| Summary: | It is known that copepods can sense solar UV and avoid it vertically or horizontally, but no in situ studies have been documented to monitor their responses to diurnal solar radiation changes. Here, we provided in situ evidence that zooplankton sense changes in solar radiation during a diurnal solar cycle. By comparing the abundance of the zooplankton in a shaded water column with that in the non-shaded adjacent area, we found that, on a cloudy day with low solar radiation levels, the ratios of zooplankton biomass in the shaded areas to those in nearby non-shaded water ranged from 0.90 to 1.49. However, on sunny days with high solar radiation levels, the ratios ranged from 0.83 to 2.88, with the amount of zooplankton in the shaded water being higher than that in the non-shaded area and higher during the periods of higher irradiance levels. These results indicated that the horizontal migration of zooplankton may be a protective strategy against stressful solar radiation.
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