Photosynthetic physiology and physicochemical forcing in the Arabian Sea, 1995

The Arabian Sea, characterized by strong seasonal monsoonal forcing as well as prolific mesoscale features, is a complex hydrographic environment for phytoplankton photosynthesis. Previous studies have demonstrated seasonal cycles in both primary production and photosynthetic biomass that may be related to patterns of photosynthetic efficiency, but the magnitude of water column efficiency is high throughout the year suggesting that other mechanisms may be operating. To uncover the patterns and regulation of photosynthetic efficiency on the synoptic scale (basin-wide, seasonal dynamics), here we focus on the relationship between physicochemical forcing such as nitrate concentrations and mixed layer depth and photophysiology. These observations, which are from a transect from the coast of Oman to about 1400 km offshore in the Arabian Sea at 65E, were made on five US JGOFS Arabian Sea Process cruises conducted between March and December 1995. Parameters of photosynthesis as quantified by pigmentation, photosynthesis-irradiance curves and spectral absorption properties were spatially and temporally variable, but demonstrate several marked trends. During the early Southwest Monsoon, when nitrate concentrations were elevated, maximum quantum yield of carbon uptake (φC,max, 0.051±0.024 mol C mol Q−1) and light-utilization coefficients (αB, 0.040±0.016 mg C mg Chl a−1 h−1 (μmol Q m−2 s−1)−1) were increased and non-photosynthetic pigment indices (NPPi, 0.339±0.089 g g−1) and chlorophyll a-specific mean absorption (σChl a, 0.032±0.012 m2 mg Chl a−1) were typically reduced compared to the Spring and Fall intermonsoon seasons (φC,max, 0.020±0.009; αB, 0.025±0.011; NPPi, 1.065±0.173; σChl a, 0.058±0.018). These patterns are in part driven by some significant correlations between mixed layer depth and nitrate concentrations and NPPi, φC,max, αB and σChl a at the synoptic scale. However, other factors not measured here such as time-lagged processes may also be important to the overall variability of photosynthetic physiology. In general, photosynthetic efficiency as measured by φC,max suggests phytoplankton populations that are exceptionally photosynthetically efficient during the SW Monsoon period, while at other times of the year efficiency may be a contributing factor to the limitation of primary production.