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The modern distribution of invasive amphipods of Baikal (Gmelinoides fasciatus (Stebbing, 1899), Micruropus possolskii Sowinsky, 1915) and Ponto-Caspian origin (Pontogammarus robustoides Sars, 1894, Chelicorophium curvispinum (Sars, 1895)) in Lake Ladoga is described. The heterogeneity of the distribution of quantitative indicators of amphipods in different parts of the lake was established. The most widespread species in the littoral zone was G. fasciatus. A decrease in the contribution of G. fasciatus in the bottom communities of macrozoobenthos was noted. The dominant role of G. fasciatus was observed only in those parts of the lake where other species of invasive amphipods were not established yet. The range of Ponto-Caspian amphipods is limited by the boundaries of the Volkhov Bay. A significant increase in the quantitative development of P. robustoides and active southward spread of M. possolskii along the west coast of the lake indicate the ongoing restructuring of littoral biocenoses.

Amphipods play the leading role among alien species in Lake Ladoga. In particular, the Baikal species Micruropus possolskii Sowinsky, 1915 is becoming increasingly important here. The purpose of this paper is to present updated data on the appearance of M. possolskii in Lake Ladoga and on the temporal and spatial dynamics of its development and expansion in this water body. The species was initially recorded in the Shchuchiy Bay of Lake Ladoga in 2003. Its spread to the south to the Petrokrepost Bay (where it was recorded in 2017) is shown. The further expansion of the species in Lake Ladoga and its significant quantitative development in the colonized biotopes and possible expansion to Lake Onega and the Neva Bay of the Gulf of Finland are expected.

The issue of the formation of the compound composition of low-molecular-weight organic compounds in aquatic macrophytes, which make up their low-molecular-weight metabolism, has hardly been studied when introducing them into new habitats. The study aims to compare the component composition of the low-molecular-weight metabolome of sago pondweed (Potamogeton pectinatus L.) on the low-molecular-weight organic compounds contained in the essential oil from the invasive population in the Lake Ladoga and the populations from the native range in Astrakhan Region, Russia. Gas chromatography–mass spectrometry was the major research method. Upon the invasion of P. pectinatus into Lake Ladoga, there was a change in the content of various groups of low-molecular-weight organic compounds in the essential oil and a change in the complex of major components. Fatty acids (29.3–40.0%) and ketones (14.5–18.5%) prevailed in the low-molecular-weight metabolome of sago pondweed in the lakes of the native range. Ketones (27.4%) and aldehydes (18.1%) were the main groups in this species inhabiting Lake Ladoga. About a third of the compounds in the composition of the low-molecular-weight metabolome were specific both to the invasive population of the sago pondweed in the Lake Ladoga and to the population from the lakes of the native range. The plasticity of the metabolism of P. pectinatus allows it to adapt to a wide spectrum of abiotic conditions and different biological environments and to settle in new habitats, primarily those under anthropogenic impact.

The difference of the evenness index (DE′) is offered for an assessment of the ecological state of zoobenthos communities. The index is deduced on the basis of the Shannon indexes calculated according to abundance and biomass data and differentiates between these informational estimations (Shannon diversity) of evenness of species in any given community. Conclusions made by Pianka about the predomination of organisms with r and K life strategy in communities impacted and unimpacted by ecological stress, as well as the ABC-curves method suggested by Warwick, were used as the basis of the functioning mechanism for the suggested index. The examples of index applicability are demonstrated by an assessment of materials collected in freshwater, estuarine, and marine waterbodies during one-time surveys and long-term monitoring observations. The results are compared with zoobenthos assessments made on the basis of some other indexes. Conclusions concerning the efficiency of the DE′ have been made and some of its advantages over other indexes are shown.

