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Many aquatic ecosystems in densely populated deltas worldwide are under stress from overexploitation and pollution. Global population growth will further increase these pressures in the coming decades, while climate change may amplify the consequences for chemical and ecological water quality. Meteorological variations are a major driver for changes in water quality. Climate change projections foresee higher temperatures and larger extremes in wet and dry periods. Still, the impact of climate change and climate variability on water quality is only poorly understood. In this study, we investigated the integrated effects of climatic variability on the chemical and ecological quality of groundwater and surface water in the sandy part of the Netherlands. We especially exploited the dense monitoring information from Water Board Aa en Maas to evaluate the water quality response on the past 50 years of climate change and climatic variability. Our results show a direct effect of climate extremes on the leaching of nutrients from agriculture. The 2018-2020 drought for example reduced nutrient concentrations in summer, but the nutrient losses increased in the subsequent wet winter seasons and in the first next wet summer of 2021. In addition, extreme wet conditions give nutrient load pulses and strongly reduce oxygen concentrations which can have both instant and long term effects on downstream ecology. The long-term trends (1990-2022) showed a general improvement in water quality due to reduce inputs, although an accelerated increase in water temperature since 2010 makes the system more vulnerable to eutrophication.
Rozemeijer et al. (2024) studied this question.