Time Series Measurements and Algorithm Development at the FRONT Site on the New England Continental Shelf.

Funded by NASA SIMBIOS contract # 99011.

Principal investigators:  J. Ru Morrison† Heidi M. Sosik†

Collaborators:  Daniel L. Codiga§ and Scott M. Gallager†.

†  Woods Hole Oceanographic Institution Woods Hole, Woods Hole, MA 02543
§  Dept. of Marine Sciences, University of Connecticut, 1084 Shennecosett Rd., Groton, CT 06340


The optical properties of case II coastal ocean are influenced by a complex mixture of seawater constituents and a wide variety of physical processes.  Especially in regions where formation of physical and optical fronts is frequent, but temporally and spatially variable, this complexity makes interpretation of ocean color signals subject to large uncertainty.  The goal of the proposed research is to determine which processes and optically important constituents must be considered to explain ocean color variations associated with coastal fronts on the New England continental shelf.  To accomplish this goal we propose to carry out extensive time series sampling and to perform algorithm development and evaluation in collaboration with the NOPP-supported FRONT program.  FRONT is a three-year multi-disciplinary effort initiated in late 1999 aimed at understanding and modeling physical and biological processes associated with frontal formation and persistence at a study site located at the mouth of Long Island Sound.  The observational network includes an array of acoustically linked physical moorings and the Autonomous Vertically Profiling Plankton Observatory (AVPPO, a profiling mooring), which we have outfited with a suite of optical sensors.  Our efforts will focus on collection of a complete set of optical property observations and constituent analysis necessary for evaluating bio-optical algorithms for retrieving inherent optical properties and phytoplankton characteristics.  These include band-ratio algorithms, semi-analytical radiance inversion algorithms and algorithms which incorporate chlorophyll fluorescence.



Figure 1:  FRONT location and structure.

A)     SeaWiFS chlorophyll concentration (blue low, orange high, and clouds gray) from 6 October 1997.  The black lines show the position of fronts using the URI edge detection algorithm.
B)    Regions of frontal probability greater than 5 % are shown for January in blue and July in violet.  Colored dots and rings show the location and range of telemetering antenna for the FRONT array.
C)     The FRONT array showing locations of profiling moorings, ADCPs, and repeater nodes.  The distance between nodes is nominally 5 km.


Ed(z,?) Downwelling vector irradiance (413, 443, 490, 511, 555, 666, 683 nm)
Ed(0+,?) Incident irradiance (413, 443, 490, 511, 555, 666, 683 nm)
Lu(z,?) Upwelling radiance (413, 443, 490, 511, 555, 666, 683 nm)
aac9(z,?) Absorption (412, 440, 488, 510, 532, 555, 650, 676, 715 nm) from WETLabs ac-9
cac9(z,?) Beam attenuation (412, 440, 488, 510, 532, 555, 650, 676, 715 nm) from WETLabs ac-9
aa?(5,?) Absorption (442, 555) from HOBILabs a-?eta
bb(5,?) Backscattering (442, 555) from HOBILabs a-?eta
chl-fl(z) Chlorophyll fluorescence with a Wetstar fluorometer
c(z,660) Beam attenuation coefficient at 660 nm with SeaTech transmissometer
T(z), S(z) Temperature and conductivity with SeaBird probes

Figure 2:  Mooring schematic and photos.


Hydrographic data and Inherent Optical Properties

Figure 3:  From the initial four day AVPPO deployment, as part of the NOPP project, hydrographic data from the mooring showed the regular occurrence of colder, less saline water at semi-diurnal timescales, Panels A and B.  This was also apparent in the bio-optical data, from both the fluorometer, Panel C, and the ac-9 on the AVPPO, Panels D-F, as well as a diurnal signal.  Decomposition of the spectral absorption measured by the ac-9, achieved by assigning characteristic shapes to algal and non-algal fractions and minimizing the SOSD between predicted and observed spectra, suggested that the majority of the variability in the optical signal was due to changes in the non-algal component, Panels G and H (absorption values are at 440 nm).  The non-algal slope, S, was also retrieved using the decomposition and was within the range of previously observations (mean=0.0120 nm-1, std=2.5×10-4, N=1274).

ADCP Measurements

Figure 4:  Current vectors, measured using an ADCP mounted on the AVPPO winch sled, superimposed upon interpolated temperature data indicated that the colder less saline water was associated with a southeasterly flow.  Both a semi-diurnal and diurnal component can be seen in the current measurements shown by the greater magnitudes of the southerly flows around the middle of the days in the data shown.  Fourier transformation of the sea surface elevation showed the presence of M2, S2, O1, and K1 tidal harmonics.

Remote Sensing Reflectance

Figure 5:  The remote sensing reflectance from the AVPPO, obtained from measurements using OC-200 radiometer heads, showed two distinct shapes most clearly seen by the ratio of 550 to 510 nm, Panel A.  On all days, higher ratios were associated with colder less saline waters and lower ratios with warmer more saline waters, Panel B.  Most of the variability in the ratio appeared to be associated with variations in the the non-algal component from decomposition of spectral absorption measurements, Panel C.  There appeared to be no relationship with algal absorption estimates from the decomposition, Panel D.


Acknowledgments: This work was supported by NOPP, the SIMBIOS project (Morrison and Sosik), and NSF, Biological Instrumentation (Gallager).  Anne Canaday assisted with data collection and instrument calibration.