Ecology: From Individuals to Ecosystems, 4th Edition

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The effect of the WWTP release was evident on the river stage of downstream sites 1—5 , which reached minima during the early morning and then rose sharply until noon remaining high until the evening Fig. Despite the daily fluctuation in the surface water, it was possible to characterise sites 1, 3 and 5 as UW zones and sites 2, 4 and 6 as DW zones Fig. Therefore, we could accept that daily fluctuations in the surface water level did not affect substantially the general physicochemical and vertical hydrodynamic conditions at each study site during the study period.

The rate at which the daily amplitude of temperatures decreased was greater in sites 1, 3 and 5 than in sites 2, 4 and 6 Fig. Therefore thermal extinction depth, and consequently surface water influence, did not penetrate so deep at sites 1, 3 and 5. These results agree with the obtained streambed vertical-flux values and redox conditions.

Streambed vertical-flux values after VFLUX2 routines were similar using both the Hatch and the Kerry Amplitude method, thus, only the results from Hatch amplitude method are reported here. At sites 1, 3 and 5 the upward flux of water between temperature sensors negative mean values, Fig. In contrast, at sites 2, 4 and 6 downward flux of water into deeper layers positive mean values, Fig. Thermal extinction depth TED is marked as a red dotted-line between the two temperature sensors in which the amplitude of daily temperature variation becomes undetectable.

Positive VF mean values indicate downward flux, negative values indicate an upward flux. After WAIC routines, biomass and production models included vertical hydrodynamic conditions as a single factor, depth as a continuous covariate and the interaction between depth and taxonomic group.

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In the case of the diversity model, depth and vertical hydrodynamic conditions were kept as effective parameters, while the interaction was not included. Each matrix contains the numeric Bray-Curtis similarity value that resulted from comparing assemblage structure composition and abundance between sections.

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Similarity values range from maximum similarity 1. The red dotted line marks the depth at which differences with the upper layers were significant for the ANOSIM analysis. This is represented schematically in the panels on the far right.

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Furthermore, the reduction in biomass and secondary production with increasing depth varied significantly depending on the taxonomic group Fig. Eumetazoa invertebrates showed the most abrupt reduction in biomass and secondary production with depth, followed by ciliates, and finally, flagellates. The vertical hydrodynamic conditions were also an important predictor variable in our models, at least for biomass and secondary production.

In contrast, diversity values did not show any clear relationship with vertical hydrodynamic conditions hydrodynamics as the non-significant coefficient in Poisson-GLMM. We were able to detect the depth at which the benthic assemblage is replaced by the hyporheic assemblage in our system. Therefore, it was also possible to delineate the boundary between the BZ and the HZ purely based on biocenosis characteristics, and determine that the position of this boundary in the sediment differed depending on the direction of water flow.

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Our results showed that, at least for the limit between benthos and hyporheos our main response , vertical differences in assemblage structure were significantly higher than horizontal differences comparing between sites. We also found that the degree of change between the surface and deeper layers differed considerably depending on the vertical hydrodynamic conditions the similarity analysis 1 — Bray-Curtis index; Fig. In contrast, under DW conditions, community structure was more consistent and organisms colonized deeper layers of the streambed Fig.

Regardless of the depth at which the boundary between both communities appeared, similarity of benthos and hyporheos was consistent between sites Supplementary Material: Fig. The decline of biomass, secondary production and diversity of streambed populations with increasing depth has been widely reported in the literature and our results support these findings 12 , 13 , 16 , 23 , 30 , 40 , 65 , However, no previous publications have quantified the relationship of these responses with depth in the streambed across a range of taxa with varying body sizes eumetazoan invertebrates, ciliates and flagellates.

Our study clearly showed that the decline of biomass and secondary production was dependent on the taxonomic group; taxa with smaller body sizes, lower metabolic rate and higher tolerance to low oxygen levels penetrated deeper into the sediment. Schmid-Araya 23 also found that the abundance of flagellates increased in comparison to larger ciliates at deeper layers in the streambed. As previously suggested 17 , including the taxonomic group interaction term notably improved previous predictive models on depth—related biomass and secondary production of streambed assemblages and highlighted the necessity of including small fauna, such as ciliates and flagellates, when determining the total production of streambed systems It is intuitive that the larger body-size of eumetazoan invertebrates constrains their ability to colonise agglomerated sediments 18 , 19 , 21 and that the smaller size of ciliates and especially flagellates enables them to colonise the more compacted pore-space in deeper layers and dominate the assemblage in terms of biomass and secondary production.

