1.4 Aquatic biomes

Aquatic biomes are diverse ecosystems covering much of Earth's surface. They're crucial for supporting life and maintaining ecological balance. These biomes are characterized by water as the dominant feature, influencing physical, chemical, and biological processes within them.

Freshwater and marine biomes are the two main categories, each with distinct characteristics. Understanding these ecosystems is vital for conservation efforts and managing human impacts on aquatic environments, including pollution, overfishing, and climate change.

Types of aquatic biomes

• Aquatic biomes are diverse ecosystems that cover a significant portion of the Earth's surface and play crucial roles in supporting life and maintaining ecological balance
• Aquatic biomes are characterized by the presence of water as the dominant feature, which influences the physical, chemical, and biological processes within these environments
• The two main categories of aquatic biomes are freshwater biomes and marine biomes, each with distinct characteristics and ecological communities

Physical characteristics of aquatic biomes

Light penetration in aquatic environments

• Light penetration in aquatic environments depends on factors such as water clarity, depth, and the presence of suspended particles or dissolved organic matter
• The photic zone is the upper layer of water where sufficient light penetrates to support photosynthesis, while the aphotic zone receives little to no sunlight
• Light attenuation with depth creates vertical gradients in aquatic ecosystems, influencing the distribution and productivity of aquatic organisms

Temperature variations in aquatic habitats

• Temperature variations in aquatic habitats are influenced by factors such as latitude, depth, season, and water circulation patterns
• Thermal stratification occurs in lakes and oceans, creating distinct layers with different temperatures (epilimnion, thermocline, and hypolimnion)
• Temperature affects the metabolic rates, growth, and distribution of aquatic organisms, as well as the solubility of gases and nutrients in water

Dissolved oxygen levels

• Dissolved oxygen is essential for the survival of aquatic organisms and is influenced by factors such as water temperature, salinity, and biological activity
• Oxygen enters the water through diffusion from the atmosphere and photosynthesis by aquatic plants and algae
• Oxygen levels can vary vertically and horizontally in aquatic ecosystems, with hypoxic or anoxic conditions occurring in deep or stagnant waters

Nutrient availability and distribution

• Nutrients such as nitrogen, phosphorus, and silica are essential for the growth and productivity of aquatic organisms
• Nutrient availability in aquatic ecosystems is influenced by factors such as weathering of rocks, atmospheric deposition, and biological processes (nitrogen fixation, decomposition)
• Nutrient cycling in aquatic environments involves the uptake, assimilation, and regeneration of nutrients by various organisms and biogeochemical processes

Freshwater biomes

Rivers and streams

• Rivers and streams are flowing freshwater ecosystems that transport water, sediments, and nutrients from land to larger water bodies
• The physical characteristics of rivers and streams, such as flow velocity, substrate type, and channel morphology, influence the distribution and diversity of aquatic organisms
• Riparian zones along rivers and streams provide important habitats and ecological services, such as nutrient cycling, flood control, and wildlife corridors

Lakes and ponds

• Lakes and ponds are standing freshwater ecosystems that vary in size, depth, and nutrient status (oligotrophic, mesotrophic, or eutrophic)
• The littoral zone along the shoreline supports diverse aquatic plants and invertebrates, while the pelagic zone in the open water is dominated by plankton and fish
• Lakes and ponds undergo seasonal changes, such as thermal stratification and mixing, which influence nutrient cycling and biological productivity

Wetlands and marshes

• Wetlands and marshes are transitional ecosystems between terrestrial and aquatic environments, characterized by waterlogged soils and adapted vegetation (emergent, submergent, or floating plants)
• Wetlands provide critical habitats for wildlife, including migratory birds, amphibians, and fish, and serve as nurseries for many aquatic species
• Wetlands perform important ecological functions, such as water purification, carbon sequestration, and flood control, making them valuable ecosystems for conservation and management

Marine biomes

Coastal and intertidal zones

• Coastal and intertidal zones are dynamic environments at the interface between land and sea, influenced by tides, waves, and sediment transport
• Intertidal habitats, such as rocky shores and sandy beaches, support diverse communities of organisms adapted to the changing conditions of exposure and submersion
• Coastal ecosystems, such as estuaries and salt marshes, are highly productive and serve as nurseries for many commercially important fish and shellfish species

Coral reefs

• Coral reefs are diverse and complex marine ecosystems found in shallow, tropical waters, built by coral polyps and their symbiotic algae (zooxanthellae)
• Coral reefs support an incredible diversity of life, including colorful fish, invertebrates, and marine mammals, making them hotspots of marine biodiversity
• Coral reefs provide important ecosystem services, such as coastal protection, fisheries, and tourism, but are threatened by climate change, ocean acidification, and human activities

