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

Oceans and Ice Branch, Code 971, NASA Goddard Space Flight Center, Greenbelt. Maryland 20771, USAABSTRACT: A model was developed to assess the impact of chromophoric dissolved organic matter(CDOM) on phytoplankton production within the euphoric zone. The rate of depth-integrated dailygross primary productivity within the euphotic zone (f, GPE.,) was evaluated as a function of date,latitude, CDOM absorption (a_ _X_M) characteristics, chlorophyll a (chl a) concentration, vertical stratifi-cation, and phytoplankton sensitivity to UV radiation (UVRI. Results demonstrated that primary pro-duction was enhanced in the upper -30 m of the water column by the presence of CDOM, where pre-dicted increases in production due to the removal of damaging UVR more than offset its reductionresulting from the absorption of photosynthetically usable radiation. At greater depths, where littleUVR remained, primary production was always reduced due to removal by CDOM of photosyntheti-cally usable radiation. When CDOM was distributed homogeneously within the euphoric zone, J',GPP,,_was reduced under most bio-optical (i.e. solar zenith angle, chl a and CDOM absorption, and ozoneconcentration) and photophysiological (i.e. sensitivity to UVRI conditions because the predicted reduc-tion in primary production at depth was greater than the enhancement of production at the surface.A reduction in f, GPP,._ was also predicted when CDOM or phytoplankton was restricted to near-surfacewaters (~30 m) and CDOM absorption was moderate [a 0.015 m t]. {_GPP_,_, however, wasenhanced when CDOM or phytoplankton was restricted to a very shallow surface layer (-I0 m}, evenit CDOM absorption was high lac.t_OM(_) at 450 nm _0. 07 m _]. Changes in /_GPP,,, resulting from thepresence of CDOM were only slightly sensitive to ozone concentrations. In well-mixed waters wherethe flux of UVB is relatively high, such as in the Southern Ocean when the ozone hole is present, thepresence of CDOM should result in little or no enhancement of _,GPP,,,, although phytoplankton pro-duction would be expected to increase somewhat in surface waters.KEY WORDS: CDOM. UV. Photosynthesis. Primary productionINTRODUCTIONRecent studies have demonstrated that ambientlevels of ultraviolet radiation (UVR, 280 to 400 nm) cansubstantially decrease rates of carbon fixation byphytoplankton (Karentz et al. 1991, Cullen et al. 1992,Helbling et al. 1992, Smith et al. 1992, Holm-Hansenet al. 1993a). At normal ozone {Oa) concentrations, i.e.344 Dobson Units (DU), UVR has been found to reduceprimary productivity in surface waters by as much as50% (Cullen et al. 1992, Holm-Hansen et al. 1993b,Cullen & Neale 1994). Because seawater strongly*E-mail: kevin@shark.gsfc. nasa.gov**Present address: Satellite Research Laboratory, NOAA/NESDIS, E/RA13, Washington, DC 20233, USA© Inter-Research 1996Resale of full article not permittedattenuates UVR, particularly at shorter wavelengths,UVR inhibition is most severe near the sea surface. Forexample, along 50 ° S in mid December, a normal levelof UVR was estimated to reduce phytoplankton pro-duction by 57 % at a depth of 1 m, while UVR inhibitiondecreased to <5% at 30 m (Arrigo 1994). Inhibitionassociated with UVR is due primarily to UVA (320 to400 nm), which is more abundant at the sea surfaceand is attenuated less strongly in the water columnthan the more damaging UVB (280 to 320 nml. Esti-mates of UVR inhibition resulting from O:_ depletionrange from an additional 1 to 12% of depth-integrateddaily primary productivity {Smith et al. 1992, Holm-Hansen et al. 1993a, Arrigo 1994), due to the increasedatmospheric transmission of UVB (UVA is not affectedby changes in 03).