remote sensing of coastal habitats: challenges: adjacency effects atmospheric correction (no null...

Remote sensing of coastal habitats:

Challenges: Adjacency effects Atmospheric correction (no null NIR band)

Currently addressed by Europeans (e.g. Belcolour)Atmospheric correction – similarity spectra (Rudick),

Doron & Babin).

Use of Red & NIR bands to obtain [chl]Development of algorithms for [Sediment]In-situ validation

Impetus for European effort: cross-EU litigation.

Related contributions:

Linking in-situ optical properties and coral-reef biogeochemistry (see below).

Linking in-situ optical properties to particle composition and dynamics in coastal environments (resuspension-aggregation-settling) - OOXIX

Optical properties related to biogeochemistry:

DOM, Hydrocarbons – fluorescence (UV-ex, VIS-em), absorption.

TSS, TSM, POC – attenuation, scattering, ocean color.

Phytoplankton pigments – fluorescence, absorption, ocean color.

Particulate size distribution – spectrum of attenuation, near forward scattering, ocean color.

Particulate composition (index of refraction) – back-scattering to scattering ratio, degree of polarization.

Temperature –NIR radiance.

Nitrate, sulphides – UV absorption.

The effect of bottom substrate on inherent optical properties: Evidence of biogeochemical processes.

Emmanuel Boss (U. Of Maine) & Ron Zaneveld (Oregon State U.)

Special thanks to: Jim Washburn, Francois Baratange, Dick Zimmerman, Dave Burdige, and Rob Wheatcroft. Funded by ONR.

Biogeochemical processes near coral reefs:

• Sediment resuspension by currents.

• Removal of selected particulate material from the water column by benthic filter feeders such as corals and sponges.

• Emergence/disappearance of organisms (‘particulate material’) from the sediment into/outoff the water column.

• Mechanical resuspension of sediment by benthic organisms and water column organisms such as fish.

• Release of colored particulate and dissolved waste products into the water by in-water organisms (e.g. zooplankton and fish) and by release of dissolved waste products by organisms (e.g. coral and sponges).

• Diffusive and advective (when sediment is resuspended) flux of colored dissolved material released from sediment pore water.

Using optics for studying biogeochemical processes

Advantages:

• In-situ.• Non-invasive.

Disadvantages:

• Only net effect of by all processes can be measured. • ‘bulk’ analysis: all particles of given sizes (as determined by pre-

filters) are measured together.

Methods:

CTDac-9

inlet

Data loggers

Human Sea-Sore

Can sample very close to the bottom.

Optical properties and their relation to biogeochemical parameters:

Attenuation at a red wavelength: (c(660)):

•Indicator of total suspended particulate mass (volume or concentration. CDM absorption at this wavelength is negligible).

•In the oceanic environment found to correlate well with POC.

•To a lesser degree it also varies with size distribution and composition.

Optical properties and their relation to biogeochemical parameters:

CDM absorption and ‘flavor’:

•In the coastal ocean CDM absorption is an indicator of the concentration of dissolved organic carbon, though the exact relationship varies between environments.

• Exponential slope of ag, indicator of CDM “flavor”. A steep slope (~0.02) represents “fulvic”-like or low molecular weight material, while a smaller slope (~0.01) represents “humic”-like or high molecular weight material (Carder et al., 1989, Blough and Del Vecchio, 2002).ag= ag(0)*exp(s( - 0))

Optical properties and their relation to biogeochemical parameters:

Chlorophyll absorption:

•[chl][apg(676)-apg(650)]/0.014

•indicator, at a given light level, of phytoplankton biomass.

•Note: can be affected by non pigmented material (organic detritus and inorganic particles) when [chl] is low, since for it apg(676)<apg(650).

Optical properties and their relation to biogeochemical parameters:

– exponent of cp, indicator of tendencies of the particulate size distribution. The parameter will be larger when the sample is dominated by small particles.

cp= cp(0) (0-

Sampling site:

Lee stocking Isl.Bahamas:

Fringe reefs

Seagrass beds (dark)

Narrow outlets, characterized by ooids sediments in shallow areas (bright).

Bahamas banks

Exuma Sound

Results: Horizontal gradients in optical properties across coral-sand boundaries (N=15):

The variability in all properties was higher above the reef.

Attenuation was larger over sand (13/2).

Attenuation slope was larger over reef (12/3).

CDM absorption was larger over reef (13/2).

[chl] was larger over sand (11/4).

Implications: Reef is patchy. Reef filters [chl] containing large particles and produces CDM.

The variability in all properties was higher above the reef

Attenuation was larger over sand

CDM absorption was larger over reefAttenuation was larger over sandAttenuation slope was larger over reef

CDM absorption was larger over reefAttenuation was larger over sandNote: gradient in physical properties less than 0.01kg/m3

Results: Vertical gradients in optical properties above reef and sand: (N=11, Nreef=6, Nsand=5):

The variability at 10cm was higher than at any depth further away from bottom.

Attenuation increased away from reef (7/4).

Attenuation spectral slope decreased away from bottom substrate (9/2).

CDM absorption was largest near substrate (11/0).

CDM spectral slope increased away from substrate (11/0).

[chl] increased away from bottom substrate (7/4).

Implications: ‘bottom’ filters out large, [chl] containing particles. CDM with low spectral slope is produced in/on substrate.

Particle settling and resuspension

The vertical distribution of particles above the reef is opposite what is observed in a typical BBL.

Results:Role of bottom substrate in vertical gradient between pore waters and overlying bank waters:

Results:Role of Bahamas banks in export of DOC to Exuma Sound, time series of ~36hrs:

Inverted s-ag relationship relative to continental coast.DOC fluxed has higher spectral slope.

Conclusions:

Bottom processes affect optical properties.

Optical measurements are sensitive enough to infer biogeochemical processes associated with bottom substrates.

Local benthic processes can result in large-scale gradients between environments as observed in CDM absorption between the bank waters and the adjacent Exuma Sound.

ac-9 based :

Coulter counter based :

N=N(D0)D- dD

cp= cp(0) (0-

y = 1.0926x - 0.1271R2 = 0.9898

0

4

8

12

0 5 10

measured Chl (g/L)

estim

ated

chl

((a

676-

a 650

)/0.0

14)

Karp-Boss,2001, data from Oregon coast (N=25)

The variability at 10cm was higher than at any depth further away from bottom:

Blough and Green (1995)

Gradients in CDM between pore waters and overlying waters:

First generation sampling method…

Spinrad et al. (1983): Bishop (1999):

Example of a measurement series of pore waters:

Taken during ebbing tideDefinition of salinity may change in pore water (Hales).