Threats to Nursery Areas
Marine nursery areas are rapidly becoming degraded around the globe -- many have been lost altogether. Sometimes the loss is natural (e.g. hurricanes and naturally-occurring climate change), but more often than not the impacts are human-induced. These anthropogenic impacts are direct, as through the in-filling of wetlands, or are indirect, as via the diversion of freshwater from estuaries, or the poisoning of organisms from land-based sources of pollution.
A. Habitat conversion. Habitat is lost, usually permanently, when coastal development and marine resource use is destructive or unsustainable. The greatest factor leading to loss of marine nursery habitats is through conversion of wetlands, including marshes and mangrove forests, for coastal development and construction135 136. A new and rapidly growing form of coastal development that severely impacts nursery areas is uncontrolled building of shrimp ponds and other aquaculture / mariculture sites137 138. Dredging of waterways also causes nursery habitat loss139. Finally, humans increasingly cause the loss of nursery habitat through destructive fishing practices such as blast fishing (the use of underwater explosives) and trawling (dragging of weighted nets along the sea floor)140 141, as well as through the construction of infrastructure such as jetties, groynes and seawalls which can alter long-shore drift and result in changes in sand/sediment budgets142.
B. Resource extraction within the nursery area. Marine nurseries may be disrupted for specific resources such as mangroves or oysters. Although such extractive uses may remove only a small proportion of the habitat, if these resources contribute to the physical structure, the habitat may lose its ability to support ecosystem services such as the provision of nursery habitat143 144. Mangrove depletion for firewood is an example of such a resource extraction145. In this case, habitat loss occurs without deliberate conversion of the environment for the purposes of coastal development. Resource extraction can also undermine the nursery function of such habitats when prey availability is reduced. The removal of small bait fishes from an estuarine nursery area is a good example of this sort of threat146. In the Caribbean, the depletion of grazers such as parrot fish in the Caribbean resulted in turf algae dominating many reef areas147.
Resource extraction can also target the species that utilize the habitat as a nursery area. For example, in some instances the aquarium fish industry relies on wild stock to support the trade rather than culturing fish expressly for aquaria. Fisheries directed at spawning stock of those species that utilize nursery areas can also have an effect, even when the fishing occurs outside the nursery area148. This is because some members of the biological community in nursery habitats have special roles to play in maintaining ecological interactions, such as keystone species (a species whose presence is essential to maintain a large number of other species within the community)149 150. For example, the removal of fish and invertebrates that graze algae off seagrasses can cause destruction of seagrass beds when heavy algal mats subsume the seagrass meadows151 152.
C. Habitat alteration. Human activities also impact nursery areas indirectly, by causing the alteration and degradation of distant habitat that then impacts the nursery habitat. This is an insidious and largely unseen threat, noticed only once the cumulative effects of degradation has altered or destroyed these ecosystems.
Eutrophication: Another picture emerges when run-off into streams and rivers sends pollutants into estuaries and other coastal habitats. Perhaps the greatest impact that such threats pose is eutrophication, or the addition of a large enough quantity of fertilizers, sewage, and other non-natural nutrients to change the processes occurring in these ecosystems158. Eutrophication can cause algal blooms and algal overgrowth of substrates, rendering both the water column and growing surfaces unsuitable for other species159 160. Typically, eutrophication is caused by fertilizers, agricultural waste, human sewage, and other forms of anthropogenic nutrients. It is commonplace in virtually all coastal waters near areas of human habitation161 162 163 164.
Since nitrogen is typically the limiting nutrient in marine systems, its affect on coastal ecology has been the source of much study. Recent research indicates that even in cases where nutrient levels are not significantly increased, the form that nitrogen takes in human-dominated ecosystems is predominantly inorganic, whereas in undisturbed ecosystems, the organic form makes up the bulk of nitrogen165. This has significant ramifications for organisms in these altered ecosystems, which utilize organic and inorganic nitrogen quite differently166.
Despite the value of nursery areas to support the tourism industry, coastal tourism development often uses nursery habitats such as estuaries, mangroves, marshes, and atoll lagoons for waste disposal, degrading these areas and rendering them useless in providing ecosystem services such as nursery habitat167 168.
Sedimentation: Sedimentation is another form of pollution caused by run-off. As a result of clear-cutting of forests in watersheds, removal of wetlands in riparian areas, road building, and other land use and management practices, rain washes soil into streams and rivers, which then deposit sediments above normal levels in coastal areas169. Such sedimentation can dramatically alter nursery habitats by increasing turbidity, lowering light penetration, and physically suffocating filter-feeding organisms170 as well as fish eggs.
Other Pollutants: Additional pollutants borne by rivers into coastal areas include debris, heavy metals, hydrocarbons, and other persistent organic pollutants, which have toxic effects on fish and wildlife and can rapidly build up through bioaccumulation up the food chain171 172. Species themselves can be genetically “polluted” from aquaculture operations in which farmed organisms escape and breed with wild stock – a phenomenon which is now beginning to wreak havoc on species and nursery habitat communities in many parts of the world173. Often introduced organisms change the structure of nursery habitat by physically displacing native vegetation174 or physically change the habitat (e.g. tilapia changes stream structure when it breeds, making it difficult for native species to use the habitat). Although the introduction of exotics increases biodiversity, it often results in homogenization of the system since the exotics often dominate the system, reducing the systems resilience and in many cases the ability of the ecosystem to provide a range of ecosystem services175.Climate Change: Finally, climate change has already altered nursery habitat and is expected to cause dramatic changes in the future. Global warming changes the temperature and salinity of estuary and nearshore nursery habitats, rendering them inhospitable to species with narrow temperature tolerances176. Warming can also exacerbate the problem of eutrophication, leading to algal overgrowth, fish kills, and even what are known as “dead zones”177 178. Warming can lead to heat stress, making it impossible for some organisms to reproduce. Ozone depletion may also affect nursery areas, through the increased radiation that causes the mortality of surface-dwelling plankton179. Changes in weather patters expected in some extreme scenarios of climate change, including increased precipitation in some areas, warming at the poles, and increased frequency and intensity of storm events, and sea level rise, would all affect oceanic circulation and currents and the ability of organisms to access nursery areas180.
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