INTRODUCTION: Aquatic ecosystems are key components of the Earth’s biosphere. They produce more than 50% of the biomass on our planet and incorporate at least the same amount of atmospheric carbon dioxide as terrestrial ecosystems. The primary producers in freshwater and marine ecosystems constitute the basis of the intricate food webs, providing energy for the primary and secondary consumers and are thus important contributors for the production of the human staple diet in the form of crustaceans, fish, and mammals derived from the sea. Solar UV can negatively affect aquatic organisms. The massive loss of stratospheric ozone over Antarctica during the past two decades as well as ozone depletion over the Arctic and high to mid latitudes have aroused concern about the effects of increased solar UV-B radiation on marine and freshwater ecosystems.6 Clear lakes and oceans in alpine and polar regions, where UV penetrates deep into the water column, may be particularly vulnerable.
The biological organisms in polar waters are even more at risk because of the limited repair capabilities under the inhibitory effects of low temperatures. Exposure to solar UV radiation can reduce productivity, affect reproduction and development, and increase the mutation rate in a Institut phytoplankton, macroalgae, eggs, and larval stages of fish and other aquatic animals. Consequences of decreased productivity are a reduced sink capacity for atmospheric carbon dioxide and negative effects on species diversity, ecosystem stability, trophic interactions and ultimately global biogeochemical cycles. In contrast, UV-A, in addition to being deleterious has some positive effects, as it can be used as a source of energy for photosynthesis, or in DNA-related repair mechanisms.
Solar UV radiation and penetration in aquatic ecosystems A growing number of stations and networks have shown that there has been an increase in solar UV-B radiation at the surface of and within aquatic systems8–11 which corresponds with stratospheric ozone depletion.12 Comparative measurements indicate continued increases in solar UV-B, which are masked by much larger seasonal changes and geographic differences Instrument accuracy has been improved in recent years and measurement deviations have been quantified. In addition, biological and chemical actinometers have been developed to determine UV-B doses on site during experiments and exposure.
Aquatic environments vary tremendously in their UV attenuation. Coastal areas and shallow continental shelf waters have a lower transparency than open ocean waters due to the runoff of silt and dissolved organic carbon (DOC) from shores. In open oceans the optical properties are largely determined by plankton and their degradation products,20–22 with zooplankton being an additional source of DOC.23 Owing to the high input of inorganic and decaying organic material, freshwater ecosystems usually. This false-color map represents the Earth’s carbon “metabolism”—the rate at which plants absorbed carbon out of the atmosphere during the years 2001 and 2002. The map shows the global, annual average of the net productivity of vegetation on land and in the ocean. The yellow and red areas show the highest rates, ranging from 2 to 3 kg of carbon taken in per km2 per year. The green areas are intermediate rates, while blue and purple shades show progressively lower productivity. In any given year, tropical rainforests are generally the most productive places on Earth. Still, the ongoing productivity near the sea’s surface, over such a widespread area of the globe, makes the ocean more productive than the land.