36
P. Tett
Can the plankton be said to have organization? This section attempts to answer that
question for the phytoplankton, by introducing a monitoring tool called the
Phytoplankton Community Index, or PCI.
An ecosystem is made up of many parts: in the case of loch Creran, of water,
mud, dissolved substances and populations of many species of animals, algae and
bacteria. These components are continuously changing: water is exchanged with
the sea, benthic animals reproduce, seeding the water with planktonic larvae; the
balance amongst the populations of species of phytoplankters changes with the
season. So, like humans whose every atom is said to be replaced every seven years,
ecosystems remain identifiable while subject to flux. There is a way to describe the
essence of such changing systems in terms of state variables. In the case of Creran,
these variables might include the volume and salinity of water in the loch, the concentrations of nutrients and oxygen in the water, and the abundance of species of
benthic animals and of phytoplankton. System theory states that a system is in the
same state whenever all state variables have the same value. This may seem obvious, or perhaps even tautologous, but it allows us to find ways of describing ecosystems so as to discover whether they are indeed in the same state, which is a
precursor to deducing whether the state is “good” (from the perspective of the
WFD), or “healthy”, or whether it has changed in a way that would be regarded as
an “impact” (from the standpoint of the DPSIR terminology). Now let us zoom in
to consider phytoplankton alone (but as a component of an ecosystem).
What state variables can we define to capture the essence of phytoplankton in
ecosystems? Ecologists have for a long time been interested in species diversity and
questions about number of species and the relative abundance of each species (Tett
and Barton 1995). However, the list of species of phytoplankters in a typical water
sample may be as long as several hundred, and in most cases we know little about
what particular species “do” in the pelagic ecosystem. An alternative is to consider
that there are a number of functional rôles to be played by pelagic photosynthesizers and that all these rôles must be properly played for proper functioning (and
hence health) of the ecosystem. The functions include the cycling of nutrients,
and this suggests that there is a distinction between the glassy-walled diatoms, which
use and cycle silicon as well as nitrogen and phosphorus, and most other phytoplankters, which do not use silica. Another distinction might be between small phytoplankters, which are suitable food for pelagic protozoa, and larger phytoplankters,
which offer a tasty mouthful for copepods and pelagic crustaceans. To cut a potentially
long story (Tett et al. 2003b) short, we may view the phytoplankton as being made
up of populations of a handful, or double handful, of life forms, and a healthy
“balance of organisms” being a balance of these life forms able to carry out all the
functions that the ecosystem requires of the phytoplankton, and without which it
will degrade into an unhealthy state.
This is not the place to list life forms. Indeed, we probably do not know enough
about phytoplankton ecology to make a single undisputed list. For the sake of illustration, let us take just two life forms: pelagic diatoms (PD, so called to distinguish
them from the thick-walled diatoms that normally grow on the seabed but which
can be lifted into the phytoplankton by turbulence); and medium-sized autotrophic
P. Tett
Can the plankton be said to have organization? This section attempts to answer that
question for the phytoplankton, by introducing a monitoring tool called the
Phytoplankton Community Index, or PCI.
An ecosystem is made up of many parts: in the case of loch Creran, of water,
mud, dissolved substances and populations of many species of animals, algae and
bacteria. These components are continuously changing: water is exchanged with
the sea, benthic animals reproduce, seeding the water with planktonic larvae; the
balance amongst the populations of species of phytoplankters changes with the
season. So, like humans whose every atom is said to be replaced every seven years,
ecosystems remain identifiable while subject to flux. There is a way to describe the
essence of such changing systems in terms of state variables. In the case of Creran,
these variables might include the volume and salinity of water in the loch, the concentrations of nutrients and oxygen in the water, and the abundance of species of
benthic animals and of phytoplankton. System theory states that a system is in the
same state whenever all state variables have the same value. This may seem obvious, or perhaps even tautologous, but it allows us to find ways of describing ecosystems so as to discover whether they are indeed in the same state, which is a
precursor to deducing whether the state is “good” (from the perspective of the
WFD), or “healthy”, or whether it has changed in a way that would be regarded as
an “impact” (from the standpoint of the DPSIR terminology). Now let us zoom in
to consider phytoplankton alone (but as a component of an ecosystem).
What state variables can we define to capture the essence of phytoplankton in
ecosystems? Ecologists have for a long time been interested in species diversity and
questions about number of species and the relative abundance of each species (Tett
and Barton 1995). However, the list of species of phytoplankters in a typical water
sample may be as long as several hundred, and in most cases we know little about
what particular species “do” in the pelagic ecosystem. An alternative is to consider
that there are a number of functional rôles to be played by pelagic photosynthesizers and that all these rôles must be properly played for proper functioning (and
hence health) of the ecosystem. The functions include the cycling of nutrients,
and this suggests that there is a distinction between the glassy-walled diatoms, which
use and cycle silicon as well as nitrogen and phosphorus, and most other phytoplankters, which do not use silica. Another distinction might be between small phytoplankters, which are suitable food for pelagic protozoa, and larger phytoplankters,
which offer a tasty mouthful for copepods and pelagic crustaceans. To cut a potentially
long story (Tett et al. 2003b) short, we may view the phytoplankton as being made
up of populations of a handful, or double handful, of life forms, and a healthy
“balance of organisms” being a balance of these life forms able to carry out all the
functions that the ecosystem requires of the phytoplankton, and without which it
will degrade into an unhealthy state.
This is not the place to list life forms. Indeed, we probably do not know enough
about phytoplankton ecology to make a single undisputed list. For the sake of illustration, let us take just two life forms: pelagic diatoms (PD, so called to distinguish
them from the thick-walled diatoms that normally grow on the seabed but which
can be lifted into the phytoplankton by turbulence); and medium-sized autotrophic