Changes in phytoplankton production and upwelling intensity off A Coru\xf1a (NW Spain) for the last 28\xa0years

Abstract

The analysis of a 28-year-long (1989–2016) series of monthly measurements of chlorophyll concentrations and primary production rates at a shelf station off A Coruña (NW Spain) provided evidence of changes at several time scales that were only partly related to upwelling intensity. Chlorophyll determinations were made in acetonic extracts and primary production rates by the measurement of 14C-uptake by natural phytoplankton populations in simulated in situ conditions. Wavelet analysis revealed multiple modes of variation in both series, particularly at high frequencies, but some were only significant for part of the series. For instance, the seasonal cycle was not uniform through the series despite the annual repetition of maxima and minima. At multiannual time scales, both series were divided in three quasi-decadal periods characterized by significant increases in mean values. Fluctuations in chlorophyll and primary production covaried with changes in upwelling intensity at annual scales, but annual means showed low correlation. Changes in dissolved nutrient concentrations from continental sources were the likely drivers of the observed changes in productivity at large time scales. Increases in the decadal mean rates of production and concentrations of chlorophyll were driven by increased intensity of spring blooms associated to increased nutrients and low salinity water in the surface. In contrast, blooms caused by upwelling nutrients remained unchanged along the series. This study illustrates the complexity of interactions in coastal upwelling areas at large time scales, where changes in continental nutrient inputs may affect phytoplankton production more than variations in upwelling intensity.