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The field diet of meroplanktonic decapod crustacean larvae is poorly known, despite standard use of microzooplankton as food in laboratory culture. Using callianassid shrimp Nihonotrypaea harmandi larvae collected from a 65 m deep inner-shelf location off mid-western Kyushu, Japan, between June and August 2012 and 2013 and mass-reared in the laboratory, a phytoplankton-based diet through larval development (zoeae I?VI to decapodid) was demonstrated. When the pure-cultured diatom Chaetoceros gracilis was fed to zoeae, survival rate to decapodids was 3.4 to 3.9% in 26 to 40 d at 22°C, which was comparable to previous rearing results for zoeae fed microzooplankton. Trophic enrichment factors (TEFs) from stable isotope (SI) analysis of zoeal whole-body tissue in the laboratory were 2.0‰ for δ13C and 1.9‰ for δ15N. In the field water column, diatoms dominated the nano- to micro-sized plankton, accounting for 38 to 81% of the biovolume, followed by heterotrophic protists. The trophic position (TP) estimated from amino acid-specific δ15N values for the field-collected zoeae VI was 2.1 (TPGlu/Phe) or 2.7 (TPAla/Phe), suggesting that those zoeae fed on mixtures of phytoplankton and heterotrophs including protists. Bulk SI analyses were performed for particulate organic matter (POM; proxy for phytoplankton), microzooplankton (mainly calanoid copepods), and shrimp zoeae to elucidate the diet of larvae in the water column. A shift in SI from fresh to degraded POM was determined through the incubation of field-collected POM. Based on this shift during degradation and larval TEFs, phytoplankton and their sinking detritus with heterotrophic protists were estimated to be the principal diet for those larvae residing mostly below the chlorophyll maximum layer., Marine Ecology Progress Series, 592, pp.141-158; 2018

To identify the origin of various fishes and reconstruct their migration history at the individual level, isotope analysis is a powerful alternative to artificial tagging. We used a novel individual-based methodology to reconstruct individual migratory and/or trophic shifts associated with growth based on isotopic data in the vertebral centrum of adult Japanese flounder Paralichthys olivaceus in Sendai Bay. We measured carbon and nitrogen isotope ratios (δ^13C and δ^15N) in muscle tissues, and conducted a segmental isotope analysis of bulk δ^13C (δ^13Cbulk), bulk δ^15N (δ^15Nbulk), and δ^15N of glutamic acid (δ^15NGlu) and phenylalanine (δ^15NPhe) in vertebral collagen. The δ^15NGlu and δ^15NPhe values for bone collagen revealed an increase in trophic position and a shift to lower trophic baselines (δ^15NBase: indicative of δ^15N values of primary trophic sources) for most individuals. For both δ^13Cbulk and δ^15Nbulk, we detected significant positive correlations between values for muscle and the outermost section of vertebral collagen. A nonlinear time-series analysis of δ^13Cbulk and δ^15Nbulk suggested that a combination of intrinsic (the timing of migration from the nursery to deep offshore areas in juveniles) and extrinsic (habitat and/or food qualities) factors influence the isotopic chronology. A segmental isotope analysis revealed the segregation of individuals among sampling sites at all life stages and changes in trophic positions and δ^15NBase values during growth. Our results suggest that the P. olivaceus population in Sendai Bay has both temporal and spatial structure. The temporal structure may be caused by variation in the timing of migration from the nursery to the deep offshore area in juveniles, and the spatial structure may be explained by individual variation in habitat preferences.

