Introduction: the coastal ecosystem complex as a unit of structure and function of biological productivity in coastal areas
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Coastal seas are composed of relatively independent ecosystems, such as estuaries, rocky shores, mangroves, and coral reefs. Such individual ecosystems (IEs) are linked closely to each other by the dispersal and circulation of nutrients and organic matter and the movement of organisms, and form a coastal ecosystem complex (CEC). The CEC is understood as a unit of structure and function of coastal seas. It is characterized by a high degree of species diversity and it plays a large role in fishery and aquaculture production, as it provides important marine services for humans. This special volume compiles recent ecological studies of target species and IEs, to facilitate the conservation of coastal seas and the sustainability and production of their fisheries and aquaculture based on our understanding of the structure and function of the CEC in these environments.
KeywordsCoastal sea Individual ecosystem Ecosystem complex Conservation
Capture and aquaculture production in 2016 in Japan amounted to 4.25 million t (mt) (Ministry of Agriculture, Forestry and Fisheries 2017). The capture production of wild plants and animals amounted to 3.22 mt (76% of the total production), of which 0.99 mt (23%) was from coastal seas. The aquaculture production amounted to 1.03 mt (24%), of which 0.39 mt (9%) was seaweed, 0.37 mt (9%) was composed of filter-feeding bivalves, and 0.25 mt (6%) was composed of fish and shrimp-feed culture. Considering that seaweed and bivalves grow by utilizing nutrients or phytoplankton and particulate organic matter in the sea, 94% of the capture and aquaculture production in Japan depends on marine productivity, and only 6% of the total is produced by artificial feeding. Marine production differs completely from terrestrial agriculture and livestock production in terms of dependence on wild biological productivity. Thus, we must understand and conserve the structures and functions of marine ecosystems to sustain capture and aquaculture production.
Coastal seas include estuaries, seagrass and seaweed beds, mangroves, and coral reefs, each of which is as productive as a terrestrial tropical forest. Although the coastal seas are not large, with a total area of 3102 million ha (6% of the total global surface area), they account for an estimated 38% of all global ecosystem services (Costanza et al. 1997). Anthropogenic activities have long been concentrated in coastal seas, resulting in the modification of coastal and riverine structures, eutrophication and pollution, and invasion and colonization by foreign species. Thus, coastal ecosystem functions have largely been destroyed or impaired. The restoration of coastal ecosystems to health and the sustainability of these conditions are important for societies to enjoy the significant benefits of coastal ecosystems.
The Census of Marine Life, conducted from 2000 to 2010, established that 33,629 species of plants and animals inhabit the waters around Japan; 26% of these species are molluscs, 19% are arthropods, and 13% are chordates (Fujikura et al. 2010). The seas around Japan have high degrees of species diversity in such taxa, which are essential biological resources. Diverse marine plants and animals have reproduced and maintained species populations that can be exploited as important biological resources for human societies. Extensive anthropogenic activities, however, have been detrimental to coastal ecosystems. The capture production in Japanese coastal seas in 2016 (0.99 mt) was only 44% of the 2.27 mt recorded in 1985. Further deterioration of coastal ecosystems due to progressive warming and acidification of the seas is a critical problem for the maintenance of food production and species diversity. Another serious problem on the Pacific coast of northern Japan was caused by the earthquake and tsunami that occurred on 11 March 2011. Large disturbances to the coastal ecosystems are attributable to this historical tsunami event. Investigation of the secondary succession of ecosystems that began after the tsunami event is crucial to enable understanding of how to recover fishery production in the area.
