Zouhair Lachkar

Rapid Progression of Ocean Acidification in the California Current System

Acidification Blues The increase in the concentration of atmospheric carbon dioxide threatens the health of the oceans ecosystems because of the resulting acidification of the ocean and the decrease in its carbonate saturation state. Gruber et al. (p. 220, published online 14 June) used a regional ocean model to project how the saturation state of aragonite, a form of calcium carbonate that is produced by many marine organisms, will change in the California Current System through the year 2050. The sea floor along many parts of the California coast is likely to become exposed to year-round aragonite undersaturation within the next 20 to 30 years, a situation that could severely reduce the range of habitats for marine shellfish. Rising concentrations of atmospheric carbon dioxide threaten to amplify the severity of ocean acidification in upwelling zones. Nearshore waters of the California Current System (California CS) already have a low carbonate saturation state, making them particularly susceptible to ocean acidification. We used eddy-resolving model simulations to study the potential development of ocean acidification in this system up to the year 2050 under the Special Report on Emissions Scenarios A2 and B1 scenarios. In both scenarios, the saturation state of aragonite Ωarag is projected to drop rapidly, with much of the nearshore region developing summer-long undersaturation in the top 60 meters within the next 30 years. By 2050, waters with Ωarag above 1.5 will have largely disappeared, and more than half of the waters will be undersaturated year-round. Habitats along the sea floor will become exposed to year-round undersaturation within the next 20 to 30 years. These projected events have potentially major implications for the rich and diverse ecosystem that characterizes the California CS.

Dominant role of eddies and filaments in the offshore transport of carbon and nutrients in the California Current System

The coastal upwelling region of the California Current System (CalCS) is a well-known site of high productivity and lateral export of nutrients and organic matter, yet neither the magnitude nor the governing processes of this offshore transport are well quantified. Here we address this gap using a high-resolution (5 km) coupled physical-biogeochemical numerical simulation (ROMS). The results reveal (i) that the offshore transport is a very substantial component of any material budget in this region, (ii) that it reaches more than 800 km into the offshore domain, and (iii) that this transport is largely controlled by mesoscale processes, involving filaments and westward propagating eddies. The process starts in the nearshore areas, where nutrient and organic matter-rich upwelled waters pushed offshore by Ekman transport are subducted at the sharp lateral density gradients of upwelling fronts and filaments located at ∼25–100 km from the coast. The filaments are very effective in transporting the subducted material further offshore until they form eddies at their tips at about 100–200 km from the shore. The cyclonic eddies tend to trap the cold, nutrient, and organic matter-rich waters of the filaments, whereas the anticyclones formed nearby encapsulate the low nutrient and low organic matter waters around the filament. After their detachment, both types of eddies propagate further in offshore direction, with a speed similar to that of the first baroclinic mode Rossby waves, providing the key mechanism for long-range transport of nitrate and organic matter from the coast deep into the offshore environment.

Climatic modulation of recent trends in ocean acidification in the California Current System

We reconstruct the evolution of ocean acidification in theCalifornia Current System (CalCS) from 1979 through 2012 using hindcast simulations with an eddy-resolving ocean biogeochemicalmodel forcedwith observation-based variations of wind andfluxes of heat and freshwater.We find that domain-wide pH and arag W in the top 60mof thewater columndecreased significantly over these three decades by about−0.02 decade and−0.12 decade, respectively. In the nearshore areas of northernCalifornia andOregon, ocean acidification is reconstructed to have progressedmuchmore rapidly, with rates up to 30%higher than the domain-wide trends. Furthermore, ocean acidification penetrated substantially into the thermocline, causing a significant domain-wide shoaling of the aragonite saturation depth of on average−33m decade and up to−50m decade in the nearshore area of northernCalifornia. This resulted in a coast-wide increase in nearly undersaturatedwaters and the appearance of waters with 1 arag W < , leading to a substantial reduction of habitat suitability. Averaged over thewhole domain, themain driver of these trends is the oceanic uptake of anthropogenic CO2 from the atmosphere. However, recent changes in the climatic forcing have substantiallymodulated these trends regionally. This is particularly evident in the nearshore regions, where the total trends in pH are up to 50% larger and trends in arag W and in the aragonite saturation depth are even twice to three times larger than the purely atmospheric CO2-driven trends. This modulation in the nearshore regions is a result of the recentmarked increase in alongshorewind stress, which brought elevated levels of dissolved inorganic carbon to the surface via upwelling. Our results demonstrate that changes in the climatic forcing need to be taken into consideration in future projections of the progression of ocean acidification in coastal upwelling regions.

Eddies reduce denitrification and compress habitats in the Arabian Sea

The combination of high biological production and weak oceanic ventilation in regions such as the northern Indian Ocean and the eastern Pacific and Atlantic, cause large-scale oxygen minimum zones (OMZs) that profoundly affect marine habitats and alter key biogeochemical cycles. Here we investigate the effects of eddies on the Arabian Sea OMZ — the worlds thickest — using a suite of regional model simulations with increasing horizontal resolution. We find that isopycnal eddy transport of oxygen to the OMZ region limits the extent of suboxia, so reducing denitrification, increasing the supply of nitrate to the surface and thereby enhancing biological production. That same enhanced production generates more organic matter in the water column, amplifying oxygen consumption below the euphotic zone, thus increasing the extent of hypoxia. Eddy-driven ventilation likely plays a similar role in other low-oxygen regions, and thus may be crucial in shaping marine habitats and modulating the large-scale marine nitrogen cycle.

Oxygen Minimum Zone Contrasts Between the Arabian Sea and the Bay of Bengal Implied by Differences in Remineralization Depth

The combination of high primary productivity and weak ventilation in the Arabian Sea (AS) and Bay of Bengal (BoB) generates vast areas of depleted oxygen, known as oxygen minimum zones (OMZs). The AS OMZ is the worlds thickest and hosts up to 40% of global denitrification. In contrast, the OMZ in the BoB is weaker and denitrification free. Using a series of model simulations, we show that the deeper remineralization depth (RD) in the BoB, potentially associated with organic matter aggregation with riverine mineral particles, contributes to weaken its OMZ. When the RD is set uniformly across both seas, the model fails to reproduce the observed contrast between the two OMZs, irrespective of the chosen RD. In contrast, when the RD is allowed to vary spatially, the contrasting distributions of oxygen and nitrate are correctly reproduced, and water column denitrification is simulated exclusively in the AS, in agreement with observations.