Droughts, floods and riverine biota: natural disturbance or environmental catastrophe?

We humans often think that the whole world revolves around us, and that if something affects our lives, it must be important. Droughts and floods are examples of natural events and processes that occur not uncommonly in rivers, but that cause lots of angst for humans. While the animals and plants carry on pretty much as normal, we humans get all excited, and rush around wringing our hands about how catastrophic it all is. But most of this concern is for ourselves, our livelihoods and how we will deal with the scarcity of this vital, and increasingly expensive, resource. Or if the river animals and plants are considered, the finger of blame is firmly pointed at the drought or flood and its effects. Unfortunately, it is not as simple as that.

The biota of rivers in Australia, and many other dry parts of the world, have evolved in the context of extremes of flow. The ability of some species of fish, such as the dwarf galaxiid (Galaxiella pusilla), the Queensland lungfish (Neoceratodus forsteri), or the inimitable salamanderfish (Lepidogalaxias salamandroides), to aestivate (the summer equivalent of hibernate and meaning to become dormant, usually in the absence of free water, during dry periods) is one example of a physiological adaptation to frequent, and in many cases predictable, dry periods. The life history of other species of fish, like golden perch (Macquaria ambigua) or silver perch (Bidyanus bidyanus), which produce hundreds of thousands of eggs and can reabsorb these if conditions are not right, is another example of an adaptation to taking advantage of the good times, while biding time during the bad ones. Actually, many species of native fish in the Murray-Darling Basin seem to breed during the drier, warmer months of the year because of good growing conditions at this time, which is contrary to what we thought some years ago and often surprises people used to overseas patterns.

The migration of thousands of pelicans to the vast waters of newly inundated salty Lake Eyre or other water birds to wetlands previously dry, is a further illustration of how animals take advantage and ‘cue-into’ the dry and wet periods for which Australia is so well known. When rivers start flowing, after prolonged dry periods, microinvertebrates, like copepods, cladocerans and rotifers, and some macroinvertebrates, like fairy and shield shrimps, hatch from dormant, desiccation-resistant eggs and appear in their billions. Fish, that have taken refuge in small, isolated water holes, emerge and breed – also in vast numbers – take advantage of the abundant food supply and grow quickly. The fish eat the invertebrates, and the birds in many cases eat the fish.

Of course, not all parts of Australia show such extremes, but all regions, even in the relatively wet south-east have wet and dry periods each year. In fact, many of our rivers would have naturally dried to a series of isolated pools during most summers. The Broken River, that runs through Benalla and meets the Goulburn River at Shepparton, for example, was supposedly named because it did just this: the river ‘broke’ into pools, interspersed with dry riverbed, in most summers. The animals that depend on the water, like fish, would in many cases move downstream to more permanent water, or take refuge in pools. Nowadays, river regulation does not allow most rivers to dry, and we actively managed to keep rivers wet at all times. This is called ‘anti-drought’ by Tom McMahon and Brian Finlayson of the University of Melbourne.

High flows also occur naturally and frequently, of course. In fact, most rivers would normally ‘break their banks’ once every two years, by definition…if allowed. As I have talked about in a previous post, river channels are defined largely by the flow which travels down them. And it seems that the flows which exceed the capacity of what we call the main channel, occur with relatively predictable frequencies. Like drying, we also try to manage rivers so that they don’t flood. We build levees to increase the height of river banks or construct upstream dams to ‘flood-proof’ our rivers and hold back unusually large rainfall events.

Sometimes dry periods are longer than normal and become ‘droughts’. So-called droughts do stress the inhabitants of rivers; there is no doubt. Isolated pools or water holes that gradually dry because of declining groundwater, become more concentrated with solutes, like salt, and often also have lower dissolved oxygen concentrations than flowing water. While most species of native fish, for example, can cope with quite high salinities and low dissolved oxygen levels, there comes a time when these will prevent growth and ultimately cause death. The recent ‘millennium drought’ went on for so long that there were major concerns for fish, river redgums and water birds, much of it warranted. But droughts and drying also allow the build up of organic material, both living and dead, in the dry parts of rivers. This will contribute to the energy of rivers when water returns, although drying will also change the biogeochemical make-up of the sediment. And during this same extended dry period, there appeared to be a resurgence in Murray cod (Maccullochella peelii) numbers in some parts of the Murray-Darling Basin, whereas exactly the opposite was true for carp (Cyprinus carpio). Something interesting was happening, and not all of it bad!

