October 2009 Archives

How a whale is like a tree

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SeascapeModeling has been on the road.  Last week, Nick and I traveled to the RARGOM Gulf of Maine Symposium in St. Andrews, New Brunswick.  We're now at the Society for Marine Mammalogists Biennial meeting in Quebec City.  Nick and Dan presented various aspects of our copepod-right whale modeling work.  I presented something completely different.  


For the last several years, I've been sketching diagrams, scribbling equations, and filling spreadsheets trying to figure out whether whales are like trees, at least when it comes to carbon dioxide. Seriously. Yes, I know I need a life, but this is actually really neat.  Here's how it goes:


Forests are an important reservoir of carbon on land.  Through photosynthesis, trees take carbon dioxide out of atmosphere and turn it into tree (leaves, roots, and wood).  When the carbon is locked up as tree it is no longer in the atmosphere, reducing the greenhouse effect.  This is why businesses (at least in Europe) can get carbon credits by helping preserve forests.  As a forest grows, more carbon is taken out of the atmosphere.  Conversely, if a forests burns, the carbon gets released as carbon dioxide.


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In the ocean, most of the photosynthesis takes place in single celled phytoplankton.  These cells may live a few days or weeks, so they can't really store carbon.  Instead, carbon in the ocean is stored in the bodies of larger organisms.  As the largest and longest lived animals, whales act like the trees of ocean (minus the leaves). Whaling, like a forest fire, turned hundreds of years worth of whale-carbon and returned it to the atmosphere.  Since industrial whaling stopped in the 1970s, most whale populations are now recovering and are storing more carbon.


Large fish, notably tuna and sharks, can similarly store carbon for many years.  However, even including these species, the amount of carbon stored by marine vertebrates is small compared with the total amount of forest on land.  But, whales (and large fish) have one more trick.  Once a forest becomes mature, its ability to store carbon decreases.  While there is an upper limit to how much carbon can be stored in living whale, whale populations can continue to export carbon as dead whales.  Whales have few predators, so many of the whales that suffer "natural" deaths will sink to the bottom of the ocean.  If the whale dies in deep water, its carbon will remain out of the atmosphere for thousands of years.  The amount of dead whales is related to the total number of whales, so whaling reduced the size of this carbon "sink".  By estimating the total decline in the mass of whales, assuming that whaling turned whales into carbon dioxide, and accounting for the lost "dead whale export potential", I calculated that the total carbon footprint of 100 years of whaling released an amount of carbon dioxide to the atmosphere equivalent to setting New England on fire.  The full paper, (including equations!) is here.

Yesterday, I went for a dive at the Rachel Carson Salt pond up in New Harbor, Maine. Down around 90 feet, much of the light in the visible spectrum (~400-700nm) has been attenuated. Basically, this means that the light is absorbed as it passes through water. Think about sitting under a tree on a sunny day with the sun directly overhead: the bigger the tree, and the more leaves between you and the sun, the more stuff the sunlight needs to pass through to reach you, which means less direct light reaches you under the tree. But were you sitting under a short tree with fewer leaves, more light reaches you directly, since there's less stuff in the way. The leaves on the tree are like particles in the water and the deeper you swim, the more particles there are between you and the sun (i.e. there's more stuff in the way).
  What makes this phenomenon neat, is that different colors of light are absorbed and scattered by the particles in the water at different rates, so, for example, at 90 feet, things all look pretty blue/green since the red light is absorbed/scattered faster than the blue/green light. To illustrate this, I took 2 pictures of a northern red anemone (Urticina felina) at 90 feet (or ~27m): one with natural light and one with a flash. The flash is akin to letting the sun shine directly on the anemone in the photograph, that is the red light hasn't been absorbed yet, since the light source is less than 1 ft (~.3m) from the subject.

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above: Northern red anemone (Urticina felina) shown in natural light at 90 ft depth.
below: same anemone, same location, but with a flash used to illuminate the picture
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Our biological oceanography lab has a biweekly zooplankton time-series study collected from the Darling Marine Center in Walpole, Maine.  The study samples the zooplankton at two stations: one well within the Damariscotta estuary, and the other a few miles out.  At the nearshore station, we see an estuarine community, with a diverse collection of copepods and other zooplankton.  At the offshore station, depending on the time of year, the community is dominated by the large copepod, Calanus finmarchicus.  

These two communities are characteristic of two different marine ecosystems.  The big copepods in the oceanic system provide essential prey for pelagic species ranging from herring to right whales.  The smaller, more diverse estuarine system can serve as a nursery for larval fish.  The seascape modeling lab is interested in the processes that maintain the boundary between the two types of system.

In order to characterize the nearshore-offshore gradient, we ran a cruise on Thursday, taking profiles with the LOPC at fixed intervals of roughly 1 km (see map).  We're still feeding out and reeling the LOPC cable by hand, until we get the data logger fixed.  This can be tiresome, but thanks to ongoing splicing efforts (including some last-minute work before leaving the dock), it's effective.  We have a nice transect showing the shift in size distribution from the nearshore out towards the offshore.

Additionally, it was nice to be on the water on a brisk October day.  We got an early start, catching the sunrise ferry from Peaks Island, and we saw some fair wildlife, which, hopefully, Pete will share some pictures of in a later entry.

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Sampling stations (X).

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Reeling in the LOPC and cable.

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