Photophysiology and Light Absorption Properties of the Phytoplankton Community in the Northern Gulf Of Mexico

Studies on phytoplankton photophysiology are few in the Northern Gulf of Mexico and are mainly limited to the shallow inner-shelf and the immediate plume area of the Mississippi River. Here we expand our knowledge during the course of five research cruises between January 2009 and March 2010. We explored how variations in phytoplankton optical and photosynthetic properties were related to variability in their community composition. Phytoplankton photosynthesis parameters were quantified from photosynthesis versus irradiance (P-E) curves; phytoplankton community and light absorption properties were also determined from alongside analyses of pigments and spectral absorption. The maximum rate of photosynthesis (P^B_max) normalized to chlorophyll-a showed an increasing trend from the plume to offshore waters and was significantly higher (K-S test, p<0.05) in the prochlorophyte and cyanobacteria-dominated communities. Maximum quantum yield of photosynthetic carbon fixation (Φ_cmax) differed significantly between the zones, Φ_cmax was significantly higher (K-S test, p<0.05) in diatom-dominated high chlorophyll plume environment. Both P^B_max and Φ_cmax varied significantly (p<0.05) with the blue to red ratio of phytoplankton spectral absorption (a_φ(440)/a_φ(676)) but in opposite direction (r = 0.5 and r = -0.39 respectively). However, the initial slope of the P-E curve (α^B) normalized to chlorophyll-a varied widely across the continental margin and were independent of the community composition. and were positively correlated and were strongly related to the relative abundance of diatoms in comparison to other groups. The results presented in this study will help in understanding the photophysiological basis of variability in regional primary production and thereby improve accuracies of phytoplankton primary production estimates to better predict future changes from remotely sensed data.