The role of invasive amphipods (Baikalian species Gmelinoides fasciatus (Stebbing, 1899) and Ponto-Caspian Pontogammarus robustoides G.O. Sars, 1894 and Chelicorophium curvispinum (G.O. Sars, 1895)) in transformation of the Lake Ladoga ecosystem is assessed. It is shown that presently G. fasciatus plays the main role. Invasion of G. fasciatus into Lake Ladoga has led to an increase in productivity of the littoral benthic communities and more efficient utilization of the energy coming into the littoral zone. It is confirmed that P. robustoides and C. curvispinum are established in the lake. The zone of their dwelling may extend, and the role in the littoral habitats may increase. The analysis of the ecological state of the lake with new indices of the concept for assessing the risks of aquatic species invasions reveals a high degree of transformation of the Lake Ladoga ecosystem.

Feeding selectivity and assimilability of the Baikal invader Gmelinoides fasciatus (Stebbing, 1899) in Lake Ladoga at feeding by various types of plant and animal food have been studied experimentally. The food demand has been assessed at different assimilation levels. Freshwater shrimp G. fasciatus has been shown to have distinct selective capacity with respect to different kinds of plant and animal food.

The feeding of the Baikal invader Gmelinoiudes fasciatus (Stebbing, 1899) in Lake Ladoga has been studied based on field and experimental data. The food spectrum of this amphipod was revealed; it includes many plant and animal species. Differences in the feeding of G. fasciatus living in different littoral biotopes were shown. The ecological and physiological rations of this species were determined. The effect that the bottom substrate has on the feeding rate of the amphipod was estimated. The introduction of G. fasciatus into Lake Ladoga provided a more adequate utilization of the energy accumulated by the primary producers and its transfer to higher trophic levels. This Baikal amphipod occupied a niche that had been, in effect, vacant. This made the invader capable of maintaining the quantitative parameters of its population at a very high level, and it largely reorganized the flow of matter and energy in the littoral zone of the lake.

Dynamics of the size-age structure and sexual structure of the population and characteristics of reproduction and life cycle have been studied in the Baikal invader Gmelinoiudes fasciatus (Stebbing, 1899) in Lake Ladoga. The studied characteristics display both similar features with and differences from populations of this species in other waterbodies. It is found that the size and age structure of the G. fasciatus population changes in the circannian aspect under the influence of complex of environmental factors. The sexual structure of the population is dynamic, but a sex ratio of approximately 1: 1 is maintained in the circannian cycle. Six principal periods of hatching are recognized. The main factor determining the timing of the start and termination of the reproduction period in G. fasciatus is temperature. The weight of eggs oviposited by the female over the whole life cycle (about 21.5 cal in the energy equivalent) amounts to about 60% of the female body weight.

The invasive amphipod Chelicorophium curvispinum (G. O. Sars, 1895) (Amphipoda, Crustacea) was first recorded in Lake Ladoga in the area of Volkhov Bay in August 2009. The data on quantitative development of this species and two other alien species of amphipods (Baikal Gmelinoides fasciatus (Stebbing, 1899) and Ponto-Caspian Pontogammarus robustoides G.O. Sars, 1894) in three different biotopes in the littoral zone of the lake are presented. The necessity of further detailed studies of new alien species distribution in Lake Ladoga is emphasized since their introduction can pose a serious threat to the ecosystem in the littoral of largest European lake.

It is shown that meiobenthos plays an important role in the secondary production by zoobenthos in lakes, as well as in the degradation of organic matter. In large lakes (Lake Ladoga, Lake Onega, Lake Paijanne, Lake Constance), the ratio of meiobenthic production to the production of macrobenthos is on average 50–61%. In the small Latgalian lakes (Latvia), this proportion is different: in the profundal of these lakes it varies from 92.5% in a naturally clean mesotrophic lake to 0.0004% in the most eutrophic lake, and in the littoral of lakes – from 578–1476% in mesotrophic lakes to 148–306% in eutrophic ones. The level of production of littoral meiobenthos does not depend on the trophic status of the lake, and can be equally high both in undisturbed mesotrophic lakes and in strongly eutrophicated lakes. The intensity of production of the littoral meiobenthos in oligotrophic and mesotrophic lakes, on the one hand, and in eutrophic lakes on the other, are not reliably distinguished. There is a clear tendency for a decrease of the role of profundal meiobenthos with regard to the transformation of energy flows in lake ecosystems, both with an increase in eutrophication and with an increase in the amount of organic matter in the benthal available from phytoplankton.