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Nutrient Pollution of Coastal Rivers, Bays, and Seas

Nutrient Pollution of Coastal Rivers, Bays, and Seas

Issues in Ecology Number

Issues in Ecology Number 7 Fall 2000trap silica before it reaches the coast. Thus, the concentrationof silicate in Mississippi River water entering the Gulf ofMexico decreased by 50 percent from the 1950s to the 1980seven as nitrate fluxes and concentrations increased. Eventhe silica that reaches the coast can quickly become unavailableif eutrophication leads to diatom blooms. As the diatomsdie and fall to the bottom, their silica-rich shells remainstored long term in bottom sediments.A decrease in silica availability, particularly if combinedwith increases in N, may encourage some harmful algalblooms as competition with diatoms is decreased. In allcases where long-term data are available on silica in coastalwaters, a decrease in silica relative to N or P has been correlatedwith an increase in harmful algal blooms.SOURCES OF NUTRIENTS TOCOASTAL ECOSYSTEMSHuman activities profoundly influence the global cyclingof nutrients, especially movement of nutrients to estuariesand other coastal waters. For instance, human activityhas more than doubled the rate of P delivery from land tothe oceans. Our effect on N cycling is equally immense, andthe rate of change in the human N use pattern is much greater.Our increased reliance on synthetic, inorganic fertilizers hascaused the single largest change in the global N cycle. Burgeoningfertilizer use accounts for more than half of the totalhuman-driven alteration of the N cycle.The process for making inorganic N fertilizer wasinvented during World War I, but was not widely used untilthe 1950s. The rate of fertilizer use increased steadily untilthe late 1980s, when the collapse of the former Soviet Unionled to great disruptions in agriculture and fertilizer applicationsin Russia and much of Eastern Europe. These disruptionsled to a slight decline in global N fertilizer use for a fewyears. By 1995, however, global use of inorganic N fertilizerwas again growing rapidly, with much of the growthdriven by China. Annual fertilizer use by 1996 totaled approximately83 teragrams (Tg), where one teragram equalsa million metric tons of N. Approximately half of the inorganicN fertilizer that has ever been used on earth has beenapplied during the past 15 years.Although fertilizer production and use is the humanactivity that mobilizes the largest amount of N worldwide,other human-controlled processes also convert atmosphericN into biologically available forms. These activities includethe burning of fossil fuels such as coal and oil and extensiveplantings of nitrogen-fixing crops such as soybeans, peas andalfalfa.Human fixation of N from all these activities increasedby two- to three-fold from 1960 to 1990 and continues togrow. By the mid 1990s, human activities made new Navailable at a rate of some 140 Tg per year, matching thenatural rate of biological nitrogen fixation on all the land11surfaces of the earth. Thus, the rate at which humans havealtered N availability far exceeds the rate at which we havealtered the global carbon cycle, driving up the carbon dioxideconcentration in the atmosphere and perhaps setting inmotion a warming of the earth’s climate.Human-driven changes in nutrient cycling have notoccurred uniformly around the world. The greatest changesare concentrated in the areas of high human population densityand intensive agricultural production. Some regions ofthe world – such as the Hudson’s Bay area of Canada —have experienced very little change in the flows of either Nor P to the coast, while other places have experienced tremendouschanges. Human activity is estimated to have increasedN fluxes down the Mississippi River by more than 4-fold; into the coastal waters of the northeastern United Statesgenerally, and Chesapeake Bay specifically, by some 6- to 8-fold; and into the rivers draining to the North Sea by 11-fold(Figure 6). Trends over time also vary among regions. Forexample, while the global use of inorganic N fertilizer continuesto increase, its use in the United States has risen verylittle since 1985.Wastewater vs. Nonpoint Sources of NutrientsPoint-source wastewater flows can sometimes be themajor source of N to an estuary when the watershed is heavilypopulated and small relative to the size of the estuary itself.Even in some estuaries fed by runoff from larger watersheds,sewage wastewater can be the largest source of N if thewatershed is heavily populated. For example, wastewatercontributes an estimated 60 percent of the N that entersLong Island Sound, largely because of sewage dischargesinto the sound from New York City. In most estuaries, however,N and P inputs from nonpoint sources are greater thanthose from wastewater, particularly in estuaries that haverelatively large watersheds and thus more rural land devotedto crop and livestock production as well as more area to trapN pollution from the atmosphere. For example, only onequarter of the N and P inputs to Chesapeake Bay come fromwastewater treatment plants and other such point sources.For the Mississippi River, sewage and industrial point sourcessupply an estimated 10 to 20 percent of the total N and 40percent of the total P contribution.Agriculture as a Source of NutrientsFor P, agriculture is one of the largest sources ofnonpoint pollution. For N, both agriculture and burning offossil fuels contribute significantly to nonpoint flows to estuariesand coastal waters. N from these sources can reachthe water either by direct leaching or runoff from farm fieldsor indirectly, through the atmosphere. Some N is leacheddirectly from agricultural fields to groundwater and surfacewaters. A significant amount of the N from agriculturalsources, however, travels on the wind. Globally, some 40percent of the inorganic N fertilizer that is applied to fields is

