Freshwater rock pools
Geetha Iyer
Across the world there exist within the large biomes, small spaces with unique characteristics, hosting specialized diversity of life. Such spaces or habitats as they rightly deserve to be called have hardly received any attention or recognition, except for some interested ecologists and geologists studying them. But that is changing and some of them at least have begun to be taken more seriously and studied. Through the next few articles, I introduce you to some of these specialized habitats.
Freshwater rock pools are a unique freshwater system. They occur all over the world in most biomes, especially predominant in rocky areas. They are known by various local names. South Americans call them oricangas; they are known as ‘gnammas’ in Australia, ‘lithothelms’ in Bulgaria, ‘tinajas’ in Mexico and Western USA and ‘Opferkessel’ in Germanay. Shallow rock pools with a flat base are often referred to as pans. Despite their presence worldwide they are poorly studied aquatic ecosystems. Had it not been for a visiting ecologist from UK, I would not have known the significance of, or been introduced to, the rock pools that were present in the rocky outcrops surrounding the Rishi Valley School campus.
All kinds of depression that occur in rocky substrata and contain fresh water, temporarily, permanently, or periodically are referred to as ‘freshwater rock pools’. These pools host their own unique diversity. Worldwide they are perhaps some of the oldest freshwater habitats in existence. Rock pools are precipitation dependent, i.e., they are rain-fed. The depressions in rocks (that hold water to form the pools), are formed due to weathering and erosion. The size and shape of the rock pools are dependent on the rate and pattern of weathering. The nature of the rocks will give you an idea of the age of these rock pools. Some formed on ancient mountain ranges such as the Western Ghats could be quite old while others that are carved out of sandstone or limestone hills of recent origins could be relatively young.
These are different from the small rock pools that form along the coast lines. They are also not to be confused with the different quarry ponds or pot holes in floodplains that get filled with water. Such pools are filled due to overflowing ground water or water from rivers and streams. The organisms found in such pools are brought along by the flowing water. They will not survive when these pools dry out. The freshwater rock pools are rain-fed and have organisms adapted to survive the changing conditions of the pool, including drying out.
Characteristics of rock pools
The characteristics of the rock pools including the properties of the water within are generally determined by the kind of rocks in which these pools are formed as well as the climatic conditions. For e.g., rock outcrops at various elevations in the Western Ghats are composed of different bedrocks such as laterite and basalt. Rocks in Andhra Pradesh or Madhya Pradesh and some parts of Tamil Nadu are limestone rocks. Some parts of Aravallis have sandstones, marbles, etc. So the properties that govern the ecosystem within the pools on these rocky hills will naturally be different and therefore have a different environment and support varied diversity. For e.g., sandstone and limestone rocks are easily erodible than granite rocks. Hence, the pools on such rocks face the possibility of drying out early. In general any aquatic habitat that holds water periodically will experience the cyclic wet and dry phases.
Since these are temporary wetlands, the danger of drying out is a factor to be addressed by its inhabitants. The water may be present only for a few days or may be there for the entire year. Similarly, the volume of water within these pools can fluctuate. Climatic conditions could drive the characteristics such as pH of the water, temperatures, etc. Organisms inhabiting these pools thus need to adapt to conditions ranging from desiccation to flooding, and stress tolerance due to changes in environmental conditions.
Rock pool ecosystem
The rock’s physical and chemical properties are the most important abiotic factors that influence, affect, and support the biodiversity within the pools. The local climate, seasonality, base rock, and rainfall patterns can influence water chemistry. Rock pools are oligotrophic systems that are highly sensitive to diurnal and seasonal changes. Therefore, the dynamic water chemistry influences the occurrence of various life forms.
Organisms living in rock pools are classified into two groups – active and passive dispersers, based on their life cycle and dispersal strategy. Passive dispersers stay put in the pool during dry periods or move to other pools using water, wind, or animals; whereas active dispersers don’t stay and actively move away as the pools begin to dry. They are not dependent on external agencies to move away.
Passive dispersers: Those organisms that choose to stay in the pools adopt certain strategies to combat desiccation. Certain species of Crustacea, Turbellaria, and Rotifera lay their eggs to coincide with the dry season. This is a resting stage and the eggs will hatch as soon as wetness begins. Some other species survive by forming a water filled vesicle around them. This is called encapsulation. Some species of Tardigrada form ‘tuns’. The limbs of these animals are contracted and then covered by a waxy substance on the surface that protects them from desiccation. This is called tun formation. Passive dispersers have shorter periods of development from egg to adult stage to rapidly colonize the pools unlike active dispersers that take a longer time to mature. As in all groups there are always exceptions to such rules.
Active dispersers: These are present in the rock pools during the wet phase and begin migrating before the pools dry out. They are generally insects that can fly in search of other rock pools. Some exceptions to this have also been noticed. Some of the larvae of active dispersers such as certain species of midges remain in a state of diapause to overcome the dry phase. Some like the Aedes vittatus produce desiccation resistant eggs to survive. Some dragon flies adopt risky strategies to maximize their survival. Variously called the globe skimmer, globe wanderer, or wandering glider, Pantellaflavescens, lay their eggs randomly in different pools that may be healthy or unhealthy, hoping to increase the chances of survival. The scarlet rock glider, Trithemiskirbyi, selects large and deep pools with a long wet period to lay its eggs. Some species of midges also select similar pools.
