Lecture Note 18 - Aquatic Ecosystems

Aqua means

water

Aquaticus means

relating to water

It is a water-based ecosystem in which different water-based organisms interact with other biotic and abiotic components surrounding them. These water-based organisms are completely dependent on their water-based environment for

their food, shelter, reproduction, and other requirements or services. However, the capability of the ecosystem to sustain life will be dependent on the water’s physiochemical characteristics, its quantity of dissolved sediments, levels of dissolved nutrients, etc. factors.

The aquatic ecosystem definition states it is a water-based environment, wherein, living organisms interact with both physical and chemical

features of the environment. These living creatures whose food, shelter, reproduction, and other essential activities depend on a water-based environment are known as aquatic organisms.

Water plays a significant role in the management of world-scale ecosystem processes in aquatic systems, connecting the atmosphere, lithosphere, and biosphere by transferring material between them and allowing chemical reactions to occur. Water has unique physicochemical features that reflect the water body's quality. The physicochemical characteristics of an

aquatic ecosystem determine how well it functions and how long it can support life forms. In the same way as sediments in terrestrial ecosystems provide substrate, nutrients, and a home for live aquatic resources, sediments in aquatic ecosystems are equivalent to the soil in terrestrial ecosystems. Sediments are significant catalysts in environmental food cycles and the two water quality dynamics.

The quality of sediment has a direct or indirect impact on the functioning of an aquatic ecosystem. The many physicochemical properties of sediment

determine its quality. Similarly, the biotic mix of an aquatic environment determines how well it functions. In the aquatic environment, they serve as a trophic level and a source of energy. Fish have a significant ecological role in the whole food web at the trophic level.

Some of the most common aquatic organisms are – nekton, plankton, and benthos. Additionally,

lakes, oceans, ponds, rivers, swamps, coral reefs, wetlands, etc. are a few popular aquatic ecosystem examples.

Water covers approximately 71% of the Earth's surface, making it one of the most abundant substances on our planet. This vast expanse of water is primarily found in oceans, which comprise about 96.5% of Earth's water resources. The remaining 3.5% is freshwater, found in glaciers, ice caps, rivers, lakes, wetlands, and groundwater. Oceans play a crucial role in regulating

the Earth's climate, absorbing heat and carbon dioxide from the atmosphere, and distributing them across the globe through ocean currents. They also support a rich diversity of marine life, from microscopic phytoplankton to massive whales, and provide essential resources for human activities, including food, transportation, and recreation.

Freshwater resources are essential for life on Earth, supporting ecosystems, agriculture, industry, and human consumption. However, freshwater is unevenly distributed across the planet, with some regions facing water

scarcity or contamination due to pollution, overuse, and climate change. Managing and conserving freshwater resources is critical for ensuring water security and sustainability for future generations.

Biodiversity Support:

Aquatic ecosystems are home to a diverse array of plant, animal, and microbial species. They provide habitat and refuge for countless organisms, contributing to the overall biodiversity of the planet.

Nutrient Cycling:

Aquatic ecosystems facilitate the cycling of nutrients such as nitrogen, phosphorus, and carbon. Microorganisms break down organic matter, releasing nutrients that are taken up by plants and algae. When organisms die, their remains are decomposed, returning nutrients to the ecosystem.

Water Filtration and Purification:

Wetlands, marshes, and other aquatic habitats act as natural filters, removing pollutants and sediment from water. Aquatic plants and microorganisms help to break down contaminants, improving water quality and clarity.

Climate Regulation:

Oceans play a crucial role in regulating the Earth's climate. They absorb and store heat from the sun, moderating temperatures on land and in the atmosphere. Additionally, marine algae and phytoplankton absorb carbon dioxide, helping to mitigate the effects of climate change.

Carbon Sequestration:

Aquatic ecosystems, particularly coastal habitats like mangroves and salt marshes, store large amounts of carbon in plant biomass and sediment. This carbon sequestration helps to mitigate greenhouse gas emissions and combat climate change.

