10.2 Nutrient cycling and energy flow in the oceans
3 min read•july 22, 2024
Oceans are bustling with nutrient cycles and energy flow. Carbon, nitrogen, and phosphorus move through marine ecosystems, supporting life from microscopic plankton to massive whales. These cycles are crucial for maintaining the delicate balance of ocean ecosystems.
Primary producers like form the base of marine food webs. They capture sunlight and convert it into energy-rich compounds, fueling the entire ecosystem. Factors like light, nutrients, and temperature influence ocean productivity, shaping marine life's distribution and abundance.
Nutrient Cycles in the Ocean
Nutrient cycles in oceans
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- Atmospheric CO2 dissolves in surface waters forming carbonic acid ()
- Photosynthesis by phytoplankton (diatoms, dinoflagellates) incorporates carbon into organic compounds (glucose, amino acids)
- Respiration and decomposition by bacteria and other organisms release CO2 back into the water and atmosphere
- Carbon can be stored in deep ocean waters and sediments for long periods (centuries to millennia)
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- Nitrogen gas (N2) is abundant in the atmosphere but unavailable to most organisms
- by certain bacteria (Trichodesmium) converts N2 into biologically available forms (ammonium, )
- by bacteria (Nitrosomonas, Nitrobacter) converts ammonium to nitrite () and then to nitrate ()
- by bacteria in anoxic conditions converts nitrate back into N2 gas
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- Phosphorus enters the ocean through weathering of rocks and minerals (apatite)
- Phytoplankton incorporate phosphorus into organic compounds (ATP, DNA)
- Decomposition by bacteria releases phosphorus back into the water as phosphate ()
- Phosphorus can be lost from the system through sedimentation and burial in seafloor sediments
Energy Flow in Marine Ecosystems
Primary producers in marine ecosystems
- Primary producers (phytoplankton, seaweeds, seagrasses) capture solar energy through photosynthesis
- Convert inorganic carbon (CO2) into organic compounds (carbohydrates, lipids)
- Store energy in chemical bonds of these compounds
- Primary producers form the base of marine food webs
- Provide energy and nutrients for higher
- (copepods, krill) and other primary consumers (small fish, mollusks) feed on primary producers
- Transfer energy to higher trophic levels (larger fish, seabirds, marine mammals)
Factors influencing ocean productivity
- Light availability
- Photosynthesis requires sufficient light energy (wavelengths of 400-700 nm)
- Phytoplankton are most productive in the euphotic zone (upper 200 m of the ocean)
- Factors affecting light penetration (water clarity, season, latitude) influence productivity
- Nutrient concentrations
- Primary producers require essential nutrients (nitrogen, phosphorus, iron, silica)
- Nutrient limitation (especially nitrogen and iron) can restrict primary productivity
- and mixing bring nutrients from deep waters to the surface
- Water temperature
- Temperature affects metabolic rates and growth of primary producers
- Optimal temperature ranges vary among species (polar vs. tropical phytoplankton)
- Stratification and mixing of water layers influence temperature and nutrient distribution
Importance of nutrient and energy flow
- Nutrient cycling
- Ensures the availability of essential elements for
- Maintains the balance and productivity of marine ecosystems
- Disruptions in nutrient cycles can lead to (excess nutrients) or nutrient limitation
- Energy flow
- Transfers energy from primary producers to higher trophic levels
- Supports the growth and reproduction of marine organisms
- Influences the structure and dynamics of marine food webs (trophic cascades)
- Interactions between nutrient cycling and energy flow
- Efficient nutrient cycling supports high primary productivity
- High primary productivity enables greater energy flow to higher trophic levels
- Disruptions in either process can have cascading effects on the entire ecosystem (coral bleaching, fishery collapses)
Key Terms to Review (20)
Apex Predators: Apex predators are the top predators in an ecosystem that have few or no natural enemies, playing a crucial role in maintaining the balance of their environment. These organisms often regulate populations of prey species and help to maintain the health of the ecosystem by preventing overpopulation and ensuring biodiversity. They are integral to nutrient cycling and energy flow as they occupy the highest trophic levels.
Carbon cycle: The carbon cycle is the continuous movement of carbon atoms between the Earth's atmosphere, oceans, soil, and living organisms. It involves processes like photosynthesis, respiration, decomposition, and oceanic absorption, playing a critical role in regulating global climate and sustaining life. This cycle is essential for understanding how nutrients flow through marine ecosystems and how phytoplankton contribute to primary production in the ocean.
Denitrification: Denitrification is a microbial process that converts nitrates (NO3-) and nitrites (NO2-) into nitrogen gas (N2) or, to a lesser extent, nitrous oxide (N2O), effectively removing nitrogen from the ecosystem. This process is crucial in nutrient cycling, as it closes the nitrogen cycle loop by returning nitrogen back to the atmosphere, thus helping to maintain ecological balance in marine and terrestrial environments. Denitrification plays an important role in regulating nitrogen levels, preventing nutrient overloading that can lead to harmful algal blooms and other ecological issues.
Eutrophication: Eutrophication is a process that occurs when water bodies become overly enriched with nutrients, leading to excessive growth of algae and other aquatic plants. This phenomenon can disrupt aquatic ecosystems, cause hypoxia, and threaten the biodiversity of marine life by altering food webs and nutrient cycling.
Herbivores: Herbivores are organisms that primarily consume plants or plant-based materials for their energy and nutrient requirements. In marine ecosystems, herbivores play a crucial role in maintaining the balance of energy flow and nutrient cycling by consuming primary producers, like phytoplankton and seaweeds, and facilitating the transfer of energy up the food chain.
