Modeling the effects of ultraviolet radiation on embryos of Calanus finmarchicus and Atlantic cod (Gadus morhua) in a mixing environment

Abstract

It is well established that ultraviolet radiation (UVR, 280-400 nm) harms aquatic organisms. Reductions in productivity have been reported for phytoplankton, ichthyoplankton, and zooplankton in incubations exposed to UVR. It is difficult, however, to estimate the effects of UVR in natural waters. Quantitative assessments of UVR effects on aquatic organisms require high-resolution measurements of solar irradiance and its attenuation in the water, spectral weighting functions for biological effects, and realistic descriptions of the distributions and vertical movements of particles in the water column. Using experimentally determined biological weighting functions for UV-induced mortality along with measurement-based models of solar irradiance and of vertical distributions of embryos as influenced by mixing, we modeled UVR-induced mortality in the early life stages of two key species in the upper estuary and Gulf of St. Lawrence, Atlantic cod (Gadus morhua) and the planktonic copepod, Calanus finmarchicus. G. morhua embryos are insensitive to UVR, with an average daily survival of apprx99% over numerous environmental conditions. C. finmarchicus are considerably more vulnerable, with an average survival of 90% +- 12% (SD). Lowest modeled daily survival was 59% under ambient ozone and 49% under 50% ozone loss. A sensitivity analysis allowed us to examine the relative influences of hydrographic variability, meteorological conditions, and ozone depletion on UVR-induced mortality in C. finmarchicus embryos. The modeled hydrographic and meteorological conditions are a representative range of natural variability for the St. Lawrence region during the 1997 field season, with the exception of extreme ozone depletion (50%). Effects are expressed as relative change of survival normalized to survival under a reference simulation. Similar to other studies, water column mixing and water clarity have the most significant influence on embryo survivorship, with a 3%-80% increased chance of survival when in static, compared with mixed waters, and a 3%-46% increased chance of survival when in the darkest, compared with the clearest waters. Cloudy skies increase survivorship between 1%-30%, and ozone depletion of 50% can decrease survivorship by 9%. On average, ozone depletion decreases survival by 3% and of the factors considered has the smallest influence on mortality of C. finmarchicus embryos.