Dependence of Long-Term Dynamics of Zooplankton in the Ob River on Interannual Changes in Hydrological and Hydrochemical Parameters

Here we summarize a long-term study on qualitative and quantitative composition of zooplankton (Cladocera, Copepoda, Rotifera) in the Ob River. We carried out these investigations at 13 sampling stations of the Middle and Low Ob in the years 1994, 1996, 1999, 2001, 2002 and 2009. It was found that the species richness of all zooplankton and abundance of cladocerans and rotifers is significantly determined by the temperature conditions of the month preceding sampling. In contrast, among other factors, we revealed that pH decreases as well as phosphate and nitrate concentrations increase zooplankton abundance. Dissolved oxygen and oxidizable organic substances (BOD5) were positively correlated to copepod population levels (according to abundance and biomass); on the other hand, an increase in difficult-to-oxidize substances (COD) inhibited their development. During this study, we found that high water levels had a positive influence on zooplankton richness in river itself probably due to being downstream from Ob River floodplain lakes.

The influence of thiacloprid on the feeding behaviour of the copepod, Diacyclops bicuspidatus, preying on nematodes

Nowadays, neonicotinoids are the most widespread insecticides. As neurotoxins with high toxicity to many arthropods, they provide effective pest control, but it also became apparent that they are impacting non-target organisms in freshwater habitats. This study investigated the influence of the neonicotinoid thiacloprid on the behaviour of the copepod, Diacyclops bicuspidatus, feeding on different nematode species (Caenorhabditis elegans, Plectus aquatilis, Panagrolaimus sp.) and on a natural nematode community. All experiments demonstrated that thiacloprid led to decreased feeding of D. bicuspidatus (EC50 values 116-158 μg l−1). In addition, in the experiment where nematodes from a natural sediment were offered as prey, copepod feeding was significantly reduced. The copepod feeding response was a more sensitive endpoint than acute and chronic toxicity tests with daphnids that are routinely used for regulatory risk assessment. An impact on feeding is also a parameter with immediate relevance for the copepod population (reduced fitness) and the associated ecosystem.

Oxygenation of anoxic sediments triggers hatching of zooplankton eggs

Many coastal marine systems have extensive areas with anoxic sediments and it is not well known how these conditions affect the benthic–pelagic coupling. Zooplankton lay their eggs in the pelagic zone, and some sink and lie dormant in the sediment, before hatched zooplankton return to the water column. In this study, we investigated how oxygenation of long-term anoxic sediments affects the hatching frequency of dormant zooplankton eggs. Anoxic sediments from the brackish Baltic Sea were sampled and incubated for 26 days with constant aeration whereby, the sediment surface and the overlying water were turned oxic. Newly hatched rotifers and copepod nauplii (juveniles) were observed after 5 and 8 days, respectively. Approximately 1.5 × 10 5 nauplii m −2 emerged from sediment turned oxic compared with 0.02 × 10 5 m −2 from controls maintained anoxic. This study demonstrated that re-oxygenation of anoxic sediments activated a large pool of buried zooplankton eggs, strengthening the benthic–pelagic coupling of the system. Modelling of the studied anoxic zone suggested that a substantial part of the pelagic copepod population can derive from hatching of dormant eggs. We suggest that this process should be included in future studies to understand population dynamics and carbon flows in marine pelagic systems.