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Periphyton grazing by Amnicolalimosa: An enclosure-exclosure experiment

Periphyton grazing by Amnicolalimosa: An enclosure-exclosure experiment An enclosure-exclosure experiment was performed in Nonquit Pond, Rhode Island to test the effect of grazing by Amnicola limosa (Say) (Gastropoda) upon lentic periphyton. Periphyton from enclosures with A. limosa had higher organic content in May and July, and lower standkg crops in May, July, August, and September than periphyton from exclosures without A. limosa. Small diatom 6 18 pm long, other than Cocconeis lacenEEJ--abundances on glass slides were significantly lower in + t e enclosures than in tile exclosures. COCCO- neis abundances were not significantly affected by grazing. Cocconeis relative abundances on glass slides exposed in the littoral zone of Nonquit Pond also increased with increased grazing pressure from A. limosa. Because characteristics of Nonquit Pond do not seem to bF unusual, and grazer abundances were not extreme, these data may have general importance to the study of lentic periphyton conanunity structure. Introduction The importance of herbivores in determining the littoral zone floral cornunity structure and productivity has been demonstrated for the marine enviromnt (e. g. Castenholz, 1961 ; Paine, 1969; Paine and Vadas, 1969; Menge, 1976; Nicotri, 1977; Lubchenco, 1978), but there have been few such studies dealing with freshwater lit- toral ccmmunities. Furthermore, most studies either directly or indirectly dealing with the effects of grazers upon periphyton have been performed in lotic environments. Douglas (1958), while inves- tigating the ecology of diatoms in a small stream, observed an in- verse relations hi^ between the densi tv of the diatom Achnanthes and the abundance of ihe larval trichopte;an Agapetus fusci es. Patrick (1970) reported that the snail Physa heterostropha TdL e upon most diatom species except Cocconeis placentula. The result of this sel- ection was large populations of Cocconeis and a community w'ith reduced diversity. Elwood and N-72) estimated gastropod grazing to limit periphyton production rates by controlling peri- phyton standing crop in a stream. Dickman (1973) observed peri- phyton to be influenced by the oligochaete Stylaria lacustris which ingested mainly diatoms, apparently avoiding filamentous algae. During a road salt loading simulation, Dickman and Cochnauer (1978) 5 1 March. 1981 Journal of Freshwater Ecology. Volume 1. Number 1 Copyright 1 1981 by Oikos Publishers. Inc. attributed higher diversity at a salt-free station to the higher grazer abundances observed there. Eichenberger and Schlatter (1978) attributed deterioration of previously established vegetation in outdoor channels to grazing pressure. Both Calow (1973) and Kehde and Wilhm (1972) observed no effect of gastropod grazing in deter- mining algal species composition or diversity in a stream, altllough grazing did result in higher concentrations of chlorophyll - a in the attached flora. One of the first major studies designed to investigate the effects of grazers upon freshwater lentic periphyton was that of Flint and Goldman (1975). They found that the crayfish Pacifastacus leniusculus recycle nutrients and either increase or decrease peri- phyton primary productivity, depending upon crayfish population density. In a study similar to the present one Hunter (1980) dem- onstrated that after 45 days freshwater pulmonate snails reduced lentic periphyton standing crop and algal diversity. Chlorophyll a and nitrogen content . mg dry weighte1 were significantly increases by the assembledge of snail grazers at near natural densities. In laboratory experiments Kesler (1979) demonstrated that grazer organisms could affect lentic periphyton standing crop and species composition. Observations of changes in periphyton standing crop and species composition in the field with changing grazing pressure were consistent with predictions made from laboratory experiments. The present paper reports the results of manipulative field experi- (Say) (Gas- ments in which the effects of the grazer Amnicola limosa tropoda) were demonstrated. These effects are usemxplain changes in relative abundances of two diatom groups on field-exposed glass slides from April to September, 1978 and 1979. Methods All sampling was performed in the littoral zone of Nonquit Pond, Rllode Island, a highly stained (71 = 310), acidic reservoir (pH 5.8- 7.2) of low alkalinity 6 0.1 meq . 