The influence of crevice size on the protection of epilithic algae from grazers

1. This study investigated how the size of crevices might affect their effectiveness as refuges for diatom‐dominated algal assemblages from the grazing minnow Campostoma anomalum. 2. Crevice size was manipulated by making fired‐clay substrates, using moulds to produce eight substrates with pits from 1.17 to 22.0 mm diameter. Non‐pitted clay‐stones and limestone were also tested. Cages were used to control the access of Campostoma to arrays of the 10 different algal‐colonised substrates. The grazing treatments were: open and grazed, caged and ungrazed, and a grazed cage control. The experiment was replicated in eight large outdoor tanks. After 3 weeks, substrates were brushed and chlorophyll a concentrations of the removed algae and the algae remaining in pits were measured. 3. The experiment was field‐validated by exposing arrays of substrates to grazing Campostoma in five pools of a limestone stream. 4. The clay‐stone and limestone substrates accrued similar algal biomass and assemblages. 5. Smaller crevices provided more protection against grazing than larger crevices. Specifically, pits with openings smaller than 2 mm protected the enclosed algal assemblages in both the tank and field experiments. Larger pits provided less protection and pits over 7 mm in diameter were heavily grazed and may even be preferentially grazed by Campostoma. 6. None of the tested pit sizes were protective against larval chironomid grazers in the tank experiment, demonstrating that differences in the grazer size influence the effectiveness of crevice refuges.     

Grazing and the distribution of sediment particle sizes in artificial stream systems

Recent research has shown that grazing by the algivorous minnows of the genus Campostoma can have a significant effect on both structural and functional parameters in stream ecosystems, influencing algal height and type, primary productivity, carbon dynamics, bacterial biomass and the size fractionation of benthic organic matter. This study was undertaken to evaluate the effect of minnow grazing on benthic particulate organic matter (BPOM) under the controlled conditions available in experimental streams. For this study, four grazed and four ungrazed artificial streams were sampled for BPOM on two dates. The samples were partitioned into large, medium, fine and ultrafine fractions via wet filtration. Fish grazed systems had significantly larger percentages of the fine fraction and significantly smaller percentages of the ultrafine fraction. The ability of grazers to alter particle size distribution is important to overall stream organic matter dynamics because recent studies have shown the importance of particle size in determining bacterial numbers and activity, which in turn influence fundamental stream process like respiration and organic carbon dynamics.     

Responses of stream algae to grazing minnows and nutrients: a field test for interactions

Summary Previous studies have shown that an algivorous grazing minnow (Campostoma anomalum) is the major herbivore in Brier Creek, a hardwater stream in south central Oklahoma. In summer and autumn schools of Campostoma virtually eliminate algae from substrate surfaces in deeper areas of some pools. The pool-to-pool distributions of algae and Campostoma reported for this stream could occur if nutrient limitation permits grazing by Campostoma to outrun algal growth. To test this hypothesis, mesh pens were built to exclude Campostoma from substrates in each of four typical Campostoma pools. N+P+K lawn fertilizer was added daily to two of the four pools; the other two, which received no fertilizer additions and which were not visibly affected by fertilizer transported downstream from the pools enriched with nutrients, served as controls. Algae accumulated rapidly on natural substrates and on unglazed ceramic tiles in grazer-exclusion pens in pools receiving N+P+K additions and more slowly in pens in both control pools. Periphyton biomass on grazed substrates in all four pools remained low throughout the experiment. Hence, Campostoma at normal densities were able to outrun algal growth even when nutrients were added. Eleven days after the experiment started, I determined biomass, biomass-specific net primary productivity, and areal net primary productivity of periphyton on substrates exposed to all combinations of grazer (+,0) and nutrient (+,0) treatments. Grazing increased biomass-specific primary productivity, prevented accumulation of biomass, and decreased areal primary productivity of periphyton. Additions of N+P+K increased biomass-specific net primary productivity of grazed and ungrazed periphyton and markedly increased biomass of periphyton on substrates protected from Campostoma. Although food supply for Campostoma appeared to be greater with nutrient additions, condition of Campostoma in pools receiving N+P+K was not significantly different from Campostoma collected from control pools 35 days after the experiment started. I conclude that although nutrient supply limits biomass-specific primary productivity of periphyton in Brier Creek, nutrient limitation in this stream exacerbates, rather than causes, the visually conspicuous pool-to-pool complimentary distribution of algae and Campostoma: in this stream, grazing by Campostoma at natural densities can outrun periphyton growth even when nutrients are added.     

