Combined effects of local habitat, anthropogenic stress, and dispersal on stream ecosystems: a mesocosm experiment

Abstract

The effects of anthropogenic stressors on community structure and ecosystem functioning can be strongly influenced by local habitat structure and dispersal from source communities. Catchment land uses increase the input of fine sediments into stream channels, clogging the interstitial spaces of benthic habitats. Aquatic macrophytes enhance habitat heterogeneity and mediate important ecosystem functions, being thus a key component of habitat structure in many streams. Therefore, the recovery of macrophytes following in-stream habitat modification may be prerequisite for successful stream restoration. Restoration success is also affected by dispersal of organisms from the source community, with potentially the strongest responses in relatively isolated headwater sites that receive a limited amount of dispersing individuals. We used a factorial design in a set of stream mesocosms to study the independent and combined effects of an anthropogenic stressor (sand sedimentation), local habitat (macrophytes, i.e., moss transplants), and enhanced dispersal (two levels: high vs. low) on organic matter retention, algal accrual rate, leaf decomposition, and macroinvertebrate community structure. Overall, all responses were simple additive effects with no interactions between treatments. Sand reduced algal accumulation, total invertebrate density, and density of a few individual taxa. Mosses reduced algal accrual rate and algae-grazing invertebrates, but enhanced organic matter retention and the number of detritus and filter feeders. Mosses also reduced macroinvertebrate diversity by increasing the dominance by a few taxa. Mosses reduced leaf mass loss, possibly because the organic matter retained by mosses provided an additional food source for leaf-shredding invertebrates and thus reduced shredder aggregation into leaf packs. The effect of mosses on macroinvertebrate communities and ecosystem functioning was distinct irrespective of the level of dispersal, suggesting strong environmental control of community structure. The strong environmental control of macroinvertebrate community composition even under enhanced dispersal suggests that re-establishing key habitat features, such as natural stream vegetation, could aid ecosystem recovery in boreal streams.

Aquatic bryophytes play a key role in sediment-stressed boreal headwater streams

“…First, we expected that bryophyte cover and richness are negatively impacted by fine sediments. We further expected that bryophyte cover and fine sediments both contribute importantly to stream community composition, with bryophytes having a generally stronger effect of the two (see Turunen et al, 2018), especially for diatoms and macroinvertebrates. Fungal community composition was expected to be more driven by water chemistry than by bryophyte or fine sediment cover, since water chemistry typically controls fungal assemblages more strongly than substrate (Niyogi et al, 2003;Tolkkinen et al, 2015).…”

“…In many stream ecosystems, aquatic macrophytes modify erosion and sedimentation regimes and create habitat for other organisms (Sand-Jensen, 1998;Sand-Jensen & Pedersen, 1999). Macrophytes generally increase habitat heterogeneity and thus support diverse and abundant invertebrate communities (e.g., Taniguchi et al, 2003;Suurkuukka et al, 2014;Turunen et al, 2018). Macrophytes also provide refuge from predation and harsh environmental conditions such as floods or droughts (Rantala et al, 2004;Huttunen et al, 2017).…”

“…In boreal streams, bryophytes and particularly mosses often dominate macrophyte communities. Bryophytes retain fine and coarse organic matter (Muotka & Laasonen, 2002;Turunen et al, 2018) and thus indirectly fuel detritus-based stream food webs. While it is known that bryophytes provide important habitat for stream invertebrates (Suren, 1993) and buffer invertebrate communities against variability (Huttunen et al, 2017) their importance to other ecosystem components, such as microbial communities and stream ecosystem functions remains poorly understood.…”

“…The excess of fine sediments clog interstitial spaces and impair hyporheic flow exchange within the stream bed, with adverse ecological consequences on, for example, crevice-dwelling invertebrates and gravel-spawning fishes, such as salmonids (Jones et al, 2012;Sear et al, 2016). The instability and scouring of sediments may hinder the establishment of periphyton and interfere with ecosystem functions (McKie & Malmqvist, 2009;Louhi et al, 2017;Turunen et al, 2018). A particularly harmful consequence of excessive sedimentation is that it reduces bryophyte abundance in streams (Matthaei et al, 2006;Turunen et al, 2017).…”

“…A particularly harmful consequence of excessive sedimentation is that it reduces bryophyte abundance in streams (Matthaei et al, 2006;Turunen et al, 2017). Indeed, experimental evidence suggests that bryophyte loss may have a stronger effect than sedimentation on stream invertebrates and ecosystem functions (Turunen et al, 2018), but the relative importance of these two stressors in field conditions remains unexplored.…”