Isotopic compositions of animal tissue are an intrinsic marker commonly used to trace animal origins and migrations; however, few isotopes are effective for this purpose in marine environments, especially on a local scale. The isotope ratio of the lanthanoid element neodymium (Nd) is a promising tracer for coastal animal migrations. Neodymium derives from the same geologic materials as strontium, well known as an isotopic tracer (87Sr/86Sr) for terrestrial and anadromous animals. The advantage of the Nd isotope ratio (143Nd/144Nd, expressed as εNd) is that it varies greatly in the ocean according to the geology of the neighboring continents, whereas oceanic 87Sr/86Sr is highly uniform. This study explored the utility of the Nd isotope ratio as a marine tracer by investigating the variation of εNd preserved in tissues of coastal species, and the causes of that variation, in a region of northeastern Japan where the bedrock geology is highly variable. We measured εNd and 87Sr/86Sr in seawater, river water, and soft tissues of sedentary suspension feeders: the mussels Mytilus galloprovincialis and Mytilus coruscus and the oyster Crassostrea gigas. We also measured concentrations of three lanthanoids (La, Ce, and Pr) in shellfish bodies to determine whether the Nd in shellfish tissue was derived from solution in seawater or from suspended particulates. The εNd values in shellfish tissue varied regionally (−6 to +1), matching the ambient seawater, whereas all 87Sr/86Sr values were homogeneous and typical of seawater (0.7091–0.7092). The seawater εNd values were in turn correlated with those in the adjacent rivers, linking shellfish εNd to the geology of river catchments. The depletion of Ce compared to La and Pr (negative Ce anomaly) suggested that the Nd in shellfish was derived from the dissolved phase in seawater. Our results indicate that the distinct Nd isotope ratio derived from local geology is imprinted, through seawater, on the soft tissues of shellfish. This result underscores the potential of εNd as a tracer of coastal marine animals.

Food-web complexity often hinders disentangling functionally relevant aspects of food-web structure and its relationships to biodiversity. Here, we present a theoretical framework to evaluate food-web complexity in terms of biodiversity. Food network unfolding is a theoretical method to transform a complex food web into a linear food chain based on ecosystem processes. Based on this method, we can define three biodiversity indices, horizontal diversity (DH ), vertical diversity (DV ) and range diversity (DR ), which are associated with the species diversity within each trophic level, diversity of trophic levels, and diversity in resource use, respectively. These indices are related to Shannon's diversity index (H'), where H' = DH + DV - DR . Application of the framework to three riverine macroinvertebrate communities revealed that D indices, calculated from biomass and stable isotope features, captured well the anthropogenic, seasonal, or other within-site changes in food-web structures that could not be captured with H' alone.

The stable N isotopic composition of individual amino acids (SIAA) has recently been used to estimate trophic positions (TPs) of animals in several simple food chain systems. However, it is unknown whether the SIAA is applicable to more complex food web analysis. In this study we measured the SIAA of stream macroinvertebrates, fishes, and their potential food sources (periphyton and leaf litter of terrestrial C3 plants) collected from upper and lower sites in two streams having contrasting riparian landscapes. The stable N isotope ratios of glutamic acid and phenylalanine confirmed that for primary producers (periphyton and C3 litter) the TP was 1, and for primary consumers (e.g., mayfly and caddisfly larvae) it was 2. We built a two-source mixing model to estimate the relative contributions of aquatic and terrestrial sources to secondary and higher consumers (e.g., stonefly larva and fishes) prior to the TP calculation. The estimated TPs (2.3-3.5) roughly corresponded to their omnivorous and carnivorous feeding habits, respectively. We found that the SIAA method offers substantial advantages over traditional bulk method for food web analysis because it defines the food web structure based on the metabolic pathway of amino groups, and can be used to estimate food web structure under conditions where the bulk method cannot be used. Our result provides evidence that the SIAA method is applicable to the analysis of complex food webs, where heterogeneous resources are mixed.

The northern East China Sea (ECS) serves as a spawning and nursery ground for many species of fish and squid. To clarify the basis of the food web in the northern ECS, we examined the nitrate (NO3) dynamics along four latitudinal transects based on stable nitrogen and oxygen isotopes of NO 3 (δ15NNO3 and δ18ONO3) and temperature-salinity dynamics in both winter (February 2009) and summer (July 2009 and July 2011). The δ15NNO3 and δ18ONO3, which were distinctly different among the potential NO3 sources, were useful for clarifying NO3 sources and its actual usage by phytoplankton. In winter, Kuroshio Subsurface Water (KSSW) and the Yellow Sea Mixed Water (YSMW) predominantly contributed to NO3 distributed in the shelf water. In the surface water of the Okinawa Trough, NO3 from the KSSW, along with a temperature increase caused by an intrusion of Kuroshio Surface Water (KSW), seemed to stimulate phytoplankton growth. In summer, Changjiang Diluted Water (CDW), Yellow Sea Cold Water Mass (YSCWM), and KSSW affected the distribution and abundance of NO 3 in the northern ECS, depending on precipitation in the Changjiang drainage basin and the development of the YSCWM in the shelf bottom water. Although isotopic fractionation during NO3 uptake by phytoplankton seemed to drastically increase δ15NNO3 and δ18ONO3 in summer, relatively light nitrate with δ15NNO3 lower than expected from this fractionation effect might be explained by contribution of atmospheric nitrogen and/or nitrification to NO3 dynamics in the surface and subsurface layers. If the latter were a dominant process, this would imply a tightly coupled nitrogen cycle in the shelf water of the northern ECS., Biogeosciences, 11(4), pp.1297-1317; 2014

