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

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Herbivory,current velocity and algal regrowth: how does periphyton grow when the grazers have gone?

1. An experiment conducted in streamside channels was used to document the regrowth of grazed periphyton. Our objective was to determine the relative importance of current velocity, grazing duration, and grazer type in shaping the trajectory of algal and periphytic regrowth. 2. The grazing mayflies Baetis bicaudatus and Epeorus longimanus were used alone and in combination to create three grazing treatments at slow, medium and fast current (2–5, 15–20 and 30–40 cm s^?1, respectively). Duration treatments consisted of 2, 4, 6, 8, 10 days of grazing. Chlorophyll a and ash‐free dry mass (AFDM) accumulation on grazed tiles was measured (as periphytic AFDM and chlorophyll a, respectively) at 2, 4, 6, 8 and 10 days following the removal of grazers. 3. Chlorophyll a and AFDM was best predicted by interactions between current velocity, grazing duration and regrowth time. 4. The two grazer species did not differ in their effect on Chlorophyll a and AFDM during the period of periphytic regrowth that followed grazing. 5. Longer grazing duration reduced periphytic biomass, but also accelerated algal regrowth, and this growth enhancement was more pronounced at slower current velocities. 6. Data from this study suggest that herbivory can have important historical effects on periphytic accrual.     

Periphyton responses to invertebrate grazing and riparian canopy in three northern California coastal streams

SUMMARY. 1. Field experiments were conducted to examine the impact of grazing invertebrates on periphyton biomass in twenty-one pools across three northern California coastal streams (U.S.A.): Big Sulphur Creek, the Rice Fork of the Eel River, and Big Canyon Creek. Periphyton accrual on artificial substrate tiles was compared in each stream between two treatments: those elevated slightly above the stream bottom to reduce access by grazers (= platforms) and those placed directly on the stream bottom to allow access by grazers (=controls).
2. Crawling invertebrate grazers (cased caddisflies and snails) were numerically dominant in each stream (86% of all grazers in Big Sulphur Creek, 61% in the Rice Fork, 84% in Big Canyon Creek). Platforms effectively excluded crawling grazers, but were less effective in excluding swimming mayfly grazers (Baetidae).
3. Periphyton biomass (as AFDM) on tiles was significantly lower on controls compared to platforms for the Rice Fork, an open-canopy stream, and Big Sulphur Creek, a stream with a heterogeneous canopy. In contrast, no grazer impact was found for Big Canyon Creek, a densely shaded stream. Here, extremely low periphyton biomass occurred for both treatments throughout the 60 day study.
4. The influence of riparian canopy on periphyton growth (i.e. accrual on platforms), grazer impact on periphyton, and grazer abundance was examined for Big Sulphur Creek. As canopy increased (15–98% cover), periphyton biomass on platforms decreased. In contrast, canopy had little influence on periphyton accrual on controls; apparently, grazers could maintain low periphyton standing crops across the full range of canopy levels. The abundance of one grazer species, the caddisfly Gumaga nigricula , was highest in open, sunlit stream pools; abundance of two other prominent grazers, Helicopsyche borealis (Trichoptera) and Centroptilum convexum (Ephemeroptera), however, was unrelated to canopy.     

The effect of two scales of habitat architecture on benthic grazing in a river

1. This experiment studied the effects of differing levels of the complexity of substratum architecture at two spatial scales on the distribution and abundance of benthic algae and invertebrates, and the strength of the trophic interaction between invertebrate grazers and algae. Some estimates of the effects on invertebrate colonization rates were also made. 2. Four levels of microhabitat architectural complexity were created using artificial substrata (clay tiles) and placed in Mountain River, Tasmania, in two riffle types (bedrock and boulder-cobble) of differing large-scale substratum complexity. After a colonization period, invertebrate grazers were removed from half the tiles to measure the effects of grazing. Invertebrates on the tiles were also counted and identified. At the end of the experiment, algae were removed from the tiles and analysed for chlorophyll a. 3. Invertebrate grazers did not reduce algal biomass during the experiment, and microhabitat-scale architecture influenced algal biomass more strongly than riffle-scale architecture. Highly complex microhabitat architecture increased algal biomass by providing more surface area, but once standardized for surface area, algal biomass decreased as the complexity of microhabitat architecture increased. 4. Microhabitat-scale architecture was also predominant in determining invertebrate density and the identity of the dominant grazer species. In contrast to algal biomass, invertebrate densities and species density increased with the complexity of microhabitat architecture, suggesting that refuges from flow (and possibly predation) were as important to river invertebrates as the distribution of their food source. 5. Riffle-scale architecture had some effect on the colonization of two slow-moving grazer taxa, but, overall, the colonization processes of slow-moving grazers were determined mostly by the complexity of microhabitat-scale architecture.     

