The potential multi-faceted impacts of solar UVR on biogeochemical cycles have been assessed in this chapter using a combination of observations and modeling evaluations. In addition to the well-known effects of stratospheric ozone depletion on UVR, recent research indicates that other co-occurring global environmental changes in climate, land use and deposition are affecting the interactions of UV with element cycling. For example, it is now well-documented that CDOM, concentrations of which are sensitive to climate and land use changes, plays a key role in the interactions of UVR with aquatic biogeochemistry. CDOM controls the penetration of UV into many freshwater systems and the ocean and seasonal changes in CDOM concentrations linked to factors such as precipitation changes and photochemical and biological decomposition have important effects on aquatic UV exposure. Large scale changes in oceanic CDOM may be driven by El Nino-Southern Oscillation (ENSO) events that periodically alfect oceanic mixed layer depth, upwelling, and mixing dynamics. Significant differences have been observed in the spectral, chemical, physical, and photochemical properties of the CDOM in freshwaters and the sea. These differences, which usually become apparent in transects along estuarine and coastal environments, can be attributed to differences in sources, as well as in UV-induced direct and photochemically-altered microbial decomposition.

UV exposure generally inhibits phytoplankton photosynthesis and recent results indicate that, on the average, BWFs for such inhibition are similar for mid-latitude and Antarctic phytoplankton. UV also indirectly affects phytoplankton photosynthesis through its effects on the biological availability of iron and other trace metal nutrients.

UV exposure affects microbial decomposition processes both through direct inhibition of bacterial activity as well as through effects on the biological availability of carbon and nitrogen substrates. Models of UV interactions with phytoplankton and bacteria indicate that factors such as vertical mixing dynamics and mixed layer depth have important effects on damage and repair. The net effects of UV exposure on the biological availability of CDOM are dependent on its source. Most current evidence indicates that UV exposure stimulates the lability of terrestrially-derived CDOM but reduces the availability of algal-derived organic matter.

CDOM also can be directly photodecomposed by solar UVR to DIC, carbon monoxide, and various carbonyl-containing compounds. This direct photo-decomposition is accompanied by oxygen consumption although oxygen appears not to be directly involved in some of the photoreactions. Terrestrially-derived and open ocean CDOM have dissimilar quantum yield spectra for CO photoproduction, indicating that open ocean CDOM may produce CO less efficiently. Simulations using mathematical models indicate that UV induced decomposition potentially can consume all of the riverine CDOM that enters coastal regions. Observations of changes in UV-sensitive isotopic and lignin content are consistent with the simulations.

UVR can potentially affect nitrogen cycling through inhibition of nitrogen fixation and indirectly through changes in the biological availability of iron, an element that stimulates the growth of certain nitrogen-fixing cyanobacteria. UV also initiates the conversion of persistent DON into nitrogenous compounds that are readily assimilated by aquatic organisms.

Sulfur cycling is affected in a variety of ways, including UV photoinhibition of organisms such as bacterioplankton and zooplankton that affect sources and sinks of DMS and UV-initiated CDOM-sensitized photoreactions that oxidize DMS and produce carbonyl sulfide. Metal cycling also interacts in many ways with UVR via direct photoreactions of dissolved complexes and of metal oxides and indirect reactions that are mediated by photochemically-produced ROS. Photoreactions can affect the biological availability of essential trace nutrients such as iron and manganese, transforming the metals from complexes that are not readily assimilated into free metal ions or metal hydroxides that are available. Such photoreactions can enhance the toxicity of metals such as copper and can initiate metal redox reactions that transform non-reactive ROS such as superoxide into potent oxidants such as hydroxyl radicals.

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