11 Freshwater Biodiversity: Spatial Patterns

Dr. Felix Bast

1.      Learning outcomes

1.1     To learn the overview and importance of freshwater biodiversity

1.2     To learn about various ecosystem goods and services provided by the freshwater biodiversity

1.3     To learn about zonal classification of freshwater biodiversity

1.4     To learn about various lentic, lotic, aquifer and glacier systems of the world

1.5     To learn about threats and challenges that are being faced by freshwater biodiversity

2.    Concept map

3.      Description

3.1. Introduction

Freshwater is defined as water with salinity less than 0.05% or 0.5 PPT (parts per thousand; i.e., 0.5 grams per kg). Salinity between 0.5 PPT to 30 PPT is termed brackish water, which is found in estuaries, or river mouths. Typical salinity of seawater is 31 to 38 PPT. Out of the total aquatic resources of planet earth freshwater resources constitute merely 0.01%. Out of the total freshwater, more than 98% is stored in massive ice sheets of Antarctica, and Greenland, so the available freshwater for the sustenance of organism that are dependent on it is extremely scarce. Freshwater encompass merely 0.8% of the surface of planet earth. Yet, freshwater support 100,000 species out of 1.8 million described species, which is 6% of the overall biodiversity. Forty percent of global fish biodiversity occur in freshwater habitats. Approximately one thirds of all vertebrate species depend directly on the freshwater resources for its life, demonstrating the key role played by freshwater for the support of biodiversity. Freshwater ecosystems, also known as limnetic ecosystem, can be classified into either lentic biomes (encompassing stagnant waters, such as ponds, lakes etc) or lotic biomes (encompassing running waters such as streams, rivers etc).

3.2. Ecosystem goods and services provided by the freshwater biodiversity

Freshwater biodiversity provides a number of tangible goods and intangible services to the humanity. Freshwater fisheries is an important industry providing food and employment to a large number of human population. In addition, freshwater habitats houses a number of unique life forms the value of which might not be tangible at present, waiting to be revealed in the future. The existence value as well as bequest value as discussed in the case of marine biodiversity are also applicable here. The genetic resources of freshwater biodiversity might have potential applications in human wellbeing including agriculture and pharma industries. Earlier days wetlands where considered to be wasteland, with government providing free hand for its reclamation (habitat alteration), leading to tremendous loss of freshwater biodiversity. However, the intangible importance of the rich biodiversity offered by wetlands and other freshwater habitats are revealing itself to be highly valuable. One estimate for the value of freshwater biodiversity suggest USD 6579 x 109 per year, which exceeded value of all other non-marine biodiversity combined. Freshwater resources support potable water for human consumption and almost the entire realm of agriculture, and therefore plays a crucial role in the sustenance of human beings. Freshwater ecosystems also have crucial importance for human health, as waterborne illnesses prevalent in tropics (where it constitutes 80% of all illnesses) are directly linked with the health of freshwater ecosystems.

3.3. Major limnetic systems

Limnetic systems of the world can be grouped into lentic or lotic systems.

3.3.1.    Lentic systems

Lentic ecosystems of the world encompass stagnant freshwater and associated biodiversity. Examples include lacustrine habitat (habitat of freshwater lakes), ponds and wetlands. Lacustrine and other lentic habitats can further be divided into various ecological zones, as illustrated in Fig. 1.

Figure 1. Various ecological zones and habitats of lentic or lacustrine ecosystem

The shallow region of stagnant water nearest to the shore is called littoral zone. This zone is inhabited by various primary producers including algae, moss, aquatic plants like water lillies, Hydrilla, Water hyacinth, Lotus etc., snail, tortoise, ducks, swans, snakes, fungi, viruses and so on. Littoral zone, due to its shallow nature, is the most speciose in a lacustrine ecosystem, and is usually entirely photic (the light could penetrate till the bottom), supporting benthic algae, periphyton (complex mixture of algae, cyanobacteria, bacteria, detritus attached to submerged surfaces) and aquatic plants. However, during rains, the littoral zone receive abundant runoff with sediments, increasing the turbidity of the water column thereby limiting the extent of photic zone.

