WEB FOR LAKE MEAD, NV
webs may be described in terms of both energy and nutrient (carbon, nitrogen
flows (Figure 15). Although the process typically begins with sunlight-driven
by algae and plants, balanced nutrition is also required to sustain life.
For example, we cannot live strictly on sugar, despite its high caloric content,
irrespective of what our kids may argue.
two basic life-sustaining processes in lakes, just as on land; photosynthesis
Green plants capture energy from sunlight to convert nonliving, inorganic
dioxide, water, and mineral compounds) into living, organic
plant tissue. Lake photosynthesizers
include algae and macrophytes.
Together, they are the primary producers, because they create the organic
material required by most other organisms for nutrients and energy.
Oxygen, the waste product of photosynthesis, adds to the oxygen
supplied to the lake by the atmosphere. In water layers where photosynthetic
rates are very high, such as during an algal bloom, the water may become
supersaturated. That is, the oxygen content may exceed 100% of saturation
with respect to the amount the water could hold if it was allowed to
equilibrate with the atmosphere. This saturation value, in turn, depends
on the temperature of the water. Colder water can hold more O2
than warmer water. During periods of stratification,
the only potential source of O2 to the deeper zones of the
lake is photosynthesis. This occurs only if light penetrates below the
In lakes where light does not penetrate below the thermocline, there
is no internal source of oxygen to the deeper waters.
light, algae and higher plants need oxygen, carbon
dioxide (CO2), and mineral nutrients to survive and grow.
Except for a very few species of blue green algae, most are unable to
survive in anoxic
(no O2) water. CO2 is virtually always available
and comes from the weathering
of carbonate rocks, such as limestone, in the watershed, diffusion
from the atmosphere (very important in softwater,
rain sensitive lakes), and from the respiration of organic matter
by all of the organisms in the lake (see below). Dissolved mineral nutrients
are absorbed from the water by algae and from the water and the sediments
by higher plants. Typically, the most important nutrients are phosphorus
and nitrogen, because they are present in very low concentrations unless
there are sources of pollution ( see trophic
state section) and are typically low enough to limit the growth
of algae. Other minerals essential to life, such as the major ions (calcium,
magnesium, sodium, and potassium) and certain trace metals (iron, cobalt,
molybdenum, manganese, copper, boron, and zinc), are usually present
at sufficient concentrations. Silicon is required by diatoms
and a few other groups of algae and is usually, though not always, present
at sufficient levels. Another mineral required by all living things,
sulfur (in the form of sulfate), is typically not deficient in lakes.
The whole interaction
of photosynthesis and respiration by plants, animals, and microorganisms represents
the food web. Food webs are usually very complex and, in any one lake ecosystem,
hundreds of different species can be involved. Because the available energy
decreases at each trophic level, a large food base of primary producers (mostly
plants) is necessary to support relatively few large fish.
may die and decompose or be eaten by primary
consumers the second trophic level. This link in the food
chain typically involves zooplankton grazing on algae but also includes
larval fish eating zooplankton and a variety of invertebrates that eat
attached algae (periphyton)
and higher plants. Other animals, such as small fish, secondary
consumers (third trophic level) eat the primary consumers and thus
are considered secondary consumers. Still larger consumers such as large
fish, ospreys, and people are tertiary
consumers (fourth trophic level). Thus, energy and nutrients originating
from the photosynthetic production of biomass
and energy cascade through the food web (Figure 15). There is some recycling
of nutrients back up to the top of the cascade. Respiration, the oxidation
of organic material, releases the energy that was originally captured
from sunlight by photosynthesis. Both plants and animals respire to
sustain their lives, and in doing so, consume oxygen. Microorganisms
(bacteria and fungi) consume a large fraction of available oxygen in
of excreted and dead organic material.
sinks for plant and animal wastes, but they also recycle nutrients for
photosynthesis. The amount of dead material in a lake far exceeds the
living material. Detritus is the organic fraction of the dead material,
and can be in the form of small fragments of plants and animals or as
dissolved organic material. In recent years, scientists have recognized
that zooplankton grazing on detritus
and its associated bacterial community represent an additional important
trophic pathway in lakes.