Ecophysiological Responses Of Two Copepods To Ultraviolet Radiation And Ocean Acidification

The oceans have absorbed a large amount of CO2 emitted by humans,leading to ocean acidification.In the meantime,the depletion of atmospheric ozone and the shallowing of the upper mixed layer of the oceans bring about an increase in the amount of UV radiation received by plankton in this layer.Copepods are the key zooplankton,participating in the process of ocean food webs by feeding on phytoplankton.In this paper,Acartia spinicauda and Calanopia thompsoni off the coast of Xiamen were studied,and their ecophy sic logical responses to UV radiation and elevated pCO2 projected for future ocean acidification have been examined.The main results are as follows:A.spinicauda and C.thompsoni were able to avoid UV radiation actively.A.spinicauda was mainly sensitive to UV-B,while C.thompsoni evaded both UV-A and UV-B radiation.These behavioral characteristics were not affected by increase pCO2/pH drop.Under different PAR levels,they showed the opposite tropisms:A.spinicauda is a positive phototaxis species,mainly distributed in areas with higher light intensity,and C.thompsoni is a negative phototaxis species that mainly distributed in the lowlight intensity area.UV radiation inhibited copepods by reducing the rates of grazing,clearance,and ultimately aggravated their death.The grazing of A.spinicauda was mainly inhibited by UV-B irradiance,while UV-A-induced inhibition was low.After being exposed to UV radiation,the grazing and clearance rates of A.spinicauda gradually recovered within 48 h under PAR condition.In contrast,the grazing of C.thompsoni was more affected by UV-A than UV-B.Under the influence of UV,the grazing and clearance rates decreased over time,both by about 50%compared with the initial level.The grazing and clearance rates of A.spinicauda and C.thompsoni exposed to full-spectrum of solar radiation were significantly reduced by UV radiation.And UV radiantion did not affect respiration rate significantly.Under the combined impacts of high CO2 and UV radiation,the rates of grazing,clearance and respiration of two copepods changed in different ways.The CO2-inducedacidification increased the respiration rate of these two copepods,but there was no significant difference compare to the eontrol.As for C.thompsoni,its grazing and clearance rates increased under influence of UV radiation after 72 h exposure,by 34%and 39%,respectively.Besides,mortality rate under the high CO2 increased within 72h after the UV exposures,though there was no significant difference from the controlIn summary,UV radiation inhibited the rates of grazing and clearance of A.spinicauda and C.thompsoni,and exacerbated their deaths.However,both A.spinicauda and C.thompsoni had sensitivity of UV radiation,so they could actively migrate away from high UV exposures.In addition,acidification did not aggravate the negative effects of UV within the experimental periods.