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Title: The position of the Gulf Stream and lake nitrate concentrations in southwest Ireland.
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Eleanor Jennings
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Department of Zoology and Centre for the Environment,
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School of Natural Sciences,
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Trinity College Dublin, Dublin 2,
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Republic of Ireland.
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Norman Allott*
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Department of Zoology and Centre for the Environment,
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School of Natural Sciences,
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Trinity College Dublin, Dublin 2,
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Republic of Ireland
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*Phone: +353 1 608 1642
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Fax:
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Email: nallott@tcd.ie
+353 1 671 8047
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Running Head: Gulf Stream and SW Ireland
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Abstract
A positive relationship was observed between winter nitrate concentrations in two lakes in
SW Ireland and the latitudinal position of the Gulf Stream position in the previous spring.
Weaker but statistically significant relationships were apparent between the Gulf Stream
position and weather variables, as well as soil moisture levels, in the same year. Wind speed,
cloud cover and precipitation in May and June were negatively related to the Gulf Stream
position in April. In contrast, air temperature and sunshine hours in May and June and the
magnitude of the soil moisture deficit in June were positively related. There was also a
positive correlation between the magnitude of the early summer soil moisture deficit and lake
nitrate concentrations in the following winter. This three way linkage implies that the
concentration of winter nitrate in these lakes is influenced by a sequence of related factors that
are initiated by the latitudinal position of the Gulf Stream. In this sequence the position of the
Gulf Stream appears to influence early summer weather in SW Ireland which in turn dictates
the extent of moisture deficit in catchment soils and, consequently, the degree of nitrate loss
to surface waters in the autumn. This connection, between events in the Atlantic Ocean,
weather systems in the North West Atlantic and processes in catchment soils in SW Ireland
has implications for both the quality and quantity of biota in lakes and catchments in the
region.
Key words: Gulf Stream Index, nitrate, catchment, Ireland, climate, soil moisture deficit
2
Introduction
The increase in climate related research in recent decades has highlighted the influence of
large-scale global phenomena on variation at ecosystem level. Large-scale phenomena which
have been linked to ecosystem variability in the NE Atlantic region include the latitudinal
position of the Gulf Stream current, as measured by the Gulf Stream North Wall index
(GSNW index) (Taylor and Stephens, 1980). Variation in the latitudinal position of the Gulf
Stream is itself a response to large-scale atmospheric changes, in particular to fluctuations in
the North Atlantic Oscillation (NAO) two years previously and, to a lesser extent, to the El
Nino/Southern Oscillation (ENSO) (Taylor and Stephens, 1998; Taylor and Gangopadhyay,
2001). The latitudinal position of the Gulf Stream has been shown to be statistically related to
zooplankton numbers in the NE Atlantic (Taylor and Stephens, 1980; Taylor et al., 1992), in
the North Sea (Taylor, 1995) and in Lake Windermere in the U.K. (George and Taylor, 1995;
George, 2000). A correlation has also been noted between the position of the Gulf Stream and
vegetation growth in the U.K. (Willis et al., 1995).
There is evidence that the causal mechanism for these relationships is the influence of the
Gulf Stream position on weather patterns in NW Europe (Taylor et al., 1992; Taylor, 2002;
Taylor et al., 2002). Taylor (2002) reported that sea-level pressure and the distribution of
storm tracks over the NE Atlantic in spring and to a lesser extent in November differed from
the norm in years when the Gulf Stream is in an extreme northerly or extreme southerly
position. However, direct correlations between the Gulf Stream position and weather
characteristics in the region are typically poor (Taylor et al., 1992; George and Taylor, 1995;
Willis et al., 1995; Taylor et al., 2002). It has been suggested that organisms such as
zooplankton act as integrators of a complex weather signal and therefore show a higher
3
correlation to the Gulf Stream position than do individual weather variables (George and
Taylor, 1995; Taylor et al., 2002).
