1 Introduction. 1.1 What is a wetland?

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1 1 Introduction 1.1 What is a wetland? All around the world ( Fig. 1.1 ) there are places with shallow water or saturated soils whose vegetation is dominated by species of plants that are found nowhere else in the surrounding uplands. Plants sticking out of the water or wet soil are usually the most conspicuous feature of such places, but closer inspection often reveals the presence of plants floating on the surface of the water, and even plants growing under the water. There are also groups of animals, especially birds, which are more-or-less restricted to these areas. These places, with their unique flora and fauna, are wetlands. The term wetland is a relatively new term that encompasses many kinds of wet areas that are locally called marsh, swamp, fen, slough, bog, glade, hammock, vlei, jheel, etc. Some of these areas are very large and cover hundreds or thousands of square kilometers and contain a variety of vegetation types, e.g. the Florida Everglades, the Hudson Bay Lowlands, the Pantanal, the Okavango Delta, the Mekong Delta, the Danube Delta, etc., while others can be very small with areas less than a hectare, e.g. prairie potholes, California vernal pools, Australian billabongs. There are an incredible variety of wetland types ( Fig. 1.2 ) that result from differences in their sources of water (hydrology) and location on the landscape (geomorphology). They range from moss-dominated bogs in the arctic to tree-dominated tropical floodplains in the Amazon Basin ( Finlayson and Moser 1991 ). In wet climates, e.g. in northwestern Europe, some kind of wetlands can be found on almost any landform from depressions to highland hills ( Fig. 1.2 ), but in dry climates, e.g. in southern Africa and most of Australia, wetlands are often restricted to river channels and depressions. In intermediate climates, e.g. in the north central United States, most wetlands are associated with either depressions or slopes ( Fig. 1.3 ). Because of minor differences in their hydrology, a variety of vegetation types can be found in similar landscape settings in the same area ( Fig. 1.4 and 1.5 ).

2 2 BIOLOGY OF FRESHWATER WETLANDS Wetlands more than 10% Wetlands % Wetlands less than 0.5% Islands with substantial wetlands Figure 1.1 Global distribution of wetlands based on Gore ( 1984 ). From Williams ( 1990 ), with permission from John Wiley & Sons. Wetlands have features in common with nearby aquatic ecosystems, especially their microbiota (bacteria, algae, invertebrates ( Chapter 3 )), and with nearby terrestrial ecosystems, especially their macrobiota (plants, birds, and mammals ( Chapter 4 )). Nevertheless, they have two features that together make them unique. The first of these is anaerobic soils. Anaerobic soils develop because dissolved oxygen in water-saturated soils is quickly depleted by microorganisms ( Chapter 2 ). It is their anaerobic soils that distinguish wetlands from terrestrial systems, such as grasslands and deciduous forests. Not only are the soils anaerobic, but oxygen is often absent, or found only in very low concentrations in the water above these soils. Oxygen is the limiting factor for many plant and animal species in wetlands. Consequently, many organisms that live in wetlands must have anatomical, morphological, physiological, or behavioral adaptations to enable them to acquire, conserve, store, or find oxygen ( chapters 3 and 4 ). It is the close proximity of aerobic and anaerobic environments in wetlands that make them unique and that is responsible for their distinctive floras, faunas, soils, and functions. The second feature that distinguishes wetlands from other aquatic systems is large plants, collectively called macrophytes ( Chapter 4 ). Macrophytes, which mostly have growth forms (tree, shrub, grass, fern, moss, etc.) similar to those in terrestrial ecosystems, are major structural components and

3 1. Basin within a plain (flat) 4. Flat adjoining a channel Basin, permanently inundated=lake Flat, seasonally waterlogged=palusplain Flat, seasonally flooded = floodplain Channel, permanently flooded = river 2. Flat within a broad valley 5. Flat adjoining a channel Flat, seasonally inundated = floodplain Flat, seasonally waterlogged = palusplain Flat, seasonally waterlogged = palusplain Channel, seasonally flooded = creek 3. Basin within a plain, adjoining a slope 6. Tops of highland surfaces 7. Slopes on highlands / hills Slope, seasonally waterlogged = paluslope Seasonally waterlogged surfaces blanketing the uplands=palusmont Flat, seasonally flooded = floodplain Basin, permanently flooded = lake Seasonally waterlogged slopes = paluslope Figure 1.2 Landforms and the wetland types associated with them. From Semeniuk and Semeniuk ( 1995 ), with permission from Springer Science+Business Media B.V.

