Chapter IV, Section C, Item 3:  Eutrophication

Other than climate change, overfishing, and the physical loss of the ecosystem itself through habitat destruction, the greatest human stress on aquatic ecosystems and their biota is nutrient pollution. Unlike the other stresses, nutrient pollution can readily be expressed in terms of the energy demand for its assimilation or treatment. Ironically nutrients that are critical to sustaining life, particularly the limiting-nutrients nitrate and phosphorus, are the “pollution” ... anything in excess. Limiting-nutrients in abundance create life in abundance, and it is this life in abundance that causes aquatic ecosystem stress and even aquatic biota mass death. The enrichment of nutrients and sediment in waterways is a natural process called eutrophication, part of natural succession that transitions lacustrine and estuarine ecosystems first into wetlands and eventually into terrestrial ecosystems. But the accelerated rate of eutrophication in response to extreme nutrient loading creates impacts far beyond these local ecosystems, well into marine environments.

The life in abundance creates plant and algal life out of balance. Plants and algae photosynthesize and create oxygen, but they also respire like animal life, consuming oxygen to produce the energy needed to maintain their systems and grow. In addition, consumers of the living, and decomposers of the dead, respire and consume oxygen. In the end, the net effect is for dissolved oxygen levels in the water to drop precipitously, killing most aquatic animal life. The system feeds back on itself as the dead decompose, consuming more oxygen. The eutrophication problem ultimately leads to hypoxia, a technical term for what could aptly be named the dissolved oxygen crisis. Eutrophication is not alone in creating the dissolved oxygen crisis. As with most gases, dissolved oxygen levels decrease with increasing temperature. Thus other stressors contribute to hypoxia, including climate change and heat discharges. In fact, the potential exists for climate change to cause a global oceanic hypoxia crisis.

Hypoxia from nutrient loading is already creating dead zones that reach far into the marine ecosystem. In his article, The Oil We Eat, Richard Manning describes a 200 square mile dead zone in the Gulf of Mexico extending from the mouth of the Mississippi River, in response to nutrient loading from the river’s discharge. The source of nutrient pollution is clearly agricultural, both from the disposal of animal waste into waterways as well as the application of fertilizers. The source is non-point, and the dead zone the collective effect of the entire Midwest farm belt.

The energy liability of the eutrophication problem can be assessed by calculating the energy needed to neutralize the nutrient loads. As an example, one nutrient, nitrate, from one river, the Mississippi, has an immense energy cost. But even this cost is a small fraction of the total nutrient pollution problem. Nitrate is naturally either assimilated into plant proteins, or reduced by bacteria into nitrogen gas. In either case, a reduction reaction occurs, requiring an electron donor, i.e. requiring energy. The energy is provided symbiotically by the plants or bacteria in exchange for a critical, otherwise life-limiting nutrient. But the energy required to neutralize excess nutrients is an energy stress on life at large, the ecosystem. The average nitrate concentrations and water flows in the lower Mississippi are nearly immutable, once the last of the major tributaries have made their contributions. At that point, the USGS has been monitoring the river’s characteristics at its Vicksburg, Mississippi gaging station for over 100 years, including the measure of the nitrate concentration and total water discharge. The multiple of this concentration and flow yields the load of nitrate to the Gulf, over 3.6 million kilograms per day. The energy required to reduce this mass is the equivalent of 1.1 million gallons of gasoline per day.

As with other non-point pollution problems, the best medicine for nutrient pollution is prevention. Reducing the source of nutrients in the first place is the best prevention, by promoting crop rotation, reducing or targeting the application of fertilizers, and controlling the discharge of animal waste. But human land use will always produce excess nutrient levels in runoff. The ultimate goal is to prevent these nutrients from discharging into waterways. To that end, a simple solution exists, but requires cooperation from all the inhabitants of the watershed. Riparian zones are buffers of shrubs, trees, and shoreline aquatic plants maintained between human land use and the waterways that serve to absorb nutrient runoff and soil erosion. The effect of riparian zones is a dramatic increase in water quality, while the zone itself creates habitats and promotes a diversity of life at the water’s edge.

The alternative, to not address nutrient loading, is a reduction of biodiversity in aquatic ecosystems, including the marine environment.

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