Nitrate-nitrogen (NO3-N) export from catchments is known to be strongly influenced by
climatic factors, including antecedent weather conditions (Scholefield et al., 1993; Stronge et
al., 1997; Reynolds and Edwards, 1995; Worrall and Burt, 1999). The relationship is complex
and includes the combined effects of temperature and moisture availability on rates of
mineralization, plant uptake, nitrification and denitrification (Scholefield et al., 1993; Stronge
et al., 1997). A positive relationship is often reported between the quantity of NO3-N leached
from a soil and the magnitude and duration of a soil moisture deficit (SMD) in the previous
summer (Garwood and Tyson, 1977; Scholefield et al., 1993; Stronge et al., 1997).
Exceptionally high NO3-N export has also been noted in many studies following drought
years (e.g. Garwood and Tyson, 1977; Casey and Clarke, 1979; Reynolds and Edwards,
1995). Winter NO3-N in some European lakes have been linked to the North Atlantic
Oscillation (NAO), a large scale climatic phenomena that is known to strongly affect the
winter weather in Europe (Monteith et al., 2000; George et al., 2004a; George et al., 2004b)
However, no relationship was found between the NAO and lake NO3-N in SW Ireland in an
earlier investigation (Jennings et al., 2000).
As part of an on-going exploration into the influence of large-scale phenomena on lake and
catchment processes, relationships between the Gulf Stream position and long-tem data sets
from two lakes in the south-west of Ireland have been examined. In this paper we explore the
relationships between the position of the Gulf Stream and weather variables in the region and
the relationships between winter NO3-N in the lakes and local weather effects. In addition, we
report on the relationship between the latitudinal position of the Gulf Stream and lake NO3-N.
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Materials and methods
The three Lakes of Killarney are situated in County Kerry in SW Ireland (Fig. 1). The
catchment of 553 km2 consists of two contrasting components: an area of upland mountain
peat which lies to the south and west (the Upper Catchment) and drains through the two
smaller lakes, Upper Lake and Muckross Lake, into the larger Lough Leane, and an area to
the east of the lakes which is predominantly agricultural and drains into Lough Leane via the
Rivers Flesk and Deenagh. Landuse in the smaller Deenagh sub-catchment is almost
exclusively dairy and dry stock grazing. Dairy and dry stock grazing dominates the northern
section of the Flesk sub-catchment while the southern section consists of upland peat with
small areas of forestry. There have been no major changes in landuse or vegetation during the
study period. The catchment is very close to the Atlantic coast and is therefore strongly
influenced by the temperate oceanic climate that predominates in the region. Muckross Lake
receives drainage only from the upland peat section of the catchment and consequently has a
low level of total phosphorus and chlorophyll a (Table 1). The concentration of phosphorus
and chlorophyll a are higher in Lough Leane owing to nutrient loading from the agricultural
catchment in the east. Algal blooms are periodically a problem in the lake.
Sampling for a suite of chemical and physical variables has been carried out on the lakes in
most years since the mid 1970s. Surface NO3-N concentrations (1984-2004) from open lake
sites situated at the deepest points in Muckross and Lough Leane were used in the following
analysis. Surface NO3-N concentration was measured spectrophotometrically following
reaction with sulphosalicylic acid from 1984 to 1992 (Standing Committee of Analysts,
1981), by ion-selective electrode in 1994 (Lough Leane only) (American Public Health
Association, 1999) and spectrophotometrically following cadmium reduction from 1997 to
2004 (American Public Health Association, 1999). Sampling frequency was 1-2 times per
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month. Examination of the combined dataset indicated no shift in concentration related to the
change in methods of analysis. Mean NO3-N data for January and February were used in this
study as representative of mid-winter concentrations. No data were available between 1993
and 1997 inclusive for Muckross and for 1993, 1995 and 1996 for Lough Leane. Nitrate
concentrations in Lough Leane were measured by a semi-quantitative comparator method
(Mackereth, 1963) between 1976 and 1983. However, these data are not used in the statistical
analysis.
