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The Influence of Riparian Shade on Lowland Stream Water Temperatures in Southern England and Their Viability for Brown trout

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Suitable thermal conditions in streams are necessary for fish and predictions of future climate changes infer that water temperatures may regularly exceed tolerable ranges for key species. Riparian woodland is considered as a possible management tool for moderating future thermal conditions in streams for the benefit of fish communities. The spatial and temporal variation of stream water temperature was therefore investigated over 3 years in lowland rivers in the New Forest (southern England) to establish the suitability of the thermal regime for fish in relation to riparian shade in a warm water system. Riparian shade was found to have a marked influence on stream water temperature, particularly in terms of moderating diel temperature variation and limiting the number of days per year that maximum temperatures exceeded published thermal thresholds for brown trout. Expansion of riparian woodland offers potential to prevent water temperature exceeding incipient lethal limits for brown trout and other fish species. A relatively low level of shade (20–40%) was found to be effective in keeping summer temperatures below the incipient lethal limit for brown trout, but ca. 80% shade generally prevented water temperatures exceeding the range reported for optimum growth of brown trout. Higher levels of shade are likely to be necessary to protect temperature-sensitive species from climate warming. © Crown copyright 2010.
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THE INFLUENCE OF RIPARIAN SHADE ON LOWLAND STREAM WATER
TEMPERATURES IN SOUTHERN ENGLAND AND THEIR VIABILITY
FOR BROWN TROUT
y
S. B. BROADMEADOW,
a
J. G. JONES,
b
T. E. L. LANGFORD,
b
P. J. SHAW
b
*and T. R. NISBET
a
a
Centre for Forestry and Climate Change, Forest Research, Alice Holt Lodge, Wrecclesham, Farnham, Surrey, GU10 4LH, UK
b
Centre for Environmental Sciences, School of Civil Engineering and the Environment, University of Southampton, Southampton, SO17 1BJ, UK
ABSTRACT
Suitable thermal conditions in streams are necessary for fish and predictions of future climate changes infer that water temperatures may
regularly exceed tolerable ranges for key species. Riparian woodland is considered as a possible management tool for moderating future
thermal conditions in streams for the benefit of fish communities. The spatial and temporal variation of stream water temperature was
therefore investigated over 3 years in lowland rivers in the New Forest (southern England) toestablish the suitability of the thermal regime
for fish in relation to riparian shade in a warm water system. Riparian shade was found to have a marked influence on stream water
temperature, particularly in terms of moderating diel temperature variation and limiting the number of days per year that maximum
temperatures exceeded published thermal thresholds for brown trout. Expansion of riparian woodland offers potential to prevent water
temperature exceedingincipient lethal limits for brown trout and other fish species. A relatively low level of shade (20–40%) was found to
be effective in keeping summertemperatures below the incipient lethal limit for brown trout, but ca. 80% shade generally prevented water
temperatures exceeding the range reported for optimum growth of brown trout. Higherlevels of shade are likely to be necessary to protect
temperature-sensitive species from climate warming. #Crown copyright 2010.
key words: brown trout; climate change; habitat management; riparian shade; water temperature
Received 14 August 2009; Revised 3 November 2009; Accepted 4 December 2009
INTRODUCTION
Temperature regulates nearly all bio-chemical processes and
is therefore a key factor determining the suitability of
aquatic environments for organisms, particularly in affecting
the behaviour, distribution and growth of poikilothermic
aquatic organisms (Coutant, 1976; Langford, 1990). The
threat of global warming has consequently stimulated con-
siderable interest in climate effects, via air temperature, on
water temperature (Mackey and Berrie, 1991; Stefan and
Sinokrot, 1993; Eaton and Scheller, 1996) and the consequences
of an altered thermal regime for river biota (Weatherley et al.,
1991; Webb and Walsh, 2004; Hari et al., 2006).
The spatial and temporal thermal regime of a river is
sensitive not only to a range of natural factors (geology,
hydrology, topography and climate; e.g. Elliott, 2000;
Rutherford et al., 2004) but also to anthropogenic impacts
(riparian land use and abstraction; e.g. Bourque and
Pomeroy, 2001). As one of many factors affecting the
ecology of fish, temperature is both significant and
multifarious. Fish respond to all aspects of the temperature
regime, including the maxima and minima, seasonal and diel
fluctuations, rates of change and the duration of extreme
thermal events. Moreover, temperature can affect the
metabolism and growth of invertebrates (Briers et al.,
2004), the timing of emergence (Durance and Ormerod,
2007) and community structure (Daufresne et al., 2004),
although thermal effects on emergence are not always
apparent (Langford and Daffern, 1975). Potentially, altera-
tions of stream thermal regimes may, therefore, alter the
food resource available to predators, notably fish. Other
important factors such as water quality and discharge also
interact significantly with water temperature (Durance and
Ormerod, 2009).
