1. Aquatic Ecosystem Overview:
We need to understand the physical (e.g. hydrodynamics) and chemical environment that ultimately control the productivity, interactions, and evolution of life in aquatic ecosystems.
Organisms reside in habitats characterized by their physicochemical setting, perform a particular niche or function, and represent a particular trophic level.
Individual organisms are members of a population, which interacts with other populations within a community.
Ecosystems may possess multiple community types based on groupings of common habitat and/or niche characteristics.
Littoral (fringe with light) - Benthic (bottom)
Plankton (drifting) - Nekton (swimming)
Detritus (dead organics) - Groundwater (subsurface; hyporheic)

2. The Lake Example:
Temperate lake formed by geological faulting, depicted during the summer season.
Density differences due to temperature result in stable layers, or strata. This process is called stratification.
Know your lake regions for benthic and open water habitats.
Why the difference in O2 and CO2?
(Cole, 1994; Fig 2.1)

3. Biological Productivity (Trophic Status)
Volumetric units: mg C/m3/d
Areal units: mg C/m2/d
(Rawson Diagram):
(Cole, 1994; Fig 1.1)

4. Productivity and Trophic Status
Ecosystem productivity is generally classified according to trophic states for convenience (see Dodds pp ):
Oligotrophic (low)
Mesotrophic (medium)
Eutrophic (high)
These states represent ranges within a continuum of potential productivity within a system.
The impetus for identifying trophic state came from observations that certain biota (from algal to fish species) were often associated with specific nutrient levels.
Being able to classify systems is convenient for comparison of different areas as well as for defining management goals and reference points.
Trophic state is generally defined as a function of phytoplankton biomass / chlorophyll concentrations, nutrient concentrations, and/or water clarity.

5. Probability Distribution
(Dodds, 2002; Fig. 17.2)

6. Fixed Boundary Classifications
OECD = Organization for Economic Cooperation and Development
Total P = phosphorous in dissolved and solid forms; inorganic and organic.
Mean Chl = annual average of chlorophyll a.
Mean Secchi = average depth to which a black and white disc can no longer be seen.
(Dodds, 2002)

7. Continuous Value Scale (Carlson, 1977)
(Dodds, 2002; Fig. 17.2)

9. Stream Ecosystems
Unless flow is slow enough, stream systems usually do not have a truly planktonic biomass of primary producers. Trophic state is measured by:
Attached benthic algae (periphyton) biomass
Nutrient (N & P) concentrations
Due to dependence of streams on terrestrial (allocthonous) organic matter, primary producers are not always a good indicator of system productivity.