Physiological mortality of planktonic ciliates: Estimates, causes, and consequences

Abstract

AbstractContrasting physiological mortality with predator‐induced mortality is of tremendous importance for the population dynamics of many organisms but is difficult to assess. This is especially true for tiny organisms, such as most protists, which do not leave any carcasses behind. I performed a meta‐analysis using planktonic ciliates as model organisms to estimate the maximum physiological mortality rates (δmax) across pelagic ecosystems in relation to environmental and biotic factors. The median δmax of planktonic ciliates was 0.62 d−1 and did not differ between marine and freshwater species. Maximum ciliate mortality rates were species‐specific and affected by their specific growth rates (rmax), cell volume, and ability to encyst. Cyst‐forming species had, on average, higher δmax than species unable to encyst. Maximum mortality rates were positively related to rmax, but, in contrast to the latter, δmax appeared unaffected by temperature. I conclude that (1) in the ocean, physiological mortality is more critical for controlling ciliate population size than ciliate losses imposed by microcrustacean predation, but (2) in many lakes, the opposite holds; (3) cyst formation is an effective ciliate trait to cope with the high mortality of motile cells upon starvation. The lack of a temperature effect on δmax deserves further study; if correct, planktonic ciliates may take advantage of rising ocean and lake temperatures, with important implications for the pelagic food web.