Biomass density in Oussudu lake at five locations
The Oussudu watershed is situated at 11°57' North and 77°45 ' East on either side of theborder separating the Union Territory of Puducherry and the Indian state of Tamil Nadu (Figure 1). Apart from playing a crucial role inrecharging the ground water aquifers, the Oussudu watershed also harbors rich flora and fauna (Chari and Abbasi, 2000; 2002; 2005). This watershed supports Puducherry's largest inland lake Oussudu which is also called - Ousteri (a Tamil language hybrid ofOussudu and eri, meaning Oussudu lake) with a surface area of 8.026 Km2 and shore line length of 14.71 Km2.Oussudu lake is such an important wintering ground for migratory birds that it has beenidentified as one of the heritage sites by IUCN (Interactional Union for Conservation of Nature) and has been ranked among the most important wetlands of Asia (Scott 1989).
In the recent past, Oussudu lake and its watershed have been subject to enormous pressuresdue to the increasing population, industrialization and urbanization. The resultant inputs of pollutants – rich in nitrogen and phosphorous – has provided aquatic weeds an opportunity to grow uncontrollably in the lake to the exclusion of other flora. This has led to a defacing of the lake by large patches of ipomoea (Ipomoea carnia) and other weeds.
2.1. Biomass estimation
The biomass estimation was done using the total harvest method as per APHA (2005). Brassrings of 31 cm diameter and 0.5m length were used as a sampling units. These rings wereplaced at 5 representative sites (Figure 2). All the macrophytes thatwere within the circumference of the rings were then harvested, segregated, identified, packed inpolythene covers and labeled appropriately. Some of the samples included grossly decayedplant material which had become unidentifiable. Such biomass was recorded as 'mixed phytomass'.
The samples were washed under the running tap to remove the debris and silt and were placedin a cloth bag. To this bag a piece of strong thread was tied and the bag was swirled till all the excess water was removed by the centrifugal force due to the swirling action. At
this point the sampleswere weighted for theirfresh weight, also called the wet weight. The samples were then ovendried at 105° C to a constant weight, and theirdry weight was taken
The moisture content was calculated as follows:
2.2. Remote sensing and GIS
The area covered by Ipomoea was estimated using remote sensing and GIS. A satelliteimagery, IRS-ID L1SS Ill, was processed using the image processingsoftware Image Analyst 8.2and the GIS software MapInfo Professional 5.5 (Abbasi and Abbasi, 2010a). The image (Figure 1) was then classified for the land cover / land use categories as per the system adopted from Avery and Berline (1992).
The classified image was interpreted by means of visual observation (on-site verification). Five locations were chosen for biomass essay on the basis of achieving representativeness in terms of a) lake depth, b) extent of infestation, and c) proximity to population clusters.
3. Results and discussion
The dominant phytomass species at each of the five locations and the overall biomass density at each location are presented in Table 1. Lake-wise averages, computed on this basis, are presented in Table 2. This data, as well as visual observations indicate that Oussudu lake is heavily infested with Ceratophyllum demersum and Hydrillaverticillata ─ two of the world's most dominant submersed weeds. The weeds form such densemats in some parts of the lake that it is impossible to cast dragnets for capturing fishes there(Chari and Abbasi, 2005).
|Site||Depth (m)||Seechi depth (m)||Dominant macrophyte||Fresh weightg m-2||Dry weightg m-2||Moisture content (%)|
|M1||0.48||0.34||Ceratophyllum sp.||2576||3 17||87.7 %|
|Hydrilla sp.||676||74||89. 1%|
|Mixed phytornass||555||6 1||89.1%|
|M5||0.06||--||Cera tophyllum sp.||849||11 7||86.2%|
The species, Ceratophyllum, is the most widespread and present at all the sites (Table 1,Figure 3). The fresh weight of this species varies between 268 g m-2and 2576 g m-2, withan average of 999 g m-2. The dry weight varies between 31 g m-2and 317 g m-2,with anaverage of 122 g m-2 (Table 2, Figure 3). The moisture content, with respect to fresh weight,varies between 89.4% and 87.67%, with an average of 88.1% (Table 2, Figure 5).
|Phytomass species||Average fresh weight(g m-2)||Average dry weight(g m-2)||Average moisture content (%)|
Like Ranuncules, Nymphea, and Vallisneria, Ceratophyllum is known to precipitate lime. Also, this species is capable of utilizing bicarbonate ions as a source of carbon (Gupta, 1987).
