Limnetic crustaceans (i.e. those which live in the open water of lakes and ponds) were investigated in Esthwaite water and adjacent lakes. These are represented by two groups, Copepoda and Cladocera

Smyly, J.P.

Aquatic Commons

70 FORTY-SIXTH ANNUAL REPORT 
LIMNETIC CRUSTACEANS IN ESTHWAITE WATER AND ADJACENT 
LAKES 
W. J. P. SMYLY 
The word 'limnetic' refers to a community of organisms, mostly 
microscopic in size, which live in the open water of lakes and ponds. In this 
community, the crustaceans are represented by two groups, Copepoda and 
Cladocera. These animals feed on other planktonic organisms and are 
themselves eaten by both fish and invertebrate predators. 
Research on zooplankton in the Windermere laboratory over the last 
twenty years has centred mainly on discovering and quantifying the factors 
which regulate population abundance. The problem has been tackled (i) by 
comparing events in different lakes, (2) by laboratory experiments and (3) 
by field experiments, e.g. in enclosures such as the Lund tubes. 
Evidence about annual changes in Esthwaite Water comes from two 
main sources. Goulden (1964) studied the abundance of cladoceran 
remains in the bottom sediments and found evidence that cladoceran 
populations change little over hundreds of years, and changes that do occur 
appear to be slow gradual shifts in relative abundance of two species 
within the same genus, probably in response to changing environmental 
conditions. Recent work begun in 1956 (Smyly 1972) provides evidence of 
more rapid cyclical changes which may temporarily reverse some more 
general trend. Trends in mean annual abundance of the two species of 
cyclopoid copepod in this lake show (Fig. 1) that the dominant species, 
Mesocyclops leuckarti (Claus), has been declining in abundance, while the 
DIRECTOR'S REPORT 71 
scarcer of the two species, Cyclops abyssorum Sars, began in 1963 a 
recovery which persisted subsequently for two or three years, before 
declining again. Because C. abyssorum is found mostly in deep cold lakes in 
the world at large and also in most lakes of the English Lake District, and 
M. leuckarti tends to be found in the less deep and less cold lakes both here 
and elsewhere, Smyly (1968) suggested that C. abyssorum was an 
indigenous species and M. leuckarti an invader from the south penetrating 
northwards with amelioration of climate. Recent work however provides 
little evidence to support the hypothesis that M. leuckarti is a successful 
competitor in process of replacing C. abyssorum. On the contrary, under 
certain conditions, C. abyssorum seems well-adapted to maintain a 
superiority, e.g. in Rydal Water, and to recover lost ground, as in Esthwaite 
Water. We seem to have here a delicate balance between two adaptable 
species with similarities in life-cycle but also differences, e.g. in body-size, 
periodicity and vertical distribution, all strategies for successful 
coexistence. The question has to be asked therefore why they co-occur in 
only three out of eighteen lakes. One possible explanation lies in the 
carnivorous habits of the copepodid instars of both species. Two advantages 
are gained by switching from a herbivorous to a carnivorous diet: more 
efficient conversion of energy from food to body-tissue is possible, and the 
opportunity arises to prey on herbivorous instars of the slower-growing 
species. Success could be associated with water-temperature and its 
influence on growth-rate, cooler water favouring C. abyssorum and warmer 
water favouring M. leuckarti. According to this hypothesis, short-term 
fluctuations are to be expected, while in the long run one species should 
predominate and eventually exclude the other. 
One feature common to almost all limnetic crustaceans is the possession 
of a resting stage. In several species a resting stage is obligatory, i.e. the 
resting stage is essential to the continued existence of that species; in others 
it is facultative, i.e. the species can produce a resting stage if necessary but 
does not always do so, or may regularly produce a resting stage but not be 
dependent on it. Daphnia for example produces resting eggs in spring and 
early winter in Esthwaite Water but is present all through the year, 
whereas in Grasmere it disappears from the limnetic zone from late 
December to early May, when it reappears as small individuals hatched 
from resting eggs laid in the previous year. The two species which have 
been most thoroughly studied, Cyclops abyssorum and Mesocyclops 
leuckarti, also have resting stages but these are in the final copepodid 
instars, chiefly stage V. M. leuckarti leaves the limnetic zone altogether in 
winter and spends from October to February dormant in the bottom 
sediments. C. abyssorum never completely deserts the limnetic zone, and in 
some lakes has a resting stage in the bottom sediments from August to 
February. Most such lakes are less than 20 m deep but one, Thirlmere, is 
much deeper (42 m). This exception rules out a direct correlation between a FIG. I . Mean annual totals of Cyclops abyssorum, (a), and Mesocyclops leuckarti, (1), in 
Esthwaite Water from 1956 to 1971. 
72 FORTY-SIXTH ANNUAL REPORT 
resting stage in this species and some factor shared by all the shallower 
lakes. At the present time, we do not know how dormancy in copepods is 
initiated, and not much about how it is maintained and finally ended. Low 
temperatures seem essential for maintenance though there comes a time 
beyond which dormancy cannot be prolonged. We also know that if it is 
ended prematurely, for example by a rise in temperature, dormancy cannot 
