Tomoya Iwata

A global experiment suggests climate warming will not accelerate litter decomposition in streams but might reduce carbon sequestration

The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding influence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO(2) production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a significant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback.

Seasonal subsidy stabilizes food web dynamics: Balance in a heterogeneous landscape

Resource subsidies from external habitats can substantially affect the food web dynamics of local habitats. In this paper, we explore a mathematical model that is tailored for a stream food web, studied by Nakano and colleagues, in which consumers, in situ prey and subsidies all show seasonal fluctuation. The model reveals that the food web dynamics are stabilized if subsidies increase in summer when in situ productivity is low. Consumer dynamics are stabilized because subsidies complement seasonal resource deficiency. In situ prey dynamics are stabilized because subsidies indirectly balance the predation pressure by consumers, with seasonal change in prey carrying capacity. In summer when prey carrying capacity is low, seasonally abundant subsidies indirectly decrease predation pressure, whereas in winter, with high prey carrying capacity, scarce subsidies increase the predation pressure. Our results suggest that temporal productivity differences between spatially linked habitats are important to promote the stability of food web dynamics in a landscape context.

Prey use by web-building spiders: stable isotope analyses of trophic flow at a forest-stream ecotone

A forest-stream trophic link was examined by stable carbon isotope analyses which evaluated the relationship of aquatic insects emerging from a stream to the diets of web-building spiders. Spiders, aquatic and terrestrial prey, and basal resources of forest and stream food webs were collected in a deciduous forest along a Japanese headwater stream during May and July 2001. The δ13C analyses suggested that riparian tetragnathid spiders relied on aquatic insects and that the monthly variation of such dependence is partly associated with the seasonal dynamics of aquatic insect abundance in the riparian forest. Similarly, linyphiid spiders in the riparian forest exhibited δ13C values similar to aquatic prey in May. However, their δ13C values were close to terrestrial prey in both riparian and upland (150 m away from the stream) forests during June to July, suggesting the seasonal incorporation of stream-derived carbon into their tissue. In contrast, araneid spiders relied on terrestrial prey in both riparian and upland forests throughout the study period. These isotopic results were consistent with a previous study that reported seasonal variation in the aquatic prey contribution to total web contents for each spider group in this forest, implying that spiders assimilate trapped prey and that aquatic insect flux indeed contributes to the energetics of riparian tetragnathid and linyphiid spiders.

Linking stream habitats and spider distribution: spatial variations in trophic transfer across a forest–stream boundary

In headwater streams, many aquatic insects rely on terrestrial detritus, while their emergence from streams often subsidizes riparian generalist predators. However, spatial variations in such reciprocal trophic linkages remain poorly understood. The present study, conducted in a northern Japanese stream and the surrounding forest, showed that pool–riffle structure brought about heterogeneous distributions of detritus deposits and benthic aquatic insects. The resulting variations in aquatic insect emergence influenced the distributions of riparian web-building spiders. Pools with slow current stored greater amounts of detritus than riffles, allowing more benthic aquatic insects to develop in pools. The greater larval biomass in pools and greater tendency for riffle insects to drift into pools at metamorphosis resulted in an emergence rate of aquatic insects from pools that was some four to five times greater than from riffles. In the riparian forest, web-building spiders (Tetragnathidae and Linyphiidae) were distributed in accordance with the emergence rates of aquatic insects, upon which both spider groups heavily depended. Consequently, the riparian strips bordering pools had a density of tetragnathid spiders that was twice as high as that of the riparian strips adjacent to riffles. Moreover, although limitations of vegetation structure prevented the aggregation of linyphiid spiders around pools, linyphiid density normalized by shrub density was higher in habitats adjacent to pools than those adjacent to riffles. The results indicated that stream geomorphology, which affects the storage of terrestrial organic material and the export of such material to riparian forests via aquatic insect emergence, plays a role in determining the strength of terrestrial–aquatic linkages in headwater ecosystems.

Trophic cascading effects of predatory fish on leaf litter processing in a Japanese stream

A manipulative field experiment to test for trophic cascading effects of predatory fish on detritus processing by benthic invertebrates was performed in stream channels running through a wetland forest in northern Japan. To control for fish effects on benthic invertebrates, two simple treatments (fish-present and fish-absent) were established for 4 weeks, with two common predatory fish, rainbow trout (Oncorhynchus mykiss) and freshwater sculpin (Cottus nozawae), being introduced into and excluded from stream cages. At the end of experiment, the biomass of the dominant detritivore, an amphipod (Jesogammarus jezoensis), was significantly less in the fish-present treatment (0.56 g m–2 in dry mass on average) than that in the fish-absent treatment (1.32 g m–2), there being no significant treatment effect evident for the second-dominant detritivore, coleopteran larvae (Optioservus kubotai). The loss of oak leaves (Quercus crispla) from litter bags in the fish-present treatment (0.31 g week−1 in dry mass on average) was significantly less than in the fish-absent treatment (0.54 g week−1). Predator-induced lower biomass and likely lowered foraging activities of the J. jezoensis were responsible for the suppression of litter processing efficiency. In contrast, the standing crop of fine particulate organic matter did not differ significantly between the treatments. The experimental results revealed that the predatory fish had an indirect but significant effect on leaf litter processing in the stream.

Timescale hierarchy determines the indirect effects of fluctuating subsidy inputs on in situ resources.

