Marine Biology 94, 597-603 (1987) M a r i n e ~ ............... B i o l o g y

�9 Springer-Verlag 1987

Experimental test of an allometric method for estimating potential copepod production

S. M c Clatchie *

Oceanography Department, Dalhousie University; Halifax, Nova Scotia B3H 4J1, Canada

Abstract

Phytoplankton and zooplankton, collected in November 1984 at the deepest point of Bedford Basin, Canada, were used in a 2 mo experiment in the Dalhousie tower tank to test the validity of estimating potential copepod produc- tion from allometric equations based on body size and temperature under food-saturated conditions ("maximum instantaneous growth, MIG" method). The development of the plankton community in the mesocosm was similar to fall and winter assemblages in a coastal Nova Scotian inlet. Predicted instantaneous rates of potential production were compared with average production of a mixed copepod assemblage measured over weekly intervals in the absence of predators. Predictions exceeded measured production by 1.9 to 136 times, but generally agreed within an order of magnitude. Whereas the MIG method facilitates automated estimation of potential copepod pro- duction, this advantage must be offset by the relative loss of accuracy compared to more labour-intensive methods. The method appears to be most useful for mesoscale surveys or neritic regions where the aim is to quickly obtain an order of magnitude estimate of secondary production.

Introduction

The characterization of plankton communities using a biomass size spectrum (Sheldon etal., 1972, 1973; Platt and Denman, 1977, 1978; and others) is a method where- by energy flow in the pelagic system can be quantified without recourse to the species level. The advantage of this approach lies in circumventing the complexity of in-

* Present address: Portobello Marine Laboratory, P.O. Box 8, Portobello, Otago, New Zealand

dividual species-interactions in ecosystems, to gain a broader perspective through the use ofallometric relationships. The allometric approach has been used to predict zooplankton excretion rate (Peters and Rigler, 1973; Ikeda, 1985), respiration rate (Banse, 1979, 1982; Ikeda, 1985), filtration rate (Huntley, 1980; Knoechel and Holtby, 1986) and production (Borgmann, 1982; Huntley and Boyd, 1984), as functions of body mass and temperature. Allometric relationships can be used to generate all the major terms in an energy budget, with varying degrees of accuracy (production = ingestion x assimilation efficiency - respira- tion - excretion). This assumes that assimilation efficiency is constant, and that food concentrations are known.

Attempts have been made to estimate instantaneous rates of potential copepod production in mixed assem- blages using particle size, temperature and food concentra- tion as independent variables in a predictive equation (Conover and Huntley, 1980; Huntley, 1980, 1985; Hunt- ley and Boyd, 1984). If this approach can be validated, potential copepod production could be rapidly and ef- ficiently estimated using automated methods (Boyd, 1985). In the only experimental verification of the method, Huntley (1985) concluded that the "maximum instanta- neous growth" (MIG) method was able to estimate the potential growth rate of Calanus pacificus (i.e., production in the absence of mortality) as accurately as cohort analysis in the range 12 ~ to 22 ~

I used the plankton size-spectrum and temperature data from a 59 d experiment in the Dalhousie University tower-tank mesocosm to compare production estimated by the MIG method with measured production. I tested the hypothesis that, in the absence of food limitation, predic- tions based on the MIG method provide a useful ap- proximation to measured average production. The ex- periment was run under predator-free conditions, since the MIG method does not allow for predation losses or loss due to advection or other causes. Although the mesocosm plankton assemblage is not identical to a natural as- semblage, it is sufficiently similar to natural conditions to

598 S. McClatchie: Copepod production

permit calibration of the technique. The main advantage is that the same plankton populations can be repetitively sampled, eliminating the uncertainties imposed by sam- pling advected populations in the field (Mullin, 1982).

Materials and methods

Tank setup and inoculation

The experiment was conducted over 59 d in the Aquatron tower tank facility. The tank is 10.46 m deep, 3.66 m in diameter and holds 117.1 m 3 of seawater. It was divided vertically into two columns by a polyethylene divider, and was thermally insulated. Lighting was provided by an array of four 1 000 W phosphor-coated metal halide lamps (CGE MV 1 000/C/BUH), timed to produce a 12 h light: 12 h dark photoperiod. Surface-light intensity was 10 to 407o of the midday summer sun [(2.8 to l l . 2 )x 10 -3 W cm-2], with a spectral quality similar to that of sunlight (Balch et at., 1978). A more detailed description of the tower tank is given in Lane and Collins (1985).