Depth distribution of Campostoma grazing scars in an Ozark stream

Synopsis Campostoma spp., widespread and abundant herbivorous minnows of eastern North America, produce distinctive grazing scars when feeding on algae attached to natural substrates in streams. These scars are particularly prominent upon the low growth forms of blue-green algae that dominate the attached algal flora of many upland streams. In one stream pool in the Ozark uplands of Oklahoma, numbers and sizes of grazing scars coincided with numbers and sizes of individual Campostoma that occurred across a depth gradient, demonstrating that the information contained in the scars can provide quantification of microhabitat use and grazing intensity of these important herbivores. The results also support the hypothesis that in environments free of aquatic predators, larger fish use deeper parts of available stream habitats, particularly if threats from terrestrial or avian predators exist.     

Benthic microalgal diversity enhanced by spatial heterogeneity of grazing

This study presents model experiments on the effect of the spatial pattern of herbivory on primary producer diversity. Microalgal biofilms (periphyton) were exposed to different mixtures of two benthic herbivores, the isopod Idothea chelipes and the gastropod Littorina littorea. The herbivores are similar in their feeding selectivity but differ strongly in the spatial pattern of grazing. Idothea did not increase the spatial heterogeneity of algal cell densities beyond the level of ungrazed controls (<1 order of magnitude between local minima and maxima at the 1 mm² scale). Littorina grazing, in contrast, created a pronounced spatial heterogeneity with maximum:minmum ratios of almost 3 orders of magnitude. When algae were exposed to mixtures of both gazers, the spatial heterogeneity of microalgal cell densities increased with an increasing proportion of Littorina in the herbivore mixture. Algal species richness, diversity and evenness also increased with increasing proportions of Littorina, and was highly significantly correlated with the spatial heterogeneity of cell densities. Received: 25 May 1999 / Accepted: 14 September 1999     

Temporal patterns of grazer-periphyton interactions in laboratory streams

SUMMARY. 1. The snail Juga silicuta (500 m^?2) and the caddisfly Dicosmoecus gilvipes (50 m^?2) were introduced into separate laboratory streams on days 1, 9, 16 and 28 of algal development. The mayfly Baetis spp. (500 m^?2) was introduced on days 1 and 16, and two streams did not receive grazers. We assessed the interaction between succession in the pcriphyton, herbivore type and time of encounter in a 40-day experiment. 2. In ungrazed streams, the chlorophyte Scenedesmus obliquus was the most abundant early colonizer. The relative abundance of diatoms increased after day 9, and at day 40 the algal assemblage consisted of a thick mat of diatoms and S. obliquus with an overstorey of filaments of the chlorophyte Stigeoclonium tenue. In general, introductions of grazers at any stage altered this pattern by removing biomass, accelerating the replacement of S. obliquus by diatoms, and suppressing the growth of filaments. Grazing also reduced the relative abundance of the larger diatom Nitzschia oregona but increased the relative abundance of the smaller adnate diatoms Nitzschia frustulum var. perpusilla and Navicula minima. 3. Dicosmoecus decreased algal biomass and altered successional trajectories to a greater degree than either Juga or Baetis. Dicosmoecus rapidly grazed the entire substrate, whereas Juga and Baetis only cleared patches in the assemblages. Little alteration in algal development was observed in the Baetis streams after day 16, probably because (he periphyton assemblages attained a size and structure that prevented effective grazing by Baetis. 4. The patchy grazing patterns of Juga and Baetis resulted in more diverse algal assemblages than either the Dicosmoecus grazed or ungrazed streams. In natural streams, the temporal and spatial pattern of grazing relative to the developmental stage of the periphyton may contribute to maintaining a mosaic of algal patches in different serai stages.     

How do grazers affect periphyton heterogeneity in streams?

