The Occurrence of "Green Tides" - a Review
9
1990); nutrients associated with sewage eftluents discharged from urban
areas (Letts and Richards 1911; Wilkinson 1963; Sawyer 1965; Buttermore
1977; Wilson et al. 1990). Probably the last two systems are responsible for
the majority of land-derived nutrients entering coastal waters and are,
therefore, primarily responsible for eutrophication processes.
The relative contribution made by these two anthropogenic nutrient
sources will, however, vary geographically. Not surprisingly, large catchment areas, such as the Po River Delta in Italy, are major sources of
agriculturally-based nutrients (Justic 1987; Pugnetti et al. 1990), whilst
nutrient contributions from urban areas are largely derived from sewage
discharges (e.g. Letts and Richards 1911; Wilkinson 1963; Sawyer 1965;
Bellan 1970; Buttermore 1977).
Although these sources represent quite a substantial input of nutrients
into coastal waters, given suitable dilution factors and widespread dispersal, they would not give rise to eutrophication phenomena. Indeed, in some
oligotrophic seas such as the Mediterranean, the overall effect of such added
nutrients on primary production would probably be considered beneficial
(Friligos 1987). It is only when the nutrients become entrapped in coastal
systems and relatively quite high levels are reached that the recipient waters
can be termed eutrophic and ecological imbalances ensue. Not surprisingly,
therefore, reports of eutrophication phenomena are generally confined to
largely enclosed water systems, both artificial and natural, where water
exchange/movement is reduced. These include, for example, basins (Nasr
and Aleem 1948), loughs (Letts and Richards 1911), bays (Fletcher 1974;
Wilson et al. 1990; Park 1992), harbours (Nicholls et al. 1981) and lagoons
(Buttermore 1977; Sfriso 1987; Wilson et al. 1990). However, given the right
hydrodynamic conditions, eutrophication phenomena can also occur on
apparently more open coasts, e.g. in the Bay of st. Brieuc, Brittany, as a
result of nutrient entrapment by tidal currents (Piriou and Menesguen
1990). Similar hydrodynamic phenomena might explain reports of "green
tides" in some bays on the relatively exposed south-east tip of England
(Fletcher 1974). There have also been a number of reports of coastal
eutrophication problems arising as a result of hydrographical alterations,
construction work, sand movement etc. limiting tidal interchange (Sawyer
1965; Fahyet al. 1975; Buttermore 1977; Sfriso et al. 1987; Ruiz et al. 1990;
Park 1992). In the Venice Lagoon, northern Adriatic, this was the result of
channel excavations (Sfriso et al. 1987). In a shallow lagoon in Hobart,
Tasmania, and in the Rogerstown Estuary, Ireland, it was the result of
causeway constructions (Buttermore 1977 and Fahy et al. 1975
respectively), whilst in Boston Harbour it was the result of a channel
becoming blocked and preventing fresh ocean water entering (Sawyer
1965). Hydrodynamic changes resulting in habitat changes were also
9
1990); nutrients associated with sewage eftluents discharged from urban
areas (Letts and Richards 1911; Wilkinson 1963; Sawyer 1965; Buttermore
1977; Wilson et al. 1990). Probably the last two systems are responsible for
the majority of land-derived nutrients entering coastal waters and are,
therefore, primarily responsible for eutrophication processes.
The relative contribution made by these two anthropogenic nutrient
sources will, however, vary geographically. Not surprisingly, large catchment areas, such as the Po River Delta in Italy, are major sources of
agriculturally-based nutrients (Justic 1987; Pugnetti et al. 1990), whilst
nutrient contributions from urban areas are largely derived from sewage
discharges (e.g. Letts and Richards 1911; Wilkinson 1963; Sawyer 1965;
Bellan 1970; Buttermore 1977).
Although these sources represent quite a substantial input of nutrients
into coastal waters, given suitable dilution factors and widespread dispersal, they would not give rise to eutrophication phenomena. Indeed, in some
oligotrophic seas such as the Mediterranean, the overall effect of such added
nutrients on primary production would probably be considered beneficial
(Friligos 1987). It is only when the nutrients become entrapped in coastal
systems and relatively quite high levels are reached that the recipient waters
can be termed eutrophic and ecological imbalances ensue. Not surprisingly,
therefore, reports of eutrophication phenomena are generally confined to
largely enclosed water systems, both artificial and natural, where water
exchange/movement is reduced. These include, for example, basins (Nasr
and Aleem 1948), loughs (Letts and Richards 1911), bays (Fletcher 1974;
Wilson et al. 1990; Park 1992), harbours (Nicholls et al. 1981) and lagoons
(Buttermore 1977; Sfriso 1987; Wilson et al. 1990). However, given the right
hydrodynamic conditions, eutrophication phenomena can also occur on
apparently more open coasts, e.g. in the Bay of st. Brieuc, Brittany, as a
result of nutrient entrapment by tidal currents (Piriou and Menesguen
1990). Similar hydrodynamic phenomena might explain reports of "green
tides" in some bays on the relatively exposed south-east tip of England
(Fletcher 1974). There have also been a number of reports of coastal
eutrophication problems arising as a result of hydrographical alterations,
construction work, sand movement etc. limiting tidal interchange (Sawyer
1965; Fahyet al. 1975; Buttermore 1977; Sfriso et al. 1987; Ruiz et al. 1990;
Park 1992). In the Venice Lagoon, northern Adriatic, this was the result of
channel excavations (Sfriso et al. 1987). In a shallow lagoon in Hobart,
Tasmania, and in the Rogerstown Estuary, Ireland, it was the result of
causeway constructions (Buttermore 1977 and Fahy et al. 1975
respectively), whilst in Boston Harbour it was the result of a channel
becoming blocked and preventing fresh ocean water entering (Sawyer
1965). Hydrodynamic changes resulting in habitat changes were also