Hydrobiologia special issue: metrics for water quality and defining phosphorus targets to avoid toxic algae.
A special issue of Hydrobiologia presents conclusions from the EU-funded WISER project, developing and reconciling metrics based on biological parameters for assessing Water Framework Directive quality status of surface waters, and for recovery of deteriorated waters. A further paper in the Journal of Applied Ecology examines whether target phosphorus levels can be defined for lakes to avoid the development of toxic algae.
There is general agreement that bio-indicators are a valid and pertinent method for assessing the ecological status of surface waters, that is whether lakes, rivers and estuaries are negatively affected by human impacts (pollution, modification of water system morphology and functioning, water extraction), or whether they are in a state similar to their natural quality. Bio-indicators are metrics of ecological quality status based on assessment of different BQEs (Biological Quality Elements), for example phytoplankton populations, macrophytes, macro-invertebrates and fish.
Intercalibrating bio-indicator systems
The WISER project reviewed 297 different biotic assessment metrics for rivers, lakes and estuaries, based on information provided by water authorities in 28 European countries. The authors (A) note that the high number of metrics suggests that the methods used are adapted to regional situations, for example predominant stressors (environmental issues), local species and local scientific understanding and historic data sets. However, the consequence is that results are not directly comparable.
The intercalibration of methods developed through the EU Water Framework Directive implementation, has shown that the aggregated assessment results, that is when different BQE indicators are combined to define overall “Ecological Status”, are generally comparable across Europe.
Restoring Europe’s water quality
The EU Water Framework Directive requires that water bodies identified as not achieving “Good Ecological Status” should be restored, so that their water chemical quality and ecosystem biodiversity and functioning are comparable to (only a sight deviation from) “reference” conditions (pre-disturbed state). The conclusions of the first River Basin Management Plans in member states show that non-achievement of “Good Ecological Status” is the situation for many waters in Europe, particularly in densely populated regions where almost all rivers and lakes fail “Good” status. Restoration of water bodies will therefore be a challenging task across Europe for the coming decades.
The key questions concerning water restoration identified by the authors are: what is the (spatial) extent of restoration measures needed and how long will it take after implementation of these measures before “Good Quality Status” is restored ?
The authors note that after significant or intense deterioration of water quality status, affecting diversity of biological populations, 15 – 25 years may be needed for most water systems to re-attain a biotic diversity and ecosystem functioning comparable to the pre-disturbed state (Good Ecological Status).
They also emphasise that there may be a need to reconsider the reference conditions, and consequently restoration targets, because climate change could accentuate eutrophication effects.
Assessing specific metric systems
In a further paper in Hydrobiologia (B), the authors compare 11 biological quality metrics developed from over 2000 European lakes. The strongest and most sensitive indicators for assessing eutrophication pressure were phytoplankton chlorophyll-a (which is the traditional metric, and effectively measures total aquatic algae), as well as more complex indicators based on algal species composition and relative growth, a macrophyte species index and the Nordic fish index.
The authors use total phosphorus as the only indicator of eutrophication pressure in this paper, and the correlation between total P and phytoplankton chlrolophyll-a (primary producers) has been known for a long time. This study shows again that phytoplankton biomass is the first and most direct indicator of eutrophication pressure. Benthic (lake floor) invertebrates and fish are more indirect indicators, responding through secondary effects such as availability of phytoplankton or phytoplankton-derived organic matter as food or changes in light, oxygen availability or habitat. Macrophytes are more slowly impacted, because they rely on nutrients in sediments and are affected by nitrate availability, as well as being impacted by changes in light (water transparency).
The authors note that other indicators such as cyanobacteria blooms or the European lake fish index are less strongly correlated to total phosphorus at the European level, but remain valid indicators of eutrophication, and may be better metrics for individual lakes. In particular, they measure effects which are of relevance to sustainable water use. Specific metrics may give a better measurement of particular pressures or water use impacts, for example macrophytes or littoral benthic algae. In general, a combination of several metrics will give the best picture of overall lake quality status.