With respect to fish, salmonids are undeniably of high
importance in UK rivers (e.g. Salmon and Freshwater
Fisheries Act, 1975; Butler et al., 2009) and markedly
affected by physico-chemical conditions in rivers (Hendry
et al., 2003). All salmonids have high metabolic rates and
oxygen demand, but brown trout (Salmo trutta L.) are
amongst the most temperature-sensitive of British native fish
species, typically thriving in temperatures below 208C with
an upper thermal limit of 24–308C (Elliott et al., 1995). High
RIVER RESEARCH AND APPLICATIONS
River Res. Applic. 27: 226–237 (2011)
Published online 25 January 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/rra.1354
*Correspondence to: P. J. Shaw, Centre for Environmental Sciences, School
of Civil Engineering and the Environment, University of Southampton,
Southampton, SO17 1BJ, UK. E-mail: ps@soton.ac.uk
y
This article, The Influence of Riparian Shade on Lowland Stream Water
Temperatures in Southern England and their Viability for Brown Trout, was
written by Sarah Broadmeadow and Tom Nisbet of the Forest Research
Department. It is published with the permission of the Controller of Her
Majesty’s Stationery Office, the Queen’s Printer for Scotland and the Forest
Research Department.
#Crown copyright 2010.
water temperature can be lethal to brown trout; they are
particularly at risk during periods of low flow with
concurrent reduced oxygen levels and increased pollutant
concentrations (Environment Agency, 2006). The threat of
climate warming to native cold-water fish species across
southern England has been recognized as a key priority for
research and action by UK Government (DEFRA, 2005).
Behavioural adjustment to variation in water temperature
has also been shown in many fish species. When subject to
thermal stress, fish will move to cooler water (Schulz and
Berg, 1992; Langford, 1990). In the US, chinook salmon
(Oncorhynchus tshawytscha) have been shown to use cool-
water refugia to maintain their core body temperature as
much as 2.58C below the mid-river temperature (Berman
and Quinn, 1991); lake populations are known to move
deeper to cooler water in the summer (Reynolds and
Casterlin, 1979).
Average summer temperatures are predicted to rise by
4–58C across southern England by the 2080s as a result of
climate change (UKCIP02 HadCM3 scenario; Hulme et al.,
2002). Air temperature is often used to predict stream water
temperature (Webb and Nobilis, 1997) and it is probable that
a48C increase in air temperature would lead to increases in
water temperatures of a similar magnitude. The predicted
consequences of this change are habitat loss and local
species extinction (Eaton and Scheller, 1996; Durance and
Ormerod, 2007). There are few long-term records of water
temperature from the southern, lowland UK and no data on
the responses of lotic fish to either temporal or spatial
thermal discontinuities. Elsewhere (e.g. the Upper Rho
ˆne),
however, an increase of water temperature of 1.58Cover
20 years has been associated with shifts in the population of
thermophilic fish and invertebrate species (Daufresne et al.,
2004).
Shade cast by riparian vegetation can substantially modify
the thermal regime of a watercourse (Malcolm et al., 2004;
Caissie, 2006) and, therefore, influence the potential survival
of sensitive fish such as salmonids during extreme
conditions. As and when the regional climate alters (e.g.
Hulme et al., 2002), the influence of riparian shade is likely
to become increasingly important in protecting trout and
salmon populations from thermal stress. Information is
lacking, however, on stream water temperatures across a
range of habitats in southern lowland catchments. This study
was devised, therefore, to determine the potential of shading
by riparian trees to moderate stream thermal regimes and
thereby contribute to the future management of suitable
conditions for salmonids. Specifically, the study aimed to (1)
characterize and quantify the spatial and temporal variation
of stream water temperature in relation to riparian shade
cover and (2) consider the potential of riparian shade as a
management tool to modify stream thermal regimes for
salmonids under predicted regional climatic conditions.
MATERIALS AND METHODS
Field sites
The two study catchments are located in the New Forest,
southern England, and form part of its drainage system
(Langford, 1996). The Ober Water, a tributary of the river
Lymington, flows south into the Solent; Dockens Water is a
tributary of the river Avon (Figure 1). Both rivers are low
conductivity and circumneutral 3rd order streams (Table I).
They have catchments of low relief with soft substrates
dominated by clay, sand and gravel derived from similar
geologies, arising within Barton clays and passing into
Barton sands in the lower reaches (Environment Agency,
1998). The high ground is formed by drift geology, with
river terrace deposits of sand and gravel, and alluvial
accumulations of fine sediments and peat in the valleys
(Environment Agency, 1998). Both catchments are domi-
nated by surface drainage, which makes them more
responsive to fluctuations in air temperature and solar
insolation than groundwater-dominated systems. Land use
comprises a mixture of broad-leaved woodland, coniferous
plantations, heaths, lowland mires, forest lawns and
improved pasture. The level of riparian shade ranges from
complete woodland cover over both banks to highly grazed,
open lawns.
The New Forest has been designated a special area of
conservation (SAC) due to the scale and combination of
internationally important and threatened lowland habitats.
