Population dynamics and production of Acanthocyclops robustus ...

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Population dynamics and production ofAcanthocyclopsrobustus(Sars) andMesocyclops leuckarti (Claus) in Tjeukemeer. J. Vijverberg & A. F. Richter.
Population dynamics and production ofAcanthocyclopsrobustus(Sars) andMesocyclops leuckarti (Claus) in Tjeukemeer J . Vijverberg & A . F . Richter Limnological Institute, Tjeukemeer Laboratory, De Akkers 47, 8536 VD Oosterzee, The Netherlands

Keywords: population dynamics, secondary production, cyclopoid copepods, simulation

Abstract The population dynamics and production ecology of the two dominant copepod species, Acanthocyclops robustus and Mesocyclops leuckarti, in Tjeukemeer (The Netherlands) were studied for three successive

years . Since copopods in Tjeukemeer show continuous recruitment, a population dynamics model `INSTAR' was developed and used to integrate field data on population density, population structure and fecundity and laboratory data on development rates and length-weight relationships . The pattern of size specific mortality indicated that both invertebrate and vertebrate predation were important in the regulation of population numbers .

Introduction This paper presents part of the results of a study on the population dynamics and production of copepods and cladocerans in Tjeukemeer, a shallow, eutrophic reservoir in the north of the Netherlands . Previous papers were concerned with length-weight relationships of copepods and cladocerans (Vijverberg & Frank 1976), the population structure and population densities of copepods (Vijverberg 1977), the effect of temperature in laboratory studies on growth and development of copepods and cladocerans (Vijverberg 1980) and the population dynamics and production of Daphnia spp . (Vijverberg & Richter 1982) . The work was carried out within the framework of an IBP project which also included studies on higher and lower trophic levels (Beattie et al. 1971) . The copepods represent about 20% of the mean annual zooplankton biomass . They dominate the cladocerans in winter and early spring and often during autumn too, although they reach their highest densities during late spring and summer . The lake is inhabited by a large population of plankti-

vorous fish, mainly by the young (0+) of various species, and older bream (van Densen 1978 ; Goldspink 1978 ; Lammens 1982) . The study deals with two copepod species which are abundant particularly during the growing season (May-September) : Acanthocyclops robustus (Sars), the dominant copepod species in the lake, and Mesocyclops leuckarti (Claus) (subdominant) . The studies on two other copepod species, Eurytemora affinis (Poppe) and Cyclops vicinus (Uljanin), both of which are abundant during spring only, will be presented in a separate paper. In order to analyse the factors regulating the production of the two former species, their population dynamics were studied and, whenever possible, related to the environmental factors . Since copepods in Tjeukemeer show continuous recruitment, just as cladocerans do, a population dynamics model was used to integrate field data on population densities, population structure and fecundity and laboratory data on development rates and length-weight relationships .

Hydrobiologia 95, 261-274 (1982) . 0018-8158/82/0953-0274/$02 .80 . © Dr W . Junk Publishers, The Hague, Printed in The Netherlands .



262 Study area Tjeukemeer is a shallow (mean depth = 1 .5 m), eutrophic, freshwater lake with a surface area of about 21 km 2 . The littoral zone is poorly developed; the index of shoreline development (D,), calculated according to Hutchinson (1957) is 1 .25 . For a detailed description of the lake, its reservoir function and the position of the sampling stations see Vijverberg (1977) and de Nie et al. (1980) .

Material and methods Field data

For three years (1969, 1970 and 1971), the lake was sampled using a 5 1 Friedinger sampler at fixed stations at weekly intervals, except during ice cover. This resulted in 130 collecting dates . At each station two samples were taken, one just below the surface and the other just above the bottom, and these were concentrated using a 80 µm mesh plankton gauze . Population densities, relative abundance of instars, mean size of instars, sex ratio and fecundity were measured (Vijverberg 1977) . The copepodites were measured under the microscope from the anterior end of the cephalothorax to the tip of the furcal ramus . We did not discriminate between the different naupliar instars . Initially the nauplii of the four copepod species were counted together ; this gave a total naupliar density . Later, the naupliar density of each species was estimated from the relative abundance of the first copepodite instar of that particular species . Density for each day was computed by linear interpolation between sampling dates . Water temperatures measured daily showed a pronounced seasonality . The highest temperatures, 23-24 'C, were recorded during July and August . There was an ice cover during winter lasting 1-3 months . The monthly water temperatures, expressed as day-degrees above 10 'C, are given by Vijverberg & Richter (1982) in Table 1 . Laboratory estimates

