Effects of stream grazers with different functional traits on the spatial heterogeneity of periphyton mats

Introduction

Periphytic algae are the most important primary producers in stream ecosystems (Minshall, 1978; Vannote et al., 1980; Lange et al., 2011). Grazer-periphyton interactions in stream environments have been used as model systems to understand more general producer-herbivore interactions (Hart & Robinson, 1990; Feminella & Hawkins, 1995; Doi, Katano & Kikuchi, 2006; Doi & Katano, 2008; Katano & Doi, 2014). The influence of resources and grazing on the composition of periphyton assemblages can be assessed both taxonomically and with regard to functional groups (Passy, 2008), two parameters that can be affected by light, nutrients, and grazing regimes (Johnson, Tuchman & Peterson, 1997; Hollingsworth & Vis, 2010; Lange et al., 2011).

The impact of grazing on the spatial complexity of periphyton has been shown to vary widely depending on grazer functional traits, including mouthpart morphology, aggregation behavior, and grazer activity (Liess & Hillebrand, 2004; Wellnitz & Poff, 2006). Grazing on periphytic mats results in a feeding trace that may change mat structure and heterogeneity (Pringle, 1990; Palmer, 1995; Wellnitz & Poff, 2006). Periphytic mat heterogeneity in turn may influence the fitness of grazer species by affecting their growth rates (Pringle, 1990; Palmer, 1995).

Insect grazers have different mouthpart morphologies and the impact of their feeding on periphytic mat structure has typically been evaluated by taking into account their mouthpart morphologies (Karouna & Fuller, 1992; Merritt & Cummins, 1996). However, other functional traits, such as feeding behavior, may also be important (Karouna & Fuller, 1992; Arens, 1994; Wellnitz & Ward, 2000). Therefore, to better understand the effects of grazing on periphytic mat structure, the effects of other functional traits must be evaluated.

Although the effect of grazers’ feeding traits on periphyton has been previously studied using the larvae of caddisflies and other insects (Karouna & Fuller, 1992; Arens, 1994), the effects on periphytic heterogeneity and the micro-structure of periphytic mats remains largely unknown. Investigating these effects of feeding traits is important for understanding how stream grazers shape the spatial structure of periphyton. Therefore, the aim of this study was to assess the effect of grazing by species with different feeding traits on the structure of periphytic mats, focusing on mouthpart morphology, body mass, and speed of movement, To address this aim, we conducted an enclosure experiment with three grazer species having different mouthpart morphologies, two with scraping mouthparts and one with brushing morphology. We also measured the spatial heterogeneity of chlorophyll (Chl a) and analyzed periphytic mat structure using scanning electron microscopy (SEM).

Materials & Methods

Enclosure experiment

For the enclosure experiment, an non-linear experimental channel was set up that was 10 m in total length, 0.70 m wide, and 0.40 m deep (see Katano et al., 2007; Fig. 1). The experimental channel was located near a small tributary of the Shigo-gawa stream, and the water was collected directly from the tributary. An experimental zone within the channel (1 m long; 7,000 cm⁻² area) was established 2.5 m downstream of the water inlet. A polystyrene foam plate (2 cm thick) was set as a mooring float. The enclosures were 24 experimental cages measuring 6 × 6 × 6 cm and constructed of 0.2 mm nylon mesh, as used in Katano et al. (2007; Fig. 1). To minimize any reduction in periphyton productivity, the mooring float was manually cleaned once a day during the experimental period to prevent clogging by leaf litter.

The enclosure experiment in the channel was performed from March 16 to 27, 2004 (12 d). Six clay tiles (2 × 2 × 1 cm) with periphyton were placed in each cage and incubated in the channel for two months (Fig. 1). The ash-free dry weight (AFDW) was 2.8 ± 0.27 mg cm⁻² (mean ± 1 SD, N = 4), and the chlorophyll a content was 10.1 ± 2.4 mg cm⁻². The clay tiles measured 4.5 × 4.5 × 0.4 cm for Chl a measurement (48 tiles), and 2.25 × 2.25 × 0.4 cm for SEM (12 tiles). For Chl a measurement, the study cages were randomly divided into the following four treatments; control (without larvae), M. quadriloba with 23 larvae, Glossosoma sp. with three larvae, and Epeorus sp. with three larvae. Each treatment was replicated six times.

Measurement of periphyton Chl a

The abundance of microalgal cells in each treatment was estimated by measuring the Chl a concentration. The Chl a was measured according to the UNESCO method (1966), followed by assessment of the abundance of microalgae in the experimental tile. At the end of the experiment, periphyton from each tile (N = 12 for each treatment) were removed using a toothbrush and rinsed into a 100-mL container with tap water. The samples was then filtered through a glass filter (GA-100; Toyo-roshi Co., Tokyo, Japan; pore size, 1 µm). To extract the chlorophyll, the filter was cut into small pieces and placed into a vial, containing 20 mL of 99.5% ethanol. Vials were placed in the dark at 4 °C for 24 h. Following this, the absorbance spectra of the extracted pigments were measured at 480, 630, 645, 665, and 750 nm using a MPS-2000 spectrophotometer (Shimadzu Co., Japan). For six tiles of each treatment, the coefficient of variation in Chl a content was calculated from the Chl a content in four separate areas (zones 1–4 in Fig. 2). For six tiles of each treatment, the Chl a content was measured in both the grazed and non-grazed areas to the ratios of Chl a content in grazed/total periphyton area, and also we examined grazed area (cm²) by photograph of the tile using ImageJ software ver. 1.31.

