Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 455

Movement of water and organisms has direct effects on energy flux through
ecosystems. In aquifers with very fine pore sizes, only dissolved materials,
fine particles, bacteria, and very small protozoa can move through the sed-
iments. Thus, only bacterial “producers” and primary consumers are pres-
ent. Karst aquifers have the largest channels, but there are still few trophic
levels relative to streams and lakes; however, there are more levels than in
aquifers with fine pore sizes. Predatory fish, amphibians, crayfish, or pla-
narians can be the top carnivores; four trophic levels may be the highest
number found in karst groundwaters (Culver, 1994), but usually there
are less.
Other ways to classify groundwater ecosystems are by their depth or
by their degree of connectivity with surface waters and terrestrial habitats.
Connection of groundwaters and streams has been well described in some
cases and is dependent on the type of substrate and spatial and temporal
scales being considered (Harvey and Wagner, 2000). Over the short term
(mainly less than a few months), in aquifers with low hydraulic conduc-
tivity, only shallow groundwater interacts with lakes, streams, and wet-
lands. Over longer time periods, deep groundwater can have considerable
interaction with surface waters.
Groundwater can link lakes across the landscape over years. In north-
ern Wisconsin, lakes occur in sandy glacial outwash, and groundwater
links many of them (Kratz et al., 1997). Drought leads to changes in
groundwater flow and increases in ion concentrations in some lakes. It can
take up to 5 years for drought effects on ion concentrations to move
through lakes that are substantially affected by groundwater dynamics
(Webster et al., 1996, 2000). Groundwater effects in these lakes can some-
times override the effects of climate depending on lake position (Baines
et al., 2000).
Given the difficulty of sampling groundwater habitats, detailed trophic
analyses and energetic or nutrient budgets are not as well documented as
they are for surface waters. However, knowledge of groundwater habitats
increased tremendously in the past two decades. What was a virtually un-
described group of biomes is beginning to be understood.
STREAMS
The major concepts associated with streams considered here are (i) the
flood pulse concept, (ii) autochthonous versus allochthonous production,
(iii) inverted biomass pyramids (Allen's paradox), (iv) nutrient spiraling,
and, (v) the river continuum concept. Although other important concepts
have been explored for stream ecosystems, these exemplify some of the key
features of stream ecosystem science.
The flood pulse concept is a change in paradigm from viewing floods
as disturbances that alter an ecosystem that is otherwise at equilibrium to
viewing flooding as a characteristic property of river and stream ecosystem
function. It is sometimes difficult to view flooding as a natural process in
rivers that does not need to be controlled. For example, debris flows in
small streams can exceed 10,000 m 3 of mud, logs, rocks, and sediment that
sweep through portions of steep watersheds. Such flows may appear to be