The use of large plastic enclosures, or 'Lund tubes', in lakes as a semi-natural basin for experiments, approximating to lakes within lakes, is discussed. The advantages and disadvantages of the tubes were studied, and the results of investigations into the validity of studies using such tubes and the effects of fertilisers on phytoplankton are presented

Lund, J.W.G.

English

Aquatic Commons

32 FORTY-SIXTH ANNUAL REPORT 
REVIEW ARTICLES 
EXPERIMENTS WITH LAKE PHYTOPLANKTON IN LARGE ENCLOSURES 
J. W. G. LUND 
The intrinsic characteristics of planktonic organisms (plankters) can be 
discovered by laboratory investigations. Examples are basic needs for 
growth and reproduction, the effects on growth rate of variations in the 
rate of supply of nutrients and the relation of total nutrient supply to yield. 
Generally, we are forced to study one organism at a time. In a few cases it 
may be possible to study two or more organisms in a defined medium or 
under defined conditions. However, any microcosm we set up in the 
laboratory in order to obtain essential basic data will be more or less remote 
- usually very remote - from the natural environment with its multitude of 
diverse species living in the same space, such as the plankters living in the 
open waters of lakes. Moreover we do not understand the natural 
environment fully and cannot follow all the changes occurring in space and 
time. This in no way implies a criticism of laboratory investigations, 
without which there is no hope of understanding natural events, but it does 
explain why ecologists may wish to carry out experiments with natural 
populations under conditions as natural as possible. 
The present article is about the use of large plastic enclosures in one of 
our lakes as semi-natural basins for experiments, approximating to lakes 
within lakes. Most of the work has been with phytoplankton but these 
enclosures or 'tubes', as we call them, have been used for other 
investigations and are potentially usable for studies on virtually all kinds of 
communities or groups of organisms. One drawback of the size of these 
tubes is that the extra work involved in studying the enclosed water is 
equivalent to that of studying another lake. Apart from the consequent 
demand on manpower and resources, but arising from it, a second 
drawback is that it is impossible to have so many enclosures that 
experiments can be carried out with sufficient replication for statistical 
analysis. However, the belief that satisfactory experiments with 
phytoplankton could be carried out on small samples (e.g. 100 ml) of lake 
water proved to be fallacious, and so tests were made with larger and larger 
containers. While they were suitable for short-term experiments, they all 
proved too small for those relating to the longer periods of time involved in 
investigating the periodical phases of the plankton development typical of 
lakes. The relatively high ratio of surface area to enclosed volumes of lake 
water in these containers favoured attached or benthic algae more than the 
phytoplankton. This effect was more pronounced the longer the experiment 
lasted. Since the containers were not large enough to approach the desired 
situation of a lake within a lake, the decision was made to use very much 
larger enclosures holding over 18 000 m3 of water. I do not believe that this 
PLATE 3(a). Three Lund tubes in 'Grote Rug' reservoir near Dordrecht, Netherlands (Courtesy of Dr Jan G. Booij, National Institute for Water Supply). 3(b) Margin of a tube 
in Blelham Tarn. 
DIRECTOR'S REPORT 33 
is necessarily the ideal size for all purposes or places. That almost identical 
or similar enclosures can be of value elsewhere and for somewhat different 
purposes is shown by their use by the Water Research Centre in the 
Farmoor Reservoir 2 of the Thames Water Authority, in the Norfolk 
Broads (Nature Conservancy Council 1976) and in a reservoir in the 
Netherlands (Plate
 3a). The tubes in the reservoir in the Netherlands, 
which have improved anchorages, have been successful despite its exposed 
situation (Booij & Vlugt, in press). There can be little doubt that 
modifications and special precautions are necessary in large waterbodies 
exposed to high winds. Moreover, a tube enclosed in a large piece of ice 
during a thaw would be in great danger with even a moderate wind 
blowing. 
The construction and emplacement of the original models in Blelham 
Tarn are described by Lack & Lund (1974), the modifications for the two in 
Farmoor Reservoir 2 by the Water Research Centre (Anon. 1977a & b) and 
the three in the Dutch reservoir by Booij & Vlugt (in press). Basically the 
enclosures are tubes (Fig. 1) placed vertically in the waterbody. The top of 
the tube is inflatable and has compartments or, in the newer models, 
inflatable tubes within a covering. Our tubes in Blelham Tarn, English 
Lake District, have three such rings of inflatable compartments, one below 
the other and welded to the tubes themselves. Normally only the top ring is 
inflated. The bottom of the tube also has a hollow ring containing a heavy 