Food Production:

Aquatic ecosystems support valuable fisheries and aquaculture industries, providing protein-rich food sources for billions of people worldwide. Fish, shellfish, and other aquatic organisms are important components of global food security and nutrition.

Recreation and Tourism:

Rivers, lakes, oceans, and other aquatic environments offer opportunities for recreational activities such as fishing, boating, swimming, and diving. These activities contribute to local economies through tourism and outdoor recreation.

Flood Control:

Wetlands, floodplains, and other aquatic habitats act as natural buffers against floods by absorbing and storing excess water during periods of heavy rainfall. They help to reduce the risk of flooding downstream and protect communities and infrastructure.

Habitat Connectivity:

Aquatic ecosystems provide corridors for the movement of species between different habitats. Rivers and streams, for example, allow fish and other aquatic organisms to migrate between freshwater and marine environments, supporting genetic diversity and population resilience.

Cultural and Aesthetic Value:

Aquatic ecosystems hold cultural significance for many indigenous communities and societies around the world. They inspire art, literature, music, and spiritual practices, serving as symbols of beauty, tranquility, and renewal.

Aquatic ecosystems play vital roles in maintaining the

health of our planet and supporting life in numerous ways.

Abiotic factors are non-living components of aquatic ecosystems that influence the physical and chemical conditions of the environment, ultimately

shaping the structure and functioning of these ecosystems.

Salinity:

Measures the concentration of dissolved salts in water. It varies widely in different aquatic habitats, from freshwater lakes and rivers to brackish estuaries and marine environments. Salinity affects the distribution and physiology of aquatic organisms, particularly those adapted to specific osmotic conditions.

Water Temperature:

Affects the metabolic rates, reproduction, and distribution of aquatic organisms. It also influences physical and chemical processes such as nutrient cycling and dissolved oxygen levels.

Sunlight Availability:

Is essential for photosynthesis, the process by which plants and algae produce energy from sunlight. Light availability varies with depth, water clarity, and time of day, influencing the distribution and productivity of aquatic plants and primary producers.

Dissolved Oxygen (DO):

This in water is essential for the survival of aquatic organisms, particularly fish and other aerobic organisms. DO levels fluctuate depending on factors such as temperature, water flow, and the presence of photosynthetic organisms.

pH Level:

Measures the acidity or alkalinity of water. Aquatic organisms have specific pH tolerances, and deviations from optimal pH levels can affect their survival, reproduction, and behavior. pH levels can be influenced by factors such as rainfall, runoff, and geological characteristics.

Dissolved Nutrients:

Such as nitrogen, phosphorus, and carbon are essential for the growth and productivity of aquatic plants and algae. Excessive nutrient inputs from sources such as agricultural runoff or wastewater can lead to eutrophication, causing algal blooms and oxygen depletion.

Turbidity:

Refers to the cloudiness or clarity of water, which is influenced by suspended particles such as silt, clay, and organic matter. High turbidity levels can reduce light penetration, affecting photosynthesis and the growth of aquatic plants.

Water Flow:

Including factors such as velocity, direction, and turbulence, influences habitat structure, sediment transport, and nutrient cycling in aquatic ecosystems. Flow patterns can affect the distribution of organisms and the functioning of riverine and riparian habitats.

Topography and Geology:

features such as substrate composition, slope, and elevation influence the physical characteristics of aquatic habitats. They also affect water flow patterns, nutrient availability, and habitat diversity in aquatic ecosystems.

Aquatic organisms are living organisms that inhabit aquatic ecosystems, including oceans, rivers, lakes, wetlands, estuaries, and coral reefs. These organisms have adapted

to a wide range of aquatic environments and play crucial roles in the functioning of aquatic ecosystems.

Plankton:

• Microscopic organisms that drift or float in aquatic environments.