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) of macroinvertebrates inhabiting littoral zones of lakes can serve as useful indicators of material loading from the watershed. We collected snails (Semisulcospira spp.) and bivalves (Unio douglasiae biwae Kobelt) from 29 littoral sites in Lake Biwa near the mouths of river tributaries with various human population density (HPD) and land-use patterns. The δ13C and δ15N signatures were determined for three potential food sources: particulate organic matter in the pelagic zone (PPOM), riverine particulate organic matter from tributaries (RPOM) and epilithic organic matter in the littoral zone (EOM). The stable isotope mixing model revealed that snails relied mainly on EOM, and bivalves on PPOM and RPOM. Multiple regression analysis showed that intersite variation in δ15N for snails was best explained by HPD, while variation in δ15N of EOM and nitrate was explained to a lesser extent by HPD. Comparison with isotope signatures of their food sources and riverine nutrients revealed that snails assimilated anthropogenic nitrogen from wastewater in the watershed. Our results also showed that the δ13C value of bivalves was marginally related to the fraction of paddy fields in the watersheds. In conclusion, the isotope signatures of macroinvertebrates inhabiting the littoral zone can be useful indicators of anthropogenic impacts from the watershed.

Epilithic bacterial communities play a major role in biogeochemical cycles of rivers; however, distributional patterns and controls of epilithic communities remain unclear. The objective of the present study was to examine possible environmental factors that affect longitudinal distri- butional patterns of epilithic bacterial communities in 2 rivers (Yasu and Ado Rivers) draining the Lake Biwa basin, Japan. Phylogenetic analyses were conducted using denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes and fluorescence in situ hybridization (FISH) with oligonucleotide probes targeted to small subunit rRNA. Non-metric multidimensional scaling and canonical correspondence analysis of the DGGE profile indicated that a clear shift in community composition occurred at the middle reach of the Yasu River. This shift was most closely related to variation in the nitrogen stable isotope ratio (δ 15 N) of epilithic materials (BIOENV analysis, ρw = 0.512), which is an indicator of the extent of anthropogenic nitrogen loading. The concentration of dissolved organic carbon (DOC) also affected community composition (ρw = 0.510 for the combina- tion of δ 15 N and DOC). The longitudinal pattern of the DGGE profile was less evident in the Ado River. In both rivers, FISH results indicated that Alphaproteobacteria and Betaproteobacteria gener- ally accounted for a large fraction (37 to 71%) of total bacterial abundance, indicating that species and clones affiliated with a limited number of the major phylogenetic groups occupy diverse epilithic habitats. Our results are consistent with the hypothesis that nutrients and organic matter of anthropo- genic origin are major determinants of epilithic bacterial community structure in rivers.

We examined how nitrogen-stable isotopic signatures of food web components (basal resources, primary and lower consumers, and omnivores) in rivers change with increasing levels of human population density (HPD) in their watersheds. Samples were collected from 22 rivers flowing in the Lake Biwa basin, Japan. Among three potential resources at the base of food webs (epilithon, benthic and suspended particulate organic matter), the mean isotopic values (δ15N) of the epilithon (4.5–7.8%) were consistently higher than those of other items (1.9–4.2%) and displayed the most pronounced elevation (by 3.3%) with increasing HPD. The mean δ15N values of the individual taxa of lower consumers (bivalve, snail and caddisfly) tended to increase with increasing HPD, although the pattern and the extent of the elevation were highly variable among the taxa. These results suggest a taxon-specific feature in the N source (or sources) of lower consumers. Our data suggested that human activities (e.g. nutrient loading) potentially induce changes in the N baselines of river food webs. The major N source of bivalves appeared to be shifted from suspended particulate organic matter to other items with increasing HPD. Trophic levels of goby fish (Rhinogobius sp. OR) and shrimp (Palaemon paucidens), being estimated to be at 2.4–3.8 and 2.1–3.4, respectively, did not differ significantly among rivers with different HPD levels.

