Source: Bruno et al. Role of the Hyporheic Zone in Increasing the Resilience of Mountain Streams Facing Intermittency. Water 2020, 12(7), 2034 (2020).
Full-time, part-time, surface, subsurface
Not all streams flow continuously: some don’t flow for much of the year, some don’t have surface water in parts of their channels, some change these characteristics throughout time, and some are patchy in all these regards! Streams that don’t always flow are called intermittent, often only flowing during rainy seasons or right after heavy rainfall. Even when they appear dry, however, there might be a surprising amount of water close beneath the surface in what’s called the hyporheic zone, the area between surface waters and groundwater.
The hyporheic zone acts as a distinct region, with its own sort of physical, chemical, and biological signatures separate from the stream it’s a part of! It varies in size across space and time, like streams, and is influenced by local weather patterns: if there’s not a lot of rainfall or snowmelt adding water to the landscape, the hyporheic zone might shrink in size and swell in wet times. Weather patterns are just a part of broader climate trends, so under climate change many regions will be facing increasingly dry conditions, leading to responses in their streams’ hyporheic zones.
Macroinvertebrates (“bugs,” like insects, worms, crustaceans, and mollusks) often live in the hyporheic zone when they’re really young and vulnerable or while above-ground conditions are unideal, like during a flood/drought or when there are a bunch of predators around. Bugs are integral parts of a healthy stream system and can be picky about their living conditions, so better understanding how they interact with the hyporheic zone might help better understand conditions favorable to a healthy bug community and stream system in a changing world.
Drs. Bruno, Doretto, Boano, Ridolfi, and Fenoglio sought to discern if bugs started using the hyporheic zone differently when their once year-round flowing stream started becoming intermittent. If the bugs could take advantage of a different part of the stream more, it might help the community stay resilient against their changing conditions.
Collecting bugs in the original alpine environment
The team of scientists conducted their study near the Po River in the Italian Alps, where temperatures are rising, precipitation is becoming more unpredictable, and overall hydrologic conditions are being disrupted. Within a single stream, the scientists collected data from two areas, one where water always flowed, and another downstream where water would sometimes stop flowing on the surface. Thin tubes called piezometers were installed, allowing them to measure subsurface water levels and temperatures, among other variables. Hoses were stuck down the piezometers, allowing the team to also pump out bugs living in the hyporheic waters.
Once the bugs were identified, the scientists related the communities they collected to the hydrologic conditions in which they were living: they were compared at the same depths but at different sites with different hydrologic conditions, at different depths within the same sites, and over time.
Is there enough space for everybody down there?
Having organized their data and run their analyses, the scientists observed that intermittent flow is a strong shaping force of the bug communities and that bug communities bounce back from drought instead of withstanding it.
The scientists found the greatest number of bugs and the most variability in those numbers across time in the most shallow parts of the hyporheic zone, reflective of the tolerable conditions and visitation frequency by bugs, likely due to small but frequent changes in surface water levels. Despite the difficulties faced living deeper in the hyporheic zone, that the scientists consistently found more and more types of bugs further down when surface conditions were drying was a strong indicator that the bugs would try to stay where there was water and adapt to new conditions in a pinch.
For the bugs that could remain in those deeper waters more permanently, they less affected by drought, but there were generally fewer of them. When the upper levels of the hyporheic zone were saturated, these deep-dwellers were found exploiting the area, highlighting the importance of the boundary between surface and subsurface waters as a mixing zone for not only water and organisms but for nutrients and food resources, too.
Finally, the scientists found evidence of the hyporheic zone being important to repopulating the stream once surface flows returned: those bugs have to come from somewhere, and they were found to largely be coming from the ones that were waiting things out in the hyporheic zone instead of drifting from upstream sources.
Staying resilient against poor conditions
Putting this all together, Drs. Bruno, Doretto, Boana, Ridolfi, and Fenoglio were able to provide evidence of bugs in a stream starting to become intermittent taking advantage of a part of the stream they wouldn’t have needed to rely on as heavily before. The hyporheic zone acted as a zone for bugs from the surface or deeper down to mix, take advantage of new resources, and wait droughts out until surface water flowed again and they could repopulate the stream they came from.
Streams will face increased drought under climate change, putting pressure on the bugs and other organisms residing within, but with the right kind of pressure, their communities might be able to hang on by adapting to new environments.