The majority of people, I suspect, think that rivers flow only above ground. After all, it doesn’t really make sense for rivers to flow through the ground, does it? Yet that is actually what they do. The phenomenon is called subsurface flow, and the water, and sediment that the water flows through, makes up the hyporheic zone and is habitat for a range of animals, collectively called the hyporheos (from the Greek, hypo = below, and rheos = flow). In fact, there is a whole lot going on down there!
First, let me describe the basics of how water gets down below rivers. And then I will get on to the types of animals that live down there, what they do and why we should care. When rain falls, the water seeps down into the soil partly by gravitation through relatively large spaces between particles, and partly by capillary action; a bit like the way a wick works with wax in a candle. The water moves downwards through the spaces in soil and between cracks in rock, until it hits bedrock. Then it can move no further, and so begins to fill up from there, creating the saturated or groundwater zone, and at its top, forms the water table. It rises until it gets to the surface, normally in a valley, which is the deepest region in the landscape. Once it reaches this valley, it starts to flow downwards because of a gradient, as a stream (see figure below). In dry periods, the flow is entirely due to the underlying water table. But after rainfall, runoff can greatly increase the flow of water downstream.
There is a whole area within fluvial geomorphology that considers the interactions between surface water and groundwater, but that is beyond what I want to deal with here. Nevertheless, the hyporheic zone is essentially where the groundwater and the surface water zones meet (see figure below). The water will flow slowly through the bed material if it is loose enough, and while the oxygen content is lower than at the surface (since there are animals down there respiring), it is enough to support life. The term hyporheic zone was first coined by Orghidan in 1959 (his seminal paper is now translated into English and republished in 2010), although the existence of a fauna in the hyporheic zone had been known for some time before (Karaman 1935). White (1993) defines the hyporheic zone as:
“…the saturated interstitial areas beneath the stream bed and into the stream banks that contain some proportion of channel water or that have been altered by channel water infiltration (advection).” (page 62)
(Source: White, 1993, page 63)
I don’t want to get too much into the physics and chemistry of surface flow and hyporheic flow interactions (I will leave that for another time). But it is important to understand that there are downwelling zones in rivers where water flows into the hyporheic zone from surface waters, and upwelling zones, where water flows up from the hyporheic zone into surface waters (see figure below). Instead, I’d like to just focus on the invertebrate fauna, although of course, the invertebrates need something to eat, and microbial biofilms (bacteria and fungi) are the primary source of food for these animals.
Excellent reviews of the ecology of the hyporheic zone have been published by Boulton et al. (2010) and Robertson and Wood (2010). The former includes a timeline of research in the area, and a ‘web’ of interactions of various of the physical and chemical components of rivers with the hyporheos, at various scales. It is a great summary of what influences the distribution and composition of the hyporheos.
(Source: White, 1993, page 64).
There are many invertebrates that move between surface waters and the hyporheic zones, but some species are only found in the hyporheos. These are often blind, unpigmnented and have specialised appendages, and typically breed less frequently and live longer than their compatriots up above (Gilbert et al. 1994). They are dominated by crustaceans: copepods, amphipods, cladocerans etc. Hynes (1970) and others (Pennak and Ward, 1986; Boulton et al. 1992) list the animals that compose the hyporheos as:
- Hydra (polyps in the same group as jellyfish – Cnidaria)
- Turbellaria (free-living flatworms – Platyhelminthes)
- Nematoda (round worms, more commonly parasitic)
- Oligochaeta (worms – Annelida)
- Tardigrada (water bears – fascinating little animals, capable of amazing feats of suspended animation)
- Gastropods (snails – Mollusca)
- Copepoda (mostly Harpacticoida – small, planktonic crustaceans – Arthropoda)
a – Diacyclops sp. (Cyclopoida, Copepoda), b – Harpacticoida (Copepoda), c – Frontipodopsis reticulatifrons (Acarina) (Confocal microscopy images performed at Museo Nacional de Ciencias Naturales, Madrid, by Sanda Iepure and Antonio Valdecasas, MNCN, Madrid).(Source: http://www.madrimasd.org/blogs/remtavares/2011/03/31/131598)
- Ostracoda (pea shrimps – Arthropoda)
- Chydoridae (water fleas – Cladocera, Crustacea, Arthropoda)
- Syncarida (Crustacea, Arthropoda)
- Amphipoda (scuds, Crustacea, Arthropoda)
- Hydracarina (water mites, Arachnida, Arthropoda)
- Chironomidae (midges, Insecta, Arthropoda)
- Plecoptera (stoneflies, Insecta, Arthropoda)
- Trichoptera (caddisflies, Insecta, Arthropoda)
- Ephemeroptera (mayflies, Insecta, Arthropoda)
bold = Phylum
Hynes cites Schwoerbel (1961, 1964) as finding upwards of 34,000 animals per square metre in the upper 10 cm of the hyporheos. So animals there can be enormously abundant. Some scientists have suggested that their numbers can exceed those of invertebrates in surface waters!
