By Aslan Wright-Stow
Streams and rivers are typically thought of in two dimension space, flowing from upstream to down, from high in the catchment and then out to sea. Delve a little deeper however and we find a third dimension – a vertical space underneath the streambed that’s home to many freshwater invertebrates.
Linked hydrologically to the overlying water column, the hyporheic zone sits underneath what is typically considered the bottom of a stream or river and links groundwater to surface water. Given the inability for simple direct observations and difficulties in sampling the hyporheic zone, research into this ecosystem didn’t really start until the mid-1990s. Common study techniques include sampling biota by pumping hyporheic water and animals from wells sunk to different depths, freeze-coring vertical profiles by pumping liquid nitrogen or dry ice down a steel well and extracting the resulting frozen core with lifting equipment, and retrieving vertically-stratified colonisation pots that are dug into the streambed and left to equilibrate. So hyporheic researchers tend to be burly types!
As a link between surface water and groundwater, the hyporheic zone is home to an unusual and diverse fauna. Stygophiles is the term for invertebrates that actively use the habitat within the hyporheic zone for part or all of their life cycles. The horny cased caddisfly Olinga feredayi, for example, may spend part of its lifecycle below the stream surface, and part above, or it may select not to venture into the hyporheic zone at all. The stonefly Spaniocercoides cowleyi uses the hyporheic zone during the nymph stage of its lifecycle but returns to the streambed to complete its lifecycle, whereas Harpacticoida copepods spend their entire lifecycle below the stream surface and have morphological features associated with living in constant dark – blind and unpigmented.
The number of types, and density of insects found in the hyporheic zone differs depending on the species type and the habitat quality. Excessive sedimentation blocks the exchange with surface water, effectively destroying the hyporheic zone. It is thought that different species use the hyporheic environment to different extents. Some mayflies for example, are found in decreasing numbers with depth (e.g., Deleatidium), whereas other insects are uniformly distributed throughout (e.g., Olinga and the snail Potamopyrgus antipodarum). In contrast, increasing density with depth has been observed by some caddisflies (e.g., Polyplectropus puerilis). Furthermore, seasonal changes in use of the hyporheic zone have been reported for some insects such as Olinga and Potamopyrgus with suggestions that the link is related to their lifecycle.
Insects commonly found in flowing stream water may also use the hyporheic zone as a refuge from both flow and predation impacts. Floods typically only disturb the streambed to a depth of around 15 cm meaning that in high flow events this ecosystem may act as an important refuge. In contrast, insects may also seek refuge in the hyporheic zone when there is not enough water and a stream periodically or temporarily dries up. The hyporheic zone may also provide refuge from predating fish with some studies showing a greater use of the hyporheic zone by insects in streams containing predatory fish than those that are fishless.
Whilst there is still a great deal to learn about this “3rd dimension”, we do understand that the hyporheic zone is of fundamental importance to the way a stream ecosystem functions. The challenging part in future research, as has been in the past, is figuring out the best way to “see” what is happening below the bottom of the streambed.
Aslan Wright-Stow is a Principal Technician at NIWA.