Spatial and temporal variations of photosynthetic parameters in relation to environmental conditions in coastal waters of the northern Gulf of Mexico

S.E. Lohrenz, G.L. Fahnenstiel, and D.G. Redalje


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 photosynthetic-irradiance saturation curve (Pbmax, alphaB , Ik) and to relate the observed variations to environmental conditions. Experiments to examine the importance of diel variation in upper mixed layer populations were conducted in July-August 1990 and March 1991. During July-August 1990, PBmax and Ik showed significant increases and alphaB decreased during the photoperiod in both river plume and shelf-slope populations. During March 1991, no consistent covariance of P-I parameters with local time was found, although highest values of alphaB in the river plume were observed in early morning. Seasonal variation in PBmax and alphaB were correlated with temperature. Spatial variations of photosynthetic parameters in the upper mixed layer ranged from twofold to threefold within any given cruise. Variations of photosynthetic parameters in the upper mixed layer were related to principle components derived from environmental variables, including temperature, salinity, nutrients, mixed layer depth, attenuation coefficient, and daily photosynthetically active radiation (PAR). Greater than 70% of the variation in the environmental variables could be accounted for by two principle components; the majority of this variation was associated with the first principle component, which was generally strongly correlated with salinity, nutrients, mixed layer depth, and attenuation coefficient. Correlations of PBmax, alphaB, and Ik, with the first principal component were found to be significant in some cases, an indication that the 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 principal components derived fr om depth, temperature, and mean daily PAR. For most cruises, PBmax and Ik were negatively correlated with daily PAR. This was consistent with a decrease in both PBmax and Ik with depth that could be related to decreasing daily PAR. Positive correlation s of alphaB with the first principal component for two cruises, March 1991 and April 1992, indicated an increasing trend with depth. In conclusion, relationships between P-I parameters and environmental variables in the region of study were significant in some cases, but variation between cruises made it difficult to generalize. We attributed this variation to the physically dynamic characteristic of the region and the possible effects of variables that were not included in the analysis such as species composition. Our findings do support the view that a limited set of observations may be adequate to characterize P-I parameter distributions in a given region within a restricted period of time.

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