Aquaculture Methods for Use in Managing Eutrophicated Waters
117
should normally be considered a natural and healthy response. However, in
the more extreme cases, the resulting algal biomass can create ecological
imbalances that can result in significant problems, including direct fouling
of recreational beaches and navigation channels, and the indirect impacts
of hypoxia and release of noxious gases (principally hydrogen sulfide)
during their eventual death and anaerobic decay (Briand 1987; Merrill and
Fletcher 1991). A notable form in which the proliferous biomass is sometimes observed is exemplified along the Brittany coast of France where Ulva
biomass accumulates in extensive windrows along the beaches, leading to
the use of the descriptive term "green tide". This name might well be
applied to other excess macroalgae situations in order to bring attention to
their similarities as well as the commonalities of cause, even though such
phenomena are not always the result of green algae, nor are they always
associated with tidal events (a model adapted from the concept of "red
tides" associated with dinofagellate blooms; Merrill and Fletcher 1991).
The conditions which lead to green tides typically include: (1) high
inorganic nutrient fluxes, particularly nitrogen; and (2) relatively shallow
water, without strong currents or waves (Sfriso et a1. 1987). These conditions are found throughout the world wherever bays or lagoons with
suitable physical characters are impacted by the eutrophicating affects of
agricultural runoff or human sewage discharge.
The most common macroalage leading to green tides are species of the
green alga Ulva, although significant additional biomass may come from
other green algae such as Enteromorpha or Monostroma. It is a somewhat
unique characteristic of Ulva that it can thrive without any attachment to
the substratum whatsoever. This is a particularly important point since the
types oflocations where green tides are most significant tend to have mud,
silt, or sand bottoms as a result of the relative lack of water movement. The
ability to thrive unattached, coupled with a strong ability for nitrogen
uptake from the seawater, contribute to its domination in the habitats
previously mentioned. Specific inhibitory effects of Ulva upon other algae
may also contribute to its dominance (Svirski et al. 1993).
In many places direct action has been necessary to reduce or eliminate
the damaging impacts of green tides. Such action typically consists of
harvesting or otherwise removing the offending algal mass (Fig. 4.1). This
type of harvesting has reached major proportions in some places, for
example, Venice Lagoon where amounts in excess of 200 tons/day are
removed over extended periods through spring, summer and fall seasons
(Sfriso et a1. 1987), and the Brittany coast of France, where 98 000 m 3 were
removed in 1991, involving 25% of the coastal communitites in the area
(Dion and Le Bozec, Chap. 9, this vo1.). Intervention on such a massive scale
is achieved only at tremendous expense. The harvest of Ulva from Venice
117
should normally be considered a natural and healthy response. However, in
the more extreme cases, the resulting algal biomass can create ecological
imbalances that can result in significant problems, including direct fouling
of recreational beaches and navigation channels, and the indirect impacts
of hypoxia and release of noxious gases (principally hydrogen sulfide)
during their eventual death and anaerobic decay (Briand 1987; Merrill and
Fletcher 1991). A notable form in which the proliferous biomass is sometimes observed is exemplified along the Brittany coast of France where Ulva
biomass accumulates in extensive windrows along the beaches, leading to
the use of the descriptive term "green tide". This name might well be
applied to other excess macroalgae situations in order to bring attention to
their similarities as well as the commonalities of cause, even though such
phenomena are not always the result of green algae, nor are they always
associated with tidal events (a model adapted from the concept of "red
tides" associated with dinofagellate blooms; Merrill and Fletcher 1991).
The conditions which lead to green tides typically include: (1) high
inorganic nutrient fluxes, particularly nitrogen; and (2) relatively shallow
water, without strong currents or waves (Sfriso et a1. 1987). These conditions are found throughout the world wherever bays or lagoons with
suitable physical characters are impacted by the eutrophicating affects of
agricultural runoff or human sewage discharge.
The most common macroalage leading to green tides are species of the
green alga Ulva, although significant additional biomass may come from
other green algae such as Enteromorpha or Monostroma. It is a somewhat
unique characteristic of Ulva that it can thrive without any attachment to
the substratum whatsoever. This is a particularly important point since the
types oflocations where green tides are most significant tend to have mud,
silt, or sand bottoms as a result of the relative lack of water movement. The
ability to thrive unattached, coupled with a strong ability for nitrogen
uptake from the seawater, contribute to its domination in the habitats
previously mentioned. Specific inhibitory effects of Ulva upon other algae
may also contribute to its dominance (Svirski et al. 1993).
In many places direct action has been necessary to reduce or eliminate
the damaging impacts of green tides. Such action typically consists of
harvesting or otherwise removing the offending algal mass (Fig. 4.1). This
type of harvesting has reached major proportions in some places, for
example, Venice Lagoon where amounts in excess of 200 tons/day are
removed over extended periods through spring, summer and fall seasons
(Sfriso et a1. 1987), and the Brittany coast of France, where 98 000 m 3 were
removed in 1991, involving 25% of the coastal communitites in the area
(Dion and Le Bozec, Chap. 9, this vo1.). Intervention on such a massive scale
is achieved only at tremendous expense. The harvest of Ulva from Venice