Empirical studies have shown that temporally fluctuating inputs of resource subsidies can indirectly increase or decrease the abundance of in situ resources by affecting generalist consumers that feed on both subsidies and in situ resources. By mathematical modeling, we develop a theoretical framework that can explain these variable consumer‐mediated indirect effects. We show that the hierarchy of timescales among fluctuations in the subsidy input rate and consumers’ reproductive and aggregative numerical responses predict the signs of the indirect effects. These predictions are consistent with field observations from a variety of natural systems. Our results suggest that the timescale hierarchy of ecological processes is fundamentally important for understanding and predicting indirect effects in nonequilibrium food web dynamics.

Shrimp abundance and habitat relationships in tropical rain-forest streams, Sarawak, Borneo

Microhabitat use and habitat-abundance relationships of four freshwater shrimps. Atyopsis moluccensis, Macrobrachium pilimanus, Macrobrachium trompii and Macrobrachium neglectum, were surveyed in tropical streams running through primary and secondary forests in Borneo, East Malaysia. Underwater observations revealed that A. moluccensis preferred relatively high water velocity and a boulder substrate. Macrobrachium pilimanus also preferred high water velocity and a cobble substrate, whereas M. trompii occupied stream margins with slow current and fine substrates (from POM (particulate organic matter) to pebbles). In contrast, M. neglectum was distributed relatively evenly through the stream channel. The abundance of A. moluccensis, M, pilimanus and M. trompii in the stream reaches was best explained by the abundance of boulders, cobbles and POM, respectively, suggesting that the amount of preferred microhabitat is an important factor affecting shrimp abundances in the tropical rain-forest streams. The primary-forest reaches were dominated by coarse substrates, such as cobbles and boulders, while a great proportion of the streambeds in the secondary-forest reaches were covered with sand. Owing probably to such habitat differences, the abundance of both A. moluccensis and M. pilimanus, which preferred coarse substrates, was less in the secondary- than in the primary-forest reaches. These suggested that loss of preferred habitat, namcly decreased coarse substrate availability, by sedimentation resulting from riparian deforestation had altered the shrimp assemblage structures.

Biotic and abiotic variables influencing plant litter breakdown in streams: A global study

Plant litter breakdown is a key ecological process in terrestrial and freshwater ecosystems. Streams and rivers, in particular, contribute substantially to global carbon fluxes. However, there is little information available on the relative roles of different drivers of plant litter breakdown in fresh waters, particularly at large scales. We present a global-scale study of litter breakdown in streams to compare the roles of biotic, climatic and other environmental factors on breakdown rates. We conducted an experiment in 24 streams encompassing latitudes from 47.8° N to 42.8° S, using litter mixtures of local species differing in quality and phylogenetic diversity (PD), and alder (Alnus glutinosa) to control for variation in litter traits. Our models revealed that breakdown of alder was driven by climate, with some influence of pH, whereas variation in breakdown of litter mixtures was explained mainly by litter quality and PD. Effects of litter quality and PD and stream pH were more positive at higher temperatures, indicating that different mechanisms may operate at different latitudes. These results reflect global variability caused by multiple factors, but unexplained variance points to the need for expanded global-scale comparisons.

Identification of Major Planktonic Sulfur Oxidizers in Stratified Freshwater Lake

Planktonic sulfur oxidizers are important constituents of ecosystems in stratified water bodies, and contribute to sulfide detoxification. In contrast to marine environments, taxonomic identities of major planktonic sulfur oxidizers in freshwater lakes still remain largely unknown. Bacterioplankton community structure was analyzed in a stratified freshwater lake, Lake Mizugaki in Japan. In the clone libraries of 16S rRNA gene, clones very closely related to a sulfur oxidizer isolated from this lake, Sulfuritalea hydrogenivorans, were detected in deep anoxic water, and occupied up to 12.5% in each library of different water depth. Assemblages of planktonic sulfur oxidizers were specifically analyzed by constructing clone libraries of genes involved in sulfur oxidation, aprA, dsrA, soxB and sqr. In the libraries, clones related to betaproteobacteria were detected with high frequencies, including the close relatives of Sulfuritalea hydrogenivorans.

Metabolic balance of streams draining urban and agricultural watersheds in central Japan

Empirical data that describe the metabolic balance of stream ecosystems in human-dominated watersheds are scarce. We measured ecosystem metabolism in 23 open-canopied lowland streams draining urban and agricultural areas in the Fuji River Basin, central Japan. Gross primary production (GPP) and community respiration (CR) were estimated using the diurnal dissolved oxygen (DO) change technique, with the reaeration coefficient (K2) determined from seven empirical depth-velocity equations. Because the predicted values of K2 showed variation among the depth-velocity equations, the estimates of stream metabolism also varied according to the equations. However, CR was almost always greater than GPP, resulting in negative net ecosystem production (NEP) and GPP/CR ratios below unity for most of the study reaches. Highly heterotrophic streams were found in intensively farmed watersheds, suggesting that organic matter loading from agricultural lands is likely to be a source of allochthonous carbon fueling excess respiration in the study streams. In contrast, streams draining more urbanized areas were less heterotrophic. The present results suggest that lowland streams in agriculturally developed watersheds are associated strongly with terrestrial ecosystems as a source of organic carbon. The resultant strong respiration might become the dominant process in ecosystem metabolism, as reported for headwater streams, large downstream rivers, and estuaries.