Extensive summer blooms in fjords of West Spitsbergen triggered by run-off from land

Summary

Primary production in Svalbard fjords is highly seasonal with the largest fraction of annual primary production traditionally associated with the phytoplankton spring bloom, usually peaking in May. The spring bloom is terminated when winter accumulated nutrients in surface waters are exhausted and typically followed by a post-bloom period characterized by low phytoplankton biomass. Here we report on extensive phytoplankton blooms in fjords of West Spitsbergen during late-June 2024 occurring four weeks after the peak of the spring bloom and coinciding with the snow freshet period. We focus on bloom development in Kongsfjorden and explore the environmental drivers that triggered this bloom. The bloom developed in the upper 5 – 15 m of the water column with exceptionally high chlorophyll-a concentrations of up to 25 µg L-1. Its species composition was distinctly different from the spring bloom, dominated by diatoms, and almost exclusively dominated by a pelagophyte species new to science. Surface currents, nutrient and chlorophyll-a profiles and satellite images indicate that the bloom developed inside the fjord and was subsequently advected onto the adjacent shelf. Since post-spring bloom nitrate concentrations in fjord surface waters were completely exhausted in the upper 25 m, we hypothesize that this bloom was triggered by land-derived nutrients associated with meltwater run-off, highlighting terrestrial drivers of marine primary production in fjord ecosystems. Such extensive blooms during early summer are unusual and could possibly be associated with the record melt on Svalbard in summer 2024.