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

• Published in: Progress in oceanography 2019-07, Vol.175, p.284-299
• Main Authors: Fragoso, Glaucia M., Davies, Emlyn J., Ellingsen, Ingrid, Chauton, Matilde S., Fossum, Trygve, Ludvigsen, Martin, Steinhovden, Kristine B., Rajan, Kanna, Johnsen, Geir
• Format: Article
• Language: English
• Subjects: Automated underwater vehicles; In-situ imaging; Photophysiology; Phytoplankton size structure; Suspended particles
• ISSN: 0079-6611; 1873-4472
• DOI: 10.1016/j.pocean.2019.05.001

•Phytoplankton and particles from Mausund Bank varied in concentration and size.•Vertical mixing explains the phytoplankton size variation in the bank.•Phytoplankton within bank region has high plasticity to dynamic light environment.•Copepods were potentially actively grazing on the microphytoplankton.•Intensive mixing promoted the agglomeration of particles within the bank. 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 (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) 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 stra