1 Fish Farm Wastes in the Ecosystem
13
depth reaches up to a hundred metres in clear ocean waters, such as parts of the
Mediterranean, but may be only 1 or 2 m in some very turbid coastal waters. The next
group of distinction in the typology arises from the relationship between the euphotic
zone, the seabed, water column layers, and natural and human supplies of nutrients.
A key distinction is that between waters in which the seabed is within the euphotic
zone, allowing seaweeds, seagrasses or micro-algae to flourish, and those where it
lies deeper, so requiring phytoplankton to provide the primary production. In the first
case, nutrient enrichment may lead to replacement of seagrasses or brown seaweeds
by green seaweeds or epiphytic micro-algae, and there will be concern if an increase
in phytoplankton results in less light reaching the seabed. In the second case, the seasonal pattern of phytoplankton growth, and the ecosystem’s sensitivity to nutrient
enrichment, depends on seasonal patterns of water layering.
In the second case, we need to distinguish between waters that are well-mixed
in the vertical, due to strong stirring by tidal or other currents, or by wind or surface
cooling, and waters that are layered in density as a result of surface heating or
freshwater input. The term pycnocline is used by oceanographers to refer to a zone
of strong vertical gradient in density (due to temperature or salinity) that separates
mixed layers. Phytoplankters growing above such a pycnocline are better illuminated, on average, than those in deep mixed waters. On the other hand, the upper
layer tends to become depleted in nutrients during the main season of phytoplankton growth, and this constrains micro-algal growth. Nutrients added to such an
impoverished layer can have a striking effect by fertilizing phytoplankton when
there are few planktonic animals to eat the micro-algae. Organic matter produced
during these blooms can give rise, later to an increased risk of deoxygenation when
uneaten material sinks, and decays, below a pycnocline.
At the latitude of Scotland, there is generally too little light for phytoplankton
production during the winter, and the typical pattern in coastal seas is that of a
spring bloom as the surface of the sea is warmed by the sun and forms a distinct
layer. Within this well-illuminated surface layer, algae can rapidly convert winter
nutrients into biomass. This is, typically, followed by a summer period of low biomass because of nutrient exhaustion, and sometimes by an autumn bloom as nutrients are remixed into the surface water. In the Mediterranean, in contrast, the main
seasons of phytoplankton growth are the autumn and Winter; in summer the surface
layer is typically intensely nutrient-depleted, but there may be a subsurface layer of
high chlorophyll. As demonstrated by loch Creran (Tett and Wallis 1978), layering
(Fig. 1.1) resulting from freshwater input can extend the season of phytoplankton
growth, unless the freshwater supply is so great that it brings the salinity down
below a level tolerated by marine phytoplankton or flushes the algae from the
system.
A final part of ecohydrodynamics takes into account the type of grazers on the
primary producers. This is important in relation to eutrophication, for a poor coupling
between producers and consumers can allow nutrient enrichment to stimulate a large
increase in producer biomass – red tides of dinoflagellates, or blooms of opportunistic
green seaweeds, for examples. In shallow waters, removal of pelagic micro-algae by
water-filtering benthic animals can be important, but in deeper systems the benthos
13
depth reaches up to a hundred metres in clear ocean waters, such as parts of the
Mediterranean, but may be only 1 or 2 m in some very turbid coastal waters. The next
group of distinction in the typology arises from the relationship between the euphotic
zone, the seabed, water column layers, and natural and human supplies of nutrients.
A key distinction is that between waters in which the seabed is within the euphotic
zone, allowing seaweeds, seagrasses or micro-algae to flourish, and those where it
lies deeper, so requiring phytoplankton to provide the primary production. In the first
case, nutrient enrichment may lead to replacement of seagrasses or brown seaweeds
by green seaweeds or epiphytic micro-algae, and there will be concern if an increase
in phytoplankton results in less light reaching the seabed. In the second case, the seasonal pattern of phytoplankton growth, and the ecosystem’s sensitivity to nutrient
enrichment, depends on seasonal patterns of water layering.
In the second case, we need to distinguish between waters that are well-mixed
in the vertical, due to strong stirring by tidal or other currents, or by wind or surface
cooling, and waters that are layered in density as a result of surface heating or
freshwater input. The term pycnocline is used by oceanographers to refer to a zone
of strong vertical gradient in density (due to temperature or salinity) that separates
mixed layers. Phytoplankters growing above such a pycnocline are better illuminated, on average, than those in deep mixed waters. On the other hand, the upper
layer tends to become depleted in nutrients during the main season of phytoplankton growth, and this constrains micro-algal growth. Nutrients added to such an
impoverished layer can have a striking effect by fertilizing phytoplankton when
there are few planktonic animals to eat the micro-algae. Organic matter produced
during these blooms can give rise, later to an increased risk of deoxygenation when
uneaten material sinks, and decays, below a pycnocline.
At the latitude of Scotland, there is generally too little light for phytoplankton
production during the winter, and the typical pattern in coastal seas is that of a
spring bloom as the surface of the sea is warmed by the sun and forms a distinct
layer. Within this well-illuminated surface layer, algae can rapidly convert winter
nutrients into biomass. This is, typically, followed by a summer period of low biomass because of nutrient exhaustion, and sometimes by an autumn bloom as nutrients are remixed into the surface water. In the Mediterranean, in contrast, the main
seasons of phytoplankton growth are the autumn and Winter; in summer the surface
layer is typically intensely nutrient-depleted, but there may be a subsurface layer of
high chlorophyll. As demonstrated by loch Creran (Tett and Wallis 1978), layering
(Fig. 1.1) resulting from freshwater input can extend the season of phytoplankton
growth, unless the freshwater supply is so great that it brings the salinity down
below a level tolerated by marine phytoplankton or flushes the algae from the
system.
A final part of ecohydrodynamics takes into account the type of grazers on the
primary producers. This is important in relation to eutrophication, for a poor coupling
between producers and consumers can allow nutrient enrichment to stimulate a large
increase in producer biomass – red tides of dinoflagellates, or blooms of opportunistic
green seaweeds, for examples. In shallow waters, removal of pelagic micro-algae by
water-filtering benthic animals can be important, but in deeper systems the benthos