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Seagrass decline is a worldwide phenomenon requiring managerial interventions to be reverted. However, any actions to stop or reverse decline require a proper understanding of the causes behind the decline. This chapter described the multiple man made and natural causes for seagrass decline.

By Carlos M. Duarte, Nuria Marbà (IMEDEA), Rui Santos (CCMAR)

While seagrasses are recognized as priority will be reduced or, at places, lost subjects for conservation efforts in international altogether. Seagrass loss leads to a loss of the (e.g. Rio Convention, EU’s Habitats Directive) and associated functions and services in the coastal national frameworks, there is evidence that they zone. The consequences of seagrass loss are are experiencing significant widespread decline. well documented, through observations of the Seagrasses exist at the land- margin and are changes in the ecosystem upon large-scale highly vulnerable to pressures from the world's seagrass losses. Seagrass loss involves a shift in human populations, which live disproportionately the dominance of different primary producers in along the . Human population growth, with the coastal ecosystem, which can only partially concomitant increased , hardening and compensate for the loss of . alteration of coastlines, and watershed clearing, For instance, the increased planktonic primary threatens seagrass and has resulted production with increasing inputs does not in substantial and accelerating seagrass loss over compensate for the lost seagrass production, so the last 20 years. Globally, the estimated loss of that there is no clear relationship between seagrass from direct and indirect human impacts increased nutrient loading and ecosystem primary amounts to 33,000 km2, or 18 % of the production. The loss of the sediment protection documented seagrass area, over the last two offered by the seagrass canopy enhances decades, based on an extrapolation of known sediment resuspension, leading to a further losses. Reported losses probably represent a deterioration of light conditions for the remaining small fraction of those that have occurred and seagrass plants. The extent of resuspension can many losses may remain unreported, and indeed be so severe following large-scale losses, such as may never be known because most seagrasses that experienced during the marina leave no long-term record of their existence. wasting disease, that the shoreline may be Causes range from changes in light attenuation altered. The loss of seagrasses will also involve due to sedimentation and/or nutrient pollution, to the loss of the oxygenation of sediment by direct damage and climate change. Losses are seagrass , promoting anoxic conditions in the often experienced first at either the lower or upper sediments. Seagrass loss has been shown to depth limit of the seagrass . Seagrasses result in significant loss of coastal , require an underwater irradiance generally in leading to a modification of food webs and loss of excess of 11 % of that incident in the water harvestable resources. In summary, seagrass loss surface for growth, a requirement that typically represents a major loss of ecological as well as sets their depth limit. The upslope limit of economic value to the coastal ecosystems, and is seagrasses is imposed by their requirement for therefore, a major source of concern for coastal sufficient immersion in seawater or tolerable managers. disturbance by waves and, in northern latitudes, ice scour. There is, therefore, concern that the functions seagrasses have performed in the marine

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Natural and anthropogenic causes reduces shoot density and plant biomass, and digging for clams can also exert extensive The likely primary cause of seagrass loss is damage. Many of these impacts remain reduction in water clarity, both from increased unquantified as yet, and their long term effects are nutrient loading and increased turbidity. Run-off of poorly known. In the Mediterranean, the and sediments from human activities on exploitation of marine resources, and the use of land has major impacts in the coastal regions certain types of fishing gear like bottom trawls, where seagrasses thrive. The relatively high light has detrimental effects on seagrass beds. Mussel requirements of seagrasses make them harvest in the Dutch Wadden Sea is believed to vulnerable to decreases in light penetration of be a major factor in the loss of Z. marina and Z. coastal waters. Along temperate, more noltii there. industrialized coasts, the losses of water clarity Aquaculture is growing fast in both the come from rapidly increasing inputs of Mediterranean and Atlantic European . and from waste discharge, Aquaculture has been shown to produce major atmospheric deposition, and watershed run-off. In environmental impacts, particularly due to contrast, in tropical areas, the major impacts on shading, and sediment water clarity are extensive sediment