Estuaries are a transient zones, where salt water from the sea and fresh water from a river mixes to make a brackish water ecosystem. Hence any change in sea level rise will have an adverse effect on species that thrive on this brackish system. Lagoon ecosystems support a myriad of species such as fish, shrimp and crabs. These are often very sensitive to any change in the pH range, chemical constituents, salinity and temperature, all of which are subject to change by climate change, giving rise to a host of environmental issues. Changes expected in estuarine ecosystems due to climate change are changes in the flushing regime, freshwater inputs, and water chemistry to complete inundation and loss, and the resulting loss of natural and human communities. Human communities that live near lagoons are especially susceptible to climate change as they live close to the sea. Climate change impact on the community range from loss of livelihood, humananimal conflict, loss of recreational and aesthetic value, land degradation, water issues, sanitation issues, socially insecurity, loss of shelter, nutritional deficiencies all leading to or result from poverty.
Sea level rise
Sea level rise resulting from climate change would, perhaps irreparably, change lagoon water’s salinity and increasing opacity of the water, thereby reducing penetration of sunlight. Increased water depth also restricts the amount of light reaching the bottom of the lagoon, thereby reducing the photosynthesis and productivity of aquatic plants that are the primary producers of lagoon ecosystems. Large scale inundation could make species composition, flowering seasons and germination go haywire.
The salinity level of a lagoon has a strong association with the monsoonal rains, ranging from zero, during rains, to almost ocean water salinity level during dry season, when salt water intrudes into the lagoon system. A notable long term increase in salinity levels of Nagombo lagoon in the past two decades may indicate decrease in freshwater input. Sea level rise will affect the unique brackish nature of lagoon water making the habitat unconducive to its native species. Species that are saline sensitive will die out and invasive species that favor salinity will encroach on the ecosystem. It will change the breeding patterns of aquatic species that use lagoons as breeding ground.
Changes in tidal dynamics
Changes in tidal dynamics, such as current speed, circulation flow patterns and tidal range, could have a host of ripple effects. This could reduce light penetration, increase exposure of plants at low tide and increase water column turbidity. In any lagoon the flushing rate, used to measure water exchange rate between lagoon and ocean, plays an imperative role in its biophysical environment. It affects water chemistry and sedimentation. For example, Negombo lagoon experiences a weak tide and strong freshwater input. Any degree of sea level rise coupled with anthropogenic activities that may reduce the fresh water input will tip the balance of salt water to freshwater ratio changing the salinity level and chemical constituents of lagoon water.
Changes in rainfall
As mentioned before any reduction in freshwater input will tip the balance of salt water to freshwater ratio. Other than anthropogenic activities reduction in rainfall due to climate change can also reduce precipitation. Moreover it has been observed that in the Negombo Lagoon, chlorophyll-a content increases with monsoonal rains. The concentration increases towards the head of the lagoon, indicating that freshwater from Dandugam Oya, Ja-Ela and Hamilton Canal are the sources of chlorophyll-a. Any reduction in rainfall up river will result in a reduction of chlorophyll-a, directly affecting this ecosystems productivity.
These ecosystems also harbor other coastal wetland types such as mangroves, mud flats and sea grass beds. Any temperature rise would reduce the productivity of the ecosystem and cause dieback of sea grass in the lagoon system. Increased temperature coupled with nutrient pollution resulting from anthropogenic activities could encourage the growth of invasive algae such as seaweed and phytoplankton. This in turn can reduce penetration of sunlight that other species require for survival. All these impacts could generate ripple effects on aquatic birds and economic invertebrates.
Increase in dissolved CO2
However climate change effects are not restricted to sea level rise and changes in salinity. Increased atmospheric carbon dioxide is known to cause increase in dissolved carbon dioxide in water. This will lead to a shift in species distribution. Research suggests that although increased dissolved carbon dioxide levels will lead to enhanced photosynthesis, it will be at the expense of those species with a reduced carbon-extraction capacity. For example invasive algae have been found to thrive on waters rich in dissolved carbon dioxide.
Impaired ecosystems functions
Lagoons and associated systems such as mangroves act as lungs purifying water and trapping pollutants. This vital ecosystem function will be impaired if it’s subjected to sea level or temperature rise. It will also affect its function of erosion control. Such coastal ecosystems also act as a barrier to the destructive nature of the sea, evident during natural disasters such as the Tsunami. If this ecosystem is destroyed or altered in any way irreparably due to climate change it would also impair these functions of coastal zone protection.
Species composition and richness
Chaetomorpha, the most dominant macro algae causes light deprivation, which affects the seagrass beds’ productivity. The species is tolerant to salinity variations and has displayed invasive qualities. However, these changes in species composition maybe due to anthropogenic activities and not climate change. Grapsid and ocypodid crabs are abundant in the mangroves and on mud flats of lagoons. The mud lobster inhabits mangrove stands. Hermit crabs are also abundant in estuarine environments. Polychaete abundance decreases with increasing depth and their diversity changes with the salinity. Consequently, any degree of change in water depth through sea level rise or reduction of precipitation due to climate change or salinity changes due to decreased freshwater input or increased salt water input, these species may die out. Research suggests that species such as P. indicus, P. semisulcatus and M. dobsoni breed in the sea and post larvae migrate to lagoons and stay in the lagoon till maturation, and then return to the sea for subsequent breeding and spawning. This indicates that the lagoon provides the ideal nutrient base and salinity level that are conducive to the growth of these species. And any changes in any of these environmental conditions could inhibit the propagation of these species. These are economically important shrimp species and economic implications of such population reduction are abound.
