An impact of two different flood pulses on phyto-and zooplankton communities of the two largest floodplain lakes of the Daugava River were studied in the spring and summer of 2005. Samples of phyto-and zooplankton were taken at weekly and biweekly intervals. At the end of March, a medium size pulse of spring flood was observed. At the beginning of May, it was followed by an unusually high pulse of flush floods caused by heavy rainstorms in the local drainage area. An overall increase of biomass and the number of taxa of planktonic communities during the filling and drainage phases of the spring floods was stated. The pulse of the flush floods resulted in a lower total biomass and higher species diversity, and can be regarded as a disturbance event. The high species diversity represented by a hump-shaped pattern caused by an intermediate disturbance that was measured by the rate of water level change during the floods according to Intermediate Disturbance Hypothesis (IDH). This study reflected both linear and a slight hump-shaped relationships between the rate of water level change and the number of phytoplankton and zooplankton taxa.

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Cladocerans are a considerable food source for such planktivorous fish as salmonids. In Latvia 26 lakes are categorized as priority fish lakes for salmonids and therefore studies on cladocerans as salmonid fish prey are of particular interest. For the study of salmonids' food base, i.e. zooplankton diversity, and the genetical diversity of Daphnia cucullata in four Latvian lakes zooplankton samples were taken in the summers of 2010 and 2011. The most dominant species of Cladocera were Diaphanosoma brachyurum, Daphnia cucullata, Bosmina crassicornis, Bosmina longispina, and Bosmina longirostris. The highest diversity and biomass of cladocerans were observed from June to late July. This can be explained by seasonality. The common cladoceran species such as Daphnia cucullata may be used as a good model organism for ecological genetics research. We found D. cucullata in all the investigated lakes. The genetic structure and plasticity in D. cucullata were studied using nuclear random amplified polymorphic DNA (RAPD). Carl Roth random primers PCR were used for the amplification of DNA samples. The DNA markers were polymorphic segments with band sizes from 500 to 3000 bp. The number of polymorphic DNA bands of D. cucullata in the four investigated lakes was different.

The potential distribution of tropical fish species in Eastern Europe—Gambusia holbrooki (introduced for biological control) and Poecilia reticulata (aquarium species, found in waste waters of big cities)—tend to be of particular interest in terms of global climate change. After GIS modeling of our own data and findings listed in the GBIF databases (2278 points for G. holbrooki and 1410 points for P. reticulata) using the Maxent package and ‘ntbox’ package in R, 18 uncorrelated variables of 35 Bioclim climatic parameters from CliMond dataset, it was found out that by 2090 guppies will appear in the south of Ukraine (Danube river’s estuary, as well as in several places in the Caucasus and Turkey with habitat suitability > 0.3–0.5). G. holbrooki will also slightly expand its range in Europe. Limiting factors for G. holbrooki distribution are: bio1 (Annual mean temperature, optimum +12–+24 °C) and bio19 (Precipitation of coldest quarter (mm). Limiting factors for P. reticulata are: bio1 (optimum +14–+28 °C), bio4 (Temperature seasonality), bio3 (Isothermality). Unlike G. holbrooki, guppies prefer warmer waters. Such thermophilic fish species do not compete with the native ichthyofauna, but they can occupy niches in anthropogenically transformed habitats, playing an important role as agents of biological control.

Helminth infracommunities were studied at 174 sites of Latvia in seven hosts from six amphibian taxa of different taxonomical, ontogenic and ecological groups. They were described using a standard set of parasitological parameters, compared by ecological indices and linear discriminant analysis. Their species associations were identified by Kendall's rank correlation, but relationships with host size and waterbody area were analysed by zero-inflated Poisson and zero-inflated negative binomial regressions. The richest communities (25 species) were found in post-metamorphic semi-aquatic Pelophylax spp. frogs, which were dominated by trematode species of both adult and larval stages. Both larval and terrestrial hosts yielded depauperate trematode communities with accession of aquatic and soil-transmitted nematode species, respectively. Nematode loads peaked in terrestrial Bufo bufo. Helminth infracommunities suggested some differences in host microhabitat or food object selection not detected by their ecology studies. Associations were present in 96% of helminth species (on average, 7.3 associations per species) and dominated positive ones. Species richness and abundances, in most cases, were positively correlated with host size, which could be explained by increasing parasite intake rates over host ontogeny (trematode adult stages) or parasite accumulation (larval Alaria alata). Two larval diplostomid species (Strigea strigis, Tylodelphys excavata) had a negative relationship with host size, which could be caused by parasite-induced host mortality. The adult trematode abundances were higher in larger waterbodies, most likely due to their ecosystem richness, while higher larval abundances in smaller waterbodies could be caused by elevated infection rates under high host densities.