WaterSubject

Methods to assess the physical habitat provide important tools for many aspects of river management. Hydraulic units (defined as a homogeneous patch of flow type and substrate) were described in mountain streams of Central Argentina and the distribution of macrozoobenthos in these habitat units was analyzed. Four streams from the upper Carcarañá River Basin (Córdoba, Argentina) were sampled in two hydrological periods. Hydraulic units (as substrate and flow type), current velocity, depth, macrophytes and macroalgae were assessed. Three benthic samples were taken in each hydraulic unit. A total of 12 hydraulic units were registered, which varied seasonally in their proportional abundance. The highest values of taxonomic richness, total abundance, diversity and evenness were found in the low-water period. The most heterogeneous hydraulic units (characterized by substrate of diverse grain size) presented the highest richness, diversity and evenness, whereas the highest total abundance was observed in hydraulic units with homogeneous substrate, such as bedrock or gravel sand. Canonical correspondence analysis grouped samples and taxa mainly in relation to the hydraulic units, and temporal variation in macroinvertebrate assemblages was observed. We found that the interaction between hydrological and geomorphological conditions affected benthic assemblages and that their organization is important at a mesoscale. Therefore, hydraulic units may be considered important tools in assessing stream integrity in lotic systems of central Argentina.

Anthropogenic activities have strong effects on aquatic ecosystems leading to widespread modification of the physical habitat and consequently, of biotic communities and ecological functioning. The physical habitat of stream biota is a spatially and temporally dynamic entity, determined by the interaction of the structural features of the channel and the hydrological regime. Methods to assess the physical habitat provide important tools for many aspects of river management, including river health monitoring, determination of river restoration strategies, and biodiversity assessment.

Riverine systems can be studied considering a variety of spatial scales. The scale in which knowledge is currently most lacking is at ‘mesoscale’. This is a level of detail larger than the micro-habitat and smaller than river habitat features such as riffles and pools. The mesoscale range from one to tens of meters and typical time scales of development are seasonal or yearly, from 1 month to a year. The consideration of stream ecology at this scale may be important, because it is expected to provide useful tools in management and river rehabilitation.

Recent parallel developments in the fields of stream ecology and geomorphology have provided an opportunity to link ‘biological’ and ‘physical’ definitions of habitat. In ecology, definitions of the ‘functional habitat’ or the ‘mesohabitat’ concept describe habitat units, made up of substrate or vegetation types, which are identified as distinct by their macroinvertebrate assemblage. On the other hand, physical habitat is also known as ‘hydraulic biotope’ or ‘hydraulic unit’ by geomorphologists. According to these authors, these habitat units are patches of relatively homogeneous flow and substrate character and they are nested within geomorphic units such as riffles and runs. Hydraulic units are discharge (flow stage) dependent features, whereas geomorphic units are relatively more stable in the short term. The usefulness of the procedures proposed by geomorphologists will depend on demonstrating that hydraulic units have ecological relevance so that patches of similar surface flow type, substrate and aquatic vegetation support similar biotic assemblages.

The Carcarañá River Basin is one of the most important fluvial systems of the central region of Argentina. Studies of water quality, drift and functional community structure have already been performed. However, research on macroinvertebrate assemblages associated with particular habitat units considering a mesoscale, is still lacking. Consequently, the characterization of local macroinvertebrate assemblages associated with different habitat units turns out to be essential, in order to use habitat assessment as a predictive tool for the evaluation of human actions on lotic ecosystems.

The main purpose of this study is to provide the answer to the following question: how valuable is the hydraulic unit concept to describe macroinvertebrate community structure in the study streams? Since it is known that substrate and flow influence macroinvertebrate distribution, and hydraulic units are defined by these two variables, we hypothesized that macroinvertebrate assemblages from different hydraulic units are distinct and therefore, these habitat units define the macroinvertebrate community. In this study, we identified and characterized dominant hydraulic units, and we examined the distribution of macroinvertebrate assemblages in these habitat units in order to test their ecological importance in mountain streams of central Argentina.

showed that the main source of variation was the faunal composition in the hydraulic units. This may indicate that the study streams have quite similar hydraulic units and that macroinvertebrate assemblages differ more in relation to habitat variables at a mesoscale than in relation to particular characteristics of each stream at a larger scale. found similar results. Although they observed many ubiquitous taxa in all mesohabitats and reaches, the assemblages were more different among mesohabitats.

Temporal variations in the assemblages were less important, although some differences were observed, as was assessed by DCA in each stream. Similar results have already been reported but in studies of Patagonian streams, an important temporal variation of macroinvertebrate assemblages in each habitat was observed.

In this study, we found that some of the hydraulic units characterized by the same substrate but different flow allocated quite similar macroinvertebrate assemblages, since they were not clearly separated in the CCA. This result may indicate that probably flow type may not be the appropriate variable to define habitat units in all cases. However, this result was not found in all hydraulic units characterized by the same substrate; therefore, assemblages from most of the hydraulic units were distinct.

Each habitat unit of a lotic system is associated with a particular macroinvertebrate assemblage, whose composition is determined mainly by substrate character and flow type. According to the preference for one substrate or another is determined at first by the oviposition behavior of the organisms. But this distribution may be modified during the life of aquatic organisms by multiple factors, which make them move by passive drift or active migration. The relative importance of variables that determine organism distribution in different habitats can change in relation to the stage of the organism’s life history.

Our results suggest that the interaction between hydrological and geomorphological conditions affects benthic assemblages and that their organization is importat at a mesoscale. Mesohabitats provide a means of categorising streams at a scale which has ecological relevance, but which can also be used to improve stream assessment for management purposes. If habitat units possess predictable and distinct biological communities, study and management of streams would be facilitated. Furthermore, greater confidence and efficiencies in sampling programs would result, and the emphasis presently placed on habitat restoration and conservation would have more a biological basis. In this sense, hydraulic units may be considered to be important tools in assessing stream integrity in lotic systems of central Argentina.