Geoscience Reference
In-Depth Information
Table 12.1
(
Continued
)
n
◦
of classes (minimum to maximum)
Description
2
PRESSURES
BARR
Presence of dikes or dams upstream/downstream
modifying the ratio depth/width of water column
4 classes (non-existent to large dam)
PAST
Grazing intensity in the river bed and banks
4 classes (no grazing to bovine or equestrian
grazing)
POLU
Pollution (solid waste) in river bed and banks and
evidence of eutrophication in water
4 classes (non polluted to algae, turbidity and
solid detritus)
1
Data source: National Environmental Agency (Ag encia Portuguesa do Ambiente, 2008)
2
Data source: Field assessment for all variables, except ALTI and ORDE where topographic maps were used
Results
geographical location of each group is indicated in
Figure 12.1. The TWINSPAN analysis established
six groups of stream segments from several
geographical locations and different combinations
of the most frequent species. However, a small
overlap between some of the most frequent
species did occur (Tables 12.3 and 12.5). SIMPER
analysis indicated that a small group of bryophyte
species were responsible for the similarity
between segments of the same TWINSPAN
group. On average 3-4 species were responsible
for determining a group and this confirms the
importance of certain indicator species, some of
which are unique to a group but also appear
in
Structure
Bryophytes were found in every stream segment,
covering on average about 43% (SD
=
32.07)
of the available rocky substrata and representing
a substantial proportion of the total macrophyte
cover (mean
=
56.77%; SD
=
32.33). The mean
diversity of bryophyte species for each segment,
expressed by the Brillouin Index, was 2.1 (SD
=
0.50), showing an intermediate value of diversity
of these communities.
Table 12.2 shows the significant correlations
established between environmental parameters
and several aspects of bryophyte and macrophyte
occurrence. Overall, the correlations between
environmental parameters and each bryophyte
Division are limited but generally higher (r
classification
analysis
as
'pseudo-species'.
Examples
include
Cinclidotus
fontinaloides,
Bryum
alpinum
,
Ceratodon
purpureus
and
Leptodictyum
0.3) when mosses and liverworts are aggregated
(Cob M
riparium
(Table 12.3; Figure 12.2).
L; Table 12.2). Species richness, diversity
and total macrophyte cover show less significant
correlations with the selected parameters (Table
12.2).
+
Ordination and environmental drivers
The DCA ordination supports the segregation
of TWINSPAN groups and essentially reflects
a river hierarchy, represented by bryophyte
communities from more acidic, higher altitude
streams, with greater rainfall and runoff (groups
B, C and D) to those characterizing more
mineralized watercourses at lower altitudes
(groups A, E and F; Figure 12.3). The first two
axes in the ordination diagram were strongly
correlated with the measured environmental
data. Since the correlation values of species-
environment relationships for the first four axes
Composition and groupings
A total of 292 macrophyte taxa were recorded
from the stream bed, of which 175 were
bryophytes. Mosses (134 taxa) were predominant
compared with liverworts (41 taxa). On average, 15
bryophyte taxa were found in each stream segment
(range: 2-30).
The TWINSPAN classification of bryophyte
composition is shown in Figure 12.2, while the
