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A few weeks ago, my family and I took a vacation on the Oregon coast and found the weather to be refreshingly cool with the high temperatures in the low 60s and nights which often reached into the low 50s. Everyone understood that, in this region of the coast, the water, even in midsummer,  is too cold for normal swimming, such that the brave few who entered the water always did so in wet suits. So the most frequent form of beach activity reverted to that of waiting for low tide, at which time visitors ventured out along the rocky ocean beaches  to see the holdings of the many tide pools that were carved out of stone and stocked with invertebrates.  In that region, strong tidal forces plunge the Pacific ocean against the rocky coast which  submits by giving way to neatly carved  stone and sand tide pools that nestle along the beach and usually harbor a rich array of invertebrates.   Near the tide pools one could see photographers shooting scenes of starfish feeding on clams while unidentified, trapped  invertebrates scurried about for a place of safety or escape, usually just a high tide away. A tide pool is a microscopic world of violence, but everything seems to move in slow motion, beyond our tolerance to wait, watch or investigate more closely. Things in tide pools move as if marking with a geologic time scale. One would need the patience of an A.O. Wilson or Rachel Carson to gain an understanding of nature’s dynamics in the tide pool environment. Yet, one can’t help but feel some sense of security in knowing that life is abundant in the tide pool, that perhaps it’s a safe outpost of nature, seemingly untouched by man’s intrusion into the ocean ecosystems. But is that true? Maybe not!

Fig 1. A description of this figure is found at the bottom of this posting

Recently I was reading about the Oregon coast and discovered that, since 2002, the region has experienced sudden periods during the summer months in which the shallow ocean water dramatically loses oxygen levels below those required to sustain normal marine life. The first occurrence of this event took place between Newport and Florence along the Oregon coast, and included Yachats, the small town where we stayed. Though I did not personally see any evidence of fish or invertebrate kills, these surges of hypoxic coastal ocean water take place further out in the shallow ocean water beyond the shores and are evident at depths up to about 50 meters or so: because of the intense wave action, tidal pools probably get effective oxygenation through wave aeration; its an excellent mechanism for mixing water and air and the Pacific ocean seems very adept at creating intense wave activity. I have always appreciated how much better the Pacific ocean is at generating large, strong waves when compared to its Atlantic cousin.

When hypoxic events occur, many fish are able to swim out of oxygen depleted regions into more sustainable water, whereas the slower invertebrates are stuck, and in the case of the Oregon coast, thousands of invertebrates have been dying every summer when the ocean becomes intolerably hypoxic. You can view a Quick Time video clip of a fish/invertebrate kill photographed underwater along the Oregon coast here: it amounts to a massive kill.

Marine biologists tell us that normal ocean surface water contains 5 to 8 ml of oxygen per liter of ocean. But during these anoxic spells in Oregon, the measured oxygen level was as low as 1.4 ml/l, too low for most fish and invertebrate survival. Many regions of the world have hypoxic ocean waters, some of which have been created by eutrophication, or fertilizer runoff from intense agriculture, which produces blooms of plankton that reduce the oxygen content of the water. But the scientists who initially investigated the Oregon coast hypoxia knew that it was unlikely to be caused by eutrophication, simply because farming along the Oregon coast didn’t seem sufficient to generate significant fertilizer runoff.  Initially, marine biologists thought that they were viewing a once-in-a-lifetime event, but anoxic waters along the coast of Oregon are now an annual event and have been detected each summer since the first large scale fish and invertebrate kills of 2002. From as early as mid-April to mid-October, hypoxic water has been the rule, though fluctuations in the intensity of oxygen depletion give variance to its magnitude. To this day, the cause of this phasic oxygen deprivation is unknown, though several theories seem to be prominent among oceanographers and marine biologists. Some have even considered this phenomenon to be part of a natural, long-term cycle of ocean behavior.  But, no significant letup has occurred and in 2006, the most extreme case of anoxia took place in which coastal waters lost all detectable oxygen levels for four weeks. In that instance starfish, mussels and rockfish died in large numbers, while other, more mobile fish were able to flee the hypoxic zone, which grew to 3,000 square kilometers. Furthermore the region has been monitored for oxygen content at different depths going back to 1950 and from 1950 to 1999, no anoxic events were recorded (see Fig 1).

