Marine coastal ecosystem

, illustrating the shelf, slope and rise The Earth has approximately of coastline. Coastal habitats extend to the margins of the continental shelves, occupying about 7 percent by area of the Earth's oceans. These coastal seas are highly productive systems, providing an array of ecosystem services to humankind, such as processing of nutrient effluents from land and climate regulation. In the coastal zone, the fluxes and transformations of nutrients and carbon sustaining coastal ecosystem functions and services are strongly regulated by benthic (that is, occurring at the seafloor) biological and chemical processes. The high value of these ecosystem services is obvious considering that a large proportion of the world population lives close to the coast. In many cases, the changes alter underlying ecological functions to such an extent that new states are achieved and baselines are shifted. The United Nations has also declared 2021–2030 the UN Decade on Ecosystem Restoration, but restoration of coastal ecosystems is not receiving appropriate attention. ==Coastal habitats==

Intertidal zone Intertidal zones are the areas that are visible and exposed to air during low tide and covered up by saltwater during high tide. There are four physical divisions of the intertidal zone with each one having its distinct characteristics and wildlife. These divisions are the Spray zone, High intertidal zone, Middle Intertidal zone, and Low intertidal zone. The Spray zone is a damp area that is usually only reached by the ocean and submerged only under high tides or storms. The high intertidal zone is submerged at high tide but remains dry for long periods between high tides. Other types of estuaries also exist and have similar characteristics as traditional brackish estuaries. The Great Lakes are a prime example. There, river water mixes with lake water and creates freshwater estuaries. This can be seen as, of the 32 largest cities in the world, 22 are located on estuaries as they provide many environmental and economic benefits such as crucial habitat for many species, and being economic hubs for many coastal communities. Lagoons Lagoons are areas that are separated from larger water by natural barriers such as coral reefs or sandbars. There are two types of lagoons, coastal and oceanic/atoll lagoons. A coastal lagoon is, as the definition above, simply a body of water that is separated from the ocean by a barrier. An atoll lagoon is a circular coral reef or several coral islands that surround a lagoon. Atoll lagoons are often much deeper than coastal lagoons. Most lagoons are very shallow meaning that they are greatly affected by changed in precipitation, evaporation and wind. This means that salinity and temperature are widely varied in lagoons and that they can have water that ranges from fresh to hypersaline. Coral reefs are being heavily affected by global warming. They are one of the most vulnerable marine ecosystems. Due to marine heatwaves that have high warming levels coral reefs are at risk of a great decline, loss of its important structures, and exposure to higher frequency of marine heatwaves. File:The Coral Reef at the Andaman Islands.jpg| File:Global distribution of coral, mangrove, and seagrass diversity.png| File:Dugong Marsa Alam.jpg| Bivalve reefs bivalve reefs Bivalve reefs provide coastal protection through erosion control and shoreline stabilization, and modify the physical landscape by ecosystem engineering, thereby providing habitat for species by facilitative interactions with other habitats such as tidal flat benthic communities, seagrasses and marshes. Vegetated Vegetated coastal ecosystems occur throughout the world, as illustrated in the diagram on the right. Seagrass beds are found from cold polar waters to the tropics. Mangrove forests are confined to tropical and sub-tropical areas, while tidal marshes are found in all regions, but most commonly in temperate areas. Combined, these ecosystems cover about 50 million hectares and provide a diverse array of ecosystem services such as fishery production, coastline