[Explainer] What are freshwater rock pools and who lives in them?

Temporary wetlands are small and shallow aquatic habitats that, more often than not, dry annually or periodically. The period or length of time water is present within them is referred to as hydroperiod. One such ephemeral wetland is the freshwater rock pool that is beginning to garner the attention of researchers and conservationists.

Although freshwater rock pools are more common on rocky outcrops, which are areas of land with visible exposed rocks, they can be observed in most rocky biomes. They originate as depressions formed as a result of weathering of the parent rocks which then fill with water. The most distinguishing factor of these pools is that they depend on precipitation for their existence. Because they are rain-dependent, they go through cyclic phases of wetness and dryness.

They are not to be confused with rock quarry ponds or holes dug out of rocks in flood plains which are fed by groundwater, i.e., water that comes from rivers, streams, etc., and contain organisms brought in with the flow of water. Freshwater rock pools, being rain/precipitation-fed, are habitats for organisms uniquely adapted to their cyclic wet-dry conditions.

They are also not to be confused with some of the rock pools one sees on the coasts. Some of the coastal pools found on rocky shores may be rain-fed but are also served by the water brought in by waves thus being a mix of freshwater and marine habitats.

What are some of the kinds of freshwater rock pools?

The rate and pattern of the weathering process determine the size and shape of the rock pools. This weathering process is dictated by local climates, rock composition, and geological features of the rocks that endow them with varying degrees of resistance to erosion. Therefore, there are different kinds of freshwater rock pools known by different names.

Pans are the more commonly known rock pools. They are shallow with a horizontal or flat floor and vertical edges.

Many pools distributed on large rocky outcrops,  can be compared to islands. The term ‘inselbergs’ describes a cluster or group of rocky pools close to one another that become connected when the pools get filled and the excess water then flows out thus joining them. Such connected pools may also be seen in rocky streams that are formed as a result of precipitation.

What kind of organisms are found in freshwater rock pools?

The ‘hydroperiods’ or the time frame during which water remains in these formations determine the kind of flora and fauna that inhabit them. Depending on the climatic conditions, water may be present for as small a period as 15 days, says Mihir Kulkarni, a researcher who studies rock pools in the Western Ghats region. The rate at which the drying process happens dictates the kind of species found in these habitats. If the rock pool dries out before the onset of monsoon, the organisms living inside it must be prepared to survive the dry phase. The rock pools in the southern-most part of Western Ghats would be receiving water from two monsoon periods. The organisms here would be differently adapted than those that may be located at places that are in the rainshadow regions or receive water during one of the monsoons only.

Despite uncertainties of hydroperiods, researchers are finding the habitat to play host to a variety of organisms. Kulkarni says that since this habitat is only beginning to be studied in detail, it is, but natural, that species new to science are being discovered in them and will continue to do so for some years to come.

There are a number of factors that contribute to the drying of rockpools. Rocks heat up quickly.  Lack of vegetation cover, rains stopping hasten the drying up process. Fluctuating daytime temperatures lead to the environment within the pools changing frequently. This creates a dynamic situation leading to adaptions for both survival as well as reproduction, to continue their lineage. Describing these conditions, Kulkarni adds that for these reasons, organisms from other freshwater wetlands such as lakes or stream-fed ponds will not survive here. The reverse is also true says Kulkarni. These are specialists.

Giving an example of the reproductive strategy adopted by some of the fauna he says, “… in some cases the organisms have short generation times where eggs hatching to adult is completed within a week. In habitats that dry up very fast species of clam shrimps are seen to complete their life cycle much faster.  In many of these species, eggs have to dry up to hatch, hence the organisms’ reproductive cycle is tuned to the hydroperiods of the pools”. These organisms cannot survive in lakes for two reasons. Water is a constant presence in most lakes; eggs can hatch only when the environment turns dry. Moreover, the fish present in lakes will predate on these eggs. Freshwater rock pools that dry up fast do not have fish.

Survival mechanisms vary. Many organisms move to new or other pools to survive. Based on their life cycle and the strategy they employ to migrate, they may be passive and active dispersers. The passive dispersers are dependent on external factors such as the wind, overflow of water or insects to move to new pools. Some angiosperms survive as tubers buried in the soil. Others produce copious flowers and pollens and ensure that pollination ensures the continuation of lineage in other pools. The active dispersers such as the insects are present in the pool when there is water and migrate as they begin to dry up.

Some survive the dry periods through encapsulation (in capsules that will contain drops of water), or as in some Tardigrades by ‘tun’ formation. Tun is a state of suspended animation of the animals. The organisms’ limbs are invaginated and the whole body retracted to a ball-like form, where the metabolism drops down to an extremely low level. This is common among Tardigrades which can survive for years as tuns.

According to a study on freshwater rock pools, by a team of Belgian scientists, about 460 aquatic animal species have been recorded from freshwater rock pools around the world. Of these around 170 species are known to be passive dispersers.

“Freshwater rock pools can support many diverse taxa which otherwise would perish”

In order to understand the diversity and dynamics within the rock pools, the ecological services they render as well as the need to conserve them, Mongabay-India spoke with Aparna Watve, IUCN SSC Red List Authority Coordinator for Western Ghats Plant Specialist Group.

