Changes in Aquatic Habitats

The diversification and expansion of vascular plants from the Middle Silurian through the Devonian had a profound effect on non-marine aquatic habitats. As a general rule, earlier habitats tended to be unstable, structurally homogeneous and unproductive. Colonization by plants of the water’s edge and progressive expansion into drier terrestrial habitats modified aquatic habitats through bank stabilization, accelerated soil formation and organic matter enrichment.

overhead and profile views of braided streamTop view (left) and cross-section (right) of an idealized braided stream channel.

Braided Streams

Geologic studies have revealed that Silurian through the Middle Devonian streams in the Catskill Formation (eastern United States) typically exhibit a braided channel morphology. As a general rule, braided streams are broad and shallow, with channels that diverge and join to form an abundance of low-lying islands. Stream sediments tend to be coarser sands that constantly shift in response to the current. The channels themselves are unstable and can migrate considerably within a matter of days or weeks.

Braided streams present a number of challenges for vegetation and aquatic animals. The constantly shifting sediments inhibit the establishment of rooted plants and bury sessile animals, while more mobile animals are subject to stranding or being carried away by the current. Organic matter accumulations, which are important food sources, also tend to be lost either via burial or downstream transport. In addition to problems presented by shifting substrates, the characteristically shallow depths of braided streams can result in dramatic shifts in water temperatures and limit space for larger animals. Finally, drops in water levels can dramatically reduce the total amount of aquatic habitat.

Channel braiding occurs when streamwater is not restrained from scouring bank sediments and migrating across a broad alluvial plain. Modern braided streams typically occur either where there is a relatively uniform and moderate gradient (or slope) or a denuded watershed. Streams with moderate gradients usually have currents strong enough to remove silts and clays sediment but leave coarser sands and fine gravel; the currents are also strong enough to remove streamside vegetation. Denuded watersheds caused by overgrazing, deforestation and/or arid climates can also cause channel braiding. Vegetation promotes the creation and retention of soils, which in turn intercept and store stormwater. Consequently, streamflows are moderated and water currents are reduced. Moreover, bank stabilized by plants and fine sediments resist scouring.

Watersheds prior to the Devonian Transformation lacked the vegetation and fine sediments that stabilize banks and moderated flow regimes. Some bank stabilization and flow moderation would have resulted from the establishment of Late Silurian and Early Devonian floras. However, the effects of these low-lying and shallow-rooting plants would have probably been limited. The advent of arborescent forms in the Middle Devonian would have increased the moderating influences of terrestrial vegetation, but the most dramatic changes would occur in the Late Devonian with the advent of deep-rooting archaeopterid progymnosperms.

profile and overhead views of meandering streamCross section (left) and top view (right) of a meandering stream.

Meandering Streams

The characteristically sinuous pattern of meandering streams result from the propensity of water to wander over a flat alluvial plain combined with the restraint imposed by banks stabilized by streamside vegetation. The water, which is confined to a single channel, slowly erodes the banks on the outside of the curves while sediment accumulates on the inside of the curves. The channel migrates, but unlike those in braided streams, changes usually occur on a scale of years or decades rather than of days or weeks.

Currently, meandering channels are found in the majority of lowland streams and most unregulated large rivers. They are also prevalent in tidally influenced wetlands. Evidence for meandering streams occur as early as the Silurian, but braided streams probably predominated until much later. The conspicuous increase in the frequency of meandering stream channels in the Late Devonian is associated with the spread of Archaeopteris, a deep-rooting tree that was dominant in many coastal and alluvial settings. The increase is also strongly associated with an acceleration of pedogenesis (soil formation) that also occurs in the Late Devonian.

Meandering streams offer a variety of habitat opportunities for aquatic plants and animals. Their relative stability allows for the establishment of streamside and aquatic vegetation, as well as a variety of smaller aquatic animals. Moreover, meandering streams offer a greater range of physical habitats than do braided streams. The inside bend of a meandering channel usually has reduced currents and shallower depths, while the outside bend has swifter currents and deeper water. Shifting sands can be common in the deeper water, but finer sediments are common on the inside bend and along the banks. Fallen trunks and branches (snags) provide additional aquatic habitat.

The gradual channel migration of meandering streams also serves to increase the variety of habitats through the creation of floodplain ponds. These lakes are formed when the channel creates a shortcut through a pronounced bend. The floodplain pond may then undergo a succession through which it slowly becomes filled with plant growth and sediment. In many cases, however, floodplain ponds are reconnected to the main channel during flooding, and an exchange of lake and river organisms can occur.

Wetlands

The first vascular plants probably evolved in semiaquatic habitats along the margins of marine, estuarine and/or freshwater systems. Some groups, most notably the trimerophytes and their descendants, became adept at the terrestrial habit, but vascular plants continued to thrive and diversify in these semiaquatic, or wetland habitats. With increasingly moderated flow regimes and the accelerated accumulation of fine sediments, it’s probable that there were substantial increases in the extent and variety of wetlands during the Late Devonian and Early Carboniferous.

One indication of the Late Devonian expansion of wetlands is the first occurrences of coal deposits. (Coal is produced when plant production exceeds decomposition.) While modest by Carboniferous standards, they indicate substantial wetlands production. Another indication is the diversification of arborescent lycopsids (e.g., Cyclostigma and Lepidodendropsis) at the end of the Late Devonian. Presumably, these large plants required a more stable environment than did the generalists plants (e.g., Rhacophyton) that they replaced.

Modern wetlands occur in a variety of marine, estuarine and freshwater settings. Some have arborescent plants or trees (e.g., mangroves and swamps), while others are more open (e.g., marshes). They are commonly among the most productive habitats we know. Many also support a wide variety of animals, most noticeably birds. They also provide crucial nursery habitat for the young of many marine, estuarine and freshwater fishes.

Late Devonian wetlands probably presented major opportunities and challenges for vertebrates. The abundance of food would have certainly been inviting, but the shallow waters would have excluded most large fishes; many wetlands fishes are either small-bodied species or the young of larger fishes that live in deeper water. Moreover, decomposition of abundant plant matter would have greatly reduced dissolved oxygen. Perhaps this was a situation ripe for the evolution of semiaquatic tetrapods.

wetlands profile

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Web:
Canadian Geographic's web page on river meandering:
www.canadiangeographic.ca/landforms/meanders.asp
PhysicalGeography.net's web page on fluvial geomorphology:
www.physicalgeography.net/fundamentals/10z.html
Scientific Papers:
Beerbower, J.R. 1985. "Early development of continental ecosystems." pp. 47-92. In: B.H. Tiffney (ed.), Geological Factors and the Evolution of Plants. New Haven: Yale Univ. Press.
Beerbower, J.R., J.A. Boy, W.A. DiMichele, R.A. Gastaldo, R. Hook, N. Hotton, III, T.L. Phillips, S.E. Scheckler, and W.A. Shear. 1992. "Paleozoic terrestrial ecosystems." pp. 205-235. In: A.K.Behrensmeyer, J.D. Damuth, W.A. DiMichele, R.Potts, H.-D. Sues and S.L. Wing (eds.) Terrestrial Ecosystems throught Time. Chicago: Univ. Chicago Press.
Scheckler, S.E. 1986. "Floras of the Devonian-Mississippian transition." In: T.W. Broadhead (ed.) Land Plants: Notes for a short course. Paleontological Society.
Image Credits:
Images are copyrighted © 2002, Dennis C. Murphy, (see Terms of Use).

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