Page 234 - Materials Chemistry, Second Edition

220 R.K. Rosenbaum et al.

species composition of the ecosystem. It may also lead to the development of toxic
phytoplankton, dynophysis, cyanobacteria or blue-green algae. When the algae die,
they sink to the bottom where they are degraded under oxygen consumption. As a
consequence, the concentration of dissolved oxygen decreases (hypoxia), which
results in biodiversity loss (flora and fauna). Ultimately, if the process is not
stopped, this will turn a lake into a swamp, that will gradually become grassland
and forest. This process occurs naturally but over a much longer time horizon.
For terrestrial systems, the most significant environmental problem in relation to
nitrogen compound loading is changes in the function and species composition of
nitrogen-poor (and nitrogen limited) ecosystems in heathlands, dune vegetation,
commons and raised bogs as a result of the atmospheric deposition of nitrogen
compounds. Forestry and agriculture may also be affected by reduced yields via
damage to forests and crops. This section however focuses on aquatic
eutrophication.

10.9.2 Environmental Mechanism

The food chain in aquatic ecosystems can be distinguished into three trophic levels:
primary producers (algae and plants producing biomass via photosynthesis), pri-
mary consumers (species consuming algae and plants, the vegetarians) and sec-
ondary consumers (species consuming primary consumers, the carnivores). In
addition to sunlight, growth of primary producers (algae and higher plants) requires
all of the elements which enter into their anabolism (i.e. their synthesis of the
molecules which constitute the organisms’ cells). A molecular formula for the
average composition of an aquatic organism is C 106 H 263 O 110 N 16 P (Stumm and
Morgan 1981). Apart from the elements represented in this formula, minor quan-
tities of a large number of other elements are required, e.g. potassium, magnesium,
calcium, iron, manganese, copper, silicon and boron (Salisbury and Ross 1978). In
principle, the availability of any of these elements can determine the potential
extent of the growth of the primary producers in a given system. The elements
entering in greatest quantities into the primary producers (as in all other living
organisms) are carbon, C, hydrogen, H and oxygen, O. The availability of water can
limit growth in terrestrial plants, but the availability of one of the three basic
elements is rarely a limiting factor in the growth of primary producers.
The other elements which enter into the construction of the primary producers
are nutrients, as the availability of these elements in sufficient quantities is neces-
sary to ensure growth. The nutrients are classified as macronutrients (>1000 lg/g
dry matter in plants) and micronutrients (<100 lg/g dry matter in plants) (Salisbury
and Ross 1978). In rare cases, growth is limited by the availability of one of the
micronutrients, but very small quantities of these elements are required by the
primary producers, and these elements are therefore limiting only on very poor
soils. Of the macronutrients, sulphur is added to all ecosystems in fair quantities in
most of the industrialised world by the atmospheric deposition of sulphur