178
1. Wallentinus
evanescens did not occur in the area until the end of last century when it
was introduced. This species constituted an important part of the fucoid
belts in the inner fjord in the late 1980s (Bokn et al. 1992 and references
therein). There is no large river entering the area, so the effects are
mainly due to impact from direct anthropogenic discharges (of which a
large part has been sewage) and atmospheric deposition. During the
1960s and 1970s there was a considerable decline in the fucoids and an
increase in ephemeral green algal species (of the genera Enteromorpha,
Blidingia, Cladophora and Ulva) mainly due to pollution from sewage
(Rueness 1973; Bokn and Lein 1978; Klavestad 1978). Revisits in the late
1980s (Bokn et al. 1992) showed an increase in fucoid abundance in
some part of the inner Oslo fjord, especially for Fucus spiralis. On the
other hand, a decrease had occurred for Fucus evanescens, a species
which earlier had been favoured by high nutrient concentrations. Bokn
et al. (1992) also quoted a simultaneous increase in Secchi depths and
decreases in pelagic chlorophyll a concentrations and in phosphorus
load to the area, where previously the loads of phosphorus and nitrogen
had increased 13 and 6 times, respectively, between 1910 and the 1970s.
In the outer Oslo fjord area, however, Rueness and Fredrikssen (1991)
found that the macroalgal communities were more affected in the late
1980s than 40 years earlier, which they attributed to higher loads of
nutrients and organic matter. They noticed an almost total dominance of
the filter-feeding mussel Mytilus edulis at depths down to 6-10 m, and
that several species had disappeared or become rare in the area during
the last 40 years such as e.g. the belt forming Chorda ria flagelliformis,
Halidrys siliquosa, Laminaria digitata and Corallina officinalis. It is not
clear whether the dominance of the mussels was due to the effects of the
Chrysochromulina bloom in 1988 (cf. the Swedish west coast). In addition, several species had decreased in depth distribution substantially,
although the observed decrease in Secchi depths was not of the same
magnitude as the upward shift of the vegetation. The decreased depth
distribution is a common feature in other eutrophicated areas also and
thus might indicate an impact of eutrophication, seen also in the changes
of the soft-bottom fauna in the area. Further offshore macroalgae are less
affected (Karlsson 1995).
6.4.2 Other Areas Along the South and Southwest Coasts
Effects of eutrophication in several inner fjord areas have been reported
during the last decades (cf. Fig. 6.1). Examples are the Sandefjordsfjorden and Mefjorden (Iversen 1981), the Grenland fjords (Bokn 1979) and
1. Wallentinus
evanescens did not occur in the area until the end of last century when it
was introduced. This species constituted an important part of the fucoid
belts in the inner fjord in the late 1980s (Bokn et al. 1992 and references
therein). There is no large river entering the area, so the effects are
mainly due to impact from direct anthropogenic discharges (of which a
large part has been sewage) and atmospheric deposition. During the
1960s and 1970s there was a considerable decline in the fucoids and an
increase in ephemeral green algal species (of the genera Enteromorpha,
Blidingia, Cladophora and Ulva) mainly due to pollution from sewage
(Rueness 1973; Bokn and Lein 1978; Klavestad 1978). Revisits in the late
1980s (Bokn et al. 1992) showed an increase in fucoid abundance in
some part of the inner Oslo fjord, especially for Fucus spiralis. On the
other hand, a decrease had occurred for Fucus evanescens, a species
which earlier had been favoured by high nutrient concentrations. Bokn
et al. (1992) also quoted a simultaneous increase in Secchi depths and
decreases in pelagic chlorophyll a concentrations and in phosphorus
load to the area, where previously the loads of phosphorus and nitrogen
had increased 13 and 6 times, respectively, between 1910 and the 1970s.
In the outer Oslo fjord area, however, Rueness and Fredrikssen (1991)
found that the macroalgal communities were more affected in the late
1980s than 40 years earlier, which they attributed to higher loads of
nutrients and organic matter. They noticed an almost total dominance of
the filter-feeding mussel Mytilus edulis at depths down to 6-10 m, and
that several species had disappeared or become rare in the area during
the last 40 years such as e.g. the belt forming Chorda ria flagelliformis,
Halidrys siliquosa, Laminaria digitata and Corallina officinalis. It is not
clear whether the dominance of the mussels was due to the effects of the
Chrysochromulina bloom in 1988 (cf. the Swedish west coast). In addition, several species had decreased in depth distribution substantially,
although the observed decrease in Secchi depths was not of the same
magnitude as the upward shift of the vegetation. The decreased depth
distribution is a common feature in other eutrophicated areas also and
thus might indicate an impact of eutrophication, seen also in the changes
of the soft-bottom fauna in the area. Further offshore macroalgae are less
affected (Karlsson 1995).
6.4.2 Other Areas Along the South and Southwest Coasts
Effects of eutrophication in several inner fjord areas have been reported
during the last decades (cf. Fig. 6.1). Examples are the Sandefjordsfjorden and Mefjorden (Iversen 1981), the Grenland fjords (Bokn 1979) and