Hydrological stressors are caused by alterations to hydrological regimes – the timings, amounts and flows of water through a water body – and are caused by a number of pressures and drivers:
- Water abstraction - water is abstracted from rivers, lakes and groundwater aquifers across Europe for use in agriculture, industry, energy production and urban development,
- Climate change - alterations to rainfall and temperature patterns across Europe as a result of climate change are causing widespread hydrological alterations, with increased flooding in some areas, and longer, intensified drought periods in others,
- Barriers - the construction of barriers such as dams, weirs, locks for hydropower, transport, flood protection or freshwater supply projects along river channels fragments water flows and reduces ecological connectivity through a catchment,
- Channelisation and embankments - alterations to water body channels, beds and shorelines due to flood defence construction, urban development, coastal land reclamation, dredging and straightening of river channels for transport can all cause morphological pressures which in turn cause hydrological alterations and stressors,
- Land use - water flows diverted to aid land drainage, mitigate flooding or create irrigation reservoirs alter hydrological dynamics,
- Hydropeaking - hydropower projects generate electricity in response to demand, meaning that their operation – and the water they subsequently release – can be highly variable, leading to ‘peaks’ of water flows being released downstream, often at high velocity, causing significant variations in river level and flows.
Effects on ecological status of aquatic ecosystems
Hydrological stressors can cause a number of impacts on the status of aquatic ecosystems and the ecosystem services they provide.
Habitat alteration and loss - many organisms are adapted to live in specific hydrological regimes, and alterations to water flow and depth may cause a loss of suitable habitat. Hydrological alterations can alter nutrient concentrations, salinity, sediment erosion and deposition, and water temperature, creating multiple stressor effects which further alter habitat quality which may be detected through alterations to ecosystem health (e.g. macrophyte diversity) and ecosystem service provision (e.g. provision of edible fish species).
Hydropeaking and population loss - hydropeaking can cause significant ‘drift’ of organisms (macroinvertebrates, fish) along and across river channels, and potential strandings as water levels quickly recede. Such stressors may be detected by changes in community composition (e.g. of macroinvertebrates and fish such as grayling) downstream of hydropower projects.
Overabstraction and loss of ecosystem services - the (over)abstraction of water from surface and groundwaters can compromise ecosystem health, water quality and the ongoing provision of ecosystem services such as clean drinking water.
Embankments and loss of land-water connectivity - the human restriction of water flows across flood plains and reductions in water levels in lakes and rivers can decouple a water body from its wider landscape, leading to reductions in riparian vegetation, flood plain biodiversity, and nutrient and sediment exchanges.
Common combinations as multiple stressors
Hydrological stressors are frequently found to act in combination with nutrient stressors (a chemical stressor) in lakes, rivers and transitional waters. Hydrological-nutrient stress is the most common of all stressor combinations – particularly in rivers and transitional waters – as nutrient concentrations (e.g. from diffuse pollution of fertilisers) are often increased when water levels and flows are lowered and reduced, and decreased when water levels and flows are increased. Hydrological-morphological is another frequent stressor pair, as alterations to the course, channel and shoreline of a water body often alter its hydromorphological dynamics (Nõges et al 2015; REFORM)
The European FP7 REFORM project focused on the ecological impact of hydrological and morphological stressors in streams and rivers to identify appropriate restoration measures. Piniewski et al (2017) reviewed the impacted of floods and droughts on invertebrates and fish and conclude that biota resistance to floods was found to be lower than the resistance to droughts. The impact of the most common pressures have been summarised in conceptual schemes on hydrological regime alteration, river fragmentation and morphological alteration (Garcia de Jalon et al. 2013), while Wolter et al. (2013) synthesised the impact on macroinvertebrates, macrophytes and fish.
How lake and reservoir ecosystems located in these Mediterranean climates have been affected by changes to water levels and salinity in the past has been reviewed by Jeppesen et al. (2015). Water level reduction often results in higher nutrient concentrations, higher phytoplankton biomass and lower water transparency in both shallow and deep lakes and reservoirs. Similarly, the authors found that increases in lake salinity often “markedly alter the community composition of phytoplankton, zooplankton, macrophytes and fish and often lead to a decrease in the biomass and diversity of each of these organism groups.”
When water level drops nutrient concentrations in lakes generally rise, because although there is less ‘nutrient pollution‘ from runoff of fertiliser and waste from surrounding towns and fields, the nutrients already in the shrinking lake are likely to be concentrated. In many cases, this can lead to eutrophication. In particular, shallower lakes with increased water temperatures might experience blooms of cyanobacteria, and especially of toxin-producing species such as Microcystis. Such cyanobacteria blooms have become common in Doiran Lake on the border between Greece and Macedonia, as a result of lowered lake levels due to agricultural abstraction (Krstić & Aleksovski 2016).
Many studies found that when lake levels dropped, macrophytes flourished due to increased light levels and reduced turbidity (the ‘cloudiness’ of the water). This is not always the case. In the coastal Lake Biviere di Gela in Sicily, Italy, reduced water inflows – as a result of abstraction for irrigation – led to the lake getting shallower and shifting from a clear, macrophyte-dominated ecosystem to one that was more turbid and phytoplankton-dominated. Even when lake levels increased, the lake remained dominated by phytoplankton blooms, and the macrophytes did not re-establish themselves, possibly due to a decrease in water quality (Manno et al. 2007).
