Northern Ireland has approximately 1670 lakes, which cover 4.4% of theland surface. However, most of the water area is accounted for by the largelakes such as Lough Neagh (385 km2) and Lower Lough Erne (109.5 km ).The majority of lakes are less than 100 hectares in area. They tend to bedistributed towards the south and west of the Province, where extensivedrumlin swarms are rich in small waterbodies

Chris Gibson

Crawford Jordan

FBA Journal System (Freshwater Biological Association)

  
 LAKE SAMPLING FROM THE AIR 
 
 
11
A NEW SYNOPTIC SURVEY OF NORTHERN IRELAND LAKES: 
SAMPLING FROM THE AIR 
 
CHRIS GIBSON AND CRAWFORD JORDAN 
 
(Prof. C. E. Gibson and Dr C. Jordan,  
Department of Agricultural & Rural Development, 
Agricultural & Environmental Science Division, 
Newforge Lane, Belfast BT9 5PX, Northern Ireland.) 
 
Introduction 
Northern Ireland has approximately 1670 lakes, which cover 4.4% of the 
land surface. However, most of the water area is accounted for by the large 
lakes such as Lough Neagh (385 km2) and Lower Lough Erne (109.5 km2). 
The majority of lakes are less than 100 hectares in area. They tend to be 
distributed towards the south and west of the Province, where extensive 
drumlin swarms are rich in small waterbodies. 
In 1988–1991, 610 of the 708 lakes between one and 100 hectares were 
sampled by the Northern Ireland Lake Survey. The objective was to assess 
their conservation status based on their aquatic macrophyte flora, but in 
addition to extensive plant surveys, the water of each lake was analysed for 
a range of chemical variables. The results of the survey were presented in 
an extensive report to the Department of the Environment (Northern 
Ireland). Although the report to DoE was unpublished, a bound copy is 
held in Queen’s University Agriculture library and scientific papers based 
on the data are still being written (Smith et al. 1993; Gibson et al. 1995; 
Wu & Gibson 1996; Heegard et al. 2001). The sampling was carried out in 
the summer months with two teams each averaging two lakes per day. 
Because the survey was confined to the summer months, the nutrient data 
were a reflection of annual minimum rather than maximum values (Gibson 
et al. 1996). The survey occupied a protracted period, and at least for the 
nutrients, comparisons between lakes are not strictly valid since they could 
have been sampled months apart in the seasonal cycle and in different 
years throughout the three-year survey. Also, the data were taken at least 
eleven years ago and many changes are likely to have occurred in the 
intervening period. 
With the requirements of the Water Framework Directive in mind, it was 
felt that a new survey should be conducted in late winter when nutrient 
concentrations are likely to be a true reflection of the trophic status of the 
lake. It was also felt that the lakes should be sampled over the shortest 
possible time interval and helicopter sampling offered this possibility. A 
 CHRIS GIBSON AND CRAWFORD JORDAN 
 
 
12
trial series of eight lakes was sampled in August 2001, to test the sampling 
method and ensure that the helicopter did not produce any sampling 
artefacts such as disturbing the bottom sediments with the downdraft. This 
mission was highly successful. The samples were taken from a JetRanger 
helicopter carrying two scientists and the pilot. No problems were 
experienced with disturbance of the water, the downdraft being quite small 
from a helicopter of this type. In this article, we report on a full-scale 
survey carried out in early March 2002. 
 
The survey 
 
A list of 108 lakes to be sampled was supplied by the Department of the 
Environment, Northern Ireland (DoE), Environment & Heritage Service, 
who funded the project. Unlike the earlier Northern Ireland Lake survey, 
the lakes were not chosen by a stratified random method and so the results 
cannot be used to generalise the state of lakes in Northern Ireland. 
However, they provide a data set consistent with the objectives of the DoE 
in developing a lake classification and determining the state of some lakes 
of high conservation value. 
Sampling began on 4 March, the earliest date possible after agreement of 
the contract. Weather that week was stormy and flying was impossible on 
Wednesday 7 March. Since the helicopter was based at Newtownards, Co. 
Down, there was approximately 40 minutes travel time to the furthest 
lakes. Originally it was planned to stay overnight in Fermanagh on the 
5 March, but this was deemed inadvisable because of the bad weather. If 
this had happened, there would have been a considerable saving of travel 
time. Apart from the weather and problems in finding some lakes, 
sampling went very well. In spite of the strong winds the following 
schedule was achieved (see Fig. 1): 
 4 March 30 lakes  Fermanagh & Tyrone 
 5 March 35 lakes  Fermanagh 
 7 March  40 lakes  Armagh & Down 
 12 March 8 lakes  Antrim Plateau. 
Flying would have been possible on Friday 8 March, but flying the 
remaining eight lakes on the Antrim Plateau was postponed until 12 March 
so that the samples could be analysed during normal working hours on 13 
March. 
Finding the correct lake was not a trivial task, partly because of the 
heavy rain in the days preceding the survey. At least one lake had spilled 
over into the surrounding fields and it is probable that the area sampled 
 LAKE SAMPLING FROM THE AIR 
 
