Variability in the sub-surface light climate at ecohydrodynamically distinct sites in the North Sea

This paper shows that the sub-surface light regime in the offshore North Sea varies spatially and seasonally between different ecohydrodynamic regions, which is likely to have important implications for primary production and carbon and nutrient fluxes in different areas of the North Sea. Measurements of downward irradiance were collected using different instruments (i.e. water column-profiling instruments, semi-autonomous moorings, and remote sensing) at three ecohydrodynamically distinct sites in the North Sea: in the southern Bight (SB), at the Oyster Grounds (OG) and north of the Dogger Bank (ND). The ND site was the deepest, and had the lowest and least variable light attenuation coefficients (mean Kd(PAR) = 0.11 m⁻¹). The onset of the phytoplankton spring bloom was earlier than at the other two sites. In summer, ND had low Kd(PAR) ∼ 0.07 m⁻¹ and light penetration was shifted towards blue-green wavelengths (490–560 nm), with water itself being one of the strongest contributors to overall attenuation. In contrast, the SB site was characterised by the highest and most variable values of Kd(PAR) (mean = 0.54 m⁻¹), comparable to near-coastal waters, and the spring bloom started almost a month later than at the ND site. The vertical variability of the attenuation coefficient and the strong PAR attenuation in the blue region of the spectrum were the result of higher concentrations of phytoplankton, CDOM and SPM, due to riverine inputs, shallow depth and strong tidal mixing. The OG site showed intermediate conditions between the ND and SB sites with a mean Kd(PAR) = 0.23 m⁻¹, and deepest penetration of irradiance in the green region of the spectrum at 560 nm. The implications of these results for phytoplankton growth and ecosystem modelling are discussed.