Trophic Levels & Complexity

• Highly complex and poorly understood
• Most members are omnivores and eat at multiple levels
• Many consume different prey at different parts of life
• Epipelagic food chains longer than terrestrial food chains
• Energy transfer often 20% and more
• Some food chains shorter

The Microbial Loop

• Fueled by DOM
• DOM produced by photosynthetic nano- and picoplankton
• DOM consumed by bacteria
• Bacteria consumed by protozoan grazers
• Protozoans eaten by larger zooplankton
• Detritus (fecal pellets & larvacean houses) also important

General Concepts

• Consumer populations closely track primary production
• Primary production can be limited by light and/or nutrients

Light Limitation

• Winter at high latitudes
• Poor weather
• Self-shading

Nutrients

• Nitrates often limiting in open ocean
• Phosphates sometimes limiting
• Iron limiting in about 20% of Southern ocean
• Silicon sometimes limiting for diatoms
• Nitrogen fixing cyanobacteria sometimes a source of nitrates in tropics

Stratification & Nutrient Loss

• Nutrients continually lost as marine snow to deep ocean
• Deep ocean has excess nutrients but no light
• Because of stratification little mixing occurs

Seasonal Effects

• Overturn can occur in temperate regions during winter
• Even without overturn some mixing can occur
• Overturn usually results in a spring bloom
• Stratification reestablished in summer and phytoplankton quickly deplete nutrients
• Fall bloom may occur if cooling occurs before day length is too short
• Colder polar waters may not stratify at all in summer
• Antarctic typically most productive, Arctic less so

Upwellings and Productivity

• Ekman spirals can lead to Ekman transport of nutrients
• Caused by wind which causes water to move at 45° angle.
• Each successive layer moves at a slightly greater angle
• Ekman layer defined as portion of water column affected by wind
• Cause intense upwellings in coastal waters
• Steady upwellings occur on Pacific coast of SA, sporadically on CA coast
• Seasonal upwellings occur in some areas (monsoon season off coast of Somalia)
• Equatorial divergence zones result in upwellings
• Center of large gyres have no upwellings and are nutrient poor

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