protection, pollution buffering, as well as high rates of carbon sequestration. These carbon stocks can exceed those of terrestrial ecosystems, including forests, by several times. When coastal habitats are degraded or converted to other land uses, the sediment carbon is destabilised or exposed to oxygen, and subsequent increased microbial activity releases large amounts of greenhouse gasses to the atmosphere or water column. The potential economic impacts that come from releasing stored coastal blue carbon to the atmosphere are felt worldwide. Economic impacts of greenhouse gas emissions in general stem from associated increases in droughts, sea level, and frequency of extreme weather events. Diagram showing connectivity between a vegetated coastal ecosystem for the Penaeid prawn lifecycle indicating that valuations for harvest areas may overlook critical importance within the lifecycle. The presence of frost seems to control the demarcation – mangroves do not like frosts.   Coastal wetlands are among the most productive ecosystems on Earth and generate vital services that benefit human societies around the world. Sediment-stabilization by wetlands such as salt marshes and mangroves serves to protect coastal communities from storm-waves, flooding, and land erosion. Coastal wetlands also reduce pollution from human waste, remove excess nutrients from the water column, trap pollutants, and sequester carbon. Further, near-shore wetlands act as both essential nursery habitats and feeding grounds for game fish, supporting a diverse group of economically important species. Mangrove forests Mangroves are trees or shrubs that grow in low-oxygen soil near coastlines in tropical or subtropical latitudes. They are an extremely productive and complex ecosystem that connects the land and sea. Mangroves consist of species that are not necessarily related to each other and are often grouped for the characteristics they share rather than genetic similarity. Because of their proximity to the coast, they have all developed adaptions such as salt excretion and root aeration to live in salty, oxygen-depleted water. The soil in these marshes is often made up of mud and a layer of organic material called peat. Peat is characterized as waterlogged and root-filled decomposing plant matter that often causes low oxygen levels (hypoxia). These hypoxic conditions causes growth of the bacteria that also gives salt marshes the sulfurous smell they are often known for. Salt marshes exist around the world and are needed for healthy ecosystems and a healthy economy. They are extremely productive ecosystems and they provide essential services for more than 75 percent of fishery species and protect shorelines from erosion and flooding. In 2007, kelp forests were also discovered in tropical waters near Ecuador. Physically formed by brown macroalgae, kelp forests provide a unique habitat for marine organisms and are a source for understanding many ecological processes. Over the last century, they have been the focus of extensive research, particularly in trophic ecology, and continue to provoke important ideas that are relevant beyond this unique ecosystem. For example, kelp forests can influence coastal oceanographic patterns and provide many ecosystem services. However, the influence of humans has often contributed to kelp forest degradation. Of particular concern are the effects of overfishing nearshore ecosystems, which can release herbivores from their normal population regulation and result in the overgrazing of kelp and other algae. This can rapidly result in transitions to barren landscapes where relatively few species persist. Already due to the combined effects of overfishing and climate change, kelp forests have all but disappeared in many especially vulnerable places, such as Tasmania's east coast and the coast of Northern California. The implementation of marine protected areas is one management strategy useful for addressing such issues, since it may limit the impacts of fishing and buffer the ecosystem from additive effects of other environmental stressors. ==Coastal ecology==