However, in our system there is also a strong redox gradient with depth which may additionally constrain eumetazoan invertebrates because they are highly dependent on oxygen availability in the streambed In contrast Protozoa include taxa known to support, and even prefer, anaerobic conditions 16 , 43 ; unfortunately the level of protozoan taxonomic resolution in our study was insufficient to determine whether such groups were present in the community.

The environmental filtering concept has been traditionally applied to determine community identity at large spatial scales such as altitudinal and latitudinal gradients Our findings suggest that micro-scale filters such as body size and metabolic requirements play a role in determining the community composition of streambed sediments and that environmental filtering is an important driver of gradients in the functional characteristics of organisms i.

However, other factors that we did not measure e. Community biomass, productivity and diversity will be significantly higher in DW flow conditions in comparison to UW flow conditions. Biomass and production also differed significantly depending on the vertical flow direction, mostly supporting our second hypothesis.

In DW zones surface water ingress resulted in markedly lower redox potential at deeper depths than in UW zones. This promoted the establishment of assemblages that had greater biomass and production than those in UW zones. This idea has been hypothesised previously 71 , however, our interdisciplinary approach to determining vertical flow conditions and the resident community in the streambed sediments enabled us, for the first time, to confirm the link between streambed flow direction and productivity of streambed assemblages.

We demonstrated the role of DW sites as significant hot spots of productivity, and therefore carbon processing in freshwater systems. We did not detect any effect of vertical hydrodynamic conditions on Shannon-Wiener diversity measurements, implying that there was a proportional simplification of the streambed assemblage through depth, independent of the vertical flux. Thus, similarly to Storey and Williams 64 , depth was the strongest predictor of the assemblage diversity models. Vertical hydrodynamic conditions will determine the depth at which this boundary occurs.

Using a fine scale approach based on biocenosis features we detected the depth at which the hyporheos replaced the benthos, and showed that these two communities, composing the whole streambed assemblage, are measurable ecological entities with individual integrity. Thus our third hypothesis was supported.

Our findings provide definitive quantitative evidence for previous suggestions that the benthos is replaced by the hyporheos with increasing depth into the sediment 65 , Our study confirms that the HZ is a spatially ecotone between the surface stream and the deep groundwater 11 and that the hyporheos is a discrete and measurable biological community.

It also draws attention to the importance of including precise measurements of vertical flux direction when defining streambed system boundaries. Accurately defining natural system boundaries is an important aspect of designing ecological studies and for interpretation of results 40 , but it is also central to defining ecosystems Our study also supports the often mentioned necessity of multidisciplinary approaches in modern freshwater ecology.


This strategy enabled us to assess the interplay of two central environmental filters depth gradient and vertical hydrodynamic conditions in driving the productivity, diversity and organization of streambed assemblages. Finally, I. Change history. The error has been fixed in the paper. Supplementary information accompanies this paper at National Center for Biotechnology Information , U. Sci Rep. Published online Oct Robertson 1. Anne L.

Author information Article notes Copyright and License information Disclaimer. Ignacio Peralta-Maraver, Email: ku. Corresponding author. Received Oct 18; Accepted Oct This article has been corrected. See Sci Rep.

This article has been cited by other articles in PMC. Associated Data Supplementary Materials Supplementary material.

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Abstract A current controversy in ecology is whether biological communities are discrete biological entities or simply study units created for convenience; a debate that becomes even more heated when delimiting communities along ecotones. Subject terms: Freshwater ecology, Community ecology.

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Introduction Delineating communities has a long history reaching back to the writings of Theophrastus in the 4 th century BC 1. Open in a separate window. Figure 1. Figure 2. Vertical hydrodynamics Vertical hydrodynamic conditions at different sites were characterised by coupling the location of the thermal extinction depth, measurement of the vertical streambed fluxes, and indirect analysis of redox conditions.

Streambed community Sampling The community of invertebrates and protists inhabiting the sediments was sampled using a modified Kajak corer. Biomass, secondary production and diversity Body dimensions of all counted organisms were converted to biovolume as described by Reiss and Schmid-Araya Statistical analysis Biomass, secondary production and diversity Two linear mixed effect models LMMs were applied to test the effect of vertical hydrodynamic conditions factor with two levels: UW and DW , depth continuous covariant and taxonomic group factor with three levels: eumetazoan invertebrates, ciliates and flagellates on the responses biomass and production respectively.

Results Study-site characterisation All study sites showed a relatively similar GSD through the depth profile, aside from marginally higher proportions of interstitial clay at sites 5 and 3, and higher proportions of gravels at sites 4 and 5 Fig. Figure 3. Figure 4.