Intra- and interannual variability of nearshore phytoplankton biovolume and community changes in the northern Humboldt Current system

Abstract

Phytoplankton biomass in the northern Humboldt Current system is known tofluctuate over intra- and interannual time scales in response to environmentalvariability. General changes in the phytoplankton community are known, but aquantitative description of the link between environmental signals and observedchanges is lacking. The present study examines these links through an analysis oflong-term phytoplankton community changes in Ancon Bay, Peru ( 118S) from1992 to 2004. The correlation of several environmental signals with phytoplanktonbiovolume was explored using stepwise multiple regression and community ana-lyses. Results indicate that environmental signals of interannual periodicity, e.g.those most associated with the El Nin˜o Southern Oscillation, were responsible foroverall biovolume levels, while signals of annual periodicity (i.e. sea surface temp-erature) correlated to changes in phytoplankton taxa proportions. Specifically,diatoms (e.g. Chaetoceros spp., Actinocyclus octonarius and Skeletonema costatum) dominatebiovolume only during the coldest periods (13–168C) when upwelling is strongest,whereas dinoflagellates dominate warmer periods. During warm periods, adecrease in offshore transport or the intrusion of offshore waters (e.g. from beyondthe shelf) increases the proportion of later phytoplankton successional stages,within the bay (e.g. dinoflagellates, flagellates and silicoflagellates). These resultssuggest that cross-shelf interactions between offshore and nearshore habitats existat both intra- and interannual scales as affected by local upwelling variability, withpossible consequences for resources of the nearshore upwelling ecosystem.KEYWORDS: phytoplankton; coastal upwelling; El Nin˜o phenomena; environ-mental effects; Eastern boundary currents; Ancon Bay; Peru; Humboldt Current