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4 What Are the Effects of Nutrient Over-Enrichment?
Pages 84-112

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From page 84...
... · The productivity of many coastal marine systems is limited by nutrient availability, and the input of additional nutrients to these systems increased primary productivity. In moderation in some systems, nutrient enrichment can have beneficial impacts such as increasing fish production; however, more generally the consequences of nutrient enrichment for coastal marine ecosystems are detrimental.
From page 85...
... . Direct and indirect ecological impacts of nutrient enrichment include increased primary productivity, increased phytoplankton biomass, reduction in water clarity, increased incidences of low oxygen events (hypoxia and anoxia)
From page 86...
... As noted in a previous report from the National Research Council (NRC 1993a) , planners and managers now are often at a disadvantage because "no guidelines exist by which to determine whether coastal marine ecosystems are in fact eutrophic." That report goes on to recommend that coastal eutrophication be judged by some measure of the relationship between phytoplankton biomass (as represented by chlorophyll concentrations)
From page 87...
... A study of the benthic community in Corpus Christi Bay, Texas, indicated that dissolved oxygen concentrations less than 3.0 mg 1-~ should be the operational definition of hypoxia in that system (Ritter and Montagna 1999) , and that a single value of dissolved oxygen as a water quality standard for estuarine waters may not be appropriate.
From page 88...
... In both systems researchers have documented a seasonal increase in primary productivity in surface waters that was related to nutrients and river flow; this increase was followed by hypoxia in the bottom waters. The hypoxia onset, however, lagged peak river flow in the Gulf of Mexico and Adriatic Sea by two and four months, respectively.
From page 89...
... Shifts in Community Structure Caused by Anoxia and Hypoxia The occurrence of hypoxic and anoxic bottom waters may also lead to shifts in benthic and pelagic community structure due to the mortality of less mobile or more sensitive taxa, reduction of suitable habitat, and shifts in predator-prey interactions (Diaz and Rosenberg 1995~. Hypoxia plays a major role in the structuring of benthic communities because species differ in the sensitivity to oxygen reduction (Diaz and Rosenberg 1995~.
From page 90...
... For example, the flux of nitrogen out of the sediments is affected by the rates of Vitrification and denitrification, and these processes depend on the naturally oxic and anoxic character of the sediments. Changes in Plankton Community Structure Caused Directly by Nutrient Enrichment Nutrient over-enrichment can also change ecological structure through mechanisms other than anoxia and hypoxia.
From page 91...
... Looking beyond the major nutrients, it is also evident that phytoplankton species have variable requirements for nutritional trace elements or have different tolerances for toxic metals (Sunda 1989) , and the effects of these elements can be affected by dissolved organic matter (DOM)
From page 92...
... Another long-term perspective on nutrient enrichment on phytoplankton community structure is seen in recent data examining the abundance of dinoflagellate cysts in bottom sediments of Oslofjord, Norway (Dale et al.
From page 93...
... Although at times changes in community structure are directly the result of nutrient enrichment, sometimes they are an indirect result of other changes caused by increased nutrients. For instance, a change in the phytoplankton community in the form of selection for different species can be caused directly by increased nitrogen.
From page 94...
... ord, Norway, as indicated by the changes in the proportions of dinoflagellate cysts in sediment cores. The left panel shows the concentrations of all cysts combined, the middle panel shows the concentration of the three most important species (Lingulodinium machaerophorum, Operculodinium centrocarpum, and Peridinium faeroense)
From page 95...
... Oxygen levels can also drop to dangerous levels in "healthy" blooms due to algal respiration at night. Estuaries and nearshore waters are particularly vulnerable to low dissolved oxygen problems during warm summer months, especially in areas with restricted flushing.
From page 96...
... Nutrients can stimulate or enhance the impact of toxic or harmful species in several ways. At the simplest level, toxic phytoplankton may increase in abundance due to nutrient enrichment but remain as the same relative fraction of the total phytoplankton biomass (i.e., all phytoplankton species are stimulated equally by the enrichment)
From page 98...
... Degradation of Seagrass and Algal Beds and Formation of Nuisance Algal Mats Many coastal waters are shallow enough that benthic plant communities can contribute significantly to autotrophic production if sufficient light penetrates the water column to the seafloor. In areas of low nutrient inputs, dense populations of seagrasses and perennial macroalgae (including kelp beds)
From page 99...
... in association with nutrient enrichment have been increasing along many of the world's coastlines (Lapointe and O'Connell 1989~. Phytoplankton biomass and total suspended particles increase in nutrientenriched waters and reduce light penetration through the water column to benthic plant communities.
From page 100...
... (B) The bottom-dwelling plants of a marine ecosystem that received high rates of nitrogen input.
From page 101...
... Seagrass roots and rhizomes stabilize sediments, and their dense leaf canopy promotes sedimentation of fine particles from the water column. Loss of seagrass coverage increases sediment resuspension and causes an efflux of nutrients from the sediment to the overlying water that can promote algal blooms.
From page 102...
... even though there were only small increases in measured DIN and SRP concentrations. The growth rate of the coral reef macroalga Diclyosphaeria cavernosa, which overgrew Kaneohe Bay in the 1960s due to sewage nutrient enrichment (Banner 1974)
From page 103...
... Indirect effects on coral reefs can stem from increased phytoplankton biomass that alters the quality and quantity of particulate matter and optical properties of the water column (Yentsch and Phinney 1989~. For instance, reduced light levels associated with turbidity on reefs in Barbados depressed larval development and maturation in the coral Porites porites (Tomascik and Sander 1985~.
From page 104...
... from nutrient enrichment or the benefits of the improvements that result from reducing or reversing this process. To measure value, economists use the concept of "willingness to pay" (WTP)
From page 105...
... Water quality is a classic example of a non-market good. While individuals value water quality improvements (in the same way that they would value an increase in the consumption of shellfish, for example)
From page 106...
... 06 CLEAN CO~S~[ AIRS improvements in environmental Marty. Me applic^1Uty of these techniques varies.
From page 107...
... Thus, each provides only a partial measure of value; none is capable of providing a measure of the total economic value of a water quality improvement. The derived demand approach is applicable when water quality serves as an input into the production of a marketed good.
From page 108...
... Although this study was based on lakes rather than coastal waters, it suggests that homeowners do value water clarity improvements and are willing to pay for them. However, measures of willingness to pay based on hedonic studies of this type
From page 109...
... As water quality at one site changes, users may switch to other sites, even if switching leads to increased travel costs. Random utility models are designed to model the choices that individuals make among alternative sites, as determined by environmental quality, distance, and other site characteristics.
From page 110...
... As noted above, by focusing on a particular user population, each of the revealed preference approaches provides a partial measure of the value of improved water quality. In particular, by their nature, they can capture only the use value of an environmental resource.
From page 112...
... This suggests that ignoring non-use values will lead to significant under estimation of the total value of a water quality improvement (Box 4-3~.


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