Meteorological data were obtained for nearby stations from the Irish Meteorological Service
(Met Éireann). The stations were Muckross House, a lakeshore station at Lough Leane, and
Valentia, a station 40 km SW of Killarney. The data set available from Muckross House
consisted of daily values of maximum and minimum air temperature (1969-2002), total
rainfall (1969-2002) and sunshine hours (1969 – 1991). Sunshine hours were not recorded at
Muckross House after 1991. However, data were available from the new Met Eireann site at
Ardfert from 1990 for all years with the exception of 1993-1996 inclusive. The regression
coefficient between mean monthly sunshine hours at Muckross House and at Ardfert for the
overlap period of 18 months (1990-1991) was r2 = 0.94 (p < 0.0001). Additional daily data for
wind speed and cloud amount were obtained for the Valentia station (1966-2002).
Soil moisture deficit (SMD) was calculated using the procedure outlined in the grass growth
model developed for Irish grasslands (Brereton, 1981; Brereton et al., 1996). Potenital
evapotranspiration (PE) rates were estimated for the Lough Leane catchment based on air
temperature and sunshine data using the method of Priestley and Taylor (1972). Actual
evapotranspitaion (AE) was calculated as a proportion of potential evapotranspiration based
on the parameters of Aslyng (1965), with PE decreasing linearly for SMD less than 40 mm to
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a value of zero at a SMD of 120 mm. SMD was calculated as the cumulative difference
between rainfall and actual evapotranspiration. Nitrogen fertiliser sales figures, which are
considered by the Department to reflect fertiliser use in a given year, were obtained from the
Department of Agriculture and Food (2004).
The GSNW index (Taylor and Stephens, 1980) is commonly used to indicate the overall
position of the northern boundary of the Gulf Stream. The index is derived using principal
components analysis of sea surface temperature data at six longitudes between 65W and
79W (Taylor and Stephens, 1980). A high value indicates a more northerly position. The
GSNW index is available at http://www.pml.ac.uk/gulfstream/ for each month and year from
1966 to present. Monthly GSNW index values were used in this paper.
All statistical analysis as performed using either SPSS for Windows (11.0.1) or DataDesk
(6.1). All data sets were checked for normality using a Shapiro-Wilks test. All data had a
normal distribution with the exception of soil moisture deficit which was transformed using a
log function. A multiple linear regression approach was used to assess the influence of the
GSNW index on lake NO3-N, with NO3-N as the dependent variable and nitrogen fertiliser
sales and monthly values of the GSNW index as independent variables. The relationship
between meteorological measurements and GSNW index values was assessed using least
squares regression. Residuals were assessed for serial correlation both visually and using the
Durbin-Watson test.
7
Results
Relationship between the climate in Kerry and the position of the Gulf Stream
There were no significant relationships between the position of the Gulf Stream and the
weather in SW Ireland for the months between January and April inclusive. However, several
relationships were found between the Gulf Stream position in April and weather variables in
the May and June period (Table 2). Wind speed, cloud cover and precipitation were
negatively related to the Gulf Stream position in April, with r2 values of 0.31, 0.27 and 0.20
respectively (Table 2). In contrast, maximum daily air temperature and hours of bright
sunshine were positively correlated with the April GSNW index (r2 values of 0.22 and 0.47
respectively). These positive relationships were reflected in a correlation between PE and the
April GSNW index (r2= 0.34). The correlation between AE in May and June and the April
index was positive but not significant due to the negative effect of higher SMD on
evapotranspiration rates. However, there was also a significant positive relationship between
the mean June SMD (log transformed) and the April GSNW index in the same year (r2 =
0.43).
Relationship between climate and lake NO3-N
There were no direct relationships between winter NO3-N in the two lakes and the weather
variables that were related to the GSNW index. However, mean SMD in June (log
transformed) was positively related to the both the winter NO3-N in Muckross Lake (r2 =
0.48; p = 0.0061; d.f. = 12) (Fig. 2 A and B) and the winter NO3-N residuals for Lough Leane
(r2 = 0.36; p = 0.0231; d.f. = 12) (Fig. 3A and B).