The two sides of the divided tower tank were run in parallel, having been treated in exactly the same manner, and having the same starting conditions. Initial conditions were established by running the tower in flow-through mode for three consecutive days, water entering through the supply lines, and overflowing to drain via the scuppers at the top of the tank. Seawater entering the tank was passed through four Graver pressure sand-filters which removed particles larger than 50 #m (Lane and Collins, 1985). After three days, seawater lines were closed and the tank was maintained as a static water column. Water temperature was chilled to 8.7~ before plankton were added.

Zooplankton were collected during the night of 7 November 1984 at the deepest point (60 m) of Bedford Basin, a coastal inlet at the head of Halifax harbour (44 ~ 42.3'N; 63~ Surface-water temperature was 11 ~ at the time of field collections. Zooplankton were collected from two 10 min tows at 50 m depth using a 450 #m mesh net. An additional tow with a 330 #m mesh net was made in similar fashion. Phytoplankton were collected in four 10 rain-duration tows using a 10#m mesh net at 2 to 3 m depth. Before and after transport to the laboratory in insulated containers, ctenophores, medusae and chae- tognaths were removed by gentle screening. This was done to eliminate as many plankton predators as possible. Inoculation of both sides of the divided tower tank was done simultaneously, using a Y-shaped siphon to draw from a well-mixed bucket of plankton. Organisms were left to disperse for 24 h prior to first sampling.

Sampling procedures

Integrated water-column plankton samples were collected at weekly intervals. Phytoplankton were collected using a

l0 ~m mesh ring-net raised by hand frorn the bottom of the tank to the surface. Samples were preserved in acid Lugol's solution for species counts. Zooplankton were collected in similar fashion using a miniature bongo net fitted with 76~tm mesh to capture juveniles as well as adults. Zooplankton samples were recorded by silhouette photography before fixation in 4% buffered Formalin.

Water-column profiles of temperature, salinity and in vivo fluorescence were recorded using a five-point octo- pus sampler (Lane and Collins, 1985) attached to a pump. Water passed through opaque 2.5 cm diam hoses to a Turner Designs fluorometer and a thermosalinograph before being discharged to waste. Resolution of the pump sampler was established using 4 s chlorophyll pulses at both the long and short arms of the intake. The mean lag time (s) for arrival of the signal was calculated from:

delay = Fi" i . A t F~, (1) /=1

where F~ is the fluorescence of the ith digitized point, A t is the digitizing interval (1.26 s) and N is the number of sampling intervals from dye injection to final disap- pearance of the signal (Mackas and Owen, 1982). Delay was used to relate fluorescence, temperature and salinity to vertical location of the sampler in the tower. Smearing of the signal due to velocity shear in the hoses was estimated as the time between upward and downward crossing of ~=0.5Fmax (full-width half-amplitude) (Mackas and Owen, 1982).

Particle-sizing procedures

Phytoplankton samples were settled onto slides using the columns described by Crumpton and Wetzel (1981). Single subsamples were enumerated in their entirety for large cells, and 10 to 20 fields were counted at 500x for small cells. Diatom chains were counted as a unit. The mean equivalent spherical diameter (ESD) for each algal taxon counted was measured using interactive contouring of the cell perimeter on a Leitz Television Analysis System. Total volume concentration for each taxon was converted to carbon concentration using Strathman's equations for diatoms or non-diatoms (Smayda, 1978), and then to dry weight concentration using a carbon to dry weight ratio of 0.37.

Zooplankton samples were separated into log2 size- intervals using a sieve-shaker array with sequential mesh sizes of 500, 355, 250, 180, 125, 90, and 63 #m (Hargrave et al., 1985). Zooplankton collected from the 250, 355 and 500~tm sieves were counted in their entirety. All samples were rinsed in distilled water and dried at 60 ~ for 24 h prior to weighing.