metal chain so that it sinks into the soft bottom deposits. That it has sunk 
into the mud is determined by a diver who treads round the edge of the 
bottom of the tube to ensure that it is as deep as or deeper than his legs can 
reach. The tubes are 15 m long but reach the bottom at about 11-12 m 
below the surface, their mean depth being a little over 11 m. The extra 
length of tubing allows for the rise and fall of the tube with changes in the 
34 FORTY-SIXTH ANNUAL REPORT 
lake level. The Farmoor tubes are fixed to the bottom of the reservoir 
(Anon. 1977a, b). All the tubes are constructed of butyl rubber (Esso 
Butylite). 
Though these enclosures do behave like lakes within lakes (Lund 1972, 
1975; Lack & Lund 1974), those in Blelham Tarn have a mean depth over 
four metres greater than that of the lake itself (6.8 m) and they are artificial in 
that they do not have any shallow water in them. A third tube now under 
construction will be placed in a more marginal area of the lake so as to 
enclose also a shallow, littoral region. 
The tubes were installed in 1970 and the fact that they appear to be good 
for another 7 years attests to the durability of butyl rubber. Entry into 
them is made by rowing a keel-less boat up to the tube as fast as possible. 
The boat then slides up and over the inflation collar. Even in the regions of 
entry the original butylite is still in use. 
A preliminary experiment, which had to last over a year in order to 
answer the questions posed, showed that a natural phytoplankton would be 
maintained for an apparently unlimited period and that thermal 
conditions, such as the degree of stratification, were very similar to those in 
the lake outside (Lack & Lund 1974, Figs 4-6). This experiment also 
showed that enclosure of lake water, and so prevention of renewal of 
nutrients by the inflows of the lake, caused the phytoplankton to change 
rapidly from that of a moderately eutrophic lake to a very oligotrophic one. 
Both quantity and quality of the phytoplankton altered (Lund 1975). This 
in turn suggested that eutrophication of the lake during the last 25 years 
had not (as has been recorded for some other lakes) so enriched the bottom 
deposits that an immediate improvement in water quality could not be 
effected by regulation of the sources of enrichment. Six years later, a 
similar experiment using the same tube did not cause such a marked 
diminution of the algal populations as in 1971, nor was the composition of 
the phytoplankton the same. These differences may be caused by the 
enrichment of the deposits in this tube as a result of the fertilizers added in 
experiments during the years 1972-1976. If so, a similar condition may 
arise in the lake itself if enrichment from farming and sewage (Lund 1971, 
1978) continues, as indeed it may well do. It is also possible that this 
change, in part at least, was the result of the growth of algae derived from 
the previous year's phytoplankton. In that year fertilizers were added to the 
tube and both vegetative cells and spores of the plankters which then were 
abundant could have sedimented on to the bottom deposits and so formed 
an inoculum for the 1977 phytoplankton. 
The fertilization experiments were carried out at all times of year. 
Laboratory bioassay was carried out during much of the period, using the 
technique of Lund et al. (1971) and the results for the lake water have been 
published (Lund et al. 1975). Before an experiment started, the tubes were 
opened by deflating some of the compartments of the inflation collar so that 
DIRECTOR'S REPORT 35 
the tube water would equilibrate with that of the lake or, in summer, with 
the lake's epilimnion. Each experiment lasted several months, though it 
might include more than one fertilization. Only a few general descriptions 
or comments on some of the experiments carried out so far can be given 
here. 
Apart from winter, when physical conditions control algal growth, a 
single addition of phosphate will produce an increase in the phytoplankton, 
suggesting that phosphorus is the major element limiting algal growth in 
Blelham Tarn. For diatoms, as expected, silicon has to be added at certain 
times. The anaerobic part of the hypolimnion is enriched with both 
elements but the amount of regenerated silicate is, in total, small compared 
to that added by the inflows to an equal volume of lake water. Phosphate 
returns in variable amount from the bottom deposits each autumn, even in 
the absence of fertilization, but decreases during winter. The phosphate 
concentration in the lake water will remain similar or increase during 
winter. It may be that much of the recycled phosphate in the tubes is 
attached to ferruginous particles, because the total iron concentration also 
falls during winter. 
Laboratory bioassay using Asterionella showed that for four years it was 
necessary to add only sodium silicate, potassium dihydrogen phosphate and 
the ferric complex of ethylenediaminetetraacetic acid (EDTA), separately 
or combined, in order to promote good growth of the diatom at times when 