We performed bag experiments in a Canadian Shield lake with generally high seston (suspended food particles mainly composed of algae) carbon (C):phosphorus (P) ratios, and investigated the responses of individual and population growth of herbivorous Daphnia dentifera on their abundance with (+ P) and without (-P) a phosphorus enrichment to lake water. In both treatments, increased abundance of D. dentifera reduced seston C concentration and was accompanied by decreases in population and individual growth rates. However, P-enrichment increased seston P concentration and then reduced seston C:P ratio from 400-700 to ca 100 (by atoms). As a result, both individual and population growth rates were significantly higher in the + P treatment at all animal abundances even though seston C concentrations were similar between the treatments. The magnitude of the growth enhancement by the P-enrichment was independent of animal abundance. Stepwise regression analyses revealed that 71 and 90% of the variance in the population and individual growth rates, respectively, were explained by seston C and P concentrations, and that the contribution of the seston P concentration was roughly the same as that of seston C. Such joint effects of seston C and P indicate that food quality (P content) as well as food quantity (C concentration) can influence Daphnia not only at the level of individual growth but also at the level of population dynamics in P-limited lakes. Our results thus strongly corroborate the hypothesis that the population development of a key herbivore Daphnia in P-limited Canadian Shield lakes is inhibited by the direct effects of P-limited food on individual growth, which weaken the strength of trophic cascading interactions starting from piscivorous fish through planktivorous fish and zooplankton to algae.

Simple correlation and multiple regression analyses were performed to examine the relationship between primary productivity and environmental factors in the north basin of Lake Biwa. The primary production rates used in the analyses were estimated monthly or bimonthly during the growing season (April‐November) in 1992, 1996 and 1997 with the 13 C method. Elemental (C, N and P) contents of seston were used to assess nutrient conditions. Analyses revealed that 86% of variance in depth-integrated primary production rates (areal PP) can be explained by changes in light intensity, and sestonic C, N and P concentrations. Water temperature had no effect on areal PP. To assess relative effects of light and nutrients on PP, the P:B ratio was estimated by normalizing PP with sestonic C. The areal P:B ratio correlated most significantly with the sestonic N:P ratio, followed by light intensity. When regression analyses were made at each depth, however, the P:B ratio correlated significantly only with the sestonic N:P ratio at 0 and 1 m depths, while light intensity was also incorporated into the regressions at deeper than 2.5 m. In these regressions, the P:B ratio was negatively correlated with sestonic N:P ratio but positively with light intensity. The results suggest that the primary production rate in this lake was mainly limited by P relative to N supply rates, but was not free from light limitation in a large part of the epilimnion. In Lake Biwa, the vertical water mixing regime as well as the nutrient supply seem to be important in determining the growth and composition of primary producers, since the surface mixing layer extends into 10‐15 m depths during most of the growing season.

Long-term monitoring of ecosystem succession provides baseline data for conservation and management, as well as for understanding the dynamics of underlying biogeochemical processes. We examined the effects of deforestation and subsequent afforestation of a riparian forest of Japanese cedar (Cryptomeria japonica) on stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) and natural abundances of radiocarbon (Δ14C) in stream biota in the Mt. Gomadan Experimental Forest and the Wakayama Forest Research Station, Kyoto University, central Japan. Macroinvertebrates, periphytic algae attached to rock surfaces (periphyton), and leaf litter of terrestrial plants were collected from six headwater streams with similar climate, topography, and bedrock geology, except for the stand ages of riparian forests (from 3 to 49 yr old in five stands and >90 yr old in one reference stand). Light intensity and δ13C values of both periphyton and macroinvertebrates decreased synchronously with forest age in winter. A Bayesian mixing model indicates that periphyton contributions to the stream food webs are maximized in 23-yr-old forests. Except for grazers, most macroinvertebrates showed Δ14C values similar to those of terrestrial leaf litter, reflecting the influence of modern atmospheric CO2 Δ14C values. On the other hand, the Δ14C values of both periphyton and grazers (i.e., aquatic primary consumers) were significantly lower than that of modern atmospheric CO2, and were lowest in 23-yr -old forest stands. Previous studies show that root biomass of C. japonica peaks at 15–30 yr after planting. These evidences suggest that soil CO2 released by root respiration and dispersed by groundwater weathers carbonate substrata, and that dissolved inorganic carbon (DIC) with low Δ14C is incorporated into stream periphyton and some macroinvertebrates. The ecological response in the studied streams to clear-cutting and replanting of Japanese cedar is much slower (~20 yr) than the chemical response (