Effects of Environmental Gradients on the Biomass of Alien Cercopagis pengoi in the Neva Estuary

Biological invasions are a global trend due to globalization of the world. An important goal of invasion biology is to identify environmental characteristics that may make a region receptive to invasions. We tested the hypothesis that environmental variables have stronger effect on the biomass of the alien Cercopagis pengoi in the Neva Estuary than biotic interactions with other zooplankton species. Hierarchical cluster, principal component and Pearson correlation analyzes were used to elucidate changes within the zooplankton community along environmental gradients. The data analysis showed that the biomass of C. pengoi was higher in waters with higher salinity, low temperature, high transparency, lower chlorophyll a concentration and lower proportion of chlorophyll in particulate suspended matter. The biomass of C. pengoi showed significant positive correlations with biomasses of its potential preys. However, we did not find direct significant negative correlations between C. pengoi and larger zooplankton predators Leptodora kindtii and Bythotrephes spp. This research generally supports the hypothesis that if environmental variables are appropriate for an exotic species, then that species is likely to invade successfully, regardless of the native species already present.

Mercury-methylating bacteria are associated with zooplankton: a proof-of-principle survey in the Baltic Sea

Methylmercury (MeHg) is a potent neurotoxin that biomagnifies in marine food-webs. Inorganic mercury (Hg) methylation is generally considered to be conducted by bacteria associated with sediment or detritus, but endogenous methylation by the gut microbiome of animals in the lower food webs is another possible source. We examined the occurrence of the bacterial gene (hgcA), required for Hg methylation, in the guts of dominant Baltic zooplankters. A qPCR assay targeting thehgcAsequence in three main clades (Deltaproteobacteria, FirmicutesandArchaea) was used in the field-collected specimens of copepods (Acartia bifilosa, Eurytemora affinis, Pseudocalanus acuspesandLimnocalanus macrurus) and cladocerans (Bosmina coregoni maritimaandCercopagis pengoi). All copepods were found to carryhgcAgenes in their gut microbiome, whereas no positive amplification was recorded in the cladocerans. In the copepods,hgcAgenes belonging to onlyDeltaproteobacteriaandFirmicuteswere detected. These findings suggest that endogenous Hg methylation can occur in zooplankton and may contribute to seasonal, spatial and vertical MeHg variability in water column and food webs. Additional molecular and metagenomics studies are needed to identify bacteria carryinghgcAgenes and improve their quantification in microbiota.