The durations of egg development, and combined naupliar and combined copepodite instars in relation to temperature were measured in the

laboratory under semi-natural conditions . The methods, culture conditions and the results are given by Vijverberg (1980) . Additional culture experiments were carried out at 17 .5 ° C to assess the relative duration of the separate copepodite instars . W e took readings every 12 h and the culture conditions were the same as in the previous experiments . For calculating individual biomass, the lengthweight relationship for the cyclopoid copepods of Tjeukemeer (Vijverberg & Frank 1976) was used . (1) W = 1 .74 Lt .68 where W = weight as gg ind .-1 (organic matter minus chitine) L = body length (mm) . Population dynamics model

The technical aspects of the 'INSTAR' model were developed by one of us (A .F .R .) in co-operation with Dr . P . Hogeweg(The Netherlands) . It is a discrete event-oriented model designed to stimulate reproduction, individual growth by moulting and density changes per instar owing to birth, death or growth . The structure of the model together with some of its most important properties are summarized by Vijverberg & Richter (1982) . A more detailed account is given by Hogeweg & Richter (1982) . The model used here for the calculation of copepod birth rate, mortality rate, production and productivity is essentially the one used to study the population dynamics and production of Daphnia spp . (Vijverberg & Richter 1982) . Some minor changes were introduced to adapt the model to the life cycle events of an individual copepod (Fig . 1) . Output was organized per instar or size class (Table 1), but individuals in the model system kept acting as independent entities . The durations of the separate copepodite instars were computed from the lake temperatures, the relation between temperature and the duration of the combined copepodite instars, and from the relative duration of each specific copepodite instar at 17 .5 'C . Growth in length per moult was estimated from the mean length of the instars in the field population at that time . Somatic weight of copepodite instars is calculated using Eq . (1) . On the basis of the results of Lair (1978), we estimated the weight per egg to be 16% of

263 stars (Miller et al. 1977) . Weight increments were assessed after each moult and were used for the calculation of the standing crop biomass (B) and the biomass production per unit of time (P) . The animal becomes sub-adult in the 5th copepodite instar (CV) . Fifty per cent of the CIV instars develop into females . The number of females, percentage of females with brood and the brood size were derived from field observations (Vijverberg 1977) . Egg durations in relation to temperature and the time lapse between broods were derived from laboratory data (Vijverberg 1980) . This information is used for the estimation of standing crop biomass (B) and gonad production (P g ) . Gonad production is added to somatic production to give the total biomass production (P) . In the simulation model, mortality occurs twice a day . For each instar class the number of copepods in the simulation is compared with those in the field at the same time, the difference being mortality . This approach is similar to that of Argentesi et al. (1974) and Bernardi & Di Cola (1976), but their models are not based on the individual life cycle of zooplankton species and do not take into account the egg mortality caused by the death of ovigerous females .

BIRTH

s

Y

A DU LT? GENERATE NEWBORN $ WAIT UNTIL EGGS APPEAR CHECK D1APAUSE LL CALL . DUR. INSTAR

ADJUST L,PB

YES

YES

ADULT')

Y

CALC LONG

CALC. EGGS PB

0 NO

AGE) LO 0

Results and discussion

Fig. 1 . Simplified flow diagram of the life cycle of an individual copepod in the simulation model `INSTAR' . For explanations see text .

Duration of copepodite instars

The instar durations of A . robust us were shorter than those of M. leuckarti (Table 2) . Males developed faster than females, although the time difference was only small in the case of M. leuckarti. The duration of the A . robustus CV (9s) instar was significantly longer, compared with CV (as) and younger copepodite instars . Such a long duration

the weight of the CI ; the mean weight of the naupliar instars I-IV was assumed to be equal to the egg weight . The weight of the 5th and last naupliar instar was taken as 50% of the weight of the first copepodite instar(Burgis 1975), and its duration as 20% of the duration of the combined naupliar in-

Table 1 . Mean length (L, mm) with S .D . of naupliar and copepodite size classes . Number of observations in parentheses . Size class no . Instar A . robustus

M. leuckarti

1969 1970 1971 1969 1970 1971

1 Nauplia 0 .2 0 .2 0 .2 0 .2 0 .2 0 .2

2 Cl 0 .39 0 .42 0 .44 0 .41 0 .41 0 .41

± 0.05 (12) ± 0.04 (14) ± 0.04 (13) ± 0.01 (12) ±0 .02 (13) ± 0 .01 (14)