be restored by a return to low temperatures. 
Quantitative field sampling estimates the number of individuals present 
at a given time. This amount, the standing stock, is easily measured and 
often used to describe changes in seasonal abundance. It can however be 
misleading, because a low standing stock can arise as well from a high 
recruitment rate linked with a high mortality rate as from low recruitment 
linked to low mortality. To find out how much is actually produced, we 
have to examine separately the causes of increase from those of decrease. 
Research is currently in progress on both aspects. Smyly (1970, 1973) 
found that the growth-rate of cyclopoid copepods is affected by both 
amount and kind of food, and clutch-size is affected by food, age and body-
size. 
It is sometimes assumed that animals invariably lay eggs up to the limits 
of their capacity: this was not true of the limnetic copepod Cyclops 
abyssorum in Buttermere, where clutch-size was limited by food to about 
half of the maximal number of eggs possible. The volume of the thoracic 
region, in which the ovaries lie, was calculated from measurements of 
length, width and breadth of many adult females. The volume of the egg 
was calculated from the diameter of the, egg, allowance being made for 
swelling of the egg after extrusion from the ovary, and estimates were made 
of clutch-size. Then total yolk volume was calculated and this volume was 
expressed as a percentage of the thoracic volume. This percentage lay 
mostly between 14% and 17%, occasionally as high as 20%, in Esthwaite 
Water, a productive lake, but never exceeded 8% in Buttermere, an 
unproductive lake. If these calculations were correct, the animals in 
Buttermere were laying only about half the number of eggs which the size 
of their thorax would allow. They could have laid more but smaller eggs, 
but measurements showed that the eggs were all similar in size, regardless 
of seasons or lake of origin. To test both the accuracy of the calculations 
and the deduction that clutch-sizes of Buttermere females could be doubled, 
experiments were made by feeding these females artificially on a diet of 
newly hatched brine-shrimp nauplii, an outstandingly good food for adult 
cyclopoid copepods (Smyly 1970). On this diet, adult females from 
Buttermere increased their mean clutch-size from 12.6 to 29.8 eggs per 
clutch, and the total yolk volume expressed as a percentage of thoracic 
volume increased from 6% in the lake to 14% in the laboratory. 
Results from lake samples show that stocks and fecundity are often 
poorly correlated. Size of stock often seems to depend more on survival of 
DIRECTOR'S REPORT 73 
young after hatching than on numbers of eggs laid. Predation intensity 
varies directly with prey density, and increased fecundity may be offset by 
increased predation. Lack of food through competition with other species is 
another cause of mortality. In populations of cyclopoid copepods it is 
difficult, however, to separate the effects of mortality caused by food-
limited competition from those caused by predation, because additional 
food not only enhances directly the chances of survival of young stages but 
simultaneously reduces cannibalism by providing an alternative source of 
food. Competition is better studied between non-predatory species, 
especially under experimental conditions. Laboratory experiments are 
however often unsatisfactory; field experiments, using artificially 
manipulated populations in enclosures, offer an attractive alternative 
possibility. Observations over four years in Blelham Tarn show that, in 
enclosures holding 18 000 m3 of water, population densities of Daphnia 
hyalina were essentially similar inside and outside the enclosures in each 
year, despite the very small numbers of fish during the first two years and 
of predatory larvae of the midge Chaoborus throughout the period. In 
contrast, the calanoid copepod Diaptomus gracilis has been more abundant 
inside than outside the enclosures in all four years. There was little annual 
variation in clutch-size of either Daphnia or Diaptomus, so it seems that 
adults of both species found as much food inside the enclosures as outside. 
The implication is that rate of survival after hatching was higher for 
Diaptomus than for Daphnia inside the enclosures. Further experiments 
are planned to try and elucidate the reasons for these differences. 
I wish to thank all those who have helped in this work, in particular Mr 
T. I. Furnass, Mr J. O. Lloyd, Mrs J. Sargeaunt, Mr D. Lowson and Mr R. 
Shorrocks. 
REFERENCES 
Goulden, C. E. (1964). The history of the cladoceran fauna of Esthwaite 
Water (England) and its limnological significance. Arch. Hydrobiol. 
60, 1-52. 
Smyly, W. J. P. (1968). Observations on the planktonic and profundal 
Crustacea of the lakes of the English Lake District. J. Anim. Ecol. 
37, 693-708. 
Smyly, W. J. P. (1970). Observations on rate of development, longevity 
and fecundity of Acanthocyclops viridis (Jurine) (Copepoda, 
Cyclopoida) in relation to type of prey. Crustaceana, 18, 21-36. 
Smyly, W. J. P. (1972). The crustacean zooplankton, past and present, in 
Esthwaite Water. Verh. int. Verein. theor. angew. Limnol. 18, 
320-26. 
Smyly, W. J. P. (1973). Clutch-size in the freshwater cyclopoid copepod, 
Cyclops strenuus abyssorum Sars in relation to thoracic volume and 
food. J. nat. Hist. 7, 545-49. 


Limnetic crustaceans in Esthwaite water and adjacent lakes

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Limnetic crustaceans in Esthwaite water and adjacent lakes

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