Upward phosphorus transport by Daphnia diel vertical migration

In many lakes, zooplankton show a distinct diel vertical migration (DVM) behavior, especially during periods of stratification. Excretion products of these zooplankton could potentially cause an upward nutrient transport and consequent nutrient enrichment for phytoplankton in the epilimnion. We quantified the upward transport of phosphorus by the cladoceran Daphnia DVM experimentally by adding a radioactive tracer (33P) to the hypolimnion of large indoor mesocosms and measuring tracer accumulation in the epilimnion over time. During the daytime, when all Daphnia were found in the hypolimnion, no phosphorus transport from the hypolimnion into the epilimnion took place. As soon as the Daphnia started their upward migration, around dusk, we observed a continuous increase in phosphorus concentration in the epilimnion. The amount of phosphorus transported was in a biologically meaningful range. Our results strongly suggest that Daphnia vertical migration presents a continuous nutrient supply for the epilimnion

High trophic plasticity in the mixotrophic Mastigias papua-Symbiodiniaceae holobiont: implications for the ecology of zooxanthellate jellyfishes

International audienceThe trophic ecology of mixotrophic, zooxanthellate jellyfishes potentially spans a wide spectrum between autotrophy and heterotrophy. However, their degree of trophic plasticity along this spectrum is not well known. To better characterize their trophic ecology, we sampled the zooxanthellate medusa Mastigias papua in contrasting environments and sizes in Palau (Micronesia). We characterized their trophic ecology using isotopic (bulk δ 13 C and δ 15 N), elemental (C:N ratios), and fatty acid compositions. The different trophic indicators were correlated or anti-correlated as expected (Pearson’s correlation coefficient, r P > 0.5 or < -0.5 in 91.1% of cases, p < 0.05), indicating good agreement. The sampled M. papua were ordered in a trophic spectrum between autotrophy and heterotrophy (supported by decreasing δ 13 C, C:N, proportion of neutral lipid fatty acids (NLFA:TLFA), n-3:n-6 and increasing δ 15 N, eicosapentaenoic acid to docosahexaenoic acid ratio (EPA:DHA)). This trophic spectrum was mostly driven by sampling location with little influence of medusa size. Moreover, previous observations have shown that in a given location, the trophic ecology of M. papua can change over time. Thus, the positions on the trophic spectrum of the populations sampled here are not fixed, suggesting high trophic plasticity in M. papua . The heterotrophic end of the trophic spectrum was occupied by non-symbiotic M. papua , whereas the literature indicates that the autotrophic end of the spectrum corresponds to dominant autotrophy, where more than 100% of the carbon requirement is obtained by photosynthesis. Such high trophic plasticity has critical implications for the trophic ecology and blooming ability of zooxanthellate jellyfishes