Award: OCE-2025402

Intellectual Merit:

Island masses in the ocean disrupt prevailing atmospheric and oceanic currents, resulting in localized mixing processes that, along with terrestrial runoff, supply the upper layer of the ocean with nutrients. Consequently, phytoplankton productivity and biomass accumulation is enhanced in the vicinity of islands relative to surrounding open ocean, providing food source for zooplankton and supporting local fisheries. This phenomenon is known as the Island Mass Effect (IME). In the tropical and subtropical Pacific Ocean, one of the most oligotrophic ecosystems in the global ocean, the IME is nearly ubiquitous among the thousands of coral reef islands and atolls that it hosts. Yet, crucial knowledge gaps remain concerning the extent of the IME influence on phytoplankton and zooplankton communities beyond the coastal water into the surrounding open ocean. To address these knowledge gaps we took a nested, multi-scale approach linking broad scale ocean color remote sensing and modelled ocean currents data with detailed local field measurements of bio-optical properties, plankton imaging, environmental parameters and DNA sequencing that were collected while the vessel Tara sailed on the way to-, and from-, islands across the Pacific Ocean, as part of the Tara Pacific Expedition (2016-2018).  This allowed us to map ocean areas under the influence of the IME at the time of sampling, including tracing back open ocean patches of elevated phytoplankton biomass to the islands at which they originated, and examine how nutrient stresses, phytoplankton community composition and zooplankton biomass and composition varied between theses detached patches, the nearshore water, and the surrounding ‘background’ ocean.

We found that the detached patches sustained elevated phytoplankton biomass during their offshore transport, for periods of up to several weeks and across large distances (> 1,000 km offshore). Consequently, the spatial impact of the IME on phytoplankton biomass, and its patchiness, is significantly larger than previously thought. The high patchiness generated by the IME creates hotspots for zooplankton grazers and their predators which can enhance the overall productivity of the ecosystem. The IME acts as a naturally persistent nutrient enrichment source to the oligotrophic ocean. Particularly, we found that iron stresses were significantly reduced in water masses associated with the IME compared to the surrounding ‘background’ ocean, throughout the study area. Yet, responses of phytoplankton can be variable depending on the combination of the underlying drivers (e.g., sources of nutrient enrichment) and consequently, case by case studies, including temporal dynamics, are still needed to reveal specific underlying mechanisms. The IME is a transient phenomenon, and results from this study suggests that as patched of elevated phytoplankton biomass are advected offshore, an ecological succession and species turnover take place, resulting in variations in phytoplankton biodiversity and community composition between islands’ coastal water, offshore IME patches, and the surrounding open ocean. Finally, trophic effects of the IME on zooplankton were variable, with significant enhancement in copepods abundance and biomass (but not other taxa) in IME patches in the Western Pacific WARM Pool and in association with the Fiji Tonga archipelagos, but not in association with islands in the most oligotrophic sector of the South Pacific Subtropical Gyre. We hypothesize that there is a threshold enhancement of phytoplankton biomass below which trophic effects are too transient to detect at a basin scale with the approaches used here. This is the first study showing broader, basin-scale ecological impacts of the IME, that go beyond just the enhancement of chlorophyll a biomass.

Broder impact:

This project supported the training of one MS and one PhD students and contributed to the professional development of 16 early career scientists who participate in a workshop “Best practices for the collection and processing of ship based underway flow-through optical data" that was conducted at the University of Maine, Darling Marine Center. It also benefitted a trans-Atlantic collaboration involving researchers and graduate students visiting each other labs.

 

Last Modified: 12/20/2025
Modified by: Lee Karp-Boss