 
13
 
was outside the normal lake basin. Other lakes were difficult to identify in 
the poor weather. Some lakes were revisited on 7 March to ensure the 
sampling was correct. Subsequent analysis of the GPS data verified the 
position of the lakes and revealed that three lakes were not sampled and 
one lake was sampled twice. Lough Formal (area 8 hectares, altitude 20 m) 
was sampled twice, firstly on 5 March and again on 7 March, providing an 
insight into the likely replication errors. The results were satisfactory 
(Table 1) although the soluble reactive phosphorus (SRP) concentration 
was noticeably different between the two samples. 
FIG. 1. Lakes sampled by air in March 2001. 
Table 1. Analytical data from repeat visits to Lough Formal. SRP, soluble reactive 
phosphate; TSP, total soluble phosphorus; TP, total phosphorus; OD, optical density. 
 pH Conductivity 
µS cm-1 
SRP 
µg l-1 
TSP 
µg l-1 
TP 
µg l-1 
NO3-N 
mg l-1 
OD340 
5 March 7.0 80 4 13 17 0.11 0.204 
7 March 6.9 84 10 11 15 0.14 0.206 
 CHRIS GIBSON AND CRAWFORD JORDAN 
 
 
14
Sampling by helicopter 
Two scientists were carried in the rear seat of the helicopter. The door 
panel behind the pilot was removed to allow sampling as the windows of 
the craft are too small to allow the sampler to be deployed. However, 
flying with the door off exposes the outside scientist to considerable wind 
chill and even with appropriate warm clothing it was chilly work. During 
sampling, the helicopter hovered at approximately 7 m above the water in 
the centre of the lake. 
A sampler was developed in our test tank. It consisted of a plastic carrier 
sleeve weighted with lead. A 1 litre plastic bottle was inserted in the sleeve 
and held in place by a bungee cord. The weight of lead was trimmed to 
allow the bottle to sink slowly once it entered the water. It thus took an 
integrated sample over the depth of its travel. Although the rate of sinking 
increased as the bottle filled, tank tests showed that it took water in 
throughout its descent. A float was positioned on the rope 1 m above the 
sampler to prevent sinking below this depth. 
A list of lakes with Irish Grid References (IGR) was supplied to the 
helicopter company. All lakes to be sampled were marked on a set of maps 
for the use of the scientific staff. The helicopter pilot was responsible for 
locating the lake from his maps and for deciding on the order in which they 
were sampled. Once onsite, the pilot read out the position from the onboard 
GPS and this was noted by the inside scientist. The two scientists worked 
as a team. The scientist on the outside seat behind the pilot was responsible 
for taking the samples. His colleague on the inside seat changed the bottles 
in the sampler and kept records of the GPS reading. Lakes were flown in 
groups in two-hour sectors. At the end of the sector, the helicopter landed, 
refuelled and the scientific crew changed over if planned. After the survey, 
all GPS readings were converted into Irish Grid references and compared 
with the Irish Grid references given for the lakes in Smith et al. (1991) to 
ensure the correct lake had been sampled. 
 
Chemical analysis of lake samples 
Samples were returned to the laboratory at the end of each day and stored 
in the dark at 4°C. Analyses were carried out the following day by staff of 
the Agricultural and Environmental Science Division. Full data from the 
analyses were reported to DoE, but summary statistics are shown in 
Table 2 and Figs 2–7. 
 LAKE SAMPLING FROM THE AIR 
 
 
15
Phosphorus 
Soluble phosphorus fractions were determined after filtration through a 
pre-washed 0.45 µm pore-size membrane filter by the ascorbic acid-
molybdate method of Murphy & Riley (1962) on a filtered undigested 
sample (soluble reactive phosphorus, SRP) and a digested sample (total 
soluble phosphorus, TSP). Total phosphorus (TP) was determined after 
digestion of an unfiltered sample. Digestion was by the persulphate 
digestion method of Eisenreich et al. (1975). 
The frequency distribution of phosphorus concentrations, in the various 
fractions, is shown in Figs 2–3. It was surprising that there were so many 
low SRP values, suggesting perhaps the spring bloom had already started, 
but the TP data are more as expected with many values over 50 µg l-1. TP 
values less than 10 µg l-1 were found in lakes Atona, Cashel Upper, Lake 
110 Unnamed, Cam, Tullywannia, Navarad and Oak, and values over 
100 µg l-1 in Bresk, Muck, Fireagh, Vearty, Coranny, Unshinagh and 
Natroey. 
 
