Hydrobiologia (2006) 566:235–246 Ó Springer 2006 M.T. Furse, D. Hering, K. Brabec, A. Buffagni, L. Sandin & P.F.M. Verdonschot (eds), The Ecological Status of European Rivers: Evaluation and Intercalibration of Assessment Methods DOI 10.1007/s10750-006-0093-4
Assessment of sources of uncertainty in macrophyte surveys and the consequences for river classification Ryszard Staniszewski1,*, Krzysztof Szoszkiewicz1, Janina Zbierska1, Jacek Lesny2, Szymon Jusik1 & Ralph T. Clarke3 1
Department of Ecology and Environment Protection, August Cieszkowski Agricultural University, Piatkowska 94C, 61-691 Poznan, Poland 2 Department of Agrometeorology, August Cieszkowski Agricultural University, Piatkowska 94B, 61-691 Poznan, Poland 3 Centre for Ecology and Hydrology, Winfrith Technology Centre, DT2 8ZD Dorchester, Dorset, England (*Author for correspondence: Tel: 48-61-8466510; Fax: 48-61-8466510; E-mail:
[email protected])
Key words: macrophytes, Mean Trophic Rank, aquatic vegetation, error assessment, biodiversity, river classification
Abstract The application of macrophytes in freshwater monitoring is still relatively limited and studies on their intercalibration and sources of variation are required. Therefore, the aim of the study was to compare selected indices and metrics based on macrophytes and to quantify their variability. During the STAR project, several aspects influencing uncertainty in estimation of the ecological quality of river were assessed. Results showed that several metrics based on the indicative value of plant species can be used in evaluation of the ecological status of rivers. Among estimated sources of variance in metric values the inter-surveyor differences had the lowest effect and slightly stronger were the influences of temporal variation (years and seasons) and shading. The impact of habitat modification was the most important factor. Analysis showed that some of macrophyte-based metrics (notably MTR and IBMR) are of sufficient precision in terms of sampling uncertainty, that they could be useful for estimating the ecological status of rivers in accordance with the aims of the Water Framework Directive.
Introduction The purpose of Water Framework Directive – WFD was to established an European framework for the protection of surface waters, transitional waters, coastal waters and groundwater (Directive 2000/60/EC). It formalises the need to obtain more standardised, comparable and widespread data about aquatic ecosystems in Europe, together with estimation of the uncertainty in assessing the ecological status of water bodies. Macrophytes are one of the major groups of organisms upon which the WFD prescribes that such assessments should be made. The application of macrophytes in monitoring of freshwaters is still relatively limited
and studies on their intercalibration and sources of variation are required. Therefore, the aim of the study was to compare selected indices and metrics based on macrophytes and to quantify their variability. Studies on macrophyte variability based on replicate sampling experiments were carried out in Polish lowland rivers in the years 2003 and 2004, in the period when river vegetation was well developed (5th June–30th September). The sites selected for replicate sampling were rich in macrophyte species and encompassed a wide environmental gradient, that improved the quality of the study. Each site was pre-classified within the STAR project into one of the five WFD ecological
236 status classes. Diversity (Shannon’s index) of aquatic plant species showed unimodal distribution along the ecological status gradient, peaking at intermediate site qualities and with detectable differences between status classes. Methods based on the indicative values of plant species along environmental or stressor gradients are often used in studies of terrestial and aquatic ecosystems (Ellenberg et al., 1992; Haury et al., 1996, 1998; Allan, 1995; Dawson et al., 1998; Dawson & Szoszkiewicz, 1999; Schneider et al., 2000; Staniszewski, 2001; Szoszkiewicz et al., 2002 and others). Variation of estimation of the ecological quality of water ecosystem depends on many factors like field sampling, personal judgement, selection of reference sites and the choice and estimation of metrics representing the biological Reference Conditions for the site (Clarke, 2000). Several aspects influencing uncertainty in estimation of the ecological quality of river were detected and assessed during the STAR project. The study was designed to assess the variation due to inter-surveyor differences, temporal variation, and the influence of stressors such as shading and hydromorphological degradation.
