10.2 Nutrient cycling and energy flow in the oceans

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 phytoplankton 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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• Carbon cycle
  • Atmospheric CO2 dissolves in surface waters forming carbonic acid ($H_2CO_3$)
  • 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)
• Nitrogen cycle
  • Nitrogen gas (N2) is abundant in the atmosphere but unavailable to most organisms
  • Nitrogen fixation by certain bacteria (Trichodesmium) converts N2 into biologically available forms (ammonium, $NH_4^+$)
  • Nitrification by bacteria (Nitrosomonas, Nitrobacter) converts ammonium to nitrite ($NO_2^-$) and then to nitrate ($NO_3^-$)
  • Denitrification by bacteria in anoxic conditions converts nitrate back into N2 gas
• Phosphorus cycle
  • 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 ($PO_4^{3-}$)
  • 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 trophic levels
• Zooplankton (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
  • Upwelling 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
  1. Ensures the availability of essential elements for primary production
  2. Maintains the balance and productivity of marine ecosystems
  3. Disruptions in nutrient cycles can lead to eutrophication (excess nutrients) or nutrient limitation
• Energy flow
  1. Transfers energy from primary producers to higher trophic levels
  2. Supports the growth and reproduction of marine organisms
  3. 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)