Local Effects of a Sedentary Grazer on Stream Algae
Total Page：16
File Type：pdf, Size：1020Kb
Freshwater Biotogy (1995) 33, 401-409 Local effects of a sedentary grazer on stream algae ELIZABETH A. BERGEY Department of Entomology, University of Califomia, Berkeley, CA 94720, U.S.A. Present address and address for correspondence: Department of Botany, University of Wisconsin, Madison, WI 53706, U.S.A. SUMMARY 1. Larvae of the sedentary aquatic caterpillar Petrophila confusaiis (Walker) construct silken retreats around which they feed; outside these clearly demarcated grazed areas, stream algae are exposed to a variety of mobile grazers. Comparisons of the algal community inside and just outside grazed zones were made for third- and fifth-instar Petrophila in the South Fork of the Eel River, California. 2. Densities of both the filamentous macroalga Cladophora and diatom assemblages were significantly reduced within the grazed areas during both larval instars. Grazing of diatoms was taxonomically non-selective. 3. In spring, the grazed zones were relatively large (mean = 22.7 cm^) and visibly increased epilithic spatial patchiness. 4. Per cent composition of diatom assemblages inside and outside the grazed areas differed during the third instar but not during the fifth instar. During the third instar, the grazed zone contained more Synedra ulna (a conimon inunigrant) and less Gomphonenia olivaceum (a late-successional species). Introduction Benthic algal grazers in freshwater habitats include a to this research emphasis. Whereas mobile grazers taxonomically wide array of animals, ranging from include vertebrates (e.g. Power, Matthews & Stewart, fish and amphibians to several groups of invertebrates 1985), crustaceans (e.g. Flint & Goldman, 1975), mol- (e.g. snails, insects and crustaceans). They can be luscs (e.g. Tuchman & Stevenson, 1991), insects (e.g. characterized as being mobile grazers or sedentary Feminella & Resh, 1991), oligochaetes (e.g. Hann, grazers. Mobile grazers change location in conjunction 1991) and protozoa (e.g. McCormick, 1991), sedentary with feeding and tend to focus their grazing on distinct grazers primarily include a small number of insects patches of periphyton (e.g. Hart, 1981; Kohler, 1984; [e.g. caterpillars of the genus Petrophila (Pyralidae: Vaughn, 1986; Steinman et ai, 1987; Scrimgeour et al, Lepidoptera; e.g. Tuskes, 1977), several chironomids 1991). In contrast, sedentary grazers feed in a localized (Chironomidae: Diptera; e.g. Pringle, 1985), and the area (e.g. Hart, 1985). Although not physically fixed caddisfly genus Leucotrichia (Hydroptilidae: Trichop- to the substrate, sedentary grazers construct attached tera; e.g. Hart, 1985)]. Although few in terms of retreats and graze the surrounding area and, often, numbers of taxa, sedentary grazers are often locally the retreat itself. Alternatively, fixed retreats may be abundant (Petrophila: Peterson & Grimm, 1992; chiron- shallow mines in aquatic macrophytes, with the animal omids: Hershey et al, 1988; leucotrichia: Hart, 1985) grazing epiphytes and filter feeding (e.g. Lamberti & and may be important components in local foodweb Moore, 1984). dynamics. Studies of benthic grazing have focused almost Most sedentary grazers produce a clearly demarc- exclusively on mobile grazers. Mobile grazers include ated grazed zone on the substrate around their retreats. a much larger number of taxa than sedentary grazers Outside this area, the algal commuruty is exposed to (McAuliffe, 1984), a factor that certainly contributes mobile grazers. Thus, algal populations inside and © 1995 BlackweU Science Ltd 401 402 £. A. Bergey outside the areas grazed by sedentary grazers will be extended into a pool. The area grazed by fifth instars influenced by differences in both grazing intensity was estimated by removing cobbles with Petrophila (high and low) and grazing strategy (sedentary and pupal cases and tracing the outline of the grazed zone mobile). Sedentary grazers are especially amenable to on to clear plastic. The 30 resulting tracings were cut studies of grazer-alga interactions because they have out and weighed. fixed locations and thus need not be confined in cages Cladophora biomass next to Pefrop/ii/o-grazed zones or artificial channels (as mobile grazers are often was measured by scraping Cladophora from rocks constrained). within 20 25-cm^ quadrats. Algal samples were frozen This study examines grazing by the sedentary cater- and later thawed, dried at 105 °C for 2 days, weighed, pillar Petrophila confusalis (Walker) through examina- ashed at 550 °C for 1 h, and reweighed. Samples of tion of epilithic algae within and adjacent to areas the ash were examined microscopically for diatom grazed hy Petrophila larvae. This comparison is made frustules. separately for the third- and fifth-instar larvae, in order In addition to making comparisons of algal popula- to evaluate possible ontogenetic changes in grazing. tions inside and outside areas grazed by Petrophila larvae, differences in the impacts of grazing by third- and fifth-{final) instar larvae were examined. Algal Methods samples associated with the fifth instar were