discharge deterioration through excess organic inputs. The into coastal waters caused by watershed effects of farms and other aquaculture deforestation and clearing of the fringe. developments are of concern as areas of Worldwide, anthropogenic nutrient over- productive seagrass habitats are often targeted enrichment of coastal waters is the factor for such developments, such as in the responsible for much of the reported seagrass Mediterranean coast. Mussel culture adversely decline. The primary cause of nutrient enrichment affects Z. marina and Z. noltii beds in France. in estuarine and coastal waters is anthropogenic Seagrass beds as far as 100 m from fish cages loading from coastal watersheds. In general, can be impacted by the delivery of feed to the fish. pristine and coastal are nitrogen- The introduction of exotic marine organisms, from limited and nitrogen inputs from point and non- accidental release, vessel ballast water, hull point sources causes eutrophication. Increased fouling and aquaculture, remains an area of nutrient loading is widely acknowledged to alter concern, particularly where the introduced the structure and function of coastal ecosystems. are competitors for soft bottom substratum such In addition to nutrient inputs from land, increased as the alga Caulerpa and the fan worm Sabella nutrient inputs are also occurring in coastal areas spallanzanii. Large scale engineering projects adjacent to industrialized regions of the world have also resulted in species invasion, such as through direct atmospheric deposition of nitrogen, that by the Lesepian migrant Caulerpa racemosa, providing additional nutrients that can only be introduced through the Suez Canal. The Suez reduced at the source. Canal also allowed the introduction of the Poor land use practices also result in increased seagrass stipulacea into the soil and the delivery of vast quantities of Mediterranean. sediment into coastal waters. Removal of Direct boat propeller damage to seagrass terrestrial vegetation leads to erosion and communities has been recorded, particularly in transport of sediments through rivers and streams the Florida Keys, and is prevalent in shallow areas to estuaries and coastal waters where the with heavy boat traffic. Boat anchoring suspended particles create turbidity that reduces scars in oceanica landscapes, as do water clarity and and increase sedimentation boat moorings. Return of large temperate above levels tolerable to seagrass. -forming seagrasses to mooring scars Direct human impacts to seagrasses, in addition may take decades. Docks and piers shade to the major indirect impacts discussed above, shoreline seagrass, an effect that may fragment threaten the habitat particularly in densely the habitat. Boating may also be associated with populated areas. Direct impacts from human organic inputs in areas where boats do not have activity include: i) fishing and aquaculture, ii) holding tanks. introduced exotic species, iii) boating and The development of the coastline, particularly anchoring, and iv) habitat alteration (dredging, related to increased population pressure, leads to reclamation and coastal construction). Fishing alteration and fragmentation of habitats available methods such as dredging and trawling may for seagrasses in coastal waters. Coastal significantly affect seagrasses by direct removal. development, construction of ports, marinas and Damage to by scallop dredging groynes, is usually localized to centers of

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population. Housing developments impact coastal or geese, represent the main factor structuring , and the number of houses in a some seagrass landscapes, which are watershed has been directly correlated to rate of characteristically patchy. In contrast, some seagrass loss. Construction of the causeway at seagrass species have been able to form long- the southern end of Cockburn Sound, Western lasting meadows, with meadows of the long-lived Australia, in combination with industrial pollution, dated to > 4000 years, and destroyed existing seagrass. Construction of single clones of Zostera marina dated, using roads through shallow waters, without proper molecular techniques at 3000 years. consideration to maintaining water flow, may also In addition to natural variability of the seagrass affect circulation and lead to seagrass loss, such habitat, human intervention is becoming a major as observed in Cuban coastal waters rendered source of change to seagrass ecosystems, hypersaline by the effects of road construction whether by direct physical modification of the over shallow areas on water exchange. habitat by the growing human activity in the Dredging and reclamation of marine coastal zone (e.g. boating, fishing, construction), environments, either for extraction of sediments or or through their impact on the quality of waters as part of coastal engineering or construction, can and sediments to support seagrass growth, as remove seagrasses. Filling of shallow coastal