Physical controls on phytoplankton size structure, photophysiology and suspended particles in a Norwegian biological hotspot

The impact of the physical environment and phytoplankton size on particle types (zooplankton, biogenic matter or phytodetritus) in the water column is poorly understood. Here, we investigate how hydrography (e.g. water column stratification) impacts phytoplankton size and photophysiology across a productive coastal bank area. Additionally, we investigate how the physical environment and phytoplankton size structure influence the concentrations of plankton (e.g. copepods and diatom chains), biogenic forms (fecal pellets) and other particles (minerals, aggregates or phytodetritus) using discrete samples and in-situ optical instruments. Microphytoplankton (>20 µm), including many chain-forming diatoms, dominated (average > 90% of total size fraction) in un-stratified waters of the bank. Phytoplankton within the bank region also required more irradiance to saturate photosynthesis, as indicated by the onset light saturation parameter (Eₖ, average 297 µmol photons m⁻² s⁻¹), suggesting high plasticity to a dynamic light environment. Conversely, the contribution of nano- and picophytoplankton (<20 µm), such as flagellates increased (up to 36% of total phytoplankton size fraction) towards stratified off-bank waters. The phytoplankton community from off-bank had lower Eₖ (average 199 µmol photons m⁻² s⁻¹) and presented higher concentrations of photoprotective pigments, such diatoxanthin – used in the xanthophyll cycle to cope with light stress and potential photo-damage. Higher concentrations of copepods (>1 × 10³ counts m⁻³), fecal pellets (>1 × 10⁴ counts m⁻³) and ammonium (>0.5 µM) within the bank compared to off-bank regions, indicated that copepods were actively grazing in this region. Low stratification (average stratification index (SI) < 6 × 10⁻³ kg m⁻⁴) allowed for intensive mixing, which might have promoted the high concentration of aggregates (>5 × 10⁵ counts m⁻³) within the bank when compared to off-bank (SI off-bank > 10 × 10⁻³ kg m⁻⁴). Our results, obtained using automated techniques measured in-situ, represent an innovative approach to demonstrate that phytoplankton size and stratification influence the nature of particles found in the water column (including aggregates, fecal pellets and grazer abundances).