Water fleas are small crustaceans widely found in varied aquatic habitats. They are very sensitive to changes in the water quality of water bodies such as rivers and streams that they inhabit.

Dr Sameer Padhye, a freshwater biologist, talks to the India Water Portal on his study titled ‘Seasonal variation in functional composition and diversity of cladoceran zooplankton of a lotic eutrophic habitat from India’ published in the journal Annales de Limnologie - International Journal of Limnology that explores how these microscopic animals can greatly help in monitoring pollution levels and improving river health.

While freshwater ecosystems such as rivers and streams continue to be threatened world over due to pollution caused by human activities such as agriculture, urbanisation, industries etc, what is the state of rivers in India?

While rivers in India harbour incredible biodiversity, most of them are highly polluted due to large amounts of untreated sewage and industrial and agricultural wastes being released into its waters. This river pollution is increasingly posing a threat to river biodiversity. All the available data points to a decrease in species richness (number of species found in an ecosystem) and diversity due to increasing pollution.

At the same time, there isn’t enough information available for many freshwater plant and animal groups to give a generalised account. The environmental data does show the impact of urbanisation contributing to the degradation of water quality. Additional information on factors such as invasive species, habitat modification and its effect on the biodiversity needs to be explored in much detail.

How does pollution affect plants and animals in freshwater ecosystems such as rivers?

Pollution affects the plant and animal life at multiple levels of the aquatic food web (food cycle). It can affect by specifically removing certain elements from the food chains thereby altering the structure of food webs (food cycle).

This removal can be due to increase in the values of environmental variables like salinity and pH which exceed the physiological limits of organisms; competition by invasive species or predation by invasive species and/or removal of certain microhabitats. These factors usually contribute in combination.

What are the ways in which water pollution in rivers can be measured/detected? Why focus on biological communities or organisms living in the water?

Conventional methods on pollution monitoring focus on assessing the chemical load by using different techniques. These are highly effective and accurate, but can be relatively expensive and time consuming.

Biological communities or species living in the water also respond to changes in the local environment and hence monitoring changes in such species/ communities can also help in detecting disturbances in the aquatic systems. Studying these changes is cheaper and faster in many instances and provides information on how organisms react to pollution.

A holistic understanding of the damage caused by pollution can be obtained using a combination of physical, chemical and biological methods to assess changes to the habitats and the species affected by it.

What did your study aim at? Why did you decide to focus on river Mula in Pune?

My aim was to explore how a group of microscopic animals such as water fleas can help in assessing the health of a river. In a previous study, I had seen that seasonal changes in the diversity patterns of water fleas were indicative of eutrophication and hence I decided to explore this in a little more detail. Studying changes in certain aspects of the morphology and/or behaviour of water fleas with respect to their occurrence can also point to changes or modifications and degradation of habitats and food webs.

The study focused more on the functional diversity i.e. on the role or function of the species in their habitats rather than their identity, like their food gathering mechanism and fecundity (egg carrying capacity), which is now used more and more in field of ecology. One of my objectives also was to see if this additional level of diversity could point to degradation in the context of our rivers as well.

The Mula river, that runs through the highly urbanised Pune city has been severely modified/degraded along its stretches due to many reasons like destruction of microhabitats (removing submerged vegetation), ineffective management of sewage and other forms of garbage, high presence of industrial and agricultural effluents, high load of invasive predator species etc. These reasons made it a perfect model system to study.

Why did you focus on water fleas? Could you please describe them for us.

Water fleas or cladocerans are small crustaceans with the biggest animals being only a few millimeters in length. These animals are primarily filter feeding animals i.e. they feed on small algae by filtering them from water. These animals form a component of 'zooplankton' which consists of group of small/microscopic animals commonly found in various aquatic environments acting as primary consumers (i.e. food for animals like fish which then become food for birds) in aquatic ecosystems.

There is ample literature available which shows that these animals can be used as model systems in various biomonitoring and/or ecological studies to assess how environment affects species/communities. The best example is Daphnia, (a type of a water flea) which is a model organism in eco-toxicity tests.

What were the main findings of the study?

The study found that the variety of water flea species found in the river decreased considerably during peak summer months. Groups of water flea species performing similar functions also decreased during peak summer months with some being totally absent in the peak of summer.

The study used functional redundancy as an indicator, which can be defined as the number of species (water flea species in this study) performing similar roles found in an ecosystem that help or contribute to the functioning of the ecosystem, like for example top down control of algae.

Higher values of redundancy point to presence of many species performing similar functions that help the ecosystem thus leading to long-term stability of the ecosystem. It can also mean that, in case of any specific species loss, other species performing the same function can take their place.  Lower values of functional redundancy can be because of reduction in number of species performing similar functions that help the ecosystem due to high pollution levels, which can threaten the survival of the organism and the ecosystem in the long run.

The study found that functional redundancy decreased during peak of summer when chemical parameters like Dissolved oxygen were very less. The same phase also had very turbid waters, high nitrate values suggesting eutrophication pointing to a disturbed habitat.

How do you think will the study help in improving river health in the future?

The study aimed at exploring the suitability of new analytical techniques based on monitoring of biological organisms in understanding changes occurring in freshwater habitats such as rivers. The study found that observation of some functional characteristics of water fleas greatly helped in determining the status of river health.

These techniques are not restricted to specific animals and plants and can easily be adapted to any group of plants and animals. A comprehensive study involving many groups of plants and animals and using their functional characters to check how they can point to changes in pollution can be the way to move forward. This can help us understand what is happening to our rivers at different levels of food chains, in present times.

The paper can be accessed here

Dr Sameer Padhye currently works for Biologia Life Science LLP, Ahmednagar, Maharashtra, India. His research focuses on understanding the taxonomy and diversity of branchiopod crustaceans of India and on studying the biodiversity dynamics of certain freshwater invertebrates in eutrophicated water systems. He can be contacted on sameer.m.padhye@gmail.com