Study of photosynthetic productivity in the Northern Gulf of Mexico: Importance of diel cycles and light penetration

• The sub-pycnocline primary production is very important in the NGOM.
• Diel cycles of phytoplankton photosynthesis with depths exist in the NGOM.
• Phytoplankton in the NGOM adapt to light conditions by their own strategies.
• Phytoplankton community structure influences the measured photosynthetic parameters.

Based on 14C uptake assays, in vivo chlorophyll (chl) a fluorescence and HPLC pigment analysis, phytoplankton photosynthetic physiology and productivity in 24-h diel cycles were characterized at three stations in April and August 2012 in the Northern Gulf of Mexico. The results indicated the sub-pycnocline primary production accounted for 5–48% of the total integrated primary production, emphasizing the important influence of euphotic zone in shallow coastal areas. During the diel cycles, chl a  -specific light-saturated photosynthesis (PmaxB) as measured with photosynthesis versus irradiance curves (P–I) and the photoprotective pigment pool (diadinoxanthin, diatoxanthin, chl a) showed phytoplankton acclimation to be strongly influenced by water column structure (mixing versus stratification). Changes in chl a fluorescence and transformations between photoprotective pigments were most recognizable in surface samples. The dominate phytoplankton groups (diatoms and cyanobacteria in April and August respectively) also influenced the measured photosynthetic parameters. The Northern Gulf of Mexico is a typical coastal ecosystem with high variability of nutrients, light (intensity and attenuation) and mixing. Our study provided evidence that phytoplankton in this area are adapted to changing environmental conditions by means of fast responses as well as long-term photoacclimation strategies. Understanding the major drivers could help us to improve models involving the calculation of primary productivity, such as those focused towards understanding mechanisms controlling hypoxia.