1- ) (see Kesler, 1979 for de- tailed description). Plastic wastebaskets were used as enclosure- exclosure containers. A hole (8x8 cm) was made on each side and covered with 1.5 mm mesh plastic screen. Four such containers were secured in 15 cm deep water in the littoral zone. Before being placed in the containers, glass microscope slides (37.5 cm2 each) were suspended for 1 week in the littoral zone of Nonquit Pond. These slides were suspended 40 cm off of the bottom so grazing by A. limosa was minimized. Sixteen of these periphyton- A. -- limosa, colonized slidzs were placed in holders in each container. small (2-4 m long) gastropods common to the littoral zone, were placed in two containers at natural densities, 45-107 snails . m-2. The snails were allowed to graze for 7 days. Water temperature was Periphyton measured at the beginning and end of the grazing period. dry weight (DW) , ash- free dry weight (AFDW) , organic content, and algal abundances were determined for the grazed and control slides (Kesler, 1979). Wet mounts were made of the slides and diatom abundances were determined (80 fields at lOOx per treatment). Periphyton species composition and abundances within the littoral zone were sampled using vertically oriented glass microscope slides placed near (5-6 cm) the bottom. Two sampling regimes were employed; one exposing the slides for three-week intervals, the other exposing slides in April and collecting them periodically throughout the summer in 1978 and 1979. Changes in periphyton community composi- tion on these slides were evaluated in light of laboratory observa- tions of hicola grazing (Kesler, 1979) and with the data obtained from the enclosure-exclosure experiments. Results and Discussion The numbers of small diatoms.and Cocconeis - IIMI-~ on the grazed and control slides from the enclosure-exclosure experiments are given in Table 1. Diatoms smaller than 20 w long were designated Cocconeis placentula was as small diatoms and grouped together. counted separately from the small diatom group because a difference in susceptibility to Amnicola grazing was observed in the laboratory. The numbers of Cocconeis on the control and grazed slides were not significantly dif-ferent WM-Whitney U test, P < 0.05). These data are consistent with others reporting resistance of Cocconeis to grazers (Patrick, 1970; Moore, 1977a, 1977b; Nicotri, 1977; Hunter, 1980). This adnate diatom can cement one valve to the substratum (Patrick, 1948), a behavior that no doubt increases its resistance to grazers. Cocconeis was not present in May on either group of slides. Conversely, the numbers of small diatoms were significantly less (Mann-Whitney U test, P < 0.05) on the grazed slides. The presence of Amnicola, presumably by grazing, decreased small diatom abundances and altered the absolute and relative abundances of this diatom group. The relative abundances of Cocconeis, small diatoms, Synedra, and other diatoms on slides exposed initially in April and co- throughout the summer are given in Figures 1A and 2A. The relative abundances of diatoms on slides exposed for three-week intervals are given in Figures 1B and 2B. The calculated grazing rates of Amnicola, in terms of pg ash-free dry weight grazed . slide-1 . day-1, are also given in Figures 1-2. These grazing rates were calculated with the density and weight of snails on the slide holders, the weekly water temperatures, and the temperature-grazing function given by Kesler (1979) . The relative abundances of Cocconeis and small diatoms were different on the three-week and continuously-exposed slides in 1978 and 1979. In 1978 on the three-week slides (Figure 1B) percent abundance of Cocconeis increased with the large increase in grazing pressure. When grazing pressure declined in July and August Cocco- neis abundances similarly declined. These changes were consistent GXli the results observed in the enclosure-exclosure experiment; Cocconeis relative abundances being higher on the grazed than on the control slides. In 1979 grazing pressure on the three-week slides (Figure 2B) was less than observed in 1978. Similarly, Cocco- neis relative abundance was also lower in 1979. Again, these data =consistent with the idea that small diatoms are more susceptible to Amnicola grazing and that grazing pressure can affect the rela- tive abundances of these diatoms. Cocconeis relative abundances were higher on the continuously- exposed slides. In 1978 Cocconeis abundance increased (Figure lA), as on the three-week slides, but once established this species re- mained dominant. Because of Cocconeisfs strong attachment to the substratum there was