Rotifer responses to increased acidity: long-term patterns during the experimental manipulation of Little Rock Lake

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1 Hydrobiologia 387/388: , E. Wurdak, R. Wallace & H. Segers (eds), Rotifera VIII: A Comparative Approach Kluwer Academic Publishers. Printed in the Netherlands. 141 Review paper Rotifer responses to increased acidity: long-term patterns during the experimental manipulation of Little Rock Lake Thomas M. Frost 1, Pamela K. Montz 1, Maria J. Gonzalez 1,2, Beth L. Sanderson 1 & Shelley E. Arnott 1 1 Trout Lake Station, Center for Limnology, University of Wisconsin-Madison, WI 53706, U.S.A. 2 Department of Biological Sciences, Wright State University, Dayton, OH 45435, U.S.A. Key words: Rotifera, community, acidification, whole-lake experiments, Little Rock Lake, Wisconsin Abstract Little Rock Lake, Wisconsin, U.S.A. has been the site of a whole-ecosystem experiment since It was divided into a treatment basin that was acidified in three, two-year stages and a reference basin. The rotifer community in the treatment basin exhibited a variety of responses to the manipulation. Many species decreased in abundance under reduced ph conditions but other rotifers increased at the same time such that there were ultimately increases with acidification in total rotifer biomass, and quite conspicuously, in the proportion that rotifers comprised of total zooplankton biomass. Ten rotifer species decreased at some stage during the acidification (e.g., Kellicottia longispina, Asplanchna priodonta and Keratella cochlearis) while four species increased dramatically (e.g., Synchaeta sp. and Keratella taurocephala ). Similarity indices and total rotifer biomass differences measured between the two basins exhibited very different temporal patterns of response to acidification. Similarity decreased regularly beginning with the earliest stages of acid additions while biomass was nearly the same between the basins until the late stages of the experiment. Comparisons with other nearby lakes indicate, however, that acid conditions are not the only factors generating among-lake differences in rotifer community characteristics. Changes observed with acidification in Little Rock Lake were such that its total rotifer biomass grew more similar to that in a nearby acidicbog lake and different from that in a near-neutral-ph lake. At the same time, abundance patterns for individual rotifer species in Little Rock Lake were not particularly similar to those in the other lakes. It appears that, although they are important, acid conditions alone can not account for all observed rotifer community differences among lakes. Higher proportions of rotifer biomass and high populations of K. taurocephala do seem to be common features of many low ph habitats. Introduction Zooplankton communities vary markedly across lakes of differing acidities (e.g., Baker & Christensen, 1991; Locke, 1992; Yan et al., 1996). The rotifer components of zooplankton communities also exhibit systematic differences along ph gradients when they have been evaluated (e.g., MacIsaac et al., 1987; Brett, 1989; Siegfried et al., 1989). Despite these clear patterns, however, a number of issues remain regarding rotifers and ph including the actual nature of rotifer community responses to increasing acidity, the extent to which acidity versus other factors controls observed zooplankton community differences among lakes, and the mechanisms that underlie among-lake differences. Understanding the effects of changing ph conditions is important because of the widespread occurrence of anthropogenic acid deposition and its effects on lakes and other ecosystems worldwide (Galloway et al., 1984; Schindler, 1988; Charles, 1991). To examine these and other ph-related issues, we have been conducting an acidification experiment in Little Rock Lake (LRL), Wisconsin, USA since 1983 (Watras & Frost, 1989; Brezonik et al., 1993). Following a baseline period in 1984, LRL s treatment basin was acidified, in three, two-year phases from its original ph of 6.1 to a final target level of 4.7 while the lake s reference basin remained at natural

