Plankton community composition and productivity near the Subantarctic Prince Edward Islands archipelago in autumn

Abstract

The Subantarctic Ocean is a sink for atmospheric CO2, largely due to its biological pump, which is enhanced by the influence of the Subantarctic islands on the plankton ecosystem (the so‐called “island mass effect”). The influence of the Prince Edward Islands archipelago in the Indian Subantarctic on the surrounding hydrography and benthos has been well studied; however, over the last two decades, little attention has been paid to the functioning and productivity of its plankton ecosystem. Here, we present the first measurements of primary production at the archipelago in over 20 years and the first‐ever rates of secondary production, interpreted in the context of hydrographic, biogeochemical, and plankton community composition data. In autumn 2017, after the late‐summer bloom, nanophytoplankton dominated the near‐island waters and regenerated nutrients fuelled 76% of phytoplankton growth. Primary production and carbon export potential (inferred from nitrate uptake) reached a local maximum in the inter‐island region, which we attribute to water‐mass retention and stratification over the inter‐island plateau (a manifestation of the island mass effect). We observed a diverse mesozooplankton community, likely a remnant of the late‐summer bloom rather than representing the consumers feeding on the in situ (nano)phytoplankton. We estimate that even after the decay of the late‐summer bloom, roughly a quarter of the planktonic carbon was potentially exportable. This finding implies that the archipelago's upper‐ocean ecosystem sequesters atmospheric CO2 at least through autumn despite high rates of recycling and inefficient trophic transfer, thereby contributing to the island mass effect‐associated strengthening of the Subantarctic Ocean's biological pump.

Net Community Production in the Argentine Basin Estimated From Nitrate Drawdown Using Biogeochemical Argo Floats

“…As we do not have any measurements of iron, Si(OH) 4 − , or NH 4 + concentrations from the floats, these arguments and suggestions remain speculation. However, numerous studies in the Southern Ocean have reported similar transitions from NO 3 − uptake in spring and early summer (when light, macronutrients, and micronutrients are abundant), followed by uptake primarily of regenerated nitrogen (e.g., NH 4 + ) as iron and/or Si(OH) 4 − concentrations decrease and the phytoplankton community composition shifts from larger‐celled organisms (e.g., diatoms) to smaller‐celled organisms that may thrive in conditions of lower light and iron availability and take advantage of higher NH 4 + concentrations resulting from heterotrophic processing (e.g., zooplankton grazing) of the preceding spring bloom (Flynn et al., 2021; Lourey et al., 2003; Mdutyana et al., 2020; Smith et al., 2022; Stirnimann et al., 2021).…”

Spatial and Seasonal Variations of the Island Mass Effect at the Sub-Antarctic Prince Edward Islands Archipelago

“…Previous in situ chla measurements on the PEI shelf range between 0.01 and 2.8 mg m −3 , with only a few studies documenting values above 1 mg m −3 [26,32,46]. The phytoplankton blooms at the PEIs, as well as Bouvet and Macquarie Islands, do not seem to be as large as those at the Kerguelen, Crozet, and South Georgia Islands [7].…”

“…However, their focus on basin and larger scales and zonally averaged patterns has not necessarily provided a clear depiction of the variability at more regional and local scales around the PEIs [3,80]. Despite many earlier in situ studies of phytoplankton variability at and around the PEIs, there is still little knowledge on the seasonality of these patterns due to constrained sampling [17,26,32,46,47]. To our knowledge, the present study provides the first detailed satellite-based investigation of spatial and seasonal chla variations around the PEIs, thus improving the knowledge of local chla variability around the islands.…”

“…Numerous previous studies have examined the oceanographic and biological variability associated with the IME at the PEIs, e.g., [12,20,21,26,32,36,[42][43][44][45][46][47]. However, their findings have been derived from limited in situ observations collected during research cruises mostly restricted to April-May each year, with very few measurements in other seasons [17], and little is known about the persistence or spatial extent of the IME at the PEIs [32,46,48]. A recent study used satellite observations and reanalysis datasets to describe the seasonal cycles of Sea Surface Temperature (SST), winds and currents around the PEIs [49].…”

A Circum‐Antarctic Plankton Isoscape: Carbon Export Potential Across the Summertime Southern Ocean

“…Indeed, diatoms have been shown to take up $${{\text{NO}}_{3}}^{-}$$ faster than other phytoplankton groups at comparable substrate concentrations (Eppley et al., 1969; Hildebrand & Dahlin, 2000; Paasche et al., 1984), often achieving higher $${{\text{NO}}_{3}}^{-}$$ uptake and reduction rates than required for growth, especially in cold, $${{\text{NO}}_{3}}^{-}$$‐rich environments (Lomas & Glibert, 1999, 2000). f new was negatively correlated with the contribution to Chl‐ a of nano‐phytoplankton, which typically show a higher affinity for $${{\text{NH}}_{4}}^{+}$$ than micro‐phytoplankton (Probyn, 1985; Stirnimann et al., 2021; Wafar et al., 2004). The apparent preference of nano‐phytoplankton for $${{\text{NH}}_{4}}^{+}$$ over $${{\text{NO}}_{3}}^{-}$$ can be explained by their higher surface‐area‐to‐volume ratio, which makes them more competitive for scarce nutrients than the larger diatoms (Marañón, 2015).…”