Lowered lake levels also impact fish populations. Warmer water temperaturesdestabilising the lake thermocline can result in the loss of deep, cold water ‘refugia’ where fish can retreat from predators, sunlight and warmer, oxygen-poor water. Following a reduction of 32 metres in the depth of Lake Vegoritida in Greece between the 1950s and 2000s, populations of the native, cold-water dwelling European whitefish (Coregonus lavaretus) disappeared, and were replaced by populations of warm-water species which can survive in eutrophic conditions, such as roach and carp.
Variability in lake level also destabilises the littoral zone which can have negative effects on plant growth and fish spawning. For example, at Lake Kinneret (or the Sea of Galilee) in Israel, low water levels meant that bleak – a tiny silver fish – could not longer spawn in the stony habitats in the littoral zone, which are submerged during high water. Similarly, the same littoral zone provides habitat and shelter for young fish amongst submerged stones and vegetation. Low water levels mean that the potential of the littoral zone as a breeding location and ‘nursery’ area for young fish is lost.
Reduced rainfall means that less water enters the lake system, causing increases in salinity as solutes in the water become more concentrated (Jeppesen et al. 2014). Even a small increase in water salinity can cause a significant loss of biodiversity, and alter the ways that the ecosystem functions. Higher salinity levels put the cells of many organisms under osmotic stress: Daphnia have a low salinity tolerance while fish are least tolerant to salinity in their juvenile stages. It can be difficult to disentangle the effects of increased salinity from the effects of reduced lake levels, but many studies have reported that salinity is the most important factor in determining the ecology of Mediterranean lakes. When the salinity increase is high (e.g. from freshwater to brackish levels) its effects [on the ecosystem] may in some cases override all other environmental and pressure factors such as temperature or eutrophication. Under future climate change scenarios, salinisation of freshwater lakes may also be increased by rising sea levels.
Jeppesen E., Brucet S., Naselli-Flores L., Papastergiadou E., Stefanidis K., Nõges T., Nõges P., Attayde J.L., Zohary T., Coppens J., Bucak T., Fernandes Menezes R., Sousa Freitas F.R., Kernan M., Søndergaard, M. & M. Beklioğlu (2015). Ecological impacts of global warming and water abstraction on lakes and reservoirs due to changes in water level and related changes in salinity, Hydrobiologia 750: 201-227. DOI: 10.1007/s10750-014-2169-x (Read abstract)
Nõges P., Argillier C., Borja Á., Garmendia J.M., Hanganu J., Kodeš V., Pletterbauer F., Sagouis A., & S. Birk (2016). Quantified biotic and abiotic responses to multiple stress in freshwater, marine and ground waters. Science of the Total Environment, 540, 43-52. DOI: 10.1016/j.scitotenv.2015.06.045 (Read abstract)
Reports and publications:
EC (2007). Drought Management Plan Report: Including Agricultural, Drought Indicators and Climate Change Aspects (Download report, 3.1mb)
EEA (2009). Water resources across Europe — confronting water scarcity and drought (Download report, 4mb)
EEA (2012a). European waters - assessment of status and pressures (Download report, 28mb)
EEA (2012b). Towards efficient use of water resources in Europe (Download report, 3.9mb)
Garcia de Jalon D., Alonso C., González del Tango M., Martinez V., Gurnell A., Lorenz S., Wolter C., Rinaldi M., Belletti B., Mosselman E., Hendriks D. & G. Geerling (2013). Review on pressure effects on hydromorphological variables and ecologically relevant processes - Deliverable D1.2 of REFORM (REstoring rivers FOR effective catchment Management), a collaborative project (large-scale integrating project) funded by the European Commission within the 7th Framework Grant Agreement 282656. European Commission (External link to REFORM deliverable)
Krstić S.S. & B. Aleksovski (2016). Dominance of Microcystis spp. in Lake Dojran – a consequence of 30 years of accelerated eutrophication. Botanica Serbica, 40 (2): 119-128. (Download report, 880kb)
Manno E., Vassallo M., Varrica D., Dongarrà G. & S. Hauser (2007). Hydrogeochemistry and Water Balance in the Coastal Wetland Area of “Biviere di Gela,” Sicily, Italy. Water, Air, and Soil Pollution, 178 (1-4): 179–193. (Read abstract)
Piniewski M., Prudhomme C., Acreman M.C., Tylec L., Oglęcki P. & T. Okruszko (2017). Responses of fish and invertebrates to floods and droughts in Europe. Ecohydrology. 10:e1793. http://dx.doi.org/10.1002/eco.1793 (Read article, 1mb)
EU (2012). A Blueprint to Safeguard Europe’s Water Resources (External website)
REFORM (2015). REstoring rivers FOR effective catchment Management (External website)
Selected Freshwater blogs:
Freshwaterblog (2015). Low water and high salinity: the effects of climate change and water abstraction on lake ecosystems (External website)