Relationship between surface water NO3-N and the position of the Gulf Stream
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Winter NO3-N in Muckross Lake was found to be significantly related to the position of the
Gulf Stream as measured by the GSNW index in the previous April (r2 = 0.55; p = 0.001; d.f.
= 14) (Fig. 4 A and B; Table 3). There was no relationship between fertiliser sales and
Muckross Lake NO3-N. However, both nitrogen fertiliser sales and the position of the Gulf
Stream in April were significantly related to the winter NO3-N in Lough Leane (Table 3). The
increase in winter NO3-N in Lough Leane in recent decades coincided with an increase in the
use of fertilizer in Ireland in the same period from less than 100,000 tonnes in the early 1970s
to over 400,000 tonnes in the late 1990s (Fig. 5). The regression coefficient between the
spring GSNW index and the residuals from the relationship between the Lough Leane NO3-N
and fertilizer sales was r2 = 0.39 (p = 0.009; d.f. = 15) (Fig. 6 A and B).
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Discussion
The results presented in this paper broadly support the conclusions of Taylor (2002) regarding
the influence of the Gulf Stream position on western European weather. Taylor (2002)
reported that the distribution of storm tracks, mean sea-level pressure and the mean height of
the 500 mPa level in the NW Atlantic differed in extreme positive and extreme negative
GSNW index years in April and May. He hypothesized that during southerly index years the
Gulf Stream creates a blocking pattern in the area south of Iceland which causes fewer
cyclones to reach Ireland and the U.K in the spring. Taylor (2002) further suggested that there
would be a concurrent increase in northerly winds over the North Sea, Ireland and the U.K.,
with possibly an increase in cloud cover and lower air temperatures. Conversely, he suggested
that when the Gulf Stream is in an extreme northerly position this blocking pattern would not
occur and Ireland and the U.K. would be likely to experience increased sunshine and higher
temperatures.
In SW Ireland, the weather in May and June was sunnier and warmer, with less wind, lower
cloud cover and less rainfall in years when the Gulf Stream was in a more northerly position.
When the Gulf Stream moved to the south May and June were colder and windier, with a
concurrent increase in cloud cover, lower sunshine and higher rainfall. In addition to the
weather patterns suggested by Taylor (2002) we have reported a relationship between the
latitudinal position of the Gulf Stream and soil moisture levels in June. This relationship was
stronger that the individual relationships between either precipitation or AE and the GSNW
index. However, this probably reflects the fact that a SMD develops in a cumulative manner
and will therefore integrate the GSNW index signal over the early summer period.
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An upward trend in NO3-N concentrations in many freshwater systems in recent decades is
generally attributed to the use of nitrogen fertilizers in agricultural catchments (The Royal
Society, 1983; Vitousek et al., 1997). The influence of intensive grassland agriculture on
nutrient concentrations in Lough Leane is reflected in the relationship between NO3-N in the
lake and nitrogen fertiliser sales. Winter NO3-N in Lough Leane has risen from below 150 g
L-1 in the 1970s to consistently above 400 g L-1 in the late 1990s, tracking the increase in
fertilizer sales over this period. However, while anthropogenic inputs of nitrogen have driven
long-term trends in surface water concentrations in many catchments, the quantity of NO3-N
exported is also strongly influenced by climatic factors. These include the effect of a previous
SMD on the build-up and subsequent washout of soil nitrogen (Scholefield et al., 1993;
Reynolds and Edwards, 1995; Stronge et al., 1997, Worrell and Burt, 1999).
A SMD occurs when the rate of actual evapotranspiration exceeds the rate of rainfall
(Scholefield et al., 1993). Such a deficit has an adverse effect on grass growth when it
exceeds c.40 mm (Aslyng, 1965; Brereton et al., 1996). During low moisture conditions, plant
and microbial uptake of NO3-N decreases and eventually stops. NO3-N which has been added
to the soil as fertilizer and NO3-N present in the soil following mineralization of organic
matter remain in situ until rainfall reoccurs. Grass yield and grass nitrogen concentration have
both been found to decrease following periods of prolonged SMD in summer (Webster and
Dowdell, 1984; Lord, 1992). Rates of nitrogen mineralization in grassland soils are also
reported to be greatest when the soil is re-wetted after dry conditions (Birch, 1964; Hatch et
al., 1990). If re-wetting following drought occurs in late summer, plant nitrogen uptake will
also be slowed due to day-length limitation, further increasing the quantity of nitrogen
available for leaching to ground water. Increases in nitrate export following drier summers
have been noted in both upland peat (Reynolds and Burt, 1995) and agricultural grassland
catchments (Casey and Clark, 1979; Scholefield et al., 1993).
