12
P. Tett
1.6 Ecohydrodynamics and Sensitivity to Pressures
Although laboratory experiments can, for example, measure the concentration of
copper or zinc pyrithione that kills 50% of phytoplankton (Maraldo and Dahllöf
2004) or the amount of DAIN that must be added to generate a phytoplankton biomass in excess of the CSTT threshold of 10 mg chlorophyll m
−3 (Edwards et al.
2003), the uncontrolled variability of conditions in the sea means that it is much
harder to predict the impact of waste. For example, the food and faeces sinking from
a small salmon farm in sheltered shallow waters might rapidly blanket the seabed
beneath the farm, causing conditions to fall below those tolerable, whereas a larger
farm moored in more turbulent and deeper waters might have no visible effect on the
seabed, because the waste is dispersed by turbulence and spread over a wide area.
However, the larger farm’s waste has a greater potential to contribute to the widespread build-up of chronically harmful levels. Whereas the smaller farm may suffer
from nutrient-stimulated seaweed growth on its cages, the water body containing the
larger farm may suffer eutrophication because nutrients remain high for sufficiently
long, and over sufficient extent, for phytoplankton to benefit from them.
Such considerations lead to two key ideas: first, that the sensitivity to waste of
the waters or sea bed at a particular farm site, depend on ecohydrodynamic conditions at and around that site; second, that the impact of a particular environmental
pressure depends on the spatial and temporal scale on which that pressure is
applied. Scales are considered in the next section. Sensitivity can be roughly defined
as the ratio of impact to pressure, and ecohydrodynamics refers to the physical
conditions at a site and in a water body, and the chemical and biological conditions
that would naturally occur under such conditions. An ecohydrodynamic typology
provides a mean of classifying water bodies on the basis of such conditions. Tett
et al. (2007) proposed a typology based on four key factors: lateral exchange; vertical
mixing; illumination conditions; and the type and abundance of grazers.
The first distinction in the typology is that between open waters and partly
enclosed coastal and transitional waters, called Regions of Restricted Exchange, or
RREs. In RREs, exchange of water with the open sea is an important environmental
condition; Tett et al. (2003a) compared a number of European fjords and barrierprotected bays in which the proportion of water exchanged each day varies from
2.5% (in the Swedish Himmer fjord) to more than 200% (in the Portuguese Ria
Formosa) of the RRE’s volume at mid-tide. The exchange rate for Creran lies
between 0.1 and 0.3 d
−1 . Clearly, well-flushed RREs can accept a greater loading of
dissolved waste per unit surface area than can a poorly flushed water body, so long
as the outside sea contains a lower concentration of the polluting substance.
The availability of light for photosynthesis is an important factor. Light does not
penetrate far into water, because it is scattered by particles and absorbed by water
itself, by chlorophyll and accessory photosynthetic pigments in phytoplankton, and
by the dissolved substances than can give water a yellow or brown colour. The
euphotic zone includes the part of the water column in which there is sufficient light
for the growth of plants, seaweeds, micro-algae and photosynthetic bacteria; its
P. Tett
1.6 Ecohydrodynamics and Sensitivity to Pressures
Although laboratory experiments can, for example, measure the concentration of
copper or zinc pyrithione that kills 50% of phytoplankton (Maraldo and Dahllöf
2004) or the amount of DAIN that must be added to generate a phytoplankton biomass in excess of the CSTT threshold of 10 mg chlorophyll m
−3 (Edwards et al.
2003), the uncontrolled variability of conditions in the sea means that it is much
harder to predict the impact of waste. For example, the food and faeces sinking from
a small salmon farm in sheltered shallow waters might rapidly blanket the seabed
beneath the farm, causing conditions to fall below those tolerable, whereas a larger
farm moored in more turbulent and deeper waters might have no visible effect on the
seabed, because the waste is dispersed by turbulence and spread over a wide area.
However, the larger farm’s waste has a greater potential to contribute to the widespread build-up of chronically harmful levels. Whereas the smaller farm may suffer
from nutrient-stimulated seaweed growth on its cages, the water body containing the
larger farm may suffer eutrophication because nutrients remain high for sufficiently
long, and over sufficient extent, for phytoplankton to benefit from them.
Such considerations lead to two key ideas: first, that the sensitivity to waste of
the waters or sea bed at a particular farm site, depend on ecohydrodynamic conditions at and around that site; second, that the impact of a particular environmental
pressure depends on the spatial and temporal scale on which that pressure is
applied. Scales are considered in the next section. Sensitivity can be roughly defined
as the ratio of impact to pressure, and ecohydrodynamics refers to the physical
conditions at a site and in a water body, and the chemical and biological conditions
that would naturally occur under such conditions. An ecohydrodynamic typology
provides a mean of classifying water bodies on the basis of such conditions. Tett
et al. (2007) proposed a typology based on four key factors: lateral exchange; vertical
mixing; illumination conditions; and the type and abundance of grazers.
The first distinction in the typology is that between open waters and partly
enclosed coastal and transitional waters, called Regions of Restricted Exchange, or
RREs. In RREs, exchange of water with the open sea is an important environmental
condition; Tett et al. (2003a) compared a number of European fjords and barrierprotected bays in which the proportion of water exchanged each day varies from
2.5% (in the Swedish Himmer fjord) to more than 200% (in the Portuguese Ria
Formosa) of the RRE’s volume at mid-tide. The exchange rate for Creran lies
between 0.1 and 0.3 d
−1 . Clearly, well-flushed RREs can accept a greater loading of
dissolved waste per unit surface area than can a poorly flushed water body, so long
as the outside sea contains a lower concentration of the polluting substance.
The availability of light for photosynthesis is an important factor. Light does not
penetrate far into water, because it is scattered by particles and absorbed by water
itself, by chlorophyll and accessory photosynthetic pigments in phytoplankton, and
by the dissolved substances than can give water a yellow or brown colour. The
euphotic zone includes the part of the water column in which there is sufficient light
for the growth of plants, seaweeds, micro-algae and photosynthetic bacteria; its