26
R.1. Fletcher
1.6 Impact of Green Tides
Whilst at first acting beneficially as nutrient strippers - e.g. Welsh (1980)
described Ulva as having a great capacity to trap nutrients during shortterm intensity releases - in many situations, the excessive growths of algae
eventually have adverse ecological effects. For example, the sheer volume of
algae present can restrict water movement and velocity (Waite and Mitchell
1972a; White and Highlands 1968; Rasmussen 1990; Park 1992). This can
lead to increased sedimentation rates (Rasmussen 1990), it can interfere
with oxygen transport (Wharfe 1977; Lindahl and Hernroth 1983; Reise
1983; Rosenberg 1985; Frederiksen 1987; White and Highlands 1968; Sfriso
1987; Rasmussen 1990; Park 1992), and it can also restrict plankton
transport, with obvious implications in the supply of food for suspension
feeders (Park 1992). It will also restrict larval movement, preventing the
settlement and recruitment of polychaete and bivalve larvae (Price and
Hylleberg 1982; Reise 1983; Thrush 1986; Olafson 1988). The algal mats
would also interfere with surface feeding deposit feeders (Park 1992) and
influence epifaunal associations. Raffaelli et al. (1991) and Raffaelli et al.
(1989), for example, reported the disappearance of Corophium volutator
under high weed biomass in the Ythan estuary in Scotland as a result of the
algal mats interfering with feeding behaviour. Other authors have noted
unusually high numbers of epibenthic fauna such as Peloscolex benedeni
and Hydrobia ulvae associated with the algal mats (Wharfe 1977;
Montgomery and Soulsby 1980; Nicholls et al. 1981; Aneer 1987; Hull 1987)
with obvious ecological repercussions on food chains (Raffaelli et aL 1991).
The large mats of algae would also shade and reduce the photosynthetic
activity of underlying benthic seagrass and algal communities (Rasmussen
1990) as well as physically press down and abrade them (Rasmussen 1990;
Guist and Humm 1976). There are also reports of thick mats of the alga
Dictyosphaeria cavernosa being formed in sewage contaminated waters and
smothering all underlying reef organisms in Kaneohe Bay, Hawaii (Banner
1974; Smith et al. 1981). Reports also suggest that toxic exudates are
released from these loose-lying algal mats (Johnston and Welsh 1985;
Magre 1974) and that these might be responsible, for example, for the
high mortality of Baltic herring (Clupea harengus) eggs laid/deposited on
the surface of the algae (Aneer 1987; Rajasilta et al. 1989). In this respect, it
is interesting to note Magre's (1974) suggestion that Ulva lactuca might
produce substances which are toxic to Balanus. Finally, the physical
presence of these loose-lying algal mats on mud flats has been reported to
deter feeding wild fowl (Nicholls et al. 1981) and to interfere with
fishing methods with resultant loss of catches (Taylor and Bernatowicz 1969;
R.1. Fletcher
1.6 Impact of Green Tides
Whilst at first acting beneficially as nutrient strippers - e.g. Welsh (1980)
described Ulva as having a great capacity to trap nutrients during shortterm intensity releases - in many situations, the excessive growths of algae
eventually have adverse ecological effects. For example, the sheer volume of
algae present can restrict water movement and velocity (Waite and Mitchell
1972a; White and Highlands 1968; Rasmussen 1990; Park 1992). This can
lead to increased sedimentation rates (Rasmussen 1990), it can interfere
with oxygen transport (Wharfe 1977; Lindahl and Hernroth 1983; Reise
1983; Rosenberg 1985; Frederiksen 1987; White and Highlands 1968; Sfriso
1987; Rasmussen 1990; Park 1992), and it can also restrict plankton
transport, with obvious implications in the supply of food for suspension
feeders (Park 1992). It will also restrict larval movement, preventing the
settlement and recruitment of polychaete and bivalve larvae (Price and
Hylleberg 1982; Reise 1983; Thrush 1986; Olafson 1988). The algal mats
would also interfere with surface feeding deposit feeders (Park 1992) and
influence epifaunal associations. Raffaelli et al. (1991) and Raffaelli et al.
(1989), for example, reported the disappearance of Corophium volutator
under high weed biomass in the Ythan estuary in Scotland as a result of the
algal mats interfering with feeding behaviour. Other authors have noted
unusually high numbers of epibenthic fauna such as Peloscolex benedeni
and Hydrobia ulvae associated with the algal mats (Wharfe 1977;
Montgomery and Soulsby 1980; Nicholls et al. 1981; Aneer 1987; Hull 1987)
with obvious ecological repercussions on food chains (Raffaelli et aL 1991).
The large mats of algae would also shade and reduce the photosynthetic
activity of underlying benthic seagrass and algal communities (Rasmussen
1990) as well as physically press down and abrade them (Rasmussen 1990;
Guist and Humm 1976). There are also reports of thick mats of the alga
Dictyosphaeria cavernosa being formed in sewage contaminated waters and
smothering all underlying reef organisms in Kaneohe Bay, Hawaii (Banner
1974; Smith et al. 1981). Reports also suggest that toxic exudates are
released from these loose-lying algal mats (Johnston and Welsh 1985;
Magre 1974) and that these might be responsible, for example, for the
high mortality of Baltic herring (Clupea harengus) eggs laid/deposited on
the surface of the algae (Aneer 1987; Rajasilta et al. 1989). In this respect, it
is interesting to note Magre's (1974) suggestion that Ulva lactuca might
produce substances which are toxic to Balanus. Finally, the physical
presence of these loose-lying algal mats on mud flats has been reported to
deter feeding wild fowl (Nicholls et al. 1981) and to interfere with
fishing methods with resultant loss of catches (Taylor and Bernatowicz 1969;