no reason to expect its abundance to decrease with decreasing grazing pressure on the continuously-exposed slides. Its high density (2500-3500 cells . may have inhibited colon- ization by other species. In 1979 the pattern of Cocconeis representation on the contin- uously-exposed slides (Figur- similar to that observed in 1978 (Figure lA), although the relative abundances of Cocconeis in 1979 was lower. The calculated grazing pressure in 1979 was like- wise lower than in 1978 on the continuously-exposed slides, again supporting the idea that Amnicola grazing affected diatom abundances. These data also demonstra~iportance of grazing pressure rel- ative to the appearance of new substrates. This would be especially important for species colonizing macrophytes which grow throughout the sumner. The relationship observed in this study between Cocconeis and small diatoms was also reported by Brown and Austin (ri Cocconeis and the small diatom Achnanthes. They attributed this in- verse relationship to interspeclfic competition for space. They did not have data prior to August, but reported that Cocconeis reached a maximum percent abundance in August and September, like Jones (1978), and then decreased to less than 10% from November to <January. Achnanthes dominated at three stations from September to October and peaked later in November at a fourth station. If grazers were influencing species abundances in their study, grazer activity should have been highest in August to September due to warmer water temperature and grazer recruitment. Increased grazer activity, and selection against Cocconeis and selection for small diatoms in late summer-early fall are necessary, but not sufficient conditions to explain Brown and Austin's (1973) data. However, in light of the present study, their interpretation is open to question. Other differences in algal abundances were observed in the enclosure-exclosure experiments. The volume of filamentous algae was significantly lower on the grazed slides (Mann-Whitney U test, . pm-2 on the grazed slides in May. P< 0.001), averaging 0.32 Filamentous algae were not abundant during the other test periods. The diatoms Synedra sp. and Gomphonema sp. were present on control slides on Ma- July 16 but not on grazed slides. Kesler (1979) also reports reduction of these species by Amnicola. The adnate members of the Chaetophoraceae were not signif-lcantly affected by Amnicola grazing. The standine cro~ differences. in mcz AFDW. between control and grazed slides f&m thk enclosure-ekclosure experiments are given in Table 2. The calculated grazing rate of Amnicola, expressed as mg AFDW grazed . slide-1 . week-1 (grazing interval), is also given in Table 2. These grazing rates were calculated from the tempera- ture-grazing function of Kesler (1979) and the snail weights. On May 16 and July 16 the snails in the enclosures were not weighed, rmm2 (G = grazed; C = control; * = Table 1. Number of diatoms significant difference between G and C, P < 0.05) . May 16 July 16 Aug 8 Sept 11 Cocconeis --- --- 586 736 122 136 29 54 Small Diatoms 167 509* 1676 2477* 441 1133* 552 1417* Table 2. Comparison of calculated grazing rates and actual differences between grazed and control slides. May 16 July 16 Aug 8 Sept 11 AFDW difference between control and grazed 1.08 0.22 0.07 0.52 slides (in mg) Grazing rate (mg grazed - slide-1 week-1) 0.50 0.31 0.20 0.80 Table 3. Periphyton percent organic content (AFDW / DW) x 100 (G = grazed; C = control; * = significant difference between G and C, P < 0.05). May 16 July 16 Aug 8 Sept I1 Diatom relative abundance on glass slides exposed in Nonquit Figure 1. Pond, Rhode Island in 1978. A. Slides exposed continuously Slides exposed for three-week intervals. from April. B. I- Small Diatoms S W 200 m N W m a 100 a Y Diatom relative abundance on glass slides exposed in Nonquit Figure 2. Pond, Rhode Island in 1979. A. Slides exposed continuously from April. B. Slides exposed for three-week intervals. Small Diatoms t- z Small Diatom rK a but were assumed to weigh as much as the largest individual collected on those dates. The snails from August 8 and September 11 experiments were weighed. If the periphyton standing crop had remained constant in the control containers I would expect close agreement between the AFDW difference of control and grazed slides and the calculated graz- ing rate. However, increases in control standing crop in May and de- creases in the other months would cause these figures to diverge. Given this, the calculated grazing rates in Table 2 agree fairly well with the differences