Estuarine, Coastaland Shelf Science (1987) 25,637-645

Copepod Colonization of Natural and Artificial Substrates in a Salt Marsh Pool

Eileen Cummings and Ernest Ruber Biology Department, Northeastern Universit_v, Boston, MA 021 IS, U.S.A.

Received 6 January 1986 and in revised form 83uly 1987

Keywords: copepoda; colonization; salt marshes; substrates; Massachusetts

Pre-weighed packets of Spartina alterniflora and of plastic (polypropylene) twine were placed in a salt marsh pool and recovered on 40 dates spanning 14 months. New packets were placed out regularly to provide a contrast with ageing material. Twelve species of copepods were extracted, counted, and identified. Dry weight and Kjeldahl-nitrogen were determined for Spartina packets.

Eight species of copepods, Amphiascus pallidus, Onychocamptus mohammed,

Cletocamptus deitersei, Halicyclops sp., Harpacticus chelifer, Mesochra liltj’eborgii, Metis jousseaumei and Nitocra sp. were found in higher densities on old grass or plastic packets than on new. The quantity of material was important in that the relative attractiveness of old grass was much lower early in the second year when 7-15”,, dw and 0.7”,, nitrogen remained than early in the first year when over 60”,, dw and 2.0”, nitrogen remained. Old plastic polypropylene was equally or more attractive than old grass to 7 of 8 species, therefore, nitrogen decline in old grass was not the factor making it less attractive. Once aged, the quantity of substrate was more important than its quality. Apparently, this is due to coloniz- ation by microflora or settlement of detritus but these were not studied. The four clear exceptions to these trends were Darcythompsoniafairliensis and Eurytemoru

afinis which showed highest densities 72”,, and 50”,, of the time in new grass, Apocyclops spartinus with 70”,, in grass and equal numbers between old and new packets and Acartia tonsa a bay calanoid with 82”, of highest densities in the water column and only two occurrences out of 40 dates in the packets.

Introduction

Since the pioneering work of Burkholder (1956) there has been considerable interest in import-export budgets, the rate of decomposition of plant deritus, and the analysis of its role in salt marshes and bays (Burkholder & Bornside, 1957; Teal, 1962; Odum & de la Cruz, 1967; Kirby & Gosselink, 1976; Haines & Montague, 1979; Peterson et al., 1983).

Our recent interests have revolved around the role of Spartina detritus as a source of nutrients and as a substrate within salt marsh pools. Montagna and Ruber (1980) placed plastic-mesh litterbags containing preweighed S. alterniflora in various marsh sites and regularly censused these packets for abundance of bacteria, fungi, nematodes, diatoms, ciliates and flagellates. A sequential pattern of invasion occurred but this appeared to be correlated more with season than with stage of packet decomposition. Densities generally were more associated with the quantity than the quality of the material in the packet,

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638 E. Cummings & E. Rubrr

suggesting a more important role for this material as a substrate than as a nutrient. Montagna (1975) also found copepods to be very abundant in the packets. Although fresh litter had a higher copepod diversity than old litter his data did not permit a clear separation of the relative importance of season, subtrate quantity, and substrate nutrient value in determining the presence of copepods in litterbags. The main objective of the present study was to resolve this question.

Methods and materials

The study pool was the same one as used by Montagna and Ruber (1980). It is surrounded by dwarf to intermediate sized (30-70 cm) S. alterniflora Loisel. An old mosquito ditch lies 2 m to the side of the pool. The ditch is not connected, but tidal water does reach the pool across the narrow strip. The pool is located on the Parker River National Wildlife Refugee in Rowley, Massachusetts, about 60 km north of Boston (Ruber et al. 1981).

Pooled live S. aZternij7ora leaves were apportioned into 320 plastic-mesh bags (aper- tures 4.5 x 11.0 mm) each containing 15.0 g wet weight (2.9 g dw). On 23 June 1981, 160 bags were placed in groups of four in compartmentalized hardware cloth boxes and immersed, while the other 160 were stored frozen in plastic bags. Another 160 bags, each containing 15.0 g of plastic (polypropylene) twine were similarly boxed and immersed and 160 retained for later use. Subsequently, on each sampling date, four of the previously frozen grass packets and four plastic-twine packets were placed in the field. The surface area of the twine in the packets was 921 cm’ (calculated as a cylinder of given length) and that of the grass was 862 cm2 (taken as two-dimensional and traced on graph paper). Groups of four packets were randomly placed and recovered from the large box, but within each compartment of four, the treatments were replicates.