well as changes in the marine food webs linked to areas, known as reclamation, can directly the seagrasses. eliminate seagrass habitat and results in hardening of the shoreline, which further eliminates productive seagrass habitat, as seen Human impacts throughout Tokyo Bay. Groynes alter sediment Humans impact seagrass ecosystems both transport in the nearshore zone. Dredging through direct proximal impacts, affecting removes seagrass habitat as well as the seagrass meadows locally, and indirect impacts underlying sediment, leaving bare sand at greater that may affect seagrass meadows far away from depth, resulting in changes to the biological, the sources of the disturbance. Proximal impacts chemical and physical habitat values that include mechanical damage and damage created seagrasses support. Beach replenishment may by the construction and maintenance of impact adjacent seagrasses by delivering infrastructures in the coastal zone, as well as sediment that may shade or bury the seagrasses. effects of eutrophication, siltation, coastal Beach nourishment can also impact seagrasses engineering and aquaculture. Indirect impacts growing in areas where sediments are collected, include those from global anthropogenic changes, often at depths < 30m. such as global warming, sea level rise, CO2 and UV increase, and anthropogenic impacts on Because of the requirements of seagrass for marine biodiversity, such as the large scale adequate light and sediment conditions, they are modification of the through particularly affected by disturbances that modify fisheries. Indirect impacts are already becoming the water and sediment quality. The seagrass evident at present. shallow coastal environment is also particularly prone to physical disturbance, whether by waves The most unambigious source of human impact or turbulence associated with strong . to seagrass ecosystems is physical disturbance. Because disturbance is a sporadic phenomenon, This susceptibility derives from multiple causes, seagrass meadows are highly dynamic all linked to increasing human usage of the ecosystems. These dynamics include widespread coastal zone for transportation, recreation and loss, such as the wasting disease that led to food production. The coastal zone is becoming an catastrophic die-back of eelgrass (Zostera marina) important focus for services to society, since meadows on both sides of the Atlantic in the about 40 % of the human population presently 1930s, as well as more recent massive losses inhabit the coastal zone. Direct habitat destruction such as that in Florida Bay (USA), which is one of by land reclamation and port construction is a the largest areas of seagrass ecosystem world- major source of disturbance to seagrass wide. The causes and the possible role of human- meadows, due to dredging and landfilling activities derived impacts in such losses are still uncertain. as well as the reduction in water transparency Strong disturbances, such as damage by associated with both these activities. The hurricanes, can also lead to major seagrass construction of new ports is associated with losses. Smaller-scale, more recurrent changes in sediment transport patterns, involving disturbances, such as that caused by the motion both increased erosion and sediment of sand waves in and out of seagrass patches and accumulation along the adjacent coast. These that caused by large predators, such as changes can exert significant damage on

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seagrass ecosystems kilometres away, which can threats coastal tourism poses to seagrasses are in be impacted by both erosion and burial associated cases direct, as in some cases of purposeful with the changing sedimentary dynamics. The removal of seagrass from beach areas to operation of the ports also entails substantial “improve” beach conditions. Fortunate, there are stress to the neighboring seagrass meadows, due symptoms that coastal tourism is moving, at least to reduced transparency and nutrient and in some areas, to embrace sustainable principles, contaminant inputs associated with ship traffic and including the maintenance of ecosystem services, servicing, as well as dredging activities associated such as those provided by seagrasses, and could with port and navigation-channel maintenance. well play a role as an agent pressing for seagrass Rapid increase in sea-based transport, as well as conservation in the future. recreational boating activities have led to a major Increased nutrient inputs, causing eutrophication increase in the number and size of ports is a major component to seagrass loss (see worldwide, with a parallel increase in the below). Increased siltation of coastal waters is combined disturbance to seagrass meadows. also a major human impact on seagrass Ship activity also causes disturbance to seagrass ecosystems, which derives from changes in land through anchoring damage, which can be rather use leading to increased erosion rates and silt extensive at popular mooring sites, as