Andrew Boulton and Emily Stanley have worked on hyporheic fauna for many years. In a comment in Trends in Ecology and Evolution in 1996 (see references below), they put in a plea for considering the animals in the hyporheos, and not just the chemistry and microbes that occur down there, although they are very interesting in themselves. They succinctly summarise some of the key influences on hyporoheic fauna.
Speciose hyporheos have been collected in a diverse range of streams from large, lowland, mesic rivers to small, intermittent, desert streams. Their ubiquity and trophic diversity has led to a search to determine their role in the hyporheic zone, and to ascertain what factors influence their distribution in the sediments. As yet, we know little of individual species’ ecology but it appears that interstitial invertebrate distribution is strongly influenced by….hydraulic gradient and conductivity, and hyporheic residence time of the water (and hence key factors such as dissolved oxygen, and organic matter). Apart from the physical limits of the interstitial spaces, the pattern of subsurface water movements correlates strongly with the distribution of hyporheic invertebrates in some streams. Downwelling surface water [water moving down into the sediment from the surface], rich in dissolved oxygen and organic matter, supplies a taxonomically-richer interstitial assemblage that usually includes many facultative hyporheic species, whereas a less diverse fauna characterizes upwelling [water moving up from the sediment to the surface], hypoxic area. Microbial biofilms on the huge surface area of the sediment particles provide a primary food source for the hyporheos and are probably responsible for the bulk of hyporheic respiration. The invertebrates themselves may promote biofilm activity by grazing, as well as indirectly altering flow paths and interstitial porosity through the egestion of coarse fecal pellets that contain compacted fine sediment. (Boulton and Stanley, 1996, page 430)
Indeed, the contribution of the hyporheic zone to stream metabolism and so, production, can be similar to, or more than the stream bed. The driver of that productivity is likely dissolved organic carbon coming from the surface flow and from particulate organic matter than settles into, moves down through, the sediment. However, there is still much to be learned about contributions of the hyporheic zone to stream ecosystem function.
In recent decades, there has been a push for taking the conservation of the hyporheic fauna seriously. This fauna is often very diverse and any assessment off biodiversity of a river should take this into account. Furthermore, many species in the hyporheos are unique, rare and possibly threatened , but we don’t really know. More work needs to be done in this area.
More and more, the hyporheos is being considered in river management programs – even though a large proportion of the general community doesn’t know it even exists. Out of sight, should not, therefore, be out of mind. Especially when it comes to conservation and management.
References: Boulton, A. J., Datry, T., Kasahara, T., Mutz, M., & Stanford, J. A. (2010). Ecology and management of the hyporheic zone: stream-groundwater interactions of running waters and their floodplains. Journal of the North American Benthological Society, 29(1), 26-40. Boulton, A. J., & Stanley, E. H. (1996). But the story gets better: subsurface invertebrates in stream ecosystems. Trends in Ecology & Evolution, 11(10), 430. Boulton, A., Valett, H., & Fisher, S. (1992). Spatial distribution and taxonomic composition of the hyporheos of several Sonoran Desert streams. Archiv fur Hydrobiologie, 125(1), 37-61. Gibert, J., Danielopol, D. L., & Stanford, J. A. (1994). Groundwater ecology (Vol. 1): Access Online via Elsevier.Hynes, H. B. N. (1970). The Ecology of Running Waters. Toronto: University of Toronot Press. Karaman, S. (1935). Die fauna unterirdischen gewässer Jugoslawiens. Internationale Vereinigung für Theoretische und Angewandte Limnologie Verhandlungen, 7, 46-73. Orghidan, T. 1959. Ein neuer Lebensraum des unterirdischen Wassers: der hyporheische Biotop. Archiv fur Hydrobiologie, 55:392–414. Orghidan, T. (2010). A new habitat of subsurface waters: the hyporheic biotope. Fundamental and Applied Limnology, 176(4), 291. Pennak, R., & Ward, J. (1986). Interstitial fauna communities of the hyporheic and adjacent groundwater biotopes of a Colorado mountain stream. Archiv für Hydrobiologie. Supplementband. Monographische Beiträge, 74(3), 356-396. Robertson, A., & Wood, P. (2010). Ecology of the hyporheic zone: origins, current knowledge and future directions. Fundamental and Applied Limnology/Archiv für Hydrobiologie, 176(4), 279-289. Schwoerbel, J. (1961). Über die Lebensbedingungen und die Besiedlung des hyporheischen Lebensraumes. Archiv fur Hydrobiologie Supplement, 25, 182-214. Schwoerbel, J. (1964). Die Wassermilben (Hydrachnellae und Limnohalacaridae) als Indikatoren einer biocönotischen Gliederung von Breg und Brigach sowie der obersten Donau. Veröffentlichungen der Arbeitsgemeinschaft Donauforschung, 1(4), 386-417. White, D. S. (1993). Perspectives on defining and delineating hyporheic zones. Journal of the North American Benthological Society, 61-69.