Page 150 - Freshwater-Biology-and-Ecology-Handbook

CHAPTER 3 3.1.1 The mountain stream is a physically stressful environment because less oxygen is also associated with organic

with very fast and flashy flow, unstable rocky bed, and it
pollution. There is a greater organic loading in the chalk
stream because of its higher productivity, and this supports
is relatively cold. Its low productivity provides little food
a much larger community of species that can tolerate the low
for invertebrates. Its hard geology causes greater acidity
oxygen concentrations. As a result, the value of WHPT ASPT
in which toxic metals from the rock are present in more
is substantially lower in a chalk stream than it is in a mountain
bioavailable forms.
Few species are able to live in mountain streams and this
The example above is extreme, but even within the same
is reflected in a relatively low value of WHPT NTaxa. The stream of similar quality.
larger chalk stream is more benign with a more stable flow river catchment, natural differences in invertebrate
and riverbed, and its higher productivity provides plenty communities can cause significant variations in the values of
of food. It supports more ecological niches and therefore biotic indices independent of the impacts of human activity,
more taxa. WHPT NTaxa in the chalk stream is twice that in reflecting the natural change in character of a river from
the mountain stream. The turbulent water in the mountain headwaters to the sea or between catchments on different
stream is well aerated and shallow, so oxygen is always geologies. Even small differences in index values caused by
saturated and taxa that are very intolerant of lower oxygen natural environmental conditions can be important, because
concentrations can survive. The chalk stream has smoother the differences between Good and Moderate WFD status,
flow and is deeper, so re-oxygenation from the atmosphere ie between communities passing and failing environmental
is slower than in the mountain stream. Its luxuriant emergent quality objectives, are relatively small.
and submerged vegetation causes oxygen to become
super-saturated during the day, but when photosynthesis This phenomenon affects all biotic indices and metrics. Table
stops at night, the oxygen concentration falls dramatically 3.3 shows the range of values for a selection of biotic indices
because the plants continue to respire. Animals living in from the RIVPACS reference database from High to Good
chalk streams therefore have to be able to survive with less (best available) quality sites.
oxygen and those taxa tend to have lower WHPT values

Table 3.3
Variation of a range of indices between stream types, illustrated by the minimum and
maximum values in the RIVPACS reference database, all sites being high or best
available quality

Index Min Max

TL1 NTaxa 3 46
TL1 ASPT 3.6 8
TL2 WHPT NTaxa (AbW,DistFam) 3 56

TL2 WHPT ASPT (AbW,DistFam) 3.1 9.4
TL5 WFD AWIC(Sp) 0 14

TL4 MetTol 36.3 58.1
TL3 LIFE(Fam) (DistFam) 4.6 9.5

TL4 LIFE(Sp) 4.8 9.7
TL3 PSI(Fam) 0 100

TL4 PSI(Sp) 0 100
TL3 E-PSI(fam69) 0 100

TL4 E-PSI(mixed level) 0 100
TL2 08 Group ARMI Score 1 26
Coenagrionidae sp. juvenile

150 | Freshwater Biology and Ecology Handbook
–