Clean Coastal Waters Understanding and Reducing the Effects of Nutrient Pollution (2000) / Chapter Skim
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6 What Determines Susceptibility to Nutrient Over-Enrichment?
6 What Determines Susceptibility to Nutrient Over-Enrichment?
Pages 163-194
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From page 163... ...
. a widely accepted estuarine classification scheme is a prerequisite for a systematic approach to extending lessons learned and successful management options from one estuary to others; · such a classification scheme should allow categorization of relatively poorly known systems on the basis of a minimum suite of measurements; quantitative classifications that provide insights into the relative importance of the different factors controlling estuarine dynamics have the most potential for predictive analysis; and · a high priority should be the development of a national framework of "index sites" within which there would be an integration and coordination of environmental monitoring and research, with the goal of developing a predictive understanding of the response of coastal systems to both nutrient enrichment and nutrient reduction.
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From page 164... ...
While there are numerous similarities between all estuaries, there are also some basic differences. For example, both a drowned river valley estuary, such as Chesapeake Bay, and a bar-built estuary, such as Plum Island Sound in northeastern Massachusetts, have temporal and spatial patterns of salinity that reflect seasonal variations in freshwater discharge.
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From page 165... ...
Dilution is one of the dominant factors used to predict lake eutrophication, and Nixon (1992) showed a strong relationship between areal nutrient load, which partially accounts for dilution, and primary production in a wide variety of estuaries.
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From page 166... ...
Seagrass dominated estuaries are likely to be the most sensitive to nutrient enrichment, shown by the rapid rise in eutrophication severity as nitrogen loading is increased from 0.1 to 1.0 moles N m-2 yr-l. Salt marsh estuaries are expected to be the least sensitive to nutrient enrichment, which is illustrated by the slow rise in eutrophication severity only after nitrogen loading exceeds 1.0 moles N ~2 yr-1 (unpublished figure by C
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From page 167... ...
If both water residence time and phytoplankton turnover time are one day, there is no chance of a bloom; algae are flushed from the system as fast as they multiply. Alternatively, if the residence time is seven days and phytoplankton turnover time is one day, phytoplankton can double seven times prior to being exported and an initial algal population of 5 ,ug chl-a 1-~ can become a 640 ,ug 1-~ bloom, given no other losses.
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From page 170... ...
showed that filter feeding benthos in San Francisco Bay effectively decreased the estuarine response to nutrient loading (in terms of phytoplankton production)
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From page 171... ...
Researchers have shown that light can play a critical role in determining the response of estuarine systems to nutrient loading (e.g., Cloern 1987, 1991, 1996, 1999~. In northern San Francisco Bay, high turbidity from watershed sediment erosion reduces light levels to such an extent that primary production is light-limited year round.
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From page 173... ...
BOX 6-2 Continued WHAT DETERMINES SUSCEPTIBILITY TO NUTRIENT OVER-ENRICHMENT?
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From page 175... ...
Algal blooms are an example of an autotrophic process and net oxygen uptake is an example of a heterotrophic process. Recognizing that there are basic differences in estuarine susceptibility to nutrient over-enrichment, the development of a conceptual framework or classification scheme for organizing how to think about susceptibility will enable scientists and managers to better understand and predict the effects of human activities on estuarine and coastal ecosystems, and thus to more effectively manage human activities.
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From page 176... ...
To envision how a useful scheme might be constructed, an examination of existing schemes and their value for understanding estuarine response to nutrient loading is warranted. In reviewing existing estuarine classification systems, it is important to bear in mind that only in the past decade or so, however, have classification systems been developed to increase our understanding and ability to predict the effects of enhanced nutrient delivery to coastal ecosystems.
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From page 177... ...
Thus, this approach is of limited usefulness in understanding, predicting, and managing estuary response to nutrient loading. HYDRODYNAMIC CLASSIFICATION The hydrodynamic approach to estuarine classification focuses on the interaction in narrow estuaries of tidal currents and river flow.
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From page 178... ...
The classification became known as the Circular 39 Classification (Straw and Fredine 1956~. In 1979, the National Wetlands Inventory classification was adopted as the primary wetland classification scheme for U.S.
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From page 179... ...
A few habitat-based classification schemes have included forcing function criteria.
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From page 180... ...
Recent research on estuarine susceptibility to eutrophication suggests that stratification is a major determinant in estuarine response to nutrient loading. The Hansen and Rattray classification enables stratification to be quantified and thus may be useful in developing an eutrophication classification (modified from Hansen and Rattray 1966~.
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From page 181... ...
Linkage with hydrodynamic classification schemes is through the non-dimensional hydrodynamic parameters associated with each sediment transport forcing mode. Six transport processes are parameterized: 1)
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From page 182... ...
fractional freshwater method, which is derived from the replacement of the freshwater component of the total system volume by river flow. pop = f x—x N Vf Vt where Qf is freshwater discharge and Vf is freshwater volume in the estuary, Via is total estuarine volume and N is mean nutrient load for all estuaries.
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From page 183... ...
The next logical step would be to quantify the extent to which the ability to predict estuarine susceptibility has improved with further development of the EXP index. Analyses of the relationship between areal nutrient loading, EXP, and trophic state would illustrate whether predictive ability has improved relative to the first approximation presented by Nixon (1992~.
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From page 186... ...
These efforts, when coupled with efforts to model nutrient loading to estuaries and more rigorous quantification of tidal and stratification parameters should improve the capability to predict estuarine susceptibility to nutrient enrichment (Bricker et al.
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From page 187... ...
For instance, estuaries in Maine fell out as a cluster as did large rivers, mid-Atlantic lagoons, and south-Atlantic marsh dominated estuaries. It is not obvious why these groupings occur, but this behavior may reflect similarities in stratification and circulation in local regions or it may reflect basic differences in physiography or primary production base (e.g., salt marsh dominated lagoons versus plankton dominated drowned river valleys)
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From page 188... ...
A Proposal to Select and Use Coastal Index Sites There are too many estuaries in the United States for the nation to conduct comprehensive ecosystem studies of all those affected by nutrient enrichment. Although the federal government conducts monitoring activities at more than 15,000 sites nationwide (Pryor et al.
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From page 189... ...
The establishment of a national framework of index sites where monitoring and research are closely integrated would lead to the development of a predictive understanding of coastal system responses to anthropogenic activities, especially nutrient enrichment. At index sites, intensive monitoring and research activities would lead to a broad understanding of how and why estuaries respond as they do to nutrient inputs.
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From page 190... ...
To make major advances in understanding coastal systems and in predicting the effects of increased nutrient loading will require interdisciplinary research coordinated among investigators working within the index sites. Research should emphasize major ecological questions that stress linkages between terrestrial and coastal ecosystems.
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From page 191... ...
ADDITIONAL QUESTIONS An understanding of the response of coastal waters to nutrient loading is developing slowly. Following the scientific lead of freshwater ecologists modeling the trophic state of lakes, marine ecologists are developing a predictive understanding of some of the key parameters controlling estuarine response to nutrient loading.
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From page 194... ...
Whether the same classification schemes being developed to predict effects of increasing nutrients will work equally well in predicting reversals, if nutrient inputs are reduced, remains to be determined. As some studies have indicated, there are non-linearities and thresholds in eutrophication response to increased nutrient loading.
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