174
1. Wallentinus
They suggested that this could be due to denitrification in the sediments
or internal storages of nitrogen in the algae, while low nitrogen uptake
and denitrification rates were measured in the water phase. Unfortunately, there are no studies of the biochemical response of macro algae in
this area, but in the estuary dark green opportunistic algae are often seen
as a borderline on the rocks, indicating excess of nitrogen.
Several of the localities in Danish waters sampled by Professor Rosenvinge about 70 -100 years ago have been revisited. One of the stations is
situated on a stone reef in the northern Kattegat, between Sweden and
Denmark, where Nielsen (1991) found a high diversity of macro algae and
only a very slight impact of eutrophication, as indicated by an abundance of filter-feeding bryozoans. Several red algae had luxurious
growth, which might indicate plenty of available nutrients, but no decrease in light transmission. However, the perennial Furcellaria
lumbricalis was absent, as also observed in other areas. Although no
specimen of Corallina was found in the survey, she commented that it
did occur in the area at the time of the survey.
6.2.2 The Swedish Skagerrak Coast
So far, there are few clear effects of eutrophication in the offshore northern areas (cf. Fig. 6.1) where non-crustose algae could still be found
down to a depth of at least 32 m in the 1980s (Karlsson et al. 1992a).
Only little change was observed during the short period of the pilot study
(Karlsson et al. 1992b), except for a significant decrease of Corallina
officinalis in shallow depths. Halidrys siliquosa, which was reported to
have decreased during the late 1970s and then increased (Lundalv et al.
1986) did not show any significant changes.
On the other hand, there are several indications of changes in the
macroalgal communities in bays and inner archipelago areas along this
coast (Michaneck 1967; Rex 1976; Lundalv et al. 1986; Svane and Grondahl 1989; Isaksson and Pihl 1992; Pihl et al. 1996; H. Kautsky, pers.
comm.; C. Larsson, pers. comm.). Michaneck (1967) was one of the first
to emphasize the upward shift in the distribution of macroalgae as a
result of eutrophication, based on revisits to Gislen's diving sites of
about 40 years earlier. The other studies, some of these also based on
revisited sites, have shown the same trend in several areas as well as a
general increase in opportunistic species at the expense of the late
successional ones or large amounts of drifting ephemeral macroalgae.
1. Wallentinus
They suggested that this could be due to denitrification in the sediments
or internal storages of nitrogen in the algae, while low nitrogen uptake
and denitrification rates were measured in the water phase. Unfortunately, there are no studies of the biochemical response of macro algae in
this area, but in the estuary dark green opportunistic algae are often seen
as a borderline on the rocks, indicating excess of nitrogen.
Several of the localities in Danish waters sampled by Professor Rosenvinge about 70 -100 years ago have been revisited. One of the stations is
situated on a stone reef in the northern Kattegat, between Sweden and
Denmark, where Nielsen (1991) found a high diversity of macro algae and
only a very slight impact of eutrophication, as indicated by an abundance of filter-feeding bryozoans. Several red algae had luxurious
growth, which might indicate plenty of available nutrients, but no decrease in light transmission. However, the perennial Furcellaria
lumbricalis was absent, as also observed in other areas. Although no
specimen of Corallina was found in the survey, she commented that it
did occur in the area at the time of the survey.
6.2.2 The Swedish Skagerrak Coast
So far, there are few clear effects of eutrophication in the offshore northern areas (cf. Fig. 6.1) where non-crustose algae could still be found
down to a depth of at least 32 m in the 1980s (Karlsson et al. 1992a).
Only little change was observed during the short period of the pilot study
(Karlsson et al. 1992b), except for a significant decrease of Corallina
officinalis in shallow depths. Halidrys siliquosa, which was reported to
have decreased during the late 1970s and then increased (Lundalv et al.
1986) did not show any significant changes.
On the other hand, there are several indications of changes in the
macroalgal communities in bays and inner archipelago areas along this
coast (Michaneck 1967; Rex 1976; Lundalv et al. 1986; Svane and Grondahl 1989; Isaksson and Pihl 1992; Pihl et al. 1996; H. Kautsky, pers.
comm.; C. Larsson, pers. comm.). Michaneck (1967) was one of the first
to emphasize the upward shift in the distribution of macroalgae as a
result of eutrophication, based on revisits to Gislen's diving sites of
about 40 years earlier. The other studies, some of these also based on
revisited sites, have shown the same trend in several areas as well as a
general increase in opportunistic species at the expense of the late
successional ones or large amounts of drifting ephemeral macroalgae.