well as export from watersheds. Siltation is a particularly fisheries operation, particularly shallow trawling acute problem in other regions of the world, such and smaller-scale activities linked to fisheries, as SE Asian coastal waters, which receive the such as clam digging and use of push nets over highest sediment delivery in the world as a result intertidal and shallow areas and, in extreme of high soil erosion rates derived from extensive cases, dynamite fishing. The exponential growth deforestation and other changes in land use, and of aquaculture, the fastest growing food may be important in European waters adjacent to production industry has also led to impacts on deforested watersheds. Siltation severely impacts seagrasses through shading and physical damage seagrass meadows through increased light to the seagrass beds, as well as deterioration of attenuation and burial, leading to seagrass loss water and sediment quality leading to seagrass and, where less intense siltation occurs, a decline loss. in seagrass diversity, biomass and production. The coastal zone also supports increasing Large-scale coastal engineering often alters infrastructure, such as pipes and cables for circulation and distributions, leading to transport of gas, water, energy, and seagrass loss. Hence, seagrass meadows, communications, deployment and maintenance of previously abundant in Dutch coastal areas, are which also entail disturbance to adjacent seagrass now much reduced in surface, partially related to meadows. The development of coastal tourism, shifts of coastal waters from marine to brackish or the fastest growing industry in the world, has also freshwater regimes. led to a major transformation of the coastal zone in areas with pleasant climates. For instance, Pollution, other than that of nutrients and organic about 2/3 of the Mediterranean coastline is inputs, may be an additional source of human presently urbanized, with this fraction exceeding impacts on seagrass ecosystems, although, 75 % in the regions with the most developed seagrass appears to be rather resistant to tourism industry, with harbors and ports occupying pollution by organic and heavy metal 1250 km of the European Mediterranean contaminants. These substances may possibly coastline. Urbanization of the coastline often harm some components of the seagrass involves destruction of dunes and sand deposits, ecosystem, although such responses have not promoting beach erosion, a major problem for been examined to a significant extent. beach tourism. Beach erosion, however, does not only affect the emerged beach, and is usually Global trade has increased the mobility of marine propagated to the submarine sand colonized by species, whether purposely, such as aquarium specimens, or inadvertant, such as organisms seagrass, eventually causing seagrass loss. Wave break constructed to prevent beach erosion carried in ballast waters. The increased human- often create extensive problems, by altering long- mediated transport of species between geographically distant locations has increased the shore sediment transport patterns, further impacting the seagrass ecosystem. Extraction of incidence of invasive species. A case affecting marine sand for beach replenishment is only Mediterranean, and probably soon Eastern Pacific seagrasses, is the invasion of the Mediterranean economically feasible at the shallow depths inhabited by seagrasses, which are often by the tropical algal species Caulerpa taxifolia, impacted by these extraction activities. The which first invaded the French Mediterranean in the early 1980s, apparently released from an

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aquarium, and has been reported to have natural changes, such as unusual seawater expanded since along the French coast to reach warming, as possible triggers for the decline. the Italian and Spanish (Majorca Island) coasts. Whether indirect human impacts on global C. taxifolia grows rapidly and appears largely to processes may have played a role remains colonize areas devoid of seagrasses, but has untested. been reported to compete for space and Hence, despite clear signals of anthropogenic resources with Posidonia oceanica off Monaco, effects on climate components, the responses of being able to damage the Posidonia oceanica the seagrass ecosystem are still unclear, probably meadows. These meadows appear to be due to the still modest size of the changes particularly vulnerable to invasion by exotic experienced but also, and perhaps to a greater species when already under stress. The species extent, to the lack of adequate long-term has recently been reported in the Californian monitoring systems allowing the detection of coast, raising concerns that it can also cause responses in the seagrass ecosystem. problems to the seagrass beds there. The Mediterranean Sea has also been invaded by Halophila stipulacea across the Suez Canal, but Pathogens no damage to the local seagrass meadows has been reported. Very little is known