4 4 BIOLOGY OF FRESHWATER WETLANDS Highlands Highlands hills Very wet climates Intermediate Very dry climates Slopes Flats Channels Lowlands Basins Figure 1.3 Various landform types on which wetlands can develop under different climates. From Semeniuk and Semeniuk ( 1995 ), with permission from Springer Science+Business Media B.V. Overland flow Precipitation Evapotranspiration Water table Water table Overland flow Precipitation Evapotranspiration Ground-water inflow Ground-water inflow Stream flow Overland flow Precipitation Evapotranspiration Overland flow Precipitation Evapotranspiration Lake or river flood water Water table usually below wetland level Water table usually below wetland level Figure 1.4 Four common wetland types: depressional and slope wetlands with and without groundwater inputs. From Novitzki ( 1979 ), with permission from American Water Resources Association.

5 INTRODUCTION 5 Fen, Marsh: Large GW inflow well-drained site Cedar swamp: Moderate GW inflow moderate drainage Wet meadow: Little GW inflow poor drainage Figure 1.5 Different vegetation types in ground-water-slope wetlands in Wisconsin, USA. From Novitzki ( 1979 ), with permission from American Water Resources Association. Solar energy R Above ground plant biomass Fire Herbivores R R Roots Plant litter R Microbe-based food webs Herbivore-based food webs R R Microbes Allochthonous organic matter Peat Figure 1.6 Sources (primary production, allochthonous inputs) and losses (to herbivores, microorganisms, peat, fire) of organic matter in wetlands. R = losses owing to respiration. From Moore ( 1990 ), with permission from John Wiley & Sons. primary producers in wetlands. Algae are the major primary producers in aquatic systems (streams, rivers, lakes). Algae are also present in wetlands, where they can also be major primary producers ( Chapter 6 ). In a wetland, however, macrophytes and their litter ( Fig. 1.6 ) create much of the wetland s physical structure, and they modify its environmental conditions (e.g.

6 6 BIOLOGY OF FRESHWATER WETLANDS water and soil temperatures, water velocities, water chemistry, wind velocity ( Chapter 2 )). Because wetlands are dominated by macrophytes, functionally and structurally, they have much in common with terrestrial ecosystems. Wetlands with standing water functionally also have much in common with other aquatic systems. For many wetland plants and animals, especially those found in the water column, the chemical and physical characteristics of water per se also require adaptations to allow these organisms to move, acquire energy and nutrients, reproduce, and survive. In many wetlands, these organisms must survive or endure water-level fluctuations, ice formation, and even the periodic absence of water. As noted, many of the microorganisms, invertebrates, fish, and plants, especially algae, found in wetlands are also found in other aquatic systems, e.g. rivers and lakes. In fact, boundaries between wetlands and adjacent aquatic systems are somewhat arbitrary and difficult to delineate. Many organisms move freely across these boundaries occasionally, seasonally, or daily. Because the biology of aquatic organisms is already described in a companion volume in this series ( Brönmark and Hansson 2005 ), aquatic organisms and their adaptations will not be discussed in detail, except for some species that are common in wetlands ( Chapter 3 ). Relatively few species have the adaptations needed to live exclusively or mostly in wetlands. Only about 2 3% of angiosperm species are restricted to wetlands ( Cronk and Fennessy 2001 ) although many more species may occasionally be found growing in wetlands. Only 3% of insects are estimated to be obligate wetland species, with one or more life stages restricted to wetlands ( Ward 1992 ). Only 1 2% of birds are classified as waterfowl ( Owen and Black 1990 ) and another 3% as wading birds ( Soothill and Soothill 1982 ), two major types of birds that are restricted to or are highly dependent on wetlands. Like plants, many other species of birds are occasionally found in wetlands. For many of the biota, it is the lack of oxygen in wetlands that is the major obstacle, especially for aquatic organisms like fish and invertebrates. For the microorganisms, the range of environmental conditions in wetlands from aerobic to anaerobic enables a multitude of microorganisms, mostly bacteria, to thrive. This bacterial diversity in wetlands has a profound impact on soil chemistry and wetland nutrient cycles as described in Chapter Wetland classifi cation Several attempts have been made by national and international organizations to develop a formal definition of wetlands ( Finlayson and van der Valk 1995 ). Although they differ in wording and emphasis, they are all surprisingly consistent. According to the Committee on Characterization of