Allometrically derived potential copepod production

Huntley and Boyd (1984) derived an equation to predict maximum instantaneous growth rate (gm~x, h -1) of zoo-

S. McClatchie: Copepod production

plankton as a function of temperature (T, ~ under saturating food concentrations:

gmax =0.00226' exp (0.11 T). (2)

They suggested that this equation could be used to calculate potential production when food concentration exceeded a critical concentration ~ g carbon liter-I):

C c = 1 000 [k . w(m-n)+gmax" W(I'n)]/(a "b), (3)

where W = m g dry wt, and all other variables are a function of temperature, except a (assimilation efficiency), which is set at 0.7. b and n are the intercept and slope used to predict clearance rate as an exponential function of body weight. Similarly, k and m are the intercept and slope of the exponential function predicting respiration rate from body weight (Huntley and Boyd, 1984).

If food concentration exceeds Cc, the instantaneous rate of potential production ( P 1 , mg carbon liter 1 d 1) can be estimated as:

P1 = (0.37) (24) gmax " individuals liter 1 . ug dry wt- individual -1 / 1 000. (4)

Directly measured production

Time-averaged zooplankton production (P2, mg carbon liter -1 d -1) was estimated using pooled dry weight concen- tration from 250, 355 and 500 #m sieves:

P2 = 0.37 (Bi-Bi_l)/(ti-ti-1), (5)

w h e r e Bi is the biomass at time ti, Bi-1 is the biomass at time ti-1 and 0.37 is a carbon:dry weight conversion factor.

Results

Resolution of pump sampler

The delay between the intake and the fluorometer was 40.4 and 39.9 s for the two long arms, compared with 38.1 and 37.9 s for the two short arms of the octopus sampler (Fig. 1). When recording profiles, data were logged for an additional 38 s after the sampler reached the bottom of the profile to ensure the contents of the hose passed the sensors. Mean full-width half-amplitude (FWHA) was 7.26_+0.52 s for a 4 s duration pulse, indicating some smearing of the signal. Assuming a linear relationship between FWHA and pulse duration, FWHA for a very short pulse (1 s) was 1.82 s. The sampler was lowered slowly at 1.67 m min 1. Mackas and Owen (1982) suggest a lowering rate of 1 to 2 m rain-1 in order to define fine- scale vertical features, but were concerned with pumping systems having greater FWHA (minimum FWHA 10.8 s) and consequently lower resolution than I used. Under ideal conditions, a sinusoidal signal can be recovered without distortion if it is sampled at twice the highest frequency (Persaud and Virden, 1984). Resolution of the tower-tank pump system can therefore be estimated as 1.82 (1 min [60 s] -1) (1.67 m min -~) ~ 0.05 m.

599

4 5

40

35

3 0

2 5 .

,~ 20. o

1 0

5 / i I ,

O - i /i 4" 0 1'0 20 30 4 0 5 0 6 0

t ime s

Fig. 1. Calibration curves for pump-driven sampling system. The chlorophyll pulse was injected at 0 s. Difference in delay from long arms (L: continuous lines) and short arms (S: dashed lines) of oc- topus sampler is shown

E =-2

0 1 o! '

6

7 - i

r e l a t i v e f l u o r e s c e n c e

2 3 4 5 i i

/

6 7 8 9 J J =

Fig. 2. Maximum heterogeneity in vertical pigment structure in tower-tank water column

Vertical structure of water column

For most of the experiment, the fluorescence profile was homogeneous, showing only a gradual increase to the bottom of the tank. Near the end of the experiment, small peaks of fluorescence occurred, with a maximum at 4.4 m depth (Fig. 2). Temperatures ranged from 9 ~ to 12 ~ and were slightly warmer (0.2 ~ to 1.5 ~ at the surface than at the bottom of the tank. Salinity ranged from 29.54 to 30%0, increasing by 0.01 to 0.38%o from top to bottom of the tank.

Development of plankton populations

The concentration of food algae in the two water columns showed similar trends over time, but with some indication

600 S.McClatchie: Copepod production

1 0 0

A

8 0 -

6 0

4 0 -

2 0 -

m ^

o _~ B

1 O0 ~ ; . ~

\' S"

2 0 . . . . - . . .