0
20
40
60
80
100
10 30 50 70 90
P µ gl-1
No
 o
f l
ak
es
SRP
FIG. 2. Frequency distribution 
of concentrations for soluble 
reactive phosphorus in all the 
lakes sampled (µg P l-1). 
Table 2. Summary statistics of all lakes sampled. Average is arithmetic mean, except 
for pH which is median. 
 pH Conductivity 
µS cm-1 
SRP 
µg l-1 
TSP 
µg l-1 
TP 
µg l-1 
NO3-N 
mg l-1 
OD340 
Maximum 8.4 543 222 223 264 4.5 0.80 
Minimum 4.0 46 1 1 5 0.0 0.01 
Average 7.5 200 13 21 41 0.6 0.14 
 
 
P µg l-1 
 CHRIS GIBSON AND CRAWFORD JORDAN 
 
 
16
 
Nitrate-nitrogen 
Nitrate-nitrogen (Fig. 4) was determined after reduction to nitrite by 
copper sulphate and hydrazine (Chapman et al. 1967). The nitrate-N values 
followed a similar pattern to those for SRP in that many values were in the 
lowest category. Approximately half the lakes had concentrations of less 
than 0.25 mg N l-1 and some of the upland lakes such as Carn, Narye, 
Meenaghmore, Loughnatrosk and Loughnafanoghy contained less than 
0.025 mg N l-1. Some lakes on the other hand were markedly enriched with 
N; most notably Derryleckagh and Greenan had over 4 mg N l-1. 
 
 
FIG. 4. Nitrate-nitrogen concentrations (mg N l-1) in all lakes sampled. 
0
10
20
30
40
50
60
10 30 50 70 90
P µ gl-1
No
 o
f l
ak
es
TSP
0
5
10
15
20
25
30
10 30 50 70 90
P µ gl-1
No
 o
f l
ak
es
TP
0
1 0
2 0
3 0
4 0
5 0
6 0
0 .2 5 0 .7 5 1 .2 5 2 2 .7 5 M ore
N
o 
of
 la
ke
s
Nitra te-N
FIG. 3. Frequency distribution of concentrations for total soluble phosphorus (left) 
and total phosphorus (right), in all the lakes sampled (µg P l-1). 
P µg l-1 P µg l-1 
 
N mg l-1 
 
 LAKE SAMPLING FROM THE AIR 
 
 
17
pH & conductivity 
Electrical conductivity (Fig. 5) was determined by a Phillips PW5909 
digital conductivity meter calibrated by reference to a certified reference 
standard of 100 µS cm-1 and results were corrected to 25°C using the in-
built temperature coefficient for the meter. Measurements of pH were 
determined using an Orion EA90 ionmeter, using separate glass and KCl 
reference electrode calibrated by two certified buffer solutions. 
The pH of the majority of lakes was in the range 6.5 to 7.0 (Fig. 6), but 
there were some strikingly acid lakes, notably Loughnafanoghy, 
Loughnacrackin, Sallagh, Oak, Loughanalbanagh and Navarad which all 
had pH less than 4.5. The acid lakes were also dilute, with low 
conductivities, but not all dilute lakes were the most acid. Lakes 
Loughanalbalnagh, Mill, Shannagh, Loughnafreaghoge, Navarad and 
Carnmore all had conductivities less than 50 µS cm-1; by contrast, the 
hardest water lakes were Tullybrick and Unshinagh on the Lower Calp 
limestones of South Fermanagh with conductivities over 500 µS cm-1. The 
range of conductivity values recorded is shown in Fig. 5. 
 
Colour 
Dissolved organic matter absorbs short wavelength light strongly, but has 
little absorbance at long wavelengths.  Accordingly, colour (Fig. 7) was 
determined by measuring the optical density (OD) of an unfiltered sample 
at 340 nm and a reading at 700 nm was subtracted to give a value corrected 
for turbidity in the sample. Colour is caused by dissolved humic substances 
0
5
10
15
20
25
30
35
50 150 250 350 450
N
o 
of
 la
ke
s
Conductivity
FIG.5. Electrical conductivity (µS cm-1) in all lakes sampled. 
Conductivity (µS cm-1) 
 
 CHRIS GIBSON AND CRAWFORD JORDAN 
 
 
18
from the catchment soils. Rainfall, spring-fed or carbonate lakes typically 
have low colour whereas upland lakes with peaty catchments can be highly 
coloured. The extreme example in this survey was Lough Shannagh in the 
Mournes with OD340 of 0.74. The clearest lakes were Tullybrick, Atona and 
Fardrum, which had colour values of less than 0.04. 
 
