Effects of high latitude UV radiation on phytoplankton and nekton modelled from field measurements by simple algorithms

Abstract

Irradiance measurements from the Barents Sea and coastal areas of northern Norway were used to calibrate a simple-algorithm empirical model that simulate PAR, UVR and UVR inhibition of phytoplankton carbon assimilation. Field measurements showed that UVR levels, in addition to solar zenith angle, were controlled by variations in cloudiness and probably also ozone layer thickness. The relative amount of UV-B and UV-A to PAR increased during periods with heavy cloud cover. Inhibition of phytoplankton photosynthesis, obtained from incubation experiments, was between 50 and 75% close to the surface and was detectable down to ca. 10m depth. The common spring bloom phytoplankter Phaeocystis pouchetii was less sensitive to UVR than centric diatoms. Critical depths (D_cr) for increase in phytoplankton biomass modelled from a 40-year time series of meteorological data from Lofoten were shallower during periods with elevated UVR levels, but we suggest that total annual production is not severely affected. Nauplii of the copepod Calanus finmarchicus start to feed during the post-spring bloom period, and a delayed or prolonged spring bloom will therefore probably not affect zooplankton or cod (Gadus morhua) larvae stocks negatively. Correlation analyses between cod year class strength and modelled UVR levels (r = -0.62) in May indicate a link between years with high UV radiation (clear sky) and poor cod year classes.