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.