Coastal food webs Coastal waters include the waters in estuaries and over continental shelves. They occupy about 8 percent of the total ocean area and account for about half of all the ocean productivity. The key nutrients determining eutrophication are nitrogen in coastal waters and phosphorus in lakes. Both are found in high concentrations in guano (seabird feces), which acts as a fertilizer for the surrounding ocean or an adjacent lake. Uric acid is the dominant nitrogen compound, and during its mineralization different nitrogen forms are produced. Ecologists are increasingly recognizing the important effects that cross-ecosystem transport of energy and nutrients have on plant and animal populations and communities. A well known example of this is how seabirds concentrate marine-derived nutrients on breeding islands in the form of feces (guano) which contains ~15–20% nitrogen (N), as well as 10% phosphorus. These nutrients dramatically alter terrestrial ecosystem functioning and dynamics and can support increased primary and secondary productivity. However, although many studies have demonstrated nitrogen enrichment of terrestrial components due to guano deposition across various taxonomic groups, only a few have studied its retroaction on marine ecosystems and most of these studies were restricted to temperate regions and high nutrient waters. In the tropics, coral reefs can be found adjacent to islands with large populations of breeding seabirds, and could be potentially affected by local nutrient enrichment due to the transport of seabird-derived nutrients in surrounding waters. Studies on the influence of guano on tropical marine ecosystems suggest nitrogen from guano enriches seawater and reef primary producers. where nitrogen is a major limiting nutrient for primary productivity, they have developed specific adaptations for conserving this element. Their establishment and maintenance are partly due to their symbiosis with unicellular dinoflagellates, Symbiodinium spp. (zooxanthellae), that can take up and retain dissolved inorganic nitrogen (ammonium and nitrate) from the surrounding waters. These zooxanthellae can also recycle the animal wastes and subsequently transfer them back to the coral host as amino acids, ammonium or urea. Corals are also able to ingest nitrogen-rich sediment particles and plankton. Coastal eutrophication and excess nutrient supply can have strong impacts on corals, leading to a decrease in skeletal growth, File:Seabird ornitheutrophication coupling.png | Seabird colonies are nutrient hot spots, especially, for nitrogen and phosphorus Coastal predators s, support increased carbon uptake in plants and soils, protect coasts from storm surges and flooding, and support stability and resistance. Food web theory predicts that current global declines in marine predators could generate unwanted consequences for many marine ecosystems. In coastal plant communities, such as kelp, seagrass meadows, mangrove forests and salt marshes, several studies have documented the far-reaching effects of changing predator populations. Across coastal ecosystems, the loss of marine predators appears to negatively affect coastal plant communities and the ecosystem services they provide. Since the inception of the green world hypothesis, ecologists have tried to understand the prevalence of indirect and alternating effects of predators on lower trophic levels (trophic cascades), and their overall impact on ecosystems. Multiple lines of evidence now suggest that top predators are key to the persistence of some ecosystems. As bleak as this number is, the predators that patrol coastal systems have fared far worse. Several predatory taxa including species of marine mammals, elasmobranchs, and seabirds have declined by 90 to 100 percent compared to historical populations. Predator declines pre-date habitat declines, However, for most large bodied, marine predators (toothed whales, large pelagic fish, sea birds, pinnipeds, and otters) the beginning of their sharp global declines occurred over the last century, coinciding with the expansion of coastal human populations and advances in industrial fishing. Following global declines in marine predators, evidence of trophic cascades in coastal ecosystems started to emerge, with the disturbing realisation that they affected more than just populations of lower trophic levels. carbon sequestration, and ecosystem resilience. The idea that the extirpation of predators can have far-reaching effects on the persistence of coastal plants and their ecosystem services has become a major motivation for their conservation in coastal systems. Seascape ecology is the marine and coastal version of landscape ecology. It is currently emerging as an interdisciplinary and spatially explicit ecological science with relevance to marine management, biodiversity conservation, and restoration. Rapid advances in geospatial technologies and the proliferation of sensors, both above and below the ocean surface, have revealed intricate and scientifically intriguing ecological patterns and processes, some of which are the result of human activities. Despite progress in the collecting, mapping, and sharing of ocean data, the gap between technological advances and the ability to generate ecological insights for marine management and conservation practice remains substantial. For instance, fundamental gaps exist in the understanding of multidimensional spatial structure in the sea, and the implications for planetary health and human wellbeing. (a) diel and tidal foraging migrations, (b) ontogenetic migration of juvenile coral reef fish. File:Ecosystem cascade for structuring marine spatial planning.jpg | The ecosystem cascade to structure the stock-taking and scenario analysis steps in the marine spatial planning process. Interactions between ecosystems The diagram on the right shows the principal interactions between mangroves, seagrass, and coral reefs. Coral reefs, seagrasses, and mangroves buffer habitats further inland from storms and wave damage as well as participate in a tri-system exchange of mobile fish and invertebrates. Mangroves and seagrasses are critical in regulating sediment, freshwater, and nutrient flows to coral reefs. To compound things, removal of biomass from the ocean occurs simultaneously with multiple other stressors associated to climate change that compromise the capacity of these socio-ecological systems to respond to perturbations. Besides sea surface temperature, climate change also affects many other physical–chemical characteristics of marine coastal waters (stratification, acidification, ventilation) as well as the wind regimes that control surface water productivity along the productive coastal upwelling ecosystems. Changes in the productivity of the oceans are reflected in changes of plankton biomass. Plankton contributes approximately half of the global primary production, supports marine food webs, influences the biogeochemical process in the ocean, and strongly affects commercial fisheries. Indeed, an overall decrease in marine plankton productivity is expected over global scales. Long-term increases and decreases in plankton productivity have already occurred over the past two decades along extensive regions of the Humboldt upwelling ecosystem off Chile, and are expected to propagate up the pelagic and benthic food webs. Previous studies used this framework to assess food web robustness against species extinctions, defined as the fraction of initial species that remain present in the ecosystem after a primary extinction. These studies showed the importance for food web persistence of highly connected species (independent of trophic position), basal species, ==Coastal biogeochemistry==