Methods Site selection and sampling The main criteria used in site selection were to cover a wide range of the eutrophication gradient and wide geographic distribution (Fig. 1,Electronic supplementary material is available for this article at and accessible for authorised users.). An
Figure 1. Distribution of surveyed river sites.
additional factor influencing site selection was the overall abundance of aquatic plants. Important part of studies was selection of reference sites according to the needs of WFD and characterised with undisturbed by human activities river valley and high hydrochemical quality (Table 1). All selected sites were from comparable conditions in the sense of being from the WFD System A stream type. These streams were characterised with an upstream watershed area between 100 and 10002 km and a site altitude of up to 200 m a.s.l. The water samples were collected three times: summer 2003, autumn 2003 and summer 2004. Several water quality parameters were examined as like total phosphorus, soluble reactive phosphates, nitrates, conductivity and others. Samples were stored in ice boxes and analyses were made within 24 h. Water samples for soluble reactive phosphates and nitrates were filtered using 0.45 lm pore size.
Table 1. Differentiation of trophic parameters (TP – total phosphorus, SRP – soluble reactive phosphates, nitrates and conductivity) between reference sites and others TP mg P dm)3
SRP mg PO4 dm)3
Nitrates mg N–NO3 dm)3
Conductivity mS cm)1
Reference sites Other sites Reference sites Other sites Reference sites Other sites Reference sites Other sites Maximum 0.54
15.53
0.35
15.50
0.40
12.75
0.42
2.99
Mean
0.33
1.82
0.19
1.34
0.15
1.06
0.35
0.56
Median
0.31
0.62
0.20
0.37
0.13
0.21
0.32
0.46
Minimum
0.01
0.14
0.01
0.05
0.01
0.03
0.24
0.27
237 Selected indices and metrics as indicators of site quality
Sensitivity of indicators to various sources of error
Studies on macrophytes were undertaken in Polish lowland rivers in 2003 and 2004, in the period when river vegetation was well developed (5 June– 30 September). Field surveys were carried out according to STAR guidance for field assessment of macrophytes (Dawson, 2002) being closely related to the Mean Trophic Rank (MTR) methodology (Newman et al., 1997; Dawson et al., 1999; Holmes et al., 1999). The macrophyte assessment was based on the presence of algae, mosses, horsetails, liverworths, monocotyledonous and dicotyledonous plant species which have value as biological indicators of water quality. All submerged, free floating, amphibious and emerged plants were considered. The assessment also included the macrophytes attached or rooted on parts of the river bank substrate where they were likely to be submerged for more than 85% of the year. The presence of each species on the standard MTR survey river length of 100 m was recorded together with their percentages of area covered using the standard MTR nine point scale (Holmes et al., 1999). All of the sites were surveyed by wading along the river channel except for the Sokolda River, which was too deep to wade and for which a grapnel was used to collect plant species. All taxa were identified individually by each surveyor and additionally, samples of algae were checked by algologists from University of Lodz to confirm results. The STAR procedures for macrophyte field surveys were designed primarily to obtain MTR score but created database enabled the estimation of other macrophyte based metrics which are widely applied in the vegetation sciences. In this experiment based on the STAR protocol, in addition to MTR scores, the following four other metrics were also calculated:
The replicate survey of plant species was carried out by the group of six trained surveyors. The aim was to provide unaltered unbiased conditions for subsequent surveys by avoiding plant removal (especially for scarce species) whilst still providing accurate identification of difficult taxa. In all surveyed river sites the physical characteristics of river channel composition were recorded as percentage covers according to the STAR methodology (Dawson, 2002). During the field surveys four experiments were conducted to assess different sources of variation (Supplementary material):
1. 2. 3. 4.
Macrophyte Biological Index for Rivers – IBMR (Haury et al., 2002), Index based on Ellenberg nitrogen values for plant species (Ellenberg et al., 1992), Number of species, and Shannon diversity index (Shannon & Weaver, 1949).
inter-surveyor variation, temporal variation, influence of shading, impact of hydromorphological degradation.
