Fróðskaparrit - 01.01.1999, Side 182

186 RESPONSE TO WAVE EXPOSURE BY LITTORAL SPECIES IN THE FAROEISLANDS
CCA, species response polynomials were
developed using Expon by the reciprocal
process described by Dalby et al. (1978).
The recorded abundance of each species
and the FEV for each station were applied
to generate the first polynomials. From
these first polynomials new exposure val-
ues for each station were calculated. These
are values where the observed abundance
for each species and the estimated abun-
dance fit best for all species. In the Expon
algorithm, mean residuals (sum of square
residuals/number of species) were plotted
for all possible 1/4 steps on the biological
exposure scale (0-9). The new exposure
values selected corresponded to the mini-
mum, mean residual values. The reciprocal
algorithm was continued until only 2.7% of
the stations changed by less than 1/4 of a
step on the exposure scale. This is within
the requirement of less than 10% recom-
mended by Árrestad and Lein (1993) and
Kruskopf and Lein (1998). The final
species response polynomials were then fit-
ted and the estimated abundance for each
species listed for each biological exposure
value (BEV) from 0 (exposed) to 9 (shel-
tered). Handling of missing data, 0-values,
and the polynomial regression solution
were adapted or slightly modified from
Dalby et al. (1978) as given by Árrestad
and Lein (1993).
Diagrams illustrating the abundance of
the 23 dominant species and other species,
which covered more than 1% of the area,
were made of nine stations representing
different categories of exposure (Fig. 1 B).
The abundance was recorded at intervals of
12.5 cm from the upper limit of Semibal-
anus balanoides to the lowest water level.
The records were correlated to MLWS
(mean low water spring) according to the
tide table for the Faroe Islands (Anon.,
1996) and the air pressure. The slope of the
shore was measured in 25-cm vertical steps
by use of a surveyor’s level and a staff.
General collections of algae were made
at 49 localities, from the low-water level to
as high up as marine algae were found (Fig.
1C). These localities were grouped into
three categories of exposures: exposed (0-
2.75), moderately exposed (3-5.75) and
sheltered (6-9), based on the calculated bi-
ological exposure values (BEV). The rela-
tion between BEV and the physical expo-
sure value (PE) (linear regression, BEV = -
1.02 lnPE+7.01, R2 = 0.56) was used to es-
timate exposure for localities not included
in the calculation of the biological exposure
scale (Fig. 1 B, C). Each collection covered
a shore length of 8-50 m, which did not al-
ways correspond exactly to the stations
used for the development of the biological
exposure scale. An effort was made to col-
lect all species from all microhabitats.
Only species of Bangiophyceae, Fuco-
phyceae and Chlorophyceae, which were
found at a minimum of 33% of the locali-
ties within any one exposure group, are pre-
sented. Herbarium specimens are deposit-
ed at the Botanical Museum, Copenhagen;
the Marine Research Institute, Reykjavrk
and the Natural History Museum, Tórs-
havn.
Results
The area studied was heterogeneous in re-
spect to several physical factors. It is im-