Mongabay: Greetings, Aparna. You don many hats. Ecologist, botanist and plant taxonomist, researcher-scientist and conservationist. Your special interest as an ecologist lies with rock outcrops. Such habitats are also host to small natural features such as rock pools. Please tell us what the status of the study of freshwater rock pools in India is.

Aparna Watve: I think the hat is only of a conservation scientist. All the others are part of it, including ethnobiology and social sciences, that I put to use in plant conservation. To do real good conservation action, one needs to know taxonomy, ecology, basic biology and botany and needs to know social sciences and human-nature relations because one can build conservation on all these as pillars.

Freshwater rock pools offer a very interesting ecological “system” which can be used to study basic ecological concepts – they are small enough and well defined enough and can be easily studied for flora and fauna, which makes them favourite of ecologists. Research questions such as species richness and diversity in relation to the size of the habitat, niche specificity, metapopulation are answered using rock pool as systems. The freshwater rock pools studies in India are mainly of exploratory/descriptive nature. They concentrate on describing abiotic and biotic features of rock pools and some try to understand inter-relations between them. There are a few recent works on communities of freshwater dependent taxa (diatoms, algae, branchiopoda, beetles, frogs etc.)

Mongabay: Given the transient nature of this habitat, are there many plant species to be found here? What kind of flora is unique to freshwater rock pools?

Aparna Watve: Flora of rock pools can be of two types. One, hydrophytes or aquatics and the second, water loving species, which prefer moist areas. In general, in rocky habitats, where most areas are xeric (dry), the rock pools and surroundings are preferred habitat of many different species.

Hydrophytes include phytoplankton, diatoms, algae of various kinds, aquatic ferns such as Isoetes (which is unique to rock pools) and Marsilea, Azolla which are aquatic ferns that can colonise rock pools. Aquatic angiosperms like Weisneria triandra, Aponogeton saterensis are unique to rock pools.

Fauna can include shrimps, beetles, larvae of dragonflies, damselflies, many insects, water spiders, fish, molluscs, snakes etc.

The water loving species such as Cyperus species, Cryptocoryne species are seen around the moist edges along with Lindernia, Rotala, Bonnaya, Corynandra etc.

Mongabay: Do the nature or type of rocks influence the diversity in the pools?

Aparna Watve: There has not been enough study to show if the type of rock influences the diversity of the pools. There is an indication that they might, but it is not the main influencing factor in any case. Size of pool, temperature, seasonal changes, water chemistry and anthropogenic factors interact in complex manner to influence the diversity in the pools.

Mongabay: Scientists and researchers are of the view that freshwater rock pools deserve to be treated as keystone habitats? Why so?

Aparna Watve: I haven’t really heard too many people applying the “keystone” idea to habitat. But I can understand the reason for doing so for freshwater rock pools. Their presence can increase diversity of flora and fauna in site which is not proportional to their size. Even a small pool can support large number of aquatic and water dependent species which increases the total number of taxa at the site. It is important not only for fish, amphibians but also for bees, reptiles, birds and mammals.

I remember a case in a different landscape where a freshwater rock pool within a central Indian forest had got poisoned by mistake (probably some weedicide/herbicide bottles had been washed in it). This happened in summer and when people noticed it, there were 3-4 monkeys lying dead in the vicinity and more than 10 small dead birds (bulbul, bee eaters) were found scattered in the surroundings, probably by drinking the water. Just shows that freshwater rock pools can support many diverse taxa which otherwise would perish. 

Mongabay: From an ecological and conservation perspective do you see any advantage in declaring them as keystone habitats?

Aparna Watve: At present declaring something, anything, is not of much use unless there is some way of legally protecting it. Also, it will be difficult to separate them from surrounding landscape and declare them alone as keystone habitat. A way out will be awareness and sensitisation regarding importance of maintaining freshwater rock pools free of disturbance, giving higher conservation priority to sites that have rock pools as one of the microhabitats, and ensuring that conservation management plans emphasise protecting, managing and if needed restoring rock pools.

CITATION:

MERLIJN JOCQUE, BRAM VANSCHOENWINKEL AND LUC BRENDONCK(2010): Freshwater rock pools: a review of habitat characteristics,faunal diversity and conservation value. Freshwater Biology (2010) 55, 1587–1602. doi:10.1111/j.1365-2427.2010.02402.x

Mihir R. Kulkarni, Sameer M. Padhye, Rahul B. Rathod, Yugandhar S.Shinde & Kalpana Pai (2019): Hydroperiod and species-sorting influence metacommunity composition of crustaceans in temporary rock pools in India, Inland Waters, DOI:10.1080/20442041.2018.1548868

Brendonck, L., Jocque, M., Hulsmans, A., & Vanschoenwinkel, B. (2010). Pools” on the rocks”: freshwater rock pools as model system in ecological and evolutionary research. Limnetica, 29(1), 0025-40.

Banner image: Fairy shrimps in a temporary pool in Maharashtra. Photo by Dinesh Valke/Wikimedia Commons.