Impact of chromophoric dissolved organic matter on UV inhibition of primary productivity in the sea

A model was developed to assess the impact of chromophoric dissolved organic matter (CDOM) on phytoplankton production within the euphotic zone. The rate of depth-integrated daily gross primary productivity within the euphotic zone (integral(z)GPP(ez)) was evaluated as a function of dale, latitude, CDOM absorption (a(CDOM)) characteristics, chlorophyll a (chl a) concentration, vertical stratification, and phytoplankton sensitivity to UV radiation (UVR). Results demonstrated that primary production was enhanced in the upper similar to 30 m of the water column by the presence of CDOM, where predicted increases in production due to the removal of damaging UVR more than offset its reduction resulting from the absorption of photosynthetically usable radiation. At greater depths, where Little UVR remained, primary production was always reduced due to removal by CDOM of photosynthetically usable radiation. When CDOM was distributed homogeneously within the euphotic zone, integral(z)GPP(ez) was reduced under most bio-optical (i.e. solar zenith angle, chi a and CDOM absorption, and ozone concentration) and photophysiological (i.e. sensitivity to UVR) conditions because the predicted reduction in primary production at depth was greater than the enhancement of production at the surface. A reduction in integral(z)GPP(ez) was also predicted when CDOM or phyloplankton was restricted to near-surface waters (similar to 30 m) and CDOM absorption was moderate [a(CDOM)(450) > 0.015 m(-1)]. integral(z)GPP(ez), however, was enhanced when CDOM or phytoplankton was restricted to a very shallow surface layer (similar to 10 m), even ii CDOM absorption was high [a(CDOM)(lambda) at 450 nm similar to 0.07 m(-1)]. Changes in integral(z)GPP(ez) resulting from the presence of CDOM were only slightly sensitive to ozone concentrations. In well-mixed waters where the flux of UVB is relatively high, such as in the Southern Ocean when the ozone hole is present, the presence of CDOM should result in little or no enhancement of integral(z)GPP(ez), although phytoplankton production would be expected to increase somewhat in surface waters.