Zooplankton of Lake Srebarna and Adjacent Water Bodies (Malak Preslavets Marsh, Garvanski Marshes, Kalimok Fishponds and the Danube River)

Introduction: Zooplankton communities are key structural and functional components of lake and river ecosystems. Zooplankton represents an important link in the transfer of energy between primary producers and secondary consumers.

Scope: The present study provides an overview of zooplankton communities in Lake Srebarna and adjacent water bodies of the Danube floodplain terrace, including Malak Preslavets Marsh, Garvanski Marshes, Kalimok Fishponds (part of the Kalimok-Brashlen Protected Area), and the Danube River, over a 30-year period (1997–2026). Following the restoration of the hydrological connection between Lake Srebarna and the Danube River in 1994, systematic studies of zooplankton were initiated as part of the development of the first Management Plan for the Srebarna Managed Nature Reserve. The investigations were conducted in relation to the variable flooding regime of the Danube River.

Malak Preslavets Marsh, located near the village of Malak Preslavets, is a eutrophic lake with a relatively stable water level, supplied mainly by groundwater and precipitation. A dike with an overflow structure separates the wetland from the Danube River.

Garvanski Marshes are located within a protected area in northeastern Bulgaria, in the territories of the villages of Garvan and Popina, Silistra Province. The protected area comprises three marshes – Leshtava (also known as Garvansko Marsh), Momchila, and the Marsh immediately adjacent to the village.

The Kalimok Fishponds are located north of the village of Nova Cherna, Tutrakan Municipality, in close proximity to the Danube River. They form part of the Kalimok-Brashlen Protected Area.

The Danube River represents the common hydrological and ecological link connecting all investigated water bodies.

Materials and Methods: Nineteen sampling sites were established in Lake Srebarna, selected to characterise representative sections of the aquatic ecosystem under the variable hydrological regime of the Danube River. Four sampling sites were established in Malak Preslavets Marsh. Sampling in the Kalimok Fishponds and Garvanski Marshes was conducted during periods of high Danube water levels, including three sites in the Kalimok Fishponds and one site in Momchila Marsh. Sampling sites along the Danube River included Novo Selo, Marten, Vetren, and two additional sites located in the vicinity of the Srebarna Reserve, designated by numbers.

Zooplankton samples were collected from the surface water layer at depths of 0.1–0.8 m by filtering 50 L of water through a conical Apstein plankton net made of mill gauze with a mesh size of 86 μm. Samples were collected in plastic containers and preserved with 4% formalin. The same sampling methodology was applied to the other investigated water bodies.

Taxonomic identification of zooplankton organisms was performed using a stereomicroscope and light microscope. For the identification of selected rotifer taxa, including representatives of the genera Asplanchna, Asplanchopus, Cephalodella, Conochiloides, Dicranophorus, Harringia, and Synchaeta, the structure of the mastax was examined following digestion of soft tissues with 30% KOH.

The taxonomic status of Cladocera was determined according to Manuylova (1964), Cyclopoida according to Monchenko (2003), Calanoida according to Damian-Georgescu (1966), and Rotifera according to Kutikova (1970).

Results: Field investigations covered the period from 1997 to 2026, inclusive, and were conducted within various projects implemented by CLOE/IBER under contracts with the Executive Environment Agency and the Bulgarian National Science Fund, as well as through activities of non-governmental organisations, LTER-Bulgaria, and independent research efforts.

During the investigated period (1997–2026), a total of 271 zooplankton taxa were recorded at genus, species, and subspecies levels in Lake Srebarna, Malak Preslavets Marsh, Garvanski Marshes, the Kalimok Fishponds, and the Danube River.

Asplanchna sp. is well represented in Srebarna lake: Asplanchna sieboldii (Leydig) in 1998, Asplanchna priodonta Gosse in 1999, Asplanchna girodi Guerne in 2003, and Asplanchna silvestris Daday in 2022.

In August 2024, four Rotifera were newly recorded for Lake Srebarna: Macrochaetus subquadratus Perty, Tripleuchlanis plicata plicata (Levander), Lecane (s. str.) ungulata (Gosse), and Keratella paludosa obtusa Hauer. A possible reason for the emergence of the new species is the low level of the lake in 2024.

Eutrophication processes in Lake Srebarna continued through 2026, with the zooplankton community including species associated with more acidic conditions, such as Brachionus urceus sericus Rousselet and Trichotria truncata truncata (Whitelegge).

Conclusion: Over the 30-year study period (1997–2026), a total of 271 zooplankton taxa were recorded in Lake Srebarna, Malak Preslavets Marsh, Garvanski Marshes, the Kalimok Fishponds, and the Danube River. These included 66 taxa of Branchiopoda (42 Cladocera and 24 Copepoda) and 205 taxa of Rotifera. The long-term observations demonstrate considerable variation in zooplankton composition associated with changes in hydrological connectivity, flooding regime, and trophic conditions. The reappearance and emergence of specific taxa over the study period provide valuable indicators of ecological changes within the investigated Danube floodplain ecosystems.