Lake Erie Binational Site
Lakewide Management Plans
Lake Erie Binational Site
A Primer on Phosphorus in Lake Erie
This Primer is presented to persons interested in the present and future of Lake Erie. It is intended to present some current issues and some historical perspectives for framing discussions about those issues. The information should be easily understood by educated people without a scientific background but who have a sincere interest in understanding environmental problems and who wish to participate in the process for finding solutions. Additional and detailed information is readily available in the scientific literature for the serious student of Great Lakes ecology. The Primer uses a "Question and Answer" format. The reader is encouraged to identify additional questions and to begin to appreciate how solving one set of problems can create others.
Q: Why are we talking about phosphorus again?
A: Lake Erie has sustained an important commercial and sport fishery for a succession of species since the 1800's. Most recently, yellow perch and walleye have been the economically important species. However, radical changes in Lake Erie in the 1990's raise concerns about the future and predictability of the fish communities. For example, walleye stocks have declined by about 80% from their maximum levels in the 1980's. The supply of smelt to the commercial fishery in eastern Lake Erie has also declined by about 60%. These changes have been linked to a variety of factors, but reduced phosphorus loadings and the invasion of the lake by zebra mussels have been heavily implicated.
Although the Lake Erie ecosystem is quite complex and appears to be rapidly changing, the argument concerning zebra mussels and phosphorus loadings can be reduced to two basic premises:
Phosphorus is an important nutrient that controls the amount of algae that will grow suspended in the water. The algae, single celled plants and microscopic colonies that are collectively called phytoplankton, are the base of the aquatic food chain. Smaller quantities of phosphorus will result in smaller quantities of algae. Fewer algae will result in less food to be available to other aquatic organisms in the food chain, including perch and walleye.
Zebra mussels eat algae. To be more specific, they filter the suspended algae out of the water. What they do not use for maintenance and growth, they expel in little packets which sink to the bottom. The zebra mussels thereby remove the algae from the water which would otherwise be consumed by other organisms in the food chain. Fewer algae available in the water result in less food for the fish.
Unfortunately, solutions to stabilizing economically valuable fish populations are not so simply determined. Competing uses for Lake Erie, including drinking water, bathing beaches, recreational boating and sailing, shoreline picnicking and hiking, and fishing for other species, are also influenced by the amount of algae in the water and therefore by the amount of phosphorus and other nutrients that enter the water. Also, the behavior of the perch and walleye may change in response to secondary effects of phosphorus loadings and zebra mussels. For example, increased water clarity, a result of fewer algae cells suspended in the water, may permit more rooted aquatic plants to grow, which provide more shelter for young fish. However, the increased light also causes the more mature walleye to migrate to deeper water that is not so bright.
Q: Why did we start to reduce phosphorus levels in Lake Erie in the early 1970's?
A: The shoreline of Lake Erie was not a fun place to be in the late 1960's. The waters were green with algae, beaches were closed from bacterial contamination, decaying masses of a stringy green algae called Cladophora stunk up much of the shoreline, many fish kills were observed, and municipal water taken from Lake Erie experienced taste and odor problems from an overabundance of a type of algae called blue-greens.
Scientists from the U.S. and Canada conducted a series of investigations into the problems with Lake Erie, and they identified the importance of phosphorus as an algal nutrient. An oversupply of phosphorus was causing a huge excess of growth of algae. Several outcomes from this excess growth became apparent:
The species of algae in the water changed. The type of algae called blue-greens had become abundant. Besides smelling and tasting bad in the water, this type of algae was not particularly desired by other organisms in the Lake Erie food web.
So much algae was growing in the water during the spring and early summer, that when the algae died and sank to the bottom in the central basin in the summer, the organisms that decomposed the algae used up all the oxygen in the bottom waters, and neither fish nor aquatic insects could survive there.
Nearer to shore, on rocks or other hard surfaces, the stringy algae, Cladophora, thrived. Normally they remained attached to the rocks, but they were torn loose during storms and deposited along the shore. That was where the stinking mats of algae were coming from. In order to reclaim Lake Erie, the amount of phosphorus in the water had to be reduced.
Q: Where does phosphorus come from?
A: A better question might be "Where did the excess phosphorus come from?".
Phosphorus is a mineral that occurs naturally in the environment. It is one of the essential minerals to sustain life as we know it. In lakes and oceans, the relative amounts of phosphorus and nitrogen are important. In the Great Lakes, phosphorus is the nutrient that is usually least abundant. Most of it is readily used by the algae, and any additional amount that enters the water is soon used to create more algae. Rivers naturally transport phosphorus that has weathered from rocks or that has been transported out of soils.
Most of the excess phosphorus entering Lake Erie comes from two sources: the effluent from sewage treatment plants (STPs) and tributaries that receive agricultural runoff from farmland. Human waste contains high amounts of phosphorus that can pass through STPs. Until the mid-1970's or so, additional phosphorus in laundry detergents was also making its way through STPs. Phosphorus is one of the basic components of agricultural fertilizer, and rainstorms easily transport it from fields to streams that eventually flow into the Great Lakes.
Q: What was done to reduce phosphorus in Lake Erie?
