The island mass effect shapes the top-down and bottom-up controls on productivity and export in the Indian Southern Ocean

Rawat, Sadiyah Aboo Bakar

The island mass effect shapes the top-down and bottom-up controls on productivity and export in the Indian Southern Ocean

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Oceanography

Faculty

Faculty of Science

Abstract

At the numerous islands in the Indian Southern Ocean, bathymetric upwelling, resuspension of shallow sediments, and terrestrial runoff fertilize near-island waters with limiting nutrients such as iron and silicic acid (Si(OH)₄). Island-associated retention zones are also common, where nutrients and phytoplankton biomass can accumulate. Primary production should thus be elevated near the islands (i.e., the “island mass effect”; IME), with the higher iron availability favouring phytoplankton assimilation of nitrate over recycled ammonium, enhancing carbon export potential. Additionally, elevated Si(OH)₄ supply should support the proliferation of large diatoms that rapidly sink, transporting carbon to deeper waters. To investigate these hypotheses, we sampled 22 stations in near-island and open-ocean waters of the western Indian Subantarctic and Antarctic Oceans in late summer. Primary production near the Prince Edward Islands, Crozet Islands, and the Kerguelen Plateau was up to five-fold higher than in the surrounding open ocean, evincing a localized IME driven mainly by the Antarctic Circumpolar Current (ACC), which interacts with the near-island bathymetry to induce upwelling and form retention zones. Experimental results showing little response of phytoplankton to iron addition at both near-island and open-ocean stations suggest that this localized IME is superimposed on a broader IME influencing the whole western Indian sector. We propose that this condition is sustained partly by lateral iron supply via the ACC, extending the islands' influence beyond their immediate vicinity. Despite the apparent iron availability, rates of open-ocean productivity were lower than at the islands. We suggest that zooplankton grazing modulated the IME offshore, while at the near-island stations, phytoplankton biomass production outpaced grazing, allowing a stronger manifestation of the IME. Phytoplankton nitrate and ammonium uptake were on average higher near the islands, with preferential ammonium uptake at all stations (near-island and open-ocean) likely due to enhanced late-summer remineralization of the spring and early summer blooms. Consistent with this idea, nanophytoplankton (2.7–10 µm) dominated the biomass and productivity at all stations, with haptophytes contributing an average of >40% of the biomass. Nonetheless, large diatoms were more prevalent near the islands where we expect that Si(OH)₄ availability was higher; the resistance of this group to zooplankton grazing likely helps to explain the elevated near-island productivity. Additionally, we estimate a near-island potential carbon export flux that was two- to six-fold higher than in the open ocean. Together, our findings highlight the complex nature of the IME, with important implications for carbon export dynamics in the Indian Southern Ocean.

Keywords

Oceanography

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