The calm waters of Pashan lake in Pune city, seldom give us an idea of the hidden wonders they support. Freshwater habitats like this, harbour a wealth of biodiversity ranging from very tiny/ microscopic plants and animals to larger ones – that reside in and around the waters and depend on it for their food, reproduction and survival. These include the much larger ones like fish, amphibians like frogs, birds, reptiles, as well as tiny, often microscopic ones such as the plankton.
The term “plankton” has its roots in the Greek language, and it literally means “drifters” – referring to the many minute organisms which are carried around by water currents. Plankton comprises of both plant and animal groups referred to as phytoplankton and zooplankton respectively, and includes microscopic organisms like bacteria, some single-celled plants as well as multi-cellular animals.
Phytoplankton form the base of the aquatic food web by functioning as primary producers that also serve as food for both the tiny zooplankton and also for larger animals such as whales. What they lack in size, the plankton make up in numbers, which are crucial for maintaining the balance of freshwater ecosystem functioning.
However, urbanisation, deforestation and pollution are gradually leading to deterioration in the water quality and ecology of freshwater ecosystems in India. While restoration activities have been proposed as a solution to enhance the structure and functioning of water bodies, they can also worsen the state of the water body if done improperly and without consideration for the ecology of the water body.
Pashan lake, on the verge of death
Pashan lake seems to have met with the same fate. While the once pristine lake suffered due to urbanisation, restoration has worsened the state of the lake.
This 130-acre freshwater lake has a catchment area of 40 square kilometres, which was made by constructing a barrage on the Ram Nadi, a river that originates in Bavdhan area in Pune and flows through the city into the Mula river.
The lake met the water needs of the governer’s estate during the colonial times and fulfilled the needs of the Pashan village later. The wetland played an important role in maintaining the water tables during dry and wet periods and gradually evolved into an ecosystem of its own with dense vegetation and rare aquatic plants. Many migratory birds started visiting the lake for their feeding and temporary residence.
The fast pace of development in the 1990s leading to urbanisation, deforestation, pollution and siltation gradually led to deterioration in the water quality of the lake. This had a negative effect on the ecology of the lake and the plant and animal population residing in its waters.
Flawed restoration to blame
Considering the poor state of the lake, a restoration plan was made by the Pune Municipal Corporation (PMC) in collaboration with Naik Environment Research Institute (NERIL), Pune in 2008 under the Jawaharlal Nehru National Urban Renewal Mission (JNNURM). Restoration activities were carried out in 2004-5 and then in 2010-11, followed by beautification work around the periphery of the lake in 2013.
De-silting, creation of an artificial island, re-contouring of the lake, construction of walls and introduction of non-native fish were carried out as a part of the restoration activities that led to large scale habitat modification of the lake – drastically altering its natural shape as well as reducing the number of micro-habitats (e.g. mud flats, shallow littoral regions, natural littoral vegetation etc). These have greatly impacted the entire community of the wetland, including zooplankton, macrophytes as well as birds.
Experts have argued that the condition of the lake deteriorated further following this restoration work leading to a disturbance in the ecology of the lake and destruction of plant, animal and bird habitats around the lake.
The lake has been invaded by Pistia sp.and water hyacinth (Eichornia sp. ) and there has been a significant decrease in the number of migratory birds visiting it. While authorities blame urbanisation and increasing deposition of sewage for this deterioration, experts argue that besides these factors, flawed restoration efforts have led to further deterioration of the lake.
The struggle for survival
Deterioration in water quality has also threatened the survival of a number of large and small organisms or animal species living in the lake.
Monitoring changes in such species/ communities can help in detecting disturbances in the aquatic systems inform Mihir Kulkarni and Sameer Padhye, freshwater biologists with an expertise in studying these animals.
They share the findings of their study titled ‘Does habitat restoration disturb? A case study of a shallow urban water reservoir in western India using cladoceran zooplankton’ published in bioRxiv, a preprint server for Biology that explored the impact of these restoration activities on zooplankton, namely water fleas inhabiting Pashan lake in Pune.
What are water fleas?
Water fleas (or cladocerans) are crustaceans that include small floating or drifting animals found in oceans, seas or freshwater habitats. They cannot produce their own food and feed on phytoplankton, and/or detritus in the water for their survival. They form an important part of the food web in water bodies and larger animals such as fish depend on them for their survival. Many water fleas are sensitive to changes in the water quality of water bodies that they inhabit.
What did their study look at?
The study looked at the effects of restoration activities on the structure and function of freshwater flea communities by observing samples taken in the early and late phases of the restoration work, explain Mihir Kulkarni and Sameer Padhye. This approach allows more insights about the ecological impact as it quantifies two different aspects of the communities – diversity of species and diversity of functions.
"Species diversity (measured here as species richness) indicates the number of different species represented in the community – water fleas in this case – and is also referred to as “taxonomic diversity”. This gives an idea about the composition of the community".
"Functional diversity on the other hand, refers to the diversity in features of species (known as “traits”) which lead to different ways in which communities carry out their functions that are important for their survival and reproduction. Thus, each species in a community can have certain traits which are unique, and some which are shared with other members of the community. The higher the variation in traits, the higher the functional diversity in the community while if all members of a community share the most traits, the functional diversity of the community would be low. A low functional diversity can indicate loss of ecological functions (which could potentially cause ecosystem collapse)".
"For example, among water fleas the function of feeding – an activity which is indispensible for any organism - is carried out in different ways – some are filter feeders, which use specially modified legs to “filter” out food particles from the surrounding water, while some are scrapers, using a different modification of the legs to “scrape” food from submerged surfaces. Among filter feeders too, there is diversity in the dimensions and mode of filtration, leading to different modifications in the legs. Such diversity in function is what keeps the ecosystem running – as energy keeps moving through the food webs due to the contribution of different organisms".
"Now if due to a certain event, the scrapers are eliminated from the ecosystem, a part of the food web could collapse as there won’t be any organisms left to transfer energy from the particular region they feed in. Thus, an assessment of functional traits is important to understand the ecological health of the habitat," they explain.
What did the study find?
The study found that species richness of the water fleas (i.e. the number of different species represented in an ecological community) decreased in the later phases of the restoration work and only a small subset of the earlier observed flea communities were found to have survived the onslaught of the changes in the lake habitat.
Decrease in zooplankton species richness could have been due to structural changes in the lake topography due to destruction of the littoral zones (part of the lake near the shore) and the bottom sediments during the desilting activities undertaken as a part of the restoration process.
Destruction of bottom sediments can destroy the egg banks of zooplankton communities. While egg banks can remain viable for long periods, the most active stages of the eggs occupy the top layers of the soil, which stand the risk of being mechanically removed or capped leading to death of zooplankton communities.
The water fleas found in the early phase of the restoration process were much more functionally diverse than the late phase. Thus among the water fleas, there was more diversity in functions like food gathering techniques before restoration works finished.
This study shows evidence of diversity loss accelerated by the restoration work done on Pashan lake which resulted in deterioration of water quality and high species loss among water fleas indicating the risks to the survival of all organisms - large and small- residing in the waters of the lake and the lake ecosystem.
Threats to the lake continue to grow with increasing urbanisation and industrialisation. Urgent efforts to save/restore the lake are needed, but they should be based on realistic need assessments - informed by use of proper science and a genuine intention to bring about a change in the present condition of the lake, rather than being decided by financial or political motives!