The Pew Oceans Commission - Kenyon College

The Pew Oceans Commission - Kenyon College

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

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