Wind‐driven currents and water masses shape spring phytoplankton distribution and composition in hydrologically complex, productive shelf waters

Abstract

The Northern Patagonian shelf (35–42°S SW South Atlantic) is a transitional area between the subtropical and subantarctic domains, where the shelf shifts from source to sink of atmospheric CO2. In the southeast portion (39–42°S, 61–55°W) of this area, winds interact with different water masses and picture a complex circulation that supports two productive frontal systems: the Mid‐Shelf Front (MSF) and the Patagonian Shelf‐break Front. We combined remote sensing (e.g., monthly surface chlorophyll a over the period: September–December 2002–2021), reanalysis (winds and surface currents), and in situ measurements from three cross‐shelf cruises in spring 2016, 2017, and 2019, to assess the underlying drivers of phytoplankton distribution. The anomalous high chlorophyll concentration in the MSF in spring 2016 was related to enhanced water mass retention by N‐NE winds. Contrasting phytoplankton assemblages emerged related to stratified waters in the shelf influenced by the Patagonian Current and vertically mixed waters at the continental edge (Malvinas Current). Large diatoms prevailed in the outer shelf followed by dinoflagellates, while nanoflagellates and microzooplankton dominated in the mid‐shelf. Furthermore, thickness of the subsurface chlorophyll maximum was broader in the early spring than in a more advanced state of the bloom where the coccolithophore Emiliania huxleyi was abundant. We showed that different time‐scale physical processes—winds, surface currents and vertical mixing—are underpinning the development of spring blooms with different taxa composition in the Patagonian shelf, where different water masses define the hydrological complexity that led to patchy phytoplankton.

The phytoplankton of the Patagonian Shelf-Break Front

“…The MC intrusions play a significant role in the development of the Mid Shelf Front (between 37.5 °S and 41°S) during autumn and spring (Romero et al, 2006) through fertilisation and stimulation of phytoplankton outburst (Díaz et al, 2018;Ferronato et al, 2021). Together with the nutrient supply from the MC upwelling, phytoplankton growth in the onshore portion of the PSBF is stimulated by strong vertical stability that maintains the cells in the well-illuminated surface layers (García et al, 2008, Signorini et al, 2006Ferronato et al, 2023). Seasonal thermal cycle promotes water column stratification and marks the timing of the spring bloom initiation (Rivas et al, 2006): north from ˜45°S the bloom starts in early spring (September and October) while in the south it starts in late spring to early summer (December-January), and continues until autumn (March) (Rivas et al, 2006;Romero et al, 2006).…”

“…The occurrence of phytoplankton blooms in the PSBF have been studied from field estimations (e.g Lutz and Carreto, 1991;. Gayoso and Podestá, 1996;Carreto et al, 2003Carreto et al, , 2008 García et al, 2008;Segura et al, 2013;Ferronato et al, 2023) and satellite images of chlorophylla(Rivas et al 2006;Romero et al, 2006;Signorini et al, 2006;Dogliotti et al, 2014;Marrari et al, 2017;Ferronato et al, 2023). The organisms responsible for these high-chlorophyll levels in the PSBF have been determined through pigment analysis(De Souza et al, 2012;Moreno et al, 2012;Carreto et al, 2016;de Oliveira Carvalho et al, 2022), their bio-optical properties(Lutz et al, 2010;Ferreira et al, 2013;Segura et al, 2013), and light microscopy (e.g.García et al, 2008; Gómez 2011;Balch et al, 2014; Olguín Salinas et al, 2015;Guinder et al, 2018;Ferronato et al, 2023), mainly focusing on spring and summer seasons (September-March) when the largest blooms take place.…”

“…Few studies have focused on the composition of phytoplankton blooms and the biogeochemical drivers of their structure and distribution (e.g Garcia et al, 2008;. Gonçalves-Araujo et al, 2012;Poulton et al, 2013;Carreto et al, 2016;Smith et al, 2017;Ferronato et al, 2023) and ecophysiology (e.g Valiadi et al, 2014),. including toxin producing microalgae and their associated phycotoxins (e.g Akselman et al, 2015;Almandoz et al, 2017;Fabro et al, 2017;Guinder et al, 2018;Tillmann et al, 2019)…”