The effects of grazing by stream invertebrates on algal biomass and spatial heterogeneity were tested experimentally in flow-through microcosms with natural substrates (rocks). One experiment tested the effects of fixed densities of three species of grazers (the caddisfly Allomyia sp. and two mayflies, Epeorus deceptivus and Baetis bicaudatus) on periphyton. Baetis was tested with and without chemical cues from fish predators, which reduced grazer foraging activity to levels similar to the less mobile mayfly (Epeorus). Mean algal biomass (chlorophyll a; chl a) was reduced in grazer treatments compared to ungrazed controls, but there were no differences among grazer treatments. Algal heterogeneity (Morisita index) increased with grazer mobility, with the highest heterogeneity occurring in the Baetis-no fish treatment (most mobile grazer) and the lowest in the caddisfly treatment (most sedentary grazer). A second experiment used a three factorial design, and tested whether initial resource distribution (homogeneous vs. heterogeneous), Baetis density (high vs. low) and fish odor (present vs. absent) affected grazer impact on algal resources. Abundances of Baetis and chl a on individual rocks were recorded to explore the mechanisms responsible for the observed distributions of algae. Initial resource heterogeneity was maintained despite being subjected to grazing. Mean chl a was highest in controls, as in experiment I, and effects of Baetis on algal biomass increased with grazer density. There were no fish effects on algal biomass and no effects of grazer density or fish on algal heterogeneity. At the scale of individual rocks Baetis was unselective when food was homogeneously distributed, but chose high-food rocks when it was heterogeneously distributed. Results of these mechanistic experiments showed that Baetis can track resources at the scale of single rocks; and at moderate densities mobile grazers could potentially maintain periphyton distributions observed in natural streams.     

The influence of moss on grazers in high‐altitude streams: food,refuge or both?

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Livestock exclusion increases the spatial heterogeneity of vegetation in Colorado shortgrass steppe

Abstract. Spatial heterogeneity, an important characteristic in semi‐arid grassland vegetation, may be altered through grazing by large herbivores. We used Moran's I, a measure of autocorrelation, to test the effect of livestock grazing on the fine scale spatial heterogeneity of dominant plant species in the shortgrass steppe of northeastern Colorado. Autocorrelation in ungrazed plots was significantly higher than in grazed plots for the cover of the dominant species Bouteloua gracilis, litter cover and density of other bunchgrasses. No species had higher autocorrelation in grazed compared to ungrazed sites. B. gracilis cover was significantly auto‐correlated in seven of eight 60‐yr ungrazed exclosures, four of six 8‐yr exclosures, and only three of eight grazed sites. Autocorrelograms showed that B. gracilis cover in ungrazed sites was frequently and positively spatially correlated at lag distances less than 5 m. B. gracilis cover was rarely autocorrelated at any sampled lag distance in grazed sites. The greater spatial heterogeneity in ungrazed sites appeared linked to patches characterized by uniformly low cover of B. gracilis and high cover of C₃ grasses. This interpretation was supported by simple simulations that modified data from grazed sites by reducing the cover of B. gracilis in patches of ca. 8 m diameter and produced patterns quite similar to those observed in ungrazed sites. In the one exclosure where we intensively sampled soil texture, autocorrelation coefficients for sand content and B. gracilis cover were similar at lag distances up to 12 m. We suggest that the negative effect of sand content on B. gracilis generates spatial heterogeneity, but only in the absence of grazing. An additional source of heterogeneity in ungrazed sites may be the negative interaction between livestock exclusion and B. gracilis recovery following patchy disturbance.     

The role of light availability and herbivory on algal responses to nutrient enrichment in a riparian wetland,Alaska

We investigated how the relative availability of solar radiation in the presence or absence of grazing alters the ability of benthic algae to respond to nutrient enrichment in an Alaskan marsh. We used a factorial mesocosm experiment that included nutrient enrichment (enriched or control), grazing (grazed or ungrazed), and light (unshaded or shaded) to simulate shading by macrophytes early and late in the growing season, respectively. We found stronger effects of grazers and nutrients compared to light on benthic algal biomass and taxonomic composition. Algal biomass increased in nutrient‐enriched treatments and was reduced by grazing. Shading did not have an effect on algal biomass or taxonomic composition, but the concentration of chl a per algal biovolume increased with shading, demonstrating the ability of algae to compensate for changes in light availability. Algal taxonomic composition was more affected by grazer presence than nutrients or light. Grazer‐resistant taxa (basal filaments of Stigeoclonium) were replaced by diatoms (Nitzschia) and filamentous green algae (Ulothrix) when herbivores were removed. The interacting and opposing influences of nutrients and grazing indicate that the algal community is under dual control from the bottom‐up (nutrient limitation) and from the top‐down (consumption by herbivores), although grazers had a stronger influence on algal biomass and taxonomic composition than nutrient enrichment. Our results suggest that low light availability will not inhibit the algal response to elevated nutrient concentrations expected with ongoing climate change, but grazers rapidly consume algae following enrichment, masking the effects of elevated nutrients on algal production.     

