Abstract
High solar radiation along with extreme transparency leads to high penetration of solar radiation in the Red Sea, potentially harmful to biota inhabiting the upper water column, including zooplankton. Here we show, based on experimental assessments of solar radiation dose-mortality curves on eight common taxa, the mortality of zooplankton in the oligotrophic waters of the Red Sea to increase steeply with ambient levels of solar radiation in the Red Sea. Responses curves linking solar radiation doses with zooplankton mortality were evaluated by exposing organisms, enclosed in quartz bottles, allowing all the wavelengths of solar radiation to penetrate, to five different levels of ambient solar radiation (100%, 21.6%, 7.2%, 3.2% and 0% of solar radiation). The maximum mortality rates under ambient solar radiation levels averaged (±standard error of the mean, SEM) 18.4±5.8% h−1, five-fold greater than the average mortality in the dark for the eight taxa tested. The UV-B radiation required for mortality rates to reach ½of maximum values averaged (±SEM) 12±5.6 h−1% of incident UVB radiation, equivalent to the UV-B dose at 19.2±2.7 m depth in open coastal Red Sea waters. These results confirm that Red Sea zooplankton are highly vulnerable to ambient solar radiation, as a consequence of the combination of high incident radiation and high water transparency allowing deep penetration of damaging UV-B radiation. These results provide evidence of the significance of ambient solar radiation levels as a stressor of marine zooplankton communities in tropical, oligotrophic waters. Because the oligotrophic ocean extends across 70% of the ocean surface, solar radiation can be a globally-significant stressor for the ocean ecosystem, by constraining zooplankton use of the upper levels of the water column and, therefore, the efficiency of food transfer up the food web in the oligotrophic ocean.
Highlights
Zooplankton are essential components of the marine food web, relaying primary production from microbial algae to fish and seabirds [1]
The shapes of the relationship between mortality rate and the percent UV-B irradiance or the accumulated UV-B radiation received different, among species (Fig. 2), with most species showing an asymptotic relationship between mortality rates and the percent solar radiation or the accumulated UV-B radiation along the experiment, which was best described by the Michaelis-Menten equation (Table 1), and the mortality rate of Centropages sp. showing a linear increase with UV-B over the range of UV-B radiation tested (Fig. 2)
UV-B1/2mmax ranged from 1.5% to.50% of the UV-B radiation incident below the surface, averaging 1265.6% of incident solar radiation across taxa, corresponding to 1265.6 KJ m22. This indicates, when combined with the measured light extinction coefficients for UV-B, that the solar radiation sufficient to raise mortality to half of mmax radiation reaches down to depths ranging from 4.9 m to 29.9 m across taxa in clear coastal waters and 2.2 to 13.5 m in the more turbid waters of Obhur Creek (Fig. 3). These results confirm that Red Sea zooplankton are highly vulnerable to ambient solar radiation, with preliminary experiments, removing UV-B radiation, demonstrating that mortality is induced by the UV-B component
Summary
Zooplankton are essential components of the marine food web, relaying primary production from microbial algae to fish and seabirds [1]. Concerns on increasing exposure of marine organisms to UV-B radiation derive from consideration of the combined effects of a global increase of the UV-B radiation incident in the ocean and increased penetration into ocean waters. Evidence for increased UV-B penetration derives from indications, still awaiting confirmation, that the oligotrophic gyres, with the most transparent waters to UV-B, may be expanding in size [9] and that chlorophyll a concentration has declined globally at rates of about 1% per year, related to increasing sea surface temperature and vertical stratification [10]. For instance UV-B levels sufficient to cause significant mortality of photosynthetic plankton have been reported to penetrate down to 60 m in the oligotrophic waters of the subtropical Atlantic Ocean [11] and to 26 m in the Mediterranean Sea [12]. There is still limited understanding of the vulnerability of zooplankton to solar and UV-B radiation, as most studies have been conducted in alpine lakes and northtemperate coastal waters
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