about seagrass pathogens and Most of the impacts discussed above result from related diseases. However, some marine slime direct or indirect human intervention at the local moulds of the genus Labyrinthula have been scale. However, human activity conducive to recognised as seagrass pathogens causing the large-scale changes at the regional or global scale "wasting disease". The symptoms of seagrass also exert an important impact on seagrass infections by these fungi are the presence of small ecosystems. These effects are remarkably difficult dark brown or black lesions on the leaves, to separate from responses to background natural spreading longitudinally covering the totality of the changes of the highly dynamic coastal ecosystem. after few weeks. Usually the infections occur These impacts involve the effects of the realized on mature leaves, but during severe infections and predicted climate change, and result from young leaves may also be affected. In early 1930s changes in sea level, water temperature, UV Labyrinthula zosterae was responsible of dramatic irradiance, and CO2 concentration (see below). declines of Zostera marina meadows at both north Atlantic coasts. Since then, large scale seagrass losses due to Labyrinthula infections have not Natural vs. anthropogenic influences been observed although scattered outbreaks of Some cause-effect relationships between local this disease continue effecting Z. marina seagrass loss and direct human activities, such as meadows at local scale (Hemminga and Duarte increased nutrient and organic loading, 2000). Labyrinthula zosterae can also infect Z. constructions on the coastline or boating activity, noltii producing similar symptoms as in Z. marina. can be readily demonstrated. For instance, the However, infected Z. noltii beds do not exhibit loss of Zostera marina in a bay on the Atlantic major losses. In fact, species of the genus coast of the USA has been shown to be closely Labyrinthula occur often on different seagrass correlated to housing development. However, the beds around the world, without causing seagrass link between seagrass losses and indirect human losses. It has been suggested that the interaction influences is more elusive, since the coastal zone between seagrasses and species of slime moulds is a highly dynamic ecosystem, where many may turn detrimental for seagrasses under conditions vary simultaneously. Disturbances specific environmental conditions that make such as strong storms, hurricanes and typhoons, seagrasses already more vulnerable (Hemminga severely impact seagrass beds, to the point that and Duarte 2000). This could be the case of the they may be essential components of the wasting disease in the 1930s, which occurred dynamics of seagrass meadows. The difficulties of after a long period of high water turbidity. discriminating between sources of seagrass loss The scarce reports of seagrass diseases, are best illustrated by example. The wasting however, may not reflect a high resistance of disease decimated Zostera marina meadows in seagrasses to infections but the difficulty to detect the 1930s on both sides of the Atlantic. The them. Information about seagrass diseases could proximal cause for the loss seems to have been increase in the near future with the development an infection by a fungus, Labyrinthula zosterae, of new techniques able to detect microbial plant although it may have affected Zostera marina infections, as is occurring in the field of vascular meadows that were already stressed. Hypotheses terrestrial plants. to account for this widespread loss also point to

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Climate extremes and climate change physiological limitations setting the latitudinal limits of seagrass distribution, increasing global Global climate changes derive, at least partly, temperature will increase species diversity in from anthropogenic combustion of fossil fuels and subtropical regions and allow cold water species changes in land use with increasing to expand their geographic distribution further concentrations of dioxide and emission of towards higher latitudes, thereby increasing the other greenhouse gasses and will most likely have importance of seagrass ecosystems on the global substantial long term impacts on seagrass scale. ecosystems. Climate changes of potential The rise in temperature in the next 25 years will importance for seagrass growth and distribution result in a projected increase in sea level of include global warming, rising sea level, the between 10 and 15 cm, mostly because of increase of carbon dioxide in the atmosphere and thermal expansion of the and, to a lesser ocean, and the increasing frequency and strength extent, because of melting glaciers and ice of storms. While the increase in carbon dioxide sheets. The rise in sea level may have numerous can be predicted with relatively high precision, implications for circulation, tidal amplitude, current global warming and especially its meteorological and salinity