Penetration of solar radiation into the water column of the central subtropical Atlantic Ocean—optical properties and possible biological consequences

Abstract

The optical properties of the waters as well as the penetration of both solar ultraviolet radiation (UVR) and photosynthetically active radiation (PAR) were analyzed at different stations of the central subtropical Atlantic Ocean during the AZORES II cruise of the research vessel ‘‘Hesperides’’ in April 1999 to assess the impact of solar UVR on microorganisms populating highly transparent oceanic waters. The investigation was based on direct spectral measurements of the scalar and downward-solar irradiance between 290 and 750 nm at different depths using a temperature-stabilized double monochromator spectroradiometer (Optronic, type 754) with a highly sensitive 4p sensor connected to the entrance slit by a 20-m quartz fiber cable. In addition, the Secchi depth was measured, and water samples of different depths at each station were analyzed to determine the concentration and optical properties of phytoplankton as well as attenuating substances such as seston and gelbstoff in the column. Using the spectral irradiance data at different depths as well as the vertical irradiance profiles at different wavelengths, the following parameters were calculated: the spectral attenuation coefficients, the spectral depths of penetration to 1% of the subsurface value (‘‘1% depths’’), the 1% depths for the ranges UV-B, UV-A and PAR, as well as the water type in the Jerlov system of optical classification. The optical properties of the waters investigated can be classified into the oceanic types OI–OII in the Jerlov system, which are characterized by very small concentrations of seston and of gelbstoff, which are the main absorbers for UV radiation in natural waters. The Secchi depths varied between about 15 m (type OII) to about 45 m (type OI) showing ratios to the 1% depths of PAR of about 0.21 to about 0.31. Values of the same order were found for the depths of the maximal concentration of chlorophyll a, which varied between 45 and 100 m during midnight and between 70 and 110 m during noon, showing also significant increases with increasing transparency of the waters. In contrast, the 1% depths for the penetration of solar UV irradiance were found between 12 and 19 m (type OII) and between 26 and 31 m (type OI) for UV-B whereas their values ranged for UV-A between 29 and 65 m (type OII) and between 55 and 93 m (type OI). Thus, solar UV-B penetrates up to about 25% of the photic zone in these waters whereas for UV-A about 75–93% were found. So, solar UVR may significantly affect both the photosynthetic activity by photoinhibition as well as the composition of the kind of species populating different depths of the column by further effects such as DNA damage and inhibition of motility. The role of UVR as a selection factor is shown by a strong absorption peak in the UV range of the water samples, which occurred only when the organisms