Aquatic systems waters Watersheds

Estimation of the effects of N deposition on aquatic systems is made difficult by the large variety of forms of N found in air, deposition, watersheds, and surface waters, as well as by the myriad pathways through which N can be cycled in terrestrial and aquatic ecosystems. These complexities separate N deposition from its effects and reduce our ability to attribute known aquatic effects to known rates of N deposition. The organization of this chapter reflects this complexity. Because an understanding of the ways that N is cycled through watersheds is critical to our understanding of N effects, I begin with a brief description of the N cycle and of the transformations of N that may occur in watersheds. I then discuss the two most likely effects of N deposition (acidification and eutrophication). 

Watersheds are generally several orders of magnitude larger than the surface waters that drain them. Thus most of the atmospheric deposition that may potentially enter aquatic systems falls first on some portion of the watershed. Nitrogen may be deposited to the watershed or directly to water surfaces... 

Atmospheric N can enter aquatic systems either as direct deposition to water surfaces or as N deposition to the terrestrial portions of a watershed. Nitrogen deposited to the watershed is routed and transformed by watershed processes. It may eventually reach aquatic systems in forms only indirectly related to the original deposition. The transformations that N undergoes within the watershed (e.g., in soils, by microbial action, and in plants) play a major role in determining what forms and amounts of N eventually reach surface waters. Much of the challenge of determining when N deposition is... 

Though several studies (5-9) have described the concentrations of trace elements in surface waters, most of them did not differentiate between species of a particular element. Some (10) have considered the distribution between dissolved and particulate forms. However, few attempts (11) have been made to evaluate the distribution of metals between various solid phase components of the suspended material. The present study provides quantitative estimates of dissolved (dissolved is defined as those aquatic components that could not be removed by centrifugation from liquid phase) and various solid phase associated metal fractions in southeastern United States streams. Between November 83 and August 85, 46 bi-weekly samples were taken from six SRP associated watersheds to determine dissolved (filterable) and total element concentrations. As one of several goals of the study was to assess the impact of natural and production related activities on trace element behavior in these aquatic systems, knowledge of speciation within solid and dissolved phases was essential for data interpretation. The research described herein used sequential extraction and a thermodynamics approach to define solid and dissolved phase species of Cu, Cd, Fe, Mn, Ni and Zn. The study also evaluated the effects of natural and production related processes on the distribution of metals in aquatic systems at SRP. 

Inland water systems are more diverse and dynamic than marine systems (see Chapter 18) therefore, it becomes apparent that more emphasis should be placed on understanding microbial community dynamics in these highly variable systems. Lakes and rivers are intimately associated with their watersheds and hence receive a large influx of material from the surrounding terrestrial environments. This linkage affects the biogeochemical cycles and processes within the system as oftentimes members of the terrestrial community end up in the aquatic environment. 

The cotton landscape example presented above reveals that landscape-level risk assessment can be conducted by investigating the influence of the surrounding landscape on the emission of insecticides to the water bodies of concern in order to characterize more realistically actual exposure concentrations. This relatively simple approach addresses variability within the landscape, but pays less attention to the interactions between water bodies. A more complex approach is to assess the fate and effects of a chemical (or combination of stressors) for the entire watershed and to consider this watershed as a true continuum. The latter approach may include all water bodies within a watershed and addresses their interdependence, for example, by studying the flow of water, chemicals, matter, and organisms between these systems. An example of such a watershed approach is the study of Pandovani et al. (2004). They used a landscape-level approach to assess aquatic exposure via spray drift of chlorpyrifos-methyl in the watershed of the Simeto River in Sicily (Italy). 

The chlorophenoxy groups of herbicides includes 2,4-D, 2,4,5-T, and many other chemically related compounds. The chlorophenoxy compounds are primarily selective herbicides and comprise approximately half the total domestic herbicide market. Although 2,4-D is essentially insoluble in water, its esters are slightly water-soluble, and salts of 2,4-D are completely water-soluble. Several of these compounds are used not only for application to plant foliage and soil but also as aquatic herbicides (8). Each year hundreds of tons of these compounds are applied directly to lakes, rivers, and other surface waters for weed control. Approximately 100,000 pounds of 2,4-D granules are applied annually to the lakes in the TVA system alone (7). The herbicide 2,4-D may persist for several months in lake water whereas the esters of 2,4-D are usually broken down in a few days (I). When applied to watershed areas, the phenoxy herbicides are not likely to constitute a major water pollution hazard since the rate of bacterial degradation is sufficiently rapid to destroy them within a few days (26). However, a few of these compounds can remain in the environment for a year or more. 

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