Changing Pseudo-nitzschia and Dinophysis distributions in the North Sea and Western Approaches (NE Atlantic) and their potential use in biodiversity assessments

Pseudo-nitzschia and Dinophysis are two commonly occurring phytoplankton genera along Europe’s Atlantic coast. Species within these genera can produce toxins that accumulate in shellfish and pose health risks if consumed. Sustained monitoring of shellfish growing waters has generated decades-long time-series for these two genera, which have been considered for use in statutory biodiversity assessments as proxies for diatoms and dinoflagellates. There is also evidence that distributions are shifting; It is important to document these changes to anticipate risks for shellfisheries, and higher trophic levels. We assessed whether Pseudo‑nitzschia and Dinophysis data could support statutory biodiversity assessments and quantified distributional change through time. We compared seasonal and interannual variability in Pseudo‑nitzschia and Dinophysis with total diatom and dinoflagellate abundance at two UK stations (L4, English Channel; Stonehaven, Scotland) and used Continuous Plankton Recorder data (1960–2020) to map decadal distributions. A notable spring increase in Pseudo‑nitzschia occurred in the southern/eastern North Sea since ∼2000, followed by a post‑2010 reduction. Dinophysis remained largely summer‑restricted and declined after 2000 in parts of the eastern North Sea. Station and CPR data were complementary, but not directly comparable, reflecting differences in spatial variability and capture efficiency. We found limited evidence, however, to support proxy potential for either genus. While there were some localised seasonal correlations, they were generally inconsistent or weak. Our results show that neither genus is suitable as a proxy for diatom or dinoflagellate lifeforms in statutory biodiversity assessment, but both are potentially valuable as evidence for HAB pressure when interpreted alongside toxin measurements.