Saturday, March 20, 2010

Coastal Upwelling

In the last blog we said we would talk more about harmful algal species, but what is happening in the ocean over the last couple of weeks is a process that contributes to the formation of some algal blooms, including some harmful species. If you live in southern California, you know that we experienced very strong winds out of the north to northwest last week. Strong winds like that are typical of the springtime, but are often weaker during El NiƱo years (like this year). But those were strong winds. What is the effect of those winds on the ocean?

We could discuss the more technical aspects of wind stress and wind stress curl and their effects on the upper ocean, or various processes that contribute to divergences in the ocean that result in upwelling of deeper water. But a simplistic explanation is that when the winds blow from the north along the west coast in the northern hemisphere they accelerate the nearsurface currents toward the south and because of the rotation of the earth these equatorward currents tend to veer offshore. As they veer offshore, water needs to come from somewhere to replenish the water that is moving away from the coast. The source of that water is generally from underneath the layer of water that moves offshore, and this water comes to the surface near the coast. Because deeper water is moving upward to the surface near the coast, we refer to the process as upwelling – vertical upward transport of the water (see the schematic below). The deeper water contains nutrients (nitrogen, phosphorus, and silicate) that support the growth of the microscopic plants (phytoplankton) in the ocean when they are exposed to sunlight. As a result when significant upwelling occurs along the coast, we often see a response in the growth of phytoplankton, just like we discussed when there is significant runoff from rain. While rain runoff is conducive the growth of a group phytoplankton called dinoflagellates, upwelling tends to support more conducive to a group of rapidly growing phytoplankton called diatoms.



In general, upwelling is a very positive process in the ocean. The cooler water is important for maintaining the livable climate of coastal California, and other regions of the world. The nutrients support a biologically productive coastal ocean ecosystem that provides California’s abundant sea life. It is also part of the process where the subsurface ocean ventilates to the surface and atmosphere.

We are in the process of seeing rapid phytoplankton growth along the Southern California Coast right now. However, one of the diatom species that is becoming abundant, Pseudonitzschia australis, can produce the neurotoxin domoic acid that can cause significant damage to the hippocampus region of the brain in mammals (including humans) and birds. Marine mammals and birds feed on the fish that consume these phytoplankton and as a result, we often see the effects when these animals show up on the beach. So far we haven’t seen a lot of the toxicity, but we are watching carefully to see if it develops. If you want to follow this, you can go the Harmful Algal Bloom web page maintained by the Southern California Coastal Ocean Observing System (http://www.sccoos.org/data/habs/index.php

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