104(3):43-50. Guntenspergen, G.R., S.A. Peterson, S.G. Leibowitz, L.M. Cowardini. 2002. Indicators ... Jon Wiley and Sons, Inc. McDougall, B. K. and G. E. Ho ...
EFFECTS OF EUTROPHICATION ON WETLAND ECOSYSTEMS David W. Bressler and Michael J. Paul, PhD. Tetra Tech, Inc.
Abstract This paper provides a thorough and up-to-date description of the effects of nutrients on wetland physical, chemical, and biological processes. Wetlands are highly variable and include oligotrophic as well as naturally eutrophic systems. Nutrient additions, therefore, vary in their effects, but generally reduce the aquatic life and recreational value of these systems. As nutrient loads to a wetland increase, biogeochemical processes are especially altered and, consequently, levels of chemicals change in the water as well as in the soil, which is critical for much of the vegetation found in wetlands. These initial chemical alterations have a cascading effect on wetland biodiversity. Microbial, algal, plant, invertebrate and vertebrate communities are all affected by these changes. The changes negatively impact the uses of the wetlands: principally aquatic life and recreational (wildlife viewing, fishing) uses, but also importantly reduce the effectiveness of wetlands as effective filters that protect downstream and groundwater resources.
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1. Introduction 1.1 What are wetlands and wetland types Wetlands are a unique feature of the landscape that act as an intermediary between terrestrial and aquatic ecosystems. They are at least partially inundated with water due to any number of reasons including flooding from nearby streams, rivers or lakes, rainfall during the growing season, or groundwater seeps (Mitsch and Gosselink 2000). Soils of wetlands are saturated and anaerobic conditions are common. This is a virtually universal feature of wetlands that delimits the biology and chemistry of their soils. These anaerobic conditions result in distinctive biochemical processes, unique soils and biota in wetlands. Within this general definition of a wetland there are many variations. Swamps, bogs, marshes, and fens are all common types of wetlands. Marshes are generally considered to be fully inundated wetlands that contain primarily herbaceous vegetation rooted in hydric soils. Swamps, also fully inundated, are dominated by woody vegetation such as cypress or tupelo trees also rooted in hydric soils. Fens and bogs are both considered peatlands, wetlands that accumulate dry, partially decayed vegetation, or peat. Riparian wetlands adjacent and often connected to rivers, streams, and lakes are a common type of wetland that may undergo periods of drying and re-wetting as the adjacent waterbody levels rise and fall. These systems are not distinctly different from one another and, thus, there are many other terms (e.g., sedge meadow, pocosin, bottomland) used to describe variations of these wetland types. 1.2 Basic Feature of Wetland Structure and Function 1.2.1
Generalized wetland hydrology and physical structure
Wetland physical features can be organized into three basic elements: hydrology, soil, and vegetation (Mitsch and Gosselink 2000). Wetland hydrology is the main determinant of the overall makeup of the wetland including soil characteristics, the biochemical processes, physical attributes, and the biota that inhabit the wetland. The hydrology of a wetland is determined by climate, geology, and topography. These background conditions are what give rise to the wide variety of wetland types. Wetlands occur in a variety of settings where water is abundant and evaporates slowly – ideal conditions for forming wetlands. Landscape topography characterized by flat or rolling landscapes with less permeable soils is ideal for the formation of wetlands as they are not quickly drained. Wetlands can receive water from three potential sources: precipitation, surface flow, or groundwater. Those wetlands that are inundated due primarily to precipitation are considered “closed” systems as they receive little input from other waterbodies. Because of this closed nature these wetlands tend to be oligotrophic and, thus, support biota that are able to flourish in nutrient poor situations. Wetlands that are supplied with water from nearby lakes or streams are considered “open” systems. These systems can be highly influenced by the inflows of water which determine the chemical and physical