The streams of the open forest support internationally
threatened invertebrates species such as the southern
damselfly (Coenagrion mercuriale Charpentier 1840) and
a fish fauna comprising over 20 species (Langford, 1996).
There are important populations of species subject to
specific conservation measures including bullhead (Cottus
gobio Linnaeus 1758) and brook lamprey (Lampetra planeri
Bloch 1784).
In the past, the study streams were straightened and
dredged to improve the drainage for silviculture and for the
construction of a railway. Consequently, many of the
reaches have become incised and detached from their
floodplains (NFLP, 2006). The recent LIFE3 project
involved the restoration of floodplain and mire habitats
across the New Forest, between 2002 and 2006 (NFLP,
2006), including extensive alterations to the channel
geometry and floodplain management along the Ober
Water, and alterations to the riparian vegetation in both
study catchments. Dockens Water and Ober Water were
selected for study on the basis that they provided a range of
open and shaded stream sections. They were also known to
support a diverse fish community including thermally
sensitive salmonids (brown trout) and other species of high
conservation status such as the brook lamprey and bullhead
(Langford et al., 2010).
#Crown copyright 2010. River Res. Applic. 27: 226–237 (2011)
DOI: 10.1002/rra
SHADING AND TEMPERATURE IN LOWLAND STREAMS 227
Data collection
Between January 2005 and January 2008, water
temperature was monitored in the mainstream channel at
four locations in the Dockens Water and five sites on the
Ober Water, plus one site in the adjacent Highland Water
(Figure 1). Data were also available for 2006 for an
additional site in the Dockens Water and eight further sites in
the Ober Water. A summary of reach characteristics is
provided in Table II.
Table I. Characteristics of the Ober Water and Dockens Water catchments
Attribute Ober Water Dockens Water
Catchment area 2298 ha 2166 ha
Distance from source to confluence with larger river 13.5 km 14.3 km
Typical pH
a
7.1 7.2
Typical conductivity (mScm
-1
) 104 133
Altitude of source (AOD) 100 m 110 m
Area of forest within the catchment
b
644 ha 581 ha
(% of catchment) (28) (27)
Mean % riparian woodland cover within the catchment 27.6% 42.4%
a
Environment Agency (1998).
b
Forestry Commission (2003).
Figure 1. Map of study catchments in the New Forest, Southern England, showing location of temperature loggers and flow monitoring stations. SWT: stream water
temperature loggers. Codes refer to named locations of loggers (see Table II). This figure is available in colour online at wileyonlinelibrary.com
#Crown copyright 2010. River Res. Applic. 27: 226–237 (2011)
DOI: 10.1002/rra
228 S. BROADMEADOW ET AL.
Temperature was recorded using Gemini TinyTagPlus
data loggers with an internal encapsulated thermistor.
Loggers were secured to the stream bed using aluminium
stakes driven in the soft substrate to a depth of at least 40 cm
and positioned to drift just above the stream bed in all cases;
each was set to log water temperature at 10 min intervals.
Stated precision for the loggers is 0.28C. In November
2006, all loggers were cross-calibrated over a range of 0–
258C in a laboratory water bath and found to differ by less
than 0.78C. Spot checks for accuracy were made when the
loggers were downloaded: in situ temperature was deter-
mined using a hand held electronic thermometer (Hanna
instruments HI 145-00) and measurements were related back
to the contemporaneous logged values; there was no
evidence of drift in accuracy over the period of the study.
Due to extremely low flow during 2006, the Dockens
Water became a series of isolated pools maintained by
interstitial flow. Some loggers became exposed to air for
extended periods; data from these periods (i.e. air
temperature) were removed from the record. Some gaps
in the data record also occurred due to losses of loggers,
delays in data uploading and equipment failure.
An automatic weather station (Delta-T) was installed in
the Dockens Water catchment in March 2005. This station
provides high temporal resolution meteorological data
including rainfall, soil temperature (at 10 cm depth), air
temperature and net solar radiation, logged at 30 min
intervals.
For each reach in which stream temperature loggers were
placed, broad physical characteristics were surveyed in the
field or derived from digital OS maps (Table II). The riparian
shade within a 100 m reach upstream from each logger was
assessed using hemispherical photography. Images were
taken between July and September using a 180
8
fish eye lens
with the camera (mounted in a self-levelling gimbal)
approximately 80 cm above the streambed. The images were
analysed using Delta-T HemiView 2.1 canopy analysis
software. The software calculates several canopy structure
and gap light transmission indices. To measure the
differences in effective riparian shade between the logger
sites, the global site factor (GSF) was calculated. This
parameter represents the proportion of global radiation (sum
of direct and diffuse radiation entering through canopy gaps)
under the plant canopy relative to that in the open and,
therefore, provides a proportional estimate of potential
energy flux. The hemispherical images were also used to
calculate the mean % overhead canopy cover.
The riparian shade along the entire stream network in both
catchments was assessed using geographic information
system (GIS) software and aerial photographs taken in 2004.