2 142 ph levels. Beginning in 1991, we have been evaluating the lake s recovery from acidification (Sampson et al., 1995; Frost et al., in press). Here, we provide a detailed report of rotifer responses during the acidification phase of the experiment. We present time-series data for the entire LRL rotifer community, detailed information for a few common rotifer species, and several measures representing collective features of the entire rotifer community during the course of the experiment. In addition we compare patterns observed in LRL with those found in two nearby lakes that have been investigated as part of the North Temperate Lakes, Long-Term Ecological Research (NTL-LTER) program (Magnuson et al., 1984). We use these comparisons to test the hypothesis that acidification of LRL s treatment basin shifted its rotifer community from one similar to that occurring in a near-neutral ph lake to one found in a acid-bog system. Accepting this hypothesis would support the notion that ph levels are the primary factor controlling the structure and dynamics of rotifer communities in north temperate lakes. Methods The methods used to generate the data presented in this report have been detailed in previous papers. We only summarize these techniques here and refer to the papers that describe them in greater detail. The Little Rock Lake project was initiated in The lake s two similarly sized basins, each approximately 20 ha in surface area, were separated with a vinyl-plastic curtain in 1984 that minimized any water flow or exchange of organisms between them (Watras & Frost, 1989). Following the baseline period in 1984, acid additions were initiated in the north, treatment basin. Beginning in 1985, sulfuric acid, the dominant acid in human-influenced deposition in much of North America was mixed into the treatment basin by boat as frequently as necessary to maintain target levels during ice-free periods, usually between once every five days and once every three weeks. We maintained target-ph levels of 5.6, 5.2 and 4.7 for two years each with the last acid additions just prior to freezing in 1990 (Brezonik et al., 1993). Each of the acidification periods was initiated immediately after ice out of the first year and continued until ice out at the end of the second year. Thus the completion of the ph 4.7 period occurred in April LRL s south basin was maintained as a reference throughout the course of the experiment to provide an indication of what would have occurred in the treatment basin without the addition of acid. The NTL LTER project was among the first of what is now a worldwide network of more than 20 long-term study programs funded by the US National Science Foundation to evaluate fundamental characteristics of communities and ecosystems (Magnuson et al., 1984). We compare LRL with two of the NTL LTER primary study lakes that have been monitored since 1991 at approximately the same frequency as LRL, Crystal Lake (CL), a clear-water habitat (average Secchi depth = 7.3m ) with dilute chemistry and near neutral ph (6.0), and Crystal Bog (CB), a darkly stained sphagnum-mat dominated habitat (average Secchi Depth = 1.6 m) with high dissolved organic carbon levels and with an acidic ph (5.0). Extensive information on CL, CB and five other nearby lakes is available from the NTL LTER database through Zooplankton in all lakes were sampled, using Schindler Patalas traps equipped with 53-µm mesh buckets, at two-week intervals during ice-free periods and every five to six weeks when the lakes were ice covered. In LRL, the trap was approximately 1 m in length and samples were collected at 3 depths in each basin (Frost & Montz, 1988). Samples were processed individually but data were subsequently pooled to estimate the mean density of animals throughout the water column. For the NTL-LTER lakes, the trap was approximately 2.5 m in length and samples were usually pooled prior to counting. For these lakes, too, our data represent an estimate of the overall average density of animals within the water column (Frost & Montz, 1988). For comparisons of species among lakes, we consider only those taxa which were present on more than 5 sampling dates. Rotifer data are reported here for CL from 1984 through 1991 and for CB from 1983 through Rotifers were identified, to species in most cases, using a dissecting microscope at 50 or 100 magnification following Ruttner-Kolisko (1974) or Stemberger (1979). Biomasses of rotifers were estimated following Ruttner-Kolisko (1974) or Downing & Rigler (1984). We report data on the abundance of individual rotifer taxa in the treatment and reference basins of LRL and emphasize comparisons between the two basins as an indication of responses to acidification. We calculated a measure of community similarity for rotifers to compare the treatment and reference basins of LRL using the technique described in Frost et al.