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characteristics of the receiving waterbody. Wetlands inundated via groundwater inputs are often rich in minerals which can affect the soil and water chemistry and subsequently the biotic community characteristics. The hydrologic regime, or hydroperiod, is a major component of overall wetland ecosystem functioning. It is a wetland descriptor that is based on the fluctuation in water depth, duration, and frequency on a yearly basis (Keddy 2000). Hydroperiods among different wetlands are highly variable. In climates in which precipitation is variable from year to year such as the Midwestern U.S., hydroperiods can be unpredictable and highly variable on an annual basis. In climates with reliable weather patterns, hydroperiods may be predictable and consistently respond to seasonal changes. In other cases, hydroperiods may be responsive in the short term – rising and falling in direct response to local rainfall events (e.g., riparian wetlands of the southeastern U.S.). In open wetland systems the hydroperiod is likely to be directly related to the duration and frequency of high flows in the connected stream or river or the amount of runoff from the adjacent landscape; thus, these open systems, especially those connected to hydrologically flashy streams or rivers, are likely to exhibit frequent changes in water levels. In these open systems, the direction and velocity of flow can be especially important, influencing the kinds and amounts of materials that are transported between the adjacent terrestrial or aquatic systems and wetlands. These inputs to open wetlands can contain nutrients, toxicants, organic material, and sediment all of which can affect the structure and function of the wetland. Likewise, the outflows from these wetlands, depending on the biogeochemical processes that occur in the wetland, can have a major impact on the receiving waterbody. Low nutrient levels, different pH levels, and decreased organic material are all common characteristics of many natural wetland outflows. In closed systems or groundwater fed systems, the hydroperiod is likely to be less responsive to short, local weather events and, instead, may exhibit reliable seasonal responses based on broad annual weather patterns. 1.2.2
Anaerobic-aerobic conditions and wetland chemical reactions
One of the distinguishing features of wetlands is the soils, which are formed under saturated, flooded, or ponded conditions during the growing season for long enough to develop anaerobic conditions (NRCS 1998). Oxygen in these soils is quickly used up in aerobic microbial processes and, generally, is replenished very slowly due to the slow rate of oxygen diffusion through water. When all oxygen is used up in wetland soils, any further biochemical processes are anaerobic in nature with the exception of a thin oxidized layer at the soil/water interface and a small aerobic zone around plant roots. In normal terrestrial soils, microbial respiration is aerobic in nature. Organic carbon is oxidized (i.e., loses an electron) and oxygen serves as the terminal electron acceptor. This process results in the carbon being oxidized to a more stable state and oxygen becoming reduced to H20; energy released from the oxidized carbon is used by the organism to support metabolism and growth. Oxygen is a strong oxidizing agent (i.e., it rapidly accepts electrons from many other elements), therefore, when it is present in soils aerobic microbes dominate. Oxygen quickly diffuses through the air, therefore, as it is used up in dry, well-drained, terrestrial soils it is quickly replenished and aerobic
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respiration is able to continue. In wetlands, however, where soils are saturated for extended periods, oxygen is replenished very slowly because of its slow diffusion through water. In fact, when oxygen is consumed in a hydric soil the predominant reactions become anaerobic. Once oxygen is depleted, the next energetically efficient oxidizing agent (i.e., electron acceptor) is nitrate. Denitrifying bacteria, which use nitrate as an electron acceptor in respiration, flourish under these conditions and denitrification results in the conversion of nitrate to nitrogen gas. Once nitrate is exhausted, manganese is usually the next oxidizing agent available in wetland soils. This process of exhausting oxidizing agents continues in a wetland until, in the most reduced conditions, organic matter or carbon dioxide become the terminal electron acceptor. The products of these reactions are low-molecular-weight organic compounds and methane gas. Although anaerobic conditions are predominant in wetland soils, the thin oxidized layer that commonly occurs at the soil/water interface has an important role in overall functioning of the wetland, affecting chemical transformations and nutrient cycling. Oxidized ions are found in this layer and various aerobic processes occur here that are part of overall biochemical processes including nutrient cycling (see section next section on nutrient cycling) in the wetland ecosystem. Wetland plants have developed an ability to translocate oxygen from the upper portions of the plant that are exposed to aerobic conditions to the roots in the anaerobic soils. In this way, wetland plants are able to support belowground aerobic respiration. A portion of the root oxygen is released into the surrounding soils creating a thin aerobic zone around the plant roots. This aerobic zone around plant roots is the location of various aerobic microbial processes that play a part in chemical cycling of the wetland.