The management of biological resources in Japanese coastal seas for conservation and human utilization has been conducted in terms of species populations. That is, a management policy is implemented and a stock enhancement measure taken for each target species. In the coastal seas, however, each species reproduces and grows in the context of diverse and complicated species interactions in multiple coastal ecosystems, such as seaweed beds and sandy beaches. Enhancement measures that aim to make target species artificially dominant in coastal ecosystems, such as the seeding of coastal areas with juvenile fish and shellfish and elimination of the species’ predators and competitors, have been shown to be ineffective in enhancing stock and augmenting capture production of these species. Instead of these measures, which are typically used in terrestrial agriculture, alternative approaches are required to render natural coastal ecosystems fully functional, with the sustainable production of resource species in biotic communities. Aquatic animals and plants are characterized by vertically and horizontally dispersive advective and diffusive sea water movements. They may shift habitats from one ecosystem to another with the development of their life forms. Thus, we need to understand that coastal seas are complex systems composed of multiple relatively small ecosystems, such as river-mouth estuaries, offshore sandy beaches, and rocky seaweed beds, which are connected to each other by the cycling of material, dispersal of biotic particles, and food webs.
These individual ecosystems (IEs) have been understood to be relatively independent coastal ecosystems, and have been studied separately as units of ecological structure and function. In reality, they are adjacent and linked closely to each other in coastal sea areas, forming a complex system. The coastal ecosystem complex (CEC) composed of such IEs is a unit of biological productivity. The CEC is supplied with material from terrestrial ecosystems via streams and rivers. Oceanic bottom water also provides the CEC with nutrients through estuarine circulation driven by the outflow of river water. Nutrients, dissolved and particulate organic matter, and plankton disperse within the CEC. The production of phytoplankton, microphytobenthos, macroalgae, and seagrasses incorporates dispersive nutrients in the CEC. Diverse animal species inhabit an IE or IEs to form a biotic community with prey–predator and competitor relationships (Itoh et al. 2018). The modification of one or more IEs in the CEC or the disruption of linkages among IEs can lead to deterioration of the biological productivity of the CEC.
Ecological studies of important resources and their habitats and community structures are compiled in this special volume as case studies of biological production in the CEC.
Temperate sea bass
Fishery catches of temperate sea bass Lateolabrax japonicus have remained large in many Japanese coastal waters, in contrast to declining trends for other coastal fishes since the 1980s. Sea bass spawn in offshore waters in winter. Their pelagic eggs and larvae are transported inshore, and late-stage larvae settle in shallow (depth < 15 m) coastal waters. Juveniles utilize a variety of habitats, such as sandy and muddy inshore areas, seagrass and seaweed beds, and lower reaches of rivers, as nursery grounds. Kasai et al. (2018) report on the flexible survival strategy of juveniles, which involves the utilization of a wide range of nursery habitats and prey organisms, likely contributing to the stable stock level of this species. Fuji et al. (2018) describe the physical and biological mechanisms that allow juvenile river users to migrate from inshore areas to rivers with salt-wedge intrusions.
Japanese flounder Paralichthys olivaceus spawn at depths of 20–100 m, and their pelagic larvae are transported to shallow coastal areas. Juvenile flounder settle in sandy areas with depths < 15 m and consume mainly mysids. This feeding habit ontogenetically shifts to a piscivorous habit, and young fish leave the shallow nursery grounds for deeper waters searching for larger fish prey. It has been widely accepted that juvenile flounder inhabit shallow areas for a few months after settlement; however, Kurita et al. (2018) report that juvenile and young flounder in Sendai Bay utilize shallow feeding grounds as well as offshore nursery grounds for long periods to effectively take advantage of good feeding conditions. The characteristics of habitat use found in this study might contribute to higher recruitment success in this area.
After oceanic migration, Pacific herring Clupea pallasii return to their home coastal areas at the age of 2 years for the first spawn. The eggs are attached to seaweeds and seagrasses. The reproductive and early life ecologies of herring were surveyed in the Akkeshi area of eastern Hokkaido (Shirafuji et al. 2018) and the Miyako Bay area in northern Honshu (Yamane et al., unpublished data). In both areas, adult herring spawn in the innermost waters with dense vegetation, whereas larvae and early stage juveniles tended to inhabit waters around the spawning sites with less dense vegetation. They then move toward open bay areas with the temperature rise in the summer. Thus, the coastal habitats of herring were found to shift from the innermost waters to open bay areas with ontogenetic development.
