Estimating In Situ Zooplankton Non-Predation Mortality in an Oligo-Mesotrophic Lake from Sediment Trap Data: Caveats and Reality Check.

BACKGROUND: Mortality is a main driver in zooplankton population biology but it is poorly constrained in models that describe zooplankton population dynamics, food web interactions and nutrient dynamics. Mortality due to non-predation factors is often ignored even though anecdotal evidence of non-predation mass mortality of zooplankton has been reported repeatedly. One way to estimate non-predation mortality rate is to measure the removal rate of carcasses, for which sinking is the primary removal mechanism especially in quiescent shallow water bodies. OBJECTIVES AND RESULTS: We used sediment traps to quantify in situ carcass sinking velocity and non-predation mortality rate on eight consecutive days in 2013 for the cladoceran Bosmina longirostris in the oligo-mesotrophic Lake Stechlin; the outcomes were compared against estimates derived from in vitro carcass sinking velocity measurements and an empirical model correcting in vitro sinking velocity for turbulence resuspension and microbial decomposition of carcasses. Our results show that the latter two approaches produced unrealistically high mortality rates of 0.58-1.04 d(-1), whereas the sediment trap approach, when used properly, yielded a mortality rate estimate of 0.015 d(-1), which is more consistent with concurrent population abundance data and comparable to physiological death rate from the literature. ECOLOGICAL IMPLICATIONS: Zooplankton carcasses may be exposed to water column microbes for days before entering the benthos; therefore, non-predation mortality affects not only zooplankton population dynamics but also microbial and benthic food webs. This would be particularly important for carbon and nitrogen cycles in systems where recurring mid-summer decline of zooplankton population due to non-predation mortality is observed.

Efficiency of transfer of essential polyunsaturated fatty acids versus organic carbon from producers to consumers in a eutrophic reservoir.

One of the central paradigms of ecology is that only about 10% of organic carbon production of one trophic level is incorporated into new biomass of organisms of the next trophic level. Many of energy-yielding compounds of carbon are designated as 'essential', because they cannot be synthesized de novo by consumers and must be obtained with food, while they play important structural and regulatory functions. The question arises: are the essential compounds transferred through trophic chains with the same efficiency as bulk carbon? To answer this question, we measured gross primary production of phytoplankton and secondary production of zooplankton and content of organic carbon and essential polyunsaturated fatty acids of ω-3 family with 18-22 carbon atoms (PUFA) in the biomass of phytoplankton and zooplankton in a small eutrophic reservoir during two summers. Transfer efficiency between the two trophic levels, phytoplankton (producers) and zooplankton (consumers), was calculated as ratio of the primary production versus the secondary (zooplankton) production for both carbon and PUFA. We found that the essential PUFA were transferred from the producers to the primary consumers with about twice higher efficiency than bulk carbon. In contrast, polyunsaturated fatty acids with 16 carbon atoms, which are synthesized exclusively by phytoplankton, but are not essential for animals, had significantly lower transfer efficiency than both bulk carbon, and essential PUFA. Thus, the trophic pyramid concept, which implicitly implies that all the energy-yielding compounds of carbon are transferred from one trophic level to the next with the same efficiency of about on average 10%, should be specified for different carbon compounds.