Geoscience Reference
In-Depth Information
Salamanca (385 mm yr
−
1
) (Reiser and Kutiel,
2010). However, more important than the annual
rainfall in characterizing Mediterranean rivers is
the distribution of rainfall throughout the year.
Mediterranean areas tend to have high winter
rainfall and long dry summers. The median Dry
Days Since Last Rain (DDSLR) for northern regions
is 21 in Salamanca and 22 in Barcelona, whereas in
the more central Lisbon it is 65 and in the south of
Portugal (Loul e) it is 83 (Reiser and Kutiel, 2010).
Within Portugal this strong north-south division
between northern temperate flow regime rivers
and southern Mediterranean flow regime rivers is
evident in the classifications of aquatic macrophyte
communities (Dodkins
et al
., in press).
In the Guadiana river, which drains from Spain
into the south of Portugal, there can be four
months without flow, and up to 50% of the species
found in the channel may be terrestrial plants
(Ferreira
et al
., 2001). The drying is a continuous
and gradual process and therefore the yearly
species succession represents the hydrological
pattern within the river. The sediment within the
channel is frequently of fine texture, with a strong
association with the sediment deposited during
drying, and with the nutrient concentration when
the rain returns. Resuspension of these deposits
also results in high natural turbidity.
macrophyte species were found (Aguiar
et al
.,
2011). Birk
et al
. (2007) had developed a scoring
system to represent the association of European
macrophytes with the aquatic habitat. Using this
assessment, helophytes and hydrophytes could
be identified within the data. Helophytes and
hydrophytes represented only 12% of the total
number of taxa (Aguiar
et al
., 2011) with 105
taxa being considered suitable aquatic macrophyte
indicators. The north-south difference was evident,
with sites in the south of Portugal only having a
mean of 12.7 aquatic macrophytes per site (from
the 105 species) compared with 13.9 in the north
(significantly different at P
=
0.05).
There is a large variation in the number of
taxa used in different indicator-based indices.
For example, the Trophic Index of Macrophytes
(Schneider and Melzer, 2003) uses only 49 species,
whereas the Indice Biologique Macrophytique en
Rivi ere (IMBR) (Haury
et al
., 2006) uses 207
species. There has also been a tendency for an
increase in the number of taxa used in indices,
suggesting that indices with more taxa perform
better. For example, the LEAFPACS method
(Willby
et al.
, 2006) currently used for assessment
of macrophytes in UK rivers uses 275 taxa, whereas
the previous MTR system used only 120 taxa. The
same has occurred with diatoms, with the original
Trophic Diatom Index (Kelly and Whitton, 1995)
having only 76 taxa whereas the new TDI has 667
taxa (Kelly
et al
., 2008).
Difficulties of developing
indices for assessing
Mediterranean Rivers
Small-scale physical habitat variation
With the advent of the WFD, instead of assessment
being focused on comparison of sites upstream
and downstream of point sources, sites have to be
compared with near-natural 'reference conditions'.
The reference conditions are usually based on
actual river sites, but these are rarely close to the
monitoring station and are often on a different
river system (and in some cases may even be
located in a different country). Although rivers
are classified into types that should have similar
features, these are necessarily catchment- or reach-
scale characteristics such as width, depth, alkalinity
and slope. Physical variation at the habitat scale
such as sediment type and the range of flow types
Low numbers of species
A previous macrophyte index (Aguiar
et al
.,
2009) was rejected for use in the WFD by
the Portuguese Water Institute (INAG) since it
contained hygrophytes and terrestrial species
within the index which have a weak association
with the water column. Thus, only helophytes
(emergent) and hydrophytes (submerged or
floating) could be considered for subsequent
indices. Macrophyte surveys were conducted at
373 river sites by INAG (Aguiar
et al.
, 2008; INAG,
2008), recording macrophyte species in the river
channel and along its banks. Over a thousand
