The evaluation and classification of freshwater ecosystems by assessing their ecological status was introduced as a monitoring task almost 20 years ago by the Water Framework Directive (WFD). The adoption of the WFD has led to the intensive development of monitoring methods using all major groups of aquatic organisms [1
], making the EU member states leaders in comprehensive ecological studies of aquatic ecosystems as a determinant of environment quality. This approach has also been used for the development of biomonitoring systems in nonEU countries and even on other continents [2
Macrophytes, as a vital component of aquatic ecosystems, are among the groups of organisms considered by the WFD for river assessment. Plants are sensitive indicators of the aquatic environment, able to detect eutrophication [2
], and to some extent also acidification [10
], water flow [11
], and morphological degradation [8
]. Furthermore, aquatic plants respond to various other environmental factors, including light, temperature, and substrate [12
For the purpose of river monitoring, several systems based on aquatic plants have been developed, and some of these have been integrated into national monitoring programs. In France, the L’Indice Biologique Macrophytique en Rivière (IBMR) system [8
] was proposed, and it has been implemented in monitoring on a national scale. In Germany, several systems had been developed by the turn of the century, including Trophäe-Index Macrophyten (TIM) [6
] and the Macrophyte Index (MI) [13
]. Later, especially for the purposes of the WFD, a new method was designed, based on the Reference Index (RI) [14
]. Macrophyte monitoring of British rivers was already being tested in the 1990s using Mean Trophic Rank (MTR) [5
], and this method was tested widely in Europe, for instance in Denmark, Germany, Italy, Slovakia, Sweden [17
], Latvia [19
], and Poland [20
]. Later, further macrophyte methods were proposed and implemented in monitoring in the UK (RMNI) [21
], Denmark [22
], and Austria [23
The Polish macrophyte method was developed in 2007 and is based on calculation of the Macrophyte Index for Rivers (MIR). The method indicates the ecological status of rivers based on quantitative and qualitative evaluation of freshwater plant species. In more than a decade since its introduction, it has been modified based on the results of research projects [24
] and the EU Water Framework Directive intercalibration exercise [28
]. Although the MIR method has been in use for over 10 years, its fundamentals have not yet been described in detail in an international scientific journal. Our paper fills this gap, presenting the results of application of the most recent form of the MIR method in a selected trophy gradient. The aim was to evaluate the ability of the MIR method to detect trophic degradation in rivers and to compare its efficiency with other macrophyte metrics. Moreover, the source of the differences between metrics was analyzed. We hypothesized that the MIR method is the most advantageous method for ecological status monitoring in local conditions in Poland. Furthermore, we hypothesized that the advantage of local macrophyte methods in particular geographical conditions is a result of the use of locally adapted sets of macrophyte species and adjustment of their indicative values.
Analysis of the water quality dataset showed that the selected river sites represent a wide gradient of eutrophication, which is the major threat to rivers in Central Europe [35
] and a serious problem in many other regions. PCA revealed that the major hydrochemical gradient was strongly positively correlated with various forms of nutrients, including both forms of phosphorus and nitrogen, which are the main cause of surface water eutrophication. Moreover, this gradient was strongly positively correlated with BOD, which indicates the potential oxygen deficit in the water, accompanying the eutrophication process. The gradient obtained was also positively correlated with conductivity, which indicates a wide range of chemicals dissolved in the water. There is no direct relationship between conductivity and eutrophication, but in the central European lowlands, in the absence of a mineral and marine salt supply, a higher conductivity value usually follows nutrient enrichment [36
]. Because trophy is the factor that is most strongly indicated by macrophytes [39
], our dataset provided a very convenient way to test macrophyte metrics. Moreover, by selecting a uniform river type and excluding heavily modified river sites, a high degree of environmental homogeneity was achieved, and the impact of trophy conditions on macrophytes could be precisely evaluated.
The results obtained showed a strong relationship between macrophytes and water quality, compared with various other studies based on macrophytes [28
] and other organisms [39
]. The strong relationship was clearly due to some extent to the quality of the data based on a single river type, where a lack of typological homogeneity is regarded as one of the major sources of uncertainty in ecological assessment [43
]. Moreover, we achieved a significant reduction in interpersonal variability, because the macrophyte surveys were carried out by experienced experts trained by the present authors. This meant that interpersonal variability in our case was largely excluded, which removed a large source of uncertainty [30
]. Furthermore, other studies have demonstrated that the MIR has relatively low resistance to measurement errors, compared with other methods based on phytobenthos and macroinvertebrates [46
]. All of the aforementioned factors reducing uncertainty in our study provided favorable conditions to identify precisely the correlation between plant-based indices and environmental factors.