A digital stream network was created from the inland water
theme of the OS MasterMap
1
topography layer, which
includes all inland water features such as rivers, canals,
Table II. Mean and maximum stream water temperatures at sites on the Ober Water and Dockens Water for 2005–2007. GSF is the global site factor (see methods for details);
summer temperatures are for May, June and July; 500m US is the % riparian cover (%) over the 500m reach immediately upstream of each logger
Study reach NGR Distance from
source (km)
Riparian shade cover Summer T
mean
(8C) Summer T
max
(8C) T
mean
(8C) T
max
(8C)
Category GSF 500 m US 2005 2006 2007 2005 2006 2007 2005–2007 2005–2007
Ober water:
Mill Lawn (ML) SU230036 6.1 Open 0.989 0.3 16.4 17.4 15.6 19.3 20.5 18.0 11.6 27.7
Rooks Bridge (tributary A) (RBS) SU235038 6.4 Shade 0.181 26.1 — 15.4 — 17.5 9.9 22.0
Rooks Bridge (tributary B) (RBO) SU233036 6.4 Open 0.758 18.4 19.3 17.5 16.2 23.1 20.2 18.4 12.1 27.7
Red Rise Hill (RRH) SU238036 6.9 Shade 0.164 42.4 15.8 16.8 15.8 17.8 19.2 17.5 11.1 25.2
Markway Lawn (channel A) (ML1) SU254039 8.6 Open 0.972 19.9 17.7 15.1 22.7 18.1 10.4 34.5
Markway Lawn (channel B) (ML2) SU257039 8.9 Open 0.980 7.3 17.6 18.1 16.4 21.9 22.7 19.7 12.0 31.1
Duck Hole Bog (DB) SU263031 10.1 Partial shade 0.506 13.5 17.2 16.3 20.6 18.9 11.0 26.6
Puttles bridge (PB) SU268028 10.6 Shade 0.202 44.4 15.6 16.6 16.2 17.5 18.4 18.1 12.0 24.5
Silver Stream (SS) SU271028 11.2 Shade 0.379 64.0 16.2 15.7 18.5 17.5 10.6 24.1
Aldridge Campsite (AC) SU283033 12.2 Shade 0.170 91.5 15.5 15.2 16.8 16.4 10.6 20.6
Bolderford Bridge (BB) SU289041 13.2 Shade 0.232 81.5 15.1 15.8 15.3 16.3 17.1 16.6 11.0 22.0
Dockens Water:
Holly Hatch Lawn (HHL) SU211118 5.7 Open 0.711 18.3 15.6 16.2 15.3 17.8 18.5 17.0 10.8 26.3
Holly Hatch (HHS) SU209118 5.9 Shade 0.197 46.4 14.8 15.1 14.5 16.3 16.7 16.1 10.0 21.6
Splash Bridge (SB) SU207118 6.1 Partial shade 0.396 47.2 14.6 n.a. 14.6 16.6 n.a. 16.9 9.9 24.5
Broomy Inclosure (BI) SU202116 6.9 Partial shade 0.372 33.8 14.0 14.9 14.6 15.0 16.4 15.9 9.9 21.3
Woodford Bottom (WB) SU195112 8.0 Shade 0.128 62.2 13.5 14.0 14.3 14.9 9.9 19.8
#Crown copyright 2010. River Res. Applic. 27: 226–237 (2011)
DOI: 10.1002/rra
SHADING AND TEMPERATURE IN LOWLAND STREAMS 229
fishponds and pools. From this data set, the main stream,
tributaries and major drains were selected to create the
drainage network for each experimental catchment. A
riparian buffer 30 m wide was created along the entire length
of the network and used to clip the OS MasterMap
1
topography layer to generate a detailed map of riparian land
use.
The level of canopy shade within individual riparian
buffer polygons was classified as 0, 5, 10, 20, 30, 40, 50, 70,
80 or 100% based on the extent of riparian woodland canopy
cover using digital aerial photographs. The area weighted
mean riparian cover was determined within 100 m, 500 m,
1 km and 5 km long reaches upstream of each of the
temperature loggers; stream tributaries were included in
proportion to their relative contribution to the accumulated
flow of the stream network. Where the two methods of
assessing riparian cover overlapped (i.e. aerial photographs
and hemispherical photography), they were found to agree
well (r
2
0.756).
Summary temperature statistics were calculated for the
studied reaches to elucidate the effects of riparian shade on
daily maximum temperature (Caissie et al., 2001; Wilkerson
et al., 2005), time in excess of key threshold temperatures (to
reflect the thermal tolerance of key fish species; Elliott,
2000) and monthly mean temperatures (Stott and Marks,
2000).