it would not grow in unenriched lake water. Experiments in the tubes 
support the view that silicon and phosphorus are two of the major limiting 
factors for the growth of this and other diatoms. Experiments with adding 
iron to the tubes as the ferric complex of EDTA have given somewhat 
contradictory results. 
The algae which have become abundant when tubes were fertilized have 
included most of the major plankters of the tarn. However, a few of the 
important plankters in the lake have never grown well in the tubes, the 
most notable examples being the flagellates Ceratium hirundinella O.F.M. 
and Mallomonas caudata Iwanoff. It can be very striking to have three 
different but abundant plankton populations in the lake and the two tubes 
at the same time, for example diatoms, green algae and blue-green algae. 
Clearly, in an experiment lasting several months, the effect of 
fertilization will differ qualitatively, if not quantitatively, according to the 
season, the meteorological conditions and the effects of grazing and 
parasitism. A feature of our climate is its wetness, notably in autumn and 
winter when, moreover, there is more inflow from a given amount of 
precipitation because of the lower rate of evapotranspiration. One effect of 
a drier winter is suggested by analysis of the data on the start of the spring 
diatom bloom during the last 33 years (Lund 1978). This effect can be 
carried in a tube to the extreme condition of the waterbody receiving only 
direct precipitation. When a tube was fertilized in autumn with phosphate, 
36 FORTY-SIXTH ANNUAL REPORT 
silicate and the ferric complex of EDTA, the normal 'vernal' diatom 
maximum was reached in February when the vernal increase in the lake 
outside was only just beginning (Fig. 2). This result shows how the winter 
minimum of phytoplankton is determined by outflow as well as by light. 
Another change from the lake's spring bloom was produced by adding 
silicate, phosphate in late summer and further phosphate in autumn. 
Adding silicate alone in August had no effect but when phosphate was 
added in early September, a diatom bloom arose, together with one of blue-
green algae (Fig. 3). The further autumnal addition of phosphate had little 
effect because of fungal parasitism, grazing and short winter days. 
However, in the succeeding spring a very large population of the blue-green 
alga Oscillatoria agardhii Gom. var. isothrix Skuja arose to form the spring 
maximum. This waterbloom in the tube could be seen from far away as a 
greenish disc on the surface of the water. Further details of these 
experiments are given by Lund (1978). 
The single addition of a nutrient at a given moment is not a natural 
event. In nature, nutrients are entering the water at all times to a greater or 
lesser extent and being regenerated within the waterbody. In 1975, from 
spring to early winter, nitrogen as nitrate plus ammonium nitrogen, 
phosphorus as phosphate and silicon as sodium silicate, were added to one 
tube in amounts corresponding to those brought into the lake each week by 
its inflows, as judged from weekly analyses. The same additions were made 
to the other tube plus an extra addition of phosphate, that is it received 
twice as much phosphorus. Neither tube produced larger crops, expressed 
on a chlorophyll a basis, than the lake, after due allowance had been made 
for losses of lake plankton by outflow. However, this would not be equally 
true if, for example, carbon was used as a measure of phytoplankton 
abundance since the carbon to chlorophyll a ratio is not the same in all 
algae (e.g. for blue-green or green algae). The populations in the tubes also 
showed marked qualitative differences from those in the lake. The tube 
receiving double the amount of phosphorus was also fertilized once in early 
September with the ferric complex of EDTA, after which, until late 
October, it had a population producing waterblooms and dominated by 
Microcystis. It is interesting to note that Dr C. S. Reynolds carried out an 
experiment in 1977 based on weekly additions of nitrogen, phosphorus and 
silicon. These fertilizations were designed not just to simulate the weekly 
supplies of nutrients to the lake but to replenish the nutrient contents in a 
tube, when necessary, so that their concentrations reached certain levels. 
The results of his experiment show considerable quantitative and 
qualitative differences from those of 1975. In both the 1975 and 1977 
FIG. 2. Planktonic diatom populations in a tube fertilized in September 1972 with silicate, 
phosphate and the ferric complex of EDTA compared with that in the lake water outside. 
Vertical axis: cells per ml, logarithmic scale. 
FIG. 3. Phytoplankton of Blelham Tarn (Bl) and of a tube (Tu) in the first half of 1974. 
The tube fertilized with silicate and phosphate, August, September and November 1973. 
Vertical axis: Chlorophyll a ug 1_1, without allowance for phaeophytin; blue-green algae, 
filaments, mm (100 ul)_1 and diatoms, cells (5 ul)-1 in the 0-5 m water column. B, blue-
