LIMNOLOGY                 

                                        Essentials

LAKE BENEFITS, DISTURBANCE AND RESTORATION

 The fact that lakes occupy such a small fraction of the landscape belies their importance as environmental systems and resources for human use. They have intrinsic ecological and environmental values. Besides, human use, they are used for many commercial purposes including fishing, transportation, irrigation and industrial water supply, and function as receiving waters for wastewater effluents.

 They moderate temperatures and affect the climate of the surrounding area. By storing water they help regulate stream flow, recharge ground water aquifers and moderate droughts. They provide habitat to aquatic and semi aquatic plants and animals, which in turn provide food to many terrestrial animals, adding to the diversity of the landscape.

 The myriad ways in which humans use lakes, along with numerous pollutants generating activities, have stressed lake ecosystems in diverse ways, frequently causing impairment of lake quality for other uses. Stresses to lakes arise from easily identifiable point sources such as municipal and industrial wastewater, non-point degradation like urban and agricultural run-off within a lake's watershed, and the most insidious long-range atmospheric transport of contaminants. Major degrading factors include excessive eutrophication due to nutrient and organic matter loading; siltation due to inadequate erosion control in agricultural, construction, logging and mining activities; introduction of exotic species; acidification from atmospheric sources and acid mine drainage; and contamination by toxic (or potentially toxic) metals such as mercury and organic compounds such as poly-chlorinated biphenyls (PCBs) and pesticides. In addition, physical changes at the land-lake interface (eg. draining of riparian wetlands) and hydrologic manipulations (eg. Damming outlets to stabilise water levels) also have major impacts on the structure and functioning of these ecosystems.

 Eutrophication: Causes and consequences

Eutrophication has proved to be one of the most widespread and serious anthropogenic disturbances to aquatic ecosystems. The major cause for eutrophication is the increased loading of nutrients, especially phosphorous (P). Increasing wastewaters, introduction of phosphorous containing detergents, use of fertilisers, and erosion in the watershed are the major reasons for increased loading of nutrients. Fertilisers are the major source of nitrogen, while phosphorous in the fertiliser may be immobilised in the soil. Flooding can however transport large quantities of phosphorous due to leaching from the soil. Domestic sewage primarily results in increased P loading, containing on the average, an N:P ratio of 4:1 leading to a shift from phosphorous to nitrogen. An undesirable consequence of eutrophication is the development of cyanobacteria (blue-green algae) that tend to form dense “surface blooms”, excreting organic compounds that impart bad odour and taste, creating serious problems in drinking waters. Some of the problem-causing algae are Anabena, Aphanizomenon, Microcystis, Limnothrix and Planktothrix.

 The main effect of eutrophication in the hypolimnion and sediments is the increased consumption of oxygen. Anoxic conditions at the sediment surface and hypolimnion, impoverish the deep water fauna. This condition may also lead to a series of chemical and microbial processes like nitrate ammonification, denitrification, desulphurication and methane formation. Even pelagic fish, which release their eggs in the open water that sink to the bottom, cannot continue to reproduce normally in a lake with anoxic sediment surface. The release of phosphorous from the sediments is extremely important as it accelerates eutrophication. Fish kills may result from advanced eutrophication, initiated by elevated pH as a result of high photosynthetic rates and total ammonium concentration. Increased pH causes a shift from non-toxic ammonium ion to toxic ammonia. (see box).

Biodiversity of inland waters:

Very little is known about the biodiversity of most microbial, plant and animal groups. Rate of loss of biodiversity varies both because of differences in intensities of disturbances to the habitats as well as the number of species inhabiting particular areas.

 Water quality alterations as disturbances to biodiversity: The fresh waters of the world are experiencing accelerating rates of degradation. These major sources of disturbance impact the biodiversity of fresh waters in many ways. Direct release of chemical toxicants into the surface waters is very common. Many forms of heavy metals, inorganic nutrients and organic compounds enter the aquatic environment. Although some are inactivated by chemical precipitation or oxidation, many have long resident times. Despite dilution and dispersion in the aquatic environment, biomagnification of both metals and organic compounds is common, which increases toxicity exponentially. The biological effects of these compounds are still unknown.

