Ocean Color Remote Sensing Pete Strutton, COAS/OSU

Outline 1.What do we mean by ‘Ocean Color’? 2.How are the measurements made? 3.What parameters can be derived? 4.What are these data used for? 5.Where are the data?

Components of a remote sensing system source signal raw data processing / dissemination cal/val sensor

Ocean color (chlorophyll) Passive measurement - energy source is the Sun In contrast to altimetry, SST etc, looks at subsurface, not ‘skin’ Measures light emitted from the ocean (careful to distinguish between ‘emission’ and ‘reflection’) Actual parameter measured (raw data) is normalized water leaving radiance, often denoted nL w Most of the signal (>90%) at the satellite is NOT ocean color - atmospheric interference Also interference from other colored material in the ocean sediments, ‘colored dissolved organic matter’ (CDOM)

Chlorophyll / ocean photosynthesis LHC 2 e -  Fluorescence  f heat  h ADP+P ATP NADP + 2H + NADPH 2

The ocean color measurement and how it’s used EtEt LwLw Main signals: Atmosphere, reflection and ocean color

Measuring chlorophyll from space The instrument The satellite The Sea-viewing Wide Field of view Sensor: SeaWiFS

What SeaWiFS sees in one day The gap here is caused by the satellite tilting as it passes over the equator

Global picture of ocean and land pigments

A problem with visible remote sensing: Clouds 1 day

3 days

8 days

14 days

1 month

Clouds Severity varies with location and season When viewing multi-day composites, variable ‘sample size’ Coastal fog can have the same effect as clouds Other complicating factors Colored dissolved organic matter (CDOM) - mostly breakdown products from phytoplankton and terrestrial sources. Other components of river runoff such as sediments. Diminished in open ocean, aka Case I waters

Colored Dissolved Organic Matter

Chlorophyll fluorescence LHC 2 e -  Fluorescence  f heat  h ADP+P ATP NADP + 2H + NADPH 2  p +  f +  h = 1 Light energy not used for photosynthesis is lost as heat and fluorescence

Blue light induced chlorophyll fluorescence in Tobacco leaf. A. photographed in white light. B. taken in the low steady state of fluorescence, 5 min after the onset of illumination. The bright red fluorescing upper part of the leaf is where photosynthesis has been blocked by the herbicide duiron (DCMU). (From Krause and Weis, 1988) LHC PSI e-e-  L 683 heat (ATP & NADPH 2 ) LHC PSI  L 683 heat (ATP & NADPH 2 ) DCMU  p +  f +  h = 1

FLH vs. chlorophyll FLH vs. CDOM

Sea Surface Temperature Chl a Chl Fluorescence Line Height (°C) (mg m -3 ) (W m -2 mm -1 sr -1 ) MODIS Terra L2 1 km resolution scene from October 3 rd 2001 From OSU-COAS EOS DB Station

Chlorophyll fluorescence from space Passive measurement (sun is the initial source) Offers the possibility of phytoplankton physiology from space Also potentially a chlorophyll proxy that is unaffected by: Sediments Other colored material However, signal is very small, and our understanding is evolving

Satellite-based productivity algorithms Motivation: –Chlorophyll measurements give biomass, we want productivity (rate) –Global coverage 1000s of 14 C measurements of primary productivity have been made and continue to be made Do not accurately reflect global temporal and spatial variability Need models of primary productivity - take satellite data as input and provide integrated primary productivity as the output Allows us to quantify the spatial distribution of productivity, but also… Temporal changes at time scales from days to decades (NASA's main goal)

Other Ocean Color Parameters Particulate Organic Carbon (POC): Potentially more useful for carbon budgets than phytoplankton chlorophyll Particulate Inorganic Carbon (PIC): Indicative of a specific type of phytoplankton (coccolithophorids), common in polar waters. Photosynthetically Available Radiation (PAR) Diffuse attenuation coefficient Terrestrial biosphere products

The sensors and data SeaWiFS: Launched 1997, very successful, well-calibrated and still operating. MODIS Aqua: Launched 2002, has fluorescence channel that SeaWiFS lacks. Data available at spatial resolutions from ~1km to 9km Data available at daily resolution with the caveats previously discussed Data gateway depends on user: NASA directly, CoastWatch, others…

Some miscellaneous applications