Zhu, Kechen , Krause, Jana, Marín‐Arias, Camila, Mestre, Mireia, Höfer, Juan, Browning, Thomas J. , Achterberg, Eric P. and Hopwood, Mark J. (2024) An Impact of Cobalt on Freshwater Phytoplankton in Warming Polar Regions?. Geophysical Research Letters, 51 (23). e2024GL110065. DOI 10.1029/2024GL110065.

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Abstract

Polar freshwater ecosystems are sensitive to climate change, facing increasing temperatures and its consequences such as glacier retreat. Phosphorus and/or fixed nitrogen are widely expected to limit primary production in most freshwater environments, however the role of micronutrients is largely un‐characterized. We measured dissolved nitrate and nitrite, phosphate, cobalt, iron, manganese and zinc in a selection of Greenland and Antarctic lakes, and report the first evaluation of cobalt regulating phytoplankton growth in these systems using bioassay incubations. Results showed cobalt potentially co‐limited phytoplankton in three of the eight sites tested. A time‐series of dissolved cobalt measured in Kobbefjord (southwest Greenland) runoff corroborated low cobalt availability during summer. This was in contrast to manganese and iron which remained at high concentrations throughout summer. We hypothesize that high manganese:cobalt and/or zinc:cobalt ratios in runoff, may impede cellular cobalt acquisition, meaning cobalt co‐limitation of primary production is plausible in many polar freshwater environments.

Plain Language Summary

Primary production in aquatic environments relies on supplies of bio‐accessible nitrogen and phosphorus in addition to light availability. Glacier melt and permafrost degradation have accelerated worldwide over the past two decades, inducing changes in these resources with ensuing ecological impacts. The potential ecological role of bio‐essential metals such as cobalt has however been sparsely tested in polar aquatic environments. Here we conducted nutrient addition bioassay incubations in a selection of Greenland and Antarctic lakes, and found phytoplankton growth was potentially co‐limited by dissolved cobalt availability in three of the eight tested sites. Manganese, zinc and cobalt can compete for cellular uptake, therefore we further analyzed dissolved concentrations in runoff across a multi‐year time‐series, finding that the ratios of these elements are potentially conducive to inhibition of cobalt uptake. We hypothesize that cobalt may more broadly co‐limit aquatic primary production as a result of high manganese:cobalt and/or zinc:cobalt ratios, with potential impacts on microbial community composition.

Key Points

Concentrations of macro‐ and micro‐nutrients in a selection of Greenland and Antarctic lakes indicated cobalt deficiency for phytoplankton
Bioassay incubation results showed cobalt potentially co‐limited phytoplankton in three of the eight sites tested
A time‐series of measurements in Greenland runoff suggested high manganese:cobalt and zinc:cobalt ratios could impede cellular cobalt acquisition

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