3 143 (1995). It is based on the total of the minimum proportion of total rotifer biomass for each species in either basin of LRL. We also report rotifer species richness as the total number of taxa observed within a year and rates of species turnover indicating the appearance or disappearance of a species in a basin in any one year. Species turnovers are calculated for each year by determining the number of rotifer taxa that were recorded in a basin at any time within a year but which were then absent during the subsequent year along with the number of taxa that exhibited the opposite pattern, appearing only during the second year of a pair. The total number of appearing and disappearing taxa was then divided by the total number of taxa present at any time in the basin during the first year plus those present during the second year. This number was then multiplied by 100 to obtain the percentage change. Both species richness and turnover measures are presented in detail in Arnott et al. (in preparation). Assessing differences due to a treatment in a whole ecosystem experiment is a contentious area in terms of statistics. We have developed two methods specifically for testing such differences (Carpenter et al., 1989; Rasmussen et al., 1993) but even these have received some criticism (e.g., Stewart-Oaten et al., 1992). For this report we emphasize simple, primarily graphical, presentations of our data and have avoided any strictly statistical assessments of the significance of treatment effects. Results Responses to Acidification in LRL A total of 26 rotifer taxa were observed on at least three sampling dates in either the treatment or reference basins of LRL during the period (Table 1). Ten of these taxa appeared to decline at some stage of the acidification. Four appeared to increase, and 12 exhibited no systematic change (Table 1). Patterns of decline differed among rotifer species including those bykellicottia longispina, which decreased in 1987 during the early stages of the ph 5.2 manipulation stage (Figure 1A); Asplanchna priodonta, which declined during the ph 5.6 stage, recovered during the ph 5.2 stage, and was essentially extirpated during the ph 4.7 stage (Figure 1B); and Keratella cochlearis which declined just past midway through the second year of the ph 5.6 period (Figure 2B, Gonzalez & Frost, 1994). Patterns of increase, which frequently involved a substantially higher abundance in the treatment basin compared to the reference basin, are illustrated by Synchaeta sp. (Figure 3A) and, most dramatically, by Keratella taurocephala which shifted from a fairly minor component of overall rotifer biomass to a major portion of the rotifer community by ph 4.7 (Figure 2A, Gonzalez & Frost, 1994). The pattern for Polyarthra vulgaris typified those 12 species for which no discernible trends with acidification were detectable (Figure 3B). In contrast with the patterns for individual species, there were only subtle responses to acidification in terms of the number of species observed in either basin during any of the acidification periods. Overall, the annual species richness observed in the treatment or reference basin during each period ranged between 15 and 21 taxa and both basins were generally quite similar throughout the experiment. The number of taxa observed only declined in the treatment basin during the ph 4.7 period when 17 taxa and 15 taxa occurred in year 1 and 2, respectively, compared with 21 and 20 taxa in the reference basin during the same periods. Consistent with a response to acidification, however, the overall turnover rate for species was somewhat higher in the treatment basin compared to the reference basin (Figure 4). Overall, the net effects of changes for the entire rotifer community were such that no differences in total rotifer biomass were at all obvious between the two basins during the ph 5.6 and 5.2 phases (Figure 5A). By ph 4.7, however, the overall change was fairly dramatic and a net increase in rotifer biomass was pronounced. This pattern is even more striking, however, when the proportion of rotifers in total zooplankton biomass is considered. The increase of rotifers coupled with the decrease by other zooplankters, particularly the copepods (Frost et al., 1995), shifted the proportion of rotifer biomass from less than 20% prior to the ph 4.7 period to values that frequently exceeded 50% (Figure 6). A low proportion of rotifer biomass persisted in the LRL reference basin throughout the experiment (Figure 6). Overall, the zooplankton community decreased in total biomass by the ph 4.7 period (Frost et al., 1995) but it was increasingly dominated by rotifers as the LRL treatment basin became more acidic. The rotifer community exhibited the same overall trends as reported previously for the total LRL zooplankton assemblage (Frost et al., 1995). Responses by individual rotifers to acidification were much more dramatic than those by collective features

