Size-specific clearance rates of the ctenophore Mnemiopsis leidyi based on in situ gut content analyses

The ctenophore Mnemiopsis leidyi can consume large amounts of zooplankton prey.However, quantitative measurements of feeding rates, based on field data, arescarce. We measured the time required by the invasive M. leidyi to digest naturallyoccurring prey species in the Gullmar fjord, Sweden. Digestion times were relatedto prey size and type, number of prey in the gut and size of the predator. Largeprey species or many prey in the gut resulted in longer digestion times comparedwith small or few prey, but digestion time also varied with the size of M. leidyi. Theprey- and predator-specific digestion times were used together with in situ preyconcentrations and gut contents of M. leidyi to calculate the clearance rates.Clearance rate as a function of ctenophore size is presented for the most abundantmesozooplankton: Acartia sp., Oithona sp., Oikopleura dioica and Penilia avirostris. Onthe basis of the relation between digestion time and the carbon content ratiobetween prey and predator, we discuss the possible effects of mixed prey assemblageson the estimates of clearance rates

Cascading effects of the ctenophore Mnemiopsis leidyi on the planktonic food web in a nutrient-limited estuarine system

Increasing biomasses of gelatinous zooplankton presumably have major implications for the structure and function of marine food webs at large; however, current data on lower trophic levels are scarce, as most studies have focused on the immediate effects on zooplankton and fish larvae only. We examined the short-term impact of larvae and adults of the invasive ctenophore Mnemiopsis leidyi on a summer planktonic food web in the estuarine southern Baltic Sea, with special emphasis on the microbial loop. Grazing by M. leidyi reduced the mesozooplankton biomass, followed by increased dinoflagellate biomass in treatments with M. leidyi. While chlorophyll a increased most in the treatments with M. leidyi, small phytoplankton and ciliates decreased in all treatments. M. leidyi had a slight effect on bacterial abundance, but not on bacterial production, ectoenzymatic activities, or community composition. Undetectable levels of phosphate and a gradual accumulation of dissolved organic carbon during the experiment suggested a malfunctioning microbial loop scenario. The experiment shows that direct and indirect short-term effects of M. leidyi on the estuarine food web are limited to higher trophic levels and indicates that top-down and bottom-up consequences of M. leidyi expansions on the microbial loop will likely depend on local nutrient conditions