RESULTS
Monthly mean temperatures
Variations in monthly mean stream temperatures were
considered in terms of shaded vs. unshaded reaches and
pools vs. riffles and glides (Figure 2). Monthly mean
temperatures were shown to be highly consistent between
sites in terms of temporal trends: the timing, direction and
magnitude of month by month temperature changes were
repeated across riffles and glides, and ponds in both open and
Figure 2. Monthly mean water temperatures recorded across all monitoring sites from January 2005 to December 2007. Multiple lines show data recorded at
different sites on the Ober Water and Dockens Water
#Crown copyright 2010. River Res. Applic. 27: 226–237 (2011)
DOI: 10.1002/rra
230 S. BROADMEADOW ET AL.
shaded reaches. Stream water temperature at all sites
exhibited a sinusoidal annual pattern; in each year and at all
sites the highest water temperatures were recorded in July.
Where records were available for multiple sites of the same
type, variation was most marked during summer months and
relatively low during the winters. Peak summer mean
temperatures tended to be higher by ca.28C in open pools
than in shaded pools, although differences were greater in
2005 and 2006 than in 2007. Likewise, temperatures in open
riffles and glides tended to be higher than in equivalent
shaded reaches, but differences were again less apparent in
2007. There appeared to be little, if any, difference between
the mean monthly temperatures for pools compared to riffles
and glides for both shaded and open sites.
Three-year maximum and mean temperatures
Within the 3 years (2005–2007), the average water
temperature (T
mean
) across all sites ranged from 9.9 to
12.18C and the maximum temperature (T
max
) from 19.8 to
34.58C (Table II). Shaded or partially shaded sites were
characterized in general by lower T
mean
(typically 10–118C)
than for open sites (typically 11–128C), albeit with some
inconsistencies. Puttles Bridge, for example, is a shaded site
with relatively high T
mean
, closely comparable to other, open
sites on the Ober Water, whilst Markway Lawn (channel A)
is an open site with a relatively low T
mean
similar to other
shaded sites. Differences in T
max
were more consistent, with
values for all shaded sites <268C, compared to >268C for all
open sites.
Mean temperatures for May, June and July (hereafter
referred to as ‘summer’ temperatures) in 2005–2007 ranged
from 13.5 to 19.38C and values for shaded sites were
generally lower (13.5–16.88C) than at open sites (15.1–
19.38C; Table II). Inter-annual variation was relatively low,
varying by <38C in all but one site (Rooks Bridge tributary
A). Differences between years were generally higher for
open (typically 1–38C) compared to shaded sites (typically
<18C).
Maximum summer temperatures displayed a greater
range of 14.3–23.18C (Table II). Values for shaded sites were
generally lower (14.3–19.28C) than at open sites (17.0–
23.18C), as were inter-annual variations, which were
typically 2.5–58C for open compared to <28C for shaded
sites.
Diel variability
The mean monthly diel cycle of stream water temperature
in the open and shaded sites is exemplified by the 2006 data
for the Ober Water (Figure 3). In January 2006, the water
temperature remained around 4.28C at all the Ober Water
sites throughout the day and the amplitude of the diel stream
temperature range (<1.58C) was lower than that of air
temperature (3.58C; Figure 4). The shaded sites were
slightly cooler in the daytime and slightly warmer through
the night, but differences were consistently <0.38C
(Figure 3). By April, the monthly mean temperature for
both the open and shaded sites had increased to ca.108C and
a clear difference had emerged in the diel range; 4.88C for
the open vs. 3.08C for shaded sites (Figure 4). Shaded sites
were up to 0.68C warmer than those in the open at night but
cooler by up to 1.48C during daylight hours. By July, the
Figure 3. Mean diel variation of water (five sites) and air temperature (one
site) in the Ober Water for January, April, July and October 2006. Vertical
bars show standard error
Figure 4. Amplitude of mean diel variation of water (five sites) and air
temperature (one site) in the Ober Water for January, April, July and
October 2006. Vertical bars show standard error for water temperature
#Crown copyright 2010. River Res. Applic. 27: 226–237 (2011)
DOI: 10.1002/rra
SHADING AND TEMPERATURE IN LOWLAND STREAMS 231
mean monthly stream temperature and diel variation had
risen to 20.9 and 7.98C, respectively in the open, compared
to 18.4 and 2.78C in the shade. Daytime temperatures in
open reaches were up to 5.58C higher than those in the shade
and, unlike in January and April, open sites remained
warmer throughout the night (Figure 3). Water temperatures
cooled through the autumn and, by October, the monthly
mean had fallen to ca. 13.48C at both open and shaded sites.
The diel amplitude (Figure 4) remained higher (2.28C) in the
open than the shade (1.48C), with shaded sites cooler during
daytime (up to 0.98C), but warmer through the night (up to
0.38C; Figure 3).
Comparison of the diel variation in stream and air
temperatures revealed that maxima in the stream at open
sites were usually reached within 1–2 h of the maximum air
temperature, compared to a longer lag of 2–4 h for shaded
reaches (Figure 3). For example, water temperatures in April
peaked at 14:30 GMT in the open vs. 16:00 in the shade,
whilst in July, the open still peaked at 14:30 but the shaded
sites did not peak until 17:00.
