Using Mangroves Inland

USING MANGROVE TREES IN EUTROPHIC INLAND WATERS

KRISHNA MB




 

AN OVERVIEW

Removal of nutrients from waters entering lakes and tanks to prevent eutrophication is an important component of waste-water treatment. This can be achieved by a turnover of floating and emergent vegetation: with periodic removal or harvest of biomass from the lake, a continuous removal of nutrients from the water is possible. However, decomposing aquatic and semi-aquatic vegetation present in such lakes could itself return nutrients rapidly into the water. Therefore, there could be merit in substituting soft tissued plants with woody vegetation (trees). Trees from tropical intertidal and adjacent communities could be viable alternatives in afforesting inundated and foreshore areas of tanks and lakes. They could be much larger and capable of creating an aesthetically appealing microclimate of their own, and being woody, would be less likely to decompose and return nutrients to the water. These species could also grow better in such nutrient-rich and deoxygenated soils and sediments found in our tank and lake beds. Given that not less than twenty percent of the tanks in the larger Bangalore area could be eutrophic [1], there should be ample number of situations to try out these species. Moreover, a good tree growth in standing water could effectively 'contribute' to the 'green space' of urban Bangalore.

 

THE BACKGROUND

According to the Ramsar Convention, wetlands are defined as areas marginal between land and water, inundated or saturated frequently enough to support plant and animal communities adapted to a life dominated by water. It encompasses areas of marsh, swamp, standing or flowing water which is fresh, brackish or salty. This definition thus includes man made systems, and most wetlands in inland Peninsular India would fall into this category. In the Bangalore area, apart from the occasional seasonal stream, there are no natural wetlands. All the water bodies which occur in this area, are thus, man made [1]. A wetland of our parts basically consists of an impoundment across a stream to form a reservoir and irrigated fields down the embankment. As such these usually form a series, a number of them being found in a valley.

Remote sensing studies have revealed that wetlands account for just 4.8% of a sample area of 640 sq km covering both urban and rural zones of Bangalore [2]. The number of man made water bodies (or tanks as they are locally called), in the existing administrative area has fallen from 262 in 1960, to some 81 at present [3]. Obviously most of these were created to serve the needs of irrigated agriculture [4], but were destroyed once the pressure for land became high.

Thus, a vast majority of the wetlands in the Bangalore area occur on the outskirts, in the urban-rural fringe. The uneven distribution of Bangalore's wetlands with reference to the city at present, is obviously the result of an explosive increase in the extent of the city from around 67 sq km in 1961 to over seven times that area today. The growth of the city has not only engulfed the tank beds but also disrupted the drainage pattern in these basins. In addition, the tanks which have survived in the city have sewage flowing into them.

From a local ecological point of view, these tanks form an unique and irreplaceable system. A tank can be visualised as a basin with several zones of water of varying depths, abutting a deeper zone that lies towards the embankment. This zonation is dynamic and fluctuates with the seasons, promoting the growth of a variety of emergent, floating, anchored floating and submerged vegetation, each of which shows a preference to a particular range of water depth. These fresh water plant communities in turn accommodate a large number of small organisms like periphyton, insects, molluscs, etc. which form the staple diet of many species of invertebrates and vertebrates in the tank's food chain.

With sewage flowing into many of these urban tanks, the status of these waterbodies have changed. Many have become eutrophic and algal blooms are frequent and extensive [1]. They have lost their seasonally fluctuating water level and have attained a permanently flooded state. There could be much accumulation of sediments in the tank bed in which nutrients are trapped. These nutrients get released into the tank waters on disturbance (as when drag nets are used) and in turn lead to more algal blooms. In addition, many of these tanks (especially those within the city) have their sloping shoreline substituted with vertical stone walls. The eutrophic condition leads to extensive growth of Water Hyacinth (Eicchornia crassipes), with a loss of other structural plant forms. In a few cases, increasing growth of Ipomoea carnea is covering extensive stretches of tank beds. There could be thick impenetrable growth of tall emergent reeds, especially in tanks which are too silted up due to excavated soil and garbage dumping.

It is under these conditions that growing trees from the intertidal and adjacent communities could find merit. Being woody, they would decompose much more slowly, thus acting effectively a 'nutrient sinks' in these urban tanks. Being much larger, they could serve a greater aesthetic need of having larger green 'lung' spaces in the city. Given ground level disturbance by humans and their activity, trees might provide better substrates for urban wildlife to use these tank spaces. Since not all tanks are large, 'elevating' the substrates urban wildlife use might be a much better alternative than trying to spatially separate the habitat from areas of human use.