Reproductive success is very important for these rock pool inhabitants. Therefore, pool selection for laying eggs is important. Pools that may contain egg predators are generally avoided. This is why the strategy to choose random pools by some active dispersers is a risk-filled one.
Rock pools from India
A study of physicochemical properties of the rock pools of the Western Ghats showed that factors such as pH and conductivity followed the global pattern. It also showed that the water chemistry of the Western Ghats rock pools was mainly controlled by rock dominance and precipitation, resulting in high salinity with a moderate level of conductivity. Studies here indicated that the water quality of the rock pools was favourable for aquatic life. Diversity studies on rock pools of the Western Ghats have documented many rare as well as novel species of plants. Around 460 species of aquatic faunal species have been recorded from rock pools around the world. High degree of endemism was noticeable. Nearly 170 species are known to disperse from one pool to another as one means of escaping desiccation.
Rock pools are oligotrophic habitats, i.e., habitats poor in nutrient content. Researchers working in Panchgani have reported that these pools hold water anywhere between 15 days to four or five months. The fauna that survive here are special and adapted to withstand desiccation. They are mostly invertebrates such as crustacean, tardigrades, and some insects.
Shruti Paripatyadar, a Pune-based entomologist and her colleagues discovered 13 new species of microcrustaceans some of which were seen only in one rock pool. They also discovered flightless forms of insect species that normally would have wings. The environmental conditions have thus lead to the assemblages of organisms, both flora and fauna, unique to the pools and not found elsewhere.
Conservation
Rock pools are found on rocky outcrops, which are not seen as productive habitats by most people. Some communities in Chaukul village near Amboli, Maharashtra, have been conserving rocky outcrops for centuries. They are important for the sustenance of the local communities; but it is often not taken seriously as they are unknown habitats, invisible to the general public since there is both the absence of data and public awareness.
Despite the presence of high biodiversity and endemism these temporary pools are highly threatened in India. Inselbergs and koppjes are common in South India, where one can find many rock pools. Why are they not conserved? Because they are considered as wastelands. Because no funding is easily available to scientists for studying them. With data being deficient, it is but natural that their importance is never appreciated or understood. The 2010 Wastelands Atlas of India by the National Remote Sensing Centre and Ministry of Rural Development documented the rocky plateaus with high biodiversity value in Kolhapur, Pune, Satara, Ratnagiri, and Sindhudurg districts, as ‘barren rocky/stony waste areas’. So they are chosen for setting up windfarms for mining and other such ‘popular’ money spinning projects. With climate change wreaking destruction, how many pools or species in these rock pools will survive is a question that has no answer.
Although small in size, scientists believe that rock pools influence, in ways yet to be documented completely, the environment of the larger biomes in which they are found. Researchers are of the opinion that these pools deserve to be regarded as keystone species or keystone habitats.
References
- India’s Rocky Outcrops and Their Tiny, Mighty Lives. The Wire 2/05/2021. https://science.thewire.in/environment/indias-rocky-outcrops-and-their-tiny-mighty-lives/
- Rocky Outcrops: An ecosystem being erased before its completely discovered. https://www.downtoearth.org.in/news/water/rocky-outcrops-an-ecosystem-being-erased-before-it-s-completely-discovered-64009
- Rogers, D. C. & Padhye, S. M. 2014 A new species of Streptocephalus (Crustacea: Anostraca: Streptocephalidae) from the Western Ghats, India, with a key to the Asian species. Zootaxa 3802, 75-84. (Fairy shrimps (Streptocephalus sahyadriensis) find a home in temporary rock pools on plateau of Western Ghats. Times of India. https://timesofindia.indiatimes.com/city/pune/fairy-shrimps-find-a-home-in-temporary-rock-pools-on-plateaus-of-western-ghats/articleshow/73219130.cms?frmapp=yes&from=mdr)
- Shinde, Y.S., R. Victor & K. Pai (2014). Freshwater ostracods (Crustacea: Ostracoda) of the plateaus of the northern Western Ghats, India. Journal of Threatened Taxa 6(4): 5667-5670; http://dx.doi.org/10.11609/JoTT.o3610.5667-70
- Aboli Kulkarni et al. (2021). Vanishing waters: water chemistry of temporary rock pools of the Western Ghats, India. Water Practice & Technology Vol 00 No 0, 1 doi: 10.2166/wpt. 2021.107
- Gaikwad, S. P., Sardesai, M. M. & Yadav, S. R. 2014 Rotalasahyadrica sp. nov. (Lythraceae) from western ghats, India. Nordic Journal of Botany 32(5), 575-577.
The author is a consultant for science and environment education. She can be reached at scopsowl@gmail.com.
