🪨Biogeochemistry Unit 10 – Biogeochemistry of Aquatic Ecosystems

Key Concepts and Definitions

• Biogeochemistry studies the interactions and cycling of chemical elements between living organisms and their environment
• Aquatic ecosystems include freshwater (lakes, rivers, wetlands) and marine (oceans, estuaries, coastal zones) environments
• Biogeochemical cycles describe the movement and transformation of elements through biotic and abiotic components of ecosystems
  • Carbon cycle involves photosynthesis, respiration, decomposition, and carbon storage in sediments
  • Nitrogen cycle includes nitrogen fixation, nitrification, denitrification, and ammonification
  • Phosphorus cycle consists of weathering, uptake by organisms, and sedimentation
• Limiting nutrients are elements that restrict biological productivity when in short supply (nitrogen, phosphorus)
• Eutrophication is the excessive growth of algae and aquatic plants due to nutrient enrichment, leading to oxygen depletion and ecosystem degradation
• Redox reactions involve the transfer of electrons between chemical species, influencing nutrient availability and organic matter decomposition
• Microbial communities play crucial roles in mediating biogeochemical processes through their metabolic activities

Biogeochemical Cycles in Aquatic Ecosystems

• Carbon cycle in aquatic ecosystems is driven by photosynthesis, which converts inorganic carbon (CO2) into organic carbon, and respiration, which releases CO2 back into the water and atmosphere
• Nitrogen cycle in aquatic systems involves nitrogen fixation by cyanobacteria, uptake by phytoplankton and macrophytes, and regeneration through decomposition and excretion
• Phosphorus cycle in aquatic environments is influenced by weathering of rocks, adsorption to sediments, and biological uptake and release
• Sulfur cycle includes sulfate reduction by bacteria in anoxic sediments, producing hydrogen sulfide, and oxidation of sulfide back to sulfate in oxygenated waters
• Iron cycle is closely linked to redox conditions, with ferrous iron (Fe2+) being soluble in anoxic environments and ferric iron (Fe3+) forming insoluble oxides in oxygenated waters
  • Iron availability can limit primary productivity in some aquatic systems, particularly in the open ocean
• Silica cycle is important for diatoms, which use dissolved silica to build their frustules, and can influence the export of carbon to deep waters through the biological pump

Major Elements and Their Roles

• Carbon is the building block of organic compounds and plays a central role in the structure and function of aquatic organisms
  • Dissolved inorganic carbon (DIC) includes CO2, bicarbonate (HCO3-), and carbonate (CO32-) ions
  • Dissolved organic carbon (DOC) consists of a complex mixture of organic molecules released by organisms and decomposition processes
• Nitrogen is an essential component of proteins, nucleic acids, and chlorophyll, and its availability often limits primary productivity in aquatic ecosystems
  • Nitrate (NO3-) is the most common form of inorganic nitrogen used by phytoplankton and macrophytes
  • Ammonium (NH4+) is another important inorganic nitrogen source, particularly in nutrient-rich environments
• Phosphorus is a key element in nucleic acids, phospholipids, and energy-transfer molecules (ATP) and can limit primary productivity in freshwater and some coastal systems
  • Orthophosphate (PO43-) is the most bioavailable form of inorganic phosphorus
• Sulfur is a component of amino acids (cysteine and methionine) and plays a role in redox processes in aquatic sediments
• Iron is essential for photosynthesis, as it is a component of cytochromes and ferredoxin, and its availability can limit primary productivity in some open ocean regions
• Silica is a major constituent of diatom frustules and can influence the export of carbon to the deep ocean through the biological pump

Aquatic Ecosystem Types and Characteristics

• Lakes are large, enclosed bodies of freshwater with distinct zones (littoral, pelagic, benthic) and can be classified based on their trophic status (oligotrophic, mesotrophic, eutrophic)
  • Stratification occurs in deep lakes during summer, with a warm, well-mixed epilimnion and a cool, isolated hypolimnion
• Rivers are flowing bodies of freshwater with unidirectional current and a continuum of physical and biological characteristics from headwaters to mouth
  • Riverine ecosystems are influenced by the river continuum concept, which describes changes in energy sources, nutrient cycling, and community structure along the river course
• Wetlands are transitional areas between terrestrial and aquatic ecosystems, characterized by shallow water, hydric soils, and aquatic vegetation (marshes, swamps, bogs)
  • Wetlands provide important ecosystem services, such as water purification, flood control, and habitat for diverse species
• Estuaries are semi-enclosed coastal bodies of water where freshwater from rivers mixes with saltwater from the ocean, creating gradients in salinity, nutrients, and biotic communities
• Coastal zones are the interface between land and ocean, encompassing a variety of habitats (rocky shores, sandy beaches, seagrass beds, coral reefs) and are influenced by tides, waves, and currents
• Open ocean ecosystems are vast, deep, and relatively stable environments with distinct zones (epipelagic, mesopelagic, bathypelagic) and are largely influenced by large-scale physical processes (currents, upwelling)

