Izumi Katano

Longitudinal macroinvertebrate organization over contrasting discontinuities: effects of a dam and a tributary

Abstract Macroinvertebrate organization along a river was examined to relate biological responses to environmental changes observed across 2 discontinuities (a dam and a tributary). Benthic macroinvertebrates and a range of environmental variables were sampled from 4 study segments (above the dam, below the dam, below the tributary confluence, and in the tributary). Substrate was significantly coarser below than above the dam. In contrast, water-quality variables, such as water temperature and dissolved O2, changed little below the dam. The most striking discontinuity was substrate coarseness at the tributary confluence. Substrate below the confluence was finer than substrate below the dam and similar to the substrate above the dam. Macroinvertebrate organization differed across the 2 discontinuities. Assemblage composition above the dam was more similar to composition below the confluence than to composition below the dam. The longitudinal organization of the macroinvertebrates could be explained largely by changes in substrate characteristics and habitat preferences of the indicator species. The densities of drifting zooplankton and phytoplankton were higher below than above the dam and were higher below the dam than below the confluence. However, the density of drifting plankton did not differ between the reach immediately above the confluence and the reaches below the confluence. This result suggests that the decrease of zooplankton and phytoplankton occurred above the tributary, probably because of biological entrapment or passive deposition rather than the contribution of the tributary inflow. The dam and tributary caused contrasting discontinuities in macroinvertebrate organization. The tributary generally reversed the dam-related changes to the main stem habitat and the macroinvertebrate community. A key management implication of our study is that efforts to restore dam-related environmental impacts would be facilitated by understanding the role of tributaries downstream of the dam.

Relationships between length and weight of freshwater macroinvertebrates in Japan

Relationships between weight (W; dry weight) and length (L; head capsule width, total body length or head carapace length) were examined in 31 Japanese freshwater macroinvertebrate taxa, using the form W = aLb. The relationships were expressed as data of the lowest taxonomic level and data of higher taxonomic levels. The length–weight relationships obtained in this study were similar to those obtained in North America and Europe at the lowest taxonomic level, whereas they could be different from those obtained in North America and Europe at the higher taxonomic levels. We suggest that researchers should make their own regressions for a target taxon or use the regression for the same taxon as possible lower taxonomic level in the local area.

Distribution patterns of stream grazers and relationships between grazers and periphyton at multiple spatial scales

Abstract We examined the relationships between the distribution of dominant herbivorous insect grazers (Glossosoma larvae), environmental factors (current velocity, water depth, periphyton biomass), and grazer–periphyton interactions at multiple spatial scales (microhabitat, riffle, reach) in a stream. We used multiple regression models to explain densities of Glossosoma larvae at each spatial scale in terms of the environmental factors. All r2-values were significantly higher at the riffle than at the microhabitat or reach scales. Thus, the riffle scale provided better predictions of Glossosoma larval density than did the microhabitat (smaller) and reach (larger) scales. The r2-values of exponential regressions between grazer densities and periphyton biomass were lower at the microhabitat than at the riffle or reach scales. These results indicate that the patterns of relationships between the insect grazers and periphyton were detected more clearly at larger than at smaller scales.

Effects of biodiversity, habitat structure, and water quality on recreational use of rivers

Anthropogenic impacts on biodiversity and habitat conditions can lead to loss of ecosystem services. Cultural services of ecosystems are one of the major categories of ecosystem services, but the relationships between ecosystem conditions and human uses of their cultural services are still largely unknown. To estimate the effects of biodiversity, habitat structure, and environmental pollution on recreational uses of ecosystems, existing data sets for 109 rivers across Japan were used to analyze the relationships among recreational uses of these rivers (fishing, playing in the river, walking, and engaging in sports near the river) and their biodiversity, habitat structure, and water quality. Fish diversity, habitat structure, and water quality had significant effects on the number of people fishing and playing in the rivers. The number of people walking and engaging in sports, which typically takes place on floodplains and dikes, was significantly positively related to the size of the surrounding population rather than to biodiversity or habitat structure. However, water quality had significant effects on such uses of river ecosystems, even though these activities did not involve direct contact with river water. Overall, a decline in biodiversity and ecosystem health was related to a decrease in recreational use.

Changes in periphyton abundance and community structure with the dispersal of a caddisfly grazer, Micrasema quadriloba

We examined the larval population densities and biomass of a caddisfly grazer, Micrasema quadriloba, and the abundance and community structures of periphyton at a segment scale (7.4 km with four study sites), along a second-to fourth-order Japanese mountain stream throughout the grazer’s life cycle. In the uppermost riffle of the study segment (site 1), periphyton abundance was kept at low levels when the larvae occurred. The larval distribution spread downstream as larvae developed from first instars in May to fifth instars in January. We performed multiple regression analyses to test the effects of environmental variables and larval biomass on periphyton abundance in both the riffle of site 1 and the study segment; the results revealed that the larval biomass was significantly negatively correlated with periphyton abundance similarly in both the riffle and the study segment. In addition, both the correlation and community analyses showed that the larval biomass was significantly negatively correlated to the relative abundance of large and/or filamentous microalgae, which appeared in the uppermost layer of the periphyton mat, and that larval biomass was significantly positively correlated to the relative abundance of small diatoms, which strongly adhered to the substrate. Thus, the present study implied that the grazing of M. quadriloba larvae would regulate the abundance of periphyton in a riffle and also regulate the abundance and community structure of periphyton at the segment scale with the expansion of their longitudinal distribution.