Mutualism: Mutualism is a type of symbiotic relationship where both species involved benefit from the interaction. This kind of relationship is crucial in ecosystems, as it can enhance biodiversity and stability by allowing different species to thrive together. Mutualism plays a significant role in nutrient cycling, energy flow, and various marine interactions, showcasing how interconnected life forms can support one another for survival and ecological balance.
Net primary productivity: Net primary productivity (NPP) is the measure of the amount of organic matter, or biomass, that is produced by photosynthetic organisms in an ecosystem after accounting for the energy used in respiration. NPP is crucial because it indicates the energy available for growth and reproduction in primary producers, forming the base of the food web and supporting all marine life. Understanding NPP helps clarify how energy flows through ocean ecosystems and how nutrients cycle within them.
Nitrification: Nitrification is the biological process by which ammonia is converted into nitrite and then into nitrate, primarily by nitrifying bacteria. This process plays a crucial role in the nitrogen cycle, which is essential for maintaining the health of marine ecosystems by ensuring that nitrogen, an important nutrient, is available in forms that can be utilized by plants and other organisms.
Nitrogen Cycle: The nitrogen cycle is the series of processes by which nitrogen and its compounds are interconverted in the environment and in living organisms. This cycle is crucial for maintaining ecosystem health, as it ensures a continuous supply of nitrogen in forms that can be utilized by various marine organisms, particularly phytoplankton, which play a key role in primary production. The nitrogen cycle involves several stages, including nitrogen fixation, nitrification, assimilation, ammonification, and denitrification, connecting nutrient cycling with energy flow in ocean ecosystems.
Nitrogen fixation: Nitrogen fixation is the process of converting atmospheric nitrogen gas (N₂) into a form that is usable by living organisms, typically ammonia (NH₃) or related compounds. This transformation is essential for sustaining life, as most organisms cannot utilize atmospheric nitrogen directly and rely on fixed nitrogen for the synthesis of amino acids and nucleotides.
Ocean acidification: Ocean acidification refers to the process by which the ocean becomes more acidic due to increased levels of carbon dioxide (CO2) in the atmosphere, which dissolve in seawater and form carbonic acid. This change in pH can have significant effects on marine life and ecosystems, particularly impacting organisms with calcium carbonate structures, like coral reefs and shellfish.
Phosphorus cycle: The phosphorus cycle is the biogeochemical process through which phosphorus moves through the lithosphere, hydrosphere, and biosphere. Unlike other cycles, phosphorus does not have a significant atmospheric component, making it primarily terrestrial and aquatic. This cycle plays a crucial role in nutrient cycling and energy flow within marine ecosystems, as phosphorus is essential for the growth of organisms and the overall health of oceanic environments.
Phytoplankton: Phytoplankton are microscopic, photosynthetic organisms that live in the upper layers of the ocean and serve as a foundational component of marine food webs. They are crucial for converting sunlight into energy through photosynthesis, providing the primary source of organic matter for marine ecosystems and playing a vital role in nutrient cycling and energy flow.
Primary Production: Primary production is the process by which autotrophic organisms, primarily plants and phytoplankton, convert inorganic substances into organic matter using sunlight or chemical energy. This foundational process is critical for the energy flow in ecosystems, influencing nutrient cycling, supporting diverse marine life, and shaping overall biodiversity patterns in various habitats.
Secondary productivity: Secondary productivity refers to the generation of biomass by heterotrophic organisms in an ecosystem, primarily through the consumption of organic matter produced by primary producers. This process is essential for energy transfer within food webs, highlighting how energy flows from producers to consumers and ultimately influences nutrient cycling within marine environments.
Symbiosis: Symbiosis is a biological interaction where two different species live closely together, often benefiting one or both parties involved. This relationship can take various forms, such as mutualism, commensalism, or parasitism, and plays a critical role in the functioning of ecosystems by facilitating nutrient exchange, promoting biodiversity, and influencing species behavior and survival.
Thermohaline circulation: Thermohaline circulation refers to the large-scale movement of ocean water driven by differences in temperature and salinity, which affect water density. This circulation is a crucial component of global ocean currents, as it plays a significant role in nutrient cycling, energy flow, and the distribution of marine habitats across major ocean basins. Through these processes, thermohaline circulation influences climate patterns and marine ecosystems around the world.
Trophic levels: Trophic levels are the hierarchical positions in a food chain or food web that represent the flow of energy and nutrients through an ecosystem. Each level is defined by its role in the ecosystem, such as producers, primary consumers, secondary consumers, and so on, illustrating how energy is transferred from one level to the next.
Upwelling: Upwelling is a process in the ocean where deep, nutrient-rich waters rise to the surface, often leading to increased biological productivity. This movement of water occurs due to various factors, including wind patterns and ocean currents, and plays a crucial role in nutrient cycling and energy flow within marine ecosystems. As these nutrients are brought to the surface, they support phytoplankton growth, which forms the base of the marine food web and ultimately influences larger marine life.
Zooplankton: Zooplankton are microscopic and small aquatic animals that drift in the water column, serving as a crucial component of marine food webs. These organisms, which include a wide variety of species such as copepods, krill, and jellyfish larvae, play a vital role in transferring energy from primary producers like phytoplankton to higher trophic levels, influencing nutrient cycling and energy flow in marine ecosystems. Additionally, their sensitivity to changes in ocean conditions makes them important indicators of environmental shifts, such as warming waters.