INTRODUCTION

Macrophytes grow in or near water and are emergent, submergent, or floating, forming a vital component of lake ecosystems. However, the introduction of invasive-exotic species such as water hyacinth (Eichhornia crassipes), alligator weed (Alternanthera phyloxiroides), water lettuce (Pistia stratiotes) etc. have changed the lake dynamics significantly. In recent times the urban waterbodies are being used for the disposal of sewage, etc. Sustained inflow beyond the assimilative capacity of waterbodies has lead to eutrophication, resulting in the profuse growth and spread of invasive species. Influx of partially treated and untreated sewage has resulted in overgrowth, ageing, and subsequent decay of macrophytes creating anoxic conditions and devouring the system from life giving oxygen. This has impacted the food chain and hence the ecological integrity of the system.

Water hyacinth (Eichhornia crassipes) native to Brazil has been introduced to tropical and subtropical region [1] is amongst the fastest growing, free floating freshwater invasive weed species which derives required nutrients directly from water. Its distribution and dispersal is aided by water currents and wind. It consists of 5% dry matter with 50% silica, 30% potassium, 15% nitrogen and 5% protein [2]. Its potential negative characteristics pose a threat for the habitat quality of waterbodies. The average growth rate of water hyacinth is 10-12 g/m²d and the maximum is 45-50 g/m²/d [3, 4, 5]. During growth, water hyacinth can store N up to 909 g/m² [6]. These invasive aquatic plants form a thick ‘mat’ that restricts the exchange of oxygen across the air/water interface and also hampers algal photosynthesis resulting in reduced dissolved oxygen. The anoxic conditions under water hyacinth mats also favour the release of nitrogen and phosphorous (N and P) from sediments which may further aid the rapid growth of macrophytes [7, 8, 9]. In addition, it influences the wind-driven water movement, impeding circulation of oxygen-rich surface water [10]. Bank side grasses grow over the water hyacinth mats, anchoring the mats to the bank edges. Varieties of grasses and sedges as Cyperus and in some instances, plants like Colocasia esculenta (taro), etc. have established themselves on these mats. Once established, very large flows are required to break them up and disperse.

The southwest monsoon winds tend to push the floating macrophytes over spillways of lakes situated on their south-eastern, eastern and southwestern edges, thereby ridding the water surface free of macrophytes each year.  This natural flushing of macrophytes during monsoon associated with the phenological events was considered to be the most important short-term process for cleaning urban lakes. The macrophytes in their matured stage are infested by the mottled water hyacinth weevil that reduces about 75% of the leaf surface areas in 2-3 weeks, consequently resulting in loss of the major photosynthesizing machinery i.e. the leaves  and greatly helps in compacting the water hyacinth mass, as they also disrupt the long, spongy and bulbous stalk tissues, the plants lose their buoyancy and settles faster which is followed by leaching of plant nutrients and subsequently rapid bacterial degradation takes place which reduces the DO levels significantly and creates anaerobic conditions throughout the lake. Thus this process submerges a large quantity of organic matter which ultimately decomposes, increasing the biochemical oxygen demand (BOD) that deteriorates water quality. Dissolved oxygen falls to such low level that leads to massive fish kills [11].

Oxygen is amongst the most important of several dissolved gases vital to aquatic life. It is a principal and direct indicator of water quality in surface waters. Primary source of oxygen in surface water is from photosynthesis of aquatic plants, algae and diffusion of atmospheric oxygen across the air water interface. The dissolved oxygen content of natural water varies with the temperature, photosynthetic activities and respiration or decomposition of plants and animals [12]. On a daily basis they maintain equilibrium as per the consumption and production. The diurnal oxygen cycle varies in a sinusoidal manner with minimum values observed early in the morning and maximum concentrations at midday [13]. A decline in DO has serious implications on the health of the aquatic system, as hypoxic and anoxic conditions reduce or eliminate sensitive native fish and invertebrate species.

During aerobic decomposition, cellulosic materials are converted into carbon dioxide and water by the bacterial action. CO₂ in the dissolved form maintains equilibrium with its carbonate and bicarbonate forms and decides the C supply for the algae and aids in photosynthesis bringing manifold increase in the primary productivity of the system. Oxygen level of the waterbodies are reduced by continuous inflow of sewage, containing large loads of organic carbon, phosphates and nitrates that finally lead to profuse growth and spread of aquatic biota. Under such circumstances, aquatic plants and algae proliferate incredibly and when they die they form food for bacteria, which in turn multiply and use large quantities of dissolved oxygen. In addition to this, when plant biomass increases at the surface of the water (pelagic zone) they block transmittance of sunlight into deeper layers and diffusion of oxygen from the atmosphere into the water, thereby, reducing photosynthetic potential of submerged plants and algal species. In addition to this, their extensive root system in the water provides a large surface area for the growth of microbes which rapidly consume DO [14]. These microbes render the system more anoxic by carrying out the anaerobic digestion on a myriad of substrates. Moreover, under anoxic conditions, ammonia, iron, manganese and hydrogen sulphide concentrations can rise to levels deleterious to biota. In addition, phosphate and ammonium are released into the water from anoxic sediments further enriching the ecosystem [15].

Varthur lake, situated in the south of Bangalore, was built to store water for drinking and irrigation purposes [16].  However, over the last five decades, due to sustained influx of sewage, nutrients in the lake are now well over safe limits. Sewage brings in large quantities of C, N and P which are trapped within the system. Thislake receives about 40% of the city sewage (c.500million liters per day, MLD) resulting in eutrophication. There have been substantial algal blooms, dissolved oxygen depletion and malodour generation, apart from extensive growth and spread of water hyacinth that covers about 85% of the lake during the dry season.

Water hyacinth mats greatly reduces DO content in water under the mats [17, 8, 9] affecting aquatic diversity and productivity. Decomposition of macrophytes happens due to ageing, over-crowding, wind driven compaction, pest damage, etc. During oxidation,  microflora  utilize  detritus  C  as  an  energy  source  and reduces electron  acceptors  such  as oxygen, nitrate  and  sulphate [18]. Water hyacinth litter breaks down as a result of aerobic, anaerobic and facultative anaerobic microbial activity [19]. Bacteria accentuate degradation process and fungal decomposition under such conditions is negligible [20]. O₂ concentrations in water play an important role in the release and transformations of nutrients [21].

This paper focuses on the impact of wind induced drift, its removal during monsoon, and its rapid growth which governs the aerobic-anaerobic status of the lake and thereby brings out its relation with the water quality. The objectives of the study were to: 

1. Determine the major contributor of the BOD load that disrupts the lake’s functioning ,
2. Map oxic, hypoxic and anoxic zones based on DO levels and to understand the influence of wind induced drift of macrophytes on seasonal water quality changes and
3. Quantify nutrient loads (C and N) and their uptake by macrophytes.