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Measurements and Modeling

To determine the effects of solar radiation on marine ecosystems the penetration of UV and PAR into the water column needs to be measured (Montecino and Pizarro, 1995; Björn et al., 1996). Marine waters show large temporal and regional differences in their concentrations of dissolved and particulate absorbing substances. Jerlov (1968) classified marine waters into nine types of coastal and five types of open ocean waters depending on their transmission. The ratio between the 0.1% depths for UV-B and PAR can be used to calculate the detrimental effects on algae by solar UV-B radiation that hits the organisms in the euphotic zone (Piazena and Häder, 1997). Recent developments allow an accurate measurement of the underwater light field (Morrow and Booth, 1997). Yellow substances, chlorophyll a and other photosynthetic pigments, as well as organic and inorganic particulate material mainly cause the spectral attenuation of UV-B radiation in the water column. The modulation of open ocean underwater light field by phytoplankton has been extensively studied (Figueroa et al., 1994), but reports on the underwater light field in coastal habitats colonized by macroalgae are scarce, particularly in the UV-B range. Algal canopies modify the light quality by absorption and scattering of the incident light.

    Recently, a network of dosimeters (ELDONET) was installed in Europe ranging from Abisko in Northern Sweden to Gran Canaria with a total of 26 instruments (Santas et al., 1997). Two of the instruments are located at high altitudes and six are located under water where they operate in conjunction with a terrestrial counterpart (Fig. 4.1). These instruments record solar radiation fully automatically in three channels (UV-B, UV-A, PAR). The data are transmitted to a server in Pisa and are available to the public on the Internet in graphical and numerical form (http://power.ib.pi.cnr.it:80/eldonet/). Other networks have been installed, e.g., Biospherical Instruments Inc. (San Diego, California) is responsible for obtaining and distributing irradiance data from the U.S. National Science Foundation UV Spectroradiometer Monitoring Network (http://www.biospherical.com). Data is available from stations in San Diego (California), Ushuaia (Argentina), Barrow (Alaska) and three Antarctic stations (South Pole, Palmer and McMurdo). Great care is necessary to guarantee quality control of the light measurements (Seckmeyer et al., 1994).
 
Fig. 4.1 Locations of the terrestrial, aquatic and high altitude instruments in the ELDONET network of solar dosimeters. Circles terrestrial instruments, squares underwater instruments, triangles high altitude instruments.

Underwater UV irradiance has been measured at Northern latitudes (79°N, Spitsbergen, Norway). The measurements showed significant short term increases due to local ozone holes as that reported in winter 1994-1995 in the European SESAME campaign (Second European Stratospheric Arctic and Midlatitude Experiment) in which a depletion of 20-30% of the stratospheric ozone was observed (Ott and Amanatides, 1994).

 
Fig. 4.2 Chlorophyll concentration in the ocean. Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center

Another approach to quantify the underwater light climate is modeling (Zeng and Stamnes, 1994). Within limits the optical characteristics of the water column can be obtained from satellite data (e.g., CZCS and SeaWiFS, Fig. 4.2). These instruments cover only the visible range, but attempts have been and are being made to extrapolate the data into the UV range so as to make these remotely sensed data relevent to UV studies.

    There have been great efforts to develop techniques for measuring algal biomass by using remote sensors. Most work has focused on quantifying chlorophyll from phytoplankton in surface oceanic waters (Brown et al., 1995). Piazena and Häder (1997) discussed the applicability of remote sensing to detect and quantify phytoplankton in the water. One major obstacle for remote monitoring is the fact that overflying instruments mainly determine the surface signal. Therefore, profound knowledge of the vertical distribution of phytoplankton, as well as the distribution of algal groups is necessary to derive a quantitative analysis of biomass productivity (Piazena and Häder, 1997). Mooring of optical instruments has been used to determine phytoplankton production in the oligotrophic waters of the Sargasso Sea (Waters and Smith, 1994). Meinesz et al. (1991) have studied macroalgal biomass and distribution on the bottom of clear waters in Polynesia.

    Recently Behrenfeld and Falkowski (1997a) have evaluated models used to estimate photosynthetic rates derived from satellite-based chlorophyll concentration. In addition, they (Behrenfeld and Falkowski, 1997b) have evaluated various primary productivity models, provided a classification scheme for these productivity models and show that many of these, apparently different, models show fundamental synonymy. If equivalent parameterizations are used for satellite derived chlorophyll and the maximum chlorophyll-specific carbon fixation rate, then estimates of global annual primary production were found to be due primarily to these variables.


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