Thermal thresholds for brown trout
The number of days that stream water temperature
exceeded thermal thresholds for brown trout showed high
variation between sites (Figure 5). The upper limit for
growth of 19.18C as defined by Elliott et al. (1995) was
exceeded at most sites in each year but generally for a longer
period in the open compared to the shade. The same applied
to the number of days the incipient lethal limit for brown
trout of 24.78C (Elliott et al., 1995) was exceeded. In the
worst case, during 2006, stream temperatures exceeded
the growth limit on more than 90 days at four open sites and
the lethal limit on more than 40 days at two of these. In
contrast, stream temperatures only exceeded the lethal limit
for brown trout at one site and for 2 days during 2006.
Statistical comparisons (Mann–Witney Rank Sum test) of
the number of days over 2005–2007 that stream temperature
exceeded both limits demonstrated statistically significant
(P <0.001) difference between shaded and open reaches.
Influence of upstream vegetation
To elucidate the influence of riparian woodland within the
broader catchment, stream temperature variables (T
mean
,
T
max
and the number of days water temperature exceeded the
thermal thresholds for brown trout) were considered in
relation to riparian cover for 100 m, 500 m and 1 km reaches
upstream of the logger locations (Figure 6).
Variations in the overall T
mean
for 2005–2007 (Table II)
were largely independent of the extent of riparian woodland
cover (Figure 6A) and did not differ with the length of
upstream reach considered. Linear regression analysis (least
squares) showed that the relationships between T
mean
and %
riparian cover for 100 m, 500 m and 1 km upstream
(Figure 6A) were not statistically significant.
In contrast, there was a statistically significant linear
relationship between summer T
mean
and the level of riparian
cover over 100 m (p ¼0.006), 500 m (p ¼0.002) and 1 km
(p ¼0.002) upstream (Figure 6B). The linear regressions
derived for the three summers indicated that a 50% increase
in riparian cover over 100 m, 500 m or 1 km would likely be
associated with a reduction in T
mean
of ca.18C. Likewise,
summer T
max
values were significantly linearly related to
upstream % riparian cover (p <0.001 for 100 m, 500 m and
1 km upstream riparian shade). The best fit linear regressions
indicated that a 50% increase in upstream riparian cover
would be associated with a decrease in summer T
max
of ca.
2.0–2.38C (Figure 6C). As for summer T
mean
(Figure 6B) the
relationship between summer T
max
and upstream riparian
cover differed little when cover was considered over 100m,
500 or 1 km upstream reaches (Figure 6C).
In terms of the number of days per year that stream
temperatures exceeded critical thresholds for brown trout
(Figure 5), exceedance of the upper threshold for optimum
growth (19.18C) varied nonlinearly with upstream %
riparian cover (Figure 6D). This relationship was signifi-
cantly represented statistically (p <0.0001 for 100 m, 500 m
and 1 km upstream riparian shade) by a decay function (two
parameter hyperbolic decay) for upstream riparian cover
over 100 m, 500 m and 1 km and differed little when
considered over different distances upstream (Figure 6D).
The number of days per year that T
max
exceeded the
incipient lethal limit for brown trout (Figure 6E) also varied
nonlinearly with % riparian cover upstream. T
max
did not,
however, progressively change with % cover but only
Figure 5. Number of days per year maximum daily temperature exceeded
thermal thresholds for brown trout (Elliott and Elliott, 1995) at open and
shaded sites on the Ober Water and Dockens Water, 2005–2007 (Median:
bar; 25th and 75th percentiles: box; 10th and 90th percentiles: whiskers;
outliers: )
#Crown copyright 2010. River Res. Applic. 27: 226–237 (2011)
DOI: 10.1002/rra
232 S. BROADMEADOW ET AL.
Figure 6. Stream water temperature parameters in relation to percentage riparian upstream cover for the Ober Water, 2005–2007. (A) Mean temperature 2005–
2007, (B) Mean daily temperature during May, June and July, (C) Maximum daily temperature during May, June and July, (D) Days per year daily maximum
temperature exceeded thermal limit for growth of brown trout, (E) Days per year daily maximum temperature exceeded the incipient lethal thermal limit for
brown trout. (A1–E1) Riparian cover for 100m upstream, (A2-E2) Riparian cover for 500m upstream, (A3–E3) Riparian cover for 1000 m upstream. ~2005,
*2006, !2007 for panels B, C, D and E
#Crown copyright 2010. River Res. Applic. 27: 226–237 (2011)
DOI: 10.1002/rra
SHADING AND TEMPERATURE IN LOWLAND STREAMS 233
exceeded the lethal limit for more than two days per year at
sites with less than ca. 20% riparian cover for 500 m
upstream (Figure 6E2) and less than ca. 40% riparian cover
for 100 m (Figure 6E1) and 1 km (Figure 6E3) upstream.
