Fróðskaparrit - 01.01.2007, Síða 154

152
SOFT BOTTOM MACRO FAUNA SPECIES COMPOSITION IN FAROESE FJORDS
determining the overall healtli of ecosys-
tems.
The average species number (shown in
table 1) is 29 and the lowest number was 5
and the highest value was 66. There seems
not to be any difference related to the sedi-
ment type but shallow waters (<20 m) and
low ignition loss (<30 mg/kg) have distinc-
tively lower average species numbers in
the low values. The shallow-water depth
group has an average of 23 species while
the deeper groups (20-40m and >40m)
have 33 and 32 respectively. The group
with the lowest ignition loss shows
markedly lower average species numbers
than the other two groups, only 9 species
compared to 35 and 28 species.
A wide variety of diversity measures
are used in literature and by environmental
authorities, e.g. Simpson’s index and
Shannon diversity index. In the present
paper we use Shannon diversity index.
Compared to species richness which only
uses the number of species the Shannon di-
versity index also includes the evenness of
individuals in each species.
Shannon index:
s
H = -2^pi\ogPi
/=í
S: The number of species.
P.: The relative abundance of each species,
calculated as the proportion of individuals
of a given species to the total number of in-
dividuals in the community
n : The number of individuals in each
l
species; the abundance of each species.
N: The total number of all individuals
The average Shannon diversity index is 3.5
(Table 1). Maximum and minimum values
were 5.2 and 2.0 respectively. There is no
clear difference of diversity in the three
sediment types. The <20 m depth group
seems to have slightly lower diversity than
the groups from deeper waters, 3.1 com-
pared to 3.7 and 3.6. The same tendency
seems to be with regard to ignition loss.
The low value group (<30 mg/kg) has a
value of 2.5 compared to 3.7 and 3.4 in the
two high-value groups.
Pielou Evenness Index
log/S)
The Pielou index is a measure of how
evenly distributed individuals are among
the species tliat exist in a community. The
Pielou index is defined between 0 and 1,
where 1 represents a community with per-
fect evenness, and decreases to zero as the
relative abundances of the species diverge
from evenness.
The overall average value for Pielou
evenness was 0.74. Lowest value was 0.43
and the highest value was 0.90. When the
sediment types are compared it looks like
the silt/clay average is lower than sandy
and mixed sediments. Silt/clay sediments
have average values of 0.69 compared to
0.76 and 0.78 for mixed sediments and
sand. With depth there is no difference but
low ignition loss clearly shows a more