Results

As seen in Fig. 3, the mouthpart morphology of each grazer species was distinctive, and SEM confirmed that the Glossosoma sp. and M. quadriloba were scrapers, and Epeorus sp. was a brusher. Body mass and movement rates differed among species (Table 1), with Epeorus having the highest values (ANOVA; F = 45.8 and 6.63, P = 0.00001, Tukey; P = 0.0001, and <0.018, respectively). Growth rates showed large variation and were nott significantly different among the species (Table 1, ANOVA; F = 1.61, P = 0.232), possibly due to the relatively short duration of the experiment.

Different structures in the grazed periphyton were evident in the SEM images taken at the end of the experiment (Fig. 4). Ungrazed tiles appeared to be dominated by filamentous algae, whereas caddisfly-grazed tiles (Glossosoma sp. and M. quadriloba) were dominated by diatoms. We observed tangles of filamentous algae on Epeorus-grazed tiles, suggesting that Epeorus sp. might graze only on the upper layer of the periphyton, in contrast to the two caddisfly species. At the end of the enclosure experiment, the mat with M. quadriloba had the lowest Chl a content of the three grazer species (Fig. 5A, ANOVA; F = 5.72, P = 0.0054, Tukey; P < 0.05). Additionally, the grazing area with M. quadriloba was smaller than those with the other two species (Fig. 5B, ANOVA; F = 21.26, P < 0.001, Tukey; P < 0.001). The grazed/total area ratios of Chl a content in mats with M. quadriloba were lower than those with Epeorus sp., but were only marginally different to the mats with Glossosoma sp., possibly due to large variations of the grazed/total area ratios in Glossosoma sp. (Fig. 5C, ANOVA; F = 3.83, P = 0.045, Tukey; P = 0.054). The coefficients of variation (CV) of Chl a content in mats containing M. quadriloba was significantly higher than for both the other species and the initial control (Fig. 6, ANOVA; F = 3.81, P = 0.01, Tukey; P < 0.05).

Discussion

We found that differences in the spatial heterogeneity of periphytic mats was the result of the grazing effects of three grazer species having different mouthpart morphologies and movement rates. Our results agree with those of Liess & Hillebrand (2004), who suggested that the spatial heterogeneity of producers resulted from grazer functional diversity. Moreover, our results showed large variability in the CV (spatial heterogeneity) associated with grazing effects. Thus, the observed effects on heterogeneity were dependent on the grazer feeding traits, explaining the contrasting results obtained in previous studies on algal variability (Liess & Hillebrand, 2004).

Micrasema quadriloba, with its scraping mouthpart morphology, had the strongest grazing effect on periphytic mats in terms of Chl a abundance and CV. Micrasema quadriloba also aggregated for group feeding in the experiment in Katano et al. (2007), which may have resulted in greater spatial heterogeneity in Chl a abundance with aggregated grazing impact (Katano et al., 2007). Epeorus sp. had the weakest effect on the spatial heterogeneity of Chl a, probably because of its brushing mouthpart morphology and rapid movement rate. Glossosoma sp. had only a moderate effect on the periphyton compared to the other two species, possibly because of its scraping mouthparts with large chisel-like mandibles; the presence of Glossosoma sp. also resulted in increased heterogeneity in the grazing/total area ratio, which may also have been due to grazing using chisel-like mandibles.

Natural selection is expected to foster traits, such as mouthpart morphology and feeding behavior that enhance grazer fitness (e.g., Feminella & Resh, 1990), especially when periphytic biomass is low. Reduced periphyton can impose strong selective pressure on grazer. For example, Feminella & Resh (1990) reported that a low periphytic biomass led to reduced growth and survival of trichopteran grazers. We observed that low periphyton availability resulted in an increase in the distance travelled by Glossosoma sp. larvae. This suggests that the larvae were traveling further to maximize their food intake (Kuhara, Nakano & Miyasaka, 2001), but grazer movement will increase energy costs and consequently lower fitness. Additionally, competition among grazers is generally very intense (McNeely, Finlay & Power, 2007), and grazers change their movements, drift behavior, and diel periodicity of their feeding activity accordingly (Kohler & McPeek, 1989; Kuhara, Nakano & Miyasaka, 2001; Miyasaka et al., 2003). Therefore, the consequences of their grazing performance and competition may have resulted from the adaptations of their feeding traits for periphyton.

We found that the periphytic mat structures were greatly modified by the grazer species, and that mat structures differed according to grazer mouthpart morphology, namely scraping (two caddisfly species) and brushing (Epeorus sp.). Grazing depth is an important element for the formation of periphytic mats, as well as the community assemblages it hosts (such as bacteria and small invertebrates) and should be measured in future studies., Community structure largely depends on the structure of the periphytic mat (Pringle, 1990; Palmer, 1995; Wellnitz & Poff, 2006). Further studies are needed to evaluate the consequent effects of grazing on the entire periphytic community, including the effects on small consumers and bacteria.

Conclusions

Our study revealed that differences in the spatial heterogeneity of periphytic mats were a consequence of the different functional traits of grazer species, such as mouthpart morphology and clinging activity. The largest effect on the spatial heterogeneity and abundance of the periphyton was from a caddisfly species with a scraping mouthpart morphology. The mayfly with brushing mouthpart morphology evoked weaker effects on the spatial heterogeneity of the periphytic mats. Here, we suggest that mouthpart morphology and movement rate largely influence the spatial heterogeneity of periphytic mats. Insect grazers are the primary consumers of periphyton in streams and serve as prey for consumers in higher trophic levels (such as fish and plecopterans) (Pringle, 1990; Palmer, 1995). Therefore, understanding the feeding traits and the feeding effects of primary consumers on periphyton and their fitness may be useful for understanding trophic interactions within the larger food web.

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