Primary Production and Photophysiology of Phytoplankton Forming Subsurface Chlorophyll Maxima in a UK Summer Stratified Coastal Sea

In seasonally stratified temperate coastal and shelf seas, a subsurface chlorophyll maximum is often detectable with associated increased abundances of phytoplankton cells. These subsurface peaks in chlorophyll are generally closely associated with the seasonal thermocline, commonly occurring within or just below this density interface. UK coast and shelf sea research has often featured study of chlorophyll maxima present in summer months, yet the relative importance of primary production in these chlorophyll layers has rarely been investigated. Phytoplankton production rates and size-fractionated photophysiological measurements (>50 µm, >20 <50 µm, >10 <20 µm, >5 <10 µm, <5 µm) were made on water samples collected from subsurface chlorophyll maxima in stratified waters of the western English Channel during the summer of 2015. Carbon fixation rates were measured using ¹³C-bicarbonate incubations, and a Fluorescence Induction and Relaxation (FIRe) instrument was used to determine photophysiological characteristics of the various phytoplankton size fractions in vivo. Sampled chlorophyll maxima ranged between 2.4 – 19.0 µg L^-1, varied in thickness from less than 1 m to approximately 9 m, and phytoplankton biomass was consistently dominated by cells larger than 50 µm, e.g. Ceratium fusus. Rates of water column primary production in the western English Channel will be presented, and the significant contribution by the subsurface chlorophyll maximum will be highlighted. The photophysiological characteristics of the phytoplankton population that formed the sampled chlorophyll maxima will also be reported.