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.     

Community organization in streams: the importance of species interactions,physical factors,and chance

Summary Experimental studies were used to examine the mechanisms governing the distribution and abundance of two major patch types in unshaded reaches of Augusta Creek, Michigan (USA). One patch type is dominated by Cladophora glomerata, a macroalga potentially able to monopolize space, whereas the other type is comprised of a low-growing, epilithic microalgal lawn inhabited by several species of sessile grazers (especially the caddisflies Leucotrichia pictipes and Psychomyia flavida). Cladophora patches are absent from mid-channel sites characterized by current velocities ca. 50 cm s^–1; caging experiments indicate that their absence is due to grazing by crayfish (Orconectes propinquus). Cladophora's presence in sites with velocities >50 cm s^–1 apparently results in part because crayfish foraging activity is impaired in high flow regimes. The presence of Cladophora strongly affects various other invertebrates due to its alteration of abiotic and biotic characteristics of the microhabitat. For example, the abundance of sessile grazers (e.g. Leucotrichia and Psychomyia) that inhabit microalgal patches is negatively correlated to the abundance of Cladophora, whereas the abundance of several other invertebrates (e.g. Stenonema mayflies and Taeniopteryx stoneflies) is positively correlated to Cladophora's abundance. Therefore, in some portions of this system, crayfish act as keystone predators because of their ability to regulate the abundance of Cladophora, which in turn has strong positive and negative effects on other components of the community. Cladophora does not always monopolize space at high velocities in the absence of crayfish, however. If sessile grazers arrive at such sites before Cladophora, they can prevent its establishment. Thus, where crayfish are absent, the likelihood that a site will be dominated by either Cladophora patches or sessile grazer — microalgal lawn patches depends on two sets of stochastic processes: (1) those that create bare space (e.g. disturbance and grazer emergence); and (2) those controlling the timing of recruitment by Cladophora or grazers at these bare sites. These priority effects (i.e. the ability of grazers and Cladophora to inhibit each other's establishment) contribute to the marked spatial heterogeneity of these two patch types. Collectively, these results demonstrate how interactions between competition, predation, and physical factors can generate a complex mixture of community patterns.     

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.     

Periphyton removal related to phosphorus and grazer biomass level

1. The proliferation of nuisance periphyton in enriched streams may be dependent on the biomass of the grazing macroinvertebrates present. In the present study, the effectiveness of grazer size and biomass in controlling periphyton and the extent to which grazing effectiveness was affected by enrichment level were determined. 2. Two sets of experiments with two caddisfly grazers were conducted in laboratory channels during spring and summer 1995 and 1996. The first set tested the combined effect of phosphorus enrichment and grazing, while the second set tested the effect of variable grazer biomass on periphyton biomass. 3. Grazing reduced periphyton biomass in excess of 80%, compared to ungrazed controls. Grazers were equally effective in controlling filamentous green algae, Stigeoclonium, diatoms and small colonial greens. Near complete removal of periphyton biomass by grazing occurred at even at the lowest grazer biomass level (750 mg m^??2, i.e. approximately one-third of natural levels). 4. Grazing controlled periphyton biomass more than did enrichment with soluble reactive phosphorus (SRP). 5. Grazing rates in the phosphorus-grazing interaction experiments averaged about 6 mg chl a g invertebrate^??1 day^??1, which was similar to past work in these channels and elsewhere, while rates were about five-fold higher in the variable grazer biomass experiments. 6. Simulating effects of SRP and grazing with a calibrated model suggests that higher SRP levels would be necessary to exceed a nuisance periphyton biomass level if grazers were present. However, if grazer biomass was more than 1500 mg m^??2, a nuisance level would probably not be exceeded at any SRP.     

