Hideki Hamaoka

Sources and Exchange of Particulate Organic Matter in an Estuarine Mangrove Ecosystem of Xuan Thuy National Park, Vietnam

The spatio-temporal variations in stable isotope signatures (δ13C and δ15N) and C/N ratios of particulate organic matter (POM), and physicochemical parameters in a creek water column were examined in an estuarine mangrove ecosystem of Xuan Thuy National Park, Vietnam. The objective was to examine the factors influencing creek water properties, and the sources and exchange of POM in this important mangrove ecosystem. The diel and seasonal variations in water temperature, flow velocity, pH, dissolved oxygen, and salinity demonstrated that tidal level, season, and biological factors affected the creek water properties. Mangroves had relatively low δ15N and very low δ13C values, with respective average values of 1.5 ± 0.9‰ and −28.1 ± 1.4‰. The low mangrove leaf δ15N indicated minor anthropogenic nitrogen loading to the mangrove forests. A significant positive correlation between POM–δ13C and salinity along the axis of Ba Lat Estuary, Red River, indicated that marine phytoplankton (δ13C value, −21.4 ± 0.5‰) was the predominant source of POM at the estuary mouth. Based on the co-variation of δ13C and C/N ratios, marine phytoplankton and mangrove detritus were predominant in POM of major creeks and small creeks, respectively. During the diurnal tidal cycle, the dynamics of POM were affected by sources of organic matter, tidal energy, and seasonal factors. The contribution of mangrove detritus to POM reached a maximum at the low tide and was enhanced during the rainy season, whereas marine phytoplankton contribution was highest at high tide.

Tracing anthropogenic inputs to production in the Seto Inland Sea, Japan--a stable isotope approach.

The Seto Inland Sea (SIS) receives waste runoff from ∼24% of Japans total population, yet it is also important in regional fisheries, recreation and commerce. During August 2006 we measured carbon and nitrogen stable isotopes of particulate organic matter (POM) and zooplankton across urban population gradients of the SIS. Results showed a consistent trend of increasing δ(15)N in POM and zooplankton from the western to eastern subsystems of the SIS, corresponding to increasing population load. Principal components analysis of environmental variables indicated high positive loadings of δ(15)N and δ(13)C with high chlorophyll-a and surface water temperatures, and negative loadings of low salinities related to inputs from large rivers and high urban development in the eastern SIS. Anthropogenic nitrogen was therefore readily integrated into the SIS food web from primary production to copepods, which are a critical food source for many commercially important fishes.

Life history and ecology of the glowbelly Acropoma japonicum in the Uwa Sea, Japan

The life history and ecology of the glowbelly Acropoma japonicum a commercially important fishery resource, were investigated in the Uwa Sea, Japan. Newly settled juveniles (0*) appeared at the end of the breeding season and reached maturity in the next season, i.e. when they became yearlings (1*). The yearlings made a gametic effort comparable to that of older fish. During the breeding season, the somatic conditions were more deteriorated for males than for females. That the deterioration of male somatic conditions was not attributable to a sexual difference in energy gain from feeding suggests that the males incurred higher energetic costs of reproduction despite their relatively low gonadosomatic indices. Such an energy depletion may increase the mortality risk, resulting in female-biased sex ratios in the older age classes. Most of the 1+ fish disappeared in winter following their first breeding. For A. japonicum, the observed precocity and short life span is notable because it is a higher consumer that is generally expected to show later maturation and longer life. As likely. While fishing is one of the most influential factors, the possibility of age-specific migration toward deeper waters remains to be examined.

