Depth distributions of Fucus vesiculosus L. and Zostera marina L. as classification parameters for implementing the European Water Framework Directive on the German Baltic coast

Abstract

Abstract A classification approach was developed within the European Water Framework Directive for the outer coastal waters of the German Baltic Sea. We concentrated on the known recent presence and depth distribution of Zostera marina and Fucus vesiculosus along the German coast. According to the European Water Framework Directive the reference conditions were reconstructed based on historical data. The available databases indicate that both species formerly occurred down to 10 m depth along the whole German Baltic Sea coastline, independent on the salinity gradient. The recent depth distribution of Z. marina varied between 2.5 and 7.9 m along the German Baltic coast. Dense F. vesiculosus stands were observed only along the western part of this coast at a maximum depth of 4.7 m. Comparing the historical data sets with recent findings reveals a strong decline of depth limits for both species during the last century. Therefore, we used both species to describe the degradation of the Baltic Sea coastal waters using change in the depth distribution. The boundaries of the ecological status according to the Water Framework Directive were calculated based on modelling. Ecophysiological light demands of species and decrease of water transparency (light reduction in percent) were used to describe the degradations. This approach is robust against variation of macrophyte light requirements and is straightforward to classify recent long-term macrophyte monitoring. However, it is very sensitive against changes in the depth distribution and may result in erroneous estimations of ecological class boundaries when insufficient historical data are used as reference. This model allows the adaption of the boundaries calculations to new knowledge about historical data and ecophysiological light demand of plants. Actually, the boundaries of the classification were defined as follows: 1% light reduction represents the transition from high/pristine to good ecological status, and 5% indicates the transition from good to moderate status. At least 25% reduction corresponds to a poor status, more than 75% to a bad status.