Sammendrag
A bloom of the fish-killing haptophyte Chrysochromulina leadbeateri in northern Norway during May and June
2019 was the most harmful algal event ever recorded in the region, causing massive mortalities of farmed
salmon. Accordingly, oceanographic and biodiversity aspects of the bloom were studied in unprecedented detail,
based on metabarcoding and physico-chemical and biotic factors related with the dynamics and distribution of
the bloom. Light- and electron-microscopical observations of nanoplankton samples from diverse locations
confirmed that C. leadbeateri was dominant in the bloom and the primary cause of associated fish mortalities. Cell
counts by light microscopy and flow cytometry were obtained throughout the regional bloom within and
adjacent to five fjord systems. Metabarcoding sequences of the V4 region of the 18S rRNA gene from field
material collected during the bloom and a cultured isolate from offshore of Tromsøy island confirmed the species
identification. Sequences from three genetic markers (18S, 28S rRNA gene and ITS region) verified the close if
not identical genetic similarity to C. leadbeateri from a previous massive fish-killing bloom in 1991 in northern
Norway. The distribution and cell abundance of C. leadbeateri and related Chrysochromulina species in the recent
incident were tracked by integrating observations from metabarcoding sequences of the V4 region of the 18S
rRNA gene. Metabarcoding revealed at least 14 distinct Chrysochromulina variants, including putative cryptic
species. C. leadbeateri was by far the most abundant of these species, but with high intraspecific genetic variability.
Highest cell abundance of up to 2.7 × 107 cells L 1 of C. leadbeateri was found in Balsfjorden; the high
cell densities were associated with stratification near the pycnocline (at ca. 12 m depth) within the fjord. The cell
abundance of C. leadbeateri showed positive correlations with temperature, negative correlation with salinity,
and a slightly positive correlation with ambient phosphate and nitrate concentrations. The spatio-temporal
succession of the C. leadbeateri bloom suggests independent initiation from existing pre-bloom populations in
local zones, perhaps sustained and supplemented over time by northeastward advection of the bloom from the
fjords
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