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)

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 O 2 and CO 2 ? (Cole, 1994; Fig 2.1)

Biological Productivity (Rawson Diagram): Volumetric units: mg C/m 3 /d Areal units: mg C/m 2 /d (Cole, 1994; Fig 1.1)

Productivity and Trophic Status Ecosystem productivity is generally classified according to its trophic state for convenience: –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 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.

Probability Distribution (Dodds, 2002; Fig. 17.2)

Fixed Boundary Classifications (Dodds, 2002) 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.

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

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.