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Effects of algae on the Secchi transparency of the southeastern Mediterranean Sea
Author(s) -
Megard Robert O.,
Berman Tom
Publication year - 1989
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.1989.34.8.1640
Subject(s) - secchi disk , irradiance , pelagic zone , colored dissolved organic matter , environmental science , seawater , phytoplankton , attenuation coefficient , oceanography , mediterranean sea , attenuation , atmospheric sciences , mediterranean climate , physics , eutrophication , optics , geology , ecology , biology , nutrient
The mean transparency (37.5 m) measured by Secchi disk at “blue‐water” pelagic sampling stations in the southeastern Mediterranean Sea during 3 yr was greater than in most other seawater. Our measurements in the Mediterranean and those by others in the English Channel, Atlantic Ocean, and Pacific Ocean obey the Lambert‐Bouguer law if it is specified that transparency depends on the vertical attenuation coefficient for downward irradiance in the waveband of underwater sunlight that has lowest attenuance. Mean downward irradiance (±95% C.L.) at the depth where a Secchi disk disappears, calculated from 187 paired estimates of transparency and attenuation coefficients from these oceanic regions, is 22±1% of surface irradiance in this waveband. This value is significantly larger than the value (18%) that has been used widely since an early study by Poole and Atkins, who calculated attenuation coefficients incorrectly. As predicted by the Lambert‐Bouguer law, reciprocal transparency was proportional to the attenuation coefficient for the most‐penetrating waveband. Attenuance of this waveband and reciprocal transparency both increase linearly with increasing concentration of phytoplankton chlorophyll in pelagic water. Chlorophyll concentrations in pelagic Mediterranean water were very low (0.02–0.14 mg m −3 ), but attenuance by units of phytoplankton chlorophyll was about 10 times larger than in more productive seawater, probably because blue light was predominant and many of the phytoplankton were very small coccolithophorids (cell diameters <10 µ m) with optically dispersive cell surfaces. Measurements of transparency by Secchi disk are as accurate and precise as estimates of attenuation coefficients calculated from irradiances measured at sea with photoelectric sensors. The Poole‐Atkins equation for transparency in terms of the waveband with lowest attenuance describes the variation of transparency as well as the Duntley‐Preisendorfer equation does. Much of the information needed for interpreting satellite ocean‐color imagery could be obtained very efficiently with closely spaced measurements of transparency from surface vessels during synoptic cruises.