Application of the coastal ecosystem complex concept toward integrated management for sustainable coastal fisheries under oligotrophication

Harmonizing coastal fisheries with water-quality improvement has become an essential factor for the sustainable use of coastal ecosystem services. Here, we present the scope of our study based on an interdisciplinary approach including ecological actions, socio-economic actions and socio-psychological actions. We chose to focus on the interaction between oyster aquaculture and seagrass vegetation as a typical ecological action using the coastal ecosystem complex (CEC) concept. Coastal organisms have adapted their traits to the environment over a long period of time, so that restoration of the CEC represents reconstruction of the original process of coastal production. Subtidal seagrass vegetation with intertidal oyster reefs is the original CEC in Japan, which would be expected to enhance coastal production by improving the production efficiency without adding nutrients. A simple field experiment examining carbon and nitrogen contents and stable isotope ratios revealed that oyster spats cultivated on a tidal flat adjacent to seagrass beds had higher nitrogen contents and higher δ13C ratios than spats cultivated in an offshore area using only pelagic production. This result suggests that utilization of the CEC, which enables oysters to use both pelagic and benthic production, has potential to sustain a food provisioning service for humans, even in oligotrophic conditions.

Dual isotope study of food sources of a fish assemblage in the Red River mangrove ecosystem, Vietnam

The food source utilization and trophic relationship of the fish assemblage in the Red River mangrove ecosystem, Vietnam were examined using dual isotope analysis. The carbon and nitrogen stable isotope signatures of 23 fish species ranged from −24.0 to −15.7‰ and from 8.8 to 15.5‰, respectively. Cluster analysis based on the δ13C and δ15N signatures clearly separated the mangrove fish into five feeding groups, representing detritivores, omnivores, piscivores, zoobenthivores, and zooplanktivores, which concurred with the dietary information. The results suggested that mangrove carbon contributed a small proportion in the diets of the mangrove fish, with dominant food sources coming from benthic invertebrates, including ocypodid and grapsid crabs, penaeid shrimps, bivalves, gastropods, and polychaetes. The δ15N values showed that the food web structure may be divided into different trophic levels (TLs). The lowest TLs associated with Liza macrolepis, Mugil cephalus, and Periophthalmus modestus; 18 fish species had TLs between 3.0 and 3.8; and Pennahia argentata had the highest TL (c. 4.0).

Upwelling-like bottom intrusion enhances the pelagic–benthic coupling by a fish predator in a coastal food web

Upwelling regions where nutrients are transported from deep to surface waters are among the most productive in the oceans. Although it is well known that the upwelling affects fishery production through bottom-up trophic cascading, it remains unexplored how temporal variation in its intensity alters overall trophic energy flows within a focal food web. In the present study, we demonstrate that inter-annual variation in the intensity of upwelling-like bottom intrusion alters food web properties in coastal waters of the Uwa Sea by focusing on the levels of δ13C and δ15N for a demersal fish predator, Acropoma japonicum. This approach integrates information on prey–predator interactions. In the season following a stratification period when pelagic productivity is limited by nutrient availability, A. japonicum showed lower levels of δ13C in years with high bottom intrusion intensity than in those with low intensity. One possible cause for this isotopic depletion is that the bottom intrusion-induced nutrient supply enhances pelagic productivity and consequently facilitates a foraging shift by A. japonicum from ordinary benthic prey to supplementary pelagic prey with a lower δ13C. In conclusion, the increased intensity of bottom intrusion results in coupling of two major trophic energy flows, pelagic and benthic food chains, through the demersal predator’s foraging shift.

Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future Needs

In this chapter, we estimated carbon sequestration by seagrass and macroalgal beds, defined as the integration of annual plant tissue (organic carbon) production, in Japan. Each of the main four beds, eelgrass beds (Amamo-ba), Sargassum beds (Garamo-ba), warm-temperate kelp beds (Arame-ba), and cold-temperate kelp beds (Kombu-ba), exhibited a distinctive geographic distributional pattern along Japan’s coasts that depended on regional climate and topographic characteristics. The total area of the four beds was approximately 230,000 ha nationwide, based on an analysis of the latest satellite images and information on past distributions of the beds. Carbon sequestration of each seagrass or macroalgal bed was evaluated as integrated annual plant tissue production converted to organic carbon, which was defined by subtracting dissolved organic matter production from net primary production. Plant tissue production of the main constituent macrophyte of the seagrass and macroalgal beds was directly measured in each coastal region, and production values from past reports were also collected and utilized. Annual carbon sequestration by seagrass and macroalgal beds in Japan, expressed in a CO2-converted base, was about 4.7 million tons per year, which is comparable to the CO2 emissions of the industrial sectors of agriculture and fisheries.