regimes, coastal erosion and water implications are more difficult to foresee. Making turbidity, each of which could have major negative prediction even more complex, climate changes impacts on local seagrass performance. interact with other human-caused changes in the marine environment. The projected increase in atmospheric CO2 is expected to affect and growth of The expected increase in global temperature may seagrasses, particularly increasing seagrass have numerous effects on seagrass performance. photosynthesis. The increasing levels of dissolved Temperature affects almost every aspect of CO2 in seawater may increase the competitive seagrass , growth and reproduction advantage of seagrasses over because and also has important implications for geographic seagrasses are currently more CO2-limited than patterns of seagrass species and algae. The photosynthetic rates of light-saturated distribution. Progressively increasing temperature seagrass leaves are often limited by the may be a major threat to local populations of availability of dissolved inorganic carbon, and, seagrasses, especially if living close to their low since the concentration of CO2 in well-mixed, latitude borders of distribution, such as is the case shallow coastal waters is in equilibrium with the for Posidonia oceanica, Zostera marina and atmosphere, the increase in atmospheric CO2 , which encounter their concentration by 25 %, from 290 ppm to 360 ppm, southern (Posidonia oceanica, Zostera marina) or over the 20th century, may have led to an increase northern (Cymodocea nodosa) distributional limits in light-saturated seagrass photosynthesis by as in European waters. Seagrass distribution shift much as 20 %. There is, however, little evidence could be even greater if oceanic circulation that such physiological responses have led to changes in response to global warming, leading to observable changes in seagrass ecosystems at abrupt changes in water temperature beyond present. those directly resulting from warming, as water masses shift at the edge of present Global warming will produce an increase in biogeographical boundaries between seagrass frequency and strength of events, resulting floras. in increased coastal erosion and sediment resuspension with more turbid waters and poorer While rising temperature may have major negative light conditions for benthic plant communities. effects on local seagrass beds, there seems to be Although many species of seagrasses are less reason for concern for seagrasses on the adapted to, and can survive, periods of low light global scale. When reviewing the literature for and partial burial, storm events often reduce effects of temperature on seagrasses there seems growth and survival and require new colonization to be some bias towards the detrimental effects of by seeds to re-establish seagrass beds. high temperature and less focus on the negative Conversely, physical disturbance represents an impact of low temperatures. Seagrasses probably energetic subsidy and may be of advantage to evolved in warm waters, suggested by the high species diversity and improve growth conditions species diversity of seagrass beds in the Indo- for climax plant species. Overall, the net effect of Pacific tropical region, and although a few genera, increasing frequency and strength of storm events such as Zostera, have had great success on seagrasses is not clear. colonizing cold temperate waters, most species grow in warm waters. It is reasonable to expect The mean global sea level has risen about 10 – th that, although there may be no apparent 25 cm over the 20 century, which should have

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generated an average recession of the global Although seagrass meadows are often nutrient coastline by 10 to 25 m , and, therefore, a large limited, increased nutrient inputs can only be scale erosion of shallow marine sediments. There expected to enhance seagrass primary production is little doubt that such changes must have at very moderate levels at best. Whereas affected seagrasses, which are very sensitive to seagrasses, through their low nutrient sediment erosion, although there is little or no requirements for growth and their high capacity for direct observational evidence for these changes. internal nutrient recycling are well fitted to cope Effects of the seawater warming of 0.3 – 0.6 oC with low nutrient availability, other primary over the 20th Century on seagrasses are generally producers, both micro- and macroalgal, are more less evident than those of sea level. Temperature efficient, because of greater affinity and higher affects many processes that determine seagrass uptake rates, in using excess nutrient inputs. growth and reproduction, including Coastal eutrophication promotes photosynthesis, respiration, nutrient uptake, biomass, which deteriorates the underwater light flowering and seed germination. Although climate, and the stimulation of the growth of observations of increased flowering frequency of