SPATIAL AND TEMPORAL VARIATIONS OF PHOTOSYNTHETIC PARAMETERS IN RELATION TO ENVIRONMENTAL-CONDITIONS IN COASTAL WATERS OF THE NORTHERN GULF-OF-MEXICO

On a series of eight cruises conducted in the northern Gulf of Mexico, efforts were made to characterize temporal and spatial variability in parameters of the photosynthesis-irradiance saturation curve (P-max(B ), alpha(B), I-k) and to relate the observed variations to environment al conditions. Experiments to examine the importance of diel variation in upper mixed layer populations were conducted in July-August 1990 a nd March 1991. During July-August 1990, P-max(B) and I-k showed signif icant increases and alpha(B) decreased during the photoperiod in both river plume and shelf-slope populations. During March 1991, no consist ent covariance of P-I parameters with local time was found, although h ighest values of alpha(B) in the river plume were observed in early mo rning. Seasonal variation in P-max(B), and alpha(B) were correlated wi th temperture. Spatial variations of photosynthetic parameters in the upper mixed layer ranged from twofold to threefold within any given cr uise. Variations of photosynthetic parameters in the upper mixed layer were related to principal components derived from environmental varia bles, including temperature, salinity, nutrients, mixed layer depth, a ttenuation coefficient, and daily photosynthetically available radiati on (PAR). Greater than 70% of the variation in the environmental varia bles could be accounted for by two principal components; the majority of this variation was associated with the first principal component, w hich was generally strongly correlated with salinity, nutrients, mixed layer depth, and attenuation coefficient. Correlations of P-max(B), a lpha(B), and I-k with the first principal component were found to be s ignificant in some cases, an indication that spatial variability in P- I parameters was related to river outflow. Variation of P-I parameters in relation to depth and PAR were evaluated by regressions with princ ipal components derived from depth, temperature, and mean daily PAR. F or most cruises, P-max(B), and I-k were negatively correlated with the first principal component, which was strongly positively correlated w ith depth and negatively correlated with daily PAR. This was consisten t with a decrease in both P-max(B), and I-k with depth that could be r elated to decreasing daily PAR. Positive correlations of alpha(B) with the first principal component for two cruises, March 1991 and April 1 992, indicated an increasing trend with depth. In conclusion, relation ships between P-I parameters and environmental variables in the region of study were significant in some cases, but variation between cruise s made it difficult to generalize. We attributed this variation to the physically dynamic characteristics of the region and the possible eff ects of variables that were not included in the analysis such as speci es composition. Our findings do support the view that a limited set of observations may be adequate to characterize P-I parameter distributi ons in a given region within a restricted period of time.