3 CII & CIII 0 .52 ± 0 .55 ± 0 .59 ± 0 .50 ± 0.53 ± 0.50 ±

0 .08 (24) 0 .06 (28) 0 .08 (26) 0 .04 (24) 0 .05 (26) 0 .04 (28)

4 CIV & CV 0 .77 ± 0 .80 ± 0 .80 ± 0 .71 ± 0 .72 ± 0 .70 ±

0 .13 0 .13 0 .13 0 .08 0 .07 0 .07

(36) (42) (39) (36) (39) (42)

5

6 Q (CVI)

a (CVI) 0 .82 0 .86 0 .87 0 .76 0 .76 0 .72

± ± ± ± ± ±

0 .10 0 .08 0 .04 0 .02 0 .03 0 .04

(12) (14) (13) (12) (13) (14)

1 .12 1 .22 1 .24 0 .92 0 .94 0 .92

± ± ± ± ± ±

0 .11 (12) 0.12 (14) 0.08 (13) 0.02 (12) 0 .02 (13) 0.04 (14)



264 Table 2 . Duration in days of copepodite instars of A . robustus and M. leuckarti at 17 .5 °C, as a percentage of the duration of the combined copepodite instars (CI-CV) . Mean ± 95% CL . Number of observations in parentheses . Species

Instar no.

Duration

chronal development (Miller et al. 1977) . Isochronal development as observed here for M . leuckarti is exceptional for cyclopoid copepods . To date it had only been known for a few calanoid copepod species, mainly from marine habitats (Katona 1971 ; Johnson & Miller 1974 ; Corkett & McLaren 1978) .

(%)

(days)

Population dynamics A . robustus

M. leuckarti

CI CII

1 .2 ± 0 .13 (16) 1 .0±0 .06(16)

23 20

22 18

CIII CIV CV-6s CV-qs CI CII CIII CIV CV-6s

0 .9±0 .17(15) 0 .8 ± 0 .12 (16) 1 .2 ± 0 .20 (7) 1 .6 ± 0 .20 (9) 1 .6 ± 0 .11 (15) 1 .2 ± 0 .13 (15) 1 .5 ± 0 .25 (14) 1 .5 ±0 .25 (10) 1 .4 ± 0 .30 (9)

18 16 23

16 15

CV-Ys

1 .6 ± 0 .23 (10)

29 22 16 20 20

22 17 21 21 19

22

of CV and/ or CIV has often been observed in copepods(Eckstein 1964 ; Ivanova 1973 ; Munro 1974; Jacobs & Bouwhuis 1979 ; Jamieson 1980 ; Vidal 1980) . In M. leuckarti the copepodite instar durations are nearly constant . An exception is perhaps the relatively short duration of the CII, although differences were only significant when compared with the durations of the CI and CV (vs) . A moulting pattern in which each moult to moult phase is completed in a nearly constant time is called iso-

The changes in the population densities and population structure were simulated in three different ways: Simulation 1 . Each simulation of the population

development covered 8-9 months of field data . Thus, three simulations were needed for the three sampling years . The simulation of the A . robustus population development was started as soon as the field population reached a density of 2 .0 copepodites 1-1 . The population growth of M. leuckarti was initiated as soon as the diapause of the resting stage CV was broken, which was usually in March . Mortality is always assumed to be _>-0 . Simulation 2 . The same as Sim . 1, but mortality of

the nauplii could also be 50% copepod-1 days, occurred during the summer of 1970, due mainly to a relatively high percentage of ovigerous females in the population (Vijverberg 1977) . The seasonal pattern in the birth rates of M. leuckarti differed from that of A . robustus(Fig . 5a) . The highest rates were generally observed during July-August, except in 1971, when slightly higher rates were observed in April . The variations in birth rate reflect the seasonal changes in fecundity and water temperature . Fecundity showed two annual maxima, in April and during July-September, the first being somewhat higher than the second (Vijverberg 1977) . As water temperatures were highest during June-August, the birth rate maxima were reached usually during July-August . A somewhat different pattern observed in 1971 can be explained by the higher water temperatures in April 1971 than in April 1970 .

26 7

M . leuckart i birth rate

100 80 60 40 to

w C

20

u I0) 0 .

