Optical Characterization of the waters of the Cape Fear River Plume and Onslow Bay

The penetration, absorption, and availability of light are of great importance in assessing the physico-chemical characteristics of a water mass. Changes in the optical characteristics of water masses may be used to determine the relative influences of anthropogenic or natural impacts on the biological components of coastal system. Coastal waters can vary greatly in their optical water quality and these variations affect photosynthetic compensation depths for phytoplankton and macrophytes and set fundamental limits on the rate of primary production of coastal waters. In optical studies it is necessary to determine both inherent and apparent optical properties; apparent optical latter properties vary in relation to both the content of the water and the ambient light field. Inherent optical properties of coastal waters are affected by optically important variables such as turbidity (tripton), phytoplankton (chlorophylls, carotenoids, biliproteins, etc.), and colored dissolved organic matter (CDOM). The individual components have an additive effect to the spectral diffuse attenuation coefficient K_d(lambda)total. That is

K_d(lambda)total= K_d(lambda)water+K_d(lambda)phytoplankton+K_d(lambda)tripton+K_d(lambda)CDOM

K_d(lambda)total can be determined by measuring spectral absorbance of water samples. Partitioning the contributions to K_d(lambda)total can contribute to the refinement of algorhythms used in remote sensing of coastal waters, specifically for the purposes of monitoring the efficacy of management activities.

In this element of the Coastal Ocean Research and Monitoring Program (CORMP) we will characterize the underwater light field within the Cape Fear River plume, examine the component contributions to the optical properties of these waters, and compare them to the optical characteristics of water masses outside of the plume.

Methods:

Diffuse attenuation coefficient for Photosynthetically Active Radiation (PAR, 400-700nm) K_dPAR: At each sampling station, simultaneous scalar irradiance measurements are made in the air and in the water using spherical quantum sensors (LiCor LI-193SA) connected to a LiCor LI-1000 datalogger. The in-air sensor is mounted above a 0.6 m circular black disk on a 2-m pole. This setup minimizes variation due to boat movement and variation gue to light reflected from the water surface. A light profile is determined by lowering the in-water sensor to a series of measurement depths (0.5-1.0 m intervals) and recording average (5-10 sec) scalar irradiance from both sensors. The in-air sensor reading is used to adjust the in-water readings for changes in the incident irradiance over the course of the profile. K_dPAR is calculated from the slope of the regression of natural log-transformed percentages of surface irradiance (in water PAR:in air PAR) against depth. The data from the seven CFP stations are krigged to generate contour maps of K_dPAR.

Partitioning the spectral diffuse attenuation coefficient K_d(lambda): Surface water samples are analyzed for total and mineral solids, chlorophyll a, turbidity, and absorption by dissolved and particulate matter. Analyses of raw water versus filtered water samples is used to separate the contribution to K_d(lambda) between particulate and dissolved components of the water mass. Absorbance of particulate matter is determined by illuminating material collected from a known volume of water (500 ml to 4l) on a GF/F filter with a fibre optic light source and measuring the absorbance spectra normalized with readings from a moistened blank filter. The sample filter is then soaked in methanol for 1h to extract phytoplankton pigments, and scanned again to estimate absorption by non-algal particulate matter. Absorption is converted to units of m^-1 by multiplying by the area of the filter and dividing by the volume filtered. Absorption by colored dissolved organic matter (CDOM) will be measured on water samples filtered through a 0.2痠 Nucleopore filter. Absorbance is read using a fibre optic scanning spectrometer in 10 cm cells against distilled water blanks. Absorbance readings are multiplied by 2.303 to convert to base e and divided by 0.1 (dm m^-1).

Data Products:

The data from this study component will be used to produce an optical database for the waters of the monitored area. These optical data will be used to calibrate and to develop algorithms for remotely sensed data. Temporal and spatial changes in the optical classification within and adjacent to the River plume can be used for event-response detection. Specific optical characteristics may be used as metrics for establishing response targets for management actions.

Links to some current data:

Cape Fear River Plume Light profile data

Summary figure

3-D Summary figure

View Animation of CFP plume PAR variation

CFP March 2000 PAR k

CFP April 2000 PAR k

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Cape Fear River Plume Spectral Attenuation data

CFP February 2000 spectral k

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Onslow Bay Light profile data

OB September 2000 PAR k

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Onslow Bay Spectral Attenuation data

OB April 2000 spectral k

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References

Gallegos, C. L., D. L. Correll, and J. W. Pierce. 1990. Modeling spectral diffuse attenuation, absorption, and scattering coefficients in a turbid estuary. Limnol. Oceanogr. 35:1486-1502.

Gallegos, C. L and W. J. Kenworthy. 1996. Seagrass depth limits in the Indian River Lagoon (Florida, U.S.A.): Application of an optical model. Est. Coastal Shelf Sci. 42:267-288.

Kirk, J. T. O. 1994. Light and photosynthesis in aquatic ecosystems. 2^nd edition, Cambridge Univ. Press, New York, 401 pp.

McPherson, B. F. and R. L. Miller. 1987. The vertical attenuation of light in Charlotte Harbor, a shallow subtropical estuary, southwestern Florida. Estuar. Coastal Shelf Sci. 25:721-737.