1. Shchapova E.
   
   Institute of Lake Baikal Biology, Irkutsk State University, Lenin Str., 3, Irkutsk, 664025, Russian Federation (ru)
2. Cherkashina S.
   
   Institute of Lake Baikal Biology, Irkutsk State University, Lenin Str., 3, Irkutsk, 664025, Russian Federation (ru)
3. Rzhechitskiy Ya.
   
   Institute of Lake Baikal Biology, Irkutsk State University, Lenin Str., 3, Irkutsk, 664025, Russian Federation (ru)
4. Гассиев Д. Д. (Gassiev D.)
5. Шадрин Н. В. (Shadrin N.)

Introduction
The Baikal Natural Territory includes Lake Baikal and extreme water bodies such as swamps, saline, and hypersaline lakes. Studying their biota is essential for understanding adaptation limits, speciation, and relict species conservation. This study assesses productivity, biodiversity, and biotechnological potential of these ecosystems.

Methods
A total of 9 water bodies (swamps and lakes of varying salinity) were surveyed, with samples collected in 2024–2025. Zooplankton was filtered (25–200 L) through an Apstein net and fixed in 10% formalin. Amphipods were collected by kick sampling and fixed in 70% ethanol. Water pH, temperature, and conductivity were measured using a HANNA HI98129 meter. Dissolved oxygen was measured with an EXTECH 407510 analyzer. Species identification and counting were performed using an Olympus SZ61TR stereomicroscope. Biomass was calculated using length–weight regressions. For amphipods, the COI gene was targeted. DNA was extracted using a DNA-Sorb kit and sequenced by the Sanger method on a Nanofor-05 instrument. Sequences were aligned in Unipro UGENE using MUSCLE. Phylogenetic trees were constructed using iTOL and IQ-TREE.

Results
Extreme water bodies exhibited a wide range of abiotic conditions: pH 3.6 – 10.4, conductivity 2 – >4000 µS/cm, dissolved oxygen 0 – 31.0 mg/L (supersaturation), and summer temperatures up to 28.9°C. A total of 56 invertebrate species were identified, dominated by Arctodiaptomus salinus, Keratella quadrata and Eucyclops serrulatus. In 2024, mass occurrence of K. quadrata (up to 487,200 ind./m³) was recorded for the first time in the region. Gammarus lacustris found in several brackish lakes belonged to the East Asian genetic group by COI.

Conclusions
Extreme water bodies of the Baikal Natural Territory host high biodiversity under contrasting physicochemical conditions. G. lacustris shows potential for biotechnological applications (aquaculture, feed base).