Environmental Engineering Reference
In-Depth Information
Scientists often choose indirect measures because they are easier to make across larger
parts of a system or across more systems. As another example, the measurement of chloro-
phyll-a is often used as an indicator of primary productivity in aquatic ecosystems.
However, chlorophyll-a is not a direct measure of productivity, rather it is a measure of
the presence of a pigment used in photosynthesis, and the photosynthetic process is the
source of building biomass. Likewise, the carbon : nitrogen (C:N) ratio in soil is often used
as an indicator of litter or soil quality, and is often used to predict decomposition rates, or
rates of nitrogen cycling (see Chapters 4 and 5). To make these indirect measures useful,
empirical relationships between direct and surrogate measures must be established-
quantifying these relationships is an active area of research (see Chapter 17).
Some Tools in the Ecosystem Scientist's Toolbox
Ecosystem scientists try to answer a diverse range of questions about a wide array of
characteristics of the most varied kinds of ecosystems, using any of several scientific
approaches. It will therefore come as no surprise that ecosystem scientists use an enormous
number of specific scientific techniques in their investigations, some simple, some sophisti-
cated, some developed within the discipline, and some borrowed and adapted from other
disciplines. These techniques are far too numerous to list and discuss in an introductory
textbook. Nonetheless, several tools are worth introducing here because they are character-
istic of ecosystem science and will appear repeatedly in the coming chapters.
Balances: Mass and Charge
Mass balance (
Box 1.2
) is a major tool in ecosystem science, especially for ecosystems of
which the boundaries are defined by their watersheds. The laws of thermodynamics tell
us that matter and energy are not created or destroyed. When both inputs and outputs of
energy or matter can be measured relatively completely and accurately it is possible to
construct a mass balance and infer processes. For example, an unbalanced watershed mass
balance suggests that either the element of interest is being retained in (inputs
outputs)
.
or leaking from (outputs
inputs) the ecosystem (see Chapters 5 and 9). The watershed
mass balance approach was pioneered in the 1960s by scientists at the Hubbard Brook
Experimental Forest, New Hampshire (
Bormann and Likens 1967
), and has been used
powerfully around the world to understand the abiotic and biotic movement of a suite of
elements through ecosystems.
The other powerful “balance” tool that ecosystem scientists use is charge balance.
In water, the charges held by positive ions (cations) and negative ions (anions) must bal-
ance each other. That is, for every anion (such as chloride) in an aqueous solution, there
must be a corresponding cation (such as sodium). Why is this tool so useful? Charge bal-
ance tells us, for example, that when an anion moves through a forest soil from ground
water into a stream, it must be accompanied by a corresponding cation (see Chapter 5).
The sum of all the negative charges brought by anions must be balanced by the same
number of positive charges. Charge balance also makes it possible to check whether the
major ions in a water sample have been measured correctly; a charge imbalance tells us
that a measurement error has been made or that we have not quantified all the cations or
anions that are important in a sample.
.