Plant nutrients, particularly phosphorous and nitrogen, can lead to increased organic productivity of fresh waters. This eutrophication rate leads to enhanced rates of decomposition and in chemical conditions greatly reduce or eliminate suitable habitat for many species of plants and animals. Excessive loading of organic matter results from organic sewage from human populations, industrial effluents and agricultural run off. Another factor that contributes to reduction in habitat availability is the suspension of finely divided organic and inert inorganic matter. Erosion and transport of such substances are increasing as a large amount of vegetation between land and water is eliminated, primarily for agricultural expansions.

 Changes in biodiversity in freshwater ecosystems can arise from many disturbances. The introduction of certain competitively superior species can result in marked losses of biodiversity. Dense-floating macrophyte communities and other eutrophication associated excessive plant productivity often result in deoxygenation and reduction in habitat and elimination of plant and animal communities. Effective utilisation of freshwater resources is complicated by distribution of humans and their exploitation of regions of low water availability. Accelerating human induced changes in hydrological patterns associated with flood controls and water supply changes further obscure efficient utilisation practices. Control and reversal of degradation requires a proper economic valuation of freshwaters for efficient, conserved utilisation of water supplies for agricultural, industrial and domestic purposes.

 Shifting resilience, stability and biodiversity of freshwater ecosystems: A central feature of stability is the ability of a property (nutrient concentration, population and community) of an ecosystem to return to a steady-state equilibrium following a disturbance. Resilience or relative stability is the measure of the rate at which the property or system approaches steady state following disturbance. Resilience or relative stability in an aquatic ecosystem is governed by process rates, that is, the rates at which energy and materials are transferred among biotic components. Because much of the organic matter is detrital, most of the assimilation and decomposition metabolism is microbial (bacterial, fungal etc.). Each of the species constituting the biotic groups has a physiological range for growth, competition and reproduction within the environmental constraints. Some of these ranges differ with spatial or temporal changes in life histories of different species, which can affect species responses to disturbances.

 Physical and chemical environmental conditions mediated by a host of biogeochemical / climatic / meteorological factors regulate the bounds of nutrients and the energy flux rates. Major deviations from natural ranges arise from human or abnormal natural disturbances. The integrated ecosystem responds to major disturbances by shifting dominant metabolism from one species to alternative species as the physiological tolerance of certain species crosses the threshold limit resulting in their death. Greater biodiversity may have a greater collective effect by improving the ability for ecosystems to recover from large disturbances. Reduced biodiversity increases vulnerability by reducing the total collective physiological tolerance of the community to large habitat changes/disturbances.

 Extent and timing of disturbances and shifting stability: The differences in plant productivity result in different types of chemical composition of organic matter loaded to the lakes. Because of the large amounts and relative recalcitrance of dissolved and particulate organic detrital sources from higher plants, heterotrophic utilisation of organic matter is slowed. Alterations to the quality and quantity of dissolved organic matter entering the aquatic ecosystem from the drainage basins will influence this inherently chemically mediated metabolic stability. In this context, forested watersheds, undisturbed riparian floodplains and wetland/littoral zones are important to the metabolic stability within lake ecosystems.

 A common disturbance to lake ecosystem involves progressive increases in loading of nutrients. This increase, particularly among shallow lakes that predominate globally, a marked shift in productivity occurs from the pelagic to surfaces (attached surfaces) associated with living aquatic plants and the particulate detritus of dying (senescing) macrophyte biomass.

Biodiversity and nutrient recycling: Greater biodiversity results in greater physiological diversity to cope with natural vagaries in environmental parameters, intensifying competition for resources (like nutrients). Nutrients once acquired are intensively recycled among the attached biota and conserved. Nutrient retention is very high within micro communities and collectively, the attached habitats. The increased dependence on labile organic substrates from algae reduces resilience to disturbances.

 Predation-induced shifts in energy fluxes and biodiversity: In certain moderately productive lakes, piscivore consumption of planktivorous fishes can lead to sporadically enhanced development of zooplankton. The higher grazing rates of zooplankton can result in selective reduction of larger algae. Nutrient recycling also increases particularly as decomposition of these algae and metabolically coupled bacteria is accelerated by micro consumers and tightly retained in the biota. Here, the biodiversity among micro biota would increase and compensate for loss of larger forms by zooplankton grazing. 