Open ocean and pelagic zone

• The open ocean and pelagic zone refer to the vast expanse of water beyond the continental shelf, characterized by low nutrient concentrations and high water clarity
• The pelagic zone is divided into distinct depth zones (epipelagic, mesopelagic, bathypelagic, and abyssopelagic), each with unique environmental conditions and biological communities
• Pelagic organisms, such as plankton, jellyfish, and large predatory fish (tuna, sharks), are adapted to life in the open water, with strategies for buoyancy, locomotion, and feeding

Deep sea and benthic zone

• The deep sea and benthic zone are the regions of the ocean below the photic zone, characterized by high pressure, low temperature, and the absence of sunlight
• Benthic habitats, such as the abyssal plain and hydrothermal vents, support unique communities of organisms adapted to the extreme conditions, such as deep-sea fish, crustaceans, and chemosynthetic bacteria
• The deep sea plays a crucial role in global nutrient cycling and carbon sequestration, as well as being a repository for undiscovered biodiversity and potential biotechnological resources

Adaptations of aquatic organisms

Morphological adaptations

• Aquatic organisms exhibit a wide range of morphological adaptations to cope with the challenges of living in water, such as streamlined body shapes for efficient swimming (dolphins, sharks)
• Many aquatic animals have specialized appendages for locomotion, such as fins, flippers, or webbed feet, which help them move through the water with ease (fish, penguins, otters)
• Some aquatic organisms have evolved unique sensory structures, such as the lateral line system in fish or the echolocation abilities of dolphins, to navigate and locate prey in the underwater environment

Physiological adaptations

• Aquatic organisms have developed physiological adaptations to maintain homeostasis in the aquatic environment, such as osmoregulation to control water and ion balance (freshwater fish, marine mammals)
• Many aquatic animals have specialized respiratory structures, such as gills or swim bladders, to extract oxygen from the water efficiently (fish, crustaceans)
• Some aquatic organisms have evolved biochemical adaptations, such as the production of antifreeze proteins in polar fish or the presence of hemoglobin with high oxygen affinity in deep-sea fish, to cope with extreme environmental conditions

Behavioral adaptations

• Aquatic organisms display a variety of behavioral adaptations to optimize their survival and reproductive success in the aquatic environment, such as schooling behavior in fish to reduce predation risk and improve foraging efficiency
• Many aquatic animals exhibit migratory behaviors, such as the long-distance migrations of salmon or whales, to take advantage of seasonal resources or to reach breeding grounds
• Some aquatic organisms have evolved complex social behaviors, such as the cooperative hunting strategies of killer whales or the elaborate courtship displays of seahorses, to enhance their fitness and reproductive success

Trophic relationships in aquatic ecosystems

Primary producers

• Primary producers in aquatic ecosystems are photosynthetic organisms, such as phytoplankton, algae, and aquatic plants, that convert sunlight into chemical energy and form the base of aquatic food webs
• Phytoplankton, including diatoms and dinoflagellates, are the main primary producers in the open ocean and contribute significantly to global primary productivity
• Aquatic plants, such as seagrasses and kelp, are important primary producers in coastal and shallow marine ecosystems, providing habitat and food for a diverse array of organisms

Primary consumers

• Primary consumers in aquatic ecosystems are herbivorous organisms that feed directly on primary producers, such as zooplankton, small fish, and invertebrates (copepods, krill, snails)
• Zooplankton, including copepods and larval stages of many aquatic animals, are a critical link between primary producers and higher trophic levels, serving as a major food source for larger organisms
• Herbivorous fish, such as parrotfish and surgeonfish, play important roles in controlling algal growth and maintaining the balance of coral reef ecosystems

Secondary and tertiary consumers

• Secondary consumers in aquatic ecosystems are carnivorous organisms that feed on primary consumers, such as larger fish, squid, and marine mammals (tuna, sharks, seals)
• Tertiary consumers are top predators that feed on secondary consumers, occupying the highest trophic levels in aquatic food webs (killer whales, large sharks)
• The transfer of energy and biomass from lower to higher trophic levels is governed by ecological efficiencies, with only a small fraction of energy (typically 10%) being passed on to the next level