7 INTRODUCTION 7 Wetlands ( 1995 ), The minimum essential characteristics of a wetland are recurrent, sustained inundation or saturation at or near the surface and the presence of physical, chemical, and biological features reflective of recurrent, sustained inundation or saturation. Common diagnostic features of wetlands are hydric soils and hydrophytic vegetation. Although it is possible to define in principle what constitutes a wetland, in practice delimiting wetlands is often difficult. In large part this is simply owing to the immense number of wetland types, their dynamic nature, and the difficulty of determining the boundaries between wetlands and contiguous aquatic systems. Nevertheless, many wetland classifications have been developed, and some have been used successfully to conduct national or regional wetland inventories ( Finlayson and van der Valk 1995, Finlayson et al ). Typical of these classification systems is the one developed in the United States ( Fig. 1.7 ). It is a hierarchical system that recognizes five major wetland types (systems) based on their hydrology: marine, estuarine, riverine, lacustrine, and palustrine (shallow depressions). (In this book we will be concerned primarily with the last three.) Because aerial photographs are used to map wetlands, different vegetation classes with distinct plant growth forms (mosses and lichens, emergents, submersed aquatics, shrubs, and trees) are recognized that can be used to map wetlands in each hydrologic system. More recent classification systems, e.g. Brinson ( 1993 ), emphasize the importance of the geomorphological setting as well as the hydrology of wetlands. Wetlands exist in a landscape because some geomorphic features or landforms enable water to be stored or to flow through some portion of the landscape. Common landforms in which wetlands are found include: slopes ( Figs 1.2 and 1.3 ), where groundwater discharges; flats or plains, with saturated soils; depressions (palustrine wetlands), with or without inlets and outlets; shorelines of lakes (lacustrine); river channels and floodplains (riverine); deltas; and estuaries (estuarine). Sources of water for wetlands are normally precipitation, groundwater inflow or discharge, surface water inflow, surface water bi-directional flow (tides), or some combination of these. The major sources of water in a wetland are determined by its geomorphic setting and local climatic conditions. In turn, the sources of the water in a wetland determine not only the amount present and when it is present, but also its chemistry ( Fig. 1.8 ). The water chemistry of wetlands whose primary source of water is precipitation will be very different from that of a wetland whose primary source of water is groundwater discharge. This can have a major effect on the species composition of the vegetation and its primary production. A hydrogeomorphic classification scheme that was developed in Australia ( Semeniuk and Semeniuk 1995 ) combines landform and hydroperiod. Four hydroperiods are recognized: permanent inundation, seasonal inundation, intermittent inundation, and seasonal waterlogging; and five landforms: basin, channel, flat, slope, and highland (Fig. 1.3 ).

8 8 BIOLOGY OF FRESHWATER WETLANDS System Subsystem Class Marine Subtidal Reef Intertidal Reef Rocky shore Subtidal Reef Estuarine Intertidal Reef Streambed Rocky shore Emergent wetland Scrub-shrub wetland Forested wetland Wetlands and deep water habitats Riverine Tidal Lower perennial Upper perennial Rocky shore Emergent wetland Rocky shore Emergent wetland Rocky shore Intermittent Streambed Limnetic Lacustrine Littoral Rocky shore Emergent wetland Palustrine Moss Lichen wetland Emergent wetland Scrub-shrub wetland Forested wetland Figure 1.7 Hierarchical classification system developed for American wetlands by Cowardin et al. ( 1979 ). When combined there are 13 different wetland types that are possible. All of these wetland types can be found in very wet climates, but in very dry climates only wetlands associated with basins or depressions and channels can occur. It should be noted that in areas with very wet climates, it is possible for wetlands to overcome their geomorphological constraints. In these

