Oceanography 100
P Anderson
Chapter 13
Biological Productivity and Energy Transfer
Primary productivity
• Primary productivity is the amount of carbon (organic matter) produced by organisms
– Mostly through photosynthesis
• Energy source = solar radiation
– Also includes chemosynthesis
• Energy source = chemical reactions
Oceanic photosynthetic productivity
• Controlling factors affecting photosynthetic productivity:
– Availability of nutrients
• Nitrates
• Phosphates
• Iron
– Amount of sunlight
• Varies daily and seasonally
• Sunlight strong enough to support photosynthesis occurs only to a depth of 100
meters (euphotic zone)
Locations of maximum photosynthetic productivity
• Margins of the oceans
– Abundant supply of nutrients from land
– Water shallow enough for light to penetrate all the way to the sea floor
• Upwelling areas
– Currents hoist cool, nutrient-rich deep water to the sunlit surface
Coastal upwelling
The electromagnetic spectrum and light penetration in seawater
Water color and life in the ocean
• Ocean color is influenced by:
– The amount of turbidity from runoff
– The amount of photosynthetic pigment, which corresponds to the amount of
productivity
• Yellow-green = highly productive water
– Found in coastal and upwelling areas (eutrophic)
• Clear indigo blue = low productivity water
– Found in the tropics and open ocean (oligotrophic)
Photosynthetic marine organisms: Plants
• Seed-bearing plants
– Eelgrass (Zostera)
– Surf grass (Phyllospadix)
Photosynthetic marine organisms: Macroscopic algae
• Brown algae
– Sargassum
– Macrocystis
• Green algae
– Codium
• Red algae
– Lithothamnion
Photosynthetic marine organisms: Microscopic algae
• Microscopic algae include:
– Golden algae
• Diatoms (silica test resembles a pillbox)
• Coccolithophores (calcite plates form a spherical test)
– Dinoflagellates
• Produce a test made of keratin
• Posses a whip-like flagella
• Bioluminescence
• Exist in great abundance, creating red tides (harmful algae blooms)
Dinoflagellates and red tides
See photo on my web page red tide from summer ‘01
Regional productivity
• Photosynthetic productivity varies due to:
– Amount of sunlight
– Availability of nutrients
•
Thermocline (a layer of rapidly changing temperature) limits nutrient supply
• Examine three open ocean regions:
•
Polar oceans (>60° latitude)
•
Tropical oceans (<30° latitude)
•
Temperate oceans (30-60° latitude)
Productivity in polar oceans
• Sunlight peaks in summer
• Nutrients available nearly year-round (only weak seasonal thermocline develops)
• Productivity:
–
Peaks in spring
–
Limited by sunlight
Productivity in tropical oceans
• Sunlight strong year-round
• Nutrients limited by strong, permanent thermocline
• Productivity:
–
Steady, low rate
–
Limited by nutrients
–
Exceptions:
•
Upwelling areas
•
Coral reefs
Productivity in temperate oceans
• Sunlight varies seasonally
• Nutrients limited by thermocline
• Productivity:
–
Spring bloom limited by nutrients
–
Fall bloom limited by sunlight
Energy flow in marine ecosystems
• Categories of organisms:
– Producers
– Consumers
– Decomposers
Ecosystem energy flow and efficiency
• Energy is passed between trophic (feeding) levels
• Transfer efficiencies:
– Algae = 2%
– Other levels = 10%
Comparison between a food chain and a food web
Biomass pyramid
• At each step up the pyramid, there is/are:
– Larger organisms
– Fewer individuals
– A smaller total biomass
Ecosystems and fisheries
• Fishery = fish caught from the ocean by commercial fishers
• Largest proportion of marine fish are taken from shallow shelf and coastal waters
Fisheries and overfishing
• World total marine fish production has increased dramatically
• Overfishing occurs when adult fish are harvested faster than their natural rate of
reproduction
Fisheries and bycatch
• Some fishing practices produce large amounts of incidental catch (bycatch):
– Purse seine nets set for tuna can trap dolphins
– Driftnets (gill nets) take many unwanted species
Fisheries management
• Fisheries management seeks to maintain a long-term fishery by:
– Assessing ecosystem health
– Determining fish stocks
– Analyzing fishing practices
– Enforcing catch limits
• Fisheries management does not regulate the number of fishing vessels