Samples were taken from 1 July 1981 to 16 August 1982 as follows: weekly in April-September, every two weeks during October-November, and monthly during December, February and March. On each sampling date we collected four packets each of (a) old grass (OG) and old plastic (OP) which had been in the field since beginning of the study, (b) new grass (NG) and new plastic (NP) which had been placed in the field only on the preceding sampling date, and (c) two of an open water sample (W). The last was obtained using a Gilbert Sampler, a device which collects a 78-cm’ section of the full height of the water column (Gilbert & Ruber, 1986). Depth at the immersion site varied from 35 to 50 cm according to previous tidal stages. During recovery, litter packets were carefully slipped into separate plastic bags while still in the water.

In the laboratory, the packets were placed in a litre of water and gently swirled to suspend the organisms. The water was then strained through a plankton net (mesh 0.143 urn) and the copepods preserved in a solution of 0.3 g Rose Bengal stain per litre of 95O,, ethanol. At a later date, the copepods were transferred to glycerin in a petri dish, separated, identified and counted. The swirling technique was pre-tested by doing serial rinses followed by microscopic examination of the substrate at 40 x magnification. At least 95O,, of the copepods present were recovered in the first rinse which agreed with previous analysis (Montagna, 1975).

A commercially available software package (SPSS) was used to determine for each copepod species whether their overall distribution was affected by the packet treatment. First a one-way ANOVA tested the hypothesis that at least one of the five treatments was significantly different (P= 0.05, between groups df = 4). For those species showing such a significant difference (all) the package proceeded to a Student-Newman-Keuls multiple

Copepod colonies 639

JJ A SO ND J FM AM J JA

1981 1982 Figure 1. Percentage remaining dry weight (0) and Kjeldahl-N (+ SE) (Cl) in packets of S. alrerni’ora immersed on 23 June 1981 and collected on 40 dates up to 16 August 1982. For nitrogen, at the start n = 4, but as material was reduced packets were combined and so n = 2 from March to end of study, except where no error bars are shown then n = 1. For dry weight, n =4 throughout. Errors are omitted for clarity between 10 July and 29 July 1981 (mean error 10,6Ob), and during most of June and July 1982 (mean error 21.5”,,).

range test which indicated which treatments were significantly different (P = 0.5) and how they ranked. This test compares the calculated q value to a critical value (I’= 0.05) from the q distribution. The df of one axis of the table is the number of the range of means tested (a minimum of 2, maximum of 5) while second axis is the ANOVA error df, which varied for different species from 153 to 665 of a possible maximum of 730 which would have been the case if a species occurred in all treatments and replicates 18, on all 40 sample dates (Zar, 1974).

The grass fragments were frozen and later oven dried at 80” C for 48 h and dry weighed, then incinerated at 550” C for 2 h and the ash-free dry weight determined. Some grass samples were dried then randomly chosen to determine Kjeldahl-nitrogen. Toward the end of the study so little material remained that all packets had to be pooled for analysis. Where multiple samples were available, standard errors of the mean were calculated on the weights and, plotted as percentages (Figure 1).

Results

Only 33O,, of Spartina dry weight remained in the packets by 1 September (85 days). After this losses were much slower; 23 5, still remaining on 14 March (265 days). The material then declined somewhat more rapidly to 13 o/o on 5 May (316 days) after which it varied irregularly from 7 to 131”) with 1216 remaining on 16 August (419 days), the last sampling date (Figure 1). Kjeldahl-nitrogen began at about 2(1&, changed little for a month and then rose to 2.81t_0,2q0 on 26 August and to about 3.116 (no replicate) on 2 September. After this it declined rapidly to 2 _+ 0.05°~;j on 29 September and rose swiftly again to

640 E. Cummings & E. Ruber

TABLE 1. The distribution of 12 copepod species among five treatments over the sampling dates