High nutrient availability leads to weaker top‐down control of stream periphyton: Compensatory feeding in Ancylus fluviatilis

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The effects of grazing on the distribution and composition of low-shore algal communities on the central coast of Portugal and on the southern coast of Britain

A red algal turf is often found just below the barnacle/limpet zone of many European shores, especially on steep shores of moderate exposure. The hypothesis that grazing by limpets determines the upper limit of distribution of this red algal turf was tested on moderately exposed shores in Portugal and Britain. We also aimed to assess whether the grazing effect is modified at various spatial scales. Grazers were excluded by fences, with half-fenced and unfenced controls. Exclusion plots were rapidly colonised by green ephemeral algae in the months immediately after the beginning of the experiment (summer); these algae were later replaced by perennial algae. The percentage cover of turf-forming macroalgae showed a significant increase at both locations. The upper limit of distribution extended more than 50 cm on most of the shores studied. In contrast, control and half-fenced plots remained devoid of algae. After 2 years, ungrazed plots were mainly colonised by a red algal turf (e.g. Caulacanthus ustulatus, Gelidium spp., Laurencia pinnatifida) in Portugal, while canopy cover (Fucus serratus and Himanthalia elongata) dominated in Britain in marked contrast to the grazed plots. Physical factors acting at both local and geographical scales may explain these differences. However, although physical factors probably have an important influence on the identity, size and abundance of sublittoral fringe macroalgae, grazers play a major role in directly setting their upper limits. The effect of grazing by limpets was not consistent for all of the morphological algal groups and spatial scales considered in the present study. The effect of grazing on the cover of turf algae varied between Portugal and Britain (location scale), while effects on ephemeral and canopy algal cover varied at the shore scale within location.     

Nuisance biomass levels of periphytic algae in streams

Relative coverage of filamentous periphytic algae increased with chlorophyll a (chl a) biomass on natural substrata in 22 northwestern United States and Swedish streams. A biomass range of 100–150 mg chl a m⁻² may represent a critical level for an aesthetic nuisance; below those levels, filamentous coverage was less than 20%. Other indices of water quality (dissolved oxygen content and measures of benthic macroinvertebrate diversity) were apparently unaffected by periphytic biomass or filamentous coverage in these streams. Neither was biomass related to limiting nutrient content (soluble reactive phosphorus, SRP), as has been observed in previous experiments using bare rocks in streams and slides in artificial channels. Ambient SRP concentration may not be a useful predicter of periphyton accrual on natural substrates, due to uptake and recycling of P throughout the stream and undetermined losses such as sloughing and grazing.     

Food Limitation Affects Algivory and Grazer Performance for New Zealand Stream Macroinvertebrates

We studied herbivory and grazer performance (i.e., fitness correlates) for the hydrobiid snail Potamopyrgus antipodarum, the leptophlebiid mayfly Deleatidium spp., and the conoesucid caddisfly Pycnocentrodes aeris, common, co-occurring algivores in many New Zealand streams. Grazing effects and costs of coexisting differed among these taxa reared at ambient densities in different combinations in microcosms with algal food conditions (on clay tiles) characteristic of heavily grazed streams. The prostrate diatoms Staurosirella leptostauron, Cymbella novazealandia, and Achnanthidium minutissimum were the dominant algal species on pre- and post-grazed tiles. The relative abundance of erect physiognomic forms, dominated by Synedra ulna and Fragilaria vaucheriae, were 2–3× higher in ungrazed controls and in snail alone treatments than in other grazer treatments. The green filamentous algae Mougeotia sp. and Stigeoclonium lubricum, and the cyanophyte Merismopedia glauca were present only in ungrazed controls. Grazers significantly reduced algal community biomass in treatments by 26–52% relative to controls, except snails alone. Snails (15–30%) burrowed into surrounding sand substrates, dampening their grazing impact on tiles. Caddisflies were more effective than mayflies or snails at removing algae because of higher foraging rates, a larger body size, and an abrasive sand-grained case. Algal biomass reductions did not affect grazer growth. However, pre-pupation rates of caddisflies and emergence rates of subimago mayflies were significantly higher in caddisfly-alone and mayfly-alone treatments, respectively, than in combined-species treatments. These results imply that a limited periphytic food supply ( < 0.3 mg AFDM cm⁻²) even over a relatively brief period ( ≤ 16 d) may have population-scale consequences for co-existing P. aeris and Deleatidium spp.     