in standing crops of the grazed and control slides. These data lend confidence to the use of a laboratory-derived temperature-grazing function. Table 3 gives the mean percent organic content of the grazed and control slides. A Mann-Whitney U test revealed significantly greater organic content (P< 0.05) of periphyton on grazed slides on May 16 and July 16. I attribute these differences to the snails re- ducing the three dimensional character of the periphyton in a manner similar to that observed by Nicotri (1977). Removal of filamentous algae, Go honema, and loosely adherring material would result in a reduction + o a esion and entrapment of inorganic particles. The data presented here are a demonstration that gastropod grazers at near natural densities can affect periphyton standing crop, organic content and algal abundances in a freshwater lentic environ- ment. Because characteristics of Nonquit Pond do not seem to be unusual, and grazer abundances not extreme, this demonstration may have general importance. Acknowledgements I thank the Department of Zoology, University of Rhode Island for the use of laboratory facilities, and Dr. N.G. Hairston Jr. for comments on this manuscript. Access to Nonquit Pond was granted by the Newport Department of Water Works. Literature Cited Brown. S.D. and A.P. Austin. Diatom succession and interaction in iittoral periphyton and plankton. Hydrohiologia 43:333-356(1973). Calow, P. The food of Ancylus fluviatilis (Mull), a littoral, stone- dwelling herbivore. Oecologia 13 :113-133 (1973). Castenholz, R.W. The effect of grazing on marine littoral diatom pop- ulations. Ecology 42 : 783-794 (1961). Diclanan , bl. D. Changes in periphytic algae following bicarbonate addi- tions to a small stream. J. Fish. Res. Bd. Can. 30:1882-4 (1973). Diclanan, M.D. and M.B. Gochnauer. Impact of sodium chloride on the microbiota of a small stream. Envir. Poll. 17:109-126 (1978). Douglas, B. The ecology of the attached diatoms and other algae in a small stony stream. J. Ecol. 46:295-322 (1958). Eichenberger, E. and F. Schlatter. The effect of herbivorous insects on the production of benthic algal vegetation in outdoor channels. Verh. Internat. Verein. Limnol. 20 : 1806-1810 (1978) . Elwood, J.W. and D.J. Nelson. Periphyton production and grazing rates in a stream measured with a P-32 material balance method. Oikos 23:295-303 (1972). Flint, R.W. and C.R. Goldman. The effects of a benthic grazer on the primary productivity of the littoral zone of Lake Tahoc. Limnol. Oceangr. 20:935-944 (1975). Hunter, R.D. Effects of grazing on the quantity of freshwater Auf- wuchs. Hydrobiologia 63:251-259 (1980). .Jones, J.G. Spatial variation in epilithic algae in a stony stream Wilfin Beck) with particular reference to Cocconeis placentula. Freshwater Biology 8:539-546 (1978). Kehde, P.M. and J.L. Wilhm. The effects of grazing snails on commun- ity structure of periphyton in laboratory streams. Amer. Midi. Nat. 87:8-24 (1972). Kesler, D.H. Interaction of periphyton, the grazers Amnicola limosa (Gastropods) and Corynoneura scutellata (Dipteramutrients in a New England lake.= dissertation. University of Michigan, Ann Arbor, Michigan. 156pp (1979). Lubchenco, J. Plant species diversity in a marine intertidal commun- ity: Importance of herbivore food preference and algal competi- tive abflities. Amer. Nat. 112: 23-39 (1978). Menge, B.A. Organization of the New England rocky intertidal corn- ity; Role of predation, competition, and environmental hetero- geniety. Ecol. Monogr. 46: 355-393 (1976) . Moore, J.W. Some factors effecting algal consumption in subartic Ephemeroptera, Plecoptera and Simuliidae. Oecologia 27:261-273 f1977al. Moore; importance of algae in the diet of subartic populations of Gammarus lacustris and Pontoporeia affinis. Can. J. Zool. -- 55:m4Tf1977bl. -\- , Nicotri, b1.E. Grazing effects of four marine littoral herbivores on the microflora. Ecology 58 :1020-1032 (1977). Paine, R.T. The Pisaster-Tegula interaction: Prey patches, predator food preference and intertidal community structure. Ecology 50: 950-961 (1969). Paine, R.T. and R.L. Vadas. The effects of grazing by sea urchins Strongylocentrotus spp. on benthic algal populations. Limnol. Oceangr. 14 : 710-719 (1969). Patrick, R. Factors effecting the distribution of diatoms. Bot. Rev. 14 : 473- 524 (1948). Patrick, R. Benthic stream communities. Amer. Sci. 58:546-549 (1970). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Freshwater Ecology Taylor & Francis

Periphyton grazing by Amnicolalimosa: An enclosure-exclosure experiment

Kesler, David H.
