Ken-ichi Hayashizaki

Photosynthetic and respiratory responses of Gracilaria salicornia (C. Ag.) Dawson (Gracilariales, Rhodophyta) from Thailand and Japan

The photosynthetic and respiratory responses of Gracilaria salicornia in the subtropical waters of Japan (in Okinawa) and in the tropical waters of Thailand (in Rayong and Phuket Provinces), were studied under various conditions of irradiance, salinity and temperature. This alga showed adaptability in its photosynthetic and respiratory responses to oceanic salinity as well as to subtropical to tropical temperature. Significant differences in the photosynthetic and respiratory rates among the G. salicornia populations collected from the above sites were observed. The Phuket population showed adaptability to high irradiance and temperature, characteristic of its natural sun-exposed environment, as it exhibited the highest Ik and Ic, and lowest α and higher Pmax at 30–35°C. On the other hand, the Okinawa population demonstrated adaptability to low submarine irradiance as it had a lower Ik and Ic than either population from Thailand. Its Pmax at 20–25°C was also higher than that of the Phuket population. The Rayong population, however, showed the highest Pmax, Rd, α, Ik and Ic, suggesting its adaptability to both sun and shade light conditions in its natural environment.

Effect of shrimp farming organic waste on food availability for deposit feeder crabs in a mangrove estuary, based on stable isotope analysis

We have investigated the impact of shrimp farming waste on benthic invertebrates in a mangrove estuary in southern Thailand. Stable carbon and nitrogen isotope analyses were used to assess the contribution of aquaculture feed as a nutrient source for benthic invertebrates (fiddler crabs Uca annulipes, U. bengali, and U. forcipata) and to compare the organic origin of sediments in tributaries with and without a shrimp farm. The isotopic contributions of shrimp feed to crabs varied between the tributaries: in the tributary with the farm, the crabs had higher contributions from shrimp feed, whereas there was only a small contribution from aquaculture feed in crabs from the reference tributary. In contrast, the contribution of shrimp feed to the sediment did not differ between the tributaries, the contributions to sediments from aquaculture feed were, in general, quite low. Based on these results, we suggest that, in mangrove estuaries, changes in the food sources of several surface feeder invertebrates would be an effective indicator of the possibility that aquaculture waste was affecting the benthic ecosystem.

Short-term measurement of carbon stable isotope discrimination in photosynthesis and respiration by aquatic macrophytes, with marine macroalgal examples

Progress in the study of stable isotope discrimination in carbon assimilation by aquatic macrophytes has been slower than for other groups of primary producers, such as phytoplankton and terrestrial plants. A probable reason has been the methodologies employed for such a study: field collections or long‐term incubations, both relying on the observation of changes in carbon isotope composition of plant tissue. Here, we present a short‐term incubation method based on the change in carbon stable isotope composition in water. Its fundamental advantage over the other approaches is that the change in stable isotope composition in water in a closed system is much faster than in the plant tissue. We applied the method to investigate the relationship between carbon assimilation intensity and isotope discrimination. The results included a relatively small discrimination in respiration, a significant influence of carbon assimilation rate on discrimination, and the suggestion of HCO3− or CO2 uptake in photosynthesis. The information gathered using this method would be difficult to obtain in other ways, and so we believe that it should contribute to a better understanding of the physiology and ecology of aquatic macrophytes.

Environment determines nitrogen content and stable isotope composition in the sporophyte of Undaria pinnatifida (Harvey) Suringar

Nitrogen content and δ15N (nitrogen stable isotope ratio) were measured in different parts of the sporophyte of Undaria pinnatifida (Harvey) Suringar from two bays in the northeastern Japan. There were clear differences between the thalli collected in winter and in summer: high nitrogen content and low average δ15N were observed for the thalli collected in winter, while the opposite pattern was found for the thalli from summer. In addition, the pattern of internal δ15N distribution in the thallus changed with season. It is possible that the cause for these seasonal differences in the algae was the seasonal change in environmental conditions, because in winter water is normally rich in nitrogen, while in summer it is poor. U. pinnatifida sporophyte may be useful as an indicator of nitrogen sources in coastal waters, but consideration must be given to the effect of isotope fractionation on δ15N of the plant, especially in winter. Potential may exist for the use of different parts of the thallus to indicate nitrogen sources at different periods, but more investigation is necessary to accomplish this.