Biological Processes and Interactions

• Primary production is the synthesis of organic compounds from inorganic carbon through photosynthesis, carried out by phytoplankton, macrophytes, and benthic microalgae in aquatic ecosystems
  • Chemosynthesis is an alternative form of primary production that uses chemical energy instead of light, occurring in deep-sea hydrothermal vents and other extreme environments
• Secondary production refers to the biomass produced by heterotrophic organisms (zooplankton, fish, benthic invertebrates) through the consumption of primary producers or other organic matter
• Decomposition is the breakdown of organic matter by microorganisms (bacteria, fungi) and is a key process in nutrient recycling and energy flow in aquatic ecosystems
  • Detritus, or non-living organic matter, serves as an important energy source for many aquatic organisms
• Nutrient uptake and assimilation by aquatic organisms involve the incorporation of essential elements (carbon, nitrogen, phosphorus) into biomass
  • Nutrient limitation occurs when the availability of one or more essential elements restricts the growth and productivity of aquatic organisms
• Trophic interactions describe the feeding relationships between organisms in an ecosystem, with energy and nutrients being transferred from primary producers to higher trophic levels (herbivores, carnivores)
  • Food webs depict the complex network of trophic interactions within an ecosystem, including both grazing and detrital pathways
• Symbiotic relationships, such as mutualism (e.g., coral-zooxanthellae) and commensalism (e.g., anemone-clownfish), play important roles in the structure and function of aquatic communities

Chemical Transformations and Reactions

• Photochemical reactions are initiated by the absorption of light energy and can influence the cycling of elements and the production of reactive species in aquatic environments
  • Photodegradation of dissolved organic matter can release nutrients and alter the optical properties of water
• Redox reactions involve the transfer of electrons between chemical species and are mediated by microorganisms in aquatic sediments and water column
  • Oxidation of reduced compounds (e.g., ammonium, sulfide) releases energy that can be used by chemolithotrophic microorganisms
  • Reduction of oxidized compounds (e.g., nitrate, sulfate) occurs in anoxic environments and is coupled to the oxidation of organic matter
• Acid-base reactions influence the pH of aquatic systems and the speciation of elements, with important implications for nutrient availability and organismal physiology
  • Carbonate system buffers the pH of seawater and is influenced by the dissolution of atmospheric CO2 and biological processes (photosynthesis, respiration)
• Adsorption and desorption processes involve the attachment and release of ions and molecules to and from particle surfaces, influencing their transport and bioavailability in aquatic environments
  • Phosphate and trace metals often adsorb to iron and manganese oxides in sediments, limiting their availability to organisms in the water column
• Precipitation and dissolution reactions control the formation and removal of solid phases in aquatic systems, such as calcium carbonate (CaCO3) in coral reefs and iron sulfides (FeS) in anoxic sediments
• Complexation reactions involve the binding of ions or molecules to form stable, soluble complexes, which can influence the mobility and bioavailability of elements in aquatic environments
  • Organic ligands, such as humic and fulvic acids, can complex with trace metals and alter their speciation and toxicity

Environmental Factors and Impacts

• Temperature influences the rates of biological and chemical processes in aquatic ecosystems, with higher temperatures generally increasing metabolic rates and reaction kinetics
  • Thermal stratification in lakes and oceans can create distinct layers with different physical, chemical, and biological characteristics
• Light availability affects primary production, with the depth of the euphotic zone determined by the transparency of the water column
  • Suspended particles and dissolved organic matter can attenuate light and limit the depth of photosynthesis
• Salinity gradients in estuaries and coastal zones create unique habitats for organisms adapted to varying salt concentrations
  • Saltwater intrusion into coastal aquifers can occur due to groundwater extraction and sea-level rise, impacting freshwater resources
• Dissolved oxygen concentration is a critical factor for aquatic life, with hypoxic or anoxic conditions occurring in stratified water bodies or due to excessive organic matter decomposition
  • Eutrophication can lead to oxygen depletion in the bottom waters of lakes and coastal areas, creating "dead zones" with limited biodiversity
• Anthropogenic impacts, such as nutrient loading from agricultural runoff and sewage discharge, can greatly alter the biogeochemical cycling and ecosystem functioning of aquatic environments
  • Ocean acidification, caused by the absorption of atmospheric CO2, can have detrimental effects on calcifying organisms (e.g., corals, mollusks) and alter the carbonate chemistry of seawater
• Climate change is expected to have significant impacts on aquatic ecosystems, including changes in temperature, precipitation patterns, sea-level rise, and ocean circulation
  • Warming of surface waters can lead to increased stratification, reduced oxygen solubility, and shifts in species distributions and phenology

