Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 99

The rate of oxygen production is proportional to the growth rate of the phytoplankton
because the stoichiometry is fixed. An additional source of oxygen is reduction of
NO 3 − instead of NH 4 + for growth. When available ammonia nitrogen is exhausted

and the nutrient source is nitrate, nitrate is initially reduced to ammonia, which
produces oxygen.
Respiration is the reverse process of photosynthesis through which oxygen is
diminished in the water column as a result of algal respiration. It is normally a small
loss rate for the organism and is temperature dependent. 73
Photosynthesis depends on solar radiation; therefore, the production of dissolved
oxygen proceeds only during daylight hours. Concurrently with oxygen production
via photosynthesis, aquatic plants require dissolved oxygen for respiration, which
can be considered to proceed continuously. The photosynthesis and respiration
processes can add and deplete significant quantities of dissolved oxygen, and the
combined effect of these processes can cause diurnal and seasonal variations in
dissolved oxygen concentrations in aquatic ecosystems. 45,64,66,67 From a seasonal
perspective, photosynthesis will tend to dominate during the growing season, while
respiration and decomposition will prevail during the nongrowing period of aquatic
plants. Diurnal variations in dissolved oxygen can be induced by light, thus dissolved
oxygen could be supersaturated during the afternoon and depleted severely just
before dawn during the growing season. 45
Heavy growth of rooted and attached macrophytes can cause oxygen supersat-
uration in water. Incidental to this supersaturation, a very high pH value can occur
that is fatal to fish. 62 Rooted and attached macrophytes tend to have a greater impact
on dissolved oxygen than phytoplanktons due to two factors. 45

1. Since macrophytes are usually found in shallower water, for an equal

growth or respiration rate as phytoplankton, the impact of rooted and
attached macrophytes on a shallower system will be greater.

2. Because they are fixed in space, macrophytes tend to be more concentrated

longitudinally.

4.1.4.1.3 Oxidation of Organic Matter
The primary loss mechanism associated with organic matter is oxidation (see
Section 4.2 for details). Organic matter serves as an energy source for heterotrophic
organisms in aerobic respiration and decomposition processes. These processes
return organic matter to the simpler inorganic state. During breakdown, oxygen is
consumed and CO 2 is liberated. 45 A principal source of organic matter, other than
anthropogenic pollution and natural run-off, is detritus, produced as a result of
death of organisms. The biodegradability of organic matter is a key factor affecting
the oxidation rate. Thus, different types of organic matter; such as labile dissolved
organic matter (labile DOM), labile particulate organic matter (labile POM), refrac-
tory dissolved organic matter (refractory DOM), and refractory particulate organic
matter (refractory POM) are specified and used in models. Labile organic matter
decomposes on a time scale of days to weeks, whereas refractory organic matter
requires more time (e.g., 1 year). 45,56 Different oxidation rates are used for each of
these forms of organic matter in water quality models.