But through this channel come pouring millions of cubic metres of salt water on
each rising tide, and a slightly greater volume leaves on the ebb tide, swirling past
small islands where seals lie and black birds perch on the lookout for fish. The
outflow volume is greater because it must include the water added by rivers: in
normal circumstances only a few percent of the tidal flow, but with a major effect
on the circulation within the loch. Fresh water is less dense than salt water,
and, where it mixes with seawater forms a lighter superficial layer that floats
seawards, while the heavier saltwater, brought in by the tide, penetrates
underneath.
This circulation renews water and oxygen within the loch, and creates good
conditions for the growth of the fish and seabed animals that feed the seals and
birds. On the seabed, there were once-abundant beds of the European oyster, and
there still are extensive reefs made from the calcareous tubes of serpulid worms.
Both oysters and serpulid worms are members of the benthos. Some benthic animals feed on organic matter within seabed mud, but the oysters and serpulids get
food by filtering suspended particles. The most nutritious of these are the tiny floating algae of the phytoplankton, too small to be seen, as individuals, by the naked
human eye. These micro-algae are well known as the “grass of the sea”, the main
marine source of organic food made by photosynthesis. When my colleagues and I
studied it (Tett et al. 1985; Tett and Wallis 1978), Creran was typically rich in a
variety of phytoplankters, especially those belonging to the group known as diatoms, which absorb dissolved silica from sea-water and use it to make glassy cases
for their cells. The circulation of water through the loch provided a continuing
source of compounds of nitrogen, phosphorus and silicon; and the layering created
by the freshwater input allows phytoplankters to remain in a superficial layer that
is well-lit by sunlight for much of the year.
Phytoplankton is not the only source of organic food in Creran: seaweeds are
also important primary producers, and there is a further input of dead organic matter from rivers (Cronin and Tyler 1980; Tyler 1984). But I have described enough
to make my point: that loch Creran is an ecosystem, a term invented by Roy
Clapham in 1930, published by Arthur Tansley (1935) and defined by Eugene
Odum (1959) as
any area of nature that includes living organisms and nonliving substances interacting to
produce an exchange of materials between the living and nonliving parts…
Formally, the nonliving substances form the environment and the living organisms
form the (biotic) community; but a ecosystem is not simply environment plus community but also the interactions between and amongst them; it is both structure and
function – the food web and how it works.
Thus, the interactions in loch Creran include the biogeochemical fluxes of
organic matter and nutrients amongst the biota and between them and their surroundings; the effects of the serpulid reefs in stabilizing the seabed in Creran; the
transport of animal as well as micro-algal plankton by currents; the addition of
oxygen by algal photosynthesis and air–sea exchange, and its consumption by the
respiration of all the animals and bacteria living in the waters of the loch or on or
1 Fish Farm Wastes in the Ecosystem
5
each rising tide, and a slightly greater volume leaves on the ebb tide, swirling past
small islands where seals lie and black birds perch on the lookout for fish. The
outflow volume is greater because it must include the water added by rivers: in
normal circumstances only a few percent of the tidal flow, but with a major effect
on the circulation within the loch. Fresh water is less dense than salt water,
and, where it mixes with seawater forms a lighter superficial layer that floats
seawards, while the heavier saltwater, brought in by the tide, penetrates
underneath.
This circulation renews water and oxygen within the loch, and creates good
conditions for the growth of the fish and seabed animals that feed the seals and
birds. On the seabed, there were once-abundant beds of the European oyster, and
there still are extensive reefs made from the calcareous tubes of serpulid worms.
Both oysters and serpulid worms are members of the benthos. Some benthic animals feed on organic matter within seabed mud, but the oysters and serpulids get
food by filtering suspended particles. The most nutritious of these are the tiny floating algae of the phytoplankton, too small to be seen, as individuals, by the naked
human eye. These micro-algae are well known as the “grass of the sea”, the main
marine source of organic food made by photosynthesis. When my colleagues and I
studied it (Tett et al. 1985; Tett and Wallis 1978), Creran was typically rich in a
variety of phytoplankters, especially those belonging to the group known as diatoms, which absorb dissolved silica from sea-water and use it to make glassy cases
for their cells. The circulation of water through the loch provided a continuing
source of compounds of nitrogen, phosphorus and silicon; and the layering created
by the freshwater input allows phytoplankters to remain in a superficial layer that
is well-lit by sunlight for much of the year.
Phytoplankton is not the only source of organic food in Creran: seaweeds are
also important primary producers, and there is a further input of dead organic matter from rivers (Cronin and Tyler 1980; Tyler 1984). But I have described enough
to make my point: that loch Creran is an ecosystem, a term invented by Roy
Clapham in 1930, published by Arthur Tansley (1935) and defined by Eugene
Odum (1959) as
any area of nature that includes living organisms and nonliving substances interacting to
produce an exchange of materials between the living and nonliving parts…
Formally, the nonliving substances form the environment and the living organisms
form the (biotic) community; but a ecosystem is not simply environment plus community but also the interactions between and amongst them; it is both structure and
function – the food web and how it works.
Thus, the interactions in loch Creran include the biogeochemical fluxes of
organic matter and nutrients amongst the biota and between them and their surroundings; the effects of the serpulid reefs in stabilizing the seabed in Creran; the
transport of animal as well as micro-algal plankton by currents; the addition of
oxygen by algal photosynthesis and air–sea exchange, and its consumption by the
respiration of all the animals and bacteria living in the waters of the loch or on or
1 Fish Farm Wastes in the Ecosystem
5