collected The South Fork of the Eel River is a third-order stream on 10 June 1991 when some of the overwintering located in the Coast Range of northern California Petrophila were begiruiing to pupate. Algal samples (Mendocino Co.; latitude 39°44' longitude 123°38')- associated with the third instar were collected on 24 Local riparian vegetation is dominated by old-growth August 1991 when the next generation of Petrophila Douglas-fir {Pseudotsuga menziesii Franco) and, while was present. Sample collection and processing the banks are typically well shaded, the study reach methods were the same for both sets of samples. bordered an abandoned streambed, resulting in full Epilithic algae were collected with a brush sampler sunlight on the stream for about 7 h per day during {modified from Tuchman & Stevenson, 1980; area = summer. The 75-m-long study reach had a predomi- 1.53 cm^). For the third ins tar-associated samples, the nantly cobble-rubble substrate and the following sampler collected algae beneath the larval retreat (the characteristics during the study: width 9-12 m; depth retreat was removed prior to sampling and, once typically less than 1 m; spatial variation in current removed, its former location was not distinguishable velocity of 0.02-0.25 m s~^; and a water temperature in the field) and most of the grazed area around the of lS-17 °C in June, and 17-23 °C in August. retreat (diameter across grazed area and retreat was The life history of Petrophila confusalis in northern approximately 1.8 cm). For the fifth-ins tar-associated California has been described by Tuskes (1977, 1981). samples, the sampler was placed within the grazed Larvae build silken retreats over irregularities in rocks area surrounding the retreat (diameter across grazed or submerged wood, and graze both the underside of areas and the retreat was approximately 5.4 cm). the retreat (Kubik, 1981) and the surrounding area Samples outside the grazed area were taken adjacent (Tuskes, 1977). In preparation for pupation, fifth-instar to the grazed zone. Six {in June) or eight {in August) larvae form a flattened dome-like silk case, under replicate pairs of samples were collected and preserved which they pupate. Petrophita has two to three genera- in 6% formalin. In addition to algal samples, the larva tions per year in northern California (Tuskes, 1977). and the retreat or pupal tent associated with most The portion of the population studied consisted of samples were collected. overwintering fifth-instar larvae and third-instar lar- Processing epilithic samples included two steps: vae of tbe following generation. {i) a low-magnification quantification of filamentous Petrophila density was obtained by counting pupal algae and diatoms; and (ii) a high-magnification deter- cases {visible as light-coloured ovals in areas cleared mination of per cent composiHon of the diatoms. of Cladophora) in late June across 10 randomly located Samples were washed to remove formalin and transects in the 75-m-long study reach. Transects were adjusted to a volume of 4 ml. One millilitre of the 0.5-m wide and maximum depth was less than 1 m, well-shaken sample was added to a Sedwick-Rafter except in the two furthest upstream transects, which cell for algal quantification; the remainder of the © 1995 BlackweU Science Ltd, Freshwater Biology, 33, 401-109 Effects of a sedentary grazer on algae 403 sample was saved for diatom identification. Diatoms the two most upstream transects (3.64 and 4.50 (both living and non-living) were counted in 24 ran- individuals m~^), which were located in a pool with domly located fields at lOOx magnification with a deeper water (to 1,7 m) and slower velocity than the Whipple micrometer. Filamentous algae were identi- rest of the study reach. Excluding the pool transects, fied to genus and regarded as present or absent in mean Petrophila density was 35.64 individuals m"^ each of 100 randomly located Whipple disk fields. (SD = 10.12, n = 8). Some retreats were almost The remaining 3 ml of each sample were oxidized certainly missed in the visual counts; therefore values by; (i) warming in 30% hydrogen peroxide for 2 days; are probably underestimates. Third-instar larval densi- or (ii) heating in 30% hydrogen peroxide and adding ties were not estimated because retreats were better the catalyst potassium dichromate (an oxidative reac- concealed and difficult to count accurately. tion occurred within a few minutes). After rinsing, Algal abundances were lower inside than outside the diatoms were dried on coverslips and mounted in grazed area (Table 1). Relative numbers of Cladophora, Hyrax medium. Transects were counted at lOOOX green algal filaments other than Cladophora (primarily magnification until at least 350 individual diatoms Oedogonium, Mougeotia, and Zygnema), and blue—green had been identified. Frustule length was measured for filaments (primarily Anabaena, Calothrix, and Oscil- 100 diatoms per sample. Paired f-tests were used to latoria) were significantly lower (2-tailed paired f~test; compare mean sample sizes inside and outside the » = 6 or 8; P ^e 0.05) inside the grazed area for both Petrophila grazed areas.