Náttúrufræðingurinn - 1977, Qupperneq 63

Zonal distribution of plants and arthropods on a salt marsh
in SW. Iceland
by Agnar Ingóljsson,
Institute oj Biology, University of lceland, Grensásvegur 12, Reykjavik.
A salt marsh at Gálgahraun in Skerja-
fjördur near Reykjavík, SW. Iceland, was
studied with an emphasis on the zonal
distribution of the larger arthropods. The
marsh is flooded by fully saline water.
Two transects were placed on the marsh,
each with 11 stations, A—K, spaced at
height intervals of 15 cm. Stations de-
signated with the sarne lelter were situat-
ed at identical height levels on the two
transects. The total area studied had a
vertical extent of 1.5 m. Plant cover was
estimated on 15 September 1976 in one
lXl m quadrat at eacli station. One
pitfall trap (diameter 7 cm) was placed at
each station on 15 September 1976. The
traps were emptied on 20 September.
During this time the traps were not reach-
ed by tides, except at the lowermost sta-
tions (A), where traps were flooded once
for a short period.
The lower part ol the marsh is domin-
ated by Puccinellia maritima (Fig. 2,
Table 1), which extends over an area of
80 cm vertical extent. The lower border
of ihe Puccinellia zone (near B stations)
is submerged by about 420 tides annually,
whereas its upper border (near H sta-
tions) is submerged only about 20 limes
a year. Above this zone Festuca rubra
becomes dominant over an area of 30 cm
vertical extent. The upper part ol this
zone is submerged about once annually.
Above the Festuca zone several plant
sjiecies become common. The number of
plant species on the marsh is very low, ancl
does not increase until tlie topmost sta-
tions (K) are reached (Fig. 4), which are
never submerged completely by tides.
The amphipod Orchestia gammarellus
was captured by the traps in greater
number than any other species. It was
found in some numbers over the whole
vertical extent of the marsh, with a peak
in the central part of the Puccinellia zone
(I'ig. 3). Several staphylinid beetles were
common. Micralymma marinum occurred
throughout the Puccinellia zone with a
peak near its centre (Fig. 3). Atheta vest-
ita was common somewhat higher up
with a peak near the junction of the
Puccinellia and Festuca zones. Several
additional staphylinids, not confined to
seashore, became common at the upper
stations. Several spiders were ábundant.
Halorates reporpus was conlined to the
lower part of the Puccinellia zone and
also occurred below this (Fig. 3). Erigone
longipalpis was connnon from the lower
part of the Puccinellia zone to the centre
of the Festuca zone, while Allomengea
scopigera was found throughout the
marsh, lmt witli a peak in numbers near
the centre of the Festuca zone (Fig. 3).
Neither of the two last named species of
spiders are in Iceland conlined to the
seashore. Wasps (Hyntenoptera) were
commonly caught on the upper portion
of the marsh, witlr a peak in the Festuca
zone (Table 2), but they were not iden-
tified to species. Several dipterous llies
were common (Table 2), most of wliich
breed in decaying seaweed.
The number of animal species (or
species groups wliere identification to
species was not possible) was low on the
lowest part of the marsh, but increased
rapidly from he middle of the Puccinellia
zone to the centre of the Festuca zone,
above which there was no fnrther increase
(Fig. 4). A list of all species (or species
groups) caught in ihe pitfall traps and
not included in Table 2 is found on p.
232 (total number of specimens and sta-
tions in parentheses). Small mites (Acar-
ina) and springtails (Collentbola) obtain-
ed in the traps were ignored.
I am greatly indebted to Erling Hauge,
University of Bergen, for identifying the
spiders, and to Erling Ólafsson, University
of Lund, for identifying most of the in-
sects.
237