194
P.R. Nienhuis
intensity and nearly stops when light intensity is reduced under snowcovered ice. These results are not in agreement with the often cited
statements of Setchell (1929) that growth of Zostera is primarily controlled by temperature and stops above 20°C and below lO°C. According to
Wium-Andersen and Borum (1980) Setchell's statement should be
rejected as speculative and without any modern experimental support.
The decrease of submerged macrophytes between 1950 and 1975 and
the increase in biomass of epiphytes has been described for Randersfjord
by Mathiesen and Mathiesen (1976). Borum (1985) followed the effect of
nutrient enrichment on epiphyte development on eelgrass (Zostera
marina L.) at four localities along a nutrient gradient in Roskilde Fjord,
Denmark between March and December 1982. In the most nutrient-poor
area, epiphyte biomass followed a distinct bimodal seasonal pattern with
maxima in spring and early fall. Low nutrient availability and a high rate
of eelgrass leaf renewal kept epiphyte biomass at a low level throughout
the summer period. Unlike phytoplankton, the epiphytic community was
not stimulated by nutrient enrichment during spring, however, from May
through August, the biomass of both components increased exponentially with increasing concentrations of total N in the water. Along
the nutrient gradient, phytoplankton biomass increased 5- to 10- fold,
while epiphyte biomass increased 50- to 100- fold. Thus differences in
nutrient conditions among study sites were more clearly reflected by
epiphytes than phytoplankton.
The nitrogen supply to the Sound and the Kattegat has increased
fourfold from 1930 to 1980 and doubled in the period 1950-1980. A
review of the data shows that this doubling took place mainly after 1970
and that P supply was constant or decreased slightly during this latter
period (Anderson and Rydberg 1988). An increasing number of Danish
publications refer to macro algal blooms as the consequence of
continuing eutrophication (Steffensen 1976; Frederiksen 1987; Josefson
1990; Rosenberg et al. 1990). Geertz-Hansen et al. (1993) described the
seasonal regulation of growth rates of free-floating Ulva lactuca along a
nutrient gradient in Roskilde Fjord. Grazing by invertebrates was
negligible in the inner part of the estuary and allowed biomass accumulations of the green macro alga, whereas grazing pressure in the outer,
more saline part matched the growth rate during summer and exceeded
it by more than twofold during autumn. Reduced grazing control is
apparently an important and often overlooked factor for biomass
accumulation of free-floating macro algae under eutrophic conditions.
Strong and persistent eutrophication leads to anoxia. According to
J6rgensen (1980) the bottom water in local areas of Limfjorden
P.R. Nienhuis
intensity and nearly stops when light intensity is reduced under snowcovered ice. These results are not in agreement with the often cited
statements of Setchell (1929) that growth of Zostera is primarily controlled by temperature and stops above 20°C and below lO°C. According to
Wium-Andersen and Borum (1980) Setchell's statement should be
rejected as speculative and without any modern experimental support.
The decrease of submerged macrophytes between 1950 and 1975 and
the increase in biomass of epiphytes has been described for Randersfjord
by Mathiesen and Mathiesen (1976). Borum (1985) followed the effect of
nutrient enrichment on epiphyte development on eelgrass (Zostera
marina L.) at four localities along a nutrient gradient in Roskilde Fjord,
Denmark between March and December 1982. In the most nutrient-poor
area, epiphyte biomass followed a distinct bimodal seasonal pattern with
maxima in spring and early fall. Low nutrient availability and a high rate
of eelgrass leaf renewal kept epiphyte biomass at a low level throughout
the summer period. Unlike phytoplankton, the epiphytic community was
not stimulated by nutrient enrichment during spring, however, from May
through August, the biomass of both components increased exponentially with increasing concentrations of total N in the water. Along
the nutrient gradient, phytoplankton biomass increased 5- to 10- fold,
while epiphyte biomass increased 50- to 100- fold. Thus differences in
nutrient conditions among study sites were more clearly reflected by
epiphytes than phytoplankton.
The nitrogen supply to the Sound and the Kattegat has increased
fourfold from 1930 to 1980 and doubled in the period 1950-1980. A
review of the data shows that this doubling took place mainly after 1970
and that P supply was constant or decreased slightly during this latter
period (Anderson and Rydberg 1988). An increasing number of Danish
publications refer to macro algal blooms as the consequence of
continuing eutrophication (Steffensen 1976; Frederiksen 1987; Josefson
1990; Rosenberg et al. 1990). Geertz-Hansen et al. (1993) described the
seasonal regulation of growth rates of free-floating Ulva lactuca along a
nutrient gradient in Roskilde Fjord. Grazing by invertebrates was
negligible in the inner part of the estuary and allowed biomass accumulations of the green macro alga, whereas grazing pressure in the outer,
more saline part matched the growth rate during summer and exceeded
it by more than twofold during autumn. Reduced grazing control is
apparently an important and often overlooked factor for biomass
accumulation of free-floating macro algae under eutrophic conditions.
Strong and persistent eutrophication leads to anoxia. According to
J6rgensen (1980) the bottom water in local areas of Limfjorden