8.2 Primary production and marine food webs

Marine ecosystems rely on primary production, the process of converting inorganic carbon into organic compounds. This forms the foundation of food webs, generates oxygen, and sequesters carbon. Various producers, from tiny phytoplankton to massive kelp forests, contribute to this vital process.

Marine food webs are complex networks of energy transfer between trophic levels. Factors like light, nutrients, temperature, and grazing pressure influence primary production. Understanding these dynamics is crucial for grasping the intricate balance of life in our oceans.

Primary Production in Marine Ecosystems

Importance of primary production

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• Primary production converts inorganic carbon into organic compounds through photosynthesis or chemosynthesis forming foundation of marine food webs
• Net primary production measures total organic matter produced minus respiration providing energy for higher trophic levels
• Gross primary production represents total organic matter synthesized supporting ecosystem functions
• Process generates oxygen essential for marine life and sequesters carbon mitigating climate change
• Supports biodiversity by creating habitats and food sources (coral reefs, kelp forests)

Comparison of marine primary producers

• Phytoplankton dominate oceanic primary production with rapid turnover rates and wide distribution (diatoms, dinoflagellates)
• Macroalgae contribute to coastal production forming extensive underwater forests (kelp, sargassum)
• Seagrasses create productive meadows in shallow waters stabilizing sediments and providing nursery habitats (eelgrass, turtle grass)
• Spatial distribution varies phytoplankton throughout water column, macroalgae and seagrasses in coastal zones
• Ecosystem services differ phytoplankton global production, macroalgae and seagrasses local production and habitat creation

Marine Food Webs and Factors Influencing Primary Production

Structure of marine food webs

• Trophic levels form hierarchy: primary producers, herbivores, carnivores, top predators
• Energy transfer follows 10% rule between levels due to metabolic losses and inefficiencies
• Food web structures range from simple linear chains to complex interconnected networks
• Keystone species exert disproportionate influence on web structure (sea otters, sharks)
• Bioaccumulation concentrates pollutants up food chain affecting top predators (mercury in tuna)

Factors influencing marine production

• Light availability affects photosynthesis rates varies with depth, water clarity, and latitude
• Nutrient availability limits growth macro-nutrients (nitrogen, phosphorus) micro-nutrients (iron)
• Temperature influences metabolic rates and enzyme activity impacting production efficiency
• Water column stability affects nutrient distribution through stratification and mixing processes
• Carbon dioxide concentration impacts photosynthesis rates and ocean acidification
• Grazing pressure by herbivores exerts top-down control on primary producer populations
• Oceanic zones exhibit productivity differences coastal areas more productive than open ocean, equatorial regions vs polar regions