4 144 Figure 1. Average biomass (µg/l) of Kellicottia longispina (A) and Asplanchna priodonta (B) in the water columns of the treatment (thick line) and reference (thin line) basins of Little Rock Lake, Wisconsin, U.S.A. of the total rotifer community. Rotifer community similarity between the treatment and reference basins provides perhaps the most straightforward evidence of these differences between species-based and more collectively based variables. This similarity exhibited a strong and systematic decrease through all stages of the acidification (Figure 5B) much more pronounced than the trend in biomass difference (Figure 5A). Comparisons among lakes In terms of presence/absence, the rotifer assemblages in the LRL, Crystal Bog (CB), and Crystal Lake (CL)

5 145 Figure 2. Average biomass (µg/l) of Keratella taurocephala (A) and Keratella cochlearis (B) in the water columns of the treatment (thick line) and reference (thin line) basins of Little Rock Lake and in Crystal Bog (A) or Crystal Lake (B) (dotted lines) in Wisconsin, U.S.A. are largely similar (Table 1). We recorded 28 taxa in CLand25inCBcomparedwiththe26inLRL.Only two taxa,collotheca mutabilis and Notomata sp. were present in LRL but absent in both CB and CL. Three genera were absent from LRL but present in one of the other lakes; Anuraeopsis sp. in CL and Brachionus quadridentatus and Cephalodella sp. in CB. Otherwise, among-lake differences involved other taxa of genera that were common to at least two lakes, primarily Keratella which was more specious in CB and Polyarthra which had more species in CL. Overall, 15 taxa occurred in all three lakes (Table 1). Comparisons of abundances of individual taxa did not reveal the same common conditions among

6 146 Figure 3. Average biomass (µg/l) of Synchaeta sp. (A) and Polyarthra vulgaris (B) in the water columns of the treatment (thick line) and reference (thin line) basins of Little Rock Lake, Wisconsin, U.S.A. the lakes. For Keratella taurocephala, which had increased so substantially with acidification in the LRL treatment basin, we recorded abundances in CB that were intermediate between those in the treatment basin and those in the reference basin (Figure 2A). Its maximum abundance in CB was substantially lower than that occurring in the LRL treatment basin even during 1989 when its ph level of 5.2 was higher than the 5.0 average value for CB. For K. cochlearis, abundances in the LRL

7 Table 1. Annual average biomass values for rotifer species (µg/l throughout the water columns) in the treatment and reference basins during each of the periods during the experimental acidification Little Rock Lake, Wisconsin, U.S.A. These are followed by the slopes of standard linear regressions of annual average abundance vs. each year of the experiment for each species, the coefficient of determination, R 2, for this relationship, and our assessment of what the overall trend in the relationship was during the experiment. Also reported are whether each species was present (P), absent (A), or if another species in the same genus was present (G) in Crystal Bog (CB) and Crystal Lake (CL) Period Baseline 5.6 (I) 5.6 (II) 5.2 (III) 5.2 (II) 4.7 (I) 4.7 (II) SLOPE R 2 Change Presence in Species Basin CB CL Ascomorpha sp. T P P R Ascomorpha ovalis T G P R Asplanchna priodonta T P A R Collotheca mutabilis T A A R Conochiloides sp. T A P R Conochilus unicornis T P P R Gastropus hyptopus T G P R Gastropus stylifer T G P R Kellicottia bostoniensis T P P R Kellicottia longispina T P P R Keratella cochlearis T P P R Keratella crassa T P A R Keratella hiemalis T P P R Keratella taurocephala T P P R Lecane sp. T P P R Monostyla sp. T A P R Notomata sp. T A A R Ploesoma sp. T A P R Polyarthra dolichoptera T A P R Polyarthra renata T P P R Polyarthra vulgaris T P P R Continued on p. 148