Due to the high commercial value of the Japanese sea cucumber Apostichopus japonicus on the international market, overfishing has led to pronounced declines in landings in various waters of Japan. A good understanding of favourable habitats for this species is crucial for appropriate stock management. Minami et al. (2018) indicate that pelagic larvae in Maizuru Bay disperse from spawning areas to the entire bay area, floating in the shallow (depth < 2 m) surface zone. Juvenile sea cucumbers use steeply sloped coastal areas close to the coastline with high levels of sea-origin organic matter. Sea cucumbers tend to move to deeper areas with growth.
Larvae of many abalone species Haliotis spp. settle primarily on crustose coralline algae (CCA) after the larval swimming period of 1–2 weeks. Although metamorphosed juveniles inhabit the rocky reef ecosystem for the rest of their lives, specific habitats and ontogenetic changes remain largely unknown for most abalone species. In a review, Takami and Kawamura (2018) discuss the process and mechanism of ontogenetic habitat shifts in Haliotis discus hannai. After metamorphosing on CCA, juveniles utilize several microhabitats in the rocky reef ecosystem, with ontogenetic shifts from CCA habitats to deeper kelp beds via algal turf habitats. These habitat shifts are related closely to ontogenetic changes in diet. The authors discuss the effects of changes in rocky reef algal community structures triggered by the 2011 Great East Japan Earthquake and subsequent tsunami on abalone recruitment processes.
The larvae of the Japanese spiny turban snail Turbo cornutus settle into benthic life stages after the larval swimming period of 1–2 weeks, similar to abalone species. Larval turban snails, however, settle preferentially on algal turf of articulated coralline algae. Hayakawa et al. (2018) describe the ontogenetic habitat and diet shifts of the Japanese spiny turban snail and discuss their mechanisms, based on a field survey in Sagami Bay and feeding experiments in tanks. Although abalone species and the Japanese spiny turban snail co-inhabit rocky reefs in the same area, they utilize largely different microhabitats and diets in the rocky reef ecosystem.
Natural stocks of Manila clam Ruditapes philippinarum in Japanese coastal waters have been largely decreasing since the mid-1980s. The causes of the declines, however, are poorly understood, as fundamental ecological information (e.g., suitable food environments and habitats) remains limited. Hasegawa et al. (2018) estimated the larval dispersal to offshore waters and return to estuaries using field observations from the Akkeshi-ko Estuary and Akkeshi Bay. Ichimi et al. (unpublished data) found that the density of Manila clam is much greater on shingle beaches than in estuarine tidal flats in the Bisan Strait in the eastern Seto Inland Sea. The combined use of tidal flats and shingle beaches may be an important survival strategy for the Manila clam, preventing a catastrophic decline of its standing stock in the study area.
The kelp crab Pugettia quadridens is one of the major crustacean components of the coastal rocky reef ecosystem. Recent taxonomic studies have revealed that Pugettia quadridens had long been confused with seven or more congeners, including Pugettia quadridens quadridens and two or more undescribed species. With monthly quantitative sampling over 2 years in Sagami Bay, Ohtsuchi et al. (2018) revealed that P. quadridens ontogenetically changes its habitat from the nursery habitat of small red algal turfs in shallow (depth 1–4 m) lower subtidal areas to the adult habitat of Sargassum fusiforme beds in shallow (depth 0.1–1 m) upper subtidal areas. The kelp crab has been known to prey on juvenile abalone. However, the results suggest that their impact on abalone stocks is limited in Sagami Bay, primarily because the kelp crab’s habitat is separate from that of juvenile abalone.
The ecosystem services of seagrass beds include habitat provision for coastal organisms and carbon capture and storage functions. Hori et al. (2018) describe the nutrient support provided by seagrass vegetation to cultured oysters. They also suggest that oyster–seagrass interactions help to maintain a high level of water transparency and better sanitary conditions, resulting in the improvement of ecosystem services. Tanaka et al. (2018) describe day-night change in fish community structure in seagrass bed. Yusa et al. (2018) report variations of two seagrass pecies and their implications for grass shrimp management.
Coastal ecosystem complex
We would be more than happy if readers of this special volume thought about differences in the structure and function of CECs in warm- and cold-water areas, as well as in open-water and inland sea areas; about factors important for the conservation of CEC structure and function in these different marine areas; and about the importance and prospects of CEC ecosystem services for humans. Global warming and ocean acidification will progress more aggressively in coastal seas under the influence of intense anthropogenic activities. This special volume is expected to provide insight into the problems that we will face in the near future.