Herbivorous caddisflies,macroalgae, and epilithic microalgae: dynamic interactions in a stream grazing system

Summary 1. During the low-flow period (April–October) in sunlit pools of Big Sulphur Creek (northern coastal California), the attached algal community predictably changes from an assemblage dominated by lush, upright Cladophora glomerata filaments in spring and early summer to one dominated by epilithic diatoms and blue-green algae (together=microalgae) in late summer through early autumn. Previous studies in this stream indicated that grazing by the caddisflies Helicopsyche borealis and Gumaga nigricula maintain low algal biomass during the latter part of this period. We used a combination of in situ exclusion/enclosure experiments to examine (1) the separate and combined effects of these grazers on Cladophora and microalgal assemblages, and (2) food preferences, growth, and microdistribution patterns of grazers when offered these different algal foods. 2. Grazers exerted strong but divergent effects on algal assemblages. Selective grazing on Cladophora by G. nigricula greatly accelerated the transition from upright Cladophora to epilithic microalgae, whereas selective grazing on microalgae by H. borealis dramatically reduced biomass of these forms. Grazers were largely ineffective at reducing the non-preferred algal food source (i.e. Cladophora by H. borealis, microalgae by G. nigricula). In the case of each grazer, growth was highest on the preferred algal food. Together, the activity of these grazers produced a low-biomass assemblage dominated by microalgal cells. 3. Removal of the Cladophora overstory by G. nigricula resulted in a three-fold increase in the abundance of epilithic microalgae, the preferred food of H. borealis. Elimination of Cladophora by G. nigricula can increase food availability for H. borealis and, in so doing, can indirectly facilitate the growth of this grazer during food-limited conditions. However, microdistribution of G. nigricula shifts from high overlap with H. borealis in spring and early summer when Cladophora is abundant to low overlap in late summer after Cladophora has been eliminated. This may indicate intense competition between these species for limited epilithic algae, and a concomitant movement by G. nigricula to areas in the stream where food resources are more available.     

The effect of large macroinvertebrate herbivores on sessile epibenthos in a mountain stream

Studies of benthic invertebrates in lakes and streams suggest thatlarge-bodied herbivores are more efficient grazers than smallerones. In order to assess the effect of larger herbivores on smallergrazing invertebrates, the presence of dominant grazer taxa wasmanipulated in streamside troughs in a first order temperaterainforest stream in British Columbia. The presence of mayflies(Ameletus sp.) and tailed frog tadpoles (Ascaphustruei) reduced both algal biomass and the abundance of herbivorouschironomids (Orthocladiinae) on ceramic tiles. This confirms thatlarge mobile grazers in streams have a negative effect on smallersessile invertebrate grazers either through resource competition ordirect consumption (predation).     

Potential functional redundancy and resource facilitation between tadpoles and insect grazers in tropical headwater streams

1. We quantified production and consumption of stream‐dwelling tadpoles and insect grazers in a headwater stream in the Panamanian uplands for 2 years to assess their effects on basal resources and energy fluxes. At the onset of our study, this region had healthy, diverse amphibian populations, but a catastrophic disease‐driven decline began in September 2004, which greatly reduced amphibian populations. 2. Insect grazer production was 348 mg ash‐free dry mass (AFDM) m^?2 year^?1 during the first year of the study and increased slightly to 402 mg AFDM m^?2 year^?1 during the second year. 3. Prior to amphibian declines, resource consumption by grazers (tadpoles and insects) was estimated at 2.9 g AFDM m^?2 year^?1 of algal primary production, which was nearly twice the estimated amount available. Insect grazers alone accounted for c. 81% of total primary consumption. During the initial stages of the declines, consumption remained at c. 2.9 g AFDM m^?2 year^?1, but only 35% of the available resource was being consumed and insect grazers accounted for c. 94% of total consumption. 4. Production and resource consumption of some insect grazers increased during the second year, as tadpoles declined, indicating a potential for functional redundancy in this system. However, other insect grazer taxa declined or did not respond to tadpole losses, suggesting a potential for facilitation between tadpoles and some insects; differential responses among taxa resulted in the lack of a response by insect grazers as a whole. 5. Our results suggest that before massive population declines, tadpoles exerted strong top‐down control on algal production and interacted in a variety of ways with other primary consumers. 6. As amphibian populations continue to decline around the globe, changes in the structure and function of freshwater habitats should be expected. Although our study was focused on tropical headwater streams, our results suggest that these losses of consumer diversity could influence other aquatic systems as well and may even reach to adjacent terrestrial environments.     

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.     

A manipulative study of macroinvertebrate grazers in Hong Kong streams: do snails compete with insects?

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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.     