Region further towards the centre of lentic system is called limnetic zone. As the limnetic zone is deeper and is not directly in contact with the shore, it has a unique ecosystem and is generally less speciose than the littoral counterpart is. Surface layers encompass algae and other phytoplanktons, the main primary producers. Most of the lentic fish species lives in the limnetic zone. Invertebrate animal species including small planktonic crustaceans (like Daphnia), freshwater copepods (like Cyclops) and decapod crustaceans (like Shrimps) encompass the major limnetic fauna other than various species of fish. In addition, various insects lives on the surface of lentic system, including caddleflies, mayflies and so on. Limnetic zone typically have a very high diversity of microflora encompassing protists, bacteria, fungi and viruses.

The third zone of lentic system encompass bottom layers towards the benthic zone underneath the limnetic zone, called profundal zone. This zone is comparatively cooler year round, dimly-lit and houses various heterotrophs and scavengers. Biodiversity of profundal zone include crustacean species, crabs, eels, snails, turtle, and various species of bacteria, fungi, protozoa and viruses. As the zone is dimly-lit, an adaptation for the animals occupying this zone is a preference for olfactory and auditory senses over that of vision.

Many organisms transit across these zones though, including salamander, crocodiles and tadpoles, spending parts of their life history in various zones. Also, note that lentic systems in higher latitudes freezes during winter, virtually killing all life forms. Generally, the freezing starts from the periphery (littoral zones) towards the centre (limnetic zone) and finally towards the profundal zone. In case of massive lacustrine ecosystems, profundal zone right in the centre of lakes never completely freezes owing to the geothermal fluxes of rock at the bottom of lakes. This is the case with landlocked lakes of Antarctic oases; Priyadarshini Lake in the vicinity of India’s Maitri Station for instance.

Inland wetlands are non-permanent freshwater systems, a very important lentic ecosystem with very high diversity of species. Wetlands can further be classified as marshes, where there is no tree species, swamps where tree species exist and bogs where certain plant species exudes acidic secretions that slows down the decomposition of detritus, with a characteristic ‘unpleasant’ smell.

3.3.2.    Lotic systems

Lotic systems encompass freshwater habitats with running waters; examples include streams and rivers. Streams are typically narrow, percolating through rocks and pebbles in the mountains. Streams and run off drains joins to form rivers, which are large bodies of freshwater wider and longer than either streams or drains. Riverine ecosystem can be divided into various zones, including source zone where the river is originated- the catchment area, transition zone where the river flows at a high velocity down the mountain, floodplain zone- where river slows down and is low lying forming delta around, and finally the river mouth where it drains into a larger body of water, typically the ocean. Rivers and streams have unique ecosystems thriving along its banks, referred as riparian zone encompassing herbaceous perennials, shrubs and trees as typical vegetation.

3.3.3.   Aquifers

Subsurface stored water, the groundwater or aquifer, is a special type of limnetic system, which is traditionally neither classified into lentic nor lotic. Apart from serving as the predominant source of potable water for human consumption, the aquifers are revealing itself to be an inconspicuous subterranean freshwater ecosystem with a rich biodiversity, estimated to be around 1,00,000 species. Unique darkness-adapted organisms include a large varieties of chemoautotrophic bacteria, minute arthropod animals called Stygobites (or Stygobionts, taxa completing its entire lifecycle in subterranean groundwaters) such as Niphargus aquilex, Peracarida, Prosobranchia, Syncarida, Ostracoda, tricladida, Acari, Temnocephalida and so on.

3.3.4.    Glaciers and Ice Sheets

These are yet another, often-overlooked limnetic ecosystems, although the biodiversity is very low. Antarctic ice sheets contain more than 98% of the freshwater stock of the planet and are typically a kilometre or more thick from the rock bottom. Biodiversity encompass various ice and snow algae including the pink ice algae Chlamydomonas nivalis, cyanobacterial mats, psychrophilic bacteria and so on. In addition, recent research have revealed the existence of a number of subglacial lakes underneath the Antarctic ice sheets with a rich limnetic biodiversity of microbes. These subglacial lakes had also been demonstrated to be interlinked through a network of subglacial riverine systems; biodiversity of which remain to be described.

Based on nutrient load, limnetic systems can be grouped as oligotrophic or eutrophic habitats. Oligotrophic habitats have typically very low nutrients. Glacial meltwater lakes of Antarctica is an example of such system. In contrast, eutrophic limnetic systems have higher nutrients sustaining rich flora of freshwater algae. Excessive nutrient load of limnetic system lead to phenomenon of eutrophication where massive blooms of algae drastically reduces dissolved oxygen concentrations, leading to hypoxia and death of all other aerobic life.

