A6 Anomalous Emergence and Blooms

Topic

Anomalous upwelling occurs when wind patterns shift sufficiently to intensify or alter the rise of nutrient-rich deep waters to the surface. Depending on conditions, this anomalous upwelling of nutrients can trigger explosions of biological productivity or—in extreme cases—cause the ascent of hypoxic or anoxic waters capable of devastating the coastal ecosystems that rely on them. Climate change is intensifying coastal winds linked to the trade winds, potentially increasing upwelling in so-called eastern boundary systems—off the coasts of California, Benguela, Peru, and the Canary Islands. However, the impacts vary across these systems depending on the temperature and chemical composition of the subsurface water being brought to the surface.

The phytoplankton blooms triggered by anomalous upwelling occur when nutrient availability in the euphotic zone exceeds the losses phytoplankton suffer due to grazing by other organisms, sedimentation, and water mixing. Sverdrup’s model explains the onset of primary production for these blooms each spring: it begins when the depth of the mixed layer rises above a critical compensation depth—the point at which net biomass production, integrated across the entire illuminated water column, ceases to be positive. Crossing this critical depth is what finally initiates the spring bloom.

Among these phytoplankton blooms, there is a particularly harmful type: harmful algal blooms, caused by toxic or detrimental species that thrive when anomalous conditions regarding temperature, nutrients, and stratification coincide. These harmful algal blooms are responsible for some of the most significant impacts associated with anomalous upwelling—affecting both the fisheries upon which many coastal communities rely and the public health of those who consume the contaminated fish or shellfish.

The vertical structure of the water column where a bloom occurs is not uniform; as surface nutrients are depleted, phytoplankton cease to concentrate at the surface and shift to a deeper level, forming what is known as the subsurface chlorophyll maximum. It is precisely the simultaneous need for sufficient light and available nutrients that causes phytoplankton to accumulate at this intermediate depth, rather than at the surface or in the depths, where one of the two essential resources is lacking.

As the climate warms, oxygen minimum zones are expected to expand, and biological communities dependent on anomalous upwelling will likely shift toward higher latitudes in search of conditions resembling those of the past; furthermore, the seasonality of blooms—both ordinary ones and harmful algal blooms—is expected to shift away from historical patterns. Thus, from the nutrient upwelling that triggers the phenomenon to the subsurface chlorophyll maximum formed at depth, this entire chain of coastal biological productivity is increasingly at the mercy of an ocean changing faster than its ecosystems can adapt—causing anomalous upwelling to threaten coastal life rather than sustain it as it once did.

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