Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 460

TABLE 22.2 Summary of an Expanded View of the River Continuum Concept

Feature Headwaters Middle reaches Large rivers

Physical
Stream order 1-3 4-7 7

Discharge Low Medium High

Flooding Flashy, short, unpredictable Medium Regular, predictable

Gradient High Medium Low

Temperature Cool, constant when shaded Moderate, variable Warm, constant

Substrate Rocky, large wood Intermediate Silt, sand

Riparian canopy Dense, covering stream channel Above stream channel Important only in

open flood zone

Turbidity Low Low High

Light Low High Low

Metabolic
Photosynthesis (P) Low High Moderate-low

Respiration ( R) a ? ? ?

P/R 1 1 1

Organic carbon Coarse Intermediate Fine

CPOM/FPOM ratio b 1 1 1

Woody debris Large wood, debris dams Along margins Relatively rare, but

an important
substrate in sandy
or silty rivers
(Haden et al., 1999)

Producers
Periphyton Moderate High Low

Phytoplankton Low Low Relatively high

Macrophytes Low, but mosses may Moderate Low except in side

predominate pools

Consumer invertebrates
Shredders High Moderate Low

Filter feeders Low Moderate High

Scrapers/grazers Moderate High Low

Collector gatherers Moderate Moderate High

Predators Moderate Moderate Moderate

Fish
Diversity Low, cool water Medium High, warm water

Sight feeders High High Low

Prey Invertebrates Invertebrates, fish Invertebrates,

plankton, fish

a Relative patterns not established.
b CPOM, coarse particulate organic matter; FPOM, fine particulate organic matter.

ited by nutrients, and oxic processes predominate. On the opposite side of
the spectrum, eutrophic lakes are prone to cyanobacterial blooms, have
anoxic hypolimnia, have high rates of production in the water column, and
production tends to be limited by nitrogen (because nitrogen is lost to
denitrification) or light. Important exceptions to this classification scheme
include dystrophic lakes (with high concentrations of humic compounds)
that have low planktonic production but high macrophyte production, lim-
itation by light for the phytoplankton, and heavily anoxic sediments with
high rates of denitrification.
The classical view is that a lake ecosystem has cleanly defined bound-
aries and river inflow and outflow. The view assumes that carbon dynamics