The North Sea Coasts
193
shallow Danish waters when he published his results of Holback Fjords.
The importance of Zostera was further stressed in papers by Ostenfeld
(1908), Petersen and Jensen (1911), and Petersen (1913, 1915 and 1918).
These authors considered Zostera to be the most important primary
producer in Danish waters and the basis for life, especially for the coastal
fisheries. However, the wasting disease which struck the Danish Zostera
populations in 1932 and 1933 showed that this was not the case, since
fishing continued at the normal level, and consequently Danish research
on eelgrass more or less stopped before the second World War. Zostera
populations re-established at several localities in the 1940s and the 1950s,
but at a much lower level than before the wasting disease (Rasmussen
1977).
Rasmussen (1973) described the fluctuations of eelgrass abundance in
Isefjord: in the period 1900-1932 eelgrass was very common. The wasting
disease in 1932-1933 reduced the vegetation to almost zero. There was a
slow recovery from 1940 onwards, but it was only in the period 19601967 that the Zostera vegetation recovered to pre-1930 levels. After
1967 again a serious deterioration of the eelgrass communities was
observed.
It is difficult to disentangle natural fluctuations of Zostera populations
and changes caused by eutrophication. Eelgrass populations undoubtedly
are exposed to large fluctuations, of which the causes are hardly known.
According to Rasmussen (1973), high water temperatures may have
weakened the eelgrass population in the 1930s and hence may have
contributed directly or indirectly to their destruction. Brackish-water
populations (less than 12 -15%0 S) showed a resistance against the
wasting disease, and it may be assumed that low salinities are favourable
to generative reproduction. Seeds of Zostera marina germinate best in
water with low salinity. On the other hand, nutrients flow from the land
into the sea, thus brackish waters are more exposed to eutrophication
than saline waters.
There is accumulating evidence that the decrease of eelgrass
populations after 1960-1965 is caused by eutrophication. Eutrophication
of Danish coastal waters and consequent deterioration of eelgrass
vegetation, gave a new stimulus to the investigations of this macrophyte.
Sand-Jensen (1975) described biomass, net production and growth
dynamics in Vellerup Vig., Isefjord. Pinnerup (1980) and Bak (1979)
studied growth and production· of eelgrass at different localities in the
Limfjorden. Wium-Andersen and Borum (1980) presented data for biomass and primary production in an eelgrass (Zostera marina L.)
population in the CDresund, Denmark. It is shown that eelgrass grows
throughout the year. Growth is directly related to the incident light
193
shallow Danish waters when he published his results of Holback Fjords.
The importance of Zostera was further stressed in papers by Ostenfeld
(1908), Petersen and Jensen (1911), and Petersen (1913, 1915 and 1918).
These authors considered Zostera to be the most important primary
producer in Danish waters and the basis for life, especially for the coastal
fisheries. However, the wasting disease which struck the Danish Zostera
populations in 1932 and 1933 showed that this was not the case, since
fishing continued at the normal level, and consequently Danish research
on eelgrass more or less stopped before the second World War. Zostera
populations re-established at several localities in the 1940s and the 1950s,
but at a much lower level than before the wasting disease (Rasmussen
1977).
Rasmussen (1973) described the fluctuations of eelgrass abundance in
Isefjord: in the period 1900-1932 eelgrass was very common. The wasting
disease in 1932-1933 reduced the vegetation to almost zero. There was a
slow recovery from 1940 onwards, but it was only in the period 19601967 that the Zostera vegetation recovered to pre-1930 levels. After
1967 again a serious deterioration of the eelgrass communities was
observed.
It is difficult to disentangle natural fluctuations of Zostera populations
and changes caused by eutrophication. Eelgrass populations undoubtedly
are exposed to large fluctuations, of which the causes are hardly known.
According to Rasmussen (1973), high water temperatures may have
weakened the eelgrass population in the 1930s and hence may have
contributed directly or indirectly to their destruction. Brackish-water
populations (less than 12 -15%0 S) showed a resistance against the
wasting disease, and it may be assumed that low salinities are favourable
to generative reproduction. Seeds of Zostera marina germinate best in
water with low salinity. On the other hand, nutrients flow from the land
into the sea, thus brackish waters are more exposed to eutrophication
than saline waters.
There is accumulating evidence that the decrease of eelgrass
populations after 1960-1965 is caused by eutrophication. Eutrophication
of Danish coastal waters and consequent deterioration of eelgrass
vegetation, gave a new stimulus to the investigations of this macrophyte.
Sand-Jensen (1975) described biomass, net production and growth
dynamics in Vellerup Vig., Isefjord. Pinnerup (1980) and Bak (1979)
studied growth and production· of eelgrass at different localities in the
Limfjorden. Wium-Andersen and Borum (1980) presented data for biomass and primary production in an eelgrass (Zostera marina L.)
population in the CDresund, Denmark. It is shown that eelgrass grows
throughout the year. Growth is directly related to the incident light