Phytoplankton coastal-offshore monitoring by the Strait of Dover at high spatial resolution: the DYPHYRAD surveys

Abstract. Long-term monitoring of phytoplankton communities is essential for understanding the functioning and evolution of marine systems. This paper presents a decadal dataset on phytoplankton observations conducted along a coastal-offshore transect by the Strait of Dover, at fine spatial resolution, using an automated in vivo approach. Nine stations (∼ 1 km apart) were sampled in the sub-surface off the Slack estuary, representing the northern limit of the Marine Protected Area of “Picard Estuaries and Opal Seas” (EPMO). Since 2012, phytoplankton functional groups were characterised in vivo in sub-surface waters using multi-spectral fluorometry (Fluoroprobe, bbe Moldaenke, Gmbh) and single-cell optical analysis with a pulse shape-recording flow cytometer (CytoSense and CytoSub, Cytobuoy b.v., the Netherlands). Total phytoplankton biomass was estimated via chlorophyll a extraction and in vivo fluorescence. Spectral and functional groups were quantified in terms of abundance, size, and estimated chlorophyll a in surface waters. Weekly sampling resolution allowed us to address the community composition in order to disentangle short-term, fine spatial, seasonal, and inter-annual variability. Additionally, biogeochemical and hydrological variables: temperature, salinity, Photosynthetically Active Radiation (PAR), and nutrients (nitrate, nitrite, phosphate, silicate) were systematically measured. Over 11 years, the survey generated 1835 samples from 268 dates, averaging 167 samples per year across 24 cruises. This unique dataset provides valuable insights into phytoplankton dynamics and environmental drivers in a temperate coastal system. Free access to the dataset can be found at https://doi.org/10.17882/104524 (Hubert et al., 2025b).