Stream sediments move at low flow forming migrating ripples. These ripples can cover substantial areas where benthic communities experience erosion-resting cycles of sand grains. Sediment surface and interstitial space is colonized by meiobenthos, an assemblage of microscopic invertebrates. Here we describe how sediment migration influences the structure of the meiobenthic community. We sampled migrating and stationary sediment patches in five streams. Sediments in migrating ripple patches were characterized by coarser grain size and higher oxygen concentration, but less organic matter and chlorophyll than stationary patches. Meiobenthos was more abundant in the superficial layer of stationary sediment compared to the underlying layer, whereas comparable abundances were observed in both layers of migrating patches. This suggests that ripple migration enhances the vertical mixing of interstitial communities. Among the environmental drivers measured, meiobenthos community structure was most related to sediment transport regime: Rotatoria were more abundant in migrating patches, whereas Chironomidae, Ceratopogonidae, Copepoda and Hydrachnidia were more abundant in stationary patches. Body-size structure was affected by sediment migration, with fewer larger organisms in migrating ripples. By modifying the distribution of benthic resources and of meiobenthic consumers, ripple migration likely affects energy flow paths through benthic food webs.
The bed of fluvial ecosystems plays a major role in global biogeochemical cycles. All fluvial sediments migrate and although responses of aquatic organisms to such movements have been recorded there is no theoretical framework on how the frequency of sediment movement affects streambed ecology and biogeochemistry. We here developed a theoretical framework describing how the moving-resting frequencies of fine-grained sediments constrain streambed communities across spatial scales. Specifically, we suggest that the most drastic impact on benthic and hyporheic communities will exist when ecological and biogeochemical processes are at the same temporal scale as the sediment moving-resting frequency. Moreover, we propose that the simultaneous occurrence of streambed patches differing in morphodynamics should be considered as an important driver of metacommunity dynamics. We surmise that the frequency of patch transition will add new dimensions to the understanding of biogeochemical cycling and metacommunities from micro-habitat to segment scales. This theoretical framework is important for fluvial ecosystems with frequent sediment movement, yet it could be applied to any other dynamic habitat.
Climate change and erosion from agricultural areas cause increased drying periods and bedform migration of riverbeds, respectively, worldwide. Both sediment drying and bedform migration can independently stress the microbial community residing in the riverbed. Here, we investigated the microbial response after exposure to these stressors with a focus on long-term recovery. We conducted an in situ experiment to investigate the long-term (8 months) functional and structural recovery of benthic microbial communities from either sediment drying (episodic severe stressor) or bedform migration (frequent moderate stressor). Stressed sediment associated communities were rewetted (dried sediments) and immobilized (migrated sediments) and exposed in the River Spree (north-eastern Germany) to initiate the recovery process. We then evaluated the microbial function (community respiration, net community production and extracellular enzymatic activities) as well as the bacterial, fungal and diatom community structure (16S rRNA gene and ITS region metabarcoding, and microscopic diatom morphotype classification). We observed different recovery times for community respiration (less than 7 days) and gross primary production (more than 5 months), implying a shift toward net heterotrophy in the first few months after stress exposure. Similarly, we observed a strong autotrophic community response (particularly associated with the diatoms Navicula and Fragilaria), especially in migrated sediments. The bacterial and fungal community response to sediment drying was stronger than to bedform migration (particularly associated with the bacterium Flavobacterium and the fungi Alternaria sp. and Aureobasidium pullulans). Our results show that sediment drying and bedform migration had a significant impact on the microbial community function and structure, which persisted for several months after the stress. Due to the surprising long period of recovery, successive stress events combined with seasonal effects will likely hamper the ongoing recovery process with severe alterations to the microbial function and structure. These findings extend the concept of ecosystem resilience and stability on the dimensions of timescale and seasonal environmental variations. Legacy effects are expected to play a key role when facing future stress.
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