Decomposers and detritivores

• Decomposers and detritivores in aquatic ecosystems are organisms that break down dead organic matter and recycle nutrients back into the ecosystem, such as bacteria, fungi, and invertebrates (amphipods, polychaetes)
• Microbial decomposers, such as bacteria and fungi, play a crucial role in the mineralization of organic matter and the release of nutrients, such as nitrogen and phosphorus, into the water column
• Detritivores, such as amphipods and polychaetes, feed on particulate organic matter and facilitate the transfer of energy and nutrients to higher trophic levels in benthic food webs

Biogeochemical cycles in aquatic environments

Carbon cycle

• The carbon cycle in aquatic environments involves the exchange of carbon between the atmosphere, water, and biota through processes such as photosynthesis, respiration, and decomposition
• Aquatic primary producers, such as phytoplankton and aquatic plants, fix atmospheric carbon dioxide through photosynthesis and incorporate it into their biomass
• The burial of organic carbon in aquatic sediments, particularly in coastal and deep-sea environments, represents a significant long-term carbon sink and plays a role in regulating Earth's climate

Nitrogen cycle

• The nitrogen cycle in aquatic environments involves the transformation of nitrogen compounds, such as ammonium, nitrate, and organic nitrogen, through processes like nitrogen fixation, nitrification, and denitrification
• Nitrogen fixation, carried out by cyanobacteria and other microorganisms, converts atmospheric nitrogen gas into biologically available forms, such as ammonium, which can be used by primary producers
• Nitrification, performed by nitrifying bacteria, oxidizes ammonium to nitrite and then to nitrate, while denitrification, carried out by denitrifying bacteria, reduces nitrate to nitrogen gas under anoxic conditions

Phosphorus cycle

• The phosphorus cycle in aquatic environments involves the transfer of phosphorus between dissolved, particulate, and organic forms through processes such as weathering, biological uptake, and remineralization
• Phosphorus is a limiting nutrient in many aquatic ecosystems, and its availability can control primary productivity and the growth of aquatic organisms
• The release of phosphorus from sediments, particularly under anoxic conditions, can contribute to the eutrophication of aquatic ecosystems, leading to algal blooms and other water quality issues

Human impacts on aquatic biomes

Water pollution and eutrophication

• Water pollution, caused by the release of contaminants such as nutrients, chemicals, and pathogens from human activities, can have detrimental effects on aquatic ecosystems and human health
• Eutrophication, the excessive enrichment of water bodies with nutrients (primarily nitrogen and phosphorus), can lead to algal blooms, oxygen depletion, and the loss of biodiversity in aquatic ecosystems
• Agricultural runoff, sewage discharge, and industrial effluents are major sources of nutrients and other pollutants that contribute to water pollution and eutrophication in aquatic environments

Overfishing and habitat destruction

• Overfishing, the removal of fish and other aquatic organisms at rates faster than they can be replenished, can lead to the depletion of fish stocks, the alteration of food web dynamics, and the loss of marine biodiversity
• Destructive fishing practices, such as bottom trawling and the use of explosives or poisons, can cause significant damage to aquatic habitats, such as coral reefs and seagrass beds, and have long-lasting impacts on the associated ecological communities
• Coastal development, land reclamation, and the conversion of wetlands for human use can result in the loss and fragmentation of critical aquatic habitats, affecting the survival and reproduction of many aquatic species

Climate change and ocean acidification

• Climate change, driven by the increasing atmospheric concentrations of greenhouse gases, can have profound impacts on aquatic ecosystems through changes in water temperature, sea level, ocean circulation, and the frequency and intensity of extreme events
• Rising water temperatures can alter the distribution and abundance of aquatic species, disrupt the timing of biological events (phenology), and increase the risk of coral bleaching and disease outbreaks
• Ocean acidification, caused by the absorption of atmospheric carbon dioxide by the oceans, can have detrimental effects on marine organisms with calcium carbonate structures, such as corals, mollusks, and some plankton, and alter the functioning of marine ecosystems

Conservation and management strategies

• Conservation and management strategies for aquatic biomes aim to protect and restore the health, diversity, and productivity of these ecosystems while balancing human needs and sustainable use
• Establishing marine protected areas (MPAs) and other area-based management tools can help safeguard critical habitats, maintain biodiversity, and support the recovery of overexploited species
• Implementing ecosystem-based management approaches that consider the complex interactions between species, habitats, and human activities can promote the sustainable use of aquatic resources and the resilience of aquatic ecosystems to environmental changes
• Engaging stakeholders, such as local communities, industries, and governments, in the development and implementation of conservation and management plans can foster stewardship, support, and compliance with environmental regulations and best practices