1.2.3
Generalized wetland food web
Decomposition occurring in the soils of wetlands recycles dead primary and secondary producer material. Microbial respiration (both aerobic and anaerobic) mobilizes energy from organic carbon sources (e.g., plant detritus) allowing these populations to flourish in wetland soils. Microbial processes often transform chemicals into forms that are more useable by organisms at higher trophic levels. In addition to altering wetland chemical characteristics, microbes can also be directly associated with other types of organisms. Cyanobacteria, or blue-green algae, form complex mats with algae and bacteria in some wetlands and serve as integral components in some wetland food webs (Rejmankova et al. 2004). Primary production in wetlands can be highly variable depending on the amount of nutrients and sunlight available to algae, as well as other characteristics such as pH and water clarity. Wetland plants have developed various other adaptations both structural and physiological in nature that allow them to flourish in these unique environments. Common structural plant adaptations include shallow root systems, aerenchyma, buttressed trunks, pneumatophores, and lenticles on the stem. These types of adaptations allow oxygen to be transported from the shoots of plant to the roots where respiration occurs. Additionally, mycorrhizae, fungi associated with plant root systems, provide the plant with nutrients generated through anaerobic decomposition of dead plant material and compounds less useable to plants. Secondary producers (i.e., invertebrates,
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fish, birds, amphibians, and mammals) are dependent and intricately linked to primary producers. Algae and plants can act as a food source an also provide critical habitat and nesting material for these secondary producers (Angeler et al. 2001, Lougheed 1998, Lehman 1980). Additionally, photosynthesis and respiration by primary producers affect water and soil chemical conditions (e.g., low dissolved oxygen, high pH) that influence the behavior and survival of secondary producers (McComb and Davis 1993, Gabor et al. 1994). 1.3 Nutrient cycling in natural wetlands Nitrogen and phosphorus are essential nutrients to most organisms. Nitrogen is found in both inorganic and organic forms – particulate forms of organic nitrogen settle to the bottom; dissolved inorganic nitrogen is processed through various biochemical reactions in the water column and in the soil. The numerous forms of nitrogen are transformed in the thin oxidized soil layer at the soil/water interface, around plant roots, in the lower anaerobic soil layers, and in the water column under certain conditions. Nitrogen gas (N2 or N2O) which enters water from the atmosphere or is produced through reactions in the system, is converted by bacteria and blue-green algae to organic nitrogen (i.e., nitrogen fixation). These organic forms of nitrogen can then be converted by aerobic or anaerobic microbes to ammonium (nitrogen mineralization or ammonification) which can then be absorbed by bacteria, algae, and plants and used in cellular processes or absorbed by microbes and converted back to organic nitrogen. Under high pH conditions ammonium can be converted to NH3, at which point it leaves the system through volatilization. Through a process called nitrification, which occurs in the thin oxidized soil layer or in the aerobic region around plant roots, ammonium may also be converted by microbes to nitrite and then to nitrate which is also taken up by bacteria, algae, and plants. Nitrate and ammonium are the most common forms of nitrogen introduced to systems from anthropogenic activities and are, thus, the forms associated with most cases of wetland nutrient enrichment. Nitrate is often the more common dissolved inorganic nitrogen form exported from wetland system (especially in open systems) as it is more mobile in solution due to its negative charge and, thus, does not bind to negatively charged soil particles (Mitsch and Gosselink 2000). As described earlier, nitrate also often serves as an electron acceptor for anaerobic microbial processes. This latter process, termed denitrification can result in a loss of nitrogen from the system as it converts the nitrate to nitrogen gas which can be converted to organic nitrogen, as mentioned previously, but often exits the system before this process can occur.
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N
2
Fixation NH Inflows of N Air
N
3
2
N O, N
Fixation
Volatilization
2
2
NH + NH -
3
Surface Water Oxidized soil layer
Organic N
SON
NH +
SON
3
2
NH + 4
Nitrification NO -
Runoff, leaching NO 3
2
Downward diffusion
Upward diffusion Organic N
NO -
NO -
4
SON
Organic N
4
Algae
NO -
NH +
3
4
Denitrification
Reduced soil layer Plant uptake
N O, N 2
2
Figure 1. Nitrogen cycle in wetlands, adapted from Mitsch & Gosselink, 2000.
