NERI Technical Report no. 535: Redskaber til vurdering af miljø- og naturkvalitet i de danske farvande. Typeinddeling, udvalgte indikatorer og eksempler på klassifikation. Dahl, K. (red.), Andersen, J.H. (red.), Riemann, B. (red.), Carstensen, J., Christiansen, T., Krause-jensen, D., Josefson, A.B., Larsen, M.M., Kjerulf-Petersen, J., Rasmussen, M.B. & Strand, J. 2005. 158 p.
Summary
This report deals with the development and application of chemical and biological indicators of ecological status in the coastal zone. The overall aim of the work is to identify relevant indicators and evaluate how they can be used to assess conservation status according to The Habitat Directive and ecological quality according to The Water Framework Directive. In addition to this, the report proposes a typology for Danish coastal waters.
A typology has been developed for the Danish coastline, most of which is located in a region with a strong physical and biological gradients. It was found necessary to use different criteria for characterizing the open water coast and estuaries. The open water coast was characterized with respect to salinity, depth, wave exposure and tidal influence, whereas estuaries were characterized with respect to bottom salinity, degree of stratification, the ratio of residence time to surface runoff and sluice-control. The approach results in a division of the Danish coastal waters into 15 different types: 5 open water types and 10 estuary types. The large number of types reflects the strong salinity gradient present in the Danish coastal waters, but also that the physical factors that are relevant for defining a type, vary greatly among the Danish esuaries.
There were significant empirical relationships between the emergent macroalgal cover on the examined stone reefs in Kattegat and the nutrient load. The vegetation is primarily controlled by light. The visibility is reversed correlated to the nutrient load. A number of scenarios were calulated to demonstrate relationships between the vegetation and a variety of nitrogen loads. These relationships can be used to establish specific ecological quality objectives according to the requirements described in the Habitate Directive and in the Water Framework Directive.
The coastal vegetation and more specifically the depth distribution of macroalgae was tested as a potential indicator of water quality. We wanted to identify whether changes in nutrient load can explain changes in 1) the depth cover of the total algal community, 2) of functional algal groups and/or 3) of single species of macroalgae along depth gradients. The study is based on stastitical analyses of algal cover in relation to nutrient load, climatological data and water quality collected in the Danish National Monitoring and Assessment Programme throughout the period 1989 to 2002. The applied models considered that algal cover depends on the substratum and that cover also varies with depth, location, area and sampling year.
The cover of the total algal community was generally higher in areas with the best light conditions and the lowest concentrations of nutrients and chlorophyll. The best model explained 79% of the variation in cover between areas and demonstrated that cover was generally higher in areas with good light conditions and high salinity. Sites located in the Limfjord area generally had lower algal cover than other areas for given levels of Secchi depth and salinity. The developed models could not explain year to year changes in algal cover from changes in water quality over the monitoring period.
The functional groups encompassing filamentous and foliose macroalgae with a simple thallus are dominated by species having an opportunistic growth strategy. These algae are, in theory, favorised by increased nutrient load. The study demonstrated that the fraction of these algae was largest in the most brackish areas but showed no significant correlation to differences in water quality or nutrient load between areas or between years.
The fraction of selected species of perennial algae constituted a very small percentage of the total algal community and varied considerably between areas so the data set was inappropriate for modelling general relationships between the cover of these species and water quality.
In conclusion, the cover of the total algal community was the only tested algal variable useful as an indicator of water quality. In order to illustrate how the indicator can be used in practice to assess water quality, we applied the identified relation between algal cover and Secchi depth, then related Secchi depth to nutrient concentration and finally related nutrient concentration to nutrient load. Thereby we could relate nutrient load to algal cover. Total algal cover is, however, still not a sensitive indicator of water quality. Cover increased only 4% on average when Secchi depth increased by 1 m and algal cover did not reflect the variations in water quality, which had taken place over the last 13 years. In order to obtain better future models it is necessary to explain a larger part of the residual variation in cover data, which can be due to e.g. diver effects and which blurs relations between water quality and algal cover.
Earlier studies have demonstrated that eelgrass depth limits respond to changes in water quality. Depth limits increase as a function of increased Secchi depth and reduced concentrations of TN and differences in Secchi depth can explain up to 60% of the variation in eelgrass depth limits in Danish coastal waters. Eelgrasss depth limits are thereby among the biological indicators, which show the best-documented coupling to eutrophication in coastal areas. While the existing models are useful for assessing general relationships between depth limit and water quality and for predicting an average depth limit at a given Secchi depth, they cannot precisely predict depth limits at specific sites. There are several examples that the models are insufficient. Even though Secchi depth has lately increased in Danish coastal waters, eelgrass does not grow deeper; - in contrary, depth limits have become shallower in the estuaries. Similarly, many sites do not show a coupling between depth limits and water quality over time. The aim of this study was to improve existing predictive models based on the large data set collected under the Danish National Monitoring and Assessment Programme since 1988. However, the large data set did not provide a better predictive model, neither regarding prediction of temporal changer nor regarding precise prediction of depth limits in specific areas. Area- or site specific models did not provide a more accurate assessment of reference depth limits that did the existing overall model. Generally, the models predict shallower reference depth limits of eelgrass than indicated by historical data.
