WATER QUALITY MANAGEMENT IN CONNECTED CATCHMENT- SHALLOW LAKE SYSTEMS - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 37

environment. Thus, aquaculture has an important role to play in meeting the increasing demand for
fish. Indeed, the growth of global aquaculture is forecasted to continue for some time (FAO 2000c).
The potential negative impact due to miss-planned and uncoordinated development of these
fisheries in developing countries is quite obvious. Depending on how it's done and what species
are farmed, inland fisheries or fish ponds development has the potential to cause many problems.
These include:

Pollution within the fish ponds from wastes such as particulate matter from faecal material and
uneaten food, nutrients, and chemicals and drugs, such as pesticides, disinfectants, and
antibiotics.

Transfer of pollution to the surrounding water bodies, specifically lakes and wetlands, due to
continuous interaction and discharge of wastes from the ponds to the water bodies.

Negative impacts on lakes and surrounding water bodies of eutrophication due to excess
nutrients loadings.

Negative impacts on wild populations of fish through the escape of farmed fish and the
transfer of diseases and parasites to open water bodies, as well as negative impacts on other
wildlife.

Privatization of the fisheries and negative interactions with other stakeholders.

Eutrophication Levels and Indicators

Their usual location on the flatter, agricultural lowlands makes shallow lakes more vulnerable and
they have become turbid during this century due to eutrophication. The response of shallow lakes to
eutrophication is often catastrophic rather than smooth transition by the complete loss of submerged
plants, which are the most essential component of a pristine state of such lake.

In its simplest expression, eutrophication is the biological response to excess nutrient inputs to a lake.
The increase in biomass results in a number of effects which individually and collectively lead to
impaired water use. Meybeck et al. (1989) highlight that eutrophication is a natural process which, in
many surface waters, results in beneficial high biomass productivity with high fish yields. Accelerated,
or human-induced, change in the trophic status above the natural lake state is the common cause of the
problems associated with eutrophication. Such human induced changes may occur in any water body,
including coastal marine waters, although the progression and effects of eutrophication are also
mediated by climate. As a result warm tropical and sub-tropical lakes are more severely affected than
colder lakes.

High nutrient concentrations in a lake are derived from external inputs from the watershed. The final
biomass attained is determined primarily by the pool of nutrients available for growth at the beginning
of the growing season. The primary nutrients, such as nitrogen and phosphorus, are used until growth
is complete and the exhaustion of the pool of either one of them places a final limit on the
phytoplankton growth. By definition, the nutrient which is exhausted is the limiting nutrient in any
lake system. Meybeck et al. (1989) suggest that, in waters with an N/P ratio greater than 7 to 10,
phosphorus will be limiting, whereas nitrogen will be limiting in lakes with an N/P ratio below 7.

Changes in transparency in lakes may be caused by increasing turbidity due to increasing
concentrations of mineral material or to increasing plankton biomass. Increases in mineral matter are
caused by:

Turbid in-flow in fluvial waters with high watershed erosion rates,

Re-suspension of bottom sediment by wave action in shallow lakes, or shallow areas of lakes
(when wave height and wavelength developed during storm events allow direct interaction
with the bottom to take place), and