Environmental DNA method for estimating salamander distribution in headwater streams, and a comparison of water sampling methods.

Environmental DNA (eDNA) has recently been used for detecting the distribution of macroorganisms in various aquatic habitats. In this study, we applied an eDNA method to estimate the distribution of the Japanese clawed salamander, Onychodactylus japonicus, in headwater streams. Additionally, we compared the detection of eDNA and hand-capturing methods used for determining the distribution of O. japonicus. For eDNA detection, we designed a qPCR primer/probe set for O. japonicus using the 12S rRNA region. We detected the eDNA of O. japonicus at all sites (with the exception of one), where we also observed them by hand-capturing. Additionally, we detected eDNA at two sites where we were unable to observe individuals using the hand-capturing method. Moreover, we found that eDNA concentrations and detection rates of the two water sampling areas (stream surface and under stones) were not significantly different, although the eDNA concentration in the water under stones was more varied than that on the surface. We, therefore, conclude that eDNA methods could be used to determine the distribution of macroorganisms inhabiting headwater systems by using samples collected from the surface of the water.

Detection of an endangered aquatic heteropteran using environmental DNA in a wetland ecosystem

The use of environmental DNA (eDNA) has recently been employed to evaluate the distribution of various aquatic macroorganisms. Although this technique has been applied to a broad range of taxa, from vertebrates to invertebrates, its application is limited for aquatic insects such as aquatic heteropterans. Nepa hoffmanni (Heteroptera: Nepidae) is a small (approx. 23 mm) aquatic heteropteran that inhabits wetlands, can be difficult to capture and is endangered in Japan. The molecular tool eDNA was used to evaluate the species distribution of N. hoffmanni in comparison to that determined using hand-capturing methods in two regions of Japan. The eDNA of N. hoffmanni was detected at nearly all sites (10 eDNA-detected sites out of 14 sites), including sites where N. hoffmanni was not captured by hand (five eDNA-detected sites out of six captured sites). Thus, this species-specific eDNA technique can be applied to detect small, sparsely distributed heteropterans in wetland ecosystems. In conclusion, eDNA could be a valuable technique for the detection of aquatic insects inhabiting wetland habitats, and could make a significant contribution to providing distribution data necessary to species conservation.

Stream grazers determine their crawling direction on the basis of chemical and particulate microalgal cues.

This study aimed to determine the association between herbivore behavior and cues from producers. We used stream grazer Glossosoma larvae and determined their crawling direction in relation to chemical and visual cues from microalgae. The experimental treatments included control (no cue), particulate (chemical and particulate cues), and dissolved (chemical cue) cues from microalgae. The experimental water samples were randomly placed into either arm of a Y-shaped channel, and the crawling direction of the grazers was determined. Although the grazers crawled toward the arm containing either particulate or dissolved cues, they preferred the arm with particulate cues. This suggested that grazers responded well to both particulate (i.e., drifting algal cells) and chemical (algal smell) cues, and that particulate cues were more important for foraging. In natural habitats, grazers detect cues from producers and change their behaviors to maintain a balance between top-down and bottom-up cues.

A shady phytoplankton paradox: when phytoplankton increases under low light

Light is a fundamental driver of ecosystem dynamics, affecting the rate of photosynthesis and primary production. In spite of its importance, less is known about its community-scale effects on aquatic ecosystems compared with those of nutrient loading. Understanding light limitation is also important for ecosystem management, as human activities have been rapidly altering light availability to aquatic ecosystems. Here we show that decreasing light can paradoxically increase phytoplankton abundance in shallow lakes. Our results, based on field manipulation experiments, field observations and models, suggest that, under competition for light and nutrients between phytoplankton and submersed macrophytes, alternative stable states are possible under high-light supply. In a macrophyte-dominated state, as light decreases phytoplankton density increases, because macrophytes (which effectively compete for nutrients released from the sediment) are more severely affected by light reduction. Our results demonstrate how species interactions with spatial heterogeneity can cause an unexpected outcome in complex ecosystems. An implication of our findings is that partial surface shading for controlling harmful algal bloom may, counterintuitively, increase phytoplankton abundance by decreasing macrophytes. Therefore, to predict how shallow lake ecosystems respond to environmental perturbations, it is essential to consider effects of light on the interactions between pelagic and benthic producers.

Distribution and drift dispersal dynamics of a caddisfly grazer in response to resource abundance and its ontogeny

Stream grazers have a major impact on food web structure and the productivity of stream ecosystems; however, studies on the longitudinal (upstream versus downstream) and temporal changes in their drift dynamics and resulting distributions remain limited. Here, we investigated the longitudinal and temporal distributions and drift propensity of a trichopteran grazer, the caddisfly, Micrasema quadriloba, during its life cycle in a Japanese stream. The distribution of larvae significantly shifted downstream during the fifth instar larval stage during late winter; with periphyton abundance (i.e. their food source) showing similar shifts downstream. Therefore, our results show that the drift dispersal the caddisfly occurs in response to decline in available food resources (i.e. food-resource scarcity) and an increase in food requirements by growing individuals. Furthermore, our results show that this observed longitudinal shift in larval distribution varies through their life cycle, because the drift dispersal of fifth instar larvae was greater than that of immature larvae. The correlation between periphyton abundance and drift propensity of fourth instar larvae was not statistically significant, whereas that of fifth instar larvae was significantly negative. In conclusion, we detected an ontogenetic shift in drift propensity, which might explain the longitudinal and temporal distributions of this species.