Application of stable isotope analysis for detecting filial cannibalism

A novel type of filial cannibalism has been reported in pipefishes, in which the eggs are absorbed through the males brood-pouch epithelium. The present study explored the applicability of stable isotope analysis for the detection of paternal brood cannibalism in the seaweed pipefish Syngnathus schlegeli. As expected, the δ15N values for liver, which conveys short-term dietary information about the recent reproductive season, were higher in males than in females. In contrast, the δ15N values for muscle, which reflects longer-term feeding habits that span both the reproductive and non-reproductive seasons, did not significantly differ between the sexes. This finding indicates that males occupy a higher trophic position than females only during the reproductive season, and it is probable that this difference is a result of paternal uptake of nutrients from embryos in the brood pouch.

Production of Zostera marina with different shoot size and stand structures in the Seto Inland Sea, Japan : production in the luxuriant season in 2012

2012年６月に瀬戸内海の３つのアマモ場の，それぞれ異なる株サイズと群落構造を示す計7地点のアマモ群落において生産量を測定，比較した。株あたりの生産量は株サイズに依存し，大きい株が相対的に低い密度で生育する生野島（広島県；安芸灘）のアマモ場で50.0-73.2 mg DW shoot -1 d-1であり，小さい株が密生する平郡島（山口県；伊予灘）の株あたり生産量（7.7-27.4 mg DW shoot -1 d-1）より大きかった。面積あたり生産量において，株あたり生産量の差は株密度により相殺される傾向もみられたが，生野島のアマモ場の生産量（2.89-5.38 g DW m-2 d-1）の方が平郡島のアマモ場の生産量(1.63-2.56 g DW m-2 d-1) よりも大きかった。これら２つのアマモ場では底質に大きな相違がみられ，アマモの群落構造や生産量に影響を与えていると考えられた。すなわち，生野島の底質はほとんど泥により構成され有機物含量も高い一方で，平郡島の底質は中砂・細砂を中心に構成されより厳しい波浪環境を反映していた。 2011年秋季のアイゴの食害による消失から回復途上にある阿波島（広島県；安芸灘）のアマモ場では新たに発芽した実生由来の株がパッチ状の群落を作り，株密度も現存量も低かった。調査した群落の中で生産量は最も低かったが(0.60 g DW m-2 d-1)，その現存量回転率（6.5％ d-1）は，他群落（1.7-3.3％ d-1）のそれよりも大きかった。

瀬戸内海西部の屋代島，平郡島における海藻藻場の特性 : 特にホンダワラ類とクロメの垂直分布について

瀬戸内海西部の伊予灘と広島湾の島嶼（屋代島，平郡島）で，大型褐藻のホンダワラ類（ヒバマタ目ホンダワラ科）とクロメ（コンブ目レッソニア科）により形成される藻場の特性を調べた。対象とした藻場は，自然岩礁域およびそれに付帯する礫集積域に形成されているものが8か所，投石による人工礁に形成されているものが3か所であり，ライントランセクト法により植生の垂直構造を明らかにし，そこにみられる法則性と環境要因との関係について考察した。調査では計85種の海藻を確認し，そのうちホンダワラ類についてはヒジキ，ノコギリモク，アカモク，ジョロモク，ホンダワラ等の計14種をみとめた。総じて，ホンダワラ類は潮間帯から水深4m 程度までで優占し，クロメは14mを下限にホンダワラ類より深所まで分布した。しかし，平郡島南岸や屋代島南岸では，浅所の岩盤上部にクロメ，その下部の漂砂影響域や礫集積域にホンダワラ類が生育し，植生の垂直構造が逆転している場合もみとめられた。人工礁上では，おおむね自然岩礁・礫集積域と類似した植生が形成されていたが，特に屋代島北岸（広島湾側）では深所で沈積浮泥の影響がみとめられ，クロメの生育は不良であるか植生から欠落していた。藻場の環境特性と植生の関係を解析した結果，平均水深が浅い藻場ではホンダワラ類の平均被度が大きくなり，海底傾斜が大きい藻場ではクロメの平均被度が大きくなる傾向がみとめられた。