The Occurrence of "Green Tides" - a Review
29
waters at the sediment surface replenishing the superficial sediment
nutrient load (Nicholls and Keeney 1973; Birch et al. 1981; Gabrielson et al.
1983; Zimmermann and Montgomery 1984; Sfriso et al. 1987, 1988b; Pavoni
et al. 1990; Pugnetti et al. 1990; Lavery and McComb 1991; Hanisak 1993;
Sfriso and Marcomini 1994). Essentially this acts as an organic input into
the sediments (Zimmermann and Montgomery 1984); it also releases
nutrients back into the water column (Gabrielson et al. 1983) which can be
manifested in the occurrence of phytoplankton blooms (Sfriso et al. 1987,
1989). Pertinent to this discussion was the report of Gabrielson et al. (1983)
that Cladophora tissue decomposed into the usable forms ammonium and
orthophosphate. Also interesting was Smith and Horne's (1988) report that
Ulva had a greater affinity for nitrogen than do some plankton, thereby
depressing phytoplankton growth.
1.7 Treatment of Green Tides
Not surprisingly the world-wide occurrence and ecological impact of
"green tides" has aroused considerable concern and much consideration
has been given towards the development and implementation of various
control methods. Although there is some evidence that grazing by
invertebrates can exert some control on biomass development (GeertzHansen and Sand-Jensen 1990), in the majority of "green tide" situations
and during the peak periods of biomass production, such natural biological
control methods are totally inadequate and treatment is required.
Probably the most widely used treatment involves the physical removal of
the troublesome algal biomass. This particularly applies to situations where
large quantities of weed are present e.g. 85 000 m 3 of harvestable Ulva
deposits on beaches in Brittany (Dion 1994) and an estimated 1-1.2 million
tons of Ulva growing in the Venice Lagoon (Orlandini 1988; Sfriso et al.
1989). Usually, the algae are mechanically removed following its accumulation on the tide line, and transported to land-fill sites (Merrill and Fletcher
1991). In some situations, the algal biomass is removed in situ. For example,
special ' reaping' machines were used to remove Ulva from Odense Fjord in
Denmark (Frederiksen 1987), whilst harvesting boats were used to remove
floating masses of Ulva from the Venice Lagoon (Orlandini 1988; Merrill and
Fletcher 1991). On one occasion, an unsuccessful attempt was made to treat
Ulva mats by chemical means using quicklime and copper sulphate in
Winthrop Harbour, USA (Sawyer 1965). Certainly, harvesting provides an
efficient remedial control method and offers additional benefits in the
29
waters at the sediment surface replenishing the superficial sediment
nutrient load (Nicholls and Keeney 1973; Birch et al. 1981; Gabrielson et al.
1983; Zimmermann and Montgomery 1984; Sfriso et al. 1987, 1988b; Pavoni
et al. 1990; Pugnetti et al. 1990; Lavery and McComb 1991; Hanisak 1993;
Sfriso and Marcomini 1994). Essentially this acts as an organic input into
the sediments (Zimmermann and Montgomery 1984); it also releases
nutrients back into the water column (Gabrielson et al. 1983) which can be
manifested in the occurrence of phytoplankton blooms (Sfriso et al. 1987,
1989). Pertinent to this discussion was the report of Gabrielson et al. (1983)
that Cladophora tissue decomposed into the usable forms ammonium and
orthophosphate. Also interesting was Smith and Horne's (1988) report that
Ulva had a greater affinity for nitrogen than do some plankton, thereby
depressing phytoplankton growth.
1.7 Treatment of Green Tides
Not surprisingly the world-wide occurrence and ecological impact of
"green tides" has aroused considerable concern and much consideration
has been given towards the development and implementation of various
control methods. Although there is some evidence that grazing by
invertebrates can exert some control on biomass development (GeertzHansen and Sand-Jensen 1990), in the majority of "green tide" situations
and during the peak periods of biomass production, such natural biological
control methods are totally inadequate and treatment is required.
Probably the most widely used treatment involves the physical removal of
the troublesome algal biomass. This particularly applies to situations where
large quantities of weed are present e.g. 85 000 m 3 of harvestable Ulva
deposits on beaches in Brittany (Dion 1994) and an estimated 1-1.2 million
tons of Ulva growing in the Venice Lagoon (Orlandini 1988; Sfriso et al.
1989). Usually, the algae are mechanically removed following its accumulation on the tide line, and transported to land-fill sites (Merrill and Fletcher
1991). In some situations, the algal biomass is removed in situ. For example,
special ' reaping' machines were used to remove Ulva from Odense Fjord in
Denmark (Frederiksen 1987), whilst harvesting boats were used to remove
floating masses of Ulva from the Venice Lagoon (Orlandini 1988; Merrill and
Fletcher 1991). On one occasion, an unsuccessful attempt was made to treat
Ulva mats by chemical means using quicklime and copper sulphate in
Winthrop Harbour, USA (Sawyer 1965). Certainly, harvesting provides an
efficient remedial control method and offers additional benefits in the