Evaluation of candidate biological indicators of hydrological and morphological degradation

Work Package 3 of REFORM focuses on the impacts of hydromorphological changes on river and floodplain ecosystems. The main research objectives of deliverable 3.3 are:

  • To establish empirical relationships between the biota, flow dynamics, substrate complexity/habitat heterogeneity and sediment dynamics.
  • To select and develop candidate indicators for WFD quality elements that quantify impacts of hydrological and morphological degradation in rivers.
  • To develop novel biological indicators capable of diagnosing hydromorphological stressors in a multiple stressor environment.
  • To advise on the design of monitoring programs to detect hydrological and morphological degradation.
  • To address the importance to quantify uncertainty in biology based hydromorphological assessment and discuss the implications of uncertainty for monitoring and assessment.

The empirical relationships between biota, flow dynamics, substrate complexity and sediment dynamics are investigated in chapter 1. The performance of existing indicators/metrics to assess hydromorphological degradation has been determined in a case study of the Regge catchment in the Netherlands and a study of Danish streams (Chapter 2). Novel designs for developing ‘new’ diagnostic metrics to solve issues related to multiple stressors including hydromorphological stress are presented in Chapter 3. Subsequently chapter 4 describes how monitoring programs can be designed to assess hydromorphological degradation using biological indicators. Finally, chapter 5 adresses the importance to quantify uncertainty with its implications for monitoring when assessing the ecological status of river systems.

Conclusions and recommendations:

Empirical relationships

Invertebrates– The experiment described in chapter 1.1 showed that the effects of patchiness and flow on lowland stream caddisflies are species-specific. The wide range of effects found in the experiment indicates that small-scale heterogeneity in the form of substrate patchiness is an important driver of the community patterns in streams and that change in substrate composition or spatial arrangement of patches due to natural or anthropogenic disturbances could have considerable effects on macroinvertebrate populations.

The experimental addition of fine sediment (chapter 1.2) to the substrate affected oxygen penetration, nutrient concentrations (ammonium, nitrate, nitrite and SRP), and both benthic and hyporheic invertebrate assemblage structure. Different flow rates also affected these attributes. Nevertheless, the hypothesis that higher flows would ameliorate any effects of added fines, although generally supported for oxygen penetration, was not supported for benthic or hyporheic invertebrates (no interaction between flow type and sediment treatment). However, the rates of flow used in the mesocosms did not appear to reach the threshold required to remove fines or alter sediment dynamics.

Periphyton– Experimental results (chapter 1.3) support the assumption that fine sediment has the potential to confound investigations of other stressors on periphyton in river environments, including phosphorous concentration (Jones et al. 2014). Under reduced phosphorous conditions the chlorophyll-a and ash-free dry weight concentrations were lower with sediment treatment. Whereas, under control conditions, the sediment treatment showed increased chlorophyll-aand ash-free dry weight concentrations. This is an interesting result, and clearly needs further study, as it has potential implications for management and other studies involving phosphorous reduction in river systems.

Performance of existing indicators

Macroinvertebrates– River zonation preferences (upstream vs downstream species) have the potential to indicate hydrological and/or morphological stress (chapter 2.1). Although overlap between samples of different quality classes is apparent, locations of good hydrological or morphological quality are dominated by upstream or rhithral species, whereas downstream or potamal species are more abundant in locations of poor hydrological or morphological quality.

Macrophytes- The Danish Stream Plant Index (DVPI) declined with increased frequency of weed cutting (chapter 2.2). A significant relationship was found also between DVPI and stream morphology (channel sinuosity and profile) in small streams. Such a relationship was not detected in middle-sized and large streams most likely reflecting that weed cutting often correlates with the degree of channelization, thereby making it difficult to distinguish these two types of impacts in these stream types.

Novel metrics

Macroinvertebrates- AMOEBE provides an appealing visual tool for water managers to diagnose the cause of stream degradation, including morphological and hydrological degradation (chapter 3.1). Because AMOEBE is a diagnostic tool it makes it easier to decide on appropriate restoration measures when a stream fails to meet the ecological quality objectives. Apart from AMOEBE, macroinvertebrate traits, that are indicative of low and high fine sediment conditions, have been identified (chapter 3.4). This indicates the potential to develop a biological trait-based index for fine sediment stress. It further validates the incorporation of the metric percentage of silt dwelling species in the AMOEBE, and also offers potential for improvement of AMOEBE in the future.

Fish- For fish the overall shape of size spectra was identified as a potential novel metric for assessing impacts of hydromorphological pressures and restoration projects.

Diatoms- Results suggest, in line with deliverable 3.1, that the TDI scoring system, developed to assess the degree of eutrophication, is robust to hydromorphological stress (percentage cover of fine sediment). Furthermore, no relationship between motility (proposed metric of sediment stress based on phytobenthos) and sediment stress could be detected. However, the strong influence of fine sediment on diatom community composition indicates the potential for a robust metric relating diatoms to fine sediment.

Designs for monitoring

The results from chapter 1, 2 and 3 suggest it is possible to develop indices for fish, macroinvertebrates and diatoms that can indicate (hydrolo)morphological degradation. Results described in deliverable 2.2 clearly demonstrate the importance of both aquatic and riparian vegetation as a key physical control of river form and dynamics and a crucial component of river restoration. This, in combination with the diagnostic tool AMOEBE descibed in chapter 2, is a first step in the development of stressor specific bioassessment methods.

Uncertainty

Based on current literature it is very difficult to gain insight on the extent to which different sources of variation (i.e., natural spatial, natural temporal, and within-site variation) contribute to overall variation (Vlek, 2014). Also, information on variability varies depending on the metric, ecological status and stream type studied and the monitoring techniques applied. It is, therefore, unlikely that data on variation/uncertainty can be applied universally. So, instead of providing quantified information on variation/uncertainty, chapter 5 provides guidelines to develop a biological assessment system that is both affordable and provides managers with meaningful results. 

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