- Fuji T, Kasai A, Yamashita Y (2018) Upstream migration mechanisms of juvenile temperate sea bass Lateolabrax japonicus in the stratified Yura River estuary. Fish Sci 84. https://doi.org/10.1007/s12562-017-1167-0
- Hasegawa N, Abe H, Onitsuka T, Ito S (2018) Association between the planktonic larval and benthic stages of Manila clam Ruditapes philippinarum in eastern Hokkaido, Japan. Fish Sci 84. https://doi.org/10.1007/s12562-017-1168-z
- Hayakawa J, Ohtsuchi N, Kawamura T, Kurogi H (2018) Ontogenetic habitat and dietary shifts in Japanese turban snail Turbo cornutus at Nagai, Sagami Bay, Japan. Fish Sci 84. https://doi.org/10.1007/s12562-017-1170-5
- Hori M, Hamaoka H, Hirota M, Lagarde F, Vaz S, Hamaguchi M, Hori J, Makino M (2018) Application of the coastal ecosystem complex concept toward integrated management for sustainable coastal fisheries under oligotrophication. Fish Sci 84. https://doi.org/10.1007/s12562-017-1173-2
- Itoh S, Takeshige A, Kasai A, Kimura S, Kaplan I (2018) Modeling the coastal ecosystem complex: present situation and challenges. Fish Sci 84. https://doi.org/10.1007/s12562-018-1181-x
- Kasai A, Fuji T, Suzuki KW, Yamashita Y (2018) Partial migration of juvenile temperate seabass Lateolabrax japonicus: a versatile survival strategy. Fish Sci 84. https://doi.org/10.1007/s12562-017-1166-1
- Kurita Y, Okazaki Y, Yamashita Y (2018) Ontogenetic habitat shift of age-0 Japanese flounder Paralichthys olivaceus on the Pacific coast of northeastern Japan: differences in timing of the shift among areas and potential effect on recruitment success. Fish Sci 84. https://doi.org/10.1007/s12562-018-1180-y
- Minami K, Sawada H, Masuda R, Takahashi K, Shirakawa H, Yamashita Y (2018) Stage-specific distribution of Japanese sea cucumber Apostichopus japonicus in Maizuru Bay, Sea of Japan, in relation to environmental factors. Fish Sci 84. https://doi.org/10.1007/s12562-017-1174-1
- Ministry of Agriculture, Forestry and Fisheries (2017) Statistics of capture and aquaculture production in 2016. http://www.maff.go.jp/j/tokei/kouhyou/kaimen_gyosei/attach/pdf/index-7.pdf. Accessed 17 Nov 2017
- Ohtsuchi N, Kawamura T, Hayakawa J, Kurogi H, Watanabe Y (2018) Ontogenetic habitat shift in Pugettia quadridens on the coast of Sagami Bay, Japan. Fish Sci 84. https://doi.org/10.1007/s12562-017-1171-4
- Shirafuji N, Nakagawa T, Murakami N, Ito S, Onitsuka T, Morioka T, Watanabe Y (2018) Successive use of different habitats during the early life stages of Pacific herring Clupea pallasii in Akkeshi waters on the east coast of Hokkaido. Fish Sci 84. https://doi.org/10.1007/s12562-018-1175-8
- Takami H, Kawamura T (2018) Ontogenetic habitat shift in abalone Haliotis discus hannai: a review. Fish Sci 84. https://doi.org/10.1007/s12562-017-1169-y
- Tanaka H, Chiba S, Yusa T, Shoji J (2018) Day–night change in fish community structure in a seagrass bed in subarctic waters. Fish Sci 84. https://doi.org/10.1007/s12562-017-1172-3
- Yusa T, Shoji J, Chiba S (2018) Spatial–temporal variations in the composition of two Zostera species in a seagrass bed: implications for population management of a commercially exploited grass shrimp. Fish Sci 84. https://doi.org/10.1007/s12562-018-1182-9