Effects of an invertebrate grazer on the spatial arrangement of a benthic microhabitat

We demonstrated the effect of an aquatic herbivore on the spatial arrangement of benthic algal biomass within artificial stream channels. Transects of ceramic tiles were exposed to a gradient of snail (Physella) densities in a 30 d experiment. We observed positive effects of snails on the mean abundance of overstory algae (the filamentous chlorophyte Cladophora and associated epiphytes), an important benthic microhabitat in streams. Snails altered several aspects of the spatial arrangement of overstory algae. Snails reduced the strength of downstream gradients in overstory biomass, as well as residual variability around these gradients. Geostatistical analysis revealed that snails also reduced the strength of spatial dependence, and so reduced spatial heterogeneity of the overstory, at small scales (<40 cm). As a result, organisms inhabiting the overstory might experience a more fragmented habitat landscape at high snail densities. In addition, snails increased the scale of spatial dependence in understory algal biomass (algae remaining on tiles after overstory was removed) from 10 cm to 40 cm. Consumer effects on the spatial arrangement of a microhabitat argue for the inclusion of feedbacks between the biota and the environment in spatially-explicit models.     

INDUCTION AND REDUCTION OF ANTI‐HERBIVORE DEFENSES IN BROWN AND RED MACROALGAE OFF THE KENYAN COAST1

Herbivory is particularly intense in tropical benthic communities, suggesting preference of constitutive, rather than inducible, anti‐herbivory defense. The objective of the study was to examine whether anti‐herbivore defenses in the red alga Hypnea pannosa J. Agardh and the brown algae Sargassum asperifolium Hering and G. Martens ex J. Agardh and Cystoseira myrica (S.G. Gmelin) C. Agardh could be induced and subsequently reduced in response to grazing by the amphipod Cymadusa filosa Savigny. During a 14‐day treatment phase, algae were exposed to amphipod grazing or were left ungrazed (control). Subsequently, one subset of algae was used in feeding assays, whereas another was cultivated for additional 14 days without consumers (recovery phase). At the end of each phase, bioassays were conducted to detect defensive traits in terms of differences in consumption rates of grazed and control pieces of live algae and agar‐based food containing nonpolar algal extracts. Consumption of grazed live S. asperifolium and H. pannosa specimens was lower than of control algae. Furthermore, nonpolar extracts of grazed S. asperifolium and C. myrica were less preferred than those from control algae. Defensive responses were exclusively detected after the treatment phase, although strong preference of ungrazed H. pannosa and C. myrica over grazed conspecifics continued throughout the recovery phase. These findings suggest that phenotypic plasticity in anti‐herbivory defense of marine macroalgae 1) might be more common than previously shown, 2) could be switched on and off within 2 weeks, and 3) can be found in nonpolar algal extracts.     

Role of sediment and internal loading of phosphorus in shallow lakes

Current velocity is a pervasive feature of lotic systems, yet this defining environmental variable is rarely examined as a factor for regulating stream herbivory. To investigate how current modifies herbivory in the upper Colorado River, U.S.A., loops of electrified fencing wire were used to reduce in situ grazer densities on 30 × 30 cm tile substrates. After 45 d, electrified tiles had significantly fewer grazers (P = 0.03) and >2X more algal biomass than controls (P = 0.0002). Reduced grazing on electrified tiles yielded periphytic assemblages having more diatoms and chlorophytes, as well as greater algal species richness. Current velocity effects alone did not significantly regulate algal abundance; however, the interaction between current velocity and grazer exclusion resulted in more algae in slow vs. fast current (P = 0.02). Grazer abundances were similar between fast and slow current velocities, suggesting that grazers in the Colorado River differ in their ability to regulate algae across the current velocity gradient. Our results indicate that stream current-mediated herbivory in streams may be more important than is generally recognized.     

Seasonal changes in composition and biomass of epilithic algal floras of a chalk stream and a soft water stream with estimates of production

