Influence of seasonal freshwater streamflow regimes on phytoplankton blooms in a Patagonian fjord

Abstract

ABSTRACTLarge-scale regional phenomena and global climate trends may alter the freshwater discharge of large Patagonian rivers and could modify local circulation patterns in ways that influence phytoplankton dynamics. Modifications detected in the streamflow regime of the Puelo River (41.5° S, Patagonia, Chile) in recent decades may affect the regularity of seasonal phytoplankton blooms in Reloncaví Fjord. We examined the occurrence/frequency of spring–summer and autumn phytoplankton blooms in Reloncaví Fjord with respect to seasonal and inter-annual changes in freshwater streamflows from 2003 to 2011. Surface chlorophyll-a derived from satellite-ocean colour and phytoplankton abundances revealed that significant recurrences of autumn phytoplankton blooms (> 2 mg Chl-a m−3, > 500 cell mL−1) were associated with historical low mean freshwater streamflows, mainly in autumn (< 350 m3 s−1). On the other hand, the occurrence of spring–summer blooms was related to high streamflows (> 470 m3 s−1) that increased mixing in the upper photic layer enough to enhance phytoplankton growth. Our findings imply that the intensity of autumn blooms in Reloncaví Fjord could be modulated by streamflow strength.