13 ;~0 ~'9 45 52 59 days

Fig. 3. Relative composition of algal taxa > 5/~m equivalent spherical diameter in the two water columns (A and B) of the tower tank over time. s: silicoflagellates; d: Dinophysis sp.; c: Ceratium sp.; p: Prorocentrum sp.; ch: diatom chains; o: others

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60- 222221Ji121222212222222~2;222122222222222225; �9 �9 " . ' , ' . ' ' . " �9 ' �9 . . . . ' . - " " . ' ' " . - ' . "

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0 ? 13 20 27 48 52 60 days

Fig. 4. Relative composition of zooplankton taxa retained by 250/~m sieve in the two water columns (A and B) of the tower tank over time. cf: Calanus finmarchicus," ct: Centropages typicus; p: Pseudocalanus spp; eh: Eurytemora herdmani; h: harpacticoid cope- pods; tl: Temora longicornis; o: others

Table 1. Mean equivalent spherical diameters (ESD) _+ 1 standard deviation (SD) of algal taxa

Taxon Mean ESD (urn) SD

Microflagellates 3-5 - Euglenoid 1 5.9 0.9 Euglenoid 2 7.2 1.0 Nitzschia cIosterium 7.0 1.9 Diatom chains 13.9 11.8 Silicoflagellates 19.8 1.0 Ciliates 30.2 9.1 Prorocentrum sp. 33.2 1.5 Peridinium sp. 33.8 2.8 Dinophysis sp. 44.0 1.8 Ceratium sp. 9 1.1 5.3

that the peak algal abundance was out of phase (see Fig. 6). The size distribution of algae (Table 1) shows that species smaller than 5 #m ESD were present. Microflagel- lates, two species of euglenoid flagellates and Nitzschia closterium were generally near or below this size. These taxa were numerically dominant, but were excluded from calculation of algal food concentrations available to zoo- plankters, on the assumption that they are not captured

efficiently by copepods. This assumption is conservative, because PseudocaIanus spp. is known to filter some algae smaller than 5 ~ m ESD (Poulet and Chanut, 1975). The larger algal taxa were predominant ly diatom chains, dino- flagellates (Prorocentrum, Dinophysis and Ceratium), and the silicoflagellate Distephanus speculum (Fig. 3). Ce- ratium sp. was also not included as food in the calculation, because neritic copepods in Bedford Basin select against this taxon, which is common in the au tumn phytoplankton assemblage (Conover, 1978).

The zooplankton population was almost entirely domi- nated by copepods, although cladocerans and chaetog- naths were occasionally recorded. The copepods were predominant ly Pseudocalanus spp. and Centropages typi- cus (Fig. 4), with smaller numbers of Temora longicornis, Eurytemora herdmani, and a very few Calanus finmarchi- cus. Occasional Tortanus discaudatus and Oithona sp. were also counted. Temporal trends in copepod species com- position were similar in the two sides of the tank (Fig. 4). The total biomass ~ g dry wt 1 --~) over time was higher in Water Column B during the experiment (Fig. 5); this may have arisen from slightly uneven lighting, which resulted in Water Column B receiving more light than Column A.

S. McClatchie: Copepod production 601

,r

L

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o

10- 1.1

8- "

4 i . , - /

I � 9 �9 I

0 2 4 6 8

log2 Bug dry weight liter-1 side B

�9 1~0 1'2

Fig. 5. Comparison of phytoplankton and zooplankton biomass ~ g dry wt 1 -~) in Side A and Side B of the tower tank at each sam- pling date

25000

20000-

I

T 15000-

" 10000-

5000-

0

�9 [] �9

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o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . .

-5000 1'o ~o ~o ~o ~o 60

days

Fig. 6. Algal food concentrations minus critical food concentration (C~, Eq. 3) required for maximum growth rate of copepods, plot- ted against time. Dashed line indicates equality of algal food con- centration and Cc []: Side A; �9 Side B

" 0 >_

._r 03 :3-

J~ cn

" 0

E

40-

30~

20-

10-

o

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Fig. 7. Mean dry weight of individual copepods > 250Mm (~tg in- dividual -1) in Side A ([]) and Side B (m) of tower tank over time

100-

T ? E

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measured mg carbon m -2 d -1

