Report on Harmonisation of freshwater biological methods

Annex IV: Analysis of lake biological monitoring methods(Intercalibration metadata at 14/01/2004)CHLOROPHYLL1. The WFD allows for the use of chlorophyll a as surrogate for phytoplanktonbiomass and, thus, is considered a biological parameter, and the most frequentlymeasured in lakes. In the GIGs the percentage of lakes for which chlorophyll a ismeasured varies between 40% (Eastern Continental, EC) and 100% (Mediterranean,ME), with an overall average of 82.5% (Fig. 1). A possible explanation for a smallerpercentage of lakes with chlorophyll a data in the Eastern Continental and NorthernGIGs could be a monitoring strategy targeted to identify acidification impacts and forwhich chlorophyll a is less used.Percentage measuring Chl-a per GIG1008060%40200AL AT BA CE EC ME NOFigure 1. Percentage of lakes for which chlorophyll a is measured in the GIG (Al= Alpine;AT= Atlantic, BA= Baltic; CE= Central; EC= Eastern continental, ME= Mediterranean,NO= Northern).2. However, there is great heterogeneity both within GIG and between GIGs in termsof the sampling methods. Most lakes in the GIGs are sampled for chlorophyll abetween 2 and 12 times per year (Fig. 2), and samples are taken over the whole year(monthly) or concentrated during spring, summer, or vegetation periods (Fig. 3).45