Lakes and Watercourses

Nutrients /
eutrophication
Introduction
Elevated nutrient levels, known as eutrophication, result from an increased
influx or increased availability of plant nutrients in lakes and
watercourses. Eutrophication leads to increased production and plant
and animal biomass, increased turbidity, greater oxygen demand resulting
from the decomposition or organic matter and a change in species
composition and diversity of plant and animal communities. In most
cases the nutrients governing vegetative growth in fresh water are phosphorus
(P) and, in a few cases, nitrogen (N).
Total phosphorus, total nitrogen and the phosphorus/nitrogen ratio
are parameters used to assess lakes. Total phosphorus has been chosen
even though this includes phosphorus fixed in minerals and humus,
which is not directly available to plants. This is due to the need for an
indicator that is analytically straightforward and generally used. The
relative importance of phosphorus and nitrogen is proportional to the
quantities in which they occur, here described as the weight ratio between
the concentration of total nitrogen and that of total phosphorus.
This indicates a deficit or surplus of the two elements and shows the
potential for nitrogen fixation and for accumulation of nitrogen-fixing
cyanobacteria (”blue-green algae”). The concentration of total phosphorus,
like the N/P ratio, can be clearly linked to biological and biochemical
effects. Unlike the N/P ratio, the scale for total nitrogen concentrations,
which is also given, does not measure the effects of nitrogen
on production; it is intended to differentiate between various typical
concentrations in Swedish lakes.
The area-specific loss of nitrogen and phosphorus, respectively, is used
when assessing watercourses. Although this indicator, in principle, mainly
belongs to the criteria for the Agricultural Landscape and the Forest
Landscape, it has been included here since monitoring and calculation of
these losses are a normal and increasingly important part of the environmental
water monitoring programme and since area-specific losses and
discharge of plant nutrients are of importance to the pollution burden on
lakes and marine areas.
Area-specific losses are also an indirect predictor of production
19