DISCUSSION
The present study investigated spatial and temporal changes
in stream water temperature in two catchments of the New
Forest as water flowed through mires, heavily grazed pasture
and semi-natural woodland. Previous authors have charac-
terized the thermal regime of lowland rivers but usually
involving single locations towards the lower end of major
river systems (Smith, 1968; Boon and Shiers, 1976; Webb
and Walling, 1993; Webb et al., 2003). In the UK, much
research has investigated thermal conditions in upland
streams subject to commercial afforestation with conifer
plantations (Weatherley and Ormerod, 1990; Stott and
Marks, 2000; Webb and Crisp, 2006).
The surface water dominated streams of the New Forest
are very responsive to changes in air temperature and solar
insolation and displayed marked diel (Figure 4) and seasonal
variation (Figure 3) in water temperature, in sharp contrast to
streams dominated by ground water (Webb and Zhang,
1999). At both the open and shaded sites, the annual cycle of
solar radiation was clearly evident in monthly water
temperature records (Figure 2). In 2005 and 2006 (years
of broadly similar hydrological and climatological con-
ditions), there was a consistent pattern in the inter-site
variability (Table II) as reported for elsewhere (Webb and
Crisp, 2006). In the present study, inter-site variability in
mean and maximum temperatures reflected the level of
riparian shade at the specific reach scale.
Our observations illustrate how water temperature is
moderated by passage through open and shaded reaches. The
extent of variation, however, differs according to specific
temperature parameter. The % riparian cover upstream of a
logger location, for example, is associated with relatively
little variation in T
mean
over 3 years (Figure 6A), but exerted
a marked effect on summer T
max
(Figure 6C). Net radiation
has been shown to be the dominant heat source for streams
through the summer (Hannah et al., 2008); we suggest that
riparian shade moderates maximum water temperatures in
these catchments via reduction of direct insolation and the
consequent suppression of the amplitude of diel temperature
(Figure 4). Mean temperatures in open and shaded reaches
are little different but the response of open sites to the marked
diel fluctuation of air temperature (driven by insolation) leads to
significantly higher temperature maxima in summer months.
Riparian shade, thus, affects both the timing and the
magnitude of stream water temperature changes and
substantially moderates the thermal regime of woodland
areas compared with sites with more open vegetation (Stefan
and Sinokrot, 1993). This observation was most apparent for
sites where the development of the riparian broadleaf tree
canopy in the spring strongly influences shading of the
stream below (e.g. Figure 2). Although inter-site variability
through the winter is small, the pattern is consistent with
open sites cooler during the day and warmer at night.
From a fishery perspective, New Forest streams are
notable, having a rich fish fauna with over 20 species
recorded including priority species for conservation (bull-
head and brook lamprey). The most common species are
minnow, bullhead, stone loach, lamprey and brown trout,
with the fish biomass usually dominated by large individuals
of chub, brown trout and eel (Le Cren, 1969; Mann, 1971;
Langford, 1996; Gent, 2006). Observations in this study
demonstrated that riparian shade is important in terms of
regulating stream water temperature and the viability of New
Forest streams for salmonid fish. The number of days that
thermal thresholds for brown trout are exceeded (Figure 5),
for example, amply demonstrated the value of riparian
canopy shading at specific sites. Notwithstanding the
predicted future alterations to local climate (e.g. Hulme
et al., 2002), evidence indicates that (1) New Forest streams
already experience conditions that are potentially deleter-
ious to brown trout and (2) open reaches are more
susceptible to adverse thermal conditions than shaded
reaches (Figure 5). We also note that higher levels of shading
upstream appear to restrict the number of days that thermal
thresholds for trout are exceeded (Figure 6D and E). These
observations indicate the potential for riparian shade to
protect streams against the effects of high temperatures. It
appears that a riparian woodland cover of >80% for a 100 m
to 1 km reach could, potentially, moderate maximum
temperatures such that the upper limit for optimum growth
of brown trout is exceeded for less than about 30 days per
year (Figure 6D). Similarly, maximum temperatures could
be maintained below the incipient lethal limit for brown
trout with about a 40% cover of riparian woodland for an
upstream reach of 100 m to 1 km (Figure 6E).
In addition to brown trout, many of the other fish species
present in the New Forest are sensitive to water temperature
(Table III). Although thermal tolerances derived under
controlled ex situ conditions are not necessarily truly
representative of in situ tolerances (Malcolm et al., 2008), it
is apparent that maximum stream temperatures observed in
this study at many sites in the New Forest streams were
sufficiently high to exceed critical thermal maxima for
several fish species that are less sensitive to temperature than
brown trout (Tables II and III). On the basis of published
critical thermal maxima (Table III), it would appear that sites
such as Markway Lawn (Table II), where maximum water
temperatures have occasionally reached 34.58C, will only
remain viable for eel and chub.
