LONG-TERM SHIFTS AND RECENT PATTERNS IN LAKE OHRID’S ROTIFER COMMUNITY
Keywords:
zooplankton, abundance variability, vertical zonation, ecological monitoringAbstract
Pelagic rotifers are sensitive components of freshwater zooplankton and can provide useful evidence of ecological change. Lake Ohrid, an ancient and oligotrophic lake, offers an unusually long record for examining temporal variability in its pelagic rotifer assemblage. This study reconstructs the available evidence from published investigations conducted between 2000 and 2010, the 2019 monogonont checklist, and contemporary Hydrobiological Institute monitoring from summer 2025 to spring 2026. The historical 2000–2008 monthly series recorded 13 pelagic taxa and showed pronounced seasonal and interannual variability. Mean annual rotifer density ranged from 22,544 ind. m⁻³ in 2000 to 6,988 ind. m⁻³ in 2008, while Kellicottia longispina and Keratella cochlearis were the principal abundant taxa. The largest proportion of the rotifer population was repeatedly associated with the 20–40 m zone. The 2004–2006 investigations confirmed low abundance typical of the oligotrophic pelagial, whereas the 2010 survey identified eight rotifer taxa within a 16-species zooplankton assemblage and highlighted the ecological importance of the metalimnion. The 2019 checklist recognized 12 pelagic monogonont taxa. Recent monitoring again documented K. longispina and recurrent C. hippocrepis, together with strong seasonal restructuring of the zooplankton community. The combined evidence indicates persistence of a characteristic core assemblage rather than a simple long-term increase or decline. Because historical and contemporary datasets differ in sampling design, depth coverage and reporting units, direct regression across all periods is not statistically defensible. The most robust current signal is therefore taxonomic continuity combined with marked seasonal and interannual variability. Continued standardized monitoring of abundance, composition and vertical distribution, linked to temperature, oxygen, nutrients and phytoplankton variables, is required to determine whether recent changes represent normal variability or an emerging ecological shift.
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