situ measurements and sample collection is carried out as part of the Korea-US inDian Ocean Scientific (KUDOS) Research Program cruise (April-May 2020) on board the Korea Institute of Ocean Sciences and Technology Ship R/V Isabu, one of the only multi-disciplinary oceanographic cruises planned for the Seychelles-Chagos Thermocline Ridge region during the dipole event.