The Baltic Sea and Its Transition Zones
145
The most conspicuous change in phytobenthic communities is probably the decline of Fucus communities in most coastal areas of the Baltic.
Reports that the bladder-wrack was declining or disappearing in many
places along the Baltic coasts, particularly in Germany, Sweden and Finland, were first made by local fisherman in the early 1970s. Scientific
investigations on the phenomenon were started in the mid-1970s in
Finland (Kangas et al. 1982; Kangas 1983; Haathela and Letho 1982;
Ronnberg 1984). Here, in the Finish waters, the decline of Fucus vesiculosus was most evident in moderately exposed localities in the central
archipelago and in the semi-exposed outer archipelago areas. The
changes in Fucus communities were in most cases connected with enhanced growth or mass development of filamentous algae. Since a direct
influence of land-borne pollution could be excluded in these locations,
Kangas (1983) and Haathela and co-workers (1984) suggested that
increased nutrient levels in the 1970s originating from the open Baltic
could be the primary cause of the observed alterations following the
typical eutrophication pattern. The increase in nutrients may have resulted in an increase in the biomass of phytoplankton and periphyton, in
suspended organic materials and sedimentation, which may have had
adverse effects on the Fucus communities. The increased load of epiphytic filamentous and microscopic algae, as well as attached grazing animals, may have reduced the viability of Fucus by shading or even weighing it down. The increase in periphyton, on the other hand, offers more
food for juvenile Idotea, which may result in more grazing on Fucus by
the adults. Also, resettlement of Fucus may be hampered by algae, detritus and sediments on the bare rocks, so that the zygotes cannot attach
permanently. In the early 1980s, in some of the affected Finnish waters a
recovery of the Fucus populations was observed (Ronnberg et al. 1985).
A re-investigation in 1984 of locations in the archipelago of the
Swedish east coast studied in the 1940s (Waern 1952) showed that the
lower distribution of Fucus vesiculosus had moved upward by 2 m from
11.5 to 8.5 m, and the maximal development (bottom coverage) from
5-6 to 3-4 m, probably due to decreased light penetration (Fig. 5.6).
Also, in the algal communities growing below Fucus vesiculosus, changes
were observed: the formerly dominant Sphacelaria arctica had been replaced by red algae such as Ceramium tenuicorne and Rhodomela confervoides (N. Kautsky et al. 1986).
Similar observations were made in other parts of the archipelago
along the Swedish coast. In the Asko area (northern Baltic proper),
where extensive quantitative studies on the phytobenthic communities
were performed in 1974"': 1975 (Jansson and Kautsky 1977), Fucus had
disappeared in locations close to the mainland or declined particularly in
145
The most conspicuous change in phytobenthic communities is probably the decline of Fucus communities in most coastal areas of the Baltic.
Reports that the bladder-wrack was declining or disappearing in many
places along the Baltic coasts, particularly in Germany, Sweden and Finland, were first made by local fisherman in the early 1970s. Scientific
investigations on the phenomenon were started in the mid-1970s in
Finland (Kangas et al. 1982; Kangas 1983; Haathela and Letho 1982;
Ronnberg 1984). Here, in the Finish waters, the decline of Fucus vesiculosus was most evident in moderately exposed localities in the central
archipelago and in the semi-exposed outer archipelago areas. The
changes in Fucus communities were in most cases connected with enhanced growth or mass development of filamentous algae. Since a direct
influence of land-borne pollution could be excluded in these locations,
Kangas (1983) and Haathela and co-workers (1984) suggested that
increased nutrient levels in the 1970s originating from the open Baltic
could be the primary cause of the observed alterations following the
typical eutrophication pattern. The increase in nutrients may have resulted in an increase in the biomass of phytoplankton and periphyton, in
suspended organic materials and sedimentation, which may have had
adverse effects on the Fucus communities. The increased load of epiphytic filamentous and microscopic algae, as well as attached grazing animals, may have reduced the viability of Fucus by shading or even weighing it down. The increase in periphyton, on the other hand, offers more
food for juvenile Idotea, which may result in more grazing on Fucus by
the adults. Also, resettlement of Fucus may be hampered by algae, detritus and sediments on the bare rocks, so that the zygotes cannot attach
permanently. In the early 1980s, in some of the affected Finnish waters a
recovery of the Fucus populations was observed (Ronnberg et al. 1985).
A re-investigation in 1984 of locations in the archipelago of the
Swedish east coast studied in the 1940s (Waern 1952) showed that the
lower distribution of Fucus vesiculosus had moved upward by 2 m from
11.5 to 8.5 m, and the maximal development (bottom coverage) from
5-6 to 3-4 m, probably due to decreased light penetration (Fig. 5.6).
Also, in the algal communities growing below Fucus vesiculosus, changes
were observed: the formerly dominant Sphacelaria arctica had been replaced by red algae such as Ceramium tenuicorne and Rhodomela confervoides (N. Kautsky et al. 1986).
Similar observations were made in other parts of the archipelago
along the Swedish coast. In the Asko area (northern Baltic proper),
where extensive quantitative studies on the phytobenthic communities
were performed in 1974"': 1975 (Jansson and Kautsky 1977), Fucus had
disappeared in locations close to the mainland or declined particularly in