The other aquatic weed, Hydrilla verticillata, is found at the sites MI and M2 (Table 1, Figure 4). The fresh weight of the species varies between 5 g m-2and 676 g m-2, with anaverage of 340 g m-2. The dry weight varies between 0.75 g m-2 and 74 g m-2, with an average of 37 g m-2(Table 2, Figure 4). The moisture content, with respect to fresh weight varies between 85.6% and 89.07%, with an average of 87.3% (Table 2, Figure 5).
Hydrilla, due to its low light compensation (10 - 12 Einsteins m-2 sec-1), is known to grow even at depths where most other plants can’t thrive in the aquatic habitats (Gupta, 1987). Indeed the spread of Hydrilla shows a positive correlation with the water depth of the lake (Figure 6).
3.1. Areal coverage
According to the remote sensing and GIS studies carried out by the authors, Ipomoea covered an area of1.16 Km2,which is as much as 14% of the water-spread of Oussudu lake. Huge islands of ipomoea can be seen at the shallower portions of the lake, presenting an unseemly sight and seriously jeopardizing the beauty and recreational value of the lake, besides exacerbating the environmental degradation of the lake as elaborated in the following section.
The presence of rampaging mats of terrestrial and aquatic weeds in Oussudu indicates that thelake is highly polluted and is, as a result, becoming eutrophic or 'obese' (Abbasi and Chari, 2008; Abbasi and Abbasi, 2010 b; Figure 7).
3.2. Impact on the lake ecosystem
Colonization of Oussudu by aquatic weeds threatens to upset the lake ecosystem in several ways. These include the following:
The thick mats of the weeds prevent sunlight from reaching the submerged flora andfauna, thereby cutting off their energy source. This situation would disfavor several species leading to dwindling of their populations and causing loss of diversity.
Once weeds colonize a water body due to pollution, they deteriorate the water qualityfurther (Abbasi and Nipaney, 1993; Abbasi and Abbasi 2000; Abbasi and Abbasi 2010c). The decaying of the weeds adds to the depletion ofdissolved oxygen, and increases the BOD, COD, nitrogen and phosphorus. This also encourages growth of various pathogens which may be harmful to humans.
The spread of weeds in the lake reduces the area available to fishes and hinders theirmobility. The depletion of dissolved oxygen may result in mass fish kills or may favor only certain kinds of fishes, (which can tolerate low oxygen levels), thereby eroding the piscian diversity.
The profuse growth of weeds breaks natural water currents. Consequently the waterbecomes stagnant, favoring the breeding of mosquitoes and other disease causing vectors.
Ipomoea is knowntogive off exudates which are toxic to certain animals and plants. The extracts of decaying leaves and rhizomes of several aquaticweeds are known for their phytotoxicity (Sankar Ganesh et al., 2008).
Weeds provide ideal habitat for the growth of molluscs, which in turn choke water supplysystems (canals and pipes) and impart undesirable taste and odour to water. Mollusks such assnails, are primary hosts to blood and liver flukesthe human disease causing pathogens.These mollusks seek shelter, multiply,and find sustenance among the roots of the weeds.
4. Remedial measures
The very high net biomass production in Oussudu lake may hasten the process of wetland-to-land succession, sounding the death-knell for the lake. Hence measures to control the weeds while at the same time blocking further ingress of pollutants in the lake are both very urgent requirements. Several methods of controlling the aquatic macrophytes have been suggested andfield-tested for their effectiveness; these have been summarized in Table 3. Ofthese methods, the one based on weed foragingby the diploid grass carp (Ctenopharyngdon idella, white amur) is the most effective at controlling the growth of aquatic macrophytes and filamentous algae (Cooke et. al., 1996). Hence, using the grass carp would not only control the aquatic weeds but also the filamentousalgae of Oussudu lake.
|Treatment(one application)||Short-term effectiveness||Long-term effectiveness||Cost||Chance of negative effects|
|Drawdown of water||G||F||E||F|
The species - C.idella - was earlier introduced by Puducherry’s Department of Fisheries inOussudu lake, but is no longer present now. The triploid variant of this species, which isgenetically derived from the diploid grass carp, would preclude any possibility of the spread ofthe species.
Apart from C. idella, Tilapia zilli and T. aurea also feed voraciously on the macrophytesand the filamentous algae. Introduction of those would help in the reduction of phytomass and speed up the recovery of the lake.
Authors thank the Ministry of Water Resources. Government ofIndia, for financial support.