8 148 Table 1. Continued. Period Baseline 5.6 (I) 5.6 (II) 5.2 (III) 5.2 (II) 4.7 (I) 4.7 (II) SLOPE R 2 Change Presence in Species Basin CB CL Symchaeta spp. T P P R Trichocerca sp. T G P R Trichocerca cylindrica T G P R Trichocerca multicrimis T G P T Trichocerca birostris T O G P R Figure 4. Inter-annual species turnover rates for the rotifer community in the treatment and reference basins of Little Rock Lake, Wisconsin, U.S.A. treatment basin reached peaks comparable to those observed in CL during the first three years of the experiment but they were substantially lower during more acid stages of the experiment than those occurring during some CL periods (Figure 2B). K. cochlearis abundances in the LRL reference basin were usually lower than the peaks reached in the other two basins and were substantially lower than the high values that were recorded during some periods in CL, particularly For both of these Keratella species, which we considered to be key indicators of responses in the LRL experiment (Gonzalez & Frost, 1994), these comparisons provide little support for the hypothesis that acidification simply shifted the LRL rotifers from a situation occurring in CL to one in CB. Considering the proportion of rotifer biomass in the zooplankton community gives a somewhat different impression, however. The proportion of rotifer biomass was fairly similar among CL, LRL reference basin, and LRL treatment basin during the initial stages of the experiment (Figures 6 and 7). The rotifer proportion of biomass was lowest in CL. Acidification did appear to shift the biomass of the total rotifer assemblage in the LRL treatment basin to be similar to that occurring in Crystal Bog (Figures 6 and 7). Overall, we accept our hypothesis regarding the comparisons among the lakes at the community level, in terms of the proportional biomass of the rotifers in the zooplankton community, but not at the species level, because there were substantial differences in the occurrence patterns of individual rotifer taxa among the habitats we considered. Discussion Several rotifer taxa exhibited early and dramatic responses to the acidification of LRL. These changes were generally consistent with patterns observed in other low-ph lakes. Reduced abundance of K. cochlearis and high populationsof K. taurocephala s have been reported in several acid lakes (e.g., Brett, 1989; Siegfried et al., 1989). The inability of laboratory bioassays conducted in parallel with the LRL manipulation to predict the K. taurocephala response (Gonzalez & Frost, 1994) is all the more surprising given the common dominance of this species in acid situations. It certainly appears to exhibit high populations in many low ph habitats. Measures of the collective rotifer community s responses to acidification gave a different impression than the individual-species reactions. Changes in total rotifer biomass were only evident during the most acid phase of the experiment (Figure 5A). There were no differences between the treatment and reference

9 149 Figure 5. Five-point moving average values for (A) the difference in total rotifer biomass (mg/l) for the treatment reference basins and (B) similarity indices calculated between the two basins of Little Rock Lake, Wisconsin, U.S.A. basins in either the ph 5.6 or 5.2 stages but total rotifer biomass increased markedly during the ph 4.7 stage. This increase occurred despite the declines of numerous rotifer taxa and could be attributed largely to a shift in the abundance of K. taurocephala (Frost et al., 1995). Community-level responses supported

