Osamu Nishimura

Long-term sedimentation patterns influence the ecological succession of shallow lakes. However, human-induced impacts can disrupt these processes, leading to prolonged hysteresis. Using historical sedimentation data, we simulated the future terrestrialization of Lake Izunuma-Uchinuma, a Ramsar-listed wetland in Japan. The results indicated that ecotone recovery would take over 150 years, highlighting the strong legacy effects of shoreline vegetation loss. To accelerate restoration, we implemented an integrated approach that combined water-level management with sediment stabilization structures, including fences and coconut mat rolls. Over three years, these interventions successfully restored shoreline sediment accumulation, facilitated the re-establishment of Zizania latifolia (from 328 m² to 1537 m² in Ecotone 1), and improved water quality and waterbird use. Waterbird abundance significantly increased (p < 0.05) in the treated zones, and sediment exposure led to a reduction in COD release, indicating improved substrate conditions. Our results suggest that proactive ecotone restoration strategies, including hydrological regulation and sediment management, are essential in lakes where natural recovery is hindered by long-term sedimentation deficits and water-level changes. This study highlights the importance of integrating these measures to mitigate hysteresis and enhance ecosystem resilience in degraded shallow lakes.

Seagrass is a key primary producer in coastal ecosystems; however, most studies on seagrass-benthos interactions have focused on subtidal zones. Some species such as Zostera japonica grow in intertidal flats; however, their ecological functions remain unclear. Understanding whether intertidal seagrass beds contribute to benthic abundance and diversity can provide insights that facilitate tidal flat conservation. The present study clarifies the role of intertidal Z. japonica as a food source for benthos. Field surveys were conducted in an intertidal flat in Matsushima Bay, Japan. Five benthic species (Batillaria cumingii, Umbonium costatum, Phacosoma japonicum, Nereididae, and Paguroidea) were identified. Carbon and nitrogen stable isotope ratios (δ¹³C, δ¹⁵N) and fatty acid compositions of sediment organic matter, seawater, and target benthos were analyzed to determine food sources. The results showed that B. cumingii actively consumed Z. japonica-derived organic matter present in both seagrass and sandy sediments. Z. japonica also influenced bacterial community structure, providing a favorable habitat for Nereididae. Filter feeders (U. costatum and P. japonicum) exhibited minimal reliance on Z. japonica-derived organic matter. The findings suggest that, similar to subtidal seagrass ecosystems, intertidal seagrass meadows support benthic communities by supplying organic matter and enhancing bacterial production.

内湾底層の溶存酸素消費につながる粒状有機物の沈降過程での化学組成や分解活性の変化を明らかにするため，宮城県志津川湾において異なる水深帯から沈降性と浮遊性の粒状有機物を採取した．沈降物の長鎖多価不飽和脂肪酸（C_≧20PUFA）含有量は比較的高く，C_≧20PUFAを相対的に多く含む有機物が沈降していることがわかった．また，沈降物中のC_≧20PUFA含有量は水深方向に減少し，沈降過程で易分解性の粒状有機物が分解によって減少していると考えられた．沈降物のC_≧20PUFA含有量と酸素消費速度との正の関係から，水深方向への分解に伴う粒状有機物の酸素消費活性の低下が示された．以上より脂肪酸組成は，内湾の比較的浅い海域であっても沈降過程における易分解性の粒状有機物の組成や分解性の変化を高い感度で捉えられる有用な指標になると考えられる．

火力発電所6施設の冷却系水路を対象に，3回の採水調査と計18水路系統71清掃機会の廃棄物処理量データを基に，潜在的餌料としての流入有機物量と水路付着生物の餌要求量の比（S/D）を推定した．その結果，S/Dは，付着生物の生産/バイオマス比を基に0.48―23.8（中央値3.55），付着生物のろ過摂食量を基に0.21―19.2（中央値1.99）とそれぞれ推定された．水路への有機物流入量や付着生物の餌要求量と同量の有機物を除去するのに，水路取水口前面海域にカキを水深10mまで垂下養殖しろ過摂食させる場合，必要面積は数万m²のオーダーと推計された．現状では生物付着対策のための取水口前面海域の利用に向けては，利害関係者間の調整等が容易ではないと考えられる．しかし，他の生態系サービスの創出と関連付けることで本アプローチへの理解と実現の可能性を高められるかについて検討の余地がある．

Bivalves serve as an important aquaculture product, as they are the source of essential fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in our diet. However, their cultivation in the wild can be affected by fouling organisms that, in turn, affect their EPA and DHA content. The effects of fouling organisms on the EPA and DHA contents of cultivated bivalves have not been well documented. We examined the effects of fouling organisms on the EPA and DHA contents and condition index of cultured oysters, Crassostrea gigas, in an aquaculture system. We sampled two-year-old oysters from five sites in Shizugawa Bay, Japan, in August 2014. Most of the fouling organisms were sponges, macroalgae, and Mytilus galloprovincialis. A significant negative relationship existed between the DHA content in C. gigas and the presence of sponges and macroalgae. A lower C. gigas EPA content corresponded to a higher M. galloprovincialis fouling mass and a lower C. gigas condition index. This can be explained by dietary competition between C. gigas and M. galloprovincialis for diatoms, which were the main producer of EPA in our study sites. Our findings indicate that fouling organisms likely reduce the EPA and DHA content in cultivated oysters. Therefore, our results suggest that the current efforts to remove fouling organisms from oyster clusters is an effective strategy to enhance the content of EPA and DHA in oysters.