 

ADAPTATIONS OF MANGROVE AND ASSOCIATED TREES WHICH COULD MAKE THEM SUITABLE FOR URBAN USE

The word "mangrove" has generally been used to refer to either the constituent plants of tropical intertidal forest communities or to the community itself. In a more limited sense, it is used to refer to tropical trees restricted to intertidal and adjacent communities, while the term "mangal" is used to refer to the community itself. The term "mangrove" is therefore used in the restricted sense here. Even tidal influence can be interpreted narrowly, simply to mean the shoreline inundated by the extremes of tides, or it can more widely refer to river-bank communities where tides cause some fluctuation but no salinity. The term is therefore used in its wider sense here.

To quote Tomlinson [5]: 'Most mangroves grow very well in fresh water and some penetrate considerable distances inland along river banks where water is permanently fresh and tidal fluctuations are small or absent... It is generally assumed that mangroves are excluded from terrestrial communities by competition with other kinds of plants that do not carry the burden of features that are adaptive only in stressed environments'.

There is evidence that the enzyme systems of these halophytes neither require salt or are particularly resistant to it. Tomlinson [5] gives a good review of salt balance as well as the biology of mangroves, and the readers are referred to it. Only some four genera of mangrove tree species like Avicennia are known to eliminate salt through salt glands. Others may have only epidermal structures that somewhat resemble these salt glands. The non-secreters are assumed to avoid salt at the water absorption stage itself.

Mangrove substrates are variable: from firm to soft mud, which can be rich in organic matter. In muddy substrates a disturbance produces a strong smell of hydrogen sulphide, indicating the completely anaerobic property of water-logged soil [5]. This tolerance would be of value in trying them out in inland urban eutrophic waters.

Thus, mangroves would be tree species which have the capability to grow under permanently inundated conditions, in soft mud and sediments which could be quite deoxygenated, and at high salt concentrations (when compared to that of fresh water). All these would not mean that they require these conditions to grow: literature suggests that it is only an adaptation for harsh conditions, which perhaps they could even do well without, when artificially planted.

 

USE OF WETLANDS BY BIRDS

Amongst the water bird species which occur in Bangalore and its environs [6], there can be some five broad groups which can be recognised based on wetland zones they frequent. Though most species have their preferences, there is some amount of overlap in the usages of these microhabitats. Some species also make use of more than one microhabitat. The five groups are as follows:

Open water birds
Waders and shoreline birds
Meadow and grassland birds
Birds of reedbeds and other vegetation
Birds of open airspace above wetlands

The open water birds are those which keep to the unvegetated open water zone which is generally found in the middle of water bodies, away from the shoreline. Ducks, geese, grebes, cormorants, kingfishers, terns, gulls, and Pelican tend to keep to this zone. Except for the ducks and the geese, all the others are fish eating birds generally.

Stilt, Greenshank, sandpipers, storks, ibises, Spoonbill, herons, and egrets tend to make use of the shallow waters to wade in and feed. All are long legged, long necked birds which keep to the bare open shorelines, picking up animal matter by either a wait and strike strategy like the herons and egrets, or by actively probing and searching for animals. The larger birds like the herons, egrets, and storks take vertebrate food while the smaller forms feed on invertebrates. The taller birds wade into deeper waters while the short and the short legged birds keep to shallow waters or wet mud.

Rails, bitterns, Coot, jacanas, moorhens, snipe, Painted Snipe, etc make use of vegetated portions of a wetland. Birds like the jacanas keep to floating vegetation, their elongated toes enabling their weight to be distributed over floating leaves. Purple Moorhens are especially attracted to Water Hyacinth covered patches, which they feed on. Bitterns and snipe make use of reed covered areas with standing water beneath, while moorhens and Coot can be seen even swimming out onto the open water leaving the vegetation. Other rails usually keep to dense cover.

Swallows and pratincoles hawk flying insects over water, while wagtails, lapwings and plovers pick insects and other invertebrates off the ground in grasslands, meadows and fields bordering water bodies. Birds of prey too make use of the air space over the wetland, searching for suitable animals either perched on a vantage point or by flying over the wetland.

In addition to these birds, there are those like weaver-birds and munias which do make use of wetland and other marginal areas to feed and breed. The tend to make use of the bushy and tree vegetation along the waterline to build their nests, and even roost. Swallows which forage widely make use of reed beds to roost-in along with wagtails.

Invariably, there are both specialist and generalist users of the various habitats that birds make use of. Therefore, a variety of habitats and micro-habitats are required for the survival of a variety of species [7].

