Ecosystem quality and trends in groundwater-dependent ecosystems, 2018

Grassland and marsh ecosystems most affected by reduced groundwater levels

In many areas the water table has been lowered for agricultural and residential land uses or is drawn down by drinking water abstraction, which can lead to lower groundwater levels in natural areas as well, resulting in desiccation. Reduced groundwater levels in the spring is a major reason for the loss of rare species in ecosystems.

The national trends in average spring groundwater levels (1999-2018) are stable in natural and semi-natural grassland and open dunes. The trends in heath, marsh and forest ecosystems are slightly downwards. Marsh ecosystems are particularly affected by falling groundwater levels, which reduces the availability of soil moisture to plants and leads to other desiccation effects, such as changes to the physical, hydrochemical and microbiological properties of the soil. A possible explanation for the reduction in moisture availability, together with a reduction in nutrient availability in marshland, is the process of natural succession and ageing in which the accumulation of organic matter results in a lowering of the groundwater level and an increase in the influence of rainwater; the marsh becomes drier, nutrient poorer and slightly more acid.

More than 10% of the total area of natural and semi-natural ecosystems, about 40% of the area of groundwater-dependent ecosystems, is desiccated (in terms of reduced groundwater levels). Locations where the average spring groundwater level is currently considered to be reduced are found mainly in the areas of sandy soils. The management types wet heath, wet grassland and meadows, wet dune slack and wet forests are particularly susceptible to desiccation and are often affected. In some areas rewetting measures have been able to reduce or eliminated the effects, examples being the Bargerveen, Haaksbergerveen, Korenburgerveen and Wooldse Veen bogs. Although not all environmental conditions in these sites have been restored yet, major hydrological interventions in these areas have raised the groundwater levels sufficiently for the vegetation to recover.

Presence of plants indicative of environmental conditions

Direct measurements of soil and water (groundwater levels) can be used to determine the extent to which changes in environmental conditions occur and whether or not current conditions are suitable for the sustainable conservation of ecosystems. However, the availability of measurement data is limited. Environmental conditions can also be estimated indirectly from the presence of certain plant species and changes in species composition, because plants have specific environmental requirements. The vegetation composition found in an area is therefore indicative of the prevailing environmental conditions at site level. A complicating factor is that vegetation responds later to changes in environmental conditions. It is possible that no recovery in vegetation composition is observed - and in some cases vegetation composition may even deteriorate - because environmental pollution is still too high and has not declined at all in recent years.

In the 'monitoring and evaluation methodology' document (van Beek et al., 2018), the provinces and conservation management organisations have set down how environmental conditions can be determined. In anticipation of the first set of monitoring data from the provinces, these and the two indicators below provide a picture of current environmental conditions. Assessments of environmental conditions (good, moderate, bad) are based on comparisons between the current situation and the requirements of the characteristic vegetation types of the management types.

Results of policy to reduce desiccation

The Netherlands has international commitments to meeting the goals of the Convention on Biological Diversity, the Birds and Habitats Directives (Natura 2000) and the EU Biodiversity Strategy. In the Nature Pact (Natuurpact) the national and provincial governments have agreed to raise ecological quality through the creation of the national ecological network and through more intensive restoration and management measures to improve water and environmental conditions.

Since 1989, various measures have been introduced to prevent further reduction in groundwater levels and desiccation impacts and to restore hydrological conditions, first under the subsidy scheme for effect-oriented measures (EGM) and in recent years under the 'quality initiative for nature and landscape' (SKNL) and the Integrated Approach to Nitrogen (PAS). Previous surveys of reduced groundwater levels and desiccation by the Association of Provinces and the Institute for Inland Water Management and Wastewater Treatment (IPO/RIZA, 2005) and progress reports on the Rural Areas Investment Budget (ILG) have shown that progress has been slow and the measures taken have not been sufficient for the task at hand. One of the reasons for this is that land holdings had to be purchased before water levels could be raised. In addition, securing support for the measures has proved to be problematic, because the consequences for land uses are considerable (such as agricultural damage resulting from higher groundwater levels and wet soils).

In the Nature Pact (EZ, 2013) the national and provincial governments have agreed to maintain ecological quality within the national ecological network by providing a sufficient level of standard conservation management and to raise ecological quality by intensifying efforts for temporary or permanent restoration measures aimed at improving water quality and environmental conditions. Many of the restoration measures are not only geared to removing nutrients, but also aim to combat acidification and reduced groundwater levels/desiccation. The availability of nutrients depends not only on the current deposition of nitrogen, but also on reduced water levels and the quality of surface water and groundwater. Nitrogen deposition raises the acidity of soils in natural areas, leading to the decline or disappearance of plant and animal species in those areas. Soil acidity can also be influenced by changes in hydrological conditions, such as a loss of base-rich groundwater seepage or the accumulation of organic matter in the humus layer. Acidification, eutrophication and desiccation are environmental factors that can reinforce each other.

• Ecosystem quality and trends in nitrogen availability, 1994-2021
• Ecosystem quality and trends in soil pH, 2018