9 INTRODUCTION 9 Oligotrophic Mesotrophic Total nutrient availability Production Decomposition Swamp Eutrophic Bog Poor fen Moderate rich fen Freshwater marsh Saline wetland Tidal marsh Treed Sphagnum Brown mosses Base cations, ph, alkalinity Water flow Extreme rich fen Peatlands Moss dominated Wetlands N,P in surface water Water level fluctuation Figure 1.8 The relationship between Canadian wetland types and major water chemistry, biotic, and hydrological gradients. From Zoltai and Vitt ( 1995 ), with permission from Springer Science+Business Media B.V. situations, the wetlands that may originally have developed in a basin can, through the accumulation of peat, fill up the basin and begin to expand into the surrounding uplands turning them into a wetland: a process known as paludification. These types of wetlands themselves become local landforms (Charman 2002 ). 1.3 Wetland inventories Wetlands are found on every continent except Antarctica, and in almost every climatic zone on these continents ( Fig. 1.1 ). Because no detailed inventory of the world s wetlands has ever been done, only rather crude estimates of the total area of wetlands are available ( Table 1.1 ). On the basis of a compilation of wetland inventories around the world, Finlayson et al. (1999 ) estimate that the total area of wetlands is around ha ( km 2 ). Lehner and Döll ( 2004 ), using a variety of maps and other databases, estimated that the total area of wetlands is about ha ( km 2 ). Both are considered to be underestimates. Overall, about 5 6% of the land surface of the earth is covered with wetlands. The distribution of these wetlands is uneven, with major wetland areas in the arctic and subarctic regions of North America, Europe, and Asia, and in association with large tropical rivers and lakes in South America and Africa ( Fig. 1.1 ). Probably

10 10 BIOLOGY OF FRESHWATER WETLANDS Table 1.1 Estimates of global wetland areas ( 10 6 hectares). Region Global Wetland Inventory ( 1999 ) Global Lakes and Wetlands Database (2004) Africa Asia Europe Neotropics North America Australia and Oceania Total area Data from Finlayson et al. (1999 ) and Lehner and Doll (2004 ). Adapted from the Millennium Ecosystem Assessment (2005 ). more than half of all wetlands are peatlands ( Charman 2002 ). Peatlands are distinguished from other kinds of wetlands by their soils, which are layers of partly decomposed plant material (peat). Because soils around the world have been mapped in much more detail than wetlands, maps of wetland (hydric) soils can be used to estimate the total area of wetlands around the world. According to FAO/UNESCO soil classification, there are four major types of wetland soils: histosols (peat soils); gleysols (mineral wetland soils); fluvisols (floodplain alluvial soils); and several kinds of temporarily flooded soils. There are estimated to be ha of histosols, ha of gleysols, ha of fluvisols, and ha of temporarily flooded soils in the world. Based on soil data, the total area of wetlands is estimated to be around ha. This is significantly higher than other current estimates ( Table 1.1 ), and about double the estimate of Lehner and Döll ( 2004 ). However, it is not necessarily the case that all areas mapped as having wetland soils are still wetlands. Soils may continue to be mapped as hydric soils even after the wetland that produced them has been drained. In fact, soil maps have been used to estimate wetland losses. In Iowa, for example, Miller et al. ( 2009 ) showed that the wetlands of the Des Moines Lobe could be easily identified and mapped based on their hydric soil legacy. Most of them were no longer being mapped as wetlands by the US National Wetland Inventory, however. In reality, 95% of them had been drained. The most detailed inventories of wetlands have been carried out in the United States, Canada, and some countries in Western Europe ( Finlayson and van der Valk 1995, Finlayson et al ). What is clear from these inventories is that large areas of wetlands have been lost, typically owing to drainage and conversion to farmland, but also to many other human activities, including stream channelization, dam construction, mining, filling for development, and sedimentation. Estimates of wetland loss range from about 50% (53% for the United States ( Dahl 1990 )) to more than 90% for New Zealand ( Dugan 1993 ). An assessment of wetland losses by continent ( Maltby 2009 )

