Do current European lake monitoring programmes reliably estimate phytoplankton community changes?

Victoria Bergkemper,Thomas Weisse

doi:10.1007/s10750-017-3426-6

Victoria Bergkemper, Thomas Weisse

Open Access

https://doi.org/10.1007/s10750-017-3426-6

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Abstract

Many European lakes are monitored according to the EU Water Framework Directive (WFD), with focus on phytoplankton biomass and species composition. However, the low-frequency WFD monitoring may miss short-term phytoplankton changes. This is an important issue because short-term extreme meteorological events (heat waves and heavy rain) are predicted to increase in frequency and intensity with climate change. We used records from Lake Mondsee (Austria) from 2009 to 2015 to test if a reduction from monthly to seasonal sampling affected the average annual phytoplankton biovolume. Furthermore, we combined inverted light microscopy, FlowCAM and flow cytometry to estimate the effect of sampling during extreme events on average phytoplankton biovolume. Relative to monthly sampling, seasonal sampling significantly overestimated phytoplankton biomass. A heat wave in 2015 and two episodes of heavy rain in 2015 and 2016 caused species-specific changes; biovolumes of chlorophytes and the filamentous cyanobacterium Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek increased significantly during the heat wave. Using live material with FlowCAM and flow cytometry, we detected small and fragile cells and colonies that were either ignored or underrepresented by analysing fixed samples with light microscopy. We suggest a modified sampling and analysis strategy to capture short-term changes within the phytoplankton community.

Highlights

Phytoplankton plays a key role at the base of pelagic food webs and its biomass and species composition are directly related to the trophic state of water bodies
Phytoplankton is one of the four biological quality elements used for the assessment of the ecological status of European surface waters according to the European Union Directive 2000/60/EC (EC, 2000) better known as Water Framework Directive (WFD)
Annual phytoplankton biovolumes calculated from monthly sampling differed significantly (Wilcoxon signed-rank test, P = 0.016) from biovolumes estimated from seasonal sampling during 2009–2015 (Table 1)

Summary

Introduction

Phytoplankton plays a key role at the base of pelagic food webs and its biomass and species composition are directly related to the trophic state of water bodies. Phytoplankton is one of the four biological quality elements used for the assessment of the ecological status of European surface waters according to the European Union Directive 2000/60/EC (EC, 2000) better known as Water Framework Directive (WFD). The WFD requires that phytoplankton species composition, abundance and biomass are measured to assess the ecological status of lakes. Most EU countries use assessment procedures that evaluate phytoplankton taxa in relation to nutrient conditions. The EQR is constrained to values ranging from zero (the worst possible status) to 1 (excellent) and evaluates the current ecological status relative to a reference condition, i.e. an undisturbed, quasi-pristine status for the respective water body. The ‘normalised Ecological quality ratio’ (nEQR) is the mean value of the metric converted to a normalised EQR scale, where the classes are spaced (e.g. the boundary value between moderate and good status is always 0.6 and the boundary value between good and high status is always 0.8) (http://dd.eionet.europa.eu/ dataelements/latest/resultNormalisedEQRValue)

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