In Bangalore, there would be a gradient in the kind of microhabitats which would be provided by the sequence of tanks from within the city to the outskirts and the adjoining rural areas. The city tanks like Ulsoor and Yediur are walled, and others newly engulfed by the city would have lost the associated meadows and sloping shoreline. As one moves outwards, the sloping shorelines are seen but meadows are absent, until one reaches the rural areas where all zones could be present. This would have a major influence on the kind of birdlife that would be present in the urban tanks.

 

THE STATUS OF WATER BODIES AND WETLANDS IN BANGALORE

A survey of tanks within a forty kilometre radius of the GPO in 1995 [1] revealed that nearly a third of the tanks in the Bangalore area had lost their sloping margins and were either walled or had cut margins. Of the thirty tanks which had high water levels during the survey in the month of January, twenty five had sewage waters flowing into them [1].

A survey of electrical conductivity (EC) of tank waters [1] indicated that twenty-one percent of the tanks had waters with the conductivity between one and two mmohs/cm, while 70% of the tanks showed lower EC. The urban tanks showed high EC, while three had abnormally high values with the maximum being 3.2 mmohs/cm. Since conductivity is an indicator of dissolved salts, the higher values recorded would be detrimental to the growth of sensitive plants. Chloride concentration was also more in tanks with sewage contamination.

By and large, the tanks in the Bangalore area are not deep; the embankments could be in the region of some two to five metres relative height as is indicated on the Survey of India topo sheets. During summer, many of the rain-fed tanks would tend to go dry from evoporation and water usage. The annual open-pan evoporation could itself amount to well over a metre to almost a metre and a half. However, the number of tanks still used for irrigation would just amount to a third of the total number surveyed [1].

 

THE FEATURES OF URBAN WETLANDS WHICH COULD BE OF USE FOR THE SILVICULTURE OF MANGROVE AND ASSOCIATED TREES

The features of urban tanks mentioned in the preceding parts make them ideal to try out mangrove species. Most have lost the sloping shoreline that is invariably found in the rural tanks used for irrigation. Instead they have steeply sloping margins. They are not used for irrigation purposes, and their waters could be eutrophic and even perhaps septic in some cases. Not all urban wetlands are large: the distance from the shoreline to the centre of the tank is generally quite small so that birds like ducks which are found day-roosting on the open waters would be disturbed by human activity and even presence on the shore. In those tanks where there is sewage inflow, the seasonal fluctuation in the level of water is lost and they tend to attain a permanently flooded state. The EC of these waters contaminated with sewage is higher, indicating a higher level of dissolved salts. Most of the tanks within the city which are in such a state have poor diversities, as is shown by the variety and numbers of birds [1,7], unless they are large or well protected and isolated from human activity on the shore.

 

THE ADVANTAGE OF SUCH TREES FOR BIRDS AND OTHER MACRO FAUNA IN URBAN SETTINGS

There are many advantages of having mangrove trees in urban tanks. First and foremost, they could grow in standing water forming living islands, which could be effectively used by birds and other urban wildlife. Birds could use these as substrates for roosting and breeding. Bats would find the blooms of many of these trees a source of food, as would the nectarivorous birds. They may be in a position to regenerate on their own, and spread, to there need not be a constant planting effort. Being woody, overgrowth would never be a problem, since even firewood has a good value today. Being able to withstand inundation, there is no need for periodic drying of the lake, which many other non-mangrove species might require. They are not thorny, hence unlike Acacia nilotica extensively used for foreshore plantations, they pose no threat to flying birds to get entangled in the thorns (there have been a few such reports). The knee and supporting roots could give additional perch sites for birds like herons and egrets while foraging. They could inhibit excessive growth of Water Hyacinth by a simple process of shading, as also algal blooms, while reeds could still perhaps grow in shallow water between trees where shading is not much. The green tree canopy could provide a continuum with the other tree cover of the city in spite of what substrate they themselves grow on.

 

THE POSSIBLE BENEFITS OF USING TREES FROM INTER-TIDAL AND ASSOCIATED COMMUNITIES FOR WASTE WATER TREATMENT

Growing beds of emergent plants or reeds, or cultivating Water Hyacinth and such fast growing floating plants in pre-treatment tanks have been suggested and tried to remove plant nutrients from wastewater. These processes require a lot of maintenance action if plants are not to die and decompose in situ. The shortest time soft tissued plants could take to decompose, as for example during composting, would be around three weeks. Woody tissue, on the other hand, would never decompose so fast. Hence substituting soft tissued plants with woody vegetation could be a better alternative to remove nutrients from waste water.

