Many natural and human-induced events create disturbances in seagrasses throughout the world, but quantifying losses of habitat is only beginning. Over the last decade, 90000 ha of seagrass loss have been documented although the actual area lost is certainly greater. Seagrasses, an assemblage of marine flowering plant species, are valuable structural and functional components of coastal ecosystems and are currently experiencing worldwide decline. This group of plants is known to support a complex trophic food web and a detritus-based food chain, as well as to provide sediment and nutrient filtration, sediment stabilization, and breeding and nursery areas for finfish and shellfish.We define disturbance, natural or human-induced, as any event that measurably alters resources available to seagrasses so that a plant response is induced that results in degradation or loss. Applying this definition, we find a common thread in many seemingly unrelated seagrass investigations. We review reports of seagrass loss from both published and ‘grey’ literature and evaluate the types of disturbances that have caused seagrass decline and disappearance. Almost certainly more seagrass has been lost globally than has been documented or even observed, but the lack of comprehensive monitoring and seagrass. mapping makes an assessment of true loss of this resource impossible to determine.Natural disturbances that are most commonly responsible for seagrass loss include hurricanes, earthquakes, disease, and grazing by herbivores. Human activities most affecting seagrasses are those which alter water quality or clarity: nutrient and sediment loading from runoff and sewage disposal, dredging and filling, pollution, upland development, and certain fishing practices. Seagrasses depend on an adequate degree of water clarity to sustain productivity in their submerged environment. Although natural events have been responsible for both large-scale and local losses of seagrass habitat, our evaluation suggests that human population expansion is now the most serious cause of seagrass habitat loss, and specifically that increasing anthropogenic inputs to the coastal oceans are primarily responsible for the world-wide decline in seagrasses.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/7D2BFEDB7A3C5206CAD61A94BAD32853/S0376892900038212a.pdf/div-class-title-natural-and-human-induced-disturbance-of-seagrasses-div.pdf

Natural and human-induced disturbance of seagrasses

Submerged vegetation respond to increased nutrient loading through a shift from slow-growing seagrasses and large macroalgae to fast-growing macroalgae, and the ultimate dominance of phytoplankton at high nutrient loadings. This shift reflects a change from nutrient to light limitation along the eutrophication gradient. Slow-growing seagrasses and large macroalgae are good competitors when nutrients are limiting because they have relatively low nutrient requirements, are able of efficient internal nutrient recycling, and can access the elevated nutrient pools in the sediment. Fast-growing macroalgae and phytoplankton are superior competitors when light is limiting because they are positioned closer to the water surface, and capture and use light more efficiently. The important ecosystem consequences of altered nutrient regimes derive from the shift in dominant vegetation types. Slow-growing seagrasses and large macroalgae are longevous, decompose slowly, and experience only moderate grazing losse...

Submerged aquatic vegetation in relation to different nutrient regimes

http://faculty.umb.edu/anamarija.frankic/eeos476/Class%20Materials/Habitats/burkholder2007.pdf

Seagrasses and eutrophication

An understanding of risks to biodiversity is needed for planning action to slow current rates of decline and secure ecosystem services for future human use. Although the IUCN Red List criteria provide an effective assessment protocol for species, a standard global assessment of risks to higher levels of biodiversity is currently limited. In 2008, IUCN initiated development of risk assessment criteria to support a global Red List of ecosystems. We present a new conceptual model for ecosystem risk assessment founded on a synthesis of relevant ecological theories. To support the model, we review key elements of ecosystem definition and introduce the concept of ecosystem collapse, an analogue of species extinction. The model identifies four distributional and functional symptoms of ecosystem risk as a basis for assessment criteria: A) rates of decline in ecosystem distribution; B) restricted distributions with continuing declines or threats; C) rates of environmental (abiotic) degradation; and D) rates of disruption to biotic processes. A fifth criterion, E) quantitative estimates of the risk of ecosystem collapse, enables integrated assessment of multiple processes and provides a conceptual anchor for the other criteria. We present the theoretical rationale for the construction and interpretation of each criterion. The assessment protocol and threat categories mirror those of the IUCN Red List of species. A trial of the protocol on terrestrial, subterranean, freshwater and marine ecosystems from around the world shows that its concepts are workable and its outcomes are robust, that required data are available, and that results are consistent with assessments carried out by local experts and authorities. The new protocol provides a consistent, practical and theoretically grounded framework for establishing a systematic Red List of the world’s ecosystems. This will complement the Red List of species and strengthen global capacity to report on and monitor the status of biodiversity

/pdf/scientific-foundations-for-an-iucn-red-list-of-ecosystems-13bcgazomi.pdf

Scientific foundations for an IUCN Red List of ecosystems.

Environmental impacts of dredging on seagrasses: a review.

Other chapters underline the importance of seagrasses to our nearshore ecosystems, for example in supporting fish populations and modifying sediment movement wave energy, and it is clear that seagrasses are important components of major Australian ecosystems. While in the natural environment a variety of factors affect the dynamic processed involved in the establishment, maintenance and erosion of seagrass meadows in an unstable, often high-energy environment, one can only view with concern the more or less irreversible loss of meadows, often over extensive areas, which have taken place as a consequence of man's activities. It is this 'cultural' decline in seagrass meadows which is the subject of this chapter, which addresses case studies drawn from several States, reviews mechanisms which may be responsible for seagrass decline, and addresses management considerations.

The most extensively documented example of seagrass decline on the western coast has occurred in Cockburn Sound, a marine embayment 30kmsouth of the capital city, Perth (Figure 12.1). A decision was taken to locate a major industrial development around the Sound, which is the only extensive, sheltered, deep-water area close to Perth on an otherwise inhospitable and high-energy coastline. A basin almost 20m deep, the Sound is protected on the east by Garden Island, and on the north and south by shallow banks covered by 3-5 m of water; these features restrict the exchange of water between the basin and the open ocean. A dredged channel through the northern bank allows access by deep-draught vessels.

Development began in 1954. On the mainland several major industries were established, including an oil refinery, blast furnace and steel-rolling mill, superphosphate factory, and processing plants for alumina and nickel. Garden Island became the site for a major naval facility. Industrial effluents began to enter the Sound, and to the north a sewage treatment plant began discharging in the mid 1960's.

Ten species of seagrasses were associated with the Sound. Posidonia sinuosa (formerly included in Posidonia australis) formed extensive meadows, with Posidonia australis, Amphibolis antarctica and Amphibolis griffithii at the edges of P. sinuosa meadows or in more turbulent, disturbed areas. Posidonia angustifolia is included tentatively; though no longer found there it was probably present around limestone rocks as it is in adjoining waters.

D.A. Steffensen

Papers

Decline of seagrasses