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
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
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
Testing models: measuring plankton in open oceans • Techniques for studying plankton consumers • Plankton levels may be dropping (NASA study)
Marine “Snow” krill