Eisuke Kikuchi

Seasonal increase of methane in sediment decreases δ13C of larval chironomids in a eutrophic shallow lake

Recent studies have shown that larval chironomids assimilate 13C-depleted carbon derived from biogenic methane by feeding on methane-oxidizing bacteria (MOB). The dietary contribution of MOB is known to be maximized in the autumn overturn period or winter in eutrophic dimictic lakes due to the increase of MOB biomass following the supply of oxygenated water, but in polymictic lakes, such seasonal variability has not been revealed. We investigated the seasonal patterns of larval δ13C and methane concentrations in the sediment of a eutrophic polymictic lake, Izunuma, Japan. Larval δ13C decreased in late summer and autumn. Methane concentrations above a 6 cm depth peaked in late summer or autumn, while those in the 10–11- and 20–21-cm layers peaked in October. Negative correlations between methane concentrations in the 5–6/10–11-cm layers and larval δ13C were found. This suggests that an increase in the supply of methane stimulated the activity of MOB in a polymictic lake, where water above the lake bottom rarely became anoxic because of frequent overturn, thus increasing the dietary contribution of MOB to larval chironomids.

Size-dependent ontogenetic diet shifts to piscivory documented from stable isotope analyses in an introduced population of largemouth bass

Piscivorous largemouth bass (Micropterus salmoides) have been introduced in several regions outside of their native range in North America, resulting in significant disturbance to native fish communities. This species exhibits an ontogenetic diet shift from zooplanktivory to piscivory as juveniles. An early switch to piscivory allows 0+ bass to increase their growth rate prior to winter, resulting in reduced mortality. However, little is known about the dietary switch at the population level during the first year. We used carbon stable isotope analyses to examine the diets of age 0+ individuals in Lake Izunuma, Japan. The onset of the shift to piscivory occurred at a smaller size than in native or other non-native areas [>40xa0mm total length (TL)]. We found a positive correlation between TL and δ13C throughout summer and autumn. Small individuals had δ13C values that were similar to those of zooplankton, whereas large individuals had δ13C values that were similar to those of cyprinid prey species. This suggests that the smaller 0+ individuals remain zooplanktivorous until late autumn, whereas the larger individuals shift to piscivory as early as June, soon after the breeding season ends. Our results also suggest that a significant number of 0+ bass failed to switch to piscivory until the winter of their first year, despite the smaller size threshold for the onset of piscivory.

Isotopic evidence for the spatial heterogeneity of the planktonic food webs in the transition zone between river and lake ecosystems

Resources and organisms in food webs are distributed patchily. The spatial structure of food webs is important and critical to understanding their overall structure. However, there is little available information about the small-scale spatial structure of food webs. We investigated the spatial structure of food webs in a lake ecosystem at the littoral transition zone between an inflowing river and a lake. We measured the carbon isotope ratios of zooplankton and particulate organic matter (POM; predominantly phytoplankton) in the littoral zone of a saline lake. Parallel changes in the δ 13C values of zooplankton and their respective POMs indicated that there is spatial heterogeneity of the food web in this study area. Lake ecosystems are usually classified at the landscape level as either pelagic or littoral habitats. However, we showed small-scale spatial heterogeneity among planktonic food webs along an environmental gradient. Stable isotope data is useful for detecting spatial heterogeneity of habitats, populations, communities, and ecosystems.

Ecological Consequences of the Tsunamis Caused by the Great East Japan Earthquake and Subsequent Disturbance Events in a Shallow Brackish Lagoon in Sendai Bay, Japan

The impacts of the tsunami caused by the Great East Japan Earthquake and subsequent disturbance events were examined in Gamo Lagoon, a shallow brackish lagoon in Sendai Bay, Japan. A major tsunami with a height of 7.2 m struck the lagoon on 11 March 2011, followed by a 2-month estuary closing and a typhoon-induced flood within a year. These events induced drastic changes in the salinity and sediment characteristics as well as a washout of vegetation. After the tsunami, the plant community was characterized chiefly by the proliferation of “pioneer species.” The annual helophyte Suaeda maritima became dominant in the bare high-tide zone created through the disappearance of the reed marsh. Sand dune vegetation (SDV) had recovered only in patchily after 4 years, and the community was characterized by the invasive species Cakile edentula. The density and species richness of macrozoobenthos changed after each disturbance event (i.e., the tsunami, estuary closing, and typhoon). Macrozoobenthic density recovered within 6 months after the tsunami and typhoon, chiefly due to the density overshoots of several opportunistic taxa. Though tsunami- and typhoon-induced changes in macrozoobenthic community structure were significant, by 3 years post-tsunami, the community had begun to recover toward pre-tsunami conditions. These results emphasize both the vulnerability and resiliency of estuarine biotic communities against large pulsed disturbance events.

Hypoxia within macrophyte vegetation limits the use of methane-derived carbon by larval chironomids in a shallow temperate eutrophic lake

Methane-derived carbon (MDC) can subsidize lake food webs. However, the trophic transfer of MDC to consumers within macrophyte vegetation is largely unknown. We investigated the seasonality of δ13C in larval chironomids within Nelumbo nucifera (Gaertn.) and Trapa natans var. Japonica (Nakai) vegetation in the shallow, eutrophic Lake Izunuma in Japan. Over the past several years, N. nucifera has rapidly expanded across more than 80% of the lake surface. Prior to the expansion of N. nucifera (2007–2008), a previous study reported extremely low larval δ13C levels with peak sediment methane concentrations in August or September. After the expansion of N. nucifera (2014–2015), we observed extreme hypoxia as low as or lower than 1xa0mgxa0l−1 among the macrophyte coverage during June and August. During August and September, no larvae could be found among N. nucifera, and larvae in T. natans showed relatively high δ13C levels (>u2009−u200940‰). In contrast, larvae were markedly 13C–depleted (down to −u200960‰) during October and November. The renewed supply of oxygen to the lake bottom may stimulate MOB activity, leading to an increase in larval assimilation of MDC. Our results suggest that macrophyte vegetation can affect the seasonality of MDC transfer to benthic consumers under hypoxic conditions in summer.

Zoobenthos are minor dietary components of small omnivorous fishes in a shallow eutrophic lake

We examined whether small omnivorous fishes (smaller than ~100mm long) integrate littoral, pelagic and benthic pathways in a shallow, eutrophic lake (Lake Izunuma, Japan). The surface of the lake was covered by a dense vegetation of floating-leaved macrophytes, and small species dominated the icthyofauna. We determined the δ13C and δ15N ratios of five dominant species of small omnivorous fishes. Using a stable isotope analysis in the R mixing model, we determined the possible contribution of three potential food sources (epiphytic algae, zooplankton and zoobenthos (larval chironomids)) to omnivorous fish tissue compositions. Four omnivorous fishes (Gnathopogon elongatus elongatus, Pseudorasbora parva, Biwia zezera and Tridentiger obscurus) subsisted largely on epiphytic algae and zooplankton, whereas zoobenthos contributed little to their diets. Acheilognathus rhombeus subsisted mostly on epiphytic algae. Thus, in this shallow, eutrophic lake, omnivorous fishes incorporated both littoral and pelagic production into the food web, but rarely benthic production. The dominant benthic chironomid larvae often burrow several centimetres into the sediment, and the low dietary contribution of zoobenthos to small fishes may be due to inefficiency at foraging on buried benthos associated with fish body size.