Land-use change in catchments changes the hydrology of rivers and streams and increases nutrient loads to lagoons. Rivers draining clear catchments, or those with extensive urbanization, show 'flashier' flow patterns with water levels rising and falling quickly after rainfall. The hydro-logical balance and water residence times of the lagoons are altered as a result. While nutrient loads are generally proportional to catchment area (Figure 1), loads from cleared agricultural or urban catchments are higher than those from forested catchments, the nutrient loads being proportional to the amount ofcleared land or the human population in the catchment. Carbon, nitrogen, and phosphorus loads all increase; C loads from wastewaters may lead to biochemical oxygen demands (BODs) and anoxia, while increased N and P loads stimulate algal blooms and
1 10 100 1000 10 000 100 000 Catchment area (km2)
Figure 1 The empirical relationship between catchment area (km2) and the total nitrogen load (tonnes per year) to their associated coastal lagoons. Data from the catchments of 19 coastal lagoons on the east coast of Australia. Details of data sources are given in Harris GP (1999) Comparison of the biogeochemistry of lakes and estuaries: Ecosystem processes, functional groups, hysteresis effects and interactions between macro- and microbiology. Marine and Freshwater Research 50: 791-811.
the growth of epiphytes in seagrasses. A further problem is the fact that forested catchments tend to export organic forms of N and P (which are less biologically active in receiving waters), whereas cleared and developed catchments tend to export biologically available inorganic forms of N and P. Thus, both nutrient loads and the availability of those loads increase when catchments are cleared and developed.
N is in many cases (particularly in warmer coastal waters) the key limiting element in lagoons because of high denitrification efficiencies in sediments and long water residence times in summer. In temperate waters N and P may be co-limiting or the limitation may vary seasonally and on an event basis. Overall the climate regime, geomorphology, and biogeochemistry of coastal lagoons seem to lead to extensive N limitation and deni-trification is an important process which determines many ecological outcomes. The effect of land-use change on N loads is therefore a key area of concern. A considerable amount of work has been done on the export of N from catchments around the world. Catchments tend to retain on average about 25% of the N applied to them and export about 75%. There are both latitudinal and seasonal factors which affect this figure. Catchment exports on the eastern coast of North America show an effect of latitude, with warmer, southern catchments with perennial vegetation exporting about 10% of applied N and more northerly catchments with seasonal vegetation growth exporting as much as 40% of applied N, particularly in winter. P exports tend to come primarily from sewage and other wastewater discharges, and also from erosion and agricultural runoff. Catchment loads show evidence of self-organized pattern and process in catchments - nutrient loads and stoichiometries change over time at all scales and the distribution of inflowing nutrients may be fractal.
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