#Crown copyright 2010. River Res. Applic. 27: 226–237 (2011)
DOI: 10.1002/rra
234 S. BROADMEADOW ET AL.
This study has shown that expanding the cover of riparian
woodland could be used as a means to moderate temperature
maxima for the potential benefit of the New Forest’s fish. At
present, the level of riparian shade within wooded reaches in
the study catchments effectively moderates stream tem-
peratures below the incipient lethal limit for brown trout
(Figure 6E). Our evaluation of riparian shade does not
distinguish between deciduous and coniferous trees
(Figure 1). However, water temperatures follow a repeating
seasonal cycle (Figure 2) and critical thermal thresholds for
brown trout tend to be exceeded only when broadleaved
trees are in full leaf. There appears to be no advantage,
therefore, in creating coniferous riparian woodland to
provide year-round shade.
The existing spatial distribution of riparian shade reflects
the ‘mosaic’ of habitats across the New Forest, with sizeable
areas of open, unshaded ‘lawns’ between extended reaches
of woodland providing a high level of riparian shade. The
level of riparian shade within open lawns that typically
extends over a distance of 1 km or more is generally less than
30% (provided by occasional riparian shrubs). The results
suggest that it should be possible to maintain these important
open habitats but also improve the protection for fish by
targeted planting of riparian trees to achieve the desired level
of shade. At the same time, habitat management for the
benefit of fish should not ignore the broader needs of the
New Forest’s stream biota. The southern damselfly
Coenagrion mercuriale, for example, requires channels that
are open and unshaded, with abundant marginal vegetation,
and their population densities may be negatively affected by
riparian trees (Rouquette and Thompson, 2005). This study
also highlights the need to try and retain some tree cover
within the extended riparian zones of clearfelled conifer
plantations until planted broadleaved trees can provide
adequate shade to prevent summer temperature maxima
exceeding critical thermal thresholds for fish.
Management of the New Forest should, of course, be
planned in full recognition of future climate change.
Increased stream temperature has been predicted as a
probable consequence of climate change (Mohseni et al.,
1998; Ducarne, 2008) and some evidence of a warming trend
has already been reported elsewhere in UK (Langan et al.,
2001; Durance and Ormerod, 2007). The results of this study
add to evidence of the effectiveness of riparian woodland in
regulating stream temperature (Malcolm et al., 2008) and
supports its use as a tool for climate change adaptation
(Broadmeadow and Nisbet, 2004, Scottish Natural Heritage,
2004; Forestry Commission, 2006). Our findings indicate
that planting new riparian woodland to achieve ca. 20%
canopy cover along at least a 500 m reach of small, surface
water dominated streams, could be effective in preventing
current summer maximum water temperatures from
exceeding lethal limits for salmonids and other fish. Higher
levels of riparian woodland cover are likely to be needed to
address future climate warming; a predicted air temperature
rise of 4–58C (Hulme et al., 2002) would likely lead to
thermal thresholds for fish being exceeded more frequently
and for longer. Planning and implementation of tree planting
to ameliorate climate change impacts on fish requires fuller
understanding of the relationship between air and stream
water temperatures in the context of riparian cover. A further
study will elucidate this relationship for lowland streams in
the New Forest, such that future planting can be targeted in
catchments known to currently support salmonid fisheries,
specifically at spawning sites that are presently open and
exposed to direct insolation.
ACKNOWLEDGEMENTS
The authors would like to acknowledge Matt Wilkinson and
Matt Williams for their assistance in the field. The authors
Table III. Range of thermal tolerances for adult fish of the species found in Ober Water and Dockens Water. Optimum range is that over which
feeding occurs and there are no external signs of abnormal behaviour; critical thermal maximum is the highest temperature tolerated with no
mortality over 7 days (‘incipient lethal temperature’), dependent on the experimental conditions such as the acclimation temperature and the
rate of warming
Species Optimum range (8C) Critical thermal maximum (8C)
Brown trout Salmo trutta
a
4–19 19–24
Brook Lamprey Lampetra planeri
b
12 —
Bullhead Cottus gobio
d
4–27 24–28
Eel Anguilla anguilla
e,f
8–29 30–39
Minnow Phoxinus phoxinus
f
13–25 23–31
Chub Leuciscus cephalus
f
8–25 27–39
Pike Esox Lucuis
f
9–25 29–34
Stone Loach Nemacheilus barbatulus
c
5–28 24–29
a
Elliott, 1981;
b
Maitland, 2003;
c
Elliott et al., 1994;
d
Elliott and Elliott, 1995;
e
Sadler, 1979;
f
Ku
¨ttel et al., 2002.
#Crown copyright 2010. River Res. Applic. 27: 226–237 (2011)
DOI: 10.1002/rra
SHADING AND TEMPERATURE IN LOWLAND STREAMS 235
thank Derek-John Gent for the use of his Tiny Tag loggers
and Will Parke for his permission to locate the meteorolo-
gical equipment on his land.
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An analysis and review of the ecological effects of power stations and other heated discharges on fresh and saline waters of the world. Includse effects of other temperature rises cause by hydro-electricity.. So far the only single author book on the topic. Little research has been done since the book was written so it is still very relevant though 25 years old.
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