• Include phytoplankton (photosynthetic algae) and zooplankton (small animals).

• Form the base of the aquatic food chain, providing food for larger organisms.

  • Have limited swimming ability and are carried by currents.

Nekton:

• Aquatic organisms capable of active swimming and moving independently of water currents.

• Include fish, marine mammals, turtles, and squids.

• Have well-developed locomotive adaptations, such as fins or limbs, to navigate through water.

  • Play crucial roles as predators, prey, and competitors in aquatic ecosystems.

Benthos:

• Organisms that live on or in the bottom substrate of aquatic ecosystems.

• Include benthic algae, invertebrates (such as snails, clams, and worms), and some fish.

• Adapted to life in sediments or attached to submerged surfaces.

• Play roles in nutrient cycling, sediment dynamics, and habitat structure.

Decomposers:

• Organisms that break down organic matter into simpler compounds.

• Include bacteria, fungi, and detritivores (such as earthworms and crustaceans).

• Decompose dead plant and animal material, recycling nutrients back into the ecosystem.

• Important for nutrient cycling, organic matter decomposition, and maintaining ecosystem health.

Freshwater refers to water with low concentrations of dissolved salts, typically found in rivers, lakes, wetlands, and groundwater. It is essential for various human activities, including

drinking, agriculture, industry, and recreation. Freshwater ecosystems support diverse aquatic life and play crucial roles in nutrient cycling, flood control, and water purification.

One important characteristic of freshwater is its low salinity, typically less than 0.5 parts per thousand (ppt). This distinguishes it from seawater, which has a higher salinity level. Freshwater is essential for various purposes, including drinking, agriculture, and supporting diverse aquatic life in rivers, lakes, wetlands, and groundwater systems. Its relatively low

salinity makes it suitable for human consumption, irrigation, and industrial use, but also requires careful management to prevent contamination and ensure sustainability. Additionally, freshwater ecosystems play vital roles in regulating the Earth's climate, supporting biodiversity, and providing essential ecosystem services for human well-being.

Lentic ecosystem (lacustrine ecosystem):

The freshwater ecosystem that includes water bodies that are static or still like lakes, and ponds. This type of ecosystem consists of floating or rooted plants and algae as vegetation. Water animals like shrimps, crabs, and some varieties of fish are found. Other than these land animals like snakes, alligators, crocodiles, and amphibians like frogs, and salamanders are also found.

Lotic ecosystems (riverine ecosystem):

The freshwater ecosystem includes water bodies with rapid flow of water like streams and rivers. They have phytoplankton along with insects like beetles, mayflies, etc., and other species like dolphins, eel, trout, otters, etc.

Swamps and Wetlands:

The freshwater ecosystem that is partially or fully covered by water like marshes, swamps, and bogs. These areas house wide varieties of organisms like black spruce, water lilies, dragonflies, green heron, northern pike, crocodiles, etc.

Lentic ecosystems, such as lakes and ponds, form through various geological, climatic, and hydrological processes over time. They often originate from depressions or basins created by geological forces such as tectonic activity, glacial erosion, or volcanic activity. These depressions collect water from precipitation, runoff, or groundwater seepage, gradually filling with water to form

standing bodies of freshwater. Climate factors such as temperature and precipitation influence the water balance and hydrology of lentic ecosystems, while biotic factors such as vegetation and organic matter contribute to ecosystem development and productivity. Over time, lentic ecosystems evolve and undergo changes in physical, chemical, and biological characteristics, influenced by factors such as sedimentation, nutrient cycling, and interactions among species.

Oligotrophic: Root: "Oligo" comes from the Greek word "oligos," meaning "few" or "little." Meaning: Oligotrophic ecosystems are characterized by

low nutrient levels, particularly low concentrations of nitrogen and phosphorus. These environments typically have clear water with low levels of algae and aquatic plant growth due to limited nutrient availability. Oligotrophic waters often support relatively low biodiversity but may have high water quality and are often prized for their scenic beauty and recreational value.

