Introduction

Biogeochemical cycles describe the complex interaction of biological, chemical, and physical processes that control the exchange and recycling of matter and energy at and near the Earth's surface. Considerable recent research has demonstrated that the cycles are sensitive to solar UVR in a variety of ways. A primary objective of this paper is to present and analyze interactions between UV and aquatic biogeochemistry, taking into account the fact that other co-occurring global environmental changes can influence the UV effects. Interest in this topic has been stimulated in part by declines in stratospheric ozone over the past two decades that have resulted in increases in solar UV-B radiation (280-315 nm) reaching the Earth's surface. Current projections indicate that return of the ozone layer thickness to pre-1980s levels may not occur for another 50 years.

Aquatic biogeochemical cycles are sensitive to other factors that affect underwater UV exposure such as UVR attenuation in the water, mixing and stratification. Underwater UV exposure is thus sensitive to global changes in climate, land use and other human activities that affect aquatic transport, composition and optical properties. These effects are changing the UV-A (315-400 nm) as well as the UV-B spectral region so the discussions here include the biogeochemical effects that are influenced by the entire solar UV spectrum.

Some of the many interactions between solar UV and aquatic biogeochemical cycles are illustrated in Figure 1. Biological processes are directly affected through increased damage to DNA, and damage to other molecules, including proteins, RNA, membrane-associated molecules, and chlorophyll. Resulting effects include changes in photosynthesis and respiration by phytoplankton and alterations in microbial respiration, biological availability of carbon and nitrogen, sulfur cycling, metal redox reactions and organic pollutant dissipation. Geochemical processes that are strongly influenced by UVR include: formation of greenhouse and chemically important trace gases (carbon dioxide (CO2), carbon monoxide (CO), nitric oxide (NO), DMS, carbonyl sulfide (COS)); con-

Altered precipitation

Acid, iron, & peroxide deposition

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