Grazer species effects on epilithon nutrient composition

1. Field and laboratory experiments were conducted to investigate the excretion stoichiometry of nitrogen (N) and phosphorus (P) of two benthic macroinvertebrate grazers, the crayfish Orconectes propinquus and the snail Elimia livescens, that differ in body stoichiometry (mean body molar N : P 18 and 28, respectively). Crayfish excretion had a significantly higher ammonium : soluble reactive phosphorus (SRP) ratio in the laboratory and in three natural streams than did snails, as predicted by ecological stoichiometry theory. 2. In greenhouse recirculating artificial streams, treatments consisting of crayfish, snails, or no grazers were used to examine responses in dissolved nutrient concentrations and epilithon nutrient composition and limitation. SRP concentrations depended upon the grazer species, with the snail treatment having a higher SRP concentration than other treatments (P < 0.05). Dissolved inorganic N was not affected by grazers, but appeared to be rapidly incorporated in epilithon. 3. Epilithon N content was dependent upon the grazer species present, with the crayfish treatment having a significantly higher N content than other treatments (P = 0.001). No grazer species effects on epilithon P content were found. However, both grazer treatments had significantly lower epilithon P content than the no‐grazer treatment. 4. Traditionally, studies have focused on how grazer‐induced structural changes to epilithon can alter epilithon nutrient dynamics, but this structural mechanism could not solely explain differences in epilithon nutrient contents and ratios in the present study. Our results rather suggest that benthic grazers can alter epilithon nutrient composition and limitation via nutrient excretion. Consequently, macroinvertebrate grazers may serve as ‘nutrient pumps’ that partly regulate the availability of nutrients to algae in stream ecosystems.     

Cascading effects of predatory fish on the composition of benthic algae in high‐altitude streams

Cascading effects of predators can affect ecosystem properties by changing plant biomass, distribution and assemblage composition. Using data from field surveys and whole‐stream experiments we tested the hypothesis that predatory trout change assemblage composition of benthic algae in high‐elevation streams mediated by grazer behavior. Field surveys revealed that the taxonomic composition of algal assemblages differed significantly between streams that contained trout and those that were fishless; but comparisons of palatable versus unpalatable algal taxa between fish and fishless streams were equivocal because of high natural variability. Therefore, we tested for a behavioral (non‐consumptive) trophic cascade experimentally by adding brook trout chemical cues to six naturally fishless streams for 25 days and compared responses of grazers and algae to six reference streams without fish cues added. Algal response variables included rates of change in the abundance of three physiognomic categories, from most palatable (attached erect and prostrate diatoms) to least palatable (non‐diatoms), as determined from food selectivity analyses of the most common grazers (mayflies and caddisflies). Fish cues did not affect the mean densities or changes in densities of total grazers or any individual grazer species. However, in streams where fish cues were added, rates of accrual of attached erect diatoms, which was the preferred algal type for the grazer most vulnerable to trout predation (Baetis), were higher and their densities increased significantly faster with increasing densities of this grazer species than in reference streams. Results of his experiment support the hypothesis that predator induced suppression of grazer foraging behavior, rather than cascading effects of top predators on grazer density, may contribute to variation in the composition of algal assemblages among streams by allowing proliferation of most palatable algal species.     

Does light intensity modify the effect mayfly grazers have on periphyton?

1. A factorial experiment was conducted in artificial outdoor streams to quantify the effects of irradiance (two levels) and two mayfly grazers (four densities of each) on periphytic community structure. The mayflies were Ecdyonurus venosus (Heptageniidae), a grazer using brushing mouthparts, and Baetis spp. (Baetidae) a grazer which uses mandibles and maxilla to scrape and gather periphyton. The experiment ran for 16 days. 2. Grazer densities in channels approximated those existing in a shoreline habitat in the River Sihl, Switzerland. Light treatments were natural (daily mean = 810 μmol m^–2 s^–1) and shaded (daily mean = 286 μmol m^–2 s^–1). 3. Higher irradiance increased total algal abundance by a factor of 4. Algae most affected were prostrate/motile and erect diatoms, filamentous chlorophytes and Hydrurus foetidus. 4. Both species of mayfly reduced periphytic and algal biomass. Mayfly–mayfly interactions, however, were associated with statistical increases in algal biovolume and chlorophyll-a content, indicating that the two grazers may have interfered with one another as their densities increased. The mayfly–mayfly interaction did not influence periphytic ash-free dry mass (AFDM). Light modified the influence of Ecdyonurus such that this mayfly produced greater reductions in algal biovolume under high irradiance. 5. Despite efforts to exclude other grazers, chironomids colonized experimental channels. Chironomid biomass was approximately eight times less than mayflies across treatments and was positively correlated with all measures of periphytic abundance, suggesting that these grazers were responding to periphyton rather than controlling it. Chironomids were also associated with an increase in the abundance of diatoms having a prostrate/motile physiognomy. The only physiognomy to show a negative relationship with chironomid biomass was the thallus type, a form which comprised less than 1% of the algal biovolume across channels. 6. Ecdyonurus and Baetis had distinct influences on algal physiognomy. Ecdyonurus, for example, reduced adnate, stalked and Achnanthes-type physiognomies, but was associated with a significant increase in the abundance of filamentous chlorophytes (primarily Ulothrix sp.). Baetis reduced erect, Achnanthes-type and thallus physiognomies. Neither mayfly influenced the abundance of prostrate/motile diatoms; a physiognomy that comprised 21% of the algae in channels. 7. Light and mayfly interactions affected algal community structure. The interaction of Ecdyonurus with light had a negative effect on erect diatoms, filamentous chlorophytes and the thallus physiognomy, but a positive effect on stalked and Achnanthes-type physiognomies. Baetis interacting with light had a positive effect on adnate diatoms. 8. Although both mayfly taxa influenced periphytic community structure, physiognomy was not a good predictor of algal susceptibility to grazing. The type of substratum to which an alga is attached (detritus or algal filaments vs hard surfaces) and location within the periphytic matrix may be better indicators of vulnerability to grazing than physiognomy.     