Packets

W” NP OP NG OG

Amphiascus pallidus Mean densityb Rank’ O,;Rank I“

Onychocamptus mohammed Mean Rank ?&Rank 1

Acartia tonsa Mean Rank Of0 Rank

Darcythompsonia fairliensis Mean Rank o,C,Rank 1

Apocyclops spartinus Mean Rank O:, Rank 1

Cletocamptus deitersei Mean Rank ?&Rank 1

Eurytemora afinis Mean Rank ‘&Rank 1

Halicyclops sp. Mean Rank %Rank 1

Harpacticus chelifer Mean Rank ObRank 1

Mesochra lilljeborgii Mean Rank %Rank 1

Metis jousseaumei Mean Rank 96Rank 1

0.4 15.0 104.0 9.0 44.0 -d 3 1 3 2 0.0 4.0 64.0 4.0 27.0

0.1 0.2 0.4 0.2 0.7 2 2 2 2 1 6.0 21.0 21.0 15.0 37.0

0.1 0.0 -c -p 0.0 1 2 2 2 2

86.0 0.0 7.0 7.0 0.0

0.1 -e 0.1 2.0 0.4 2 2 2 1 2 3.0 3.0 3.0 72.0 17.0

1.1 3.0 3.0 5.0 5.0 2 2 2 1 1 I.0 16.0 12.0 33.0 37.0

0.9 2.4 9.5 3.3 8.6 3 2 1 2 1 7.0 21.0 23.0 20.0 29.0

I.0 2.7 1.4 1.2 1.3 2 1 2 2 2 7.0 50.0 13.0 13.0 17.0

0.6 1.6 2.5 1.2 1.3 2 2 1 2 2 3.0 25.0 37.0 8.0 27.0

0.5 2 0.0

0.2 2 3.0

0.3 J

o-o

6.6 1 9.0

1.7 2

18.0

8.2 2

13.0

7.6 3.8 8.2 1 1 1

53.0 13.0 24.0

7.7 1

40.0

2.6 2

25.0

58.0 1

79.0

2.0 2

15.0

3.5 2 1 .o

5.7 2 6.0

Copepod colonies 641

TABLE 1. Continued

Packets

lv NP OP NG OG

Nitocra sp. Mean Rank “,Rank 1

0.3 9.8 24.3 3.7 28.0 2 2 1 2 1 0.0 12.0 34.0 8.0 46.0

Total Mean 0.5 4.3 18.2 2.9 8.8 Rank J 3 1 3 2 0 Rank <, 1 10.0 16.0 32.0 17.0 24.0

“W, water column; NP, OP, new, old polystyrene packets; NG, OG, new, old grass packets. ‘Mean density per packet or equivalent volume (14.3 cm’) of all dates (40). ‘Ranking (1 =highest) for all samples by Student-Newman-Keuls test (x = 0.05). ‘Percentage of all samples within which this treatment had the highest copepod density. ‘Between 0 and 0.049 copepods per packet. ‘Analysis gives ambiguous ranking.

k - K * . WATER

+ OLD PLASTIC n3- CJ NEW GRASS

it .

El- +

z Z J

J A S ON DJ F MA MJ J A 1981 1982

Figure 2. Distribution of Acartia toma a bay calanoid showing no attraction to packets and being almost entirely in the water column.

a peak of 3.70, (no replicate) on 27 October and to 3.4t0.2% on 10 November. It remained high (2.5-3:&) until 24 March after which it declined rapidly to 0.75 +_0.3O;, on 26 May. For the remainder of the study it varied irregularly, being in most cases (8 of 11) below lo, (Figure 1).

Twelve species of copepods occurred frequently enough to demonstrate some patterns in time or space. Ignoring season, trends in packet variability are presented in three ways, (1) mean copepod density, (2) the ranked attractiveness of the packets as determined by the Student-Newman-Keuls multiple range test, and (3) a ranking based solely on the percentage of dates on which a treatment yielded the highest density (y/o Rank 1) (Table 1).

Several patterns were observed. One species, Acartia tonsa, was found almost entirely in the water column (W) and mainly in the warm months (Table 1, Figure 2). Three other

642 E. Cummings & E. Rubrr

WATER

----- NEW PLASTIC - OLD PLASTIC - OLD PLASTIC

-----NEW GRASS - OLD GRASS

J A SO,N D J F M,tsF J J A

Figure 3. Distribution of Amphiuscuspallidus among five subsamples taken from 1 July 1981 to 16 August 1982. For each subsample n =4, except ‘ water ’ where n=Z. Data from W have been reduced and expressed as number per 14.3 ml which is the equivalent packet volume. Old is greater than New and OP is greater than OG.

species, Apocyclops spartinus, Eurytemora afinis and Cletocamptus deitersei, were also common in W, but their densities in other experimental treatments were usually higher (Table 1). Some species showed a clear orientation towards aged packets (OG, OP). This trend was most pronounced in Amphiascuspallidus (Table 1, Figure 3), Metis jeusseaumei,

Mesochra lilljeborgiiand Nitocra sp. and to a somewhat lesser, but still pronounced, degree in Harpacticus chelafer, Onychocamptus mohammed, Halicyclops sp. and Cletocamptus deitersei(Table 1). Among these, Nitocra and Onychocamptus had slightly higher densities and frequencies in OG (grass) while the rest, except Halicyclops, favored OP (plastic) by a considerable margin (Table 1).