epiphytes and opportunistic macroalgae, which Posidonia oceanica in the Mediterranean have further shade and suffocate seagrasses. The been tentatively linked to the seawater alleviation of nutrient limitation, together with the temperature increase, there is little evidence at proliferation of phytoplankton and epiphyte present to suggest any impact of increased biomass as a result of increased nutrient inputs temperature as a result of global warming at imply that coastal eutrophication leads to a shift present. The temperature increase may further from nutrient limitation to light limitation of impact seagrass ecosystems through effects on ecosystem production, enhanced through other components, such as an increased competitive interactions between different types of respiratory rate of the associated microbial primary producers for light. These effects result in communities. Stimulation of microbial respiration seagrass loss, particularly in the deeper portions would further enhance the problems derived from of the meadows. The effects of overgrowth by high organic inputs to seagrass sediments. phytoplankton, epiphytes and macroalgae may be Summer UV irradiance has greatly increased at attenuated by heavy , which can buffer the high latitudes, and an increase in the north- negative effects of eutrophication. Eutrophication temperate zone is also becoming evident. may, furthermore, have negative effects directly Increased UV levels are expected to negatively derived from the high resulting nutrient impact shallow, particularly intertidal, seagrasses. concentration, for high and ammonium concentrations may be toxic to seagrasses. Eutrophication Whereas research on the effects of eutrophication on seagrass meadows has focussed on the Widespread eutrophication of coastal waters effects of reduced light quality, the deterioration of derived from the excessive nutrient input to the the sediment conditions may also play a critical sea, is leading to a global-scale deterioration of role in enhancing the loss of seagrasses. the quality of coastal waters, which is identified as Seagrass sediments are typically rich in organic a major loss factor for seagrass meadows materials, due to the enhanced particle deposition worldwide. Human activity presently dominates and trapping under seagrass canopies compared the global nitrogen cycle, with anthropogenic to adjacent bare sediments. Microbial processes fixation of atmospheric nitrogen now exceeding are, therefore, stimulated in the seagrass natural sources and anthropogenic nitrogen now , which, if sufficiently intense lead to dominating the reactive nitrogen pools in the the depletion of oxygen and the development of atmosphere, and therefore rainwater, of bacterial communities with anaerobic metabolism, industrialized and agricultural areas. Hence, which release by-products, such as sulphide and anthropogenic nitrogen dominates the nitrogen methane, that may be toxic to seagrasses. In inputs to watersheds, with the human domination order to avoid such toxicity effects, seagrasses of nitrogen fluxes being reflected in a close pump a significant fraction of the photosynthetic relationship between nitrate export rate and oxygen produced to the roots, which release human population in the world’s watersheds. oxygen to maintain an oxidized microlayer at the Tertiary water-treatment plants only achieve a surface. However, eutrophication reduces partial reduction in nitrogen inputs to the sea, for seagrass primary production both through shading nitrogen inputs to the coastal zone are already and seagrass loss, thereby reducing the oxygen dominated by direct atmospheric inputs in heavily seagrass roots may release. This allows industrialized or agriculture areas. anaerobic processes and the resulting metabolites

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to accumulate closer to the root surface, increasing the chances of toxic effects to seagrass. At the same time, the increased pelagic primary production leads to a greater input of organic matter to the sediments, enhancing microbial activity and the sediment oxygen deficit, which may increase the production of metabolites from anaerobic microbial metabolism. Both these processes result in the deterioration of the sediment environment to support seagrass growth, leading, through its interaction with the consequences of reduced light availability, to accelerated seagrass loss. Eutrophication effects on sediments may be more acute where sewage is the dominant source of nutrients, for this is discharged along with a high organic load, stimulating microbial activity. Aquaculture activities are becoming increasingly prominent in the shallow, sheltered coastal waters where seagrass meadows abound. Shading and high Figure 5.1. Propeller scars in intertidal seagrass inputs of organic matter from fish cages have meadows. Photo: R. Santos. been shown to lead to seagrass decline below and around fish cages, through processes In shallow coastal zones where recreational