1 Fish Farm Wastes in the Ecosystem
33
from disturbance. Ecosystems, which have the emergent property of homeostasis,
are most healthy when their self-regulatory ability is fully functioning, and ecologists argue that this requires an appropriate balance of organisms performing different functions within the ecosystem. When the balance is disturbed to the extent
that the ecosystem is no longer able to self-regulate properly, and is in danger of
collapse or becoming something else, then it is unhealthy. This view envisages an
internal rather than an external reference for good status. An unhealthy ecosystem
is quite obviously not good for the organisms that form part of it, nor for sustainable
human use, and it is clearly undesirable for humans to bring about such disturbances to the balance of organisms.
Mageau et al. (1995) propose that ecosystem health has quantifiable components
of vigour, organization, resistance to disturbance, and resilience. Tett et al. (2007)
explore ways in which these components might be monitored in marine ecosystems, focusing on the relationship between organization and vigour that is shown
diagrammatically in Fig. 1.6. The terms can be illustrated by considering the impact
of fish farm organic waste on the benthic community underneath a salmon farm at
the start of a 2-year cycle.
Initially the benthic community contains a mixture of species and the full range
of “guilds” of functional types, such as burrowers and filter feeders. The first result
of extra organic input is that existing animals are better fed, and so grow and multiply better. Initially, then, the vigour of the community, as measured by the flow of
energy through it, increases. As extra organic matter continues to arrive, however,
the burrows of animals that pump aerating water through the sediment become
blocked, and these animals either die or move away. Oxygen levels within sediment
pore water begin to decrease, creating conditions in which fewer species of animals
can survive: those which do survive, typically small, specialized worms, have
plenty of food and grow numerous. Under very high levels of organic input, all animal life is impossible, and bacteria capable of surviving in oxygen free conditions
multiply, consuming all available oxygen and then turning to other compounds that
they can use to oxidize organic matter. They may, for example, use the sulphate ions
in seawater for this purpose, excreting either sulphur (which makes a white layer
on the seabed) or the gas, hydrogen sulphide, which is poisonous to most multicellular animals including fish and humans. There may be a high flow of energy
through the seabed, but little of it is put to good purpose within the ecosystem – at
least, judged from the standpoint of multicellular animals, so that vigour is much
decreased. Certainly organization, measured by the taxonomic and functional variety of the benthos, has much decreased.
The resistance of the benthic community to the pressure of increased organic
input is shown by the community’s initial increase in vigour with load; it is when
the burrowers are overwhelmed that this resistance begins to be exceeded and
organization begins to decline markedly – a state of affairs captured by the cartoon
of the Pearson–Rosenberg paradigm in Fig. 1.3(a).
Now, let us assume that, as required by regulation in Scotland, the impacted
benthic zone is confined to a small Allowable Zone of Effect, and that after 2 years
the farm is moved to a new site. Experience has shown that the benthic community
33
from disturbance. Ecosystems, which have the emergent property of homeostasis,
are most healthy when their self-regulatory ability is fully functioning, and ecologists argue that this requires an appropriate balance of organisms performing different functions within the ecosystem. When the balance is disturbed to the extent
that the ecosystem is no longer able to self-regulate properly, and is in danger of
collapse or becoming something else, then it is unhealthy. This view envisages an
internal rather than an external reference for good status. An unhealthy ecosystem
is quite obviously not good for the organisms that form part of it, nor for sustainable
human use, and it is clearly undesirable for humans to bring about such disturbances to the balance of organisms.
Mageau et al. (1995) propose that ecosystem health has quantifiable components
of vigour, organization, resistance to disturbance, and resilience. Tett et al. (2007)
explore ways in which these components might be monitored in marine ecosystems, focusing on the relationship between organization and vigour that is shown
diagrammatically in Fig. 1.6. The terms can be illustrated by considering the impact
of fish farm organic waste on the benthic community underneath a salmon farm at
the start of a 2-year cycle.
Initially the benthic community contains a mixture of species and the full range
of “guilds” of functional types, such as burrowers and filter feeders. The first result
of extra organic input is that existing animals are better fed, and so grow and multiply better. Initially, then, the vigour of the community, as measured by the flow of
energy through it, increases. As extra organic matter continues to arrive, however,
the burrows of animals that pump aerating water through the sediment become
blocked, and these animals either die or move away. Oxygen levels within sediment
pore water begin to decrease, creating conditions in which fewer species of animals
can survive: those which do survive, typically small, specialized worms, have
plenty of food and grow numerous. Under very high levels of organic input, all animal life is impossible, and bacteria capable of surviving in oxygen free conditions
multiply, consuming all available oxygen and then turning to other compounds that
they can use to oxidize organic matter. They may, for example, use the sulphate ions
in seawater for this purpose, excreting either sulphur (which makes a white layer
on the seabed) or the gas, hydrogen sulphide, which is poisonous to most multicellular animals including fish and humans. There may be a high flow of energy
through the seabed, but little of it is put to good purpose within the ecosystem – at
least, judged from the standpoint of multicellular animals, so that vigour is much
decreased. Certainly organization, measured by the taxonomic and functional variety of the benthos, has much decreased.
The resistance of the benthic community to the pressure of increased organic
input is shown by the community’s initial increase in vigour with load; it is when
the burrowers are overwhelmed that this resistance begins to be exceeded and
organization begins to decline markedly – a state of affairs captured by the cartoon
of the Pearson–Rosenberg paradigm in Fig. 1.3(a).
Now, let us assume that, as required by regulation in Scotland, the impacted
benthic zone is confined to a small Allowable Zone of Effect, and that after 2 years
the farm is moved to a new site. Experience has shown that the benthic community