Journal of Freshwater Ecology , Volume 1 (1): 9 – Mar 1, 1981
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Taylor & Francis
Copyright
Copyright Taylor & Francis Group, LLC
ISSN
2156-6941
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0270-5060
DOI
10.1080/02705060.1981.9664016
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Abstract

An enclosure-exclosure experiment was performed in Nonquit Pond, Rhode Island to test the effect of grazing by Amnicola limosa (Say) (Gastropoda) upon lentic periphyton. Periphyton from enclosures with A. limosa had higher organic content in May and July, and lower standkg crops in May, July, August, and September than periphyton from exclosures without A. limosa. Small diatom 6 18 pm long, other than Cocconeis lacenEEJ--abundances on glass slides were significantly lower in + t e enclosures than in tile exclosures. COCCO- neis abundances were not significantly affected by grazing. Cocconeis relative abundances on glass slides exposed in the littoral zone of Nonquit Pond also increased with increased grazing pressure from A. limosa. Because characteristics of Nonquit Pond do not seem to bF unusual, and grazer abundances were not extreme, these data may have general importance to the study of lentic periphyton conanunity structure. Introduction The importance of herbivores in determining the littoral zone floral cornunity structure and productivity has been demonstrated for the marine enviromnt (e. g. Castenholz, 1961 ; Paine, 1969; Paine and Vadas, 1969; Menge, 1976; Nicotri, 1977; Lubchenco, 1978), but there have been few such studies dealing with freshwater lit- toral ccmmunities. Furthermore, most studies either directly or indirectly dealing with the effects of grazers upon periphyton have been performed in lotic environments. Douglas (1958), while inves- tigating the ecology of diatoms in a small stream, observed an in- verse relations hi^ between the densi tv of the diatom Achnanthes and the abundance of ihe larval trichopte;an Agapetus fusci es. Patrick (1970) reported that the snail Physa heterostropha TdL e upon most diatom species except Cocconeis placentula. The result of this sel- ection was large populations of Cocconeis and a community w'ith reduced diversity. Elwood and N-72) estimated gastropod grazing to limit periphyton production rates by controlling peri- phyton standing crop in a stream. Dickman (1973) observed peri- phyton to be influenced by the oligochaete Stylaria lacustris which ingested mainly diatoms, apparently avoiding filamentous algae. During a road salt loading simulation, Dickman and Cochnauer (1978) 5 1 March. 1981 Journal of Freshwater Ecology. Volume 1. Number 1 Copyright 1 1981 by Oikos Publishers. Inc. attributed higher diversity at a salt-free station to the higher grazer abundances observed there. Eichenberger and Schlatter (1978) attributed deterioration of previously established vegetation in outdoor channels to grazing pressure. Both Calow (1973) and Kehde and Wilhm (1972) observed no effect of gastropod grazing in deter- mining algal species composition or diversity in a stream, altllough grazing did result in higher concentrations of chlorophyll - a in the attached flora. One of the first major studies designed to investigate the effects of grazers upon freshwater lentic periphyton was that of Flint and Goldman (1975). They found that the crayfish Pacifastacus leniusculus recycle nutrients and either increase or decrease peri- phyton primary productivity, depending upon crayfish population density. In a study similar to the present one Hunter (1980) dem- onstrated that after 45 days freshwater pulmonate snails reduced lentic periphyton standing crop and algal diversity. Chlorophyll a and nitrogen content . mg dry weighte1 were significantly increases by the assembledge of snail grazers at near natural densities. In laboratory experiments Kesler (1979) demonstrated that grazer organisms could affect lentic periphyton standing crop and species composition. Observations of changes in periphyton standing crop and species composition in the field with changing grazing pressure were consistent with predictions made from laboratory experiments. The present paper reports the results of manipulative field experi- (Say) (Gas- ments in which the effects of the grazer Amnicola limosa tropoda) were demonstrated. These effects are usemxplain changes in relative abundances of two diatom groups on field-exposed glass slides from April to September, 1978 and 1979. Methods All sampling was performed in the littoral zone of Nonquit Pond, Rllode Island, a highly stained (71 = 310), acidic reservoir (pH 5.8- 7.2) of low alkalinity 6 0.1 meq . 