A: In 1972 the governments of Canada and the United States signed the Great Lakes Water Quality Agreement (GLWQA) to protect and restore the waters of the Great Lakes. A specific component of the objectives for the GLWQA was to reduce phosphorus loadings to Lake Erie to control the nuisance algae growth and to retain oxygen in the bottom waters of the central basin all year, including late summer. Over the next few years, a series of mathematical models were constructed and compared. The results suggested that if no more than 11,000 metric tons of phosphorus per year were added to Lake Erie, nuisance algae growth would be curtailed, and a sufficiently small amount of algae would be produced in the central basin so that the bottom waters would retain enough oxygen in late summer to support fish and aquatic insects. Phosphorus concentrations in the water should eventually stabilize at about 15 ug/l in the western basin, and 10 ug/l in the central and eastern basins. To put these findings into perspective, from 1967 to 1972, annual loadings of phosphorus averaged about 24,000 metric tons, more than double the maximum allowable amount.
An intensive, expensive assault on phosphorus loadings was initiated. Over $8 billion (U.S., adjusted to 1990) has been spent on new sewage treatment plant construction and on upgrading existing facilities to make them more efficient at removing phosphorus from domestic sewage. The amount of phosphates allowed in laundry detergents has been restricted in Canada and in the U.S. drainage basin to Lake Erie. Millions of dollars have been spent on demonstration projects and other incentives to urge farmers to change tillage practices to conserve soil and reduce stormwater runoff. The industrial discharge of phosphorus has been controlled through the wastewater permitting process.
Q: What happened after all the money and time was spent reducing phosphorus loadings to Lake Erie?
A: The reversal of the effects of excess phosphorus in Lake Erie is now considered a world-class success story. Total phosphorus loadings appear to have stabilized at around 10,000 metric tons, ranging higher in wet years due to increased runoff from agriculture and overflows from sewage treatment plants, and lower in dry years. This compares favorably with the modeled loadings of not more than 11,000 metric tons to achieve the goals of the GLWQA. Springtime concentrations of phosphorus in the western basin in 1995 averaged about 25 ug/l, higher than the predicted concentration of 15 ug/l, but considerable variation exists year to year. In the central and eastern basins in 1995, phosphorus concentrations averaged 11 and 8 ug/l, respectively. Concentrations of about 10 ug/l in the central basin have been measured since the late 1980's. Over the past 20 years, the average algal biomass has been reduced by approximately half, and there has been a shift from a dominance of undesirable blue-greens to more desirable algal species. Taste and odor problems in drinking water have largely been eliminated, as have massive fish kills and piles of rotting Cladophora along the shoreline. Once again, residents of the Lake Erie basin are returning to the shoreline for fishing, boating, swimming, and other recreation.
This is not to say that all problems have been solved. In 1995 and 1996 very large growths of the blue-green algae, Microcystis, occurred in the western basin, and at some nearshore areas Cladophora still grows well. Tests of the bottom waters of the central basin in late summer in 1997 showed that some of the area was still without oxygen, although the extent and duration was not as severe as during the 1980's.
Q: What are some of the really perplexing problems now facing Lake Erie, even though the phosphorus control programs were so successful?
A: It is those darn zebra mussels and our inability to know how Lake Erie will ultimately adjust to them that underlie many of the problems. Our basic fear is that they are so disrupting the normal food web that our favorite fish, perch and walleye, will no longer be as abundant as we would like them to be. The zebra mussels, along with their close relatives, the quagga mussels, are clearly (pun intended!) diverting the food energy in the suspended algae to the bottom of the western basin and the nearshore areas of the central basin. That phenomenon may be causing a change in the normal movement of phosphorus, which would typically cycle from the water to the food chain to the bottom and back again. By interrupting this cycle, the zebra mussels may be preventing some phosphorus from reaching the eastern basin such that fewer algae are growing there and fewer fish can be supported in the food chain.
While it is tempting to suppose that an increase in the phosphorus concentration in the water will result in more fish, the consequences of increased phosphorus loadings are not at all that clear. The models that were used to determine the maximum tolerable annual load of phosphorus to Lake Erie did not account for the influence of such a major ecosystem disrupter as the zebra mussel, and many serious questions remain unanswered. For example, more phosphorus might result in more algal growth, but will the extra growth just be used by even more zebra mussels? Will increased phosphorus result in a return to problems associated with blue-greens and excessive growths of Cladophora? Should additional phosphorus be allowed in the eastern basin, but not in the western or central basins?
Other factors also confound a resolution to the debate, but they are important for defining long term goals and environmental management practices for Lake Erie. For example, the agricultural practices that reduce the amount of phosphorus running off of fields also reduces the quantity of pesticides that find their way to Lake Erie. If increases in phosphorus loadings are encouraged, are we willing to deal with an increase in toxic chemicals entering the lake also? Also, the objective of the GLWQA for restoring the bottom waters of the central basin to aerobic conditions in late summer may need to be clarified or refined. More recent evidence indicates that at least part of the central basin may have experienced periods of very low oxygen concentrations even before European settlement of the surrounding watershed. Should we continue to determine the allowable amount of phosphorus to enter Lake Erie based in part on this oxygen problem?
Q: What happens next?
A: Many of the decisions affecting the future of Lake Erie will be determined over the next few years. The opinions of concerned citizens are very important to the environmental managers, and we should seek opportunities to discuss the issues and to be part of the process for protecting and restoring this great lake.