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Paper 03 Biodiversity and Conservation

Freshwater Biodiversity: Spatial Patterns

Paper No: 03 Biodiversity and Conservation

Module: 12 Freshwater Biodiversity: Spatial Patterns

Development Team

Principal Investigator

&

Co- Principal Investigator

Prof. R.K. Kohli

Prof. V.K. Garg & Prof. Ashok Dhawan

Central University of Punjab, Bathinda

Paper Coordinator

Dr. Sunil Mittal


Content Writer

Dr. Felix Bast


Content Reviewer Dr. Sunil Mittal


Anchor Institute

Central University of Punjab

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Description of Module

Subject Name Environmental Sciences

Paper Name Biodiversity and Conservation

Module

Name/Title Freshwater Biodiversity: Spatial Patterns

Module Id EVS/BC-III/12

Pre-requisites

Objectives

1. To learn the overview and importance of freshwater biodiversity

2. To learn about various ecosystem goods and services provided by the

freshwater biodiversity

3. To learn about zonal classification of freshwater biodiversity

4. To learn about various lentic, lotic, aquifer and glacier systems of the

world

5. To learn about threats and challenges that are being faced by

freshwater biodiversity

Keywords

Littoral, Benthic, Limnetic, Riverine, Lacustrine, Glacial meltwater, Subglacial

lake, Aquifer

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Module XX: Freshwater Biodiversity: Spatial Patterns

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

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2. Concept map

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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

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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.

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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

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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

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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

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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.

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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.

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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

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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

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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

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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.

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