RESUMEN
Many aquatic ecosystems in densely populated delta areas worldwide are under stress from overexploitation and pollution. Global population growth will lead to further increasing pressures in the coming decades, while climate change may amplify the consequences for chemical and ecological water quality. In this study, we explored the effects of climatic variability on eutrophication of groundwater, streams, rivers, lakes, estuaries, and marine waters in the Netherlands. We exploited the relatively dense monitoring information from the Dutch part of the Rhine-Meuse delta to evaluate the water quality response on climatic variability, in combination with anthropogenic pressures. Our results show that water quality of all water systems in the Netherlands is affected by climate variability in several ways: 1) through the process of global climate change (mainly temperature and sea level rise), 2) through changes Atlantic ocean circulation patterns (more southwestern winds), 3) through changes in continental precipitation and river discharge fluctuations, and 4) through local climatic fluctuations. The impact of climate variability propagates through the hydrological system 'from catchment to coast'. The fluctuations in water quality induced by climatic variability shown in this study give a preview for the potential effects of climate change.
RESUMEN
Submerged macrophytes play a key role in north temperate shallow lakes by stabilizing clear-water conditions. Eutrophication has resulted in macrophyte loss and shifts to turbid conditions in many lakes. Considerable efforts have been devoted to shallow lake restoration in many countries, but long-term success depends on a stable recovery of submerged macrophytes. However, recovery patterns vary widely and remain to be fully understood. We hypothesize that reduced external nutrient loading leads to an intermediate recovery state with clear spring and turbid summer conditions similar to the pattern described for eutrophication. In contrast, lake internal restoration measures can result in transient clear-water conditions both in spring and summer and reversals to turbid conditions. Furthermore, we hypothesize that these contrasting restoration measures result in different macrophyte species composition, with added implications for seasonal dynamics due to differences in plant traits. To test these hypotheses, we analyzed data on water quality and submerged macrophytes from 49 north temperate shallow lakes that were in a turbid state and subjected to restoration measures. To study the dynamics of macrophytes during nutrient load reduction, we adapted the ecosystem model PCLake. Our survey and model simulations revealed the existence of an intermediate recovery state upon reduced external nutrient loading, characterized by spring clear-water phases and turbid summers, whereas internal lake restoration measures often resulted in clear-water conditions in spring and summer with returns to turbid conditions after some years. External and internal lake restoration measures resulted in different macrophyte communities. The intermediate recovery state following reduced nutrient loading is characterized by a few macrophyte species (mainly pondweeds) that can resist wave action allowing survival in shallow areas, germinate early in spring, have energy-rich vegetative propagules facilitating rapid initial growth and that can complete their life cycle by early summer. Later in the growing season these plants are, according to our simulations, outcompeted by periphyton, leading to late-summer phytoplankton blooms. Internal lake restoration measures often coincide with a rapid but transient colonization by hornworts, waterweeds or charophytes. Stable clear-water conditions and a diverse macrophyte flora only occurred decades after external nutrient load reduction or when measures were combined.
