ESTUARINE FISH FARMING
I45
7. Allochthonous Sources of detritus
(a) Marine seston, both plankton and detritus, carried into the
estuary ; (b) marsh and swa,mp vegetation-the contribution of Spartina, for example, has already been mentioned; (c) water-side plants
and forested slopes of estuaries, which in autumn drop large quantities
of leaves into the water, and in the tropics, mangrove leaves, (the
detritus in the stomachs of grey mullet taken in English estuaries
shows the prevalence of material derived from the leaf-fall) ; (d) riverborne plankton and organic debris, including sewage. The problems
posed by sewage disposal are too well-known to need discussion here ;
they are an important source of organic material, and in excess create
the serious problems of oxygen deficiency. The River Thames, in
London, in the days of the first Queen Elizabeth, ran so clear that it
was celebrated in poetry; in Queen Victoria’s reign it was so foul that
Parliament once had to go into recess because of the unbearable stench
of the river alongside. Only during the last decade have remedial
measures re-introduced enough oxygen to give a clean river. Rotting
leaf mould washed into the rivers is another source of detritus.
8. The nutrient trap
Ketchum (1967-5) writes that water circulation in estuaries frequently has a two-layered flow. The surface layer diluted with river water
escapes seaward, while salt water creeps upstream along the bottom.
On contact with salt water, the fresh-water organisms will, in most
cases, die and sink to counter-current depths in which they will be
distributed around as detritus in various stages of break-down. Marine
organisms carried into the estuary with the salt counter-current will
also in most cases die on contact with fresh water, and so add to the
supply of detritus. The inflowing current may add nutrients directly,
since it may have come from depths below the zone of photosynthesis.
I n these ways nutrients can become trapped in the estuary, and build
up unusually high concentrations.
Cooper and Steven (1948) suggested imitating this process to enrich marine fishponds made by the damming of a large tidal sea-loch,
like the Loch Craiglin in Gross’s fertilizer trials (Orr, 1947). If such
a barrier had two sluices-one a t the bottom to allow the entry of
nutrient-rich bottom water on the rising tide, and one a t the top to
allow of the discharge, on the falling tide of the spent surface waterthen the loch would become a nutrient trap in which fertility would
accumulate without much risk of the formation of a deep de-oxygenated
layer of water.
I45
7. Allochthonous Sources of detritus
(a) Marine seston, both plankton and detritus, carried into the
estuary ; (b) marsh and swa,mp vegetation-the contribution of Spartina, for example, has already been mentioned; (c) water-side plants
and forested slopes of estuaries, which in autumn drop large quantities
of leaves into the water, and in the tropics, mangrove leaves, (the
detritus in the stomachs of grey mullet taken in English estuaries
shows the prevalence of material derived from the leaf-fall) ; (d) riverborne plankton and organic debris, including sewage. The problems
posed by sewage disposal are too well-known to need discussion here ;
they are an important source of organic material, and in excess create
the serious problems of oxygen deficiency. The River Thames, in
London, in the days of the first Queen Elizabeth, ran so clear that it
was celebrated in poetry; in Queen Victoria’s reign it was so foul that
Parliament once had to go into recess because of the unbearable stench
of the river alongside. Only during the last decade have remedial
measures re-introduced enough oxygen to give a clean river. Rotting
leaf mould washed into the rivers is another source of detritus.
8. The nutrient trap
Ketchum (1967-5) writes that water circulation in estuaries frequently has a two-layered flow. The surface layer diluted with river water
escapes seaward, while salt water creeps upstream along the bottom.
On contact with salt water, the fresh-water organisms will, in most
cases, die and sink to counter-current depths in which they will be
distributed around as detritus in various stages of break-down. Marine
organisms carried into the estuary with the salt counter-current will
also in most cases die on contact with fresh water, and so add to the
supply of detritus. The inflowing current may add nutrients directly,
since it may have come from depths below the zone of photosynthesis.
I n these ways nutrients can become trapped in the estuary, and build
up unusually high concentrations.
Cooper and Steven (1948) suggested imitating this process to enrich marine fishponds made by the damming of a large tidal sea-loch,
like the Loch Craiglin in Gross’s fertilizer trials (Orr, 1947). If such
a barrier had two sluices-one a t the bottom to allow the entry of
nutrient-rich bottom water on the rising tide, and one a t the top to
allow of the discharge, on the falling tide of the spent surface waterthen the loch would become a nutrient trap in which fertility would
accumulate without much risk of the formation of a deep de-oxygenated
layer of water.