Increased inorganic nutrient status (particularly in relation to nitrate and phosphate availability) leads to changes in phytoplankton composition and biomass. Although such nutrient changes are normally considered in relation to human activities (anthropogenic eutrophication - Chapter 10), they may also occur as natural processes.
The term 'eutrophication' was originally used in reference to the natural ageing process of lakes, and for a long time it was assumed that oligotrophic lakes were ' primitive' , 'evolving' to eutrophic water bodies as part of a natural progression. Although this has certainly occurred in some cases, the long-term continuation of oligotrophic lakes such as Lake Baikal (Russia) indicates it is not universal.
In lakes where 'natural eutrophication' occurs, long-term inflow of water carries soluble nutrients into the lake where they are taken up by phyto-plankton, sequestered within biomass, and deposited onto sediments in a process of continued accumulation. This leads to increased 'internal loading' and an overall increase in nutrient status. The degree to which such accumulated uptake can occur depends partly on the mean annual concentration of nutrient which occurs in the system. In the case of phosphorus, for example, this may be expressed (Vollenweider and Kerekes, 1980):
where [PA] is the mean annual phosphorus concentration in the water body, rp and rw are the respective mean retention times of phosphorus and water in the system, and [P]q is the average phosphorus concentration in the inflow. The values rp and rw depend on the internal hydrology (area loading, water circulation, mean depth) of the lake, and [P]q depends on the external hydrology and catchment area. In sub-polar lakes such as Lake Baikal,
Benthic algal biofilms dominate the river environment and contain a complex mixture of algae including green algae, blue-greens, and diatoms. The algal composition of biofilms is influenced by a range of interrelated environmental factors including water flow, water depth, light regime, chemistry, nutrient status (eutrophication), pollution, and grazing. The effects of these parameters have been investigated particularly in relation to diatoms (Lock et al., 1984; Round, 1993) since these organisms:
• occur throughout all rivers - diatoms comprise the largest and most prevalent group of algae in river systems,
• can be rapidly and easily sampled,
• have species that are very sensitive to water quality (chemistry), eutrophication, and pollution - the ecological requirements of species are well documented,
• have a rapid growth cycle and respond rapidly to perturbation of the environment.
Rivers form a chemical and biological continuum, with increases in nutrient content from the upper reaches to the estuary. Longitudinal changes in physical and chemical parameters are paralleled by qualitative and quantitative changes in the algal biofilm. These have been summarized in some detail for diatoms by Round (1993), who has suggested longitudinal subdivision of increasingly eutrophic rivers into five major zones (Table 3.11).
The increasing state of eutrophication seen in Table 3.11 would be typical of a river that commences with clean water at the source (Zone 1), followed by increasing input of nutrients (agriculture,
Table 3.11 Diatom transition along a river course with increasing nutrient pollution (information taken from Round, 1993)
Physical Parameters and Water Quality
Dominant Diatom Species division into five major zones
General Aspects of Biofilm
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