1- ) (see Kesler, 1979 for de- tailed description). Plastic wastebaskets were used as enclosure- exclosure containers. A hole (8x8 cm) was made on each side and covered with 1.5 mm mesh plastic screen. Four such containers were secured in 15 cm deep water in the littoral zone. Before being placed in the containers, glass microscope slides (37.5 cm2 each) were suspended for 1 week in the littoral zone of Nonquit Pond. These slides were suspended 40 cm off of the bottom so grazing by A. limosa was minimized. Sixteen of these periphyton- A. -- limosa, colonized slidzs were placed in holders in each container. small (2-4 m long) gastropods common to the littoral zone, were placed in two containers at natural densities, 45-107 snails . m-2. The snails were allowed to graze for 7 days. Water temperature was Periphyton measured at the beginning and end of the grazing period. dry weight (DW) , ash- free dry weight (AFDW) , organic content, and algal abundances were determined for the grazed and control slides (Kesler, 1979). Wet mounts were made of the slides and diatom abundances were determined (80 fields at lOOx per treatment). Periphyton species composition and abundances within the littoral zone were sampled using vertically oriented glass microscope slides placed near (5-6 cm) the bottom. Two sampling regimes were employed; one exposing the slides for three-week intervals, the other exposing slides in April and collecting them periodically throughout the summer in 1978 and 1979. Changes in periphyton community composi- tion on these slides were evaluated in light of laboratory observa- tions of hicola grazing (Kesler, 1979) and with the data obtained from the enclosure-exclosure experiments. Results and Discussion The numbers of small diatoms.and Cocconeis - IIMI-~ on the grazed and control slides from the enclosure-exclosure experiments are given in Table 1. Diatoms smaller than 20 w long were designated Cocconeis placentula was as small diatoms and grouped together. counted separately from the small diatom group because a difference in susceptibility to Amnicola grazing was observed in the laboratory. The numbers of Cocconeis on the control and grazed slides were not significantly dif-ferent WM-Whitney U test, P < 0.05). These data are consistent with others reporting resistance of Cocconeis to grazers (Patrick, 1970; Moore, 1977a, 1977b; Nicotri, 1977; Hunter, 1980). This adnate diatom can cement one valve to the substratum (Patrick, 1948), a behavior that no doubt increases its resistance to grazers. Cocconeis was not present in May on either group of slides. Conversely, the numbers of small diatoms were significantly less (Mann-Whitney U test, P < 0.05) on the grazed slides. The presence of Amnicola, presumably by grazing, decreased small diatom abundances and altered the absolute and relative abundances of this diatom group. The relative abundances of Cocconeis, small diatoms, Synedra, and other diatoms on slides exposed initially in April and co- throughout the summer are given in Figures 1A and 2A. The relative abundances of diatoms on slides exposed for three-week intervals are given in Figures 1B and 2B. The calculated grazing rates of Amnicola, in terms of pg ash-free dry weight grazed . slide-1 . day-1, are also given in Figures 1-2. These grazing rates were calculated with the density and weight of snails on the slide holders, the weekly water temperatures, and the temperature-grazing function given by Kesler (1979) . The relative abundances of Cocconeis and small diatoms were different on the three-week and continuously-exposed slides in 1978 and 1979. In 1978 on the three-week slides (Figure 1B) percent abundance of Cocconeis increased with the large increase in grazing pressure. When grazing pressure declined in July and August Cocco- neis abundances similarly declined. These changes were consistent GXli the results observed in the enclosure-exclosure experiment; Cocconeis relative abundances being higher on the grazed than on the control slides. In 1979 grazing pressure on the three-week slides (Figure 2B) was less than observed in 1978. Similarly, Cocco- neis relative abundance was also lower in 1979. Again, these data =consistent with the idea that small diatoms are more susceptible to Amnicola grazing and that grazing pressure can affect the rela- tive abundances of these diatoms. Cocconeis relative abundances were higher on the continuously- exposed slides. In 1978 Cocconeis abundance increased (Figure lA), as on the three-week slides, but once established this