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The results in this paper highlight the effect of soil moisture levels in summer on nitrate
export in the following autumn and winter and link this effect to shifts in the position of the
Gulf Stream. We suggest that the lagged relationship reported in this paper between winter
NO3-N and the GSNW index in the previous April actually reflects the influence of the Gulf
Stream position on the magnitude of the June SMD with subsequent effects on NO3-N in the
two lakes in the following winter. The existence of lagged response between NO3-N export in
autumn and winter and the occurrence of a SMD in the previous summer is widely recognized
(Garwood and Tyson, 1977; Scholefield et al., 1993; Reynolds and Edwards, 1995). The
occurrence of drier summer conditions has also been found to be a significant factor
influencing NO3-N concentrations in Lough Neagh in Northern Ireland (Stronge et al., 1997),
while a relationship between the maximum summer SMD and groundwater NO3-N in the
following winter, has been reported from Cork in SW Ireland (Richards, 1999).
Our analysis suggests that the concentration of winter nitrate in the Killarney Lakes is
influenced by a sequence of related factors that are initiated by the latitudinal position of the
Gulf Stream. Specifically, the position of the Gulf Stream appears to influence early summer
weather in SW Ireland which in turn dictates the extent of moisture deficit in catchment soils
and, consequently, the degree of nitrate loss to surface waters in the autumn. These
observations illustrate how a large-scale phenomenon of ocean circulation in the western
Atlantic can mediate the concentration of an important macro nutrient in lakes. The role of
nitrogen as an influence on both the quantity and quality of lake phytoplankton is widely
recognised (e.g. Sommer, 1989; Downing et al., 2001). It can therefore be reasonably
proposed that the position of the Gulf Stream is one of the influences on the phytoplankton of
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these lakes. These findings are likely to apply to other catchments and lakes in SW Ireland
and perhaps elsewhere near the Atlantic coast of Western Europe.
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Acknowledgements
We acknowledge financial support from the European Commission’s Environment and
Sustainable Development Programme under contract EVK1- CT -, 2002 - 00121 (CLIME).
We wish to thank Kerry County Council (in particular David Lenihan), William Quirke,
Helena Twomey, Áine Ni Súilleabháin, Pascal Sweeny and the UCD Killarney Valley project
for providing data from Lough Leane and Muckross Lake. We also would like to thank the
two anonymous reviewers for their very helpful comments and Arnold Taylor and Glen
George for advice on the paper.
19
Table 1 Characteristics of Muckross Lake and Lough Leane, County Kerry, SW Ireland.
Lake area
Mean depth
Retention time
Annual average TP
Average max. chl a
(ha)
(m)
(year)
(range) g L-1
g L-1
Muckross
275.3
31.3
0.82
2-8
4.5
Leane
1988.6
13.4
0.57
13-25
17.3
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Table 2 Summary of significant regressions of data from SW Ireland (mean or sum of daily
values for the period 1st May-30th June) on the position of the Gulf Stream in April as
described by GSNW index. Wind speed (m s-1) and cloud cover (oktas) are from Valentia
(1966-2002); air temperature (oC) and precipitation (mm) (1969-2002) are from Muckross;
sunshine hours are from Muckross (1969-1990) and Ardfert (1991-2002). PE, AE and SMD
were calculated using data from Muckross and Ardfert.
