206
c. den Hartog and R.C. Phillips
1977; Den Hartog 1987). The disease was ascribed to the activity of some
fungi, particularly to a virulent pathogenic organism, Labyrinthula macrocystis, recently recognized as a separate species, 1. zosterae (Muehlstein et al.
1991). In the last decade, the Zostera beds along the North American Atlantic
coast suffered again from a wasting disease epidemic (Short et al.1987, 1988).
Along the European west coast, no decline attributed to 1. zosterae could be
ascertained, although this organism was isolated from almost every population that was investigated, even from geographically isolated populations of
Z. marina in the Adriatic Sea (Den Hartog et al. 1996) and the White Sea
(Maximova and Den Hartog, unpubl.). We have grave doubts that 1. zosterae
is the primary cause of the wasting disease. Damage patterns similar to those
in Labyrinthula-infected plants have been found in herbarium specimens
collected long before wasting disease was ever observed. Further, Labyrinthula has also been isolated from Zostera noltii and Z. japonica. Its very wide
distribution in the northern Atlantic and in the northern Pacific, and also in
populations isolated from the main distributional area over thousands of
years, does not lend support for considering it an organism dangerous
enough to bring Zostera to the brink of extinction. It is one of the first
organisms to develop in senescent Zostera tissue. The main feature of the
wasting disease is that it develops not only in the senescent oldest leaves, but
also in the youngest leaves, confronting the plants with a serious metabolic
problem.
Tutin (1938) correlated periods of eelgrass reduction with excessively
cloudy years, implying that eelgrass became weakened due to light reduction
in the North Atlantic in the early 1930s. In 1996, Den Hartog called this the
light-deficit hypothesis. In Western Europe, the wasting disease started in
spring 1932. This was the second of two consecutive years with a considerable
light deficit during the main growing season of Zostera marina. Starved plants
were again exposed to low light in combination with rising water temperatures (higher water temperatures also noted by Rasmussen 1977), stressing
the plant's metabolism to the limit. Add to this the effects of pollution,
turbidity, and other human activities, and circumstances were right for
damage to the plants. In this scenario, the precocious development of
Labyrinthula is a consequence of the environmentally induced poor metabolic condition of the plants and not the cause of the disease. For this reason,
the term 'wasting disease' should be replaced by the more neutral term
'wasting phenomenon'.
Other large-scale losses of seagrass beds have been recorded from the
eastern coast of northern America and Australia. Orth and Moore (1983)
recorded a massive decline of all submerged macrophytes, including the
seagrasses, from Chesapeake Bay, and regarded it to be the result of human
impact by agricultural and industrial waste. Poiner et al. (1989) described the
disastrous effects of the cyclone "Sandy" that struck the seagrass beds along
c. den Hartog and R.C. Phillips
1977; Den Hartog 1987). The disease was ascribed to the activity of some
fungi, particularly to a virulent pathogenic organism, Labyrinthula macrocystis, recently recognized as a separate species, 1. zosterae (Muehlstein et al.
1991). In the last decade, the Zostera beds along the North American Atlantic
coast suffered again from a wasting disease epidemic (Short et al.1987, 1988).
Along the European west coast, no decline attributed to 1. zosterae could be
ascertained, although this organism was isolated from almost every population that was investigated, even from geographically isolated populations of
Z. marina in the Adriatic Sea (Den Hartog et al. 1996) and the White Sea
(Maximova and Den Hartog, unpubl.). We have grave doubts that 1. zosterae
is the primary cause of the wasting disease. Damage patterns similar to those
in Labyrinthula-infected plants have been found in herbarium specimens
collected long before wasting disease was ever observed. Further, Labyrinthula has also been isolated from Zostera noltii and Z. japonica. Its very wide
distribution in the northern Atlantic and in the northern Pacific, and also in
populations isolated from the main distributional area over thousands of
years, does not lend support for considering it an organism dangerous
enough to bring Zostera to the brink of extinction. It is one of the first
organisms to develop in senescent Zostera tissue. The main feature of the
wasting disease is that it develops not only in the senescent oldest leaves, but
also in the youngest leaves, confronting the plants with a serious metabolic
problem.
Tutin (1938) correlated periods of eelgrass reduction with excessively
cloudy years, implying that eelgrass became weakened due to light reduction
in the North Atlantic in the early 1930s. In 1996, Den Hartog called this the
light-deficit hypothesis. In Western Europe, the wasting disease started in
spring 1932. This was the second of two consecutive years with a considerable
light deficit during the main growing season of Zostera marina. Starved plants
were again exposed to low light in combination with rising water temperatures (higher water temperatures also noted by Rasmussen 1977), stressing
the plant's metabolism to the limit. Add to this the effects of pollution,
turbidity, and other human activities, and circumstances were right for
damage to the plants. In this scenario, the precocious development of
Labyrinthula is a consequence of the environmentally induced poor metabolic condition of the plants and not the cause of the disease. For this reason,
the term 'wasting disease' should be replaced by the more neutral term
'wasting phenomenon'.
Other large-scale losses of seagrass beds have been recorded from the
eastern coast of northern America and Australia. Orth and Moore (1983)
recorded a massive decline of all submerged macrophytes, including the
seagrasses, from Chesapeake Bay, and regarded it to be the result of human
impact by agricultural and industrial waste. Poiner et al. (1989) described the
disastrous effects of the cyclone "Sandy" that struck the seagrass beds along