Caridad de Hoyos

Phytoplankton structure and dynamics in Lake Sanabria and Valparaiso reservoir (NW Spain)

The aim of this work is to compare the composition and seasonality of the phytoplankton population in a natural oligotrophic lake (Lake Sanabria) and a mesotrophic reservoir (Valparaíso). Both ecosystems are located on the Tera river course (NW Spain), which runs along an area of ancient metamorphic and plutonic rocks. Some physical and chemical parameters, chlorophyll a and phytoplankton biovolume were studied from monthly samples collected at different depths during the periods 1987–1989 (Lake Sanabria) and 1991–1992 (Valparaíso). Phytoplankton biovolume and chlorophyll a concentration were about five times higher in Valparaíso than in Lake Sanabria. Species composition (and main phytoplankton groups) were different. Valparaíso was highly dominated by diatoms and Lake Sanabria by cryptophytes and small chlorophytes. In spite of the fact that both sites were nitrogen limited, heterocystous cyanophytes (Anabaena sp.) were detected only in Valparaíso. The relationships between phytoplankton structure and trophic level, hydrological conditions and nitrate content are discussed.

Sustaining recreational quality of European lakes: minimizing the health risks from algal blooms through phosphorus control.

1. A safe, clean water supply is critical for sustaining many important ecosystem services provided by freshwaters. The development of cyanobacterial blooms in lakes and reservoirs has a major impact on the provision of these services, particularly limiting their use for recreation and water supply for drinking and spray irrigation. Nutrient enrichment is thought to be the most important pressure responsible for the widespread increase in cyanobacterial blooms in recent decades. Quantifying how nutrients limit cyanobacterial abundance in lakes is, therefore, a key need for setting robust targets for the management of freshwaters. 2. Using a dataset from over 800 European lakes, we highlight the use of quantile regression modelling for understanding the maximum potential capacity of cyanobacteria in relation to total phosphorus (TP) and the use of a range of quantile responses, alongside World Health Organisation (WHO) health alert thresholds for recreational waters, for setting robust phosphorus targets for lake management in relation to water use. 3. The analysis shows that cyanobacteria exhibit a non-linear response to phosphorus with the sharpest increase in cyanobacterial abundance occurring in the TP range from about 20 µg L-1 up to about 100 µg L-1. 4. The likelihood of exceeding the World Health Organisation (WHO) ‘low health alert’ threshold increases from about 5% exceedance at 16 µg L-1 to 40% exceedance at 54 µg L-1. About 50% of the studied lakes remain below this WHO health alert threshold, irrespective of high summer TP concentrations, highlighting the importance of other factors affecting cyanobacteria population growth and loss processes, such as high flushing rate. 5. Synthesis and applications. Developing a more quantitative understanding of the effect of nutrients on cyanobacterial abundance in freshwater lakes provides important knowledge for restoring and sustaining a safe, clean water supply for multiple uses. Our models can be used to set nutrient targets to sustain recreational services and provide different levels of precaution that can be chosen dependent on the importance of the service provision.

Defining chlorophyll-a reference conditions in European lakes.

The concept of “reference conditions” describes the benchmark against which current conditions are compared when assessing the status of water bodies. In this paper we focus on the establishment of reference conditions for European lakes according to a phytoplankton biomass indicator—the concentration of chlorophyll-a. A mostly spatial approach (selection of existing lakes with no or minor human impact) was used to set the reference conditions for chlorophyll-a values, supplemented by historical data, paleolimnological investigations and modelling. The work resulted in definition of reference conditions and the boundary between “high” and “good” status for 15 main lake types and five ecoregions of Europe: Alpine, Atlantic, Central/Baltic, Mediterranean, and Northern. Additionally, empirical models were developed for estimating site-specific reference chlorophyll-a concentrations from a set of potential predictor variables. The results were recently formulated into the EU legislation, marking the first attempt in international water policy to move from chemical quality standards to ecological quality targets.

A phytoplankton trophic index to assess the status of lakes for the Water Framework Directive

Despite improvements in wastewater treatment systems, the impact of anthropogenic nutrient sources remains a key issue for the management of European lakes. The Water Framework Directive (WFD) provides a mechanism through which progress can be made on this issue. The Directive requires a classification of the ecological status of phytoplankton, which includes an assessment of taxonomic composition. In this paper, we present a composition metric, the plankton trophic index, that was developed in the WISER EU FP7 project and demonstrate how it has been used to compare national phytoplankton classification systems in Northern and Central Europe. The metric was derived from summer phytoplankton data summarised by genus from 1,795 lakes, covering 20 European countries. We show that it is significantly related to total phosphorus concentrations, but that it is also sensitive to alkalinity, lake size and climatic variables. Through the use of country-specific reference values for the index, we demonstrate that it is significantly related to other national phytoplankton assessment systems and illustrate for a single European (intercalibration) lake type how it was used to intercalibrate WFD boundaries from different countries.