Dependent
variable
Independent
variable
Coefficient
SE
T
df
p
r2
Mean wind speed
GSNW index
-0.278
0.070
-3.96
35
0.0003
0.31
Mean cloud cover
GSNW index
-0.171
0.047
-3.58
35
0.001
0.27
Sum precipitation
GSNW index
-23.216
8.264
-2.81
32
0.0084
0.20
Max. air temperature
GSNW index
0.346
0.115
3.01
32
0.005
0.22
Mean sunshine hours
GSNW index
0.352
0.077
4.57
28
<0.0001
0.47
Mean PE
GSNW index
0.111
0.029
3.81
28
0.0007
0.34
Mean AE
GSNW index
0.057
0.030
1.91
28
0.0662
0.12
Log10 June SMD
GSNW index
0.173
0.042
4.49
28
0.0001
0.43
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Table 3 Results of multiple regressions of Muckross Lake and Lough Leane NO3-N
concentration (g L-1) on annual N fertiliser sales (1000 tonnes N y-1) and the GSNW index
for the previous April.
Lake
Muckross
Leane
Coefficient
SE
T
p
df
Overall R2
(adjusted)
Constant
76.06
63.27
1.2
0.2507
13
0.50
N fertiliser sales
0.10
0.17
0.59
0.5629
April GSNWI
19.98
4.97
4.02
0.0015
-292.56
161.90
-1.81
0.0909
15
0.53
N fertiliser sales
1.83
0.44
4.19
0.0008
April GSNWI
65.21
15.74
4.14
0.0010
Model terms
Constant
22
Figure legends
Figure 1: Map showing location of the Lakes of Killarney (Upper Lake, Muckross Lake and
Lough Leane) and their catchment (SW Ireland). Meteorological stations
Figure 2: Time series plot (A) and scatter plot (B) of mean January-February NO3-N (g L-1)
in Muckross Lake (1984 to 2003) and the mean June SMD (log transformed) in the previous
year (1983-2002).
Figure 3: Time series plot (A) and scatter plot (B) of mean January-February NO3-N residuals
in Lough Leane (1984 to 2003) and the mean June SMD (log transformed) in the previous
year (1983-2002).
Figure 4: Time series plot (A) and scatter plot (B) of mean January-February NO3-N (g L-1)
in Muckross Lake (1984 to 2004) and the position of the Gulf Stream in the previous April as
described by the GSNW index (1983 to 2003).
Figure 5: Time series plot of mean January-February NO3-N (g L-1) for Lough Leane and
annual fertiliser sales (1000 tonnes N yr-1) (1976 to 2004).
Figure 6: Time series plot (A) and scatter plot (B) of mean January-February NO3-N residuals
for Lough Leane (1984 to 2004) and the position of the Gulf Stream in the previous April as
described by the GSNW index (1983 to 2003).
23
Figures
Ireland
N
Deenagh
Killarney
Ardfert
Killarney
L. Leane
Valencia
Muckross Met Station
40 km
Flesk
Upper
Catchment
Muckross
Lake
0
10
20
Figure 1
24
kms
Lakes of Killarney
Figure 2
25
2.0
100
1.5
0
1.0
-100
-200
0.5
A
Log10 previous June SMD
NO3-N
200
0.0
2002
1999
1996
1993
1990
1987
-300
1984
Mean Jan-Feb nitrate residuals
Mean Jan-Feb nitrate residuals
2.5
SMD
300
200
r2 = 0.36
p = 0.023
100
0
-100
Leane
B
-200
0
0.5
1
1.5
2
Log10 previous June SMD
Figure 3
26
Figure 4
27
Figure 5
28
MeanJan-Feb nitrate residuals
2.0
1.0
0.0
-1.0
-2.0
200
100
-3.0
A
2002
1999
-4.0
1996
1993
1990
1987
Nitrate
GSNWI
Previous April GSNW index
3.0
Leane
1984
Mean Jan-Feb nitrate residuals
250
200
150
100
50
0
-50
-100
-150
-200
-250
r2 = 0.39
p = 0.009
0
-100
-200
-5.0
B
-3.0
-1.0
1.0
3.0
Previous April GSNW index
Figure 6
29