Phytoplankton community structure and depth distribution changes in the Cariaco Basin between 1996 and 2010

•We observed an increase in depth-integrated chl a concentrations of 16.4% from 1996 to 2010.•High biomass concentrations deepened from near surface waters to depths below 55 m.•Phytoplankton diversity increased by ~14% while seasonal variability in the community composition decreased.•Changes were a direct response to a decline in upwelling intensity and a deepening of the euphotic zone.

Phytoplankton community structure at the Cariaco Basin Time-Series site (10°30′N, 64°40′W) within the Cariaco Basin was examined using high performance liquid chromatography (HPLC) and microscopy between January 1996 to October 2000, and July 2006 to December 2010. Microscopy results from the upper 55 m indicated that the abundance of microphytoplankton (>20 µm) decreased by more than 5-fold from 1996 through 2010, accompanied by a change in diatom composition and a 4-fold reduction in large diatom cell numbers, particularly during upwelling periods. HPLC results also indicate a decline in diatoms, but not nearly to the same extent. Rather, HPLC data suggests a substantial reorganization of the phytoplankton community. Between the two time periods, there was a significant deepening of high biomass concentrations from near surface waters to depths below 55 m. At the same time, phytoplankton diversity increased by 14%, while seasonal variability in community composition declined. Vertical photopigment distributions of nanophytoplankton (<20 µm) further showed significant overall increases in coccolithophore, cryptophyte, and other phytoflagellate concentrations. The deepening in phytoplankton biomass below the mixed layer, the relative increase in smaller taxa, and the increasing homogeneity in phytoplankton distributions over time are likely a direct response to a reduction in upwelling intensity and a deepening of the euphotic zone associated with a decrease in the average trade winds of the region from 2006-2010, compared to 1996-2000. Surprisingly, the overall decline in large cells dominated by diatoms and an increase in smaller taxa has led to no significant change in the export of particulate organic carbon (POC) to depth, contrary to many studies that predict increasing stratification with future climate change scenarios will reduce POC export and carbon sequestration in the deep ocean.