Concluding remarks 
The data set obtained could not have been obtained in any other way. A 
synoptic data set of this kind is very unusual and will be used in the second 
phase of the project to calibrate nutrient export models. For this, the 
samples provide a snapshot akin to the satellite imagery and digital aerial 
photographs that will be the basis of the land use classification for the lake 
0
1 0
2 0
3 0
4 0
0 .0 5 0 .1 5 0 .2 5 0 .3 5 0 .4 5 M o r e
N
o 
of
 la
ke
s
Co lo u r  ( O D3 4 0 )
FIG.6. Values obtained for pH. 
 
 
0
10
20
30
40
50
60
70
80
4.0 4.5 5.0 5.5 6.0 6.6 7.0 7.5
N
o 
of
 la
ke
s
pH
FIG.7. Colour values (OD340). 
 
pH 
Colour (OD340) 
 
 LAKE SAMPLING FROM THE AIR 
 
 
19
catchments. The lakes also provide a wide range of values for all the 
variables measured, which is helpful in verification. Aside from the 
scientific advantage of sampling lakes at the same stage of the annual 
cycle, it is very efficient. Samples were analysed very close to the time of 
sampling thus reducing the problems of handling and storage that would 
arise if the lakes were sampled over several weeks by traditional methods. 
In addition, the economics of helicopter sampling are favourable. One 
hundred and ten lakes sampled in this way over four days with two 
scientific staff cost less than £100 per lake. The helicopter hire was £450 
+VAT per hour. To be sure of achieving our objective, we negotiated a 
fixed price for the job with the company. This was probably wise but, 
under more favourable conditions and with less travel time, considerable 
savings could have been made in helicopter time thereby reducing the price 
per lake sampled. On the other hand, sampling by boat on the lake would 
cost approximately 110 man-days plus subsistence, which equates to £250 
per lake. Sampling from the lake margin without a boat would allow more 
lakes to be sampled in a day, but even so, the cost of helicopter sampling 
still compares favourably. This is especially true if you include the 
substantial extra effort required in a land based survey to collect samples 
and transport them back to the laboratory, and the added analytical costs of 
storage for later analysis. 
 
Acknowledgements 
We acknowledge with gratitude the contributions of John McNally, who 
was responsible for the organisation of the flights, flew on three days and 
carried out much of the laboratory work, and Phillip Dinsmore, who flew 
two sectors and organised the nutrient and major ion analysis. Thanks also 
to his team Victor Duffin, Gareth Parke and Louise Davis who contributed 
to both sampling and analysis. Anne Marie Coyle performed the invaluable 
role of data administrator and ground liaison, Joanna Girvan flew three 
sectors in the cold seat, Jim Stevens sanctioned his staff’s involvement, 
flew one sector and first suggested the idea of helicopter sampling for 
Northern Ireland. Bill Clarke designed and built the sampler and did much 
of the initial groundwork for the study in August 2001. Finally, the 
financial support of DoE (EHS) made the project possible. 
 
References 
Eisenreich, S. J., Bannerman, R. T. & Armstrong, D. E. (1975). A 
simplified phosphorus analysis technique. Environmental Letters 9, 43-
53. 
 CHRIS GIBSON AND CRAWFORD JORDAN 
 
 
20
Chapman, B., Cooke, G. H. & Whitehead, R. (1967). Automated analysis: 
the determination of ammoniacal, nitrous and nitric nitrogen in river 
waters, sewage effluents and trade effluents. Journal of the Institute of 
Water Pollution Control 77, 478-491. 
Gibson, C. E., Wu, Y., Smith, S. J. & Wolfe-Murphy, S. A. (1995). 
Synoptic limnology of a diverse geological region: catchment and water 
chemistry. Hydrobiologia 306, 213-227. 
Gibson, C. E., Foy, R. H. & Bailey-Watts, A. E. (1996). An analysis of the 
total phosphorus cycle in some temperate lakes: The response to 
enrichment. Freshwater Biology 35, 525-532. 
Heegard, E., Birks, H. H., Gibson, C. E., Smith, S. J. & Wolfe-Murphy, S. 
(2001). Species environmental relationships of aquatic macrophytes in 
Northern Ireland. Aquatic Botany 70, 175-223. 
Murphy, J. & Riley, J.P. (1962). A modified single solution method for the 
determination of phosphate in natural waters. Analytica Chimica Acta  
27, 31-36. 
Smith, S. J., Wolfe-Murphy, S. A., Enlander, I. J. & Gibson, C. E. (1993). 
The lakes of Northern Ireland: an annotated inventory. H.M.S.O., 
Belfast, 1993. 31 pp. 
Wu, Y. & Gibson, C. E. (1996). Mechanisms controlling water chemistry 
of small lakes in Northern Ireland. Water Research 30,178-182. 
 


A new synoptic survey of Northern Ireland lakes: sampling from the air

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A new synoptic survey of Northern Ireland lakes: sampling from the air

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