Hydrology Generally, for a given drainage basin, the greater the rainfall, the greater the quantity of water and nutrients transported to a waterbody over the annual cycle. Transport of sediments does not increase indefinitely with rainfall. Sediment trasport and erosion is usually maximal at 300 mm annual precipitation; about 50 mm annual runoff. In tropical/subtropical regions, the highest level of biological productivity are usually seen in lakes and reservoirs two to three months after the rainy season. Geology Chemical composition is influenced by the geological composition, size and topography of the drainage basin. In watersheds with little or no cultural impact, P is usually supplied to a waterbody by direct atmospheric precipitation and by weathering and associated runoff. Phosphate occurs in igneous rocks in the range of 0.07-0.13%. Sedimentary rocks are generally highest in phosphates. Other things being equal, differences in nutrient regime are related to the soil fertility. Upstream reservoirs serve as sink for sediments, and downstream reservoirs do not show as much nutrients. Nutrient inputs are greater in watersheds wih steeper slopes. Nitrate in rainfall can increase nitrate in lakes and reservoirs. Biological nitrification and denitrification can affect nutrient flux. Anthropogenic factors Lake Washington experienced severe phytoplankton problems resulting from sewage inputs. Following complete diversion, it has recovered. Other things being equal, lakes located in watersheds with substantial amounts of agricultural or urban land usage are usually more eutrophic than lakes located in forested watersheds. Fundamental factors affecting nutrient and sediment loads from non-point sources include land form, land use intensity, and usage of materials. Land form characteristics refer to soil texture, soil chemistry, type of soils, drainage density, and slope. Soil texture is the single most important factor affecting nutrient input to the Great Lakes. Greatest surface runoff and associated nutrient inputs was correlated with clayey soils. Calcareous soils often produce high P concentrations in land runoff.

FACTORS RELATED TO THE WATERBODY

The characteristics of the waterbody can modify the effects of basic factors. Mean depth of lake and hypolimnion can substantially affect the impacts of increased nutrient loads to a waterbody The oxygen content in the hypolimnion during periods of thermal stratification, and the related processes of water quality deterioration depend to a large degree on the mean depth of the hypolimnion. For the same quantity of phytoplankton produced in the euphotic zone, the oxygen consumption per unit volume will usually be much greater in waterbodies with small hypolimnetic mean depths.

In Baltic lakes, a waterbody of mean depth greater than 18 m is a prerequisite for oligotrophic conditions. In-lake nutrient sources The role of bottom sediments can be a major concern. In oligotrophic lakes, a substantial portion of the imported nutrients is retained in the sediments. A net deposition of P into the bottom sediments also occurs annually in many eutrophic lakes and reservoirs. In overloaded lakes, P can be released from the sediments. Sudden reduction in input of P will prompt release from the sediments. Release of P from sediments involves physical, chemical, and biological mechanisms. Such factors as redox conditions, nitrate concentration, mineralization, gas bubble formation, bioturbation, effects of phytoplankton and macrophytes, different sediment characteristics, high pH values, diffusion and wind turbulence have been mentioned. Water temperature and water renewal are very important. These induced chemical and microbiological processes which regulate the exchange of substances between the sediments and the water column. Flushing rate The extent to which nutrients accumulate in a waterbody also depends greatly on both rainfall and flushing rate. For example, in closed lake basins in arid regions, dissolved P concentrations can be as high as levels in waste treatment lagoons. If the inflow volume Q of a lake or reservoir is very high compared to its volume V, phytoplankton can be flushed out of the waterbody before they can grow to nuisance levels. Experience suggests that a hydraulic residence time greater than about 3 days is a prerequisite for excessive phytoplankton growths. Biological controls Zooplankton can be used to control phytoplankton. This is an example of "biomanipulation." This technique requires detailed knowledge of the food-web structure of a lake or reservoir. Macrophyte growths Macrophytes cannot compete with phytoplankton in lakes and reservoirs containing very high dissolved P and N concentrations. This is due to the shading effects of high algal densities. Submerged macrophytes are usually not present in waterbodies exhibiting dense phytoplankton growths. One must be careful that control measures to reduce phytoplankton growth does not produce light conditions favorable to excessive growth of phytobentos. Rooted macrophytes depend more on the nutrient content of the sediment than of the water column. Macrophytes are relatively insensitive to control by nutrient flushing. One must consider the overall impact of controlling macrophytes vs controlling phytoplankton.

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