Chara-lakes with higher macrophyte diversity than vascular lakes: evidence from lacustrine ecosystems of the Pisz Forest

Charophytes are crucial components of freshwater ecosystems, yet their role in enhancing biodiversity remains underexplored. This study investigates the hypothesis that charophytes act as catalysts for the biodiversity of submerged macrophytes in lakes of the Pisz Forest, a protected Natura 2000 area in north-eastern Poland. We analysed macrophyte diversity across 46 lakes, comparing two habitat types: 3140 – hard oligo-mesotrophic waters with benthic vegetation of Chara spp., and 3150 – natural eutrophic lakes with Magnopotamion or Hydrocharition vegetation. Our findings indicate that lakes dominated by charophytes (habitat 3140) are characterised by significantly greater light penetration and deeper macrophyte colonisation. These conditions supported a greater proportion of macrophyte cover relative to lake area and resulted in a higher number of charophyte communities and total number of macrophyte communities. Biodiversity, as measured by the Shannon-Wiener index, was positively correlated with community richness and showed a unimodal relationship with the share of both vascular plants and charophytes, indicating a threshold beyond which diversity declines. Although vascular plant communities were similar across habitat types, charophyte-dominated lakes exhibited greater overall biodiversity. Moreover, most of the protected (3 out of 4) and red-listed macrophyte species found in the Pisz Forest were charophytes (7 out of 10), underscoring their ecological importance. These findings confirm the hypothesis that charophytes enhance aquatic biodiversity and highlight the conservation value of Chara-lakes in Central Europe.