A changing Arctic – Glacier meltwater runoff shifts phytoplankton community structure and collapses zooplankton in a high Arctic fjord (Kongsfjorden, Svalbard)

The high Arctic is one of the most sensitive regions to climate warming, which not only increase temperatures but also causes a substantial increase in freshwater discharge into coastal areas, mainly due to accelerated glacier melting. These meltwater runoffs alter the physical and chemical conditions of the water column, reducing light availability, changing light spectrum, decreasing salinity, and bringing in organic and inorganic particulate and dissolved matter. Effects of glacier meltwater on the structure and the functioning of coastal Arctic plankton communities were assessed by performing an in situ microcosm experiment in the Kongsfjorden, located on the west coast of Spitzbergen in the Svalbard Archipelago in Summer 2022. Two levels of meltwater runoff, moderate and strong, were tested by adding meltwater from two land-terminating glaciers to natural Kongsfjorden coastal waters in microcosms. Throughout the experiment, physical and chemical variables, as well as plankton abundance, diversity and metabolism, were monitored during 11 days. As expected, simulated runoffs significantly decreased salinity and light availability, but also pH, while they enhanced both N:P ratio and silicate concentrations. Meltwater runoffs significantly affected the phytoplankton community structure, favoring microphytoplankton - particularly diatoms - over nanophytoplankton; and increasing diatom diversity while decreasing that of dinoflagellate. The zooplankton community was even more affected than phytoplankton as both proto- and metazooplankton abundances strongly declined. Notably, copepod nauplii, the dominant metazooplankton group in the fjord during the experiment, almost disappeared. This suggests a considerable impact of glacier meltwater runoffs on plankton food web functioning and a weakening of zooplankton’s top-down control on phytoplankton under glacier meltwater runoff conditions. In contrast, the abundances of viruses, bacteria, picophytoplankton, and heterotrophic nanoflagellates were not significantly impacted by the simulated runoffs, suggesting a stronger resistance of these components of the microbial food web. However, based on oxygen measurements, gross primary production was not significantly affected by the treatments, indicating a strong functional resistance of the phytoplankton community and suggesting reduced light induced by the runoffs may have limited any increase in gross phytoplankton photosynthesis. Our findings indicate that the intensification of glacier meltwater runoff due to climate warming could significantly alter the structure of coastal Arctic plankton communities. Reduced zooplankton abundances and their grazing on phytoplankton, could disrupt normal ecosystem functioning, potentially reducing the efficiency of carbon transfer to higher trophic levels. Such changes may have serious consequences for the productivity and health of coastal Arctic ecosystems in the future.