 Bioindicators - Macroinvertebrates and fish

 Bioindicators are organisms whose presence, absence or condition provides information about the environmental quality. Every organism has unique environmental requirements to be healthy and reproduce successfully. The presence or absence of healthy populations of organisms within their habitats is a sign of unique environmental characteristics. The advantage of using bioindicators over chemical and physical tests to evaluate water quality is that the presence of living organisms inherently provides information about water quality over time. Chemical and physical tests give information that is accurate only for that moment when sample is taken. The presence of a mixed population of healthy aquatic insects or fish usually indicates that the water quality has been good for some time. The absence of bioindicators at a site that appears good according to chemical and physical sampling might prompt further investigation for toxic contaminants or periodic degradation of water quality.

 In lakes, benthic macroinvertebrates are taken into consideration to get an idea of water quality. Benthic macroinvertebrates include aquatic insects, worms, shellfish, crustaceans and other animals without backbones that are large enough to be seen without a microscope and live at the bottom of a water body. Many species of mayfly nymphs, caddisfly larvae and stonefly larvae, for example, can survive only in swift, cool, and oxygenated water. Their presence at a sampling site is generally a sign of good water quality. Black fly larvae, midges, leeches and aquatic worms on the other hand, are quite tolerant of pollution. They can be found in waters of both good and poor quality. If they are the only types of macroinvertebrates found at a site, chances are the site is silty and has low dissolved oxygen. Such conditions might represent a polluted lake. Apart from using the presence or absence of certain indicator species to determine water quality, the number of different species can provide more information. The three most pollution intolerant orders of aquatic insects are Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies). The higher the percentage of these pollution intolerant species in relation to the percentage of tolerant species at a site, the better the water quality.

 The health of resident fish species will be indicative of overall water quality. The condition of the lake is determined by comparing the length of the fish to its weight. Heavier the fish for its length, better the condition.

 Freshwater mussels, like aquatic insects, serve as bioindicators. Each species of mussel has different environmental requirements. Some species like Elliptio complanata, the Eastern Elliptio Mussel, are more pollution tolerant than species like Margaretifera margaretifera, the Pearl shell Mussel. Freshwater mussels are filter feeders that use a siphon to pump water into their shell where they use gills to simultaneously retrieve oxygen and extract food from the flowing water. Because they move so slowly, mussels must be able to obtain their food and oxygen from one spot. If their habitat becomes inundated with pollution such as toxins or silt, some of them will most likely perish.

 Perhaps the most interesting aspect of freshwater mussel natural history is reproduction. Larval mussels, called glochidia attach to the gills or scales of fish. The glochidia develop for a while attached to the fish and eventually drop back to the bottom. The glochidia do not seem to harm the fish. Scientists think that the mussel glochidia will only survive attached, in some cases, to particular species of host fish. For example, the Pearl shell Mussel mentioned above usually uses salmon and trout as a host. If obstacles such as dams, poor water quality or over fishing interfere with the survival of the fish, they are likely to interfere with the survival of the mussels too. Because freshwater mussels can live for decades, their presence or absence can provide even more information about the history of water quality at a site.

 Restoration of aquatic ecosystems: Lakes are more susceptible to function as sinks for pollutants, which means that pollutants tend to accumulate and potentially increase the toxicity with time. The need for management of lakes and rivers reflects an inability of the ecosystems to operate in self-sustaining ways because of the interference or damage that exceed the capacities of the ecosystem for self-restoration. Most demands for management are a result of disturbances by human activities. Four major types of degradation of surface waters have occurred in recent times:

·         Nutrient loading, particularly phosphorous and nitrogen, have resulted in nutrient enrichment and excessive growth of algae and macrophytic plants. This accelerated eutrophication has led to evolving suitable remedial measures to mitigate the problems and return the lake to some state of lower productivity.

·         Soil erosion, primarily from removal of vegetation cover and agricultural activities, leading to siltation, loss of volume in the aquatic habitat, etc.

·         Excessive loading of hydrogen ions associated primarily with strong acids. These acids result from gases from fossil fuel combustion products that are dissolved in precipitation or adsorbed to particles that are deposited from the atmosphere onto surface waters. The resulting acidification of poorly buffered waters not only increases acidity and alters the osmoregulatory capacities of organisms but also alters the solubility and bioavailability of many metals and other ions of variable toxicity.