species re- mained dominant. Because of Cocconeisfs strong attachment to the substratum there was no reason to expect its abundance to decrease with decreasing grazing pressure on the continuously-exposed slides. Its high density (2500-3500 cells . may have inhibited colon- ization by other species. In 1979 the pattern of Cocconeis representation on the contin- uously-exposed slides (Figur- similar to that observed in 1978 (Figure lA), although the relative abundances of Cocconeis in 1979 was lower. The calculated grazing pressure in 1979 was like- wise lower than in 1978 on the continuously-exposed slides, again supporting the idea that Amnicola grazing affected diatom abundances. These data also demonstra~iportance of grazing pressure rel- ative to the appearance of new substrates. This would be especially important for species colonizing macrophytes which grow throughout the sumner. The relationship observed in this study between Cocconeis and small diatoms was also reported by Brown and Austin (ri Cocconeis and the small diatom Achnanthes. They attributed this in- verse relationship to interspeclfic competition for space. They did not have data prior to August, but reported that Cocconeis reached a maximum percent abundance in August and September, like Jones (1978), and then decreased to less than 10% from November to <January. Achnanthes dominated at three stations from September to October and peaked later in November at a fourth station. If grazers were influencing species abundances in their study, grazer activity should have been highest in August to September due to warmer water temperature and grazer recruitment. Increased grazer activity, and selection against Cocconeis and selection for small diatoms in late summer-early fall are necessary, but not sufficient conditions to explain Brown and Austin's (1973) data. However, in light of the present study, their interpretation is open to question. Other differences in algal abundances were observed in the enclosure-exclosure experiments. The volume of filamentous algae was significantly lower on the grazed slides (Mann-Whitney U test, . pm-2 on the grazed slides in May. P< 0.001), averaging 0.32 Filamentous algae were not abundant during the other test periods. The diatoms Synedra sp. and Gomphonema sp. were present on control slides on Ma- July 16 but not on grazed slides. Kesler (1979) also reports reduction of these species by Amnicola. The adnate members of the Chaetophoraceae were not signif-lcantly affected by Amnicola grazing. The standine cro~ differences. in mcz AFDW. between control and grazed slides f&m thk enclosure-ekclosure experiments are given in Table 2. The calculated grazing rate of Amnicola, expressed as mg AFDW grazed . slide-1 . week-1 (grazing interval), is also given in Table 2. These grazing rates were calculated from the tempera- ture-grazing function of Kesler (1979) and the snail weights. On May 16 and July 16 the snails in the enclosures were not weighed, rmm2 (G = grazed; C = control; * = Table 1. Number of diatoms significant difference between G and C, P < 0.05) . May 16 July 16 Aug 8 Sept 11 Cocconeis --- --- 586 736 122 136 29 54 Small Diatoms 167 509* 1676 2477* 441 1133* 552 1417* Table 2. Comparison of calculated grazing rates and actual differences between grazed and control slides. May 16 July 16 Aug 8 Sept 11 AFDW difference between control and grazed 1.08 0.22 0.07 0.52 slides (in mg) Grazing rate (mg grazed - slide-1 week-1) 0.50 0.31 0.20 0.80 Table 3. Periphyton percent organic content (AFDW / DW) x 100 (G = grazed; C = control; * = significant difference between G and C, P < 0.05). May 16 July 16 Aug 8 Sept I1 Diatom relative abundance on glass slides exposed in Nonquit Figure 1. Pond, Rhode Island in 1978. A. Slides exposed continuously Slides exposed for three-week intervals. from April. B. I- Small Diatoms S W 200 m N W m a 100 a Y Diatom relative abundance on glass slides exposed in Nonquit Figure 2. Pond, Rhode Island in 1979. A. Slides exposed continuously from April. B. Slides exposed for three-week intervals. Small Diatoms t- z Small Diatom rK a but were assumed to weigh as much as the largest individual collected on those dates. The snails from August 8 and September 11 experiments were weighed. If the periphyton standing crop had remained constant in the control containers I would expect close agreement between the AFDW difference of control and grazed slides and the calculated graz- ing rate. However, increases in control standing crop in May and de- creases in the other months would cause these figures to diverge. Given this, the