Protected Areas Have Not Halted Macrophyte Biodiversity Loss and Eutrophication in Northwest German Standing Waters

Questions How have species richness, functional composition and community structure of aquatic macrophytes in standing waters changed since the mid‐20th century? Do these changes differ between oligotrophic and mesoeutrophic waters, and to what extent do protected areas conserve macrophyte diversity? Location Northwest Germany. Methods We analysed temporal changes in 76 macrophyte species across 119 waters. Sites were originally surveyed in the mid‐20th century with resurveys in the early 21st century; 55 sites also had late 20th century records. Changes in species richness, growth forms, ecological indicator values and individual species occurrences were assessed using permutational paired t ‐tests and generalised linear mixed models. Shifts in community composition were evaluated with NMDS, permutational multivariate analysis of variance and temporal beta‐diversity partitioning. Results Mean site‐level richness declined by 27% with losses mostly of threatened and oligotrophic‐water specialists. Generalists also declined in mesoeutrophic waters but slightly increased in historically oligotrophic waters. Within protected areas, richness was consistently higher while experiencing similar declines. Only larger systems showed any recovery and that since the late 20th century. Of the 60 species recorded in both main periods, occurrence probability across sites decreased for 24 species and increased for three. Community turnover within sites was high, predominantly driven by losses. Composition shifted towards indicators of eutrophic–hypertrophic conditions, that is, from submerged to floating‐leaved forms and towards higher proportions of free‐floating pleustophytes. Conclusions Continued eutrophication, sediment accumulation and increased turbidity have reshaped macrophyte communities, reducing their diversity even within protected sites. Small and oligotrophic waters are especially vulnerable. Conservation strategies should address not only rarity but also functional diversity. Limiting diffuse nutrient inputs, restoring wide buffer strips and promoting extensive habitat management are essential to halt declines and support recovery.