at the seafloor with a consequent loss of benthic fauna. Hypoxic systems tend to lose many long-lived higher organisms and biogeochemical cycles typically become dominated by benthic bacterial processes and rapid pelagic turnover. However, if hypoxia does not occur, benthic fauna tends to increase in biomass with eutrophication. In addition, nutrients and carbon are retained in biomass and transformed from organic to inorganic forms through metabolic processes. Bioturbation, including sediment reworking and burrow ventilation activities (bioirrigation), redistributes particles and solutes within the sediment and enhances sediment-water fluxes of solutes. Bioturbation can also enhance resuspension of particles, a phenomenon termed "bioresuspension". Together, all these processes affect physical and chemical conditions at the sediment-water interface, and strongly influence organic matter degradation. When up-scaled to the ecosystem level, such modified conditions can significantly alter the functioning of coastal ecosystems and ultimately, the role of the coastal zone in filtering and transforming nutrients and carbon. ==Artisan fisheries==

Artisanal fisheries use simple fishing gears and small vessels. The world fisheries crisis is among those consequences, which cuts across fishing strategies, oceanic regions, species, and includes countries that have little regulation and those that have implemented rights-based co-management strategies to reduce overharvesting. over an unprecedented geographic scale to manage the diverse coastal benthic resources using a co-management strategy. These TURFS are used for artisanal fisheries. Over 60 coastal benthic species are actively harvested by these artisanal fisheries, with species that are extracted from intertidal and shallow subtidal habitats. The Chilean TURFs system brought significant improvements in sustainability of this complex socio-ecological system, helping to rebuild benthic fish stocks, However, the situation of most artisanal fisheries is still far from sustainable, and many fish stocks and coastal ecosystems show signs of overexploitation and ecosystem degradation, a consequence of the low levels of cooperation and low enforcement of TURF regulations, which leads to high levels of free-riding and illegal fishing. It is imperative to improve understanding of the effects of these multi-species artisanal fisheries which simultaneously harvest species at all trophic levels from kelp primary producers to top carnivores. ==Remote sensing==

Coastal zones are among the most populated areas on the planet. As the population continues to increase, economic development must expand to support human welfare. However, this development may damage the ability of the coastal environment to continue supporting human welfare for current and future generations. The management of complex coastal and marine social-ecological systems requires tools that provide frameworks with the capability of responding to current and emergent issues. and even thwarted. but they are expensive and often do not provide adequate spatial and temporal coverage, while remote sensing can provide cost-effective solutions, as well as data for locations where there is no or only limited information. Coastal areas are the most dynamic and productive parts of the oceans, which makes them a significant source of human resources and services. Coastal waters are located immediately in contact with human populations and exposed to anthropogenic disturbances, placing these resources and services under threat. These concerns explain why, in several coastal regions, a rapidly increasing number of observing systems have been implemented in the last decade. Expansion of coherent and sustained coastal observations has been fragmented and driven by national and regional policies and is often undertaken through short-term research projects. This results in significant differences between countries both in terms of sustainability and observing technologies, methods and research priorities. ==Regime shifts==

Marine ecosystems are affected by diverse pressures and consequently may undergo significant changes that can be interpreted as regime shifts. However, under some circumstances, the ecosystem may undergo changes that modify the system's structure and function and this process can be described as a shift to a new regime. intense fishing exploitation or both. Criteria used to define regime shifts vary and the changes that have to occur in order to consider that a system has undergone a regime shift are not well-defined. Normally, regime shifts are defined as high amplitude, low-frequency and often abrupt changes in species abundance and community composition that are observed at multiple trophic levels (TLs). These changes are expected to occur on a large spatial scale and take place concurrently with physical changes in the climate system. the North Sea, and the Baltic Sea. In large upwelling ecosystems, it is common to observe decadal fluctuations in species abundance and their replacements. These fluctuations might be irreversible and might be an indicator of the new regime, as was the case in the Northern Benguela ecosystem. It has recently changed in the abundance of coastal pelagic species such as sardine, chub mackerel, horse mackerel, blue jack mackerel and anchovy. Moreover, in the last decades, an increase in higher trophic level species has been documented. The causes underlying changes in the pelagic community are not clear but it has been suggested that they result from a complex interplay between environmental variability, species interactions and fishing pressure. while others documented their weakening. A 2019 review of upwelling rate and intensity along the Portuguese coast documented a successive weakening of the upwelling since 1950 that lasted till mid/late 1970s in the north-west and south-west and till 1994 in the south coast. An increase in upwelling index over the period 1985–2009 was documented in all studied regions while additionally upwelling intensification were observed in the south. ==Threats and decline==

