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.

Predation follows competition in depth selection behaviour of Cladocera in a deep lake (E Poland)

Predation follows competition in depth selection behaviour of Cladocera in a deep lake (E Poland)Observations of the vertical distribution of Cladocera in the pelagic zone of the deep Lake Piaseczno (38.8 m) focused on 6 species:Diaphanosoma brachyurum, Daphnia longispina, D. cucullata, Bosmina coregoni, B. longirostris, andChydorus sphaericus.The animals displayed clear depth preferences, and their migration patterns were limited to small depth amplitudes. The results show that vertical habitat segregation of cladocerans in the lake consists mostly in the mechanism of competition for accessible food resources, rather than in the mechanism of predation avoidance.

Plankton, Chemistry, and Physics of Lakes in the Churchill Falls Region of Labrador

Of 10 lakes and a reservoir in the Churchill Falls region of western Labrador studied in 1970–71, nine lakes were 12 m or less deep, humic, thermally unstratified, and had Secchi disc transparencies between 3.5 and 8.25 m. Maximum surface temperatures of 16–17 C in the nine lakes were generally reached by mid-August. Michikamau Lake (area 1980 km2, depth 84 m) rarely exceeded 12 C and had a Secchi disc transparency of 13 m. Conductivities and alkalinities (as CaCO3) ranged from about 13 μmho/cm and less than 6.5 mg/liter, for lakes draining granite, gneiss, or quartz bedrock, to about 30 and 15, for those draining gabbro or slate. Maximum live phytoplankton biomasses in 1970 ranged from 70 mg/m3, in Michikamau Lake, to 739 mg/m3, in Ossokmanuan Reservoir, and, in general, there was a positive correlation with conductivity. Diatoms were the most abundant group, in particular Asterionella formosa, Tabellaria fenestrata, and Rhizosolenia eriensis; however, flagellated Chrysophyta, mainly species of Dinobryon, were more abundant in the reservoir than in the natural lakes. Predominant zooplankters included Diaptomus minutus, Cyclops scutifer, Holopedium gibberum, Daphnia longiremis, D. galeata mendotae, and Bosmina coregoni. Maximum seasonal standing crops ranged from 1600 to 8910 individuals/m3, with the highest densities being found in the reservoir. The chemical and biological data suggested that the reservoir 8 yr after filling, showed little evidence of trophic upsurge and was possibly entering a period of trophic depression.

Diel Vertical Movements and Feeding of Underyearling Sockeye Salmon and the Limnetic Zooplankton in Babine Lake, British Columbia

The use of a high frequency echo sounder and a midwater trawl in Babine Lake, British Columbia, in 1967 revealed a precise diel vertical movement of underyearling sockeye salmon. From early July to September, from about 1.5 hr after sunrise to 1.5 hr before sunset, the young sockeye were in two distinct layers at about 20 and 35 m, each layer about 6 m thick. About 1.5 hr before sunset the two layers began to ascend. About 0.5 hr after sunset all fish were within 3 m of the surface, and during darkness they were dispersed throughout the top 5–15 m with most fish being below the thermocline. Soon after the first light of dawn the fish usually tended to move toward the lake surface and then descended rapidly to the daytime depths.By early October the pattern had changed markedly. During daylight the fish were still found in roughly two layers at about 24 and 40 m. However, the evening ascent did not commence until about 0.5 hr before sunset, the ascent was much slower, and the fish did not come to the surface but were dispersed between 9 and 27 m. This change in behaviour was temporally associated with a cooling of the epilimnion and a decrease in intensity of feeding.Although the zooplankton of Babine Lake is composed of eight major species, only Bosmina coregoni and the calanoid copepod Heterocope septentrionalis displayed pronounced diel vertical movements. The former inhabited the surface waters during the day and were found in maximum abundance at 12–21 m at night. The latter was found at 21–30 m during the day and the surface waters at night. The major part of the zooplankton standing stock was found above 9 m.Young sockeye fed most intensively in the evening as they approached the surface and again at dawn just as they commenced the descent. At those times the most common food item was Daphnia longispina, followed by B. coregoni and H. septentrionalis. These three species were strongly selected by young sockeye, since they were numerically much less abundant than other limnetic zooplankton species. In August, terrestrial insects were occasionally of major importance. At midday the upper layer of sockeye (about 20 m) was feeding on H. septentrionalis whereas the lower layer (about 35 m) was not feeding or was feeding at a low intensity.The double layer of young sockeye during daylight occurred at least in the North Arm of Babine Lake and may be related to feeding. In July and August the fish in the upper layer had less food in the pyloric section of the stomach from 0900 to 1300 hr than did fish in the lower layer, whereas the volumes of food were similar in the two layers by late afternoon. Fish in the upper layer had much more fresh H. septentrionalis in the cardiac section from 1200 to 1800 hr than did fish in the lower layer. The ecological significance of the diel vertical movement is discussed briefly.