Phosphorus often limits primary productivity and occurs in both organic and inorganic and soluble and insoluble forms (Mitsch and Gosselink, 2000). A large portion of the phosphorus in wetlands is usually tied up in insoluble organic and inorganic forms, depending on the mineral and organic content of the soil in the wetland (Mitsch and Gosselink 2000). The only form of phosphorus that is available for biological uptake is the soluble inorganic form. Orthophosphates (H2PO4-, HPO42-, PO4-3) are the main form of soluble inorganic phosphorus found in wetlands (often termed soluble reactive phosphorus [SRP]), and the main form associated with anthropogenic nutrient enrichment. All other forms of phosphorus (i.e., soluble and insoluble organic and insoluble inorganic P) must be transformed to soluble inorganic forms in order for them to serve as nutrients in the wetland food web. Under aerobic conditions, P will complex and precipitate with various chemicals such as iron, calcium, and aluminum. Unlike nitrogen, phosphorus does not serve as an oxidizing agent; however, P is linked to the reduction/oxidation of Fe and under reducing conditions is freed from iron complexes, and bound under oxidizing conditions. . Phosphorus can also become unavailable to biota through binding to clay and peat particles and by being incorporated into living matter through uptake by microbes, algae, and plants. Phosphorus is most bioavailable at slightly acidic to neutral pH levels; in acid situations P tends to be bound to aluminum and iron; in basic soils P is complexed with calcium and magnesium. Bioavailability and the resulting trophic effects, thus, can be influenced by inputs to a wetland from natural
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or anthropogenic sources that may alter the pH, dissolved oxygen, or other chemical levels, as well as actual P levels. Inflows (runoff, tides, etc) Air Plant/microbial uptake Surface Water
Sedimentation Particulate organic P
Oxidized soil layer
-3
PO 4
SOP
SOP
-3 PO 4
Adsorption precipitation
Particulate inorganic P, including Ca-P, Fe-P, Al-P
Upward diffusion
Reduced soil layer
Particulate organic P
SOP
-3 PO 4
Anaerobic release of P
Plant uptake
Figure 2. Phosphorus cycle in wetlands, adapted from Mitsch and Gosselink (2000). 1.4 Nutrient enrichment as a threat to wetlands Wetlands often serve as nutrient sinks, filtering out bioavailable nutrients from receiving waters, storing them in sediments and converting them to organic forms, which may be stored or exported in less available forms (Reddy et al., 1993, Craft and Casey 2000, Brenner et al. 2001). This filtering ability of wetlands has resulted in the creation of artificial wetlands specifically to remove nutrients and other undesirable chemicals from stream and effluent waters (Bavor and Roser 1995). In some cases, natural wetlands have been used for this same purpose (Jeng and Hong 2005). Although wetlands remove nutrients from inflows, the nutrients, once in the wetland, can cause substantial changes to the biological assemblages as well as to the overall functioning of the system. As human populations and consumption have increased, anthropogenic influences on wetlands have also increased (Verhoeven et al. 2006). Nutrient enrichment, one component of this human influence, has been a major cause of reduced biodiversity and changes in community structure and composition of wetlands worldwide (Keddy 2000). Nutrient loading has had a particular influence on wetlands in Europe and North America, where intensive agriculture and livestock industries are prevalent (Brinson and Malvarez 2002). Although wetlands are not completely destroyed by excessive nutrient loading, as with anthropogenic hydrologic or geomorphic alteration (e.g., channelization, filling), significant nutrient enrichment can lead to major changes in a wetland ecosystem, resulting in essentially two different systems prior to and after enrichment. 8
Nutrient enrichment has been shown to influence all trophic levels within a wetland ecosystem and effects include changes in species abundance, displacement of species, reduced species diversity, and shifts in community structure and composition (Piceno and Lovell 2000, Murkin et al. 1991, Boeye et al. 1997, Hann et al. 1997, Guntenspergen 2002). The structure of a wetland is the result of natural conditions to which biotic communities have adapted. The species that inhabit a particular wetland are those that outcompete other species and most efficiently use the available resources. Nutrients are essential for life and, thus, a key component in overall ecosystem functioning. The nutrient supply is one of the resources that successful wetland species exploit. In oligotrophic wetlands (i.e., low fertility), native species have developed adaptations (e.g., plants with very dense fine root systems; nutrient conservation and storage adaptations) that allow them to thrive in nutrient poor systems and outcompete species incapable of taking advantage of the existing conditions. When nutrient enrichment occurs, species that have adapted to pre-enrichment conditions are no longer at a competitive advantage and are overrun by species adapted to higher nutrient loadings. While phosphorus and nitrogen are required by wetland organisms, exposure to abnormally high amounts (i.e., nutrient enrichment) represents a critical change in the overall environment. All trophic levels from soil microbes to vertebrates demonstrate some response to that enrichment. Just as biological communities are different between naturally different systems such as marshes and swamps, so too are the biological communities different between pre- and post-enriched conditions within a single wetland. 