Aquatic Chemistry Controlling Nutrient Cycling: Redox and O - Freshwater Ecology: Concepts and Environmental Applications - page 214

In this method, reagents are added that cause iodine to react with O 2 mol-
ecules, forming iodate, IO 3 . The iodate is then titrated back to iodine,
and the amount of titrant necessary to react with the iodate is directly pro-
portional to the O 2 concentration (Eaton et al., 1995). The Winkler method
is more time-consuming but is often more accurate and more precise than
use of standard O 2 electrodes. The Winkler method requires collecting
samples without introducing O 2 . Van Dorn samplers (see Chapter 6) and
other methods provide techniques for collecting samples and not exposing
them to the atmosphere.
Oxygen is difficult to detect at low concentrations (Fenchel and Finlay,
1995), so it is problematic to determine whether or not an environment is
strictly anoxic. Strict anoxia is required for some biogeochemical processes.
Some organisms are extremely sensitive to low O 2 concentrations and must
live in stringently anoxic conditions. Specialized dyes and sample handling
methods are used to sample for strict anoxia.

scales, O 2 concentrations are not constant. When photosynthesis domi-
nates, O 2 concentrations exceed saturation, but when respiration domi-
nates a habitat could become anoxic in the absence of any O 2 input from
the atmosphere.

PHOTOSYNTHESIS

Photosynthesis is the process that provides the energy to run most
ecosystems. Light, temperature, and nutrients all control photosynthetic
rates. Light is generally the predominant factor over the short term.
First, some of the terminology used to describe photosynthetic processes
must be explained. Because respiration and photosynthesis are both oc-
curring simultaneously, even within individual photosynthetic organisms,
we can distinguish between net and gross photosynthesis as follows:

net photosynthesis gross photosynthesis respiration

Net photosynthesis is the photosynthesis that occurs in excess of the res-
piratory demand of the community. Gross photosynthesis is the total
amount of photosynthesis that occurs. Net photosynthesis is sometimes re-
ferred to as primary production. This equation is used frequently in aquatic
ecology (Example 11.1).
Photosynthesis-irradiance (P-I) relationships describe the effects of
light on photosynthetic rate. Several parameters are generally used to de-
scribe this relationship (Fig. 11.9A). These P-I parameters include the res-
piration rate (O 2 consumption in the dark), the compensation point (where
gross photosynthesis equals respiration),

(the initial slope of the line),
P max (the maximum photosynthetic rate), and (a parameter describing