1 Fish Farm Wastes in the Ecosystem
15
discharging waste into the sea, within which concentrations are allowed to exceed
those specified by a far-field EQS. In general, it is easy to see benthic impact
(Nickell et al. 2003) but difficult to detect pelagic impact on this scale, although it
is sometimes possible to find a local increase in ammonia and a decrease in dissolved oxygen (Gowen and Bradbury 1987), and, in the case of shellfish farms, a
local decrease in chlorophyll.
In the simple case of a fish farm in waters without tides or residual currents, the
zone A scale is shown by the footprint of the cage on the sea, i.e., the area impacted
by sinking waste, and a relatively small volume of water around the farm, the
dimensions of which are set by the intensity of eddy diffusion. Under these unfavourable conditions the scale’s dimensions are unlikely to exceed twice those of the
farm. Now let us add a persistent current, which will transport the imaginary
oranges in a downstream plume, broadening as it moves away from the farm. If the
main flows are tidal, the oranges will move in an ellipse, returning after one complete tide to somewhere near their starting point, so that in this case, zone A for
dissolved waste may be several kilometres long. We may take the (slightly over) 12
hours of a tidal cycle in NW European waters as the upper limit to the zone A
timescale, and on this timescale it is impossible for added nutrients to impact on the
plankton, although fast-acting chemical toxins may harm plankton before they are
diluted by dispersion outside the zone. In order to apply this idea to non-tidal
waters, such as those in the Mediterranean, we keep the half-day timescale and
consider the limits of the zone in the water column as that reached by the oranges
during this time. Unless the farm is sited in very energetic waters, the benthic footprint will likely be obvious, and smaller than the pelagic zone A.
The main basin of loch Creran provides an example of a stratified zone B scale
water body and a region of restricted exchange. The residence time of water within
this basin has been estimated as about a week (Tett 1986), although the contents
of the surface layer leave the loch more quickly, within about 3 days, because of
the freshwater driven, tidally enhanced, circulation described earlier. Such residence times are sufficient for nutrients to turn into planktonic algae before the latter are flushed out of the loch, and it is this, and the existence of stratification, that
makes the loch potentially sensitive to the effects of nutrient enrichment. Extra
growth of phytoplankton might be controlled by the grazing of the abundant seashore and seabed animals in Creran, and by the pelagic protozoans found in the
water column. Except during times when benthic animals release their larvae into
the water, the effect of crustacean zooplankton is small, because these animals
tend to get flushed from Creran before they can complete their life cycles within
the loch.
The Firth of Lorne, with which loch Creran exchanges, is a much larger body of
water. The residence time of this water is not well known, but it is probably in the
order of weeks or longer – sufficiently long for nutrients to become phytoplankton
and then be grazed and recycled. Thus it is an example of a zone C scale water
body, and provides the boundary conditions for loch Creran – that is to say, the
water that enters Creran from the Firth already contains a certain amount of nutrients and phytoplankton, depending on the season, and enrichment or grazing within
15
discharging waste into the sea, within which concentrations are allowed to exceed
those specified by a far-field EQS. In general, it is easy to see benthic impact
(Nickell et al. 2003) but difficult to detect pelagic impact on this scale, although it
is sometimes possible to find a local increase in ammonia and a decrease in dissolved oxygen (Gowen and Bradbury 1987), and, in the case of shellfish farms, a
local decrease in chlorophyll.
In the simple case of a fish farm in waters without tides or residual currents, the
zone A scale is shown by the footprint of the cage on the sea, i.e., the area impacted
by sinking waste, and a relatively small volume of water around the farm, the
dimensions of which are set by the intensity of eddy diffusion. Under these unfavourable conditions the scale’s dimensions are unlikely to exceed twice those of the
farm. Now let us add a persistent current, which will transport the imaginary
oranges in a downstream plume, broadening as it moves away from the farm. If the
main flows are tidal, the oranges will move in an ellipse, returning after one complete tide to somewhere near their starting point, so that in this case, zone A for
dissolved waste may be several kilometres long. We may take the (slightly over) 12
hours of a tidal cycle in NW European waters as the upper limit to the zone A
timescale, and on this timescale it is impossible for added nutrients to impact on the
plankton, although fast-acting chemical toxins may harm plankton before they are
diluted by dispersion outside the zone. In order to apply this idea to non-tidal
waters, such as those in the Mediterranean, we keep the half-day timescale and
consider the limits of the zone in the water column as that reached by the oranges
during this time. Unless the farm is sited in very energetic waters, the benthic footprint will likely be obvious, and smaller than the pelagic zone A.
The main basin of loch Creran provides an example of a stratified zone B scale
water body and a region of restricted exchange. The residence time of water within
this basin has been estimated as about a week (Tett 1986), although the contents
of the surface layer leave the loch more quickly, within about 3 days, because of
the freshwater driven, tidally enhanced, circulation described earlier. Such residence times are sufficient for nutrients to turn into planktonic algae before the latter are flushed out of the loch, and it is this, and the existence of stratification, that
makes the loch potentially sensitive to the effects of nutrient enrichment. Extra
growth of phytoplankton might be controlled by the grazing of the abundant seashore and seabed animals in Creran, and by the pelagic protozoans found in the
water column. Except during times when benthic animals release their larvae into
the water, the effect of crustacean zooplankton is small, because these animals
tend to get flushed from Creran before they can complete their life cycles within
the loch.
The Firth of Lorne, with which loch Creran exchanges, is a much larger body of
water. The residence time of this water is not well known, but it is probably in the
order of weeks or longer – sufficiently long for nutrients to become phytoplankton
and then be grazed and recycled. Thus it is an example of a zone C scale water
body, and provides the boundary conditions for loch Creran – that is to say, the
water that enters Creran from the Firth already contains a certain amount of nutrients and phytoplankton, depending on the season, and enrichment or grazing within