Environmental control of phytoplankton distribution and photosynthetic performance at the Jan Mayen Front in the Norwegian Sea
The Jan Mayen Front is located in the Norwegian Sea to the east of the Jan Mayen Ridge and separates warm, salty Atlantic water and colder, less salty Arctic water. The effects of the light regime, hydrographical conditions and nutrients on the variations of chlorophyll a (chl a), quantum efficiencies of photochemistry in PSII (Fv:Fm) and effective absorption cross-section of PSII (σPSII) at the Front were studied in June 2007. Stratified waters were seen on both sides of the Front and lowered nutrient concentrations were seen shallower than 10-20 m. The lowest values of the spectral diffuse attenuation coefficient, Kd (λ), were found at 500-550 nm (0.07-0.16 m- 1), while Kd (465 nm) ranged between 0.08 and 0.17 m- 1 and Kd (380 nm) between 0.13 and 0.20 m- 1. Chl a concentrations seldom exceeded 1.0 mg m- 3 outside pure Atlantic Water, while elevated concentrations (3-4 mg m- 3) developed at depth (20-30 m) east of the Front in Atlantic Water. For the upper 100 m N/P, Si/P, N/Si and POC/PON ratios were 15.2, 8.0, 1.7 and 6.2, respectively. The quantum efficiency was strongly influenced by nutrients, suggesting nutrient limitation of phytoplankton biomass at the Front in June, but also light inhibition probably was a contributing factor in the upper part of the water column. High quantum efficiencies (0.5) and effective absorption cross sections (> 700 Å2 quanta- 1) were seen to the east of the Front and at depth (20-40 m) in stratified Atlantic waters. We therefore conclude that the Jan Mayen Front did not have a stimulatory effect on phytoplankton biomass enhancement and photosynthetic performance. This is in part due to the weak horizontal density front caused by density compensation of temperature and salinity characteristics of the adjacent water masses, and the associated weak vertical mixing.