Carstensen, J., Krause-Jensen, D., Dahl, K. & Henriksen, P. 2008: Macroalgae and phytoplankton as indicators of ecological status of Danish coastal waters. National Environmental Research Institute, <st1:place w:st="on"><st1:PlaceType w:st="on">University</st1:PlaceType> of <st1:PlaceName w:st="on">Aarhus</st1:PlaceName></st1:place> . 90 pp. - NERI Technical Report No. 683.
During the implementation of the EU Water Framework Directive, an intercalibration of selected metrics of the biological quality elements was undertaken at a limited number of sites. This report describes a method for establishing ecological status classes for phytoplankton in more areas and evaluates several macroalgal indicators and their calculated indicator values for ecological status class boundaries.
In the first part of the report, estimates of nitrogen inputs from <st1:place w:st="on"><st1:country-region w:st="on">Denmark</st1:country-region></st1:place> to the Danish straits since 1900 combined with expert judgement of the general environmental conditions of Danish waters during different time periods were used to establish nitrogen inputs representing reference conditions and boundaries between the five ecological status classes. These reference conditions and class boundaries were transformed into nitrogen concentrations in the water in several fjords and coastal localities by the use of site-specific relations between nitrogen inputs and nitrogen concentrations.
An existing macroalgal model was refined in the second part of the report. The model describes the following variables: i) the total algal cover, ii) the cumulative algal cover of the total algal community, opportunistic species or late-successional species, iii) the fraction of opportunistic species and iv) the number of late-successional species. All macroalgal variables responded to changes in total nitrogen but also to changes in salinity which emphasises the need for setting different targets depending on salinity. The strongest responses to changes in nitrogen concentration and the least variability were found for the indicators 'total algal cover', 'number of late-successional species' and fraction of opportunists'. Ecological status class boundaries were established for all the macroalgal variables in a number of Danish estuaries and coastal areas.
A Spanish macroalgal index based on 'cover', 'proportion of opportunists' and 'species richness' was tested using Danish data. Each component of the index responded to nutrient gradients but the index needs adjustment of especially the scoring system in order to be applicable to Danish conditions.
In the third part of the report site-specific correlations between concentrations of nitrogen and chlorophyll a (chla) were used to define reference conditions and ecological status class boundaries for the phytoplankton metric 'mean summer concentration of chla' in several Danish estuaries and coastal areas. The relationship between chla and nitrogen concentrations varied from site to site and reflected the bio-available fraction of total nitrogen. A relationship was demonstrated between reference conditions and good-moderate boundaries for eelgrass depth limits and the corresponding values for chla.
Precision of the two different chla indicators 'summer mean' and '90-percentile was evaluated. The 90-percentile was substantially more uncertain than the mean or median indicators, particularly for small sample sizes but also for large sample sizes.
Full report in pdf format (7,402 kB)