·         Introduction of toxic materials such as heavy metals, chlorinated hydrocarbons and radioactive materials, the sources of which are often diffuse (non-point) sources, making their control very difficult.

Lake management and restoration:

Lake management and restoration are focussed mainly on excessive nutrient loading and land management.

Lake restoration efforts have therefore been directed towards reducing the loading of phosphorous and to some extent nitrogen to the surface waters by advanced wastewater treatment, diversion and land management or reducing the phosphorous load in wastewaters by reducing the phosphorous content in detergents. Prevention of nutrient pollution and eutrophication is a long-term solution. Reductions in the nutrient loading require evaluation of diffuse and point sources within the drainage basin and a systematic program of reduction and control.

 Nutrient control:

Some of the common methods that are used singly or in combination to reduce the nutrient availability are listed below:

Nutrient removal by advanced treatment and land management: Regulation and reduction in external loadings of nutrients to eutrophic lake is the best of long-term corrective measures. Advanced wastewater treatment is practical only where used water is collected and phosphorous removed to quantities that will not alter the lake ecosystem. Agricultural land management practices including manure storage and changes in tillage can also reduce nutrient loading.

Nutrient diversion: Occasionally diversion of the major nutrient loadings is adequate to restore a eutrophic lake. In situations where the primary source of nutrient loadings is defined, wastewater or storm water may be diverted without appreciable alteration of lake hydrology. Diversion is most likely to be successful in rapidly flushed lakes, where internal nutrient loading from sediments is small.

 Dilution and flushing:

If a major source of nutrient-poor water is available, addition of such water to an enriched lake may be adequate to dilute nutrient levels to suppress algal productivity. If sufficiently large quantities can be added, the water alone can be adequate to flush out algal cells and maintain a lower productivity by dilution. Lower algal biomass can be maintained even with nutrient rich water if flow is adequate to wash out the algae faster than algal growth rates. These methods are used in conjunction with induced lake circulation, chemical precipitation and biological manipulations.

 Phosphorous precipitation and inactivation:

Recovery from eutrophication in which essential nutrients are reduced to low concentrations can be a slow process if lake water renewal times are long and nutrient releases from the sediments to the epilimnion are large. Phosphorous is precipitated by clay and carbonate particles present in water. Aluminium also precipitates phosphorous. Aluminium sulphate (alum) or sodium aluminate forms a precipitate of aluminium phosphate or a colloid of aluminium hydroxide, depending on the pH and alkalinity. In both cases, phosphorous is bound and transported to the sediments without aluminium toxicity if neutral conditions are maintained. Further addition of aluminium to the sediments retards phosphorous migration to the overlying water.

 Sediment oxidation:

High bacterial decomposition of organic matter in sediments results in anaerobic conditions. These reducing conditions lead to reduced iron and release of associated phosphates into interstitial sediments and overlying water. Additions of nitrate as an alternative electron acceptor for oxygen delays the reduction of iron and release of phosphate.

 Aeration:

Two methods have been employed to aerate water of stratified lakes, artificial circulation and hypolimnetic aeration. Oxygenation can induce precipitation of phosphorous, iron and manganese and lower ammonium content and increase pH. Artificial water circulation by pumps, jets, bubbled air or other gases, destratifies the water strata, which reduces the competitive advantages of cyanobacteria. Internal loading of phosphorous often declines from renewed aerobic conditions at the sediment-water interface if adequate iron is present. Otherwise iron has to be added.

 Sediment removal:

Shallow eutrophic lakes can often be restored by removal of sediments. This technique simultaneously removes large quantities of nutrients stored in the sediments, increasing the mean depth of the basin and removing toxic substances.

 Food-web manipulations:

The management of food web structure by manipulation of animal herbivory, predation and nutrient recycling can influence the qualitative and quantitative composition of algae on a seasonal basis. Nutrient concentrations and loading rates set limits on the types of phytoplankton and their productivities. Changes in zooplanktonic herbivore types and densities can alter algal biomass and species composition below those anticipated by the nutrient loadings to the lake ecosystem. The phytoplankton productivity is usually rapidly replaced by smaller algal species that may be less obvious for human uses of the water bodies.

