Springtime size-fractionated primary production across hydrographic and PAR-light gradients in Chilean Patagonia (41–50°S)

•Primary production across hydrographic and PAR-light gradients in Chilean Patagonia.•Primary production displayed a latitudinal pattern decreasing southwards.•Large and small-cell phytoplankton dominated the northern and southern Patagonia, respectively.•Large-cell dominated the primary production in warmer, more saline waters exhibit greater PAR.•PAR attenuation in the water column is one of the key factors shaping the primary production. We combined on-deck and in situ measurements and satellite-derived data to study the spatial variability of springtime size-fractionated primary production and chlorophyll-a biomass along gradients of hydrographic conditions and surface PAR in central and northern Chilean Patagonia (41–50°S). This extensive and fragmented region encompasses numerous fjords and channels, as well as the northern and southern icefields (46–47°S, 48–52°S). Primary production displayed a latitudinal pattern decreasing southwards (6-fold lower), particularly toward areas influenced by rivers with a nival regime. Micro-phytoplankton (>20μm) dominated the primary production (57–93%) and chlorophyll-a (43–91%) of northern sites, where warmer and more saline surface waters exhibit greater PAR irradiance. Small phytoplankton cells (50% of carbon fixation and chlorophyll-a in the southernmost sites, especially those located near glaciers and major rivers, where surface temperature, salinity, and PAR irradiance were lowest. The long-term (2002–2012) average field of springtime PAR derived from satellite imagery showed a southward increase in longitudinal gradients, which indicates that spatial changes in surface light attenuation along this region are largely driven by glacier-derived freshwater inputs. A principal component analysis of surface temperature, salinity, and PAR produced an ordination of sites that was consistent with spatial changes in the balance of oceanic versus riverine influence on surface conditions along this region. Total primary production was significantly correlated (r=0.61, p=0.007) with the first principal component, which explained 65% of joint variability in hydrographic conditions and PAR. The same principal component clearly separated sites in northern Patagonia where micro-phytoplankton dominated total primary production – along the Reloncavi fjord and Inner Sea of Chiloe – from those located further south where other size fractions were equally or more important. We stress the need to includ