BIOOPTICAL MODELING OF PRIMARY PRODUCTION - CONSEQUENCES OF VARIABILITY IN QUANTUM YIELD AND SPECIFIC ABSORPTION

Much interest is currently focused on attempts to estimate aquatic pri mary production at a variety of spatial and temporal scales using mode ls which require parameterization of phytoplankton photophysiology. Th e work described here investigates in situ variability in physiologica l properties which are important for many bio-optical models of photos ynthesis: the maximum photosynthetic quantum yield, the pigment-specif ic absorption coefficient and the product of the effective functional absorption cross section for photosystem II and the rate limiting turn over time for carbon fixation. Measurements were made on samples colle cted from 2 depths within the euphotic zone at 29 stations in the Cali fornia Current System (USA) during the winter to early spring of 1992. Within this region, even during a period of relatively low physical f orcing, all 3 parameters were found to be as highly variable as previo usly documented for a wide range of laboratory culture conditions and natural environments. Significant trends with environmental factors su ch as light, temperature and nutrient conditions were consistent with previous results from controlled laboratory studies of phytoplankton p hysiology. The consequences of the observed variability for estimation of primary production were investigated through the use of 2 bio-opti cal models, one expressed in terms of total phytoplankton absorption a nd the second accounting for only photosynthetically active absorption . Both models were sensitive to the observed variability in physiology , with greater sensitivity to the achieved photosynthetic quantum yiel d compared to the specific absorption coefficient. Model estimates wer e significantly less sensitive to observed variability in the absorpti on coefficient when only the photosynthetically active absorption was included, but little difference in sensitivity to quantum yield variab ility was found between the 2 models. Despite the documented mesoscale variability in phytoplankton optical and photosynthetic properties, m odel results suggest that it may be reasonable to use constant but rep resentative parameter values for larger or regional scale estimates of primary production.