 Control of macrophyte biomass:

Excessive growth of macrophytes, particularly certain exotic species like water hyacinth (Eichhornia crassipes), hydrilla and duckweeds, can curtail or eliminate the use of lakes.

 Draw down of water level: The draw down of water level is a multipurpose restoration technique for the management of aquatic plants in lakes and to modify the habitat for fish populations.

 Mechanical removal: Aquatic macrophyte communities can be controlled by numerous manual and mechanical methods. Harvested plants are preferably removed from water, simultaneously removing appreciable quantities of nutrients.

 Biological controls: Introduction of phytophagous insects and fish or plant pathogens such as fungi and viruses has been used as species-specific control agents to reduce the biomass of macrophytes.

 Chemical controls: A number of general and moderately selective herbicides have been effectively applied to control aquatic macrophytes. Such compounds are used in combination with other control methods.

 The term restoration means the reestablishment of predisturbance aquatic functions and related physical, chemical and biological characteristics. The objective is to emulate a natural, self-regulating system that is integrated ecologically with the landscape in which it occurs. Often, restoration requires one or more of the following processes: reconstruction of physical conditions, chemical adjustment of the soil and water; and biological manipulation, including the reintroduction of absent native flora and fauna.

 The conservation and protection involves not only buffering wetlands from direct human pressures, but also maintaining important natural processes that operate on wetlands from outside, which may be altered by human activities. Management towards this end should emphasize the long term sustenance of historical, natural wetland functions and values. Restoration is thus a good opportunity to manage wetlands for broad wildlife goals, as restored wetlands provide enhanced wildlife benefits, in addition to other benefits, concurrently.

 The preliminary step that has to be implemented in restoring lakes for their long-term sustenance includes:

 As an extension of the restoration programme, watershed management practices are essential for proper land use, protecting land against all forms of deterioration, conserving water for farm use, proper management of local water for drainage, flood protection and sediment reduction and increasing productivity from all land uses. Key steps for best management practices include:

 The restoration programs with an ecosystem approach through Best Management Practices (BMPs) helps in correcting point and non-point sources of pollution. This along with regulations and planning for wildlife habitat and fishes helps in arresting the declining water quality and the rate of loss of wetlands. These restoration goals require profound planning, authority and funding along with financial resources and active involvement from all levels of organisation (Governmental and Non-Governmental Organisations (NGOs), research organisations, media, etc.) through interagency and intergovernmental processes all made favourable in innovating and initializing the restoration programs. Network of educational institutions, researchers, NGO's and the local people are suggested to help restore the fast perishing wetland ecosystem and conserve those at the verge of extinction by formulating viable plans, policies and management strategies.

 Current scenario

Wetlands do face the tragedy of commons, as is evident from present quality and steep decline in their numbers. The prime reason for this state is mainly due to lack of coordination among many agencies involved in the management and appropriate legal measures to protect these ecosystems. As of today, wetlands are not delineated under any specific administrative jurisdiction. Some wetlands are protected after the formulation of the Wildlife Protection Act. However, it is ineffective and most are in grave danger of extinction. Effective coordination between the different ministries (energy, industry, fisheries revenue, agriculture, transport and water resources) is essential for the protection of these ecosystems.

 Prevailing laws are ineffective as far as the protection or conservation of aquatic ecosystems is concerned as most of them indirectly touch wetland protection (fragmented approach);

 India, in spite of being a signatory to the Ramsar Convention on Wetlands and the Convention of Biological Diversity, there is no significant development towards sustaining these ecosystems, either due to lack of coordination among agencies involved or lack of awareness of the values of wetlands among the policy makers and implementation agencies. The effective management of these wetlands requires a thorough appraisal of the existing laws, institutions and practices. The involvement of various people from different sectors is essential in the sustainable management of these wetlands.

 A clear understanding of limnology and geographic information systems (GIS) will help in devising better monitoring mechanisms of the physical, chemical and biological characteristics along with spatial and temporal aspects of wetland resources. Management based on accurate knowledge and increased awareness of wetland issues involving all stakeholders and components of ecosystem help in long-term conservation of these fragile ecosystems. This also would enhance the function and value of the system in terms of natural and socioeconomic factors to satisfy critical resource needs of the human population.