1 Fish Farm Wastes in the Ecosystem
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dinoflagellates (MAD, a name that emphasizes the need to distinguish photosynthesizing dinoflagellates from their relatives that live by eating other micro-organisms,
and which excludes certain large-bodied dinoflagellates characteristic of summer
in deep, temperature-layered, waters). Sampling loch Creran at a particular time,
counting the phytoplankters in these samples using a microscope, and assigning the
relevant counts to these life forms, gives a pair of values: an abundance of the PD
and an abundance of the MAD. Next, draw a pair of axes: one for the abundance
of the PD and another, at right-angles, for the abundance of the MAD. (For reasons
of statistical methods, we actually use the logarithm of abundance.) Onto the resulting Cartesian co-ordinate system, plot the point specified by the abundances of PD
and MAD on the date of sampling. It is this point that defines the state of the
ecosystem – or at least of its phytoplankton components – on that day.
This, however, is not enough. It is a characteristic of phytoplankton in temperate
seas that the absolute and relative abundance of phytoplankter life forms changes
with the seasons, and we must take account of this. So we continue to take samples
from Creran and to plot additional points until we have several years worth of data
displayed on the PD-MAD axes Now we can see that a graph linking the points
makes loops on the “PD-MAD” surface, and we can define the “state” of the loch
Creran ecosystem as being the area on the PD-MAD diagram that is occupied by
all these points. I have made such a diagram in Fig. 1.7, using data obtained from
1979, 1980 and 1981. If we assume that the loch was in a natural and healthy state
during these years, and if Creran is typical of its WFD coastal water type, we can
argue that this diagram defines a “type-specific reference condition” for the balance
of organisms in the phytoplankton. This is the approach taken by the PCI-LF: a
“state-space” diagram of this sort is made for a reference condition; an envelope is
drawn about this reference condition; and the PCI value is measured by plotting
new data onto the same diagram and counting the proportion of new points that fall
outside the reference condition envelope (Tett 2006).
We are in course of doing this with more recent data from loch Creran, and some
early results are shown in part (d) of the diagram. What is to be expected in a case
when a fish farm added sufficient nutrients to disturb the balance of organisms?
Because the nutrients would be compounds of nitrogen and phosphorus, but not
silicon, they would favor dinoflagellates rather than diatoms; and hence new points
should be found more towards the top (the MAD axis) than the right-hand side (the
PD axis) of the diagram. This is to some extent what we seem to be finding,
although it also seems that diatom abundance has decreased. That decrease might
be the proximate cause of the decrease of chlorophyll in loch Creran that was mentioned previously.
I have taken this detour into details of how to assess change in phytoplankton in
order to penetrate a little deeper into some of the theory underlying the “ecosystem
approach” and to show how such theory may contribute to the practical matter of
assessing ecological quality. There are two ways in which a PCI might be used. If
it is to be used to quantify the health of the phytoplankton, ecologists need better
knowledge of the relationship between organization and vigour in pelagic communities. To be more concrete, which parts of the PD-MAD surface represent a healthy
37
dinoflagellates (MAD, a name that emphasizes the need to distinguish photosynthesizing dinoflagellates from their relatives that live by eating other micro-organisms,
and which excludes certain large-bodied dinoflagellates characteristic of summer
in deep, temperature-layered, waters). Sampling loch Creran at a particular time,
counting the phytoplankters in these samples using a microscope, and assigning the
relevant counts to these life forms, gives a pair of values: an abundance of the PD
and an abundance of the MAD. Next, draw a pair of axes: one for the abundance
of the PD and another, at right-angles, for the abundance of the MAD. (For reasons
of statistical methods, we actually use the logarithm of abundance.) Onto the resulting Cartesian co-ordinate system, plot the point specified by the abundances of PD
and MAD on the date of sampling. It is this point that defines the state of the
ecosystem – or at least of its phytoplankton components – on that day.
This, however, is not enough. It is a characteristic of phytoplankton in temperate
seas that the absolute and relative abundance of phytoplankter life forms changes
with the seasons, and we must take account of this. So we continue to take samples
from Creran and to plot additional points until we have several years worth of data
displayed on the PD-MAD axes Now we can see that a graph linking the points
makes loops on the “PD-MAD” surface, and we can define the “state” of the loch
Creran ecosystem as being the area on the PD-MAD diagram that is occupied by
all these points. I have made such a diagram in Fig. 1.7, using data obtained from
1979, 1980 and 1981. If we assume that the loch was in a natural and healthy state
during these years, and if Creran is typical of its WFD coastal water type, we can
argue that this diagram defines a “type-specific reference condition” for the balance
of organisms in the phytoplankton. This is the approach taken by the PCI-LF: a
“state-space” diagram of this sort is made for a reference condition; an envelope is
drawn about this reference condition; and the PCI value is measured by plotting
new data onto the same diagram and counting the proportion of new points that fall
outside the reference condition envelope (Tett 2006).
We are in course of doing this with more recent data from loch Creran, and some
early results are shown in part (d) of the diagram. What is to be expected in a case
when a fish farm added sufficient nutrients to disturb the balance of organisms?
Because the nutrients would be compounds of nitrogen and phosphorus, but not
silicon, they would favor dinoflagellates rather than diatoms; and hence new points
should be found more towards the top (the MAD axis) than the right-hand side (the
PD axis) of the diagram. This is to some extent what we seem to be finding,
although it also seems that diatom abundance has decreased. That decrease might
be the proximate cause of the decrease of chlorophyll in loch Creran that was mentioned previously.
I have taken this detour into details of how to assess change in phytoplankton in
order to penetrate a little deeper into some of the theory underlying the “ecosystem
approach” and to show how such theory may contribute to the practical matter of
assessing ecological quality. There are two ways in which a PCI might be used. If
it is to be used to quantify the health of the phytoplankton, ecologists need better
knowledge of the relationship between organization and vigour in pelagic communities. To be more concrete, which parts of the PD-MAD surface represent a healthy