The Occurrence of "Green Tides" - a Review
27
Norton and Mathieson 1983; Rosenberg 1985; Perez-Ruzafa et al. 1991;
Park 1992).
Additional problems are reported to occur when these algal mats are
removed by tide/current movements and drift to new localities. For
example, they can be deposited in deeper waters (Rasmussen 1990), this
representing a short-term nutrient loss to the local ecosystem; however, as
shown for the Baltic, nutrients accumulated in deep waters are eventually
mixed up to the surface layers through thermohaline convection (Fonselius
1969). The mats can also be deposited on the shoreline, including bathing
beaches and spoiling them with resultant aesthetic, ecological and
economic problems (Sawyer 1965; Hanks 1966; Horne and Nonomura 1976;
Casabianca-Chassany 1984; Rosenberg 1985; Jeffrey et al. 1992; Park 1992;
Jeffrey 1993), notably the release of pungent smells during their decay
(Wilkinson 1963; Sawyer 1965; Hanks 1966; Southgate 1972; Fletcher 1974;
Guist and Humm 1976; Horne and Nonomura 1976; Buttermore 1977;
Sfriso et al. 1987; Park 1992) and expensive clearance operations associated
with their removal (Fletcher 1974). There have also been reports oflarge
floating drift weed deposits blocking power-station intake ducts (Southgate
1972) and intake pipes on ships (Park 1972).
A number of authors do, however, note some beneficial aspects of the
loose-lying, algal mats (Cowper 1978). They do, for example, represent a
very important sink for nutrients and, therefore, playa vital role in nutrient
recycling processes (Birch et al. 1981; Sfriso et al. 1988b; Pugnetti et al.
1990). They add considerably to the detritus system and contribute to the
dissolved organic carbon pool (Cowper 1978). They can also physically
provide food, habitat and shelter to many small invertebrates and fish and
act as important spawning and nursery grounds for many small fish
(Kimura et al. 1958; Cowper 1978; Norton and Mathieson 1983).
Particular problems arise, however, during the subsequent and
sometimes catastrophic decomposition of the algal mats. This usually
occurs during periods of high temperatures and calm conditions when
oxygen demand dramatically increases e.g. during night-time respiration
(Perkins and Abbot 1972; Casabianca-Chassany 1989; Rasmussen 1990).
Widespread axonic conditions ensue which have considerable ecological
consequences (Frederiksen 1987; Sfriso et al. 1987, 1988b; Rasmussen
1990). For example, this reduction in oxygen and the associated excessive
development of sulphide on the underlying mud surface, is reported to have
a detrimental effect on the underlying mud infauna. Perkins and Abbot
(1972) noted that the bivalve molluscs Cardium (Cerastoderma) edule and
Macoma balthica moved up to the surface to avoid deeper anaerobic
conditions; Nephtys also emerged. Many of these animals died, or being
unprotected, were predated by wading birds. Similar events were described
27
Norton and Mathieson 1983; Rosenberg 1985; Perez-Ruzafa et al. 1991;
Park 1992).
Additional problems are reported to occur when these algal mats are
removed by tide/current movements and drift to new localities. For
example, they can be deposited in deeper waters (Rasmussen 1990), this
representing a short-term nutrient loss to the local ecosystem; however, as
shown for the Baltic, nutrients accumulated in deep waters are eventually
mixed up to the surface layers through thermohaline convection (Fonselius
1969). The mats can also be deposited on the shoreline, including bathing
beaches and spoiling them with resultant aesthetic, ecological and
economic problems (Sawyer 1965; Hanks 1966; Horne and Nonomura 1976;
Casabianca-Chassany 1984; Rosenberg 1985; Jeffrey et al. 1992; Park 1992;
Jeffrey 1993), notably the release of pungent smells during their decay
(Wilkinson 1963; Sawyer 1965; Hanks 1966; Southgate 1972; Fletcher 1974;
Guist and Humm 1976; Horne and Nonomura 1976; Buttermore 1977;
Sfriso et al. 1987; Park 1992) and expensive clearance operations associated
with their removal (Fletcher 1974). There have also been reports oflarge
floating drift weed deposits blocking power-station intake ducts (Southgate
1972) and intake pipes on ships (Park 1972).
A number of authors do, however, note some beneficial aspects of the
loose-lying, algal mats (Cowper 1978). They do, for example, represent a
very important sink for nutrients and, therefore, playa vital role in nutrient
recycling processes (Birch et al. 1981; Sfriso et al. 1988b; Pugnetti et al.
1990). They add considerably to the detritus system and contribute to the
dissolved organic carbon pool (Cowper 1978). They can also physically
provide food, habitat and shelter to many small invertebrates and fish and
act as important spawning and nursery grounds for many small fish
(Kimura et al. 1958; Cowper 1978; Norton and Mathieson 1983).
Particular problems arise, however, during the subsequent and
sometimes catastrophic decomposition of the algal mats. This usually
occurs during periods of high temperatures and calm conditions when
oxygen demand dramatically increases e.g. during night-time respiration
(Perkins and Abbot 1972; Casabianca-Chassany 1989; Rasmussen 1990).
Widespread axonic conditions ensue which have considerable ecological
consequences (Frederiksen 1987; Sfriso et al. 1987, 1988b; Rasmussen
1990). For example, this reduction in oxygen and the associated excessive
development of sulphide on the underlying mud surface, is reported to have
a detrimental effect on the underlying mud infauna. Perkins and Abbot
(1972) noted that the bivalve molluscs Cardium (Cerastoderma) edule and
Macoma balthica moved up to the surface to avoid deeper anaerobic
conditions; Nephtys also emerged. Many of these animals died, or being
unprotected, were predated by wading birds. Similar events were described