Inter-surveyor variation was estimated during summer 2003 by comparing the macrophyte scores achieved by three independent and fully trained surveyors for the same 26 river sites. Temporal variation was assessed by surveying 26 sites in summer and autumn 2003 and again in summer 2004. The variation between years was assessed by comparing the field surveys in June/July 2003 with those in summer 2004 on the same 26 river sites. Impact of seasonal variation was estimated by comparing the vegetation and derived metric values recorded in early summer (June/July) 2003 with the plant cover and values in early autumn (September) 2003. For any particular site, the same person carried out all three surveys, so that differences were focused on temporal sources of variation; and to avoid effects of spatial differences between surveys, the starting point was coordinated with GPS, maps and detailed drawn plans. Influence of shading and hydromorphological degradation on river ecosystem was estimated in the separate experiments in 2004 (5 June–10 August). The sensitivity of the MTR method to shading (caused mainly by trees growing along rivers) was estimated by surveying matched pairs of sites on the same rivers in summer 2004. On each of 23 river stretches, macrophytes were surveyed in two sites within several hundred
238 meters of each other, one unshaded and one shaded. The two matched sites had very similar environmental conditions in terms of water depth and width, current velocity, hydromorphological conditions and substrate. Absence of pollution discharge between pairs was checked. Sixteen matched pairs of sites (modified and unmodified) on different rivers or river sections were selected to test the impact of physical modifications of river channel (e.g., bridges, reinforcements, regulations) on the MTR score. The two sites within each pair were selected to be within 1 km of each other but representing different classes of hydromorphological degradation. The series of experiments enabled the assessment of natural background variation focusing mainly on temporal sources of variation (differences between years and seasons of the year) and influence of physical parameters like hydromorphological degradation and shading. To estimate the influence of individual factors (surveyors, seasons, years, shading, modifications) on trophic indices and biological diversity, the Wilcoxon Matched-Pairs Signed-Ranks Test was used on the appropriate set of paired sites (Siegel & Castellan, 1988). It is a nonparametric test to estimate whether the parts of a pair differ in size and does not require population with normal distribution. The software programme STARBUGS – STAR Bioassessment Uncertainty Guidance Software (Clarke, 2004) was used to test the effect of using particular status class boundaries on the status obtained for sites (initially without any assessment of uncertainty). The ecological status class assessment for individual metrics is evaluated as normalised Ecological Quality Ratios (EQRs) involving the ratio of the observed metric values (O) to the Reference Condition values (E1) of the metric (Formula 1). Formula 1. Ecological Quality Ratio. EQR ¼
O E0 E1 E0
where: O – observed value, E1 – value of metric for which EQR=1 (Reference Condition value), E0 – value of metric for which EQR=0 (Extreme bad status value). By setting the E0 values to zero, and the E1 values to the RIVPACS-type model expected value
under Reference Conditions, the EQR values become RIVPACS-type O/E ratios of the observed (O) to expected (E) values of metrics or biotic indices (Formula 2) (Clarke et al., 1996; Wright et al., 2000; Clarke et al., 2002; Clarke, 2004). The probability of misbanding a site was tested for the whole possible range of EQR values using STARBUGS software. Class limits and level of uncertainty based on undertaken field studies were used in the setup of the programme. Class limits for the Ecological Quality Ratios (EQRs) were proposed for three macrophyte metrics (MTR, IBMR and Ellenberg index) as specified in Table 5. The criteria of setting the particular class limits was the evenness of probability of misbanding a site of each EQR class. The probability of misbanding a site of each true EQR was simulated using observed value (O) designed to provide EQR values covering the full range from extreme bad (EQR=0) to reference condition (EQR=1). The uncertainty standard deviations in the observed metric values were based on the estimated standard deviation due to the effects of one or more of inter-surveyor and temporal variation, shading and morphological modifications. The standard error (SE) of the mean of a metric’s values for the reference sites was used as the uncertainty standard deviation in the estimated expected value (E) (i.e., reference condition) for the metric to simulate errors (R) in the expected value (Formula 2). Formula 2. Simulated O/E ratio of the observed (O) to expected (E) values. O=E ¼
OþS EþR
where: O/E – estimated ratio of the observed (O) to expected (E) values, O – observed value of metric, S – random value due to sampling or other sources of variation, E – expected (reference condition) value of metric, R – random error for reference condition value. The estimated probabilities of misbanding a site based on the STARBUGS simulations for the whole possible range of true EQR are presented graphically (Fig. 4). Each figure shows the effect of considering a different source of detected uncertainty (inter-surveyor and temporal variation, shading and morphological transformations).
239 Results Comparison of selected indices and metrics as indicators of site quality In total, 227 plant species were recorded during the macrophyte surveys and monocotylodynes and dicotylodynes were the dominant groups. Among Pteridiophytes three species were found, and seven Bryophytes. Seventy of the identified taxa are indicators listed in the MTR protocol and thus have been assigned values of STR (Species Trophic Rank) covering the full range of the water trophic gradient – from oligotrophic (STR=9 and 10) to eutrophic species (STR=1 and 2). The most common species were Elodea canadensis Michaux., Lemna minor Linne´, Glyceria maxima (Hartman) Holmberg, Phalaris arundinacea Linne´, Sparganium emersum Rehmann and Nuphar lutea (Linne´) Smith. The surveyed rivers were rich in plant species (from 17 to 85 taxa per site). There were statistically significant differences between status classes in the number of plant species present (ANOVA F4,198=8.61, p
