Route to Bay recovery more complex than thought
The Chesapeake Bay Program has just completed a two-year review of progress on reducing nutrients, the nitrogen and phosphorous which is being overloaded to the Bay and the source of many of its problems. Muchattention has been given to how well we are meeting the goal to reduce the loadings of these nutrients to the Bay by 40 percent between 1985 and 2000, and the results to date are impressive.
A number of additional steps are being proposed to assure we meet the goal by 2000, and still further efforts are in the offing to evaluate whether the 40 percent reduction will be enough everywhere, given what we are experiencing with high concentrations of nutrients in the soils in some areas, especially where there is widespread use of animal manures.
All of this is key to the recovery of the Bay. It is the necessary focus by all the Bay partners to achieve our current goals, and to move beyond them if necessary to secure the future of the Chesapeake. But for those of us in the trenches, the two years of work that went into the reevaluation also provided some fascinating insights into just how a natural system as complex as the Chesapeake watershed goes through the recovery process.
Like any patient recovering from a severe illness, the path is neither straight nor always predictable. And although each case is different, there is much to learn from what we are seeing (and what we can predict we will see) which is applicable to other patients -in this case, to other watersheds, estuaries and coastal ecosystems. For example, we have understood for years that the recovery of the system requires reductions in both phosphorous and nitrogen. Although both nutrients play a role throughout the ecosystem, phosphorous is the nutrient of greatest concern in freshwater-dominated parts such as rivers above the head of tide, and nitrogen is relatively more important in the more saline areas, such as the main Bay. But we didn't understand the dynamics and interactions of the two chemicals very well.
It turns out that when you reduce phosphorous to help the rivers, you actually reduce the capacity of the river to absorb nitrogen as it passes through. It works sort of like this (I am not a scientist, so it is easy for me to oversimplify.): The lower levels of phosphorous in the rivers means less food to grow oxygen-depleting algae, which is good. But that algae also has the capability to absorb some of the nitrogen as it grows, so its absence in rivers means more nitrogen reaches the Bay. How much? Our models estimate about 9 million pounds a year; to put that in perspective, our overall 40 percent reduction goal seeks to reduce nitrogen to the Bay by 74 million pounds per year, so this is a significant added load, and makes the restoration effort that much more challenging.
Another thing we have a better handle on now is a phenomenon called "ground water lag time." Although it varies from place to place depending on geology and soils, much of the nutrient over-enriched runoff into the Bay enters the creeks, streams and rivers from ground water. Although storms often cause runoff directly into watercourses, the more normal pattern is for rain to seep into the ground, enter the water table and migrate to the nearest stream. In fact, many of the agricultural and other management practices we encourage are intended to minimize sheet runoff and direct the water into the ground.
While this reduces erosion and direct loadings of nutrients from storms, ground water moves very slowly, so that when you reduce the use of fertilizer, manures and other sources of nutrients, the beneficial effects are not immediately seen. It takes time to clean out the earlier higher nutrient loadings from the ground water and to replace them with the cleaner water.
We used to hope that this "ground water lag time" was just a few years. But what we have learned is that in many areas it will be more like 10 years before the results of this fraction of our nutrient reduction efforts shows up in the streams and rivers and in the Bay. This means that we have to either be patient with the response to our current efforts, or we need to concentrate more on areas with short lag times and technologies that get quicker results, such as upgrades of wastewater treatment plants and other point sources.
A third lesson we have learned about the Bay is the pattern of recovery shown by some of the tidal rivers that were the most over-enriched with nutrients. The array of actions taken to reduce nutrients results in lowered levels of something called "suspended solids," as well as nitrogen and phosphorous. Suspended solids are essentially soil particles in the water column which block the light. When all three are reduced, however, the nitrogen and phosphorous that have been removed are replaced for a while by amounts of those chemicals which have accumulated in excess in the bottom sediments.
Latest estimates are that the supply of excess nutrients for resuspension will likely last two years or so, after which time the ambient nutrient levels will drop. Meanwhile, the suspended solids have dropped off, and they are not replaced from the sediments So for a period, we have clearer water with continued high nutrient levels. This means better conditions for blooms of nuisance algae and even for lower dissolved oxygen levels for the period until the sediments come clean of nutrients.
In short, we need to be prepared to see at least some of our rivers go through a period of a few years where evident conditions get worse in response to the clearer waters, before they get better from a permanent drop in nutrient levels.
Most recently, with the outbreaks of pfiesteria in tidal rivers and the resultant look at manure-handling practices in the poultry-intense watersheds of the Eastern Shore, we have begun to learn even more about the complex interactions of phosphorous and nitrogen. It appears that by focusing nutrient management plans on nitrogen in areas of heavy manure use, we have failed to take proper account of the buildup of phosphorous over time in the soils and the ground water.
The underlying chemical and economic problem is that to apply the proper level of nitrogen from manure, you must necessarily overapply phosphorous because of their relative presence in poultry litter. But the shift to phosphorous-based nutrient management planning means that considerable surplus manure will need to be disposed of, and the necessary nitrogen to make up for the shortfall in the manure will need to be purchased and separately applied. So shifting our patterns of use will require a lot of thoughtful consideration of the best ways to share the costs and the responsibility.
As we learn these and other things about how the Chesapeake and its rivers react to the actions we undertake, a couple of general lessons emerge. We need to recognize that a "40 percent everywhere" approach may sound equitable, but it may not be the most cost-effective way to proceed. More important, it simply may not do the job in some areas where the nutrients have become highly concentrated. As we get smarter, we need to focus our efforts where we can get the best results for the investment, and where we have hardcore "hot spots" that warrant additional efforts.
We are all engaged in helping to bring about the recovery of the nation's largest and most productive estuary. The bottom line is that we are dealing with an extremely complex system that is not going to recover in a straight and steady line, and is not going to respond everywhere the same way to the same level of treatment.
As the physicians in charge of this patient, we need to get beyond cold baths and bleedings. We must start using some of the sophisticated information that is becoming available to us to develop the full array of modern treatments, and to put them in place where they will do the most good, regardless of the politics within the family.
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