Fróðskaparrit - 01.01.2007, Síða 160

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SOFT BOTTOM MACRO FAUNA SPECIES COMPOSITION IN FAROESE FJORDS
tion in Skálaijørður and compared the re-
sults witli Nørrevang (1990). Sakarisson
(2000) reported on benthic invertebrates in
Faroese sillijords. Josefson (2001) com-
pared two sites on the Faroese shelf.
Species richness and abundance
Species numbers ranged from 5 to 66 but
both numbers must be considered extre-
mes. The low number was from sandy sed-
iments with low organic content and the
high number was from very mixed sedi-
ments. From Figure 2a an estimated nor-
mal range for species numbers seems to be
about 20 to 45. Stations with species num-
bers below 20 are mostly from a single lo-
cality (Sandur) with sand/pebble sediments
and very low organic content (11-14%).
Four stations show distinctly higher spe-
cies numbers than the rest. These are also
from sand/pebble sediments but with a
slightly higher organic content. Thc lowest
number of individuals was 8 and the high-
est 1655. The low numbers are from the
same stations which also have the lowest
species numbers (Sandur). The samples
with the highest numbers of individuals are
from samples 7R and 7S which are from a
depth of only six meters in Fuglafjørður.
These two samples have a very distinct
domination of Polydora ciliata and are
clearly deviant from the other samples.
This species is highly opportunistic (Gray,
1979) and high numbers of individuals can
be produced within a short time.
Diversity measures
The abundance ratio (ratio between num-
ber of individuals in a sample and the
species number in the same sample) was
between 1.3 and 20.6 with an average
value 7.7. In organically enriched sedi-
ments the abundance ratio increases with
moderately higher levels of organic mate-
rials in the sediment (Pearson et al., 1986).
This is particularly interesting as it can be
used as a quick and easy method to decide
if sediments are under influence of organic
enrichment. From Table 1 it can be seen
that the average abundance ratio (3.8) in-
creases from the low ignition loss group
(<30 mg/kg) to 7.7 and 10.5 in the higher
groups (30-60 mg/kg and >60 mg/kg re-
spectively).
The average Shannon index was 3.5
with little variance between the different
sediment types, depth and organic content
in sediment. The lowest numbers were 2.0
and the highest 5.2. Shannon diversity is
widely used as a measure to evaluate
changes to benthic communities due to or-
ganic enrichment. In most cases the Shan-
non diversity is expected to decrease with
increasing organic enrichment. From Table
1 it seems like low organic content in the
sediment has a low Shannon index (2.5)
compared to the groups with higher or-
ganic content (3.7 and 3.4). This could be
explained as a low energy environment
will respond positively with additional
species with moderately increased organic
enrichment but will fall towards zero with
further enrichment. Pearson et al. (1986)
showed the same tendency from Firth of
Clyde (Scotland). The Shannon index
should therefore be used with caution in
evaluation of change in benthic communi-
ties.