Tundi Agardy, Ph.D.
Changing freshwater ecosystem conditions and the health of the oceans
Freshwater finds its way to oceans via streams, rivers, runoff, and rain, bringing with it compounds much-needed by life in the sea and at its margins. But in addition to nutrients, freshwater also brings with it things that the oceans don’t need, in the form of pollutants.
Oceans are unfortunate in being downstream, of everything. All our chemical inputs, unused fertilizers, debris, eroded silt and topsoil, untreated sewage, medicines — from our farms, our suburbs, our cities, and our factories — eventually make their way to the world ocean. As a result, coastal seas are now described as the most chemically altered environments on earth17. It should not be surprising, given the world ocean is downstream of every watershed, and all forms of pollutants reach the seas via river inputs, atmospheric deposition, and run-off. Expanding dead zones result, endangering fisheries, biodiversity, and human health.
The indirect degradation of oceans is an increasing problem, despite government regulations on pollutants. This is partly due to the fact that the Earth’s most vital organs: riparian and coastal wetlands, seagrass beds, and mangrove forests that are its lungs, liver and kidneys acting to filter out toxins before they reach the open sea, continue to be destroyed.
This is a story not easily told, which is why this may be the biggest sleeper issue of all. How much easier to portray the plight of the great whales, or to document the decline in fisheries, or to show a coastline sullied by unsustainable development. Pollution is difficult to see, even harder to trace, sometimes ephemeral, but with long-lasting impacts. Eutrophication – the over-fertilization of nearshore waters caused by too many nutrients from fertilizers, sewage, animal waste, food processing residues – threatens to disrupt the ecological balance of coastal areas around the world. At the same time, toxins enter the marine system and reside there for long periods – until they are actively removed by mitigation (or until they enter the human food chain, leaving the marine environment to reside in our own tissues).
Non –point source pollution underlies ever-expanding “dead zones” – areas of low or no oxygen, needed to support most marine life. In the Gulf of Mexico dead zone, less than a third of the states in the 31 state watershed contribute the vast majority of the nitrogen and phosphorus delivered to the Gulf, primarily through non-point source pollution from rural run-off. Alexander concludes that nutrient reductions in the Gulf may be most efficiently achieved by managing nutrients in watersheds drained by large rivers18. Corn and soybean cultivation is the largest contributor of nitrogen to the Gulf; while animal waste combined with crops cultivation contribute most of the phosphorus.
Riparian buffer have long mitigated the effects of run-off, preventing polluted freshwaters from reaching coastal systems. But as a new study in Nature points out, small stream systems may be even more important in removing pollutants and preventing eutrophication of coastal seas. Patrick Mulholland and co-authors found that nitrate was filtered from stream water by tiny organisms such as algae, fungi and bacteria. This in and of itself was not news, however, the researchers discovered that entire stream networks are important in removing pollution from stream water, not just individual streams. However, the important role that even small streams have in removing and/or transforming nitrates (and therefore preventing eutrophication downstream, and in estuaries and oceans) can quickly be overcome by too many nitrates entering the water. Thus, there are thresholds to the ability of freshwater ecosystems to provide the important ecosystem service of maintaining water quality.
Thus freshwater ecosystems and marine ecosystems downstream are threatened by both pollutant loading and the loss of stream habitat as development continues to transform the landscape. But the situation is not hopeless. Realization is growing, and market-based mechanisms to enable better watershed management are cropping up (see next section). And perhaps the one bright light in the current worldwide economic downturn is that use of fertilizers is expected to plummet as the cost of fertilizer shoots up. The consequences for both freshwater ecosystems and marine ecosystems will be positive, as has been shown to happen in the past. When the dissolution of the Soviet Union ended state subsidies of fertilizer, and fertilizer use fell dramatically. In Cuba, for instance, coral reef health improved as fertilizer use diminished. If the end result is a forced movement to more sustainable agriculture and better opportunities for small scale growers that practice sustainable methods, this may indeed be the silver lining in the dark economic clouds that currently blanket the world.
Marine managers have long recognized that effective conservation of ocean areas and creatures requires delving into watershed management, for all the free-flowing waters of the earth either find their way into, or dramatically affect, ocean ecosystems. However, achieving this level of ecosystem-based management requires the spanning of disciplines and professions in a way that is not natural to our sectoralized science and management, and keeping the big picture, regional view very much in mind.
17 MEA..2005. Ecosystems and Human Well-Being. Ch 19. Coastal systems and coastal communities. pp
18 Differences in Phosphorus and Nitrogen Delivery to the Gulf of Mexico from the Mississippi River Basin," by the U.S. Geological Survey, USGS, published in the journal "Environmental Science and Technology