3.4. Threats to limnetic biodiversity

Major freshwater ecosystems of the world are extremely vulnerable to the activities of human beings. This is due to the several unique features of freshwater systems comparing with that of marine or terrestrial systems. Most of the lentic systems are fragmented and less voluminous, leading to a fragmented network of discontinuous and unique habitats housing extremely rich biodiversity with a number of endemic species. Fragmented lentic systems also have low gene flow (low exchange of genes between divulging populations) and inter-habitat variation (variation between different habitats) leading to local radiation (rapid diversification of species and morphological forms at local level). Situation can be compared that of island biogeography where unconnected isolated islands usually have unique and rich biodiversity with several endemic species. Endemic species are extremely vulnerable to the threats from species invasion and other anthropogenic stresses. Most of the lentic systems, such as lakes and ponds are situated at low-lying valleys. This unique landscape feature makes these systems extremely vulnerable to pollution, as a receiver of run-off water and associated sediments, nutrients and pollutants from nearby areas.

Major threats to the freshwater marine biodiversity are overexploitation, pollution, flow modification, destruction/degradation of habitats, and invasion by exotic species. In addition, a number of global threats including nitrogen deposition, global warming and shifts in precipitation patterns affect freshwater habitats and its biodiversity (Fig. 1). Overexploitation is the major threat for almost the entire vertebrate component of aquatic biodiversity, including freshwater fish species, amphibians, reptiles and mammals. Due to over fishing and unsustainable fishing practices (like trawler fishing using small net size such that juvenile stages of fishes were also being caught, or fishing during the spawning stage of life history), world’s freshwater fish species are rapidly diminishing. Several of the world’s freshwater crocodile species are already listed as threatened or endangered in IUCN’s red data book. Freshwater mammals including Gangetic river dolphins in River Ganges, and Yangtze river dolphin in china are under severe threats from human exploitation. A recent estimate suggest that amphibians including frog and toad species are the most threatened among the whole freshwater biodiversity; at least 32% of amphibian species are under the verge of extinction.

Figure 1. Major threats to freshwater biodiversity

Another major threat is water pollution. As humans are directly dependent on freshwater for potable use, vast majority of human population live on the proximity of freshwater resources, and almost all of the world’s largest cities were established on the banks of some of the largest rivers and lakes of the world. These freshwater habitats receive tremendous amount of drainage from domestic and industrial sources, adding pollutant and nutrient load of these habitats. Lentic systems like ponds and lakes are more vulnerable to the pollution, as these are static, the pollutants persist there for a long period of time. Nutrient load lead to the phenomenon of eutrophication where explosive growth of certain algal species lead to the depletion of dissolved oxygen from the water, ultimately killing almost the entire aerobic life forms. Pollution by surfactants and froth-producing algal species is a tremendous problem at well-known Indian freshwater sites including River Yamuna, and Lake Ulsoor in Bangalore. The threat of flow modification is prominent in world’s lotic systems, with majority of longest and largest rivers of the world. Construction of dams for massive hydroelectric or irrigation projects would lead to the fragmentation of aquatic system, with no exchange of biota between the sides of the barrier. Several of the fish species have longitudinal migration as part of its life history; it need to swim all the way to ocean to complete its life cycle. With the flow modification, its life cycle would be incomplete, leading to the species extinction. Water impoundment by dams is indeed a global threat to the freshwater biodiversity; a recent estimate suggest that the volume of water under retention by world’s dams amount to five times that of the rivers. A related issue is construction of shipping canals (for example, Panama Canal and Suez canal); it would bring salt water towards more interiors of landmass. Saltwater intrusion to the existing freshwater systems lead to the collapse of entire ecosystems, as the living forms in freshwater cant eliminate excess salt from their body if ambient water salinity rises. Destruction/degradation of habitats is a serious threat to the freshwater habitats. This is an especially prone threat to the wetlands of the world, which were once thought as ‘wastelands’ by the humanity, and several of such wetlands were filled and converted to habitable or cultivable areas, a process called reclamation. For example, the city of Mumbai was developed entirely by land reclamation; conversion of wetlands and salt marshes to a major metropolis. Habitat destruction of wetlands around floodplain zone of riverine systems adversely affect several life forms that require periodic lateral migrations to these wetlands as part of their life cycle.