Mesotrophic: Root: "Meso" is derived from the Greek word "mesos," meaning "middle" or "intermediate." Meaning: Mesotrophic ecosystems fall between

oligotrophic and eutrophic systems in terms of nutrient richness. They have moderate nutrient levels, allowing for a moderate amount of algal and aquatic plant growth. Mesotrophic waters typically have clearer water than eutrophic systems but may exhibit some signs of nutrient enrichment, such as increased productivity and occasional algal blooms. Mesotrophic lakes and ponds often support a diverse range of aquatic species and provide important ecosystem services.

Eutrophic: Root: "Eutrophic" is derived from the Greek word "eu," meaning "well" or "good," and "trophe," meaning "nourishment." Meaning: Eutrophic ecosystems are characterized by

high nutrient levels, particularly elevated concentrations of nitrogen and phosphorus. These nutrient-rich environments support abundant algal and aquatic plant growth, often leading to the development of dense algal blooms, murky water, and reduced oxygen levels. Eutrophic waters may suffer from degraded water quality, increased susceptibility to pollution, and a decline in biodiversity. While some eutrophic systems occur naturally, many are the result of human activities such as agricultural runoff, sewage discharge, and urbanization.

Zonation in lakes refers to the spatial distribution of different habitats and ecological communities within the lake environment. This zonation is influenced by various factors such as

depth, light availability, nutrient concentrations, and physical characteristics of the lake basin. Typically, lakes exhibit distinct zones or layers, each with its own unique environmental conditions and assemblages of organisms.

Littoral Zone:

This shallow, nearshore area extends from the shoreline to the point where rooted aquatic plants cease to grow. The zone is characterized by abundant sunlight, providing optimal conditions for photosynthesis and supporting diverse communities of submerged vegetation, benthic organisms, and small fish. It serves as important habitat for breeding, feeding, and refuge for many aquatic species.

Limnetic Zone:

Also known as the open-water zone, the zone is the deeper, offshore area beyond the reach of rooted plants. It is characterized by open water and abundant light penetration, allowing for the growth of phytoplankton and other suspended algae. The zone supports zooplankton, fish, and other pelagic organisms that inhabit the water column and feed on planktonic prey. It is an important zone for primary production and supports a diverse array of planktonic and pelagic species.

Profundal Zone:

The zone is the deepest and coldest part of the lake, where light penetration is limited or absent. It typically extends from the thermocline to the lake bottom and is characterized by low oxygen levels and nutrient concentrations. Benthic organisms such as bottom-dwelling invertebrates and decomposers dominate this zone, relying on organic matter that accumulates on the lake bottom. Fish species adapted to low-light and low-oxygen conditions may also inhabit the profundal zone.

Benthic Zone:

The zone includes the lake bottom and is inhabited by organisms known as benthos. These organisms include detritivores, filter feeders, and burrowing invertebrates that play crucial roles in nutrient cycling and sediment dynamics. The zone is influenced by factors such as substrate type, water depth, and organic matter inputs, and supports diverse communities adapted to life on the lake bottom.

Lotic ecosystems, also known as riverine ecosystems, refer to freshwater habitats characterized by flowing water, such as rivers, streams, and creeks. These dynamic ecosystems exhibit distinct physical, chemical, and biological characteristics shaped by factors such as water velocity, substrate composition, and hydrological variability. Lotic ecosystems are divided into various zones, including the source or headwaters, which are typically narrow, fast-flowing, and characterized by cold, oxygen-rich water. As rivers progress downstream, they transition through zones such as

riffles, runs, and pools, each with unique habitats and species assemblages. Riffles are shallow, fast-flowing sections with rocky substrates, while runs are deeper, smoother stretches with moderate flow rates. Pools are deeper, slower-moving areas with sediment accumulation and diverse habitats. Lotic ecosystems support diverse communities of aquatic organisms, including fish, invertebrates, and algae, adapted to the dynamic flow conditions and ecological gradients along river courses. They play crucial roles in nutrient cycling, sediment transport, and habitat connectivity, providing essential ecosystem services and supporting human well-being. However, lotic ecosystems face threats such as habitat alteration, pollution, and flow regulation, highlighting the importance of conservation and sustainable management practices to ensure their health and resilience.

