Hydrobiology

River fragmentation is a major anthropogenic disturbance that modifies hydrological conditions, influences abiotic parameters, and thus also affects phytoplankton composition, diversity, and abundance. Mountain river ecosystems are particularly vulnerable. In the local context of Caucasus rivers, disruptions of ecological continuity are also important because the flux of untreated wastewater or the discharge of drinking-water-quality effluents affects water quality and thus biota substantially. However, the response of phytoplankton communities to river fragmentation is not well studied yet. Therefore, the present study addresses such a gap in the knowledge by investigating the phytoplankton community of the heavily fragmented Hrazdan River in the central part of Armenia and evaluating the effects on its taxonomic composition, diversity, and community structure. Comprehensive analysis involving hydrochemical, hydrophysical, and phytoplankton metrics revealed that both river fragmentation and disruption of ecological continuity have a strong influence on phytoplankton communities. Factors such as severe organic enrichment, elevated nutrient loads, and critical oxygen depletion create chemical barriers that induce additional ecological fragmentation. PCA and cluster analysis showed major patterns of impacts of the interaction between physical barriers and localized anthropogenic stressors, which drive distinct taxonomic regrouping within the phytoplankton community. Overall, this study demonstrates that phytoplankton dynamics in regulated mountain rivers are primarily governed by the coupling between chemical stressors and fragmentation-induced habitat alterations, highlighting the need to conduct river restoration at various parts.

Interest is increasing to restore riparian buffer zones using native long-lived species, and black walnut trees (Juglans nigra) appear attractive candidates across a wide biogeographic range of the eastern United States of America. However, little research has investigated the potential unintended consequences of juglone (the allelopathic chemicals produced by these deciduous trees) on proximate stream communities. Our current investigation examined the colonization of stream macroinvertebrates into leaf packs containing chlorophyllous (spring canopy) leaves of one of four deciduous tree species differing in the levels of juglone and structural stiffness: black walnut, shagbark hickory, tulip poplar, and red oak. To evaluate the consistency of stream invertebrate community response across local streams, we deployed leaf packs for a week in a total of three different streams over three successive summers, then identified and enumerated the invertebrates within the packs. While there were significant differences across streams, the invertebrate communities colonizing the leaves of the different tree species was typically not different, and any differences present were unlikely caused by the level of juglones in the leaves. Thus, increased levels of juglones entering streams due to local plantings of black walnut are not likely to negatively affect the aquatic macroinvertebrates. However, the more rapid fragmentation of black walnut leaves compared with the other species examined suggests that the longevity of allochthonous coarse particulate material resources may be negatively impacted if other tree species are not interspersed with the black walnuts.

The article is a literature study supplemented with self-collected data on fish density, age and growth of smolt (Osmerus eperlanus) in Lake Mjøsa over the last 40–50 years. Smolt is an important prey fish for piscivorus brown trout, and the development of the trout population is described by catch reports. The smelt population in Lake Mjøsa declined in the period 1980 to 1990 but increased sharply in 2018 and 2023. Recruitment to the smelt population is positively correlated with air temperature in May and July. Increased smelt population is a possible reason why the large trout population also appears to have increased, while smelt has shown reduced individual growth in recent years. The trout population does not appear to have a decisive effect on the smelt population but may have contributed to increased mortality in smelt in the material from 2006 to 2022 as compared to 1980. The effect of trout stocking, made for the last time in 2021, is uncertain, but future catch reporting will provide a better answer to this.