Tiny critters: Invisible heroes of freshwater ecosystems

While we all love to take a stroll around water bodies such as rivers, lakes, ponds, streams and enjoy watching birds swooping in to catch fish or ducks wading peacefully through the waters, did you know that these waters teem with not only large, but even small animals - unseen to the naked eye, but playing an extremely crucial role in the survival of these water bodies?

These tiny critters are remarkably resilient. They can survive not only in abundant waters, but also in small drops of water, tolerate dry spells and bounce back to life during the monsoons. 

Dr Sameer Padhye, a freshwater biologist, tells the India Water Portal more about them and about his recent work on these interesting organisms.

Can you tell us a bit about freshwater bodies and the tiny critters that live in them?

Freshwater bodies are extremely biodiverse and support tiny/ microscopic animals to larger ones – that reside in and around the waters and depend on them for their food, reproduction and survival. 

These include larger ones like fish, amphibians like frogs, birds, reptiles, and microscopic ones such as zooplankton -with incredibly smart jobs that aid the very survival of other freshwater ecosystems. 

Zooplankton are a type of plankton - which include many small sized organisms (zooplankton) or plants (phytoplankton) that typically float in the water or are carried around or wander around using water currents in the water bodies and are ubiquitous, found in freshwater as well as marine waters. In fact, even a single drop of water can harbour thousands of plankton. 

 Zooplankton use a variety of feeding strategies. For example, some like Cladocera (water fleas), are primarily grazers and use their feeding appendages to filter particles from the water while some are scrapers, using a different modification of the legs to “scrape” food from submerged surfaces. Others, such as  Copepods, are selective feeders and/or predators and pick small particles or prey based on their size, shape and taste.

Could you describe a few of them for us? How do they help in maintaining the health of freshwater ecosystems? Why are they so important ?

There are many groups which come under this ‘smaller organisms’ category such as  protozoans - single celled animals that use organic carbon as a source of energy; multicellular animals like flat-worms, nematodes - mostly parasites; tardigrades commonly referred to as water bears or moss piglets , hydrozoans (hydra), rotifers, gastrotrichs and micro-crustaceans (cladocerans, copepods, ostracods) to name a few.

These animals are very important as they play a crucial role in the food chains by acting as prey for predators like fish. For example, primary producers like phytoplankton trap the sun’s energy to produce food and survive and are eaten by smaller primary consumers like rotifers and micro-crustaceans like cladocerans (water fleas) and copepods (mostly parasites like fish lice); these are preyed upon by planktivorous fish and insects which themselves are subsequently preyed upon by fish and birds.

A major shift in the biodiversity of these animals can disrupt these food chains which themselves are part of more complex ‘food webs’ and result in an eventual ecological degradation of water bodies threatening their survival and affecting health and livelihoods of humans depending on them.

Studying these changes is cheaper and faster in many instances and provides information on how organisms react to pollution. For example, our studies on the impact of water pollution on water fleas in the highly polluted river Mutha in Pune found that many of the zooplankton species could not survive the high levels of pollution in the river while some water fleas species turned red due to haemoglobin production in response to low oxygen levels in the water indicating high nutrient content in the river waters.. Similarly,  the gradual deterioration of Pashan lake quality in Pune due to faulty restoration practices led to a reduction in species richness of water fleas inhabiting the lake.

Read our findings here:

Water fleas see red as river Mula Mutha in Pune chokes

Fleas flee the Pashan lake in Pune

Could you please describe the various types of freshwater ecosystems that support them? What did your recent study from the Western Ghats find?

Surface freshwater ecosystems not only include rivers, lakes, streams, but also  ponds, pools as well as certain unique types like phytotelma (water filled spaces enclosed by plants) that exist throughout the world. Freshwater ecosystems are classified based on a number of factors including size (large like lakes or small like pools), water flow (static/lentic like ponds or flowing/lotic like streams), permanence (temporary like pools or permanent like water reservoirs) amongst others. 

Each water body in itself is divided into specific zones based on factors like temperature, light penetration, presence of aquatic vegetation, geochemical properties (pH, Dissolved oxygen) etc. For example, the littoral zone (vegetated zone) adjacent the shore of many water bodies receives a lot of sunlight, is warm, nutritionally rich and supports a range of animals including phytoplankton, zooplankton and fish. In contrast, the deep water zone (benthic zone) is cooler, receives little or no sunlight and has no aquatic vegetation yet certain crustaceans, molluscans and fishes are found inhabiting it.

Members of certain invertebrate groups like ostracod crustaceans (seed shrimps) and tardigrades (water bears) are known to live in phytotelma amongst other habitats.

Our work in the Western Ghats has revealed some very interesting results. Only a small proportion of these small organisms are known to adapt to terrestrial habitats that experience alternate periods of moist and dry spells and can survive when they remain away from surface water bodies. We have found a waterflea species that has adapted to living on land and can be found to crawl in thin water films that accumulate on mosses growing in the monsoon season! This shows the remarkable resilience and fluidity that these small animals demonstrate by being able to survive even in a drop of water. 

We have shared these findings in our recent paper published in the Journal of Crustacean Biology.

What are the threats to their existence? How will that impact biodiversity and water resources in the future?

We have personally observed many such habitats getting destroyed which had a good number of aquatic biodiversity. Some of the animals also lay their eggs in these habitats in order to survive the dry season and hatch in the next monsoon season. This often leads to the formation of ‘egg banks’ (like seed banks), holding a hidden diversity. Destruction of habitats therefore destroys this unseen diversity. 

Modification/degradation of any water body influences the diversity patterns of these animals. Many rare species are lost with increasing habitat deterioration. Since these animals are part of many aquatic food chains, a continual species loss can result in an eventual collapse of such food chains. This is evident in cases like the Pashan lake in Pune, an urban water reservoir which has been ‘beautified’ haphazardly, leading to its near complete ecological breakdown. Similarly, increasing disturbance along the rivers flowing through the urban centers like Pune has negatively impacted the riverine biodiversity.

We have to consider the importance of these organisms and the associated ecology before making more mistakes which are already affecting the way we live. Sadly, these small animals do not get as much attention as some other groups, but they are equally (and at times better) in indicating deteriorating environments. 

Dr Sameer Padhye currently works as a Research Associate at the Centre for Biodiversity Genomics, University of Guelph, Guelph ON, Canada. He can be contacted at sameer.m.padhye@gmail.com.