Yet another major issue is species invasion, invasion by exotic species. Human mediated introduction of species, especially fish species, is a major problem in world’s freshwater systems. Some well-known examples of deliberate species introduction of freshwater species include the crayfish plague in Europe, Nile perch, Lates niloticus, in Lake Victoria, and Salmonids in Southern Hemisphere lakes. These exotic species outcompete with local endemic species and ultimately drive the local species to extinction. All these threats are interlinked in a reticulate fashion, exacerbating the overall threats. For example, flow modification is linked with issues of degradation of habitat, overexploitation and species invasion. Above all, all these threats are being exacerbated with three global threats, including global warming, shifts in rain pattern and nitrogen deposition. Nitrogen deposition refers to the input of excessive nitrogen above the saturation point of soils due to high intensity agriculture. The excess nitrogen run off to the water bodies, causing eutrophication and other associated problems. Global warming lead to change in species distribution patterns, and natural species dispersal. For example, several studies have concluded that an effect of global warming in Antarctica is that several of tropical and subtropical species disperse to far south, leading to a ‘generifying Antarctica’.

There is an obvious bias on the research of freshwater ecosystems of the world, with most of the surveys and studies happening in the comparatively less speciose temperate regions of northern hemisphere. Most of the highly biodiverse freshwater systems are in India, China and Indonesia; these are under tremendous threat from expanding human population. However, these resources remain poorly documented. Three of the most well-known hotspots of freshwater biodiversity in the world are Mekong riverine system that flows through China, Myanmar, Thailand, Cambodia and Vietnam, Amazonian riverine system in Brazil, and Congo Riverine system in Democratic Republic of Congo in Africa. Rivers and wetlands of Indian Western Ghats is a notable hotspot for the freshwater biodiversity in Indian Subcontinent.

4.      Summary

4.1. World’s freshwater resources are extremely limited; it encompass only about 0.01% of overall aquatic resources and most of it are stored up in the two major ice sheets in Antarctica (East and West Antarctic Ice Sheets), yet support a large number of unique life forms.

4.2. Most of the limnetic habitats, due to the small size and fragmented nature, have a high degree of endemism, and are highly vulnerable to anthropogenic activities

4.3. Limnetic ecosystems can broadly divided into lentic (stagnant waters) and lotic (running waters) systems. Major ecological zones of lacustrine systems are littoral zone, limnetic zone, profundal zone and benthic zone. Each of these zones support several unique life forms.

4.4. Although overlooked for a long time, recent research have revealed a tremendous biodiversity residing in groundwater and aquifer networks of the world. Another, less obvious freshwater system is glaciers and ice sheets of Antarctica and Greenland.

4.5. The value of limnetic biodiversity is profound; in addition to the tangible goods of fishery resources, limnetic systems acts as a unique storage systems of highly speciose endemic biodiversity that could have future potentials.

4.6. Major threats to limnetic biodiversity are over exploitation, water pollution, flow modification, destruction/degradation of habitats and invasion by exotic species. There threats are interlinked and further exacerbated by global threats including Nitrogen deposition, shifts in rain patters and global warming.

Further e-resources and learn more

1. YouTube videos: https://www.youtube.com/watch?v=uocWdk30eU0 https://www.youtube.com/watch?v=GgIGN-Z9N6A https://www.youtube.com/watch?v=-VP5WNZK6JM https://www.youtube.com/watch?v=qbUsdMVng0M
2. IUCN resource page on Freshwater Biodiversity https://www.iucn.org/theme/species/our-work/freshwater-biodiversity
3. FAO fact sheet on freshwater biodiversity www.fao.org/docrep/017/i3157e/i3157e07.pd
4. Dudgeon, D., Arthington, A. H., Gessner, M. O., Kawabata, Z. I., Knowler, D. J., Lévêque, C. & Sullivan, C. A. (2006). Freshwater biodiversity: importance, threats, status and conservation challenges. Biological reviews, 81(2), 163-182.
5. Abell, R., Thieme, M. L., Revenga, C., Bryer, M., Kottelat, M., Bogutskaya, N. & Stiassny, M. L. (2008). Freshwater ecoregions of the world: a new map of biogeographic units for freshwater biodiversity conservation. BioScience, 58(5), 403-414.
6. Abell, R. (2002). Conservation biology for the biodiversity crisis: a freshwater follow-up. Conservation Biology, 16(5), 1435-1437.