Zooplankton responses to threats from UV and predation

Sammanfattning: Ultraviolet radiation (UV) has been present longer than any biological life, and during the development of life the radiation intensities were initially high. Eventually a vital ozone layer developed, but despite that this layer screens out most of the high energy wavelengths, some still enter the biosphere and is potentially harmful to life on earth. This thesis investigates the effects of combined threats of UV radiation and predation on zooplankton ecology. My results show that zooplankton use several different defense mechanisms when challenging threats from UV and predation. The primary defenses studied were vertical migration and accumulation of UV protective compounds. The main compounds accumulated were melanin, carotenoids and mycosporine-like amino acids (MAAs). When exposed to UV treatments the zooplankton generally increased or retained their contents of these substances and when released from UV exposure they generally reduced their contents. Exposure to fish cues lead to reduced carotenoid concentrations but did not affect the MAA concentrations. The blend of carotenoids and MAAs in copepods was, moreover, governed by the availability of MAAs in the food source with increasing carotenoid accumulation at low MAA availability. Over all, there was a negative relation between carotenoids and MAAs indicating that the blend can be optimized to meet a changing environment in terms of UV and predation threats. Furthermore, there was a trait compensation between behavioral and UV protective defenses where the zooplankton generally either used vertical migration or UV protective pigmentation, and not both defenses at the same time. Among Daphnia all individuals responded to the UV threat but only the large individuals (>0.9 mm) reacted with vertical migration in response to predation threat, indicating a size-structured response to UV and predation threats. The effects of UV on the zooplankton community were, finally, moderate and long-term population dynamics for several species of zooplankton was astonishingly similar between UV and non-UV treatments. This leads to the conclusion that zooplankton communities are well buffered in response to UV changes, due to efficient defenses like vertical migration and UV protective pigmentation, suggesting that future potential increases in UV radiation may only have moderate impact on zooplankton community composition.