The importance of inter-annual variability for management

In regions with high inter-annual climatic variability, lake management strategies should be based on scientific data covering the maximum range of lake dynamics. Lake Sanabria is a monomictic lake located in a gneiss-dominated glacial valley at 1000 m a. m. s. 1. in NW Spain. A three-year study was carried out during 1987–89 to obtain the basic ecological knowledge required for a sustainable lake management. Water and phytoplankton samples were collected from several depths of the water column at the deepest zone of the lake. Temperature and Secchi disk visibility, and total and dissolved inorganic nitrogen and phosphorus, chlorophyll a, and phytoplankton species composition and abundance (in laboratory) were determined monthly following standard limnological methods. Stratification began in March-April, with the thermocline stabilized at between 6–10 m, and lasted until November-December. The euphotic depth was higher than the mixing depth throughout the stratification period, except in the month previous to the water column mixing. The water residence time, estimated from the volume of the lake and the inflows and outflows, was between 0.48–0.67 years. However, during the months of highest rainfall (September-February) it was up to three times greater than during the same months of low rainy years. Nutrient concentrations in the water were very low throughout the year (52 μg 1−1 dissolved inorganic nitrogen, 6.1 μg 1−1 soluble reactive phosphorus, and 1.45 mg 1−1 soluble reactive silica) and dominated by the dissolved organic fractions, which was related to the inputs of organic materials from the densely forested catchment. The colour of the lake, 21–41 mg Pt 1−1, is also indicative of the relatively high amount of organic substances dissolved in the waters of the lake. A bloom of cyanobacteria (Microcystis flos-aquae, Aphanothece clathrata, Merismopedia tenuisssima, Synechococcus sp.) persisted in most of the water column during August 1988-March 1989, associated with the high residence time of the water. In contrast, the cyanobacteria bloom was limited to the mesolimnion for 3–5 months during 1987 and 1989. A model is proposed for the seasonal variation of the phytoplankton, which differentiates the seasonal trend during years of high water turnover from that of years with long residence time. Lake Sanabria is oligotrophic, based on a range of criteria (Secchi disk visibility, chlorophyll a concentration, total phosphorus concentration). However, during years of long water residence time (e.g. 1988), the annual input of total phosphorus is critical to maintain the characteristics of the lake. Short-term strategies for the management of lakes should take into account the inter-annual variability rather than averages as indicators to prevent the impact of increased nutrient inputs derived from changes in land and water use in the catchment area. Long-term strategies should also consider the potential changes in the lake’s dynamics, derived from the changes in the rainfall pattern caused by climatic changes.

Specific composition and ecology of chrysophyte flagellates in Lake Sanabria (NW Spain)

Lake Sanabria is a glacial lake located at 1000 m a.s.l. in NW Spain. Its water is characterised by a low content of mineral salts, rather low pH, and oligotrophic status, which are all currently viewed as being favorable for Chrysophyte growth and proliferation. The study of the phytoplankton at different depths of the water column for three years showed that Chrysophytes were one of the groups of algae less well-represented in the phytoplankton (1% and 8%, respectively, for numbers of cells and biovolume).Ochromonas, Pseudopedinella, Mallomonas and Dinobryon were the more representative chrysophyte genera. D. cylindricum, D. divergens and D. crenulatum were some of the Dinobryon species observed. M. akrokomos, M. crassisquama and M. costata were among the most frequent Mallomonas species. Observations on the Chrysophyte cysts from samples of the lake sediment suggest that variability of representation is higher than in planktonic samples.The maximum values of Chrysophyte biovolume were found in the nutrient-exhausted epilimnion of summer and autumn which could be related to the mixotrophic capabilities of several species.

Benefits and limitations of an intercalibration of phytoplankton assessment methods based on the Mediterranean GIG reservoir experience.

The status of European legislation regarding inland water quality after the enactment of the Water Framework Directive (WFD) originated scientific effort to develop reliable methods, primarily based on biological parameters. An important aspect of the process was to ensure that quality assessment was comparable between the different Member States. The Intercalibration process (IC), required in the WFD ensures the unbiased application of the norm. The presented results were developed in the context of the 2nd IC phase. An overview of the reservoir type definition of the Lake Mediterranean Geographical Intercalibration Group, where four types were considered divided by both alkalinity and climate, together with the results for selection of Maximum Ecological Potential sites (MEP) are presented. MEP reservoirs were selected based on pressure and biological variables. Three phytoplankton-based assessment methods were intercalibrated using data from Mediterranean countries. The Mediterranean Assessment System for Reservoirs Phytoplankton (Spain), the New Mediterranean Assessment System for Reservoirs Phytoplankton (Portugal and Cyprus) and the New Italian Method (Italy) were applied. These three methods were compared through option 3 of the Intercalibration Guide. The similarity of the assessments was quantified, and the Good/Moderate (GM) boundaries assessed. All three methods stood as comparable at the GM boundary except for the MASRP in siliceous wet reservoirs, which was slightly stricter. Finally, the main taxonomic groups represented in the phytoplankton community at MEP conditions were identified, as well as their main changes with an increasing trophic status. MEP sites are dominated by chrysophytes in siliceous wet reservoirs and by the diatoms Cyclotella and Achnanthes in calcareous ones. Cyanobacteria take over the community in both calcareous and siliceous wet reservoirs as eutrophication increases. In summary, the relevance and reliability of the quality assessment methods compared were confirmed both from an ecological perspective and a health risk management point of view.

The heat budget of Lake Sanabria (NW Spain)

The heat budget is a basie limnologieal feature of lakes (HUTCHINSON 1957). lts estimation requires a number of ealeulations whieh ean be based on different approaehes. Lake Sanabria is the largest lake in the lberian Península. However, little knowledge of its physieal eharaeteristies is available. lts heat budget was ealculated during four annual eycles to idenrify interannual variations related to climatie faetors. Further informarion about the general limnologieal eharaeteristies of Lake Sanabria ean be found in DE HoYos (1996) and DE HoYos & CoMfN (1999).