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Summary Coral reefs are highly productive ecosystems with distinct biogeochemistry and biology nestled within unproductive oligotrophic gyres. Coral reef islands have often been associated with a nearshore enhancement in phytoplankton, a phenomenon known as the Island Mass Effect (IME). Despite being documented more than 60 years ago, much remains unknown about the extent and drivers of IMEs. Here we utilized 16S rRNA gene metabarcoding as a biological tracer to elucidate horizontal and vertical influence of an IME around the islands of Mo′orea and Tahiti, French Polynesia. We show that those nearshore oceanic stations with elevated chlorophyllaincluded bacterioplankton found in high abundance in the reef environment, suggesting advection of reef water is the source of altered nearshore biogeochemistry. We also observed communities in the nearshore deep chlorophyll maximum (DCM) with enhanced abundances of upper euphotic bacterioplankton that correlated with intrusions of low‐density, O2rich water, suggesting island influence extends into the DCM.
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This content will become publicly available on December 1, 2025
A 7‐yr spatial time series resolves the island mass effect and associated shifts in picocyanobacteria abundances near O'ahu, Hawai'i
Abstract Islands in oligotrophic oceans act as local sources of nutrients. These nutrients originate from land and from deep oceanic nutrients introduced to the photic zone by tides, currents, and internal waves interacting with island bathymetry. These processes create the island mass effect (IME), in which increased chlorophylla(Chla) is found near islands compared to oceanic waters. The IME has been described via satellite observations, but the effects on phytoplankton community structure are not well documented. From 2013 to 2020, chlorophyll, nutrient, and picoplankton samples were collected from multiple depths on quarterly cruises at two sites south of O'ahu, Hawai'i.Prochlorococcus,Synechococcus, picoeukaryotes, and heterotrophic bacteria were enumerated using flow cytometry. We compared nearshore results to Sta. ALOHA, 100 km from O'ahu. Consistent with the expected IME, Chlaconcentrations were significantly enhanced at both nearshore sites compared to Sta. ALOHA.Prochlorococcusconcentrations increased with greater distance from shore, particularly below 50 m; mixed layer concentrations ofSynechococcusand picoeukaryotes significantly decreased with greater distance from shore, as did concentrations of nitrate and phosphate below the mixed layer. Heterotrophic bacteria concentrations did not show a spatial trend. Carbon‐based biomass estimates of the picoplankton population indicated that the IME‐associated Chlaincreases near the island are likely driven by larger phytoplankton classes. This study describes the IME‐associated shift in the picophytoplankton community distribution, which has implications for nutrient cycling, food web dynamics and fisheries in oligotrophic island ecosystems, and adds to the understanding of spatial heterogeneity in carbon fixation in the ocean.
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- Award ID(s):
- 2241005
- PAR ID:
- 10588585
- Publisher / Repository:
- Wiley Periodicals LLC
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 69
- Issue:
- 12
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- 2830 to 2845
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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