11 INTRODUCTION 11 suggests that regional losses of 40 90% are common on all continents with wetlands. Overall, wetland loss at the global scale is probably around 50% ( Dugan 1993 ), with losses continuing in most parts of the world. In places where wetlands are protected by law from drainage and filling, as is mostly the case in the United States, these laws have been effective in reducing losses and in stimulating the restoration of wetlands to replace wetlands that are lost owing to development (Chapter 8 ). 1.4 Summary Wetlands have three essential features: (1) shallow water or saturated soils (hydrology); (2) soils that develop under anaerobic conditions (hydric soils); and (3) unique flora and fauna adapted to environmental conditions in wetlands. It is their anaerobic soils that distinguish wetlands from terrestrial systems, and it is the presence of vegetation dominated by trees, shrubs, grasses, mosses, and other large plants that distinguishes wetlands from aquatic systems. There are many different kinds of wetlands that are characterized by different hydrology (source(s) of water and duration and timing of flooding), geomorphological setting (flats, basins, slopes, channels, etc.), vegetation (submersed aquatic beds, emergents, mosses, shrubs, trees), soils (mineral, peat), and water chemistries. Wetlands can be classified based on their hydrology, vegetation, and/or geomorphology. 1.5 Practical experiments and observations There is no substitute for first-hand familiarity with wetlands. Consequently, visiting some local wetlands, observing their flora and fauna, and doing some studies and/or experiments are strongly encouraged, and suggestions for specific observations and studies are included at the end of each chapter. Some wetlands, however, are easier to visit and study than others. It can be difficult to work in some freshwater wetlands. Emergent vegetation is often very dense and very tall. Visibility can be very limited; it is easy to get lost. The bottoms of wetlands can be poorly consolidated; this can make walking very difficult. Some wetland plant species have common names, like sawgrass and rice cut-grass; these names are well deserved, and appropriate clothing may be needed. Mosquitoes can be a problem, especially along the edges of wetlands, but rarely in their interiors. In tropical and subtropical wetlands, it is possible to encounter dangerous animals like alligators, crocodiles, or poisonous snakes. Human parasites with aquatic stages can also be a problem in some parts of the world. Finally, as the name implies, wetlands

12 12 BIOLOGY OF FRESHWATER WETLANDS are wet places. Sometimes this means only that the soil is saturated; boots may not be needed to visit such a wetland. Alternatively, some wetlands may have standing water in places two meters or more in depth; boots or a boat will be essential. Although visiting and working in wetlands is no more difficult or dangerous than visiting or working in grasslands or forests in the same region, it will be a much more pleasant and productive experience if you are properly dressed and equipped. It is not recommended that you visit most wetlands for the first time, or any time, on your own. If you are taking a university course in wetland ecology, class field trips to local wetlands will undoubtedly be part of the course. If not, the ideal way to visit wetlands is by going to nearby parks or refuges with wetlands. Wetlands in parks and refuges often have boardwalks in them that make it easy to get into the wetland without any special gear like boots or boats; they may have boat trails and boats to rent, or they may have nature trails along levees or dikes. Guided tours are often available in large wetlands like the Everglades. Local birding and natural-history clubs may occasionally offer trips to wetlands, and such trips may be the best way to first visit large wetlands not in parks or preserves. Before you visit any wetland, it is highly recommended that you learn as much about it as is feasible. There is an incredible amount of information available about specific wetlands around the world; much of it is available through the internet. Borrowing or acquiring some local guides to wetland plants, birds, and invertebrates is also highly recommended in order to get the most out of your visit Wetland classification There are many kinds of wetlands ( Fig. 1.7 ). Before visiting a wetland, it very useful to become familiar with the local wetland classification system and to determine what kind of wetland you will be visiting. Most national or regional wetland classifications systems are available on the internet. If there is no national or regional classification system, the Ramsar Convention on Wetlands (see Chapter 10 ) has a classification system that is designed for use around the world. It is available on their website. Existing topographic maps can often provide most of the data needed initially to classify a wetland. Topo maps will show whether the wetland is associated with a lake or river, or whether it is isolated. They will also show whether it has surface inflows and outflows. Soils maps are useful for determining the kinds of soil in the wetland, at a minimum if they are organic (peat) or not.