boats comparable to those of the eutrophication outlined are very numerous, the continuous damages above. related to anchoring and to propellers (Fig. 5.1) result in important vegetation losses. Mechanical loss The dredging of navigation channels in coastal systems such as estuaries and coastal Mechanical damage is an important anthropogenic cause of seagrass decline. The removal of the plants and the damage of the shoots and result in drastic reductions of seagrass cover. As important as the direct effects are the indirect impacts related to the alteration of the water circulation and sediment dynamics that may increase the erosion of the seagrass prairies, to the sediment re-suspension that increase water turbidity decreasing plant photosynthesis, to the mobilization of contaminants stored in the sediment, to the modification of sediment chemistry and nutrient availability or to the promotion of the competitive ability of species that may locally out compete the seagrasses. Because seagrasses develop in sandy bottoms they are very susceptible to trawling. Drastic Figure 5.2. Channel dredging in coastal systems. seagrass losses in European coastal zones, from Photo: R. Santos. the Northern Sea to the Mediterranean, were caused by trawling. Fishing boats harvesting fish (Fig. 5.2) may have direct mechanical effects by and clams associated to the seagrass habitat, in removing the seagrasses, but often their indirect many cases illegally have reduced severely the effects are more important. The re-suspension of seagrass cover and prevent the seagrass re- the sediments or the remobilization of toxic establishment. Seagrasses are not physically contaminants stored in the bottom may affect robust and rhizomes are likely to be damaged, seagrasses. Seagrass populations are likely to and seeds buried too deep to germinate, by survive increased turbidity for short time periods. activities such as trampling, anchoring, digging, power boat and jet-ski wash.

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However, prolonged increase in light attenuation transport or simply bury the seagrasses. will result in loss or damage of meadows. Toxic Particularly the construction of solid piers contaminants can decrease photosynthesis and perpendicular to the shore has huge impacts on nitrogen fixation, reducing seagrass growth. the sediment transport. Sedimentation will Moreover, the sliding of sediments from the increase near the pier, but important coastal channel edges to the bottom compensating the erosion will result downstream. In general, sediment removal may cause important erosion of seagrass beds are intolerant of any activity that seagrass beds that develop in the shallow banks. changes the sediment regime when the change is greater than the natural variation. A striking case Suction dredging for scallop in northern Europe of direct mechanical loss of intertidal seagrass removed the seagrass Zostera in affected areas meadows occurs in Ria Formosa coastal lagoon, while in un-dredged areas the seagrass remained southern , where 90% of the clams abundant. Dredging increases fragmentation and consumed in the country are produced. The beds destabilization of the seagrass meadows, which for clam culture are prepared by destroying the lead to reduced sedimentation and increased intertidal Zostera noltii meadows with mechanical erosion, resulting in a decline over larger areas. ploughs and covering the natural sediment with coarser terrestrial sediment (Fig. 5.3).

Conclusions

The current rate of seagrass loss illustrates the imperilled status of these ecosystems and the need for increased public awareness, expanded protective policies and active management. In order to achieve such goals it is important to focus resources to monitor seagrass habitat trends and conserve existing seagrass resources, act to attenuate the causes of seagrass loss, and develop knowledge and technologies to revert on- going seagrass decline. The widespread loss of seagrasses is largely a consequence of the rapid growth in human activities and transformation of the coastal zone, Figure 5.3. Preparing a clam culture bed over an with the resulting direct and indirect impacts on intertidal . Photo: R. Santos. seagrasses. Global population growth is concentrated in the coastal zone, which also Sediment disturbance, siltation, erosion and harbors a disproportionate fraction of the world’s turbidity resulting from coastal engineering have wealth. Indeed, some industries linked to the also been implicated in the decline of seagrass marine environment, such as tourism, maritime beds world wide. Coastal engineering, including transport and aquaculture are rapidly growing. the construction of marinas and piers or the Consequently, human activity in the coastal zone nourishment of the beaches alter the littoral is likely to continue to increase, with a potential for dynamics of the water circulation and sediment even greater impacts on seagrasses.

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