The headwaters of a river are its uppermost reaches, typically originating from springs, melting snow, or mountainous terrain. These areas are characterized by cold, fast-flowing water with high levels of dissolved oxygen. The substrate in headwater streams is often

rocky, and the surrounding landscape may be forested. Headwater streams play crucial roles in nutrient cycling, providing habitat for aquatic insects and spawning grounds for fish such as trout and salmon. They also contribute to the overall flow and water quality of river systems downstream.

The transition zone, also known as the floodplain, refers to the low-lying area adjacent to the river channel that is periodically inundated during floods. Floodplains are dynamic ecosystems that experience a range of water levels and support diverse habitats, including

wetlands, riparian forests, and grasslands. These areas are highly productive and provide essential habitat for wildlife, including birds, fish, and amphibians. Floodplains play important roles in flood mitigation, groundwater recharge, and nutrient cycling. They also serve as corridors for wildlife movement and contribute to the overall biodiversity of river ecosystems.

The mouth of a river is the point where it empties into a larger body of water, such as a lake, sea, or ocean. This transition zone marks the end of the river's journey and the beginning of its interaction with marine or estuarine environments. River mouths are characterized by

brackish water, influenced by the mixing of freshwater from the river and saltwater from the ocean. These areas support unique ecosystems, including estuaries, salt marshes, and mangrove forests, which are rich in biodiversity and provide critical habitat for fish, birds, and other marine organisms. River mouths play important roles in sediment deposition, nutrient cycling, and coastal dynamics, contributing to the resilience and productivity of coastal ecosystems.

Wetlands are transitional ecosystems where water meets land, characterized by periodic or permanent inundation, saturated soils, and unique vegetation adapted to waterlogged conditions. They provide

essential ecological functions, including flood control, water purification, carbon storage, and habitat for diverse plant and animal species.

Marshes are wetlands dominated by herbaceous vegetation such as grasses, sedges, and rushes. They occur in both freshwater and saltwater environments and

may be seasonally or permanently flooded. Marshes are important for waterfowl nesting, water filtration, and shoreline stabilization.

Swamps are wetlands characterized by forested vegetation, including trees such as cypress, mangroves, and willows. They are often found along

riverbanks, floodplains, and coastal areas. Swamps provide habitat for a variety of wildlife, including birds, reptiles, and amphibians, and play roles in flood control and carbon sequestration.

Bogs are acidic wetlands characterized by peat accumulation and specialized vegetation such as sphagnum mosses, carnivorous plants, and shrubs. They receive most of their water from

rainfall and are typically low in nutrients. Bogs support unique plant and animal communities adapted to the nutrient-poor, acidic conditions.

Fens are alkaline wetlands fed by groundwater or surface water rich in minerals. They have a higher nutrient content compared to bogs and support diverse plant communities, including

sedges, grasses, and flowering plants. Fens provide important habitat for rare and endemic species and play roles in water filtration and carbon storage.

Mangrove swamps are coastal wetlands characterized by salt-tolerant trees and shrubs known as mangroves. They occur in tropical and subtropical regions and provide critical habitat for marine

and terrestrial species. Mangrove swamps protect coastlines from erosion, support fisheries, and sequester carbon from the atmosphere.

Salt marshes are coastal wetlands dominated by salt-tolerant grasses and sedges. They occur in estuarine environments and are influenced by tides and saltwater intrusion. Salt marshes provide

important nursery habitat for fish and shellfish, filter pollutants, and buffer coastal communities from storm surges and sea-level rise.