Phytoplankton Patches at Oceanic Fronts Are Linked to Coastal Upwelling Pulses: Observations and Implications in the California Current System

Abstract

Locally enhanced biological production and increased carbon export are persistent features at oceanic density fronts. Studies often assume biological properties are uniform along fronts or hypothesize that along‐ and across‐front gradients reflect physical‐biological processes occurring in the front. However, the residence times of waters in fronts are often shorter than biological response times. Thus, an alternate—often untested—hypothesis is that observed biological patchiness originates upstream of a front. To test these two hypotheses, we explore an eddy‐associated front in the California Current System sampled during two surveys, separated by 3 weeks. Patches of high phytoplankton biomass were found at the northern ends of both surveys, and phytoplankton biomass decreased along the front. While these patches occurred in similar locations, it was unclear whether the same patch was sampled twice, or whether the two patches were different. Using an advection‐reaction framework combined with field and satellite data, we found that variations in along‐front gradients in dissolved oxygen, particle biovolume, and salinity support the conclusion that the two phytoplankton patches were different. They were only coincidentally sampled in similar locations. Backward‐ and forward‐in‐time tracking of water parcels showed that these phytoplankton patches had distinct origins, associated with specific, strong coastal upwelling pulses upstream of the front. Phytoplankton grew in these recently upwelled waters as they advected into and along the frontal system. By considering both local and upstream physical‐biological forcings, this approach enables better characterizations of critical physical and biogeochemical processes that occur at fronts across spatial and temporal scales.

Patchiness of plankton communities at fronts explained by Lagrangian history of upwelled water parcels

“…This cruise sampled an eddy-associated front, dubbed "E-Front," located approximately 200-250 km offshore of Point Conception, California. This front was positioned between an anticyclonic eddy to the west (offshore) and a cyclonic eddy to the east (inshore) (De Verneil and Franks, 2015;Stukel et al, 2017;Bednaršek and Ohman, 2015;Gangrade and Franks, 2023). The cross-frontal sampling included 2 transects (E1 and E2) with high horizontal resolution (3-5 km between consecutive stations), conducted on 4-5 August 2012 and 20-21 August 2012 respectively (Figure 1).…”

“…Using satellite-derived velocity fields, we tracked the water parcels of each transect station backward in time from the time of sampling for approximately 2 months (66 days) using the Euler method, described in Gangrade and Franks (2023). Horizontal velocity products were obtained from the Copernicus Marine Environment Monitoring Service (CMEMS; https://doi.org/10.48670/mds-00327).…”

“…Recently, many studies have employed Lagrangian approaches to describe how plankton communi-Lagrangian history explains plankton patchiness ties transform as they are transported, sometimes hundreds of kilometers in a matter of days (Lehahn et al, 2017(Lehahn et al, , 2018Messié and Chavez, 2017;Messié et al, 2022). These approaches have shown that the abundance of plankton is not only determined by their immediate environment (e.g., temperature and nutrient concentration (Mousing et al, 2016;Tzortzis et al, 2021;Haberlin et al, 2019)) but is also shaped by the conditions experienced during the previous weeks at different locations (D'Ovidio et al, 2010(D'Ovidio et al, , 2015Hernández-Carrasco et al, 2023;Gangrade and Franks, 2023). The first view-local environmental conditions determine species abundance-can be likened to the classic Eulerian concept of an "ecological niche."…”

“…We use the case study of a front in the southern CCS, characterized by an intense frontal jet and horizontally converging flow (de Verneil et al, 2019), to explore how coastal upwelling pulses propagate offshore (Gangrade and Franks, 2023) and generate plankton patchiness (Mangolte et al, 2023) on time scales of a few weeks. We evaluate the relationship between plankton distributions and the characteristics of water parcels based on two different frameworks.…”

Iron Limitation and Biogeochemical Effects in Southern California Current Coastal Upwelling Filaments

“…They have cross filament scales of 10–50 km, and along‐filament scales of 50–200 km (Zaba et al., 2021). The offshore flows make filaments efficient vehicles for transporting biogeochemical tracers from zones of upwelling to offshore waters, with nutrient‐rich conditions enabling the formation of phytoplankton blooms and fast‐moving cores which advect particulate matter offshore (Gangrade & Franks, 2023; Mohrholz et al., 2014; Muller et al., 2013; Nagai et al., 2015). Mesoscale features such as filaments can contain a predictable primary producer population gradient driven by processes of mixing, nutrient drawdown, and phytoplankton community evolution (e.g., diatom to dinoflagellate, Krause et al., 2020).…”