Among the other species, Apocyclops tended to favor grass (Table 1, Figure 4). Darcythompsonia fairliensis was found most frequently in new grass. Eurytemora afinis had its highest densities in new plastic 50% of the time (Table 1).

Discussion

The disappearance of dry weight was quite similar to that previously reported for the site (Montagna & Ruber 1980). McKee and Seneca (1982) have pointed out factors involved in

Copepod colonies 643

8

5

3

1

50

30

10

- - -- NEW PLASTIC - OLD PLASTIC

- --- NEW GRASS ?’ - OLD GRASS

1981 1982 Figure 4. Distribution of Apocyclops spartinus the dominant cyclopoid, which shows a slight excess density in grass over plastic but is fairly evenly distributed after the first three months.

such losses. Kjeldahl-nitrogen showed two notable increases. Increases of nitrogen in Spat-&a litter have been reported previously (e.g. Teal 1962; Odum & de la Cruz, 1967; de la Cruz, 1975), and are attributed to colonization by epiphytic microflora and detrital deposition. More recently, it has been shown that up to 30(‘s,, of such nitrogen may exist as non-protein compounds of various sources, and may not be readily available as nutrients to detritivores (Odum et aE., 1979).

In the following paragraphs several hypotheses are tested: (1) copepod colonization of Spartim packets is determined mainly by the amount of grass in the packet; (2) new Spurtim contains something which makes it more attractive to copepods than new plastic; (3) the amount of substrate is the only thing that matters, new plastic and old plastic being equally attractive to copepods; (4) microflora and thus copepods, colonize grass more rapidly than plastic; (5) the packets are not significantly attractive to copepods but only reflect copepod densities occurring naturally in the water column.

First, how attractive to copepods is a matrix apart from its nutrient content? Matrix importance would be tested by a diminution of the attractiveness of OG relative to NG, as OG loses weight with time and NG does not. To test this we divided populations into the first six dates of the study at the end of which 600,, of the Spartina dry weight still remained and the last nine dates which spanned roughly the same time interval one year later and during which only 7-15”,, of the Spartina remained. We compared the numbers of dates for which OG and NG had number 1 rankings for copepod choice for 11 species, excluding only Acartiu which did not occur in packets during these intervals (these data not shown). We tested the hypothesis that for number 1 rankings, OG time,/NG time, = OG time,/NG time,. This was false for 10 of 11 species for which the ratio at time 1 was greater than the ratio at time 2. The hypothesis of equality was tested and rejected at P= 0.05 by the Kolmogarov-Smirnov goodness-of-fit test. Thus the relative density of copepods in OG does drop as its quantity becomes less and the significance of grass in the packets as a matrix is supported. This agrees with Montagna and Ruber’s (1980) results for ciliates, nematodes, and bacteria.

Second, the possibility still exists that Spartina detritus begins with some qualitative nutrient characteristic (e.g. higher nitrogen) which makes it attractive to microflora and copepods. In such a case one would expect NG > NP. Examining Table 1 for mean density and (‘b Rank 1, the above is true for four cases, the reverse is true for six cases and equality exists in two cases. The null hypothesis cannot be rejected, NG being about as equally attractive as NP. One species, Darcythompsonia fairliensis, follows this pattern strongly with 72q;, of its ‘ rank 1 ’ occurrences in NG; a second, Apocyclops Spartinus, also displays significant tendencies in this direction.

Third, we can hypothesize that thematrix alone is attractive, perhaps as cover. If so, one expects NP = OP as these start and remain equal in material. Censusing Table 1 as done previously, we find instead that OP> NP clearly in five species (Mesochra, Metis, Amphiascus, Nitocra, Cletocamptus) and marginally so in two species (Onychocanzptus,

Harpacticus). The results are about equal for three species (Apocyclops, Acartia, Darcythomsoniu) and in only one case (Eurytemora) is NP clearly greater than OP. The null hypothesis cannot be rejected for the 11 species collectively, but for those species which displayed pronounced tendencies, 5 of 6 found OP more attractive than NP.