An earlier study has established reference conditions and a classification system for the indicator 'eelgrass depth limit' in Denmark. The work was based on a preliminary typology for Danish coastal waters. The aim of the present study was to establish reference conditions and a classification system for eelgrass depth limit based on a revised typology.
Reference conditions for eelgrass depth limits were defined from a large historical data material from ~1900. Reference conditions or 'high ecological quality' in type areas were described as ³ the 90% fractile of historical data from sites located in each type area. Reference conditions were also estimated for coastal areas (e.g. Øresund) based on historical estimates for these areas. Eventually, reference conditions were modelled for estuaries and coastal areas based on reference total nitrogen concentration using an empirical model. Classification of good ecological quality based on eelgrass depth limits was illustrated by 3 examples using different limitation (15%, 20% and 25% of reference condition) between good and moderate quality.
The analyses showed that classification of good environmental quality depends on 1) whether it is based on type areas, coastal areas or sites, 2) the criteria used to define reference conditions (e.g. 90% fractile of historical data) and 3) the acceptable deviation from reference conditions (e.g. 15%, 20% or 25%).
This study illustrates that eelgrass depth limits are applicable indicators of water quality under the Water Framework Directive and the Habitats Directive. The use of type areas may, however, be problematic and it is recommended to apply site-specific information of reference conditions to the extent they are available. When using historical estimates of eelgrass depth limits in the classification it is important to be aware that these represent conservative estimates of depth limits (i.e. depth limits of eelgrass belts). To classify properly, they must therefore be compared with actual depth limits of eelgrass belts rather than with maximum depth limits of eelgrass.
Faunal biomass may be a useful parameter to illustrate relationships between the pressure factor eutrophication and the response variable macrobenthic fauna. Benthic biomass in the two systems has in general doubled over the 20ieth century. Since data on nutrient load are lacking from the first half of the previous century, it is not possible to define exactly a load-determined status of conservation. However, the differences between then and present can still be used as guidance when determining the biomass level for "good status". On the other hand we can not with present knowledge use species composition as response variable. Benthic biomass is also sensitive for other impact factors than the ones induced by man. Salinity influences both species composition and biomass. Therefore, good status of coastal lagoons should be finely tuned to the expected or desirable salinity of the area. The same is true for the indicator species richness, which has been investigated in other contexts.
Hazardous substances represent a variety of different compounds, and most have only been measured in recent times, i.e. since the beginning of the 1980’ies. For this reason, one of the purposes of this report was to collect an overview of available data with respect to their geographical distribution in Danish waters and particularly in marine habitats. The most useful data is from the Danish National Monitoring and Assessment Programme, NOVA, and regional programmes from 1998-2003. From 2004, NOVA was replaced by NOVANA. Mainly heavy metals were measured periodically before the NOVA programme, but in some programmes or campaigns also other hazardous substances was measured sporadically. From NOVA started in 1998, PAHs, PCBs and chlorinated pesticides together with TBT have been measured periodically. The NOVA programme covered approximately half of the marine annex 1 habitats of Council Directive 92/43 EEC of 21 May 1992.
An important limitation in the use of hazardous substance-indicators in connection with the WFD is, that measurements usually are performed in Biota (fish and mussels, and in rare cases higher mammals and birds) and sediments. Contrary, the WFD makes use of concentrations in the water phase as target values. Therefore, this report concentrated on 3 different substance groups: Cd as an example of metals, Naphthalene as example of light- and Benzo(a)pyrene of heavy PAHs and finally Tributyltins (TBT) as an example of a pesticide, used in antifouling paints to ships. PAHs and TBT represents persistent organic pollutants (POPs), for PAH’s with a natural background level from forest fires and occasionally seepage from the seabed, whereas TBT has no natural background level but is purely anthropogenic in origin. The three organic compounds exhibit different physico-chemical properties with regard to bioaccumulation potential, degradability, water solubility and vapour pressure. Using ecotoxicological criteria, concentration levels in water, biota and sediment were aggregated, so that concentrations in biota or sediments could be used to estimate water concentrations in a common classification system, and hence used as quality targets. In addition, it was studied whether the established classification provided comparable results between the different matrices and for TBT even with a coupling to biological effects (imposex and intersex in marine mollusc). The interaction between the marine conventions, HELCOM and OSPAR, and the directives are also briefly discussed.
A number of constraints, questions and recommendations were made on several of the indicators in question which are important to consider before a more comprehensive implementation of the Water Framework Directive can be made in Danish coastal waters.
Full report in pdf format (3.773 kB).
