Aquatic–terrestrial interactions: Mosaics of intermittency, interconnectivity and temporality
Recent years have seen increased interest in ephemeral streams and the patterns of intermittency of flow on in-stream communities and ecosystem processes (Datry, Bonada, & Boulton, 2017). Given the complexity of floodplain habitats, natural stream and river systems are mosaics of intermittency with some habitats inundated by stagnant waters, some habitats influenced by flowing waters, some habitats visually dry on the surface but interconnected through the hyporheic zone, and some habitats clearly historic river channels with a legacy of interconnectivity. The complexity of studying riverine systems has increased as our understanding of the linkages between terrestrial, aquatic, semi-aquatic and temporally aquatic habitats has increased (Datry, Larned, & Tockner, 2014). One paper in this current issue (Abelho & Descals, 2019) tackles an aspect of these aquatic–terrestrial linkages, through leaf litter decomposition. The authors created a study to explore the influence of time spent on the riparian forest floor on in-stream decomposition rates, arguing that 23% of litter inputs to streams enter laterally instead of directly. The longer leaves spent on the forest floor, the slower their in-stream decomposition rates. This study helps us to understand the influence of temporality in both aquatic and terrestrial habitats on leaf litter decomposition, invertebrate colonization, and colonization and sporulation of aquatic fungi. Importantly, this study also highlights a historic bias in ecological research in which we have mainly separated aquatic and terrestrial ecosystems (Abelho, 2016) and focused on direct inputs of leaves to streams, largely ignoring the lateral transfer of leaf litter from riparian forests to streams despite several early studies (Benfield, 1997; Chauvet & Décamps, 1989; France, 1995).
Flow intermittency is an important influence on stream communities and ecosystems, and one that may increase given predicted climate changes, warmer global temperatures and decreased snowpack (Kominoski et al., 2013; Mohseni, Stefan, & Eaton, 2003). It is possible that connected hyporheic zones will become important climate refuges for stream organisms (Stubbington, 2012), and potentially sites of increased microbial and detritivore activity during periods with low or no surface flow. In contrast, we may also see situations where CPOM availability limits microbial and detritivore activity due to flushing flows associated with floods caused by more intense storm events predicted by climate change models. What we think of as the typical input of leaf litter to flowing headwater streams in the autumn may only be a small fraction of the actual litter decomposition and organic matter processing that occurs across our connected aquatic–terrestrial mosaics.
Despite decades of leaf litter decomposition studies exploring countless aspects of this important ecosystem process (Boyero et al., 2011; Webster & Benfield, 1986), we are only beginning to understand the complexity of organic matter processing at a landscape scale and under potential climate changes. There are many studies that have explored how drying and wetting of leaf litter influences decomposition rates (Abelho & Descals, 2019; Battle & Golladay, 2001; Bruder, Chauvet, & Gessner, 2011; Corti, Datry, Drummond, & Larned, 2011; Datry, Corti, Claret, & Philippe, 2011; Foulquier, Artigas, Pesce, & Datry, 2015; Glazebrook & Robertson, 1999; Langhans & Tockner, 2006; Leberfinger, Bohman, & Herrmann, 2010; Treplin & Zimmer, 2012), but only some of these studies actually quantified rates of both terrestrial and aquatic decomposition as in Abelho and Descals (2019). Even fewer have explored organic matter dynamics in ephemeral systems (Fritz, Pond, Johnson, & Barton, 2019; von Schiller, Bernal, Dahm, & Martí, 2017). While spending time in riparian systems, the abundance of leaf litter on the riparian forest floor is clear. Much of it decomposes in situ, but some is eventually blown or washed into the stream, especially during storm events. Since we are predicting a variety of changes to streams under climate change scenarios (increased periods of drying, longer periods of low flows, more intense floods), we should be predicting both more time for leaf litter to decompose on land prior to entering stream systems and more litter from land to be transported into streams and rivers. In light of these predicted changes, studies like Abelho and Descals (2019) become more important to our understanding of linked aquatic–terrestrial systems as a whole.
In addition to changes to the physical environment in which leaf litter decomposes, climate changes may also change litter quality and riparian plant species distributions. Several papers have shown that drought stress alters leaf litter quality, but differentially across species (García-Palacios, Prieto, Ourcival, & Hättenschwiler, 2016; LeRoy, Wymore, Davis, & Marks, 2014; Ogaya & Peñuelas, 2006; Sardans & Peñuelas, 2004), leading to subsequently altered decomposition rates (LeRoy et al., 2014). Drought can also influence the timing of litter inputs (Ogaya & Peñuelas, 2006) and can cause branch mortality, which may lead to altered translocation ability and further litter quality differences. Additionally, changes to climate may influence plant species distributions and ranges, alter genetic variation within riparian plants, cause local extinctions and migration (Capon et al., 2013; Kominoski et al., 2013), or alter dioecious plant sex ratios across the landscape (Hultine et al., 2016). In combination, there are many new arenas of research that will help us better understand how key ecosystem processes will be influenced by changes to intermittency, interconnectivity and temporality in linked aquatic–terrestrial ecosystems.
As scientists, we need to have clearer communication with the public about the value of riverine mosaics (Steward, Schiller, Tockner, Marshall, & Bunn, 2012), especially the aspects of these mosaics that do not look like rivers throughout the year. There are interconnected parts of rivers that are hiding in the hyporheic zone, connected side channels, oxbow lakes and the ghosts of former stream channels (Chauvet & Décamps, 1989) that can now be made visible through LiDAR and remote sensing methods (Slaughter & Hubert, 2014), and may help the public realize the dynamism of rivers.
ACKNOWLEDGEMENTS
I thank Charles Fox for the invitation to write this commentary. The Evergreen State College and National Science Foundation grant DEB #1836387 provided support during sabbatical. There are no conflicts of interest to declare.