Models of photosynthesis vs. irradiance, or P vs. I, are central to the estimation of aquatic photosynthesis from observed distributions of chlorophyll and light [Ryther Yentsch 1957, Talling 1957]. Since the advent of remote sensing from space, they have been used and modified extensively, because they are necessary components of algorithms to calculate primary productivity from ocean color [Ray Smith Bidigare Baker Brooks 1987, Bidigare Prezelin Smith scaling 1992, Platt Sathyendranath estimators 1993, Morel 1991 light]. The inhibition of photosynthesis by UV and excess PAR is well recognized [Neale aquatic 1987], and the effects of excess PAR have been incorporated into models of P vs. I [Platt Gallegos Harrison 1980, Neale Richerson 1987]. However, near-surface inhibition of photosynthesis is generally ignored in water-column models, because photoinhibition in situ is difficult to generalize on the basis of experimental measurements [Marra movement 1978, Neale aquatic 1987, Henley 1993], and because decreases in productivity associated with photoinhibition are relatively small compared with the variability encountered in remotely-sensed data. When interest is turned to UV and its effects on photosynthesis, however, the photoinhibition term is of paramount importance.
The effects of UV have recently been incorporated explicitly into a model of photosynthesis vs. irradiance [Cullen Neale Lesser science 1992]:
where 
is PAR (
) ,
is the rate of photosynthesis normalized to chlorophyll,
is the maximum attainable rate in the absence of
photoinhibition,
and 
is a saturation parameter
for photosynthesis.
The product,
,
is equivalent to 
,
the potential productivity in Eq (1).
It is essentially the same as in many models.
The inhibition term,
,
is one possible form of
in Eq. (1),
where 
(dimensionless)
is biologically weighted UV irradiance,
plus wavelength-independent biologically weighted PAR.
Descriptions of P vs. I and its inhibition by UV could take other forms,
but all, either implicitly or explicitly, would have to conform to
Eq. (1):
.
Years of work have gone into describing 
,
but as we show below, it is a major task to quantify biologically weighted
irradiance, 
,
and the functional form of its relationship with
.