Biological production response to coastal upwelling intensification: insights from a comparative modeling study

Zouhair Lachkar, zouhair.lachkar@env.ethz.ch, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland

Niki Gruber, nicolas.gruber@env.ethz.ch, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland

Zouhair Lachkar

Zouhair Lachkar won the prize for the best oral presentation by a young scientist in Workshop 2 at the recent IMBER IMBIZO II. The prize was the book "Marine Ecosystems and Global Change" edited by M. Barange et al., Oxford Univ. Press.Equatorward winds along the eastern boundaries of the Atlantic and Pacific induce the upwelling of nutrient-rich water into the euphotic zone, thereby stimulating phytoplankton growth and leading to highly productive marine ecosystems (Pauly and Christensen, 1995). While supporting very rich ecosystems, the Eastern Boundary Upwelling Systems (EBUS) are also vulnerable to various anthropogenic perturbations. Directly driven by the atmospheric circulation, these ecosystems are particularly sensitive to global climate change and its potential impact on alongshore winds. Atmospheric and sedimentary observations point toward a recent strengthening of the coastal upwelling favorable winds (Shannon et al., 1992; Schwing and Mendelssohn, 1997; Mendelssohn, 2002; McGregor et al., 2007). This upwelling intensification has been related to a global warming-induced increase in the land-sea thermal gradient (Bakun 1990), and is therefore projected to further increase in the future (Snyder et al, 2003, Diffenbaugh et al 2004). Yet, the effects of enhanced upwelling on marine ecosystems are still largely uncertain. In particular, the question of how biological productivity in these systems might respond to such wind perturbation is still unresolved. While recent observations generally show the expected positive trends in primary production in most EBUS (Kahru et al., 2009, Demarq 2009), individual ecosystems exhibit very contrasting sensitivities to comparable upwelling-favorable changes. Why do these sensitivities differ?

To answer this question, we undertook a

Figure 1: Snapshots of simulated surface chlorophyll-a in the Canary CS (left panel) and the California CS (right panel) in early April.

comparative modeling study contrasting two of the four major EBUS, namely the California Current System (California CS) and the Canary Current System (Canary CS). Our goal is to explore the dominant mechanisms controlling the response of these ecosystems to changes in upwelling-favorable winds. The comparison of these two systems provides an adequate framework for generalizing individual observations and for developing a better understanding of the underlying dynamics of EBUS ecosystems in general. We made a series of eddy-resolving simulations of the California CS and the Canary CS using the Regional Oceanic Modeling System – ROMS – coupled to a nitrogen-based Nutrient-Phytoplankton-Detritus-Zooplankton (NPDZ) biogeochemical model (Figure 1). In order to explore the effects of upwelling-favorable wind intensification on coastal productivity, we compare our standard simulations for both EBUS with those forced with increased wind stress.

Figure 2: (a) left panel: 0-100km nearshore averaged Net Primary Production (NPP) as simulated with different wind forcings in the California CS (orange) and the Canary CS (purple). (b) right panel: 0-100km nearshore inventory of nitrate in the euphotic zone as simulated under different wind forcings in the California CS (orange) and the Canary CS (purple).

The increased wind simulations show contrasting productivity responses between the California CS and Canary CS (Figure 2). Despite a substantial increase in the nutrient supply associated with the upwelling intensification, the productivity shows only a limited enhancement in the California CS relative to the Canary CS. The reason for these differences are, in part, related to the faster phytoplankton growth in the Canary CS due to warmer temperatures, which results in a more efficient use of nutrients in this system. An additional factor is the rate of water renewal, i.e. the inverse of the residence time, in the nearshore area. Using a Lagrangian diagnostic tool (Blanke and Raynaud (1997) to evaluate water mass residence times, generally we indeed found that the newly upwelled water masses stay substantially longer in the nearshore area in the case of the Canary CS, before getting subducted farther offshore (Figure 3). This enhances the buildup of biomass in the coastal zone of the Canary CS and leads to a more efficient recycling of nutrients compared to the California CS. Conversely, the substantially shorter residence times in the nearshore region of California CS is associated with a much stronger offshore export of organic matter, resulting in a less efficient buildup of biomass and a weaker recycling of nutrients in the upwelling zone.

Figure 3: Water mass residence times (in days) in the 0-100km nearshore region in the Canary CS (left) and California CS (right).

Longer water residence time in the Canary CS relative to the California CS has no single explanation.

The shelf topography and the level of eddy activity probably both contribute. The wider continental shelf in the Canary CS results in an offshore displacement of the upwelling cell, producing an area over the innershelf where the circulation has almost no cross-shore transport (Marchesiello and Estrade, 2009). This prevents coastal water from being advected offshore, increasing the water residence time in the innershelf region. The higher level of eddy activity in the California CS may also play an important role in reducing the water residence times in the coastal region of this system because of a higher eddy-induced subduction and offshore transport (Gruber et al, submitted).

Overall, our results show that factors affecting characteristic timescales of biological growth such as temperature and those related to the dynamics of the lateral circulation in coastal upwelling systems such as the topography of the continental shelf and the level of eddy activity will likely exert a strong control on the magnitude of the biological response to upwelling intensification. This study also shows that the biological response to global warming induced upwelling intensification might substantially vary from one EBUS to another, with major implications for the biogeochemical cycles and fisheries in these rich marine ecosystems.

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