Climate change: What's in the rising tide?

Colossal bridges bestride the waters of Narragansett Bay. Towns, interstate highways and suburbs sprawl along its shores and its waves are studded by thousands of pleasure boats. Yet the estuary retains a quiet beauty that befits the pious settlers who, 350 years ago, named the islands within the bay Prudence, Hope and Patience.

Extending its briny fingers through much of Rhode Island, Narragansett Bay has long juxtaposed man and nature, pollution and purity. Industrialization in the nineteenth century led to the rapid growth of cities such as Providence, a port at the bay's north end. The area now supports about a million people, all flushing their waste into the pastoral watershed. Reactive nitrogen compounds from treated sewage, industrial waste and fertilizers have poured in for decades, but remained at a relatively constant level for the past 25 years. Nevertheless, set against this steady background, a silent microbial and biochemical transformation has occurred in the bay that could have devastating ecological effects. The cause is pollution, but of an indirect sort — the changes seem to be down to global warming.

For the past three decades, the bottom sediments of the bay have mopped up much of the reactive nitrogen that humans have dumped into it. Although the fraction sequestered had been falling, this valuable natural sink has been protecting the bay and the coastal oceans from the effects of nitrate runoff. But last year the sediments abruptly stopped performing this service. Worse than that, they went into reverse. In a single summer, the bay switched from being a net sink to a net source of nitrates1.

Robinson “Wally” Fulweiler, an oceanographer at Louisiana State University in Baton Rouge, and her colleagues at the University of Rhode Island in Kingston, were among the first to notice the turning environmental tide. If Narragansett is typical of other bays, they argue, it could be the harbinger of a new threat. Shifting the effect of anthropogenic nitrogen loading beyond the immediate coastal zone could destabilize ocean ecosystems by acidifying the waters, exacerbating harmful algal blooms, killing fish and shellfish, or perhaps even powering a vicious new cycle of global warming. The studies are currently hard to interpret and some say the system is poised to rebalance itself. But if they are wrong, global warming may do more to the oceans than make them rise.

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