Biological quality elements as indicators of hydromorphological degradation
REFORM analyzed a large-scale data set, but no effects of hydromorphological degradation on indices based on algae (phytobenthos) were detected. However, indices developed to assess eutrophication stress appear robust to hydromorphological alteration. Examples are the Trophic Diatom Index (TDI) and the specific pollution sensitivity index (SPI). Analysis of a spatial dataset showed a particularly strong relationship between fine sediment and the diatom community composition. This might suggest that diatoms could be used as an indicator of fine-sediment stress. In contrast to this finding, experiments showed that phytobenthos community composition was primarily impacted by soluble reactive phosphorous concentration. Fine-sediment treatment did not significantly impact on either chlorophyll-a concentrations or ash-free dry weight (AFDM), suggesting that algal growth was unaffected.
Trait characteristics of macrophytes changed significantly in response to hydromorphological degradation in small streams. Species growing from single basal meristems declined. Species with a high overwintering capacity increased. However, with the exception of the trait heterophylly (the ability to have different types of leaves above and below the water), impacts of hydromorphological stress could not be separated from eutrophication. More traits were specific to eutrophication, which could aid in the diagnosis of main stressors in multiple-stress systems. Despite their importance in some river types, macrophytes have limited suitability as a general indicator for the range of European river types. Development of a new trait-based metric might make them suitable for lowland rivers.
Relationships between identity-based (taxonomic) ecological indicators for macroinvertebrates and measures of hydromorphological quality were generally weak. REFORM found no strong relationships between hydromorphological stress and more than 200 different existing macroinvertebrate metrics and indices. Metrics to detect hydrological impairment and hydromorphological degradation, such as MESH and LIFE, appear to be no more discriminative than metrics to detect pollution, such as ASPT and SPEAR, and metrics to detect general degradation, such as EPT. Some of the strongest relationships were those between macroinvertebrate metrics that measure the presence of taxa with a preference for specific flow or substrate conditions and indicators of these specific conditions. This reflects the potential of using trait-based metrics <hyperlink to “Trait-based metrics”> to evaluate hydromorphological conditions. However, a subsequent analysis revealed that few traits in macroinvertebrates could potentially distinguish between hydromorphological and other stressors. Key to the strength of these relationships were sampling methods and to some degree organism size. Relationships were further improved when using species traits that are more closely linked to the habitat template, including hydromorphology.
|Critical low flow||-0.58||-0.47||-0.52||-0.43||-0.55|
The MESH and LIFE metrics were specifically developed to be sensitive to hydrological stress (low flow). An analysis of stations with paired measurements of hydrology and the biological quality element macroinvertebrates showed, however, that these metrics were not particularly sensitive when compared to other metrics such as ASPT, EPT and SPEAR. The closer a value is to – 1, the worse are the conditions. Values close to +1 indicate little or no stress. It is evident from the table that the metrics are almost equally sensitive to hydrological stress and other stresses. This questions our ability to make diagnostic metrics with the necessary sensitivity towards hydromorphological stresses and other individual types of stress.
Analyses of a large-scale data set showed that 49% of the studied European freshwater fish species show a significant response to hydromorphological stress. Using conceptual models that link pressures via processes to responses, benthic fish like Cobitidae, Nemacheilidae, Cottidae or Gobiidae showed the most consistent response to hydromorphological stress. This response can be related to their dependence on substrate dynamics and low sediment mobility. Conceptual models should be viewed as a first step. More traditional metric approaches using fish have significant potential too. Nonetheless, both conceptual models and traditional metrics require significant development before they can be applied in regular monitoring. The inherent difficulty in fish methods based on number of taxa is that their use is restricted to river types with a minimum of species diversity, leaving out most small streams. In small streams, more detailed investigations on length frequency distributions can be related to hydromorphological stress. Many hydromorphological pressures (but also non-hydromorphological pressures) generate a clear response in the fish community size structures. In particular, they induce changes in the overall shape of the size spectra, which might therefore be useful as a new potential metric for assessing impacts of hydromorphological stress.
Which biological quality elements can be used?
Algae appear to be the least suitable of the biological quality elements. As a biological quality element, they primarily relate to very small scales and substrate-specific sampling. Moreover, most methods in Europe use only the algae group of diatoms, and not algae with larger growth forms such as for instance filamentous green algae. Macroinvertebrates are slightly better as indicators of hydromorphological degradation, but they appear sensitive to multiple stressors. It is thus almost impossible to single out the effects of hydromorphological conditions on community composition. As was the case for algae, macroinvertebrates are usually sampled at a relatively small scale and often on specific substrates, making any linkages to hydromorphological degradation on larger scales spurious. Macroinvertebrates represent the only biological quality elementfor which specific metrics have been developed that are sensitive to hydromorphological and hydrological degradation. However, these metrics have not been intercalibrated and tend to respond in manner similar to the response of metrics sensitive to other stressors. Macrophytes show more potential. For certain river types it must be possible to develop robust metrics that will be sensitive to hydromorphological degradation. Additionally, the key role of aquatic and riparian vegetation in shaping hydromorphological processes offers an additional argument for increased focus on this biological quality element in indicator development. Fish appears to be the most promising biological quality element. It can be used to detect hydromorphological stress, although a need remains to develop more stressor-specific metrics.