The fishing industry along the coast of Oregon has been understandably alarmed about this recurrent hypoxic condition, as fishing brings in hundreds of millions of dollars each year into the economy. But Oregon’s hypoxic summer coastal waters are part a global problem, though the causes of ocean hypoxia vary for each region and always have a local component as well. Increasingly the oxygen content of our ocean waters has been receiving more attention and there is broad agreement on the impact that global climate change may have on ocean oxygenation levels, including i)  a failure to properly mix the water column through changes in oceanic currents, that could be seriously impacted by global climate change and its effect on the natural oceanic currents which exchange cool norther waters with warmer waters near the equatorial zones and ii) the warming of the ocean water itself reduces its capacity to dissolve oxygen, a strict reality of chemical reactions.  According to the 2007 IPCC report, from the period 1961 to 2003, global ocean temperature have risen by 0.10°C from the surface to a depth of 700 meters.

Biologists believe that the magic number for oxygen comes in at about 2 ml/l, below which much of the ocean fauna cannot exist; there are now large regions of our ocean, particularly those near tropical areas, where the intermediate depths of the water have reached this level of incompatibility.  While there is plenty of evidence for an increase in the temperature of the ocean over the last fifty years, so far, there is no evidence that the normal ocean currents have been altered by global climate change conditions, at least not for the major currents we concern ourselves with. If there is a compensatory side to global climate change, it is that tropical storms, whose frequency and magnitude can be correlated with ocean water temperature, help to force mixing of the ocean water with the more oxygen rich air, serving to overcome other tendencies to form oxygen-depleted zones, though the significance of this so called “benefit” has been hard to guesstimate. Who wants to be on the sidelines cheering on another Katrina?

The Oregon coast is part of a large West Coast ocean ecosystem, in which shallow, oxygen-rich ocean water, found at depths up to about 50 meters, leads to much deeper, oxygen deficient water found beyond the continental shelf, where depths become hundreds of meters or more. Those deeper regions are poor in oxygen but rich in nutrients. Measurements of oxygen levels as deep as 600 meters have been ongoing in the Oregon region for decades, which, until 2002, did not reveal coastal water hypoxia (Fig 1, left). So, if eutrophication doesn’t explain Oregon’s coastal oxygen deficiency, what does?

The most parsimonious explanation for Oregon’s summer anoxia seems to be that the deeper oxygen minimum zone (OMZ) has been upwelling at higher rates than normal and mixing with the more superficial oxygen-enriched waters in disproportionate ways that did not happen before, but might still be part of a very long periodic cycle that could last for decades or more. Others suggest the more obvious,  that what’s going on in Oregon is a perfect storm created by changes in weather, climate and ocean currents. If so, this should alarm all of us, because it illustrates how quickly the ocean environment can change. We must remember that 71% of the surface of the earth is covered by ocean water.

This new mixing between the two pools of ocean water not only tells us that the oceans can change quickly, but that they can do so with a surprisingly quick lethal outcome. There is clearly a balance force at work here in nature with ocean water mixing that is difficult to comprehend, but mind-numbing to appreciate when it doesn’t work to its historic perfection. It’s hard not to get analytical about this observation without thinking how finely tuned it all is, how interdependent the global system is and then wonder how badly out of tune we have forced mother nature’s engine for sustaining life on the land as well as the ocean. Surely we need to learn better than we ever have that land and ocean are joined at the hip. Excessive carbon dioxide in the atmosphere is acidifying the ocean, but doing so much more and in so many different ways, most of which we cannot yet articulate. Perhaps our very survival is the biological experiment. But for this experiment, mother nature is sitting on the sidelines, as we started the ball rolling on this one.

We use models to predict the impact of global climate change, but with the oceans, we have a laboratory. We should all be jumping into the oceans and making measurements! If we can’t save the country, let’s put everyone to work saving the planet!

RFM

(below is a copy of the figure illustration taken from the Chan et al Science article (note: hydrocasts are water samples obtained from a group bottles that are coupled to one another and sunk to get samples of water at different depths)

Fig. 1. Taken from a Science Brevia paper by Chan et al (Science, 319, 920, 2008). Dissolved oxygen profiles during the upwelling season (mid-April to mid-October) in the upper 800 m of the continental shelf and slope of Oregon (42.00°N to 46.00°N). (A) 1950 to 1999 from the World Ocean Database and Oregon State University archives (n = 3101 hydrocasts, blue). (B) (A) with additional data for 2000 to 2005 (n = 834 hydrocasts, green). (C) (A) and (B) plus data for 2006 (n = 220 hydrocasts,red). The black vertical line denotes the 0.5 ml/l threshold. (Insets) Overlapping locations of hydrographic (blue, green, and red) and remotely operated vehicle (black) stations through time and the 100-m and 1000-m isobaths.