Can colored dissolved organic material protect coral reefs by reducing exposure to ultraviolet radiation?.

摘要

Although mass coral bleaching events are generally triggered by high seawater temperatures, experiments have demonstrated that corals and reef-dwelling foraminifers bleach more readily when exposed to high energy, short wavelength solar radiation (blue, violet and ultraviolet [UVR]: lambda ∼ 280 - 490 nm). In seawater, colored dissolved organic matter (CDOM), also called gelbstoff, preferentially absorbs these shorter wavelengths, which consequently bleach and degrade the CDOM. Alteration of watersheds and destruction of coastal wetlands have reduced natural sources of CDOM to reefal waters.;I tested the null hypothesis that CDOM does not differ between reefs that differ in coral health, and that water transparency to UVR is not a factor in reef health. I measured absorption of UVR and UV irradiance at various reefs in the Florida Keys that differ in distance from shore and degree of anthropogenic development of the adjacent shoreline. My results show that intact shoreline - associated reefs and inshore reefs tend to be exposed to lower intensities of UVR, and lower degrees of photic stress, than developed shoreline - associated reefs and offshore reefs. Absorption due to CDOM (a g320) was higher, and photic stress, as revealed by increased production of UV-absorbing compounds, Mycosporine -- like Amino Acids (MAAs), was lower at the surface compared to the bottom.;The following results support my conclusion: ag320 and UV attenuation coefficients (Kd's) were higher at intact compared to developed shoreline -- associated reefs, and at inshore compared to offshore reefs. Spectral slope, S, was higher at offshore compared to inshore reefs, indicating a higher degree of photobleaching of CDOM. Relative expression of MAAs was higher at developed compared to intact shoreline -- associated reefs, at offshore reefs compared to inshore reefs, and at the surface compared to the bottom. Solar energy reaching the benthos at two inshore reefs of the same depth (6m) was approximately an order of magnitude higher at the reef near developed shoreline compared to the reef near intact shoreline, and may be due to greater degree of diffuseness of the underwater light field combined with lower ag at the developed shoreline - associated reef.