Fig. 8. Comparison of predicted instantaneous production based on maximum instantaneous growth method, and measured aver- age production in Side A (t3) and Side B (=) of tower tank. 1:1 line indicates equality of measured and predicted production. Numbers above points indicate sampling day

Zooplankton production

Table 2. Comparison of instantaneous copepod production predicted from maximum instantaneous growth (MIG) method and measured average production in the two sides of tower tank

Day Predictedinstan- Measured average Predicted/ taneous potential production Measured production (mg C m -2 d -1) (mg C m -2 d -1)

Side A 13 19.2 - 11.9 - 20 24.5 4.5 5.4 27 27.9 2.2 12.7 45 27.2 0.2 136.0 52 31.0 0.8 3.9

Side B 13 33.5 13.1 2.6 20 46.3 9.3 5.0 27 91.8 34.5 2.7 45 32.4 - 14.9 - 52 34.6 18.0 1.9

The concentration of food exceeded the critical con- centration (Eq. 3) except at the end of the experiment (Fig. 6), suggesting that copepods were not food-limited, and were growing at their maximum rate. Consequently, the maximum growth rate could be calculated from Eq. 2. The mean dry weight copepod --1 decreased over time for copepods larger than 250 ~m (Fig. 7).

Direct estimates of average production between sam- piing intervals showed oscillations due to growth and mortality of populations of zooplankton in the tank. Measured average production and predicted instantaneous potential production were in relatively close agreement (Table 2). Predictions derived from the MIG method were greater than measured production by a factor of 1.9 to 12.7 times (Fig. 8), except for Day 45 in Side A of the tank, when predicted production was 136 times greater (Ta- ble 2). This large difference in the average and instan- taneous rates of change arose because the measured average production was close to zero in Side A of the tank on this date.

602 S. McClatchie: Copepod production

Discussion

Although the mesocosm plankton assemblage was not intended to mimic nature, it is reassuring to examine the similarities with Bedford Basin, to illustrate the point that experimental conditions were not unacceptably artificial In the tower tank, Pseudocalanus spp. constituted 42 to 96% of the total zooplankton numbers retained by a 250~m sieve, whereas in Bedford Basin, the proportion was 40 to 60% of zooplankton collected in 233 #m mesh net-tows during mid-November to mid-January (Conover and Mayzaud, 1975). Other copepods in Bedford Basin at this season were Oithona similis (13 to 17%), Acartia clausi hudsonica (3 to 13%), Temora longicornis (2.5 to 16%) and Eurytemora herdmani (0 to 4%) (Conover and Mayzaud, 1975). In the tower tank, E. herdmani (0 to 11%) and Centropages typicus (0 to 41%) were next in abundance to Pseudocalanus spp., Oithona sp., A. cIausi and T. longicor- nis occurred in very low numbers. A bloom of harpacti- coid copepods occurred in the tower tank in the last 10 d of the experiment, which at its maximum abundance, constituted 17 to 35% of the total zooplankton. Harpacti- coids were excluded from copepod production estimates because they are members of the wall epifauna that develops during long-term experiments.

The density of zooplankters > 250#m in the tower tank ranged from 393 m -3 to 10 008 individuals m -3, with concentrations generally increasing during the experiment, These densities compare favorably with densities of 2 000 to 8 000 individuals m -3 recorded by Conover and May- zaud (1975) in Bedford Basin in mid-November to mid- January, The size of copepods in the tower tank (6 to 38 #g dry wt individual 1) was initially heavier than, but overlapped with, the range of 4 to 13 ~g dry wt individ- ual -1 reported by Conover and Mayzaud.

Concentrations of phytoplankton in the tower tank were artificially high, ranging from 510 to 10200Bg carbon liter -~. Mayzaud et al. (1984) reported concentra- tions of particulate organic carbon during mid-November to mid-January in Bedford Basin ranging from 337 to l 216#g carbon liter-1. High phytoplankton concentra- tions in the tower tank ensured that food for grazers was not a limiting factor during the experiment, even when very numerous small cells (< 5 ~m) and large Ceratium sp. were not considered food. Neritic copepod species in Bedford Basin show similar patterns of grazing with respect to particle-size spectra, mainly ingesting 10 to 57 ~m ESD particles (Poulet, 1978). The algae considered to be food in the tower tank were in this size range (Fig. 3), although the copepods do ingest larger and smaller particles at lower rates (Poulet, 1978). The abun- dance of dinoflagellates (species of Ceratium, Prorocen- trum, and Dinophysis) in the tower tank was characteristic of the fall phytoplankton bloom in Bedford Basin (May- zaud et al., 1984). Silicoflagellates constituted 5 to 20% of the phytoplankton > 5gm during the first 59 d of the experiment, but in the final week increased in abundance, dominating the larger phytoplankton (82 to 86% of algae