Issues in Ecology Number 7 Fall 2000Early Conceptual ModelContemporary Conceptual ModelNutrient LoadingNutrient LoadingResponsesReversibleFilterDirectResponsesChange in:ChlorophyllPrimary productionMacroalgal biomassSedimentation of Organic CSi:N and N:P RatiosToxic-harmful algal bloomsPhytoplankton CommunityResponsesChange in:ChlorophyllPrimary productionSystem metabolismDissolved oxygenIndirectResponsesChange in:Benthos biomass/communityPelagic biomass/communityVascular plantsHabitat quality/diversityWater transparencyOrganic C in sedimentsSediment biogeochemistryBottom-water oxygenSeasonal cyclesMor tality of fish/invertebratesBiotic diversityFigure 9 - Our understanding of the complexityof responses by coastal ecosystemsto human-caused nutrient enrichment hasgrown considerably in the past decade. Ourearlier conceptual model focused on directresponses of coastal waters, such as stimulationof phytoplankton blooms. The contemporaryconceptual model reflects agrowing awareness of the complexity of theproblem, including recognition that the attributesof specific bodies of water (rate ofmixing between top and bottom waters, frequencyof flushing with open-ocean water,clarity, grazing pressure on algae) createenormous variations in their responses tonutrient loading and lead to a cascade ofdirect and indirect effects. Further, experiencehas shown that appropriate managementactions to reduce nutrient inputs canreverse some of the degradation caused byenrichment. (Redrawn from Cloern, 2000).volatilized as ammonia and lost to the air, either directlyfrom fertilizer or from animal wastes after crops grown withthe fertilizer have been fed to chickens, pigs, or cattle. In theUnited States, the value is somewhat lower, but still 25 percentof the inorganic N fertilizer used is eventually volatilizedto the atmosphere. This N, as well as the nitrogen-basedtrace gases such as nitric oxide released during fossil-fuelcombustion, is later deposited onto the landscape as acidrain or dry pollutants, which can run off into waterways.Thus, in contrast to P, the amount of N exported into coastalwaters from non-agricultural landscapes, including forests,can be substantial.Agriculture in the United States and other developedcountries has changed tremendously over the past 30to 50 years as production systems have become more specializedand concentrated. Overall agricultural productionhas more than doubled, yet production is occurring on lessland and on fewer but larger farms. There has been an attendantincrease in the intensification of the poultry, beef,and hog industries.Prior to World War II, farming communities tendedto be self-sufficient, in the sense that enough feed was producedlocally to meet animal requirements, and manure fromthe livestock could be recycled to meet crop fertilization needs.After the war, increased fertilizer availability and use, alongwith society’s demand for cheap meat and milk supplies, encouragedthe decoupling of grain and animal production systems.Grain production could now be sustained with inorganicfertilizer inputs. Meanwhile, livestock producers could12purchase feed commercially rather than raise crops on site tonourish their animals. By 1995, the major livestock-producingstates imported more than 80 percent of theirgrain for feed, and less than a third of the grain producedon farms today is fed to animals on the farm whereit is grownThis evolution in agriculture has resulted in a majortransfer of nutrients from grain-producing areas to animalproducingareas and, consequently, an accumulation of Nand P in soils of the animal-producing areas (Figure 7). Unfortunately,this excess of nutrient-laden manure has tendedto build up in areas with marginally productive lands, and asa result, these areas have a limited capacity to make use ofthe nutrients in manure in crop production. This fact hasexacerbated the problem of nonpoint runoff of manure nutrientsinto watersheds and ultimately, coastal waters. Animalwastes also contribute greatly to ammonia inputs to the atmosphere,and when this N is deposited back onto the landscape,it can be a significant source of N to surface andground waters that flow into coastal marine ecosystems.On average in the United States, the amount ofammonia volatilized to the atmosphere from agricultural systemsis roughly equivalent to the amount of nitrate thatleaches from crop fields into surface waters. Because thisairborne N comes in addition to the nutrients that animalwastes leak directly to surface waters, animal wastes may bethe largest single source of N that moves from agriculturaloperations into coastal waters.Fossil-fuel Combustion as a Source of Nitrogen

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