Under the rug: the limitations of dilution

5 11 2013

I’ve noticed this image making the social media rounds lately, and find it to be a striking depiction of the plastics problem.

It's getting harder to make things disappear in the ocean.

It’s getting harder to make things disappear in the ocean.

For decades, we’ve been dumping trash into the waters, swearing by the adage “dilution is the solution to pollution.” In some respects, it’s still true. The world ocean has a vast capacity to dilute toxic substances and render them less harmful. As usual, however, we’ve either underestimated or chosen to ignore our vast capacity to alter our environment. While plastic pollution is a particular interest of mine, this week I’m focused on a different sort of problem–the impact of runoff on the marine environment.

Harmful algal blooms are increasingly in the news, and I was just reading of a study in the Proceedings of the National Academy of Sciences suggesting that these blooms are no longer to be treated as extreme or unusual events, but rather as our new normal. While the paper describes a bloom of toxic algae in Lake Erie, the problem is not limited to freshwater bodies. These blooms are driven by a process called eutrophication, where fertilizer runoff from the land gives algae in the water a huge growth boost. These algae overgrow, then die, and are broken down by bacteria that consume oxygen in the process. Huge numbers of algae feed huge numbers of these decomposing bacteria, and before long, those bacteria have sucked most of the oxygen out of the water creating a dead zone that cannot support fish or anything else that needs oxygen to survive. Strong ocean currents may dissipate the runoff, diluting the fertilizer, but in more sheltered areas like bays, the problem can become quite severe. Cape Cod in Massachusetts is dealing with this problem as  septic systems leach excessive nutrients into the sandy soils and thence out to the bays.

NASA satellite image shows  a bloom of toxic algae as green scum on the surface of Lake Eerie.

NASA satellite image shows a bloom of toxic algae as green scum on the surface of Lake Eerie.

The problem is not new, though the scale of it continues to grow. I have been participating in an online workshop for community college instructors this week, and today’s web video explained the process of retrieving deep sediment cores from the seafloor. Dr. John Kirkpatrick of the University of Rhode Island pointed to a dividing line in a core of sediment from off the New England coast. There was an abrupt shift from a pale gray sediment to a dark, almost black column. That shift, dating back to the advent of intensive clear-cutting and agriculture in the region, resulted in widespread erosion of soils and other debris no longer held in place by tree roots. Manure from livestock also flowed into the sea in a similar, though less industrial version of what we see today.

It’s the nature of humans to forget what we can’t see. Out of sight, out of mind has long been our approach to ocean health. I have the same tendency, and the image of that sediment shift struck me quite strongly. Not only does it drive home the fact that nothing discharged into the ocean really disappears, it underscores the fact that the ocean’s memory is long, and we are a very young species. What we do matters to the oceans, and we can make a shift. This particular problem is manageable, and solvable.  There’s still time, though, as the poem goes, it’s later than you think.

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