10 150 Figure 6. Proportion of rotifer biomass in the total zooplankton community (excluding Chaoborus spp.) in the water columns of the treatment (thick line) and reference (thin line) basins of Little Rock Lake, Wisconsin, U.S.A. Figure 7. Proportion of rotifer biomass in the total zooplankton community (excluding Chaoborus spp.) in the water columns of Crystal Bog (solid line) and of Crystal Lake (dotted line), Wisconsin, U.S.A. the hypothesis that acidification shifted LRL s rotifer assemblage from one resembling that in Crystal lake to one like that in Crystal Bog. Species-level analyses did not support this hypothesis, however. This reinforces our previous conclusions that different scales of taxonomic resolution can give very different impressions of ecosystem patterns, particularly responses to stress (Frost et al., 1995). Acid stress can shift a community to increasing rotifer dominance but ph alone can not explain the fundamental composition of rotifer communities. It is important to note that surveys of rotifers intended to detect acidification effects would have very different sensitivities depending upon whether they were focused on individual species or on the collective properties of the rotifer community. The mechanisms that underlie the increases we recorded in total rotifer community biomass with acidification are not clear. In general, and as reported previously for several biotic responses that were investigated during the LRL experiment, direct reactions to acidification were difficult to detect and most changes could best be explained by indirect, food-web related mechanisms (Webster et al., 1992). Decreases in predation seem a likely mechanism but there are conflicting indications in the treatment basin in terms of the possible changes of rotifer predators. Increases in rotifer biomass occurred at the same time as there was a major increase of the abundance of one major rotifer predator in the treatment basin, Chaoborus (Fischer & Frost, 1997). At the same time, however, there was a concomitant reduction in the populations of other potential rotifer predators, Mesocyclops edax (Brezonik et al., 1993) and Asplanchna priodonta (Figure 1). The M. edax decline has been linked with the Chaoborus increase (Fischer & Frost, 1997) indicating a shift in food-web interactions with acidification in LRL. There was also a change in the body form of K. taurocephala that was consistent with a response to a reduced signal of predation pressure, a reduction in spine length (Gonzalez, 1992). These changes could have resulted from factors in addition to predation shifts as well. Resource availability for rotifers may have increased with acidification. There were no systematic changes in chlorophyll or primary production with acidification (Brezonik et al., 1993) but there was a decline in cladoceran biomass suggesting a potential increase in the availability of rotifer-food resources. Given this combination of events, it is only clear at this point that some shifts in the LRL food web are responsible for the overall increase in rotifer biomass. General features of the rotifer community responses in LRL are quite consistent with previous reports of patterns for the entire zooplankton community (Brezonik et al., 1993; Frost et al., 1995). The contrasting patterns in total community biomass and similarity indices are very much the same for the rotifers (Figure 5) and for all zooplankton. Decreases for several species but buffered responses by the total biomass of portions of the zooplankton community were reported for cladocerans and copepods as well as for rotifers (Frost et al., 1995). Only the marked increase that we report here (Figure 6) is specific to the rotifer elements of the community.

11 151 Responses to acidification in LRL are consistent with previous studies of low ph systems. Several of the patterns that we described here are similar to those reported in previous surveys of acid-stressed lakes (e.g., MacIssac et al., 1987; Brett, 1989). Systematic comparisons with other whole-lake manipulations also revealed a high degree of consistency in such large-scale experiments (Schindler et al., 1991). This suggests that it will be reasonable to predict the patterns to be expected for rotifer communities in other low-ph lakes from LRL and previous reports. Overall, it appears that the basic importance of rotifers in lake ecosystems increases with acidity. At the same time, the nature of species-specific responses are likely to vary among habitats. Rotifer responses to low-ph conditions appear to be consistent at a community but not necessarily a species level. Acknowledgments This research was supported by grants from the U.S. National Science Foundation Long-Term Research in Environmental Biology and Long-Term Ecological Research Programs and by the U.S. Environmental Protection Agency. It was also supported by the Wisconsin Department of Natural Resources. We thank P. Brezonik, J. Fischer, D. Knauer, S. Knight, T. Kratz, T. Meinke, M. Sierszen, C. Watras, K. Webster, and many others for their assistance throughout this project. References Baker, J. P. & S. W. Christensen, Effects of acidification on biological communities in aquatic ecosystems. In D. F. Charles (ed.), Acidic Deposition and Aquatic Ecosystems: Regional Case Studies, pp Springer-Verlag, New York, 747 pp. Brett, M. T., The rotifer communities of acid-stressed lakes of Maine. Hydrobiologia 186/187: Brezonik, P. L., J. G. Eaton, T. M. Frost, P. J. Garrison, T. K. Kratz, C. E. Mach, J. H. McCormick, J. A. Perry, W. A. Rose, C. J. Sampson, B. C. L. Shelley, W. A. Swenson, & K.E. 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