WaterSubject

Ecological optima and ranges of submerged macrophytes are, amongst other factors, assumed to be influenced by ecoregion and flow velocity. In order to test the influence of ecoregion within Europe, species indicator values of three European macrophyte river trophic indices were compared to each other. Species indicator values of the United Kingdom (UK), French and German bioindication methods are significantly correlated with each other. The most obvious difference between the three indicator systems is the number of included indicator taxa. Two species exhibit major differences in indicator values: Callitriche hamulata has a broader ecological amplitude in Germany and France than in the UK, where it is restricted to oligotrophic rivers, while Ranunculus fluitans has a broad ecological amplitude in the UK, whereas the species is restricted to eutrophic rivers in Germany and France.

In order to test if current velocity has an influence on macrophyte trophic indicator qualities, species indicator values of a river (Trophic Index of Macrophytes, TIM) and a lake macrophyte trophic index (Macrophyte Index, MI), both of them applicable in Bavaria, Germany, were compared to each other. Species indicator values are significantly correlated. The most important difference is that different species are included in lake and river indicator lists. Only approximately 60% of the total species are used in both TIM and MI. Three species exhibit a major difference in ecological optima between rivers and lakes: Ranunculus circinatus has a broader ecological amplitude in rivers whereas the species is restricted to eutrophic lakes, Myriophyllum spicatum and Nuphar lutea show the opposite reaction.

All currently used and published European macrophyte indices based on numerical species indicator values for assessment of river or lake trophic status are included in this study. Indices for assessment of ecological status according to the WFD  are not included as these indices do not indicate trophic status alone, but an overall ecological status that is not only influenced by trophic status but also by additional factors like acidification, flow regulation, etc. Thus, species indicator values can strongly vary depending on river or lake type and are therefore unsuitable for detecting differences in species ecological tolerances across Europe. Four macrophyte indices meet the above-mentioned criteria: the Macrophyte Index, the Mean Trophic Rank, the Trophic Index of Macrophytes (TIM) and the Indice biologique macrophytique en rivière.

Macrophyte Index (MI)

The MI, developed in Bavaria, Germany, is applicable in calcareous lakes in the Alps and the prealpine region. Species indicator values were slightly modified by. A total of 45 species of submerged macrophytes is included in a catalog of nine indicator groups, each exhibiting different sensitivity towards nutrient enrichment. Species allocated in indicator group 1 are restricted to oligotrophic conditions, whereas those in indicator group 5 mainly occur in eutrophic, nutrient-rich lakes or sections of lakes. The remaining seven groups range from 1.5 to 4.5 and represent transitions between these two extremes. The allocation of macrophyte species into indicator groups was based on expert opinion and literature data. The mean MI of a lake correlates with its total phosphorus concentration during circulation time.

Mean Trophic Rank (MTR)

The Mean Trophic Rank is a macrophyte-based method for assessing the trophic status of rivers, particularly focused on the impacts caused by phosphate enrichment. It is applicable throughout the UK. Each of 129 aquatic plant species is allocated a Species Trophic Rank (STR) score according to its response to eutrophication. STRs range from 1 to 10 – a numerically high value indicates that the plant is intolerant of eutrophication, whereas a low STR indicates that the plant is either tolerant of eutrophication or alternatively has no preference and is termed “cosmopolitan”. Allocation of STRs was based upon literature data and expert opinion. The Mean Trophic Rank (MTR) of a sampling site is expressed by integrating the STRs of the species present at a site as a mean value, weighted according to the relative percentage cover of the individual species.

Trophic Index of Macrophytes (TIM)

The TIM is a macrophyte-based method for assessing the trophic status of rivers. It is applicable in Bavaria, Germany. Histograms depicting species phosphorus tolerances were derived based on a weighted sum of water and sediment soluble reactive phosphorus concentrations in 331 single species macrophyte patches in Bavarian rivers. From these, TIM species indicator values were determined by weighted averaging. Indicator values are given for a total of 49 species of submerged macrophytes. In cases where fewer than seven sampling sites had been sampled for a species, literature data were used to verify or correct the indicator values. Macrophyte indicator values are given on a scale from 1 to 4; 1 indicating oligotrophic and 4 polytrophic conditions, respectively. Because indicator values of individual species were determined by weighted averaging, their range is reduced. Thus, actual species indicator values range from 1.05 (Chara hispida, Potamogeton coloratus) to 3.20 (Acorus calamus) and lean towards the center of the trophic gradient. An indication of the polytrophic category (index >3.50) is not possible. Within the polytrophic category mostly a depopulation of submerged macrophytes is observed.

Indice biologique macrophytique en rivière (IBMR)

The IBMR is a macrophyte-based method for determining the trophic status of rivers. It is applicable for natural and artificial running waters in continental France. Each of 207 taxa (some at genus level) is allocated a “cote spécifique” according to its response to eutrophication. IBMR species values range from 0 to 20; 0 indicating hypertrophic and 20 indicating oligotrophic conditions, respectively. Allocation of IBMR species values was based on expert opinion and background information on water phosphorus and ammonia concentrations in a number or rivers in France.

In order to ensure comparability of species indicator values, taxa names were screened for synonyms and harmonized if necessary. To determine if significant correlations occur between species indicator values of different macrophyte indices, a Spearman rank correlation was applied. Scatter plots are used to depict the regression equations. A deviation of±0.75 (MI), 1.5 (STR) and 3 (IBMR) in the direction of the y-axis is plotted in the diagrams, corresponding to ±15% of the maximum indicator value. Species lying more than 15% above or below the regression line are highlighted. As TIM values were determined by weighted averaging, their range is reduced and the values lean towards the center of the trophic gradient. In contrast, the range of the empirically determined STR, IBMR and MI values is not reduced. Therefore a cubed regression function was applied between TIM and the other indices.The most important differences between the river macrophyte trophic indices of the UK, France and Bavaria are the indicator taxa used and not differences in indicator values of individual species. Therefore, if an index is to be applied in a different country from where it was developed, important local indicator taxa are probably neglected. Thus, minor differences in river trophic status might be overlooked. However, as individual species indicator values are not substantially different between countries, a rough classification of river trophic status is probably possible. Most indices require a certain number or biomass of indicator taxa present at a river site in order to ensure a reliable indication. By neglecting important indicator taxa a reliable indication of river trophic status might often be impossible due to the presence of too few indicator taxa. An adjustment of indicator species lists to local conditions is thus strongly recommended. The same is true when applying local river macrophyte trophic indices to lakes or vice versa.