Testing the River Continuum Concept with geostatistical stream-network models

Abstract

The River Continuum Concept (RCC) provided one of the first unifying frameworks in fluvial ecosystem theory. While the RCC predictions held in many empirical tests, other research highlighted how the model overlooked sources of heterogeneity at different scales e.g. the effects of tributaries. Disentangling these effects requires an assessment of variation in key ecosystem variables over the longitudinal and lateral dimension of river networks. However, so far, no empirical tests have employed a spatially explicit statistical approach to this assessment.Here, we show how recently-developed spatially-explicit models for river networks can be used to test predictions of the RCC whilst taking into account cross-scale sources of heterogeneity. We used macroinvertebrate data from 195 monitoring sites from 1 st to 4 th order streams spread across the Adige River network (NE Italy). We compared theoretical expectations with empirical semivariograms that incorporated network topology to assess the continuity and patchiness in the proportion of invertebrates functional feeding groups (FFG) over Euclidean and in-stream distances. Geostatistical stream-network models were then used to quantify the influence of the longitudinal gradient relative to local-scale water quality and land-use drivers, while accounting for network spatial autocorrelation. Patterns in the semivariograms based on flow-connected relationships were characterised by a nested structure associated with heterogeneity at multiple scales. Therefore, the longitudinal variation in FFG was better described by a patchy discontinuum rather than a gradient, implying that both in-stream processes and landscape factors influenced stream ecosystem function. The overall shift in FFG along the longitudinal profile was generally consistent with the RCC predictions, although the best models often included water quality and local land-use predictors. Stream-network models further indicated that up to 90% of residual variation (mean=50%) was accounted for by spatial autocorrelation, especially among flow-connected communities. Accounting for such autocorrelation not only improved model performance relative to non-spatial approaches, but indicated that most flow-connected communities were spatially correlated to some extent. This has clear implications for the assessment of the RCC tenets. This is the first test of the river continuum model that explicitly accounted for stream network topology and autocorrelation. Results indicated that in the Adige River, macroinvertebrates feeding groups exhibited heterogeneity along the longitudinal gradient, which appeared punctuated by local habitat transitions. Such transitions could be associated with artificial impoundments that alter the natural continuity of river processes, and we advocate the use of spatially explicit network models to test the RCC in more natural contexts.

Structural and Functional Variations of the Macrobenthic Community of the Adige Basin along the River Continuum

“…For example, the reliability of Dist to describe taxonomic composition of macrobenthic communities was also demonstrated by Pollice et al [18] in the river systems of Northern Italy (including river Adige basin). The decrease of predator taxa along the longitudinal gradient of the river Adige network was detected also by Larsen et al [11], who also found significant patterns for other functional groups that were not confirmed in this study.…”

“…Particularly, functional and structural indices were significantly correlated, and taxa richness was found to be the best predictor for pesticide concentrations. Larsen et al [11] studied the functional feeding habits of macrobenthic assemblages using a geostatistical method and observed that, even following a heterogeneous pattern along the longitudinal gradient, the distribution of feeding functional groups was generally consistent with the RCC. Similar outcomes were observed by Pollice et al [18] for structural composition of macrobenthic community of Adige and other alpine streams and rivers using asymmetric eigenvector maps that are used to detect the influence of directional spatial processes on the taxa distribution.…”

“…The authors state that headwater communities are mainly influenced by species sorting, while downstream assemblages are driven by dispersal-related dynamics. Beside the debate on the validity of the RCC, the role of the river continuum on shaping biotic communities is still steadily considered in river ecology [10][11][12]. Most of the other abovementioned river theories are reconcilable with the theory of RCC, introducing exceptions and adjustments to general framework of river continuum and extending the concept of river continuum beyond the original model of Vannote et al [2].…”

The River Continuum Concept: lessons from the past and perspectives for the future

“…To consider the position of the sampling sites in the river network and their spatial connectivity, beyond the mainstem view that the RCC employs, the authors used spatially explicit models using three different measures of distance: overland, flow-connected, and flow-unconnected. They found that downstream changes in FFGs were generally consistent with the predictions of the RCC, but this longitudinal variation was mostly patchy-discontinued rather than a gradient (Larsen et al 2019). Moreover, the spatial autocorrelation of spatially explicit modelling generally improved the performance of the statistical analysis.…”

Inadequate riparian zone use directly decreases water quality of a low-order urban stream in southern Brazil

“…Point 3: (25°27'43.00" S / 50°39'20.86" W) stretch with 4 m wide, on average, characterized by sandy bottom and predominantly with slab and partial presence of riparian forest. Based on the theory that rivers follow the principle of a continuum system (Larsen et al, 2019), collection points selection followed the choice criterion according to landscape composition change.…”