120 mortality rate

100 80 60 40 20

21314 51 61 71 81 9110 11 12 1121314

1969

516

71 81 91101fl112 11213

1970

4

51 61 71 81 91101 11 112 1971

Fig. 5 . Seasonal and annual variations in mean daily birth rates (b, %) per week and mean daily mortality rates (d, %) per week of M. leuckarti (Sim . 2),

The mortality rate is defined here as the percentage of copepodites (CI-CVI) dying per copepodite per day . The seasonal pattern was very similar for both species during the three years, although A . robustus reached noticeably higher maximum rates (60-120%) than M. leuckarti (40-70%) . The highest rates were usually observed during July-August, when predation pressure exercised by fish was most severe . The relatively low mortality rate in the case of M. leuckarti compared with those of A . robustus during autumn and winter is an artefact . Since in September M . leuckarti went into diapause in the bottom substrate, and thus the data on the densities of the resting stages were lacking, we assumed zero mortality for diapausing individuals . However, in this phase they were predated by the benthivorous bream, although the predation pressure was relatively mild (Lammens, pers . comm .) . Our results on birth and mortality rates are not

fully comparable with most of those reported in the literature . Firstly, we underestimated the number of females with eggs and thus the birth rate too, due to sampling errors ; secondly nearly all previous authors used Edmondson's formula (Edmondson 1960, 1974) to compute the birth and death rates, which according to Seitz (1979) and Threlkeld (1979) introduces a serious bias in the birth rate estimates, and thus also in the mortality rate estimates . This error is caused because of egg mortality as a result of mortality of adult ovigerous females . This error is particularly significant when positive size-selective predation by planktivorous fish is an important cause of death, and can therefore, lead to a considerable overestimation of the birth rate . According to Threlkeld (1979), this may cause an overestimation of the birth rate by about 50% for Daphnia . The death and birth rates found by us are very



268 Table 3. Daily means of size-specific mortality rates (d,%) of A . robustus in Tjeukemeer in different months during 1969, 1970

and 1971 (Sim . 2) . Size classes as in Table 1 . 1969 3 4 March April May June July August September October November December

0 0 8 13 9 2 17 27 33 14 3 3 4 23 13 9 12 8

1970 5

6

3 4

26 0 0 9 4 33 6 32 23 105 11 75 14 39 10 30 22 48

15 0 18 30 19 7 20 24 38 15 5 4 6 29 5 8 0 0

1971 5

6

3 4

14 12 4 13 5 64 7 29 19 60 11 93 12 15 25 103 15 25

0 6 4 29 8 12 3 44 9 35 1 39 3 4 18 7 14 0 3

5

6

31 20 2 9 10 28 9 51 11 117 9 81 15 48 10 18 13 28 1 0

Table 4 . Daily means of size-specific mortality rates (d, %) of M. leuckarti in Tjeukemeer during different months in the years

1969, 1970 and 1971 (Sim . 2) . Size classes as in Table 1 . 1969 3 4 5 March April May June July August September October November December

1971

1970 6

- - 0 0 19 100 2 3 7 21 11 4 28 20 20 14 16 15 40 1 23 26 9 7 69 12 7 27 1 34 - 0 - 0 -

3 4

5

6

0 0 6 10 11 12 15 6 13 53 6 13 10 25 49 14 1 82 0 1 140 - 0 - 0

14 13 6 10 24 39 40

-

3 4

0 11 6 38 30 21 13

0 0 12 3 5 6 9 1 0 0

5

6

64 230 1 0 15 10 9 19 14 30 18 18 42 62 -

similar to the those reported by Cummins et al. (1969) for Diaptomus siciloides and Cyclops vernaIisintheeutrophicSanctuaryLake(Penn ., U .S .A.). They also compare well with the rates given by Burgis (1971) for Thermocyclops hyalinus in the tropical and eutrophic Lake George (Uganda) . The mortality rates are also similar to those of Acartia clausii in a temperate lagoon in Wasington D .C ., U .S .A . (Landry 1978) . However, the rates observed by us are much higher than those reported for calanoid copepods in lakes in New Zealand (Chapman 1973 ; Green 1976 ; Burns 1979) . The mortality rates per size group are given in Tables 3-6 . Because of our underestimation of the egg stock, an accurate estimate of the nauplii mortality was not possible . Initial simulations (Sim . 2) to estimate the CI mortality rate showed enormous-

ly high mortality rates, often about 1 000% copepodite 1 day-1 . This seemed unrealistic when compared with the mortality rates of usually