The research showed that the MIR index in its present form is well suited for the evaluation of the ecological status of Polish rivers. It was found to reflect well the trophic degradation of rivers, which is the most serious issue affecting surface waters in this part of Europe [35
]. It was shown to respond much better to water quality changes than other coefficients of foreign origin—the Pearson’s correlation coefficient r was particularly high for the MIR index in relation to the synthetic trophic degradation gradient determined by PCA, but the values were usually also very high in the case of conductivity and phosphorus (both total and reactive). They were slightly lower for BOD, mineral nitrogen, total nitrogen, and ammonium nitrogen. The possibility of precise identification of threats related to eutrophication makes the MIR a very important tool in the process of river degradation monitoring for the requirements of the WFD, as well as a method of identifying threats caused by human activities.
The results of the completed study show that macrophyte methods developed in other countries are not as sensitive as the MIR, but still identify the undergoing eutrophication process to a large extent. The degradation gradient determined by PCA was particularly strongly detected by MTR (r = −0.79) and IBMR (r = −0.67). These indices were also strongly correlated with total and reactive phosphorus, as well as with conductivity. The obtained level of correlation was relatively high compared with other studies [37
], and in our opinion, this results from the wide gradient, the size of the analyzed database, the homogeneity of the collected data in terms of habitat background, and the reduced personal variability.
According to the results of the research, the variability of the pH factor in water is not detected by the considered macrophyte metrics. It should be noted that the tested macrophyte methods were developed this century to evaluate the ecological status of water according to the requirements of the WFD [48
]. The development of these methods was mainly focused on monitoring of the main factors responsible for the degradation of surface waters in Europe, which include eutrophication and, to a lesser extent, the hydromorphological factor [6
]. It should be pointed out that the pH gradient in the studied rivers was very narrow, as the sandy lowland rivers of the whole Eastern European Lowlands have a rather uniform geology.
Our calculations showed that the indicator values determined for the MIR better reflect the water degradation gradient than the indicator values assumed for other methods (IBMR, MTR, RMNI, and ITEM). The analyses carried out with the use of a uniform formula revealed that the detected differences in biomonitoring result not only from the different equations used, but also from the set of species and their ascribed indicator values. The significant differentiation of sets of indicator species considered by various macrophyte systems on lowland rivers has already been demonstrated in the literature [50
]. The weaker response of the foreign methods compared with the MIR persisted even when the assessment was based on a uniform equation. Indicator values of particular species differ between these methods, and it appears that those proposed for Poland (Table S1
) best reflect the local conditions of the Polish lowland rivers.
The study showed that it is justified to develop separate macrophyte systems for geographically differing countries. The MIR index was better suited for the evaluation of ecological status in Poland than foreign indicators; thus it was better adjusted to the local conditions. A stronger correlation between the MIR index and environment gradients in Polish rivers had already been indicated by other authors [51
] but the current research makes it possible to evaluate the sources of the differences. Moreover, the importance of the selection of the species set and the estimation of indicator values was proven. Therefore, for different geographic conditions, separate systems should be developed with different lists of indicator species and their indicative values should be verified according to local geographic and ecological conditions. For example, Apium nodiflorum
, which is an important species in the British MTR system [5
], is not present in the MIR system. Apium nodiflorum
is a very rare species in Poland, and therefore its ecological preferences may not be precisely verified. On the other hand, the species Berula erecta
, which also belongs to the family Apiacae
, is included in the MIR system but not in MTR, as it occurs rarely in the British Isles. The introduction of separate systems or local modifications would enable the more precise identification of threats occurring in surface waters.
The pool of taxa recorded in the lowland rivers contained a smaller number of indicator species used in the MIR system than indicators used in RMNI and ITEM. Among exclusive RMNI and ITEM indicative taxa, many are not typical macrophytes, and in many cases they can be regarded as largely terrestrial plants. This provides a convincing explanation of the precision of the MIR method in water degradation detection, since typical aquatic plants are more sensitive to the water habitat [10
]. A large proportion of emergent bank plants increases the variability of the bioindication process and prevents accurate assessment of the fluvial ecosystem.
The results were based on the category of small and medium lowland rivers, but the correlations demonstrated can also be expected to apply to other types of rivers in Poland and abroad. Our analysis focused on this category because it represents a widespread type of rivers in Europe [28
]. Moreover, the biodiversity of rivers of this type is higher than that of other rivers [37
], and analyses based on rich biological material can better reflect the processes taking place in ecosystems. It was shown by Budka [53
] that a limited pool of indicator species reduces the quantity of ecological information provided by bioindication. The use of a species-rich habitat and a very common river type in our analysis increases the universal applicability of the results.