The Northern Atlantic Coasts
173
(Pedersen et al. 1990; M. Pedersen, pers. comm.). In the first year negative effects especially on red algae were observed above the halo cline (ca.
15 m depth) due to the bloom. However, the algae had recovered in the
second year. Looking at longterm changes, no decrease in depth distributions in offshore areas was found compared to earlier investigations, but
the vegetation was quite meager compared to the highly diverse and
richly developed communities of the 1950s and 1960s. In some areas, the
common red alga Furcellaria lumbricalis had decreased drastically (Pedersen et al. 1990). In the shallow areas in the mid-Kattegat ("Middlegrund" and "Fladen") there was still a rich vegetation of macro algae
down to a depth of more than 25 m in the late 1980s. However, in one of
the areas industrial removal of sand and stones had been carried out,
creating large craters with decaying drift seaweeds (Pedersen et al. 1990;
Soderlund and Bjork 1990).
A decline in the abundance of the red alga Corallina officinalis was
observed in the mid-1980s (Karlsson 1986) in an area outside the town of
Varberg, which might indicate changes due to eutrophication. Most
other species, however, appeared to have quite normal abundances including Furcellaria lumbricalis, which has been reported as declining in
some other areas.
The Northern Kattegat. The large rivers Gota Xlv and the Nordre
Xlv bring nutrient-rich effiuents to the northernmost Kattegat coast,
which together with land runoff and discharges from sewage treatment
plants have a high impact on the archipelago on the Swedish side (cf.
Fig. 6.1).
The rivers are the main contributors of nitrogen to the area, bringing
about 10 000-20 000 t of total nitrogen annually (mainly in the form of
nitrate), of which about 35% is organic (Selmer and Rydberg 1992).
About one fourth of the water passes through the southern branch, the
river Gota Xlv, into the estuary where another 2000 t of nitrogen, mainly
ammonium, is added from the sewage treatment plant. On the other
hand, the amount of phosphorus brought by the rivers is low, and with
the high removal efficiency of phosphorus in the sewage treatment only
small amounts are added, making the area even phosphorus limited. For
many decades the nutrient enrichment has had a large impact on the
algal vegetation in the estuaries and part of the archipelago area to the
north, contributing to a dominance of green algae and cyanobacteria in
the inner archipelago area and to a great abundance of Fucus evanescens
in the outer archipelago (e.g. Lindgren 1965; Kuylenstierna 1989-1990;
I. Wallentinus, own observations). Selmer and Rydberg (1992), however,
found that there was a large sink for nitrogen in the outer archipelago.
173
(Pedersen et al. 1990; M. Pedersen, pers. comm.). In the first year negative effects especially on red algae were observed above the halo cline (ca.
15 m depth) due to the bloom. However, the algae had recovered in the
second year. Looking at longterm changes, no decrease in depth distributions in offshore areas was found compared to earlier investigations, but
the vegetation was quite meager compared to the highly diverse and
richly developed communities of the 1950s and 1960s. In some areas, the
common red alga Furcellaria lumbricalis had decreased drastically (Pedersen et al. 1990). In the shallow areas in the mid-Kattegat ("Middlegrund" and "Fladen") there was still a rich vegetation of macro algae
down to a depth of more than 25 m in the late 1980s. However, in one of
the areas industrial removal of sand and stones had been carried out,
creating large craters with decaying drift seaweeds (Pedersen et al. 1990;
Soderlund and Bjork 1990).
A decline in the abundance of the red alga Corallina officinalis was
observed in the mid-1980s (Karlsson 1986) in an area outside the town of
Varberg, which might indicate changes due to eutrophication. Most
other species, however, appeared to have quite normal abundances including Furcellaria lumbricalis, which has been reported as declining in
some other areas.
The Northern Kattegat. The large rivers Gota Xlv and the Nordre
Xlv bring nutrient-rich effiuents to the northernmost Kattegat coast,
which together with land runoff and discharges from sewage treatment
plants have a high impact on the archipelago on the Swedish side (cf.
Fig. 6.1).
The rivers are the main contributors of nitrogen to the area, bringing
about 10 000-20 000 t of total nitrogen annually (mainly in the form of
nitrate), of which about 35% is organic (Selmer and Rydberg 1992).
About one fourth of the water passes through the southern branch, the
river Gota Xlv, into the estuary where another 2000 t of nitrogen, mainly
ammonium, is added from the sewage treatment plant. On the other
hand, the amount of phosphorus brought by the rivers is low, and with
the high removal efficiency of phosphorus in the sewage treatment only
small amounts are added, making the area even phosphorus limited. For
many decades the nutrient enrichment has had a large impact on the
algal vegetation in the estuaries and part of the archipelago area to the
north, contributing to a dominance of green algae and cyanobacteria in
the inner archipelago area and to a great abundance of Fucus evanescens
in the outer archipelago (e.g. Lindgren 1965; Kuylenstierna 1989-1990;
I. Wallentinus, own observations). Selmer and Rydberg (1992), however,
found that there was a large sink for nitrogen in the outer archipelago.