Structural and functional responses of Phragmites australis to managed hydrological regimes

The regulation of water levels in recreational lakes restrains the natural hydrological variability to which littoral organisms have adapted. This study examined the morphological and photophysiological responses of Phragmites australis to water levels and fluctuations in Lake Balaton, Hungary. Morphological measurements were conducted annually between 2000 and 2024, while chlorophyll fluorescence parameters were recorded at two sites from 2012 onwards. The annual average water level was negatively correlated with aboveground biomass (ρ = −0.706), whereas annual water level fluctuation was positively associated with biomass (ρ = 0.742). Autocorrelation analysis revealed substantial temporal dependence in the morphological series, which reduced the significance of the correlations. However, the correlations in the photophysiological data remained statistically robust after correction: the annual average water level was negatively correlated with the photosynthetic capacity proxy, the maximum electron transport rate (ETR max ; ρ = −0.743), while the annual water level fluctuation was positively associated (ρ = 0.795) with ETR max . The study years were classified into four hydrological categories based on long-term averages. Thirteen of the 25 years fell within the HS category, reflecting the sustained hydrological oversaturation imposed by water management that had significant affect on plant morphology and physiology. The exploratory composite Phragmites Vigour Score (PVS), integrating morphological and photophysiological plant responses, showed strong Spearman correlations with both annual mean water level (ρ = −0.752) and annual water level fluctuation (ρ = 0.852), with water level variability emerging as the stronger and more relevant predictor within this system. These results suggest that prolonged periods of high water levels and low variability are associated with the suppression of the the structural development and photosynthetic capacity of Phragmites plants, whereas periods of plant vigour coincide with intermittent hydrological variability. Water level fluctuation is proposed as a candidate ecological indicator for emergent macrophyte functional status in regulated shallow lakes, although its system-specific adaptation is likely required. These findings support the incorporation of controlled water level fluctuations into the management of regulated shallow lakes, where ecological and recreational objectives must be reconciled.