open oceans: pelagic ecosystems ii

Open Oceans: Pelagic Ecosystems II

Global scale patterns of pelagic productivity

What are controlling processes?

• Primary Productivity– Different estimates of productivity– Gross Primary Productivity– Net Primary Productivity– Standing crop

• and Grazing Rates

What is “productivity”?

primary productivity is defined as the total quantity of carbon fixed by autotrophs a rate expressed as grams of carbon fixed per

square meter of sea-surface per unit of time gross primary production is the total

amount of organic matter produced by autotrophs

net primary production is the energy remaining after respiratory needs have been met NPP = Gross Primary Production - Respiration

Questions to consider

• Why should we care about patterns of biological productivity in oceans?

• What are the spatial patterns of productivity?

• What mechanisms promote or limit productivity?

Why should we care about productivity?

• Photosynthetic activity in oceans created current O2-rich atmosphere

• Plankton form ocean sediments & fossil fuels

• Plankton are a critical part of “carbon pump” that influences atmospheric CO2

• Phytoplankton form the base of food webs and associated biological diversity

• Limits to productivity may limit the amount of harvestable biomass from ocean ecosystems

Measuring Primary Productivity: Data

• Standing crop methods– Chlorophyll concentration (water extraction,

satellite)– Cell counts (flow cytometers)

• Rate measurements– Light-dark bottle method– Carbon-14 uptake

• Advantages & disadvantages

chlorophyll density & temperature

Link between producers & grazers

Measuring Primary Productivity: Inferences

• Each direct measure of GPP has disadvantages• Computer models integrate different parameters

to estimate rates of productivity through the water column and over time

• Model components (Field et al. Science 281:237-240)

– chlorophyll concentration– water depth in photic zone– fraction of water column where photosynthesis is

light-saturated– surface temperature

Results of productivity model

Some patterns

• average primary productivity in the oceans is ~50 g C/m2/yr

• 300 g C/m2/yr considered relatively high rate of

primary productivity 

• low rates of primary productivity typically 20 to 30 g C/m2/yr 

• What accounts for this variation?

Comparing the makeup of water and plankton

• Mean Elemental Ratios of N, and P

• Organisms: 16.0N / 1P

• Sea Water: 14.7N / 1P

• On average, seawater lacks sufficient organic N to take advantage of the available phosphorus

Limited water mixing further limits nutrient availability

Testing models: measuring plankton in open oceans

• Techniques for studying plankton consumers

• Plankton levels may be dropping (NASA study)

Changes in Productivity

From productivity to food webs: A size-based pelagic food web

Grazing pressure

Limits & Flows of NPP

Marine “Snow”

krill

A spatially realistic pelagic food web

What are links among trophic levels in pelagic systems?

Large phytoplankton vary with depth

Nutrient distribution with depth

Distribution of food web biomass

The global nitrogen cycle

Influences on biologically useful Nitrogen

Diatom mats can migrate vertically

Consequences of active transport of NO3

Links to oceanic nekton