Loss of livelihood
Lagoons have a long standing association with the fisheries industry. A range of species are caught for consumption, export as ornamental fish and as aquaculture. Lagoons are rich in finfish and shellfish (prawns, crabs and molluscs) and the lagoons and the surrounding reef areas function as major nurseries, refuges and feeding grounds for myriad aquatic species. Many of the fish species, found in abundance, are also commercially important. Penaeid shrimp species and crab species such as mud crab and sea crab are of high economic value. As mentioned before migration between the sea and the lagoon, driven by nutrient and salinity based requirements, are an integral part of the shrimp’s lifecycle. And it is these migration patterns that fishermen make use of when fishing. Fishing inside lagoons mostly targets shrimp. A large portion of the catch consists of pre-adults of seaward migrating shrimps such as penaied prawns. Any change in water salinity or nutrient base could hinder their migration habits and thereby adversely affect the fisheries industry. Adverse effects of climate change coupled with anthropogenic activities have resulted in loss of livelihood, such as fisheries and tourism. Loss of breeding and feeding grounds result in reduced fish harvest, affecting the economy of the fisher families.
Increase in salinity could cause the propagation of invasive species such as crocodiles, leading to humananimal conflict. This has been reported in the Nilwala River, where the issue has been exacerbated by sand mining which has led to a lowering or river bed allowing salt water from the sea to intrude up river.
Landward expansion of lagoons would result in land degradation. Inundation of the banks will invariably affect the mangrove ecosystems, perhaps forcing the mangroves to migrate inland, which will affect any land use patterns that currently exist. Salt water could, through the lagoon, intrude into inland water bodies and even ground water, contaminating fresh water supplies.
Adaptation and Mitigation
Climate change effects can hardly be mitigated locally but adaptive methods can be adopted locally. For example, little can be done about sea level rise, increased salinity, changes in tidal dynamics, changes in rainfall, temperature rise and changes in species composition and richness. But dredging, beach replenishment and climate resilient infrastructure, such as houses built on raise platforms, can be adopted to alleviate the effects of sea level rise. Awareness creation on natural disasters such as tsunamis, storms and cyclones coupled with a functional warning system would go a long way to reducing human casualties during such disasters. Relocation would not be a feasible alternative to deal with climate change induced social issues as these communities depend on lagoons. Loss of livelihoods can be partially solved by supplementary incomes such as ecotourism. Human-animal conflict can be managed by awareness creation about where and when to use lagoons with respect to animal behavior. Land conservation techniques, such as mangrove replanting and sand barriers, can be adopted to reduce land degradation. Anthropogenic activities have exacerbated the effects of climate change. Most local lagoons act as sinks for many anthropogenic effluents from the surrounding areas. For example, it is believed that developmental activities such as the Colombo-Katunayake Expressway have had adverse effects on the Negombo Lagoon ecosystem, intensifying climate change effects. High phosphate concentrations indicate organic pollution in the lagoon due to fertilizer runoff. Estuaries are overfished and often environmentally detrimental fishing techniques are used. This has been further exacerbated by the issue of limited resource due to the increase in number of fishers. Although trammel nets are supposed to be no longer than 600 meters, some fishermen use nets as long as 2000 meters. Unregistered shrimp farming has also led to obstruction of water flow. Fishermen, in an attempt to compensate for the reduction in size of fish, overharvest, further depleting fish resources. Sedimentation has adversely affected fisheries industry in lagoons, as it reduces aquatic species population and also bars the way of fisheries boats. Reduced water depth in the narrow inlet/exit channel results in a reduction in reduced tidal exchange and flooding. As mentioned before flushing rate has a direct relationship with the biophysical environment. As an adaptation method the sea mouth that leads to the sea can be regularly dredged to facilitate easy flow of water from the lagoon. However, the first mitigation measure should be to minimize anthropogenic activities that exacerbate climate change effects. Most of the mangrove islands have been cultivated for economic purposes and environmental protection. This could prove an effective mitigation method in the long run as it could act as a barrier to prevent salt water intrusion through the sea mouth. It could also shield coastal communities and the lagoon itself in case of a natural disaster. Research suggests that fisher folk are reluctant to accept ecotourism as they are of the notion that ecotourism related activities would hinder their livelihood, despite the fact that ecotourism may in fact generate alternate income sources for local communities. These traditional fisher folk will have to bear the brunt of climate change impacts as they have no supplementary income. Therefore awareness can be created on how ecotourism can coexist with traditional livelihood practices, in order to empower local communities.