1.5 Objective of document Because of the diversity of natural wetland types, the fine-scale responses of these systems to nutrient enrichment are nearly infinite. However, on a broader scale, responses are predictable and relatively consistent. The purpose of this review is to provide a general description of the effects of nutrient enrichment on wetlands, primarily with regard to chemical process and biological responses. Physical and chemical responses to nutrient enrichment are discussed in the following section. This then leads into a detailed discussion of biological responses, both direct and indirect, to nutrient enrichment. The final section integrates the individual component responses to provide a broader overview of wetland system responses to nutrients. 2. Physical/Chemical responses of wetlands to nutrient enrichment As nutrient loads to a wetland increase, chemical processes are altered and, consequently, levels of chemicals change in the water, as well as in the soil. These initial alterations then have a cascading effect on the trophic web of the wetland (Albright et al. 2004). Microbial, algal, plant, invertebrate, and vertebrate communities are all affected directly and/or indirectly by these changes. These ecosystem-wide changes further alter the chemical conditions within the wetland, as well as modify the physical make-up of the wetland. Examples of these systemic changes include alterations in soil characteristics and increasing organic matter and accretion rates. Considering the natural variability of wetlands throughout the world, the specific ways in which nutrient enrichment may affect
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the chemical and physical composition of a wetland are wide-ranging. There are general patterns, however, that have been identified in which common wetland types may respond chemically and physically to nutrient enrichment. Changes in biological community composition in response to nutrient enrichment can result in physical and hydrologic changes to wetlands. In some situations increased growth of algal mats can result in shading of the lower layers of the wetland water column (McDougall and Ho 1991). In other cases eutrophication may lead to increases in plant growth and increased plant biomass and resultant detritus after turnover. For example, large macrophyte beds may form in response to nutrient enrichment and may cause higher water levels and reduced flows (in open systems). These reduced flows may allow particles to settle resulting in deposits of sediment in the wetland (Kadlec 1995). These types of changes in flow patterns can affect secondary producers that are adapted to specific flow patterns. When algae and plants are in abundance, higher trophic level organisms (e.g., invertebrates, fish, birds, mammals) that feed on these materials also often flourish. The presence of these animals, especially when in abundance, can have a substantial influence on the physical characteristics of a wetland. Muskrats and beavers, for instance, are common wetland mammals that physically alter the wetland in various ways including digging holes, harvesting plants and trees, and building extensive lodges out of woody and detrital material (Weller 1981). Changes in plant community composition can also mean a loss of certain species that provide critical habitat for insects, birds, and other secondary producers. In addition to increases of the actual levels of nutrients such as nitrate and orthophosphates in wetland soils and water, parameters such as dissolved oxygen and pH are often affected by nutrient enrichment. Due to the complexity of nitrogen and phosphorus cycling described earlier, additions to these processes would be expected to affect the normal balances of chemicals. Increased nutrients in a system are processed by microbes, algae, and plants – byproducts of these reactions can alter the chemical conditions of the system. For instance, increased microbial respiration can result in greater production of nitrogen gas and carbon dioxide, and especially creating the anaerobic conditions that set the stage for so many of the biogeochemical reactions described above (Mitsch and Gosselink 2000). Increased algal and plant production can cause large changes in dissolved oxygen and pH levels (Gabor et al. 1994); and changes in primary producer community composition can alter the pathways by which nutrients are absorbed, processed, and stored (Havens et al. 1999). As these processes progressed, other potential reactions would be expected to take place such as decomposition of larger amounts of dead plant and algal material, which can further modify water and soil chemistry.