> 5 #m) in the tank. In the basin, a silicoflagellate bloom generally precedes the fall dinoflagellate bloom (Mayzaud and Taguchi, 1979).

Overall temporal trends of numerical and relative (%) abundance of taxa were similar in the two tower-tank water-columns over 60 d, but could differ markedly on any one date. This was apparently due to a phase-shift in the timing of blooms in the two sides of the tank. A bloom of Centropages ty])icus occurred in both sides of the tower, reaching 35 and 42% total zooplankton abundance, but the peak abundance was on Day 7 in Side A of the tank, and on Day 20 in Side B (Fig. 4). The concentration of algae suitable as food for grazers also showed a phase-shift, with peak abundance occurring on Day28 in Side A of the tank, and on Day 20 in Side B (Fig. 6). The temporal trend in mean size of non-harpacticoid copepods (~g drywt individual -1) in the two sides of the tank was very similar (Fig. 7).

The MIG method estimates potential copepod produc- tion at a given moment, whereas actual production of field assemblages necessarily incorporates losses due to preda- tion, advection, senescence, moulting and other losses. Consequently, it is expected that the MIG method will yield higher estimates than directly measured changes in copepod biomass over time. By eliminating as many predators as possible, a major loss term was eliminated from measured production in the mesocosm experiment. However useful this was for calibrating the allometric estimates of potential production, it is unrealistic for neritic populations of copepods. The concentrations of predators, principally Sagitta elegans and gelatinous carni- vores, in Bedford Basin can be very high. Pearre (1981) reported densities of 76 S. elegonsm -S in December in Bedford Basin. Sameoto (1973) estimated that S. elegans may consume over a third of the annual production of copepods in Bedford Basin.

Two questions arise from the comparison of the MIG method and measured production. Firstly, is an estimate of potential production useful? Huntley and Boyd (1984) considered that oceanic regions are likely to exhibit concentrations of food low enough to limit the growth of copepods, so that the MIG method is strictly applicable only to neritic areas. However, the abundance of zoo- plankton predators in neritic areas suggests that an esti- mate of potential production may have little relationship to actual production unless a correction for natural mor- tality can be obtained. One use of the MIG method may be to provide an indirect estimate of natural mortality, if this is assumed to be the difference between estimates of actual and potential production. An estimate of potential production can also be used to compare actual production of different regions if natural mortality can be estimated independently (Myers and Runge, 1983).

The second question is, is an order of magnitude estimate of potential production good enough? Clearly such an estimate is not useful if the variability in produc- tion between regions examined is less than an order of magnitude. All methods for estimating production have

S.McClatchie: Copepod production 603

limitations, so that the question should be rephrased in the context of the costs of applying different methods. The MIG method is both easier and faster than many other methods, factors that weigh heavily in its favour when regions with highly variable secondary production are studied.

Acknowledgements . C. M. Boyd, R. O. Fournier, P. Lane and P. Mudie loaned instruments and equipment for the tower-tank study. I thank N. Balch, T. Collins, and Aquatron staff for practical assistance in running the tank. I am grateful to C. Morales, C. Carver, and D. Ives for assisting with sampling and to E. G. Merinfeld for cope- pod identification. I am grateful to C. W. Burns for constructive comments on the manuscript. This research was supported by a Natural Sciences and Engineering Research Council (Canada) grant to C. M. Boyd, and a Dalhousie Graduate Fellowship to S. McClatchie. Support during manuscript preparation was provided by a New Zealand University Grants Committee Postdoctoral Fel- lowship at the University of Otago.

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Date of final manuscript acceptance: November 21, 1986. Communicated by G. F. Humphrey, Sydney