In (C), the authors compare 6 different phytoplankton based metrics for assessing eutrophication impacts in lakes. Annex V of the EU Water Framework Directive specifies that three different features of the phytoplankton BQE should be considered in assessing lake quality status: phytoplankton biomass (often proxied by chlorophyll-a) – because this directly affects water transparency and other conditions, phytoplankton composition (variety of species or taxons) and plankton bloom frequency and intensity.
As indicated above, existing methods for assessing phytoplankton abundance using chlorophyll-a have been demonstrated to be a relatively robust first indicator of eutrophication pressure, subject to appropriate calibration of measurement and sampling programmes.
Coherent methods for assessing algal taxon/species variation have been developed over recent years, based on trait-based functional classifications (grouping different types of phytoplankton according to their ecological role). These different methods today need to be benchmarked and intercalibrated.
Regarding measurement of algal blooms, the authors note that today there is still not an agreed definition, although a metric based on the actual abundance of cyanobacteria (blue-gree algae) was shown to be significantly robust and indicates water usage issues because of their potential toxicity.
Preventing eutrophication problems in lakes
In (D) the authors consider how to set water total phosphorus concentration targets for lakes at levels necessary to avoid toxic algal blooms, based on assessment of data from over 800 European Lakes. The threshold of algal blooms was defined using the World Health Organisation (WHO) criteria of “low health alert”, which corresponds to a biovolume of 2 mm3/litre of blue-green algae.
The likelihood of exceeding this WHO safety level is only around 5% at 16 µg total phosphorus per litre (summer concentration), but increases to 40% at 54 µg TP/l, showing a threshold effect for toxic algal bloom risk.
However, around 50% of studied lakes stay below the WHO alert level for blue-green algae, even with high summer total phosphorus concentrations, showing the importance of other factors affecting algal development, including water flushing rate.
The authors note that blue-greens tend to be absent in low-alkalinity lakes (soft water), so that only 5% of lakes in Northern Europe show summer blue-green blooms, compared to 37% in Central Europe where most lakes are high-alkalinity (hard water).
The authors conclude that the target TP (total phosphorus) concentration in lake water will depend on local conditions and on water usage objectives. If a low probability of blue-green blooms is an objective, then a summer total phosphorus concentration of <20 µg TP/l should be targeted.
(A) “Assessment and recovery of European water bodies: key messages from the WISER project”, Hydrobiologia (2013) 704:1–9
D. Hering, C. Feld, Dept. Aquatic Ecology, University of Duisburg-Essen, Universitätsstr. 2, 45141 Essen, Germany A. Borja, Marine Research Division, AZTI-Tecnalia, Herrera Kaia Portualdea s/n, 20110 Pasaia, Spain. L. Carvalho, Centre for Ecology & Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, UK. C.
(B) “Ecological status assessment of European lakes: a comparison of metrics for phytoplankton, macrophytes, benthic invertebrates and fish”, Hydrobiologia (2013) 704:57–74
A. Lyche-Solheim, C. Feld, S. Birk, G. Phillips, L. Carvalho, G. Morabito, U. Mischke, N. Willby, M. Søndergaard, S. Hellsten, A. Kolada, M. Mjelde, J. Böhmer, O. Miler, M. Pusch, C. Argillier, E. Jeppesen, T. Lauridsen, S. Poikane.
(C) “Strength and uncertainty of phytoplankton metrics for assessing eutrophication impacts in lakes”, (2013) 704:127–140
L. Carvalho, S. Poikane, A. Lyche Solheim, G. Phillips, G. Borics, J. Catalan, C. De Hoyos, S. Drakare, B. J. Dudley, M. Järvinen, C. Laplace-Treyture, K. Maileht, C. McDonald, U. Mischke, J. Moe, G. Morabito, P. Noges, T. Noges, I. Ott, A. Pasztaleniec, B. Skjelbred, S. Thackeray
(D) “Sustaining recreational quality of European lakes: minimizing the health risks from algal blooms through phosphorus control”, Journal of Applied Ecology 2013, 50, 315–323
L. Carvalho (1,2), C. McDonald (2), C. de Hoyos (3), U. Mischke (4), G. Phillips (5), G. Borics (6), Sandra Poikane (1), B. Skjelbred (7), A. Lyche Solheim (7), J. Van Wichelen, A-C. Cardoso (1).
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