Fourth, one can hypothesize that because of some nutrient content, microflora colonize Spartina faster than polypropylene, thus rendering the former more attractive during the early dates before it loses too much dry weight (matrix). There is no absolute way to test this because the Spartina loses dry weight so quickly, however, we examined the first six dates (remaining dw 600,,) expecting that OG > OP. This is true in five cases, the reverse in five cases and they are equal in one. Therefore, the null hypothesis (OG = OP) cannot be rejected. The data for this partial census are not presented. This analysis applies only to packet attractiveness because we did not measure and compare microflora or detritus, although accumulation of material on the packets and twine could be seen at a gross level.

Fifth, and finally, it can be hypothesized that events in the packets are only reflections of what is occuring in the water column. On the contrary, for 11 of 12 species (Acartia tonsa, a euplankter excepted) densities are much higher than in the water column in at least one and often all packet treatments (Table 1, Figures 2-4).

Montagna and Ruber (1980) found season and substrate diminution to be important for a number of groups, but were not able to assess copepod behavior in this context. We also conclude that diminution of packet material is important, as is season. Our design permits further analysis. Acartia tonsa, a bay calanoid is not attracted to the packets but is

Copepod colonies 645

usually sampled in the water column. Surprisingly, a second bay calanoid Eurytemora afinis displays some tendency toward the water column, but is captured more frequently in the packets. As expected, the two cyclopoids and eight harpacticoids are more packet than water oriented. As described earlier, species show individual variations in their orientation towards the different packets, but some trends exist. Old materials are more attractive than new so long as quantities remain equal. It does not appear to matter whether the old substrate is grass or plastic, from which we infer that colonization by microflora is important and that both substrates are substantially colonized. The nutrient lost from Spartina detritus may well stimulate microflora in the ponds, but it does not enhance copepod colonization of the Spartina litter beyond that which occurs on an inert material such as polypropylene plastic.

Acknowledgements

We appreciate access to the Parker River National Wildlife Refuge for this study under permits 5-PRR-81-2 and 5-PRR-82-2. LeBaron R. Briggs IV kindly helped us to access and interpret the SPSS package for statistical analysis.

References

Burkholder, P. R. 1956 Studies on the nutritive value of Spurtinn grass growing in the marsh areas of coastal Georgia. Bull. Torrey Bot. Club 83,327-334.

Burkholder, P. R. & Bornside, G. H. 1957 Decomposition of marsh grass by aerobic marine bacteria. Bull. Torrey Bot. Club 84,366-383.

de la Cruz, A. A. 1975 Proximate nutritive value changes during decomposition of salt marsh plants. Ifydrobiologia 47,475-580.

Gilbert, A. & Ruber, E. 1986 A water column sampler for invertebrates in salt marsh tidal pools. Estt~aries 9, 380-381.

Haines, E. G. & Montague, C. L. 1979 Food sources of estuarine invertebrates analyzed using ’ ‘C; ’ ‘C ratios. Ecology 60,48-56.

Kirby, C. J. & Gosselink, J. G. 1976 Primary production in a Louisiana Gulf Coast Spurtim ulrern~j7ora marsh. Ecology 57, 1052-1059.

McKee, K. L. & Seneca, E. D. 1982 The influence of morphology in determining the decomposition of two salt marsh macrophytes. Estuaries $302-309.

Montagna, I’. A. 1975 Rates of decomposition of Spartina alternijfora and the occurrence of associated organisms in a Massachusetts salt marsh. M.S. thesis, Northeastern University Boston, MA.

Montagna, P. A. & Ruber, E. 1980 Rates of decomposition of Spartim alrerni’ora in different seasons and habitats of northern Massachusetts salt marsh and comparison with other Atlantic regions. Esttcurzr~ 3,

61-64. Odum, E. P. &de la Cruz, A. A. 1967 Particulate organic detritus in a Georgia salt marsh estuarine ecosystem.

In Estuaries (Lauff, ed.). Publ. 83 AAAS, Washington, D.C. pp 383-388. Odum, W. E., Kirk, P. W. & Zieman, J. C. 1979 Non-protein nitrogen compounds associated with particles of

vascular plant detritus. Olkos 32,363-367. Peterson, B. J., Howarth, R. W. & Garritt, R. H. 1983 Multiple stable isotopes used to trace the flowr oforganic

matter in estuarine food webs. Science 227, 1361-1363. Ruber, E., Gillis, G. & Montagna, P. A. 1981 Production of dominant emergent vegetation and of pool algae

on a northern Massachusetts salt marsh. Bull. Torrey Bot. Club 108, 180-188. ‘Teal, J. M. 1962 Energy flow in the salt marsh ecosystem of Georgia. Ecology 43,614-624. %ar, J. 1974 Biostatistical Analysis. Englewood Cliffs, NJ: Prentice-Hall.