Phytoplankton phenology in the coastal upwelling region off central-southern Chile (35°S-38°S): Time-space variability, coupling to environmental factors, and sources of uncertainty in the estimates

Abstract

The annual cycle and phenology of phytoplankton (satellite-derived chlorophyll-a, Chl-a) in the coastal upwelling region off central-southern Chile, their time-space variation, the extent of their coupling with those of wind-driven upwelling (as Zonal Ekman Transport, ZET), Sea Surface Temperature (SST), and Photosynthetically Active Radiation (PAR) were analyzed using a ∼10 year satellite time series (2002-2012). Wavelet analysis (WA) was applied to extract the dominant frequencies of variability and their recurrence, to derive the phenological indexes, and to assess the extent of the coupling between Chl-a and environmental forcing in the annual frequency. Index estimates were obtained from minimum and maximum accumulated values in two different frequency bands, annual (WA-ANF) and all except the synoptic (WA-ALF). The annual frequency was dominant in all the variables; however, the annual cycle and phenology of Chl-a displayed higher submeso and mesoscale variability. The mean onset date of Chl-a was similar to those of PAR and ZET with WA-ALF and cross WA indicated that, for the most part, their annual cycles were coupled or coherent. Few interannual changes in Chl-a phenology were detected, including a ∼1 month longer duration (WA-ALF) during La Niña 2010-2011. The mean anomalies in the magnitudes of Chl-a and ZET during the upwelling season showed a slight but significant trend, negative for Chl-a and positive for ZET, while SST remained relatively constant. This pattern was unexpected since three La Niña-related conditions were identified during the 2007-2012 period.