THE ECOLOGY OF CLADOPHORA

Cladophora is found in a variety of marine and fresh-waters and provides habitat and food for numerous organisms. It may be the most ubiquitous macroalga in fresh-waters worldwide. This filamentous green alga can reach nuisance levels as a result of cultural eutrophication. Taxonomic identification of Cladophora species is difficult. Taxonomy may be clarified by the simultaneous culture of known taxa and material derived from field collections under the same sets of culture conditions. This should eliminate ecotypic variations in morphology. Cladophora is predominantly benthic and is often found in regions of unidirectional flow or periodic wave action. Its metabolism, and morphology are related to hydrodynamic conditions. Nitrogen and phosphorus are the most commonly reported limiting nutrients. Cladophora is a mid- to late successional species in freshwaters where it is grazer resistant. In marine communities, however, it is considered an early opportunist and relatively palatable to invertebrates. Cladophora is colonized by a wide variety of epiphytes and motile animals because it can offer protection from predation, food (in the form of epiphytes or Cladophora itself), or a substrate that is anchored against flow disturbance. Species interactions that occur within Cladophora communities include 1) competition with other primary producers, 2) top-down control of biomass, 3) association with nitrogen-fixing epiphytes, 4) grazing on epiphytes by invertebrates, and 5) complicated foodwebs in marine intertidal and freshwater communities. Because Cladophora is found in many different habitats, its ecology varies significantly with locale.     

An experimental study of the influence of periphytic algae on invertebrate abundance in a Hong Kong stream

1. Small cages (294cm²) containing unglazed clay quarry tiles were used to investigate the influence of periphytic algae on macroinvertebrate abundance in a Hong Kong stream. Algal biomass was manipulated by shading cages with plastic sheets. Individual cages were assigned to one of three treatment groups: unshaded, shaded and deeply shaded. Invertebrate densities and algal biomass within cages were monitored after 23, 37 and 65 days. 2. Multiple-regression analysis revealed that algal biomass, invertebrate morphospecies richness and total abundance declined with greater shading intensity. The responses of individual invertebrate taxa varied: some (especially Trichoptera) were unaffected by shading, whereas grazers (Baetidae, Psephenidae and Elmidae) declined as shading increased. 3. Significant regressions of the densities of individual taxa upon algal and detrital standing stocks in cages had positive slopes, but algal biomass increased during the study while detrital standing stocks declined. Abundance of invertebrates declined or remained rather stable over time. Density increases resulting from a positive association with algae were apparently offset by declines in abundance correlated with reductions in detritus. 4. Declines in algal biomass were associated with greater shading to which animals may respond directly. To uncouple the link between scarcity of algae and reduction of light intensity, the plastic covers on two groups of cages (deeply shaded and unshaded) which had been placed in the stream for 28 days were reversed so that cages which had been shaded became unshaded and vice versa. The cages were recovered on day 33, Only Coleoptera demonstrated a positive association with atgae inside cages; no relationship between population densities and algal biomass or light intensity was apparent for other taxa. However, the design may have been confounded by deposition of sediment in the cages (due to declining stream discharge) which reduced population densities of colonizers. 5. This study documents changes in invertebrate abundance and morphospecies richness in response periphyton and detritus standing stocks within patches. Summation of such responses may account for observed variations in benthic communities among Hong Kong streams which differ in the extent of shading by riparian vegetation.     

Top‐down and bottom‐up control of periphyton by benthivorous fish and light supply in two streams

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