Sometimes wet periods are of greater magnitude than normal and become ‘floods’. So-called floods certainly cause some ‘inconvenience’ for our river fauna and flora. Flooding may displace individuals downstream, because they can’t resist the current. But once the flow has reached bankfull and floods, there is an abundance of inundated habitat that is slow-flowing or still, as the water makes it way slowly across the landscape – now part of the ‘riverscape’. Some riverine animals respond to floods by breeding and/or feeding on the floodplain. Others may take advantage of food that comes back off the floodplain, although this depends on the nature of the connection of the river with the floodplain. Some species of fish, like southern pygmy perch (Nannoperca australis) or flatheaded galaxias (Galaxias rostratus), seem to use floods to move around within the floodplain. Dormant microinvertebrate eggs hatch and plant seeds germinate in response to floods and take advantage of the nutrient-rich waters on the floodplain. And the sediment that is deposited by rivers on to floodplains also promotes much animal and plant growth.

So, to riverine animals and plants, ‘droughts’ and ‘floods’ can be seen as disturbances, but not catastrophes. And, despite some knowledge, we really know so little about the roles of these events and associated processes in riverine ecosystems, that our ‘anti-droughting’ and ‘flood-proofing’ activities are almost certainly to the detriment of the native biota and perhaps beneficial to many introduced species.

I should have added above that in an unmodified landscape and riverscape, ‘droughts’ and ‘floods’ can be seen as disturbances, but not catastrophes. But once we put up barriers to movement, fish in isolated pools, introduce species, prevent drought-breaking rainfall from reaching rivers, change temperature conditions etc, the effects of droughts and floods can take on a different perspective. What were natural events from which riverine animals and plants could previously bounce back (=resilience), now can threaten populations and even species. Much of our biota, as I have described in a few examples above, is inherently resilient, because of the nature of the environment in which it is has evolved. This is not universally the case, but is probably a reasonable generalisation. But because of our modifications to rivers and to the way we interact with rivers, say through fishing, we have in many cases, compromised that resilience.

It has always puzzled me as to why the fish in the Murray-Darling Basin are doing so badly under river regulation, given the fact that they have evolved in such a harsh environment. Surely, with all their wily ways, coping with ‘droughts’ and ‘floods’ as they have for millennia, a bit of regulation here and there wouldn’t bother them. Sure, we have added insult to injury to insult in the way we have modified our rivers, but still, it didn’t quite work for me. Perhaps it is that, like in all ecosystems, the animals and plants get knocked around by disturbances, but recover after a time, and things go on pretty much as before. But now we have largely taken away that ability to bounce back, either because many species are in much smaller numbers than in previous times, or because our activities actively prevent it, and so ‘drought’ and ‘flood’ disturbances become much more significant and potentially catastrophic. Considerable conservation and management programs are now being implemented to allow more resilience in our rivers, and I sincerely hope they work. We are profoundly ignorant, however, at what spatial and time scales recovery from disturbances, like ‘droughts’ and ‘floods’, operate. Indeed, the time and spatial scales that are relevant to relationships between the dynamics of riverine animal and plant populations and the driving forces in rivers – like flow – are largely unknown. Continuing and enhanced research in this area is essential if we are to improve the health of our river ecosystems.

So, in the end, ‘droughts’ and ‘floods’ are not really a problem for riverine animals and plants…as such. They may be affected, in some cases dramatically; but they will recover. However, human activities and modifications to our rivers may have shifted the goal posts to one where recovery is much less certain.

References

Probably the best most recent reference on drought, and one of the few that looks at the ecological effects of drought, would be Sam Lake’s book, Drought and Aquatic Ecosystems: Effects and Responses, John Wiley and Sons.  I also co-edited (with Darren Baldwin) a special issue of Freshwater Biology on drought and the ecology of aquatic ecosystems that came out in 2003 and has lots of good papers in it.

Balcombe SR, Bunn SE, Arthington AH, Fawcett JH, McKenzie-Smith FJ and Wright A (2007) Fish larvae, growth and biomass relationships in an Australian arid zone river: links between floodplains and waterholes. Freshwater Biology 52, 2385-2398. Humphries P and Baldwin DS (2003) Drought and aquatic ecosystems: An introduction. Freshwater Biology 48, 1141-1146. Humphries P, King AJ and Koehn JD (1999) Fish, flows and flood plains: Links between freshwater fishes and their environment in the Murray-Darling River system, Australia. Environmental Biology of Fishes 56, 129-151. Lake PS (2003) Ecological effects of perturbation by drought in flowing waters. Freshwater Biology 48, 1161-1172. McMahon TA and Finlayson BL (2003) Droughts and anti-droughts: the low flow hydrology of Australian rivers. Freshwater Biology 48, 1147-1160. McNeil DG and Closs GP (2007) Behavioural responses of a south-east Australian floodplain fish community to gradual hypoxia. Freshwater Biology 52(3), 412-420. Nielsen DL, Hillman TJ, Smith FJ and Shiel RJ (2002) The influence of seasonality and duration of flooding on zooplankton in experimental billabongs. River Research and Applications 18, 227-237 Tonkin Z, King AJ and Mahoney J (2008) Effects of flooding on recruitment and dispersal of the Southern Pygmy Perch (Nannoperca australis) at a Murray River floodplain wetland. Ecological Management and Restoration 9, 196-201.

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