Temperature effect on carbon stable isotope discrimination by Undaria pinnatifida (Phaeophyta) in a closed experimental system

The temperature influence on carbon stable isotope discrimination (Δ) in photosynthesis by algae has not been studied taking into account the confounding effect due to photosynthetic rates. This is problematic because usually higher temperatures imply higher photosynthetic rates, and higher photosynthetic rates usually lead to a decrease in Δ. Here, we investigate the effect of temperature on Δ during photosynthesis by Undaria pinnatifida (Harv.) Suringar (Phaeophyta) in a closed system, varying temperatures between 5°C and 20°C and measuring photosynthetic rates simultaneously. There was a general trend of higher Δ for higher temperatures under the same photosynthetic rate, especially for higher photosynthetic rates. These results were consistent with the influence of phenomena related to carbon supply to the plant, like CO2 diffusion in water and through cell membranes. This influence, however, was less strong than that of photosynthetic rates on Δ (lower Δ for higher photosynthetic rates) and can be difficult to observe in nature.

Natural foods utilized by Nile tilapia, Oreochromis niloticus, in fertilizer-based fish ponds in Lao PDR identified through stable isotope analysis

Nile tilapia, Oreochromis niloticus, is one of the most popular freshwater aquaculture species in developing countries. Although formulated feed provides higher fish yield, it is expensive. Therefore, fertilizer-based fish culture using natural food sources is generally implemented in developing countries. The aim of this study was to identify the major natural foods contributing to tilapia growth in fertilizer-based fish ponds. The stomach contents of tilapia in a fertilizer-based fish pond in Lao PDR were analyzed to estimate their feeding behaviors; stable isotope ratios of carbon and nitrogen in the potential food organisms and suspended solids were measured and compared with the ratios in the fish muscle tissues. Further, the feed efficiency of chironomid larvae was compared with that of chlorella and formulated feed in a laboratory feeding experiment. Consequently, chironomid larvae were identified as the main contributors to tilapia growth in the fertilizer-based fish pond. In conclusion, benthic larvae of insects belonging to the family Chironomidae are confirmed to be a natural food source for tilapia in the fertilizer-based fish pond.

Stable C and N isotope analysis elucidated the importance of zooplankton in a tropical seagrass bed of Santiago Island, Northwestern Philippines

ABSTRACT The role of zooplankton in a tropical seagrass ecosystem was investigated in milkfish farms pollution-impacted and -unimpacted seagrass beds in Santiago Island coral reefs, Northwestern Philippines. The aim was to compare between the two sites: (1) abiotic factors and zooplankton community parameters, and (2) the trophic structure using C and N stable isotopes. Low water (98–119 mV) and sediment (–121 to −138 mV) Oxidation Reduction Potential values indicated a reducing environment in the impacted site. Zooplankton in the impacted site showed the typical community response to eutrophication (low diversity, but high total abundance due to the dominance of the cyclopoid copepod Oithona oculata), generally few elevated δ15N values, but a significant shift towards depleted 13C due to the organic enrichment of fish-farm feeds. Apart from suggesting a highly complex food web with POM and zooplankton as main food sources in the unimpacted site, the Bayesian mixing model simulation generated reduced complexity in feeding interactions between basal sources, zooplankton, and fish including adults of a key fish species, Siganus fuscescens, in the impacted sites. In this study, C and N stable isotope analysis has clarified the importance of zooplankton as fish prey in a seagrass bed food web.

Feeding ecology of two sympatric species of Acetes (Decapoda: Sergestidae) in Panguil Bay, the Philippines

Sergestid shrimps of the genus Acetes are important in global shrimp fisheries and nearshore food-webs. The feeding habits of the sympatric species Acetes erythraeus and A. intermedius from Panguil Bay, Philippines were studied using gut contents and stable isotopes analyses. Both species are omnivorous suspension feeders of plankton, macrophyte detritus, and amorphous particulate organic materials. However, the diet of adults and juveniles differed by prey type. Gut fullness differed over 24h with a night-time peak in A. erythraeus and morning and midnight peaks in A. intermedius. Over 1 year gut fullness peaked during July to September for both species, with a minor peak during January for A. intermedius. Stable isotope analysis revealed similar δ15N values, but both species showed ontogenetic and interspecific separation of δ13C. A. erythraeus seemed to assimilate highly depleted carbon food sources by mangrove-based detrital and plankton trophic pathways, whereas A. intermedius relied on plankton and macroalgal and seagrass detrital pathways. These trophic differences may be mechanisms of partitioning the feeding niche, but allotopy in the two species with A. erythraeus confined to more brackish waters and A. intermedius dwelling in more saline marine waters may indicate that habitat niche segregation underlies these contrasting carbon trophic pathways.