 

THE ADVANTAGES OF SUCH TREES FOR AESTHETICS, LANDSCAPING, FORESTRY AND LAW ENFORCEMENT IN URBAN SETTINGS

Bangalore is on a plateau, and all the water which flows, flows out. Tanks are thought to be good sources of groundwater recharge. Hence, it makes sense to retain these tanks in good condition with adequate levels of fairly good water. Again, given that land value is high in urban areas, there is a lot of pressure on tank beds from encroachments [8]. The most easily maintained deterrent to encroachments therefore, is standing water itself. Thus, trees in tanks could provide an aesthetic screen and pleasant greenery, especially in sites which are too small to have a scenic value.

 

TREES OF INTER-TIDAL AND ASSOCIATED COMMUNITIES WHICH ARE ALREADY IN CULTIVATION IN INLAND AREAS

There are lot of woody plants and trees which withstand inundation and salinity, found planted and growing well in inland areas. Many of these are from the intertidal and associated plant communities. A classic example would be the extensively grown Coconut, and Pongamia pinnata (Derris indica) both of which do well even with the roots dipping in sea water. There are others like Barringtonia and Pandanus growing well even in fresh water where there is no salinity. The botanical gardens would have littoral species like Heritiera littoralis (commonly referred to as the 'Looking Glass Tree' because of the shiny undersurface of the leaves) growing well on dry ground without salinity and salt [eg. 9]. All this would indicate that salt and salinity are not a prerequisite for these species to be cultivated. Some of the other species from the intertidal and associated communities doing well inland are: Cerbera manghas, Terminalia catappa, Calophyllum inophylum, Hibiscus tiliaceus, Thespesia populnea, to name a few.

What we need to be trying out is the core mangrove species (as some would consider) like Avicennia, Bruguiera, Ceriops, Heritiera, Nypa, Rhizophora, Sonneratia, and Xylocarpus in sewage contaminated, even possibly septic, highly eutrophic urban tanks with heavy deposits of sediments which would have accumulated over many decades.

 

REFERENCES

[1] Krishna, MB; Chakrapani, BK; Srinivasa, TS; (1996). Waterbirds and wetlands of Bangalore: a report on the status, water quality, plankton, and bird populations of the lakes in and around Bangalore and Maddur, Karnataka, India. Birdwatchers' Field Club of Bangalore and Bangalore Urban Division, Karnataka State Forest Department. Bangalore.

[2] Behera, G; Nageswara Rao, PP; Dutt, CBS; Manikiam, B; Balakrishnan, P; Krishnamurthy, J; Jagadeesh, KM; Ganesha Raj, K; Diwakar, PG; Padmavathy, AS;  Parvathy, R. 1985. Growth of Bangalore City since 1900 based on maps and satellite imagery. ISRO technical report number isro-eos-tr-55-85.

[3] Lakshman Rau, N; Issar, TP; Parthasarathy, MA; Patil, SM; Vijaya Devi; Giri Gowda, P; Shenoy, PD. 1986, 1993.  Report of the High Power Committee set up by the Government of Karnataka on 'Beautification of Bangalore'. Published by Shivashankar Engineering Co. Pvt. Ltd, Bangalore for the Government of Karnataka.

[4] Hasan, MF. 1970. Bangalore through the Centuries. Today and Tomorrow Publishers, Bangalore.

[5] Tomlinson, PB; (1986). The Botany of Mangroves. Cambridge University Press, Cambridge.

[6] Krishna, MB. 1996. The Birds of Bangalore's Wetlands. Proceedings of the Seminar on Environment related Issues of Bangalore, pp.106-114. Environment Association of Bangalore & Civil Engineering Division, Department of Space, Bangalore.

[7] Chakrapani, BK; Desai, M; George, Joseph; Karthikeyan, S; Krishna, MB; Harish Kumar, U; Naveein, OC; Sridhar, S;  Srinivasa, TS; Srinivasan, N; Subramanya, S. 1990. Survey of Irrigation Tanks as Wetland Bird Habitats in the Bangalore Area, India, January 1989. Birdwatchers' Field Club of Bangalore. Sponsored by the Karnataka State Forest Department.

[8] Jayaram, C. 1996. Management of lake systems and surface water bodies in Bangalore. Proceedings of the Seminar on Environment related Issues of Bangalore, pp.94-104. Environment Association of Bangalore & Civil Engineering Division, Department of Space, Bangalore.

[9] MariGowda, MH; Krishnaswamy, M; 1968. Plant Wealth of Lalbagh. Department of Horticulture, Government of Mysore.

[10] Krishna, MB; Chakrapani, BK; Jayaram, C. 1995. National Lakes Conservation Plan, a proposal for Bangalore. Department of Ecology, Environment and Forests, Government of Karnataka.

[Presented at the Lake 2000 seminar organised by Centre for Ecological Sciences, Indian Institute of Science, Bangalore:
http://wgbis.ces.iisc.ernet.in/energy/water/proceed/section3/paper3/section3paper3.htm]