Microscopic epilithic algae in the River Itchen at Otterbourne near Southampton and in the Ober Water in the New Forest were studied during 1984 and 1985. The River Itchen rises from chalk springs and has a steady pH near 8.2 and a mean alkalinity of 236 mg HCO₃ 1^–1; at the study site the river is about 16 m wide and 20 cm deep, with a mean flow rate of 0.33 m s^–1 and a discharge ranging through the year between 0.34 and 2.46 m³ s^–1. The Ober Water, which drains sands and gravels, has a pH between 6.9 and 7.2 and a mean alkalinity of about 50 mg HCO₃ 1^–1; at the study site it is about 6 m wide, with a mean flow rate of 0.27 m s^–1 and a discharge ranging through the year between 0.08 and 1.0 m³ s^–1.Epilithic algae removed from the pebbles that form the major part of the beds of both streams show seasonal changes in abundance and composition. Diatoms peaked in April/May and dominate the epilithic flora in both streams, comprising 70–95% of all algal cells; highest numbers of chlorophytes occurred in summer and cyanophytes increased in autumn. The species composition of the epilithic flora in the two streams was different, as was the population density; algal cell numbers ranged between 500 and 7000 cells mm^–2 of stream floor in the River Itchen and between 8 and 320 cells mm^–2 of stream floor in the Ober Water. The chlorophyll a content of epilithic algae in the River Itchen ranged between 115 and 415 mg m^–2 of stream floor, representing an annual mean biomass of about 8 g m^–2, whereas in the Ober Water a chlorophyll a content of 2.2 to 44 mg m^–2 of stream floor was found, representing an annual mean biomass of about 1 g m^–2. Cautious estimates of the annual production of epilithic algae in these streams suggest a value of about 600 g organic dry weight m^–2 in the River Itchen and about 75 g m^–2 in the Ober Water.     

Response of grazing impacts of an algivorous fish (Pseudogastromyzon myersi: Balitoridae) to seasonal disturbance in Hong Kong streams

1. Manipulative experiments were carried out in four Hong Kong streams (two shaded, two unshaded) to investigate the impact of grazing by an algivorous fish, Pseudogastromyzon myersi, on benthic algal biomass and assemblage composition. Experiments were conducted and repeated during both the dry and wet seasons to determine whether spate‐induced disturbance modified any grazing effect. Treatments comprised fish exclusion and inclusion via closed and open cages, with a no‐cage treatment used as a control for the cage effect. Treatments were maintained for 4 weeks in each experimental run. 2. Grazing by P. myersi reduced benthic algal biomass and the organic matter content of periphyton in open cages and the no‐cage treatment relative to closed cages. The similarity between open‐cage and no‐cage treatments was evidence that the overall difference among treatments was caused by limiting fish access to closed cages and not merely an artifact of caging. Grazing effects were broadly similar in all streams, but there was a significant statistical interaction between treatments and seasons. 3. Analysis of dry‐season data matched the overall trend in inter‐treatment differences, confirming the effects of grazing by P. myersi on algal biomass and periphyton organic matter. Significant differences in algal assemblage composition between closed‐cage and no‐cage treatments during the dry season reflected reductions in the abundance of erect, stalked diatoms (Gomphonema) and filamentous cyanobacteria (Homeothrix). Removal of these vulnerable overstorey algae by P. myersi resulted in greater abundance of understorey diatoms (Achnanthes and Cocconeis) in the no‐cage treatment in all streams during the dry season. The composition of algal assemblages in open cages was intermediate between the other two treatments. 4. Although fish densities were greater in all streams during the wet season, spate‐induced disturbance obscured grazing effects and there were no significant differences among treatments attributable to fish grazing. Seasonal variation in impacts of P. myersi grazing provides support for the harsh‐benign hypothesis, and confirms that biotic factors are less important controls of stream algal biomass and assemblage structure during periods (i.e. the wet season in Hong Kong) when abiotic disturbances are frequent or intense.     

Periphyton Architecture and Susceptibility to Grazing by Periwinkles (Littorina littorea,Gastropoda)

Periphyton was grown on transparent plastic substrata in the Kiel Fjord and used for short-term laboratory experiments to study the feeding selectivity of the periwinkle Littorina littorea in response to the vertical structure of the periphyton. The susceptibility of algae to periwinkle grazing was assessed by comparing the species-specific biomass within the grazing tracks of the snails to the biomass outside the tracks. After 3 weeks of incubation, the periphyton consisted of a scattered monolayer of algal cells without vertical structure. No apparent grazing could be found. After 6 weeks of incubation, periphyton consisted of a tightly attached undergrowth (mainly Cocconeis scutellum, Bacillariophyceae, and Myrionema sp., Phaeophyceae) and canopy of filamentous (Melosira moniliformis, Bacillariophyceae) and stalked forms (Achnanthes longipes, Bacillariophyceae). The unicellular diatoms Fragilaria tabulata and Stauroneis constricta grew partly on the primary substratum and partly as epiphytes on the canopy species. The canopy species and the epiphytes were decimated inside the grazing tracks, while the tightly attached undergrowth species appeared ungrazed.