green algae; C, chlorophyll a; D, diatoms. 
DIRECTOR'S REPORT 37 
38 FORTY-SIXTH ANNUAL REPORT 
treatments natural planktonic successions took place. Both experiments in 
which additions of fertilizers are made at short intervals (e.g. weekly) and 
those in which a single addition of one or more fertilizers is made are 
valuable. Each experimental system has its advantages and disadvantages. 
Apart from experiments done and observations made by other FBA 
groups (Jones, 1973, 1975, 1976; Smyly 1976; Willoughby 1974), we have 
cooperated in two major investigations which have been carried out by the 
Scottish Marine Biological Association and the Department of Biological 
Sciences of Dundee University on the kinetics of nutrient depletion of a 
natural phytoplankton and on the uptake and transformations of nitrate 
nitrogen (using 15N as a tracer) respectively. The latter study in particular 
illustrates the advantage of working with a hydrologically closed system in 
which the fate of planktonic production and transformations of matter can 
be followed. Such a system is specially useful also for studies on the 
sedimentation of live or dead algae, their incorporation into the bottom 
deposits and possible resuspension therefrom. 
The experiments described above are only a few of those that have been 
carried out and the detailed quantitative significance of many of them have 
yet to be fully analysed and interpreted. 
Though the value of such tubes is clear, in our work, as is almost 
invariably the case, a new approach to studying a problem, in this case the 
ecology of the phytoplankton, has uncovered others or exposed deficiencies 
in the work up to now. An example has been mentioned, namely the 
importance of studying the physical, chemical and biological 
interrelationships between shallow and deep water regions. The present 
location of both tubes in deep water makes the tube 'lakes' too different 
from Blelham Tarn as a whole. The results of the work of Dr Reynolds's 
team with an additional tube enclosing both shallow and relatively deep 
water will be of great interest. 
Many members of our Windermere laboratory have taken part in the 
development and use of the tubes. The National Trust kindly permitted us 
to put them in its lake. Atlantic Rubber Ltd., Altrincham, Cheshire 
constructed these and the other tubes mentioned in this article. We are 
grateful to the Department of the Environment for financial support. 
REFERENCES 
Anon. (1977a). 'Lund' tubes in Farmoor Reservoir. Water Research 
Centre. Notes Wat. Res. No. 11, 4 pp. 
Anon. (1977b). Raising 'Lund' tubes at Farmoor. Water Research Centre. 
Water Research Newsheet, Aug. p. 4. 
Booij, J. G. & Vlugt, J. C. v. d. (In press). Technical aspects of large 
model reservoirs situated in a storage reservoir in the Netherlands. 
J.Am. Wat. Wks Ass. 
DIRECTOR'S REPORT 39 
Jones, J. G, (1973). Studies on freshwater bacteria; the effect of enclosure 
in large experimental tubes. J. appl. Bad. 36, 445-56. 
Jones, J. G. (1975). Some observations on the occurrence of the iron 
bacterium Leptothrix ochracea in fresh water, including reference to 
large experimental enclosures. J. appl. Bact. 39, 63-72. 
Jones, J. G. (1976). The microbiology and decomposition of seston in 
open water and experimental enclosures in a productive lake. 
J. Ecol. 64, 241-78. 
Lack, T. J. & Lund, J. W. G. (1974). Observations and experiments on 
the phytoplankton of Blelham Tarn, English Lake District. 1. The 
experimental data. Freshwat. Biol. 4, 399-415. 
Lund, J. W. G. (1971). Eutrophication. Proc. R. Soc (B), 180, 371-82. 
Lund, J. W. G. (1972). Preliminary observations on the use of large 
experimental tubes in lakes. Verh. int. Verein. theor. Angew. Limnol. 
18, 71-77. 
Lund, J. W. G. (1975). The uses of large experimental tubes in lakes. In 
The effects of storage on water quality, 291-312. Medmenham. 
Water Research Centre. 
Lund, J. W. G. (1978). Changes in the phytoplankton of an English lake, 
1945-1977. Gidrobiol. Zh., Kiev, 14, 10-27 (in Russian). 
Lund, J. W. G., Jaworski, G. H. M. & Bucka, H. (1971). A technique 
for bioassay of freshwater with special reference to algal ecology. 
Acta hydrobiol., Krakow, 13, 235-49. 
Lund, J. W. G., Jaworski, G. H. M. & Butterwick, C. (1975)-. Algal 
bioassay of water from Blelham Tarn, English Lake District. 
Arch. Hydrobiol. (Suppl.), 49, 49-69. 
Nature Conservancy Council (1976). Second Report: 1 April 1975-
31 March 1976. London, HMSO. 
Smyly, W. J. P. (1976). Some effects of enclosure on the zooplankton in a 
small lake. Freshwat. Biol. 6, 241-51. 
Vlugt, J. C. v. d. (1976). Comparative limnological research in 'De Grote 
Rug' and model reservoirs. Hydrobiol. Bull. 10, 136-46. 
Willoughby, L. G. (1974). The ecology of lower freshwater phycomycetes 
in the tube experiment at Blelham Tarn. Verdff. Inst. Meeresforsch. 
Bremerh. Suppl. 5, 175-95. 


Experiments with lake phytoplankton in large enclosures

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Experiments with lake phytoplankton in large enclosures

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