What controls biological production in coastal upwelling systems? Insights from a comparative modeling study

Abstract

Abstract. The magnitude of net primary production (NPP) in Eastern Boundary Upwelling Systems (EBUS) is traditionally viewed as directly reflecting the wind-driven upwelling intensity. Yet, different EBUS show different sensitivities of NPP to upwelling-favorable winds (Carr and Kearns, 2003). Here, using a comparative modeling study of the California Current System (California CS) and Canary Current System (Canary CS), we show how physical and environmental factors, such as light, temperature and cross-shore circulation modulate the response of NPP to upwelling strength. To this end, we made a series of eddyresolving simulations of the two upwelling systems using the Regional Oceanic Modeling System (ROMS), coupled to a nitrogen-based Nutrient-Phytoplankton-ZooplanktonDetritus (NPZD) ecosystem model. Using identical ecological/biogeochemical parameters, our coupled model simulates a level of NPP in the California CS that is 50 % smaller than that in the Canary CS, in agreement with observationally based estimates. We find this much lower NPP in the California CS despite phytoplankton in this system having nearly 20 % higher nutrient concentrations available to fuel their growth. This conundrum can be explained by: (1) phytoplankton having a faster nutrient-replete growth in the Canary CS relative to the California CS; a consequence of more favorable light and temperature conditions in the Canary CS, and (2) the longer nearshore water residence times in the Canary CS, which permit a larger buildup of biomass in the upwelling zone, thereby enhancing NPP. The longer residence times in the Canary CS appear to be a result of the wider continental shelves and the lower mesoscale activity characterizing this upwelling system. This results in a weaker offshore export of nutrients and organic matter, thereby increasing local nutrient recycling and reducing the spatial decouCorrespondence to: Z. Lachkar (zouhair.lachkar@env.ethz.ch) pling between new and export production in the Canary CS. Our results suggest that climate change-induced perturbations such as upwelling favorable wind intensification might lead to contrasting biological responses in the California CS and the Canary CS, with major implications for the biogeochemical cycles and fisheries in these two ecosystems.

Spatiotemporal variability and long-term trends of ocean acidification in the California Current System

“…The model's performance for simulating observed sea surface T , surface chlorophyll and the mixed layer depth was already evaluated in Lachkar and Gruber (2011) and Gruber et al (2006Gruber et al ( , 2011. The model simulates the observed annual and seasonal patterns of surface temperature well with correlations of ρ = 0.98 and ρ = 0.95, respectively.…”

“…The azimuth angle represents the correlation between the observations and the modeled parameters The distance between the model point and the observation point (filled ellipse on the abscissa) indicates the normalized root mean square (RMS) misfit between model and observational estimates. Gruber et al (2006Gruber et al ( , 2012; Lachkar and Gruber (2011) and we focus on the implications of this discrepancy in the Discussion.…”

“…The analyzed simulations are those presented in Gruber et al (2012) and use the same biogeochemical parameters as those employed by Lachkar and Gruber (2011). The model considers six pools of nitrogen, i.e., nitrate (NO − 3 ) and ammonium (NH + 4 ), phytoplankton, zooplankton and two types of detritus.…”

“…1). The model grid is defined by horizontal curvilinear coordinates and a terrain-following vertical coordinate (σ ) with 32 depth levels, with enhanced resolution near the surface and nearshore on the shallow shelf (see Gruber et al, 2011;Lachkar and Gruber, 2011, for a detailed description of the model setup).…”

The organic sea-surface microlayer in the upwelling region off the coast of Peru and potential implications for air–sea exchange processes

“…Eastern boundary upwelling systems (EBUSs) like the system off the coast of Peru are characterized by high biological productivity supported by deep upwelling of nutrients and often associated with subsurface oxygen minimum zones (OMZs). The supply of oxygen to the OMZ is largely controlled by physical (i.e., diffusive and advective) mechanisms, whereas biological processes (i.e., respiration of organic matter) provide sinks (Lachkar and Gruber, 2011).…”