Research Methods and Techniques

• Water sampling involves the collection of water from various depths and locations within an aquatic ecosystem for physical, chemical, and biological analyses
  • Niskin bottles and CTD (conductivity, temperature, depth) sensors are commonly used for collecting water samples and measuring vertical profiles
• Sediment coring allows for the collection of intact sediment samples, preserving the vertical stratigraphy and enabling the study of historical changes in biogeochemical processes
  • Gravity corers and multicorers are used to collect sediment cores from the seafloor, while hand corers or push corers are used in shallow aquatic environments
• Nutrient analysis techniques, such as colorimetric assays and automated analyzers, are used to quantify the concentrations of dissolved nutrients (nitrogen, phosphorus, silica) in water samples
  • Stable isotope tracers (e.g., 15N, 13C) can be used to track the fate and transformation of nutrients in aquatic ecosystems
• Biomarkers and pigment analysis provide insights into the composition and activity of aquatic microbial communities
  • Lipid biomarkers (e.g., fatty acids, sterols) can indicate the presence of specific groups of organisms, while photosynthetic pigments (e.g., chlorophyll, carotenoids) can be used to assess phytoplankton biomass and community structure
• Microscopy techniques, such as light microscopy and electron microscopy, allow for the identification and characterization of aquatic microorganisms
  • Fluorescence microscopy can be used to visualize specific groups of organisms or to quantify bacterial abundance and activity using fluorescent stains
• Molecular tools, such as DNA sequencing and quantitative PCR, enable the study of microbial diversity, gene expression, and functional potential in aquatic ecosystems
  • High-throughput sequencing technologies (e.g., Illumina, Nanopore) have revolutionized the field of aquatic microbial ecology by providing unprecedented insights into the structure and function of microbial communities

Applications and Case Studies

• Eutrophication management in lakes and coastal areas often involves reducing nutrient inputs from external sources (e.g., agricultural runoff, sewage discharge) and implementing in-situ remediation techniques (e.g., artificial aeration, phosphorus inactivation)
  • Case study: Lake Erie, one of the Great Lakes of North America, has experienced recurring harmful algal blooms due to excessive phosphorus loading from agricultural and urban sources. Efforts to reduce nutrient inputs and improve water quality have included implementing best management practices in agricultural watersheds and upgrading wastewater treatment facilities
• Ocean iron fertilization has been proposed as a potential geoengineering strategy to enhance the biological pump and mitigate climate change by promoting phytoplankton growth and carbon sequestration in the deep ocean
  • Case study: The LOHAFEX (Loha is Hindi for iron) experiment in the Southern Ocean involved the addition of dissolved iron to a 300 square kilometer patch of the ocean to stimulate phytoplankton growth. While the experiment demonstrated enhanced primary production, the effects on carbon export to the deep ocean were limited, highlighting the complexities of large-scale iron fertilization
• Constructed wetlands are engineered systems designed to mimic the natural functions of wetlands for wastewater treatment, nutrient removal, and habitat restoration
  • Case study: The Houghton Lake Wetland Treatment System in Michigan, USA, is a constructed wetland that receives treated wastewater from a nearby community. The wetland system effectively removes nutrients (nitrogen and phosphorus) and improves water quality before discharging into Houghton Lake, demonstrating the potential of constructed wetlands for sustainable wastewater management
• Coral reef conservation and restoration efforts aim to protect and rehabilitate these valuable ecosystems that are threatened by climate change, ocean acidification, and local stressors (e.g., overfishing, pollution)
  • Case study: The Coral Restoration Foundation in the Florida Keys, USA, employs coral gardening techniques to grow and outplant endangered coral species onto degraded reef sites. By propagating corals in underwater nurseries and transplanting them onto reefs, the foundation seeks to enhance coral cover, diversity, and resilience in the face of environmental challenges
• Biogeochemical modeling is used to simulate and predict the cycling of elements and the response of aquatic ecosystems to environmental changes and management interventions
  • Case study: The European Regional Seas Ecosystem Model (ERSEM) is a complex biogeochemical model that simulates the cycling of carbon, nutrients, and oxygen in European coastal and shelf seas. The model has been used to assess the impacts of climate change, eutrophication, and fishing on marine ecosystem dynamics and to evaluate the effectiveness of management strategies, such as nutrient reduction scenarios