Many marine fauna utilise coastal habitats as critical nursery areas, for shelter and feeding, yet these habitats are increasingly at risk from agriculture, aquaculture, industry and urban expansion. Indeed, these systems are subject to what may be called "a triple whammy" of increasing industrialisation and urbanisation, an increased loss of biological and physical resources (fish, water, energy, space), and a decreased resilience to the consequences of a warming climate and sea level rise. This has given rise to the complete loss, modification or disconnection of natural coastal ecosystems globally. For example, almost 10% of the entire Great Barrier Reef coastline in Australia (2,300 km) has been replaced with urban infrastructure (e.g., rock seawalls, jetties, marinas), causing massive loss and fragmentation of sensitive coastal ecosystems. Global loss of seagrass reached around 7% of seagrasses area per year by the end of the twentieth century. A global analysis of tidal wetlands (mangroves, tidal flats, and tidal marshes) published in 2022 estimated global losses of from 1999-2019, however, this study also estimated that these losses were largely offset by the establishment of of new tidal wetlands that were not present in 1999. Approximately three-quarters of the net decrease between 1999 and 2019 occurred in Asia (74.1%), with 68.6% concentrated in three countries: Indonesia (36%), China (20.6%), and Myanmar (12%). with more than 9,736 km2 of the world's mangroves continuing to be degraded in the 20 years period between 1996 and 2016. Saltmarshes are drained when coastal land is claimed for agriculture, and deforestation is an increasing threat to shoreline vegetation (such as mangroves) when coastal land is appropriated for urban and industrial development, These accumulating pressures and impacts on coastal ecosystems are neither isolated nor independent, rather they are synergistic, with feedbacks and interactions that cause individual effects to be greater than their sums. In the year before the ecosystem restoration Decade commences, there is a critical knowledge deficit inhibiting an appreciation of the complexity of coastal ecosystems that hampers the development of responses to mitigate continuing impacts—not to mention uncertainty on projected losses of coastal systems for some of the worst-case future climate change scenarios. ==Restoration==

: 2021 to 2030 coastal ecosystems comes close to the Bonn Challenge, with the aim of increasing the global area of mangroves by 20% by 2030. ==Conservation and connectivity==

There has recently been a perceptual shift away from habitat representation as the sole or primary focus of conservation prioritisation, towards consideration of ecological processes that shape the distribution and abundance of biodiversity features. In marine ecosystems, connectivity processes are paramount, and designing systems of marine protected areas that maintain connectivity between habitat patches has long been considered an objective of conservation planning. Two forms of connectivity are critical to structuring coral reef fish populations: dispersal of larvae in the pelagic environment, and post-settlement migration by individuals across the seascape. Whilst a growing literature has described approaches for considering larval connectivity in conservation prioritisation, relatively less attention has been directed towards developing and applying methods for considering post-settlement connectivity Seascape connectivity (connectedness among different habitats in a seascape, cf. among patches of the same habitat type) is essential for species that utilise more than one habitat, either during diurnal movements or at different stages in their life history. Mangroves, seagrass beds, and lagoon reefs provide nursery areas for many commercially and ecologically important fish species that subsequently make ontogenetic shifts to adult populations on coral reefs. These back-reef habitats are often overlooked for conservation or management in favour of coral reefs that support greater adult biomass, yet they can be equally if not more at risk from habitat degradation and loss. Even where juveniles are not targeted by fishers, they can be vulnerable to habitat degradation, for example from sedimentation caused by poor land-use practices. For example, at study sites across the western Pacific, the abundance of harvested fish species was significantly greater on protected reefs close to mangroves, but not on protected reefs isolated from mangroves. Despite this evidence, and widespread calls to account for connectivity among habitats in the design of spatial management, there remain few examples where seascape connectivity is explicitly considered in spatial conservation prioritisation (the analytical process of identifying priority areas for conservation or management actions). ==See also==