Fróðskaparrit - 01.01.1999, Side 185

HVUSSU DJÓRASLØG í FØROYSKUM FIRÐUM LAGA SEG EFTIR ALDUBROTUM
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in dense populations on moderately ex-
posed shores (Fig. 4, C-F).
The species only or predominantly found
on sheltered coasts were Ascophyllum no-
dosum, Cladophora rupestris, Pelvetia
canaliculata and Verrucaria mucosa. As-
sociated with these were Polysiphonia
lanosa and Littorina obtusata with the en-
dozoic Tellamia contorta. Other species
occurring mainly on sheltered coasts were
Chondrus crispus, Chordaria flagelli-
formis, Enteromorpha compressa, Fucus
vesiculosus, Nucella lapillus and Pilayella
littoralis (Figs. 2, 3, Table 4). Most species
recorded on sheltered shores did not extend
much above the MHWS (Fig. 4, F-I).
A number of species occurred at all
grades of wave exposure. Among them
were Corallina officinalis and Mastocar-
pus stellatus, although their greatest abun-
dance was observed on exposed coasts.
Semibalanus balanoides was also found at
all exposure grades, but had the greatest
abundance on moderately exposed coasts
(Fig. 2). Palmaria palmata was found in
almost equal abundance throughout the
whole exposure range (Fig. 3), but some-
times had a patchy occurrence and was of-
ten observed in large and dense stands in
paths of running water. Patella vulgata was
observed in numbers of 50 - 100 individu-
als per m2 in most areas. Other species fre-
quently found at all exposure grades were
Acrochaetium secundatum, Acrosiphonia
arcta, Blidingia minima, Ceramium nodu-
losum, Enteromorpha intestinalis, E. linza
and Ulva lactuca (Table 4).
By coincidence, Balanus balanus (L.,
1758), Gibbula cineraria (L., 1758), Litto-
rina saxatilis (Olivi, 1792) and Ceramium
pallidum (Naegeli ex Kuetz., Maggs &
Hommers.) occurred at some of the locali-
ties selected for observation of vegetation
profiles, but were not among the dominant
species or frequently occurring algae.
Discussion
The major advantage of biological expo-
sure scales is that the abundance of long-
lived species represents an integration of
conditions over many years, thus reflecting
all local physical factors and biological in-
teractions (Raffaelli and Hawkins, 1996).
The reciprocal algorithm used in Expon for
the development of the final species poly-
nomials assigns exposure values to the sta-
tions based on the abundance of dominant
species. The changes in the exposure grade
during the process may be interpreted as ef-
fects of local conditions on wave exposure,
as long as the influence of other environ-
mental variables (e.g. instability of the sub-
strate) is negligible. As a consequence of
this, the final species polynomials based on
the biological exposure values (BEV) may
better reflect the effects of exposure on the
littoral species than similar curves based on
physical exposure values (PE). Some lo-
calities on the open shore of the Faroe Is-
lands had a lower or higher BEV than
would be expected from PE calculations, as
is also indicated by the relatively low re-
gression coefficient (0.56). Local bathy-
metric conditions or wave refraction, not
accounted for in the calculations of PE, can
explain some of the differences. Other fac-
tors not included in our data set may also
have contributed to the species variation, as