calculated grazing rates in Table 2 agree fairly well with the differences in standing crops of the grazed and control slides. These data lend confidence to the use of a laboratory-derived temperature-grazing function. Table 3 gives the mean percent organic content of the grazed and control slides. A Mann-Whitney U test revealed significantly greater organic content (P< 0.05) of periphyton on grazed slides on May 16 and July 16. I attribute these differences to the snails re- ducing the three dimensional character of the periphyton in a manner similar to that observed by Nicotri (1977). Removal of filamentous algae, Go honema, and loosely adherring material would result in a reduction + o a esion and entrapment of inorganic particles. The data presented here are a demonstration that gastropod grazers at near natural densities can affect periphyton standing crop, organic content and algal abundances in a freshwater lentic environ- ment. Because characteristics of Nonquit Pond do not seem to be unusual, and grazer abundances not extreme, this demonstration may have general importance. Acknowledgements I thank the Department of Zoology, University of Rhode Island for the use of laboratory facilities, and Dr. N.G. Hairston Jr. for comments on this manuscript. Access to Nonquit Pond was granted by the Newport Department of Water Works. Literature Cited Brown. S.D. and A.P. Austin. Diatom succession and interaction in iittoral periphyton and plankton. Hydrohiologia 43:333-356(1973). Calow, P. The food of Ancylus fluviatilis (Mull), a littoral, stone- dwelling herbivore. Oecologia 13 :113-133 (1973). Castenholz, R.W. The effect of grazing on marine littoral diatom pop- ulations. Ecology 42 : 783-794 (1961). Diclanan , bl. D. Changes in periphytic algae following bicarbonate addi- tions to a small stream. J. Fish. Res. Bd. Can. 30:1882-4 (1973). Diclanan, M.D. and M.B. Gochnauer. Impact of sodium chloride on the microbiota of a small stream. Envir. Poll. 17:109-126 (1978). Douglas, B. The ecology of the attached diatoms and other algae in a small stony stream. J. Ecol. 46:295-322 (1958). Eichenberger, E. and F. Schlatter. The effect of herbivorous insects on the production of benthic algal vegetation in outdoor channels. Verh. Internat. Verein. Limnol. 20 : 1806-1810 (1978) . Elwood, J.W. and D.J. Nelson. Periphyton production and grazing rates in a stream measured with a P-32 material balance method. Oikos 23:295-303 (1972). Flint, R.W. and C.R. Goldman. The effects of a benthic grazer on the primary productivity of the littoral zone of Lake Tahoc. Limnol. Oceangr. 20:935-944 (1975). Hunter, R.D. Effects of grazing on the quantity of freshwater Auf- wuchs. Hydrobiologia 63:251-259 (1980). .Jones, J.G. Spatial variation in epilithic algae in a stony stream Wilfin Beck) with particular reference to Cocconeis placentula. Freshwater Biology 8:539-546 (1978). Kehde, P.M. and J.L. Wilhm. The effects of grazing snails on commun- ity structure of periphyton in laboratory streams. Amer. Midi. Nat. 87:8-24 (1972). Kesler, D.H. Interaction of periphyton, the grazers Amnicola limosa (Gastropods) and Corynoneura scutellata (Dipteramutrients in a New England lake.= dissertation. University of Michigan, Ann Arbor, Michigan. 156pp (1979). Lubchenco, J. Plant species diversity in a marine intertidal commun- ity: Importance of herbivore food preference and algal competi- tive abflities. Amer. Nat. 112: 23-39 (1978). Menge, B.A. Organization of the New England rocky intertidal corn- ity; Role of predation, competition, and environmental hetero- geniety. Ecol. Monogr. 46: 355-393 (1976) . Moore, J.W. Some factors effecting algal consumption in subartic Ephemeroptera, Plecoptera and Simuliidae. Oecologia 27:261-273 f1977al. Moore; importance of algae in the diet of subartic populations of Gammarus lacustris and Pontoporeia affinis. Can. J. Zool. -- 55:m4Tf1977bl. -\- , Nicotri, b1.E. Grazing effects of four marine littoral herbivores on the microflora. Ecology 58 :1020-1032 (1977). Paine, R.T. The Pisaster-Tegula interaction: Prey patches, predator food preference and intertidal community structure. Ecology 50: 950-961 (1969). Paine, R.T. and R.L. Vadas. The effects of grazing by sea urchins Strongylocentrotus spp. on benthic algal populations. Limnol. Oceangr. 14 : 710-719 (1969). Patrick, R. Factors effecting the distribution of diatoms. Bot. Rev. 14 : 473- 524 (1948). Patrick, R. Benthic stream communities. Amer. Sci. 58:546-549 (1970).

Journal

Journal of Freshwater Ecology – Taylor & Francis

Published: Mar 1, 1981

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