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3. Biological responses 3.1 Microbes Nutrient enrichment has been shown to affect activities of microbial communities in a number of ways including denitrification, nitrification (Davidsson et al., 2002), and methanogenesis (Segers, 1998). Because of the wide variety of wetlands and the associated soils, hydrologic regimes, and physical makeup, the resident microbial communities can be highly variable (Boon et al. 1996, Sundh et al., 1997) and the response of these communities to nutrient enrichment will likely also vary. For example, in a study of five different wetland soils, Davidsson and Stahl (2000) found that peaty soils had the highest rates of denitrification and sandy soils had the lowest. A forest peat soil had low nitrification rates and produced large amounts of nitrite. Denitrification in a field peat and sandy loam soils was counteracted by production of ammonium (NH4+) and dissolved organic N, causing net N release as nitrogen gas. Because of this natural variability in nutrient processing, nutrient enrichment will likely have variable effects on wetland microbial activity. Casey and Klaine (2001), for instance, concluded that because of the organic, saturated, and presumably anaerobic nature of the soil in a particular riparian wetland, nitrate additions from a nearby golf course were most likely attenuated by microbial communities primarily through denitrification. In contrast, nitrate additions to a dryer, less consistently saturated riparian wetland soil, did not stimulate denitrification apparently because of the lack of appropriate anaerobic conditions for denitrifying bacteria to flourish (Ettema et al. 1999). Fungal species that function in decomposing plant litter can be indirectly affected by nutrient enrichment. These types of fungi may prefer particular plant species based on the nutrient status of plant tissues (Pugh and Mulder 1971, Thormann et al. 2004). Because nutrient ratios in plant tissue can be affected by eutrophication, it can be deduced that fungal species that decompose these plants may change with nutrient status as well. Microbial community response to nutrient enrichment can be highly variable depending on the aerobic and anaerobic conditions that naturally exist in a wetland and that are closely tied to hydrologic characteristics of the wetland (Davidsson et al. 1997, Sundh et al. 1997). Venterink et al. (2002) found that drying of soil stimulated N mineralization and reduced denitrification compared to saturated soils. Phosphorus release was not, however, stimulated by soil drying. Re-wetting of soil and resultant anaerobic conditions caused a significant increase in denitrification rates, as well as an increase in the pool of extractable phosphorus. They concluded that rewetting of wetland soil could potentially increase eutrophication due to resultant greater availability of phosphorus. The infiltration rate of wetland soils (i.e., the rate at which fresh water flows through the soil introducing dissolved oxygen to microbes) can also have a major impact on microbial processes and the rate and ways in which nutrients are processed. Stepanauskas et al. (1996) found that most nitrogen was removed (through denitrification) in low infiltration settings, whereas very little nitrogen was removed when infiltration was high.
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Coinciding with increased nitrate removal was the net release of ammonium, dissolved organic nitrogen, and nitrogen gas, products of denitrification and subsequent ammonification. Low oxygen concentrations in soils with low infiltration rates led to a greater decrease in the nitrification rate than in the ammonification rate, and, consequently, more ammonium leaked from the soil. Anthropogenic influences on wetland hydrology have lead, in some situations, to salinization of wetlands (Postel 1999). In these cases, nutrient enrichment also occurred, as both effects were associated with industrialization. Baldwin et al (2006) found that with increasing salinization, ammonium and Fe (II) both increased and phosphate decreased. Additionally, methanogen bacterial populations decreased. These results suggested that changes in salinity may affect how nutrients are used by wetland bacteria, thus potentially affecting bacterial community composition. Mycorrhizal colonization of plant roots can be affected by nutrient enrichment. Anderson et al. (1984) studied mycorrhizae species from a single wetland site and found that plants in dry, nutrient poor soils had different mycorrhizae then plants in wet, nutrient rich soils. This was attributed to the mycorrhizal species specific preference for particular nutrient-tolerant or nutrient-sensitive plant types. White and Charvat (1999) investigated the rate at which a single mycorrhizal species colonized a single plant species at various phosphorus levels. They found that colonization occurred only at low nutrient levels and that plant biomass did not vary, suggesting that at low nutrient concentrations mycorrhizae may assist in plant nutrient uptake. Various other studies have identified that mycorrhizal infection of plant roots is inversely correlated with phosphorus levels (Allen 1991, Amijee et al. 1993). As will be described later, wetland plant community composition can change in response to nutrient enrichment from oligotrophic to eutrophic species. These types of changes, as described in the aforementioned studies, can affect the degree of mycorrhizal root colonization. Clearly, there are many potential effects of nutrient enrichment on the wetland microbial community. The examples described here represent relatively common responses of microbial activity to nutrient enrichment. They do not encompass all possible responses. Given the complexity of the microbial communities within the vast array of differing wetland soil types and hydrologic regimes, the range of responses are large. Because of the central importance of microbes in decomposition and nutrient recycling, however, any changes in microbial activity will have large impacts on wetland function. 3.2 Algae Algal communities have been shown to respond to nutrient enrichment in a variety of ways depending on the kind of wetland, the species of alga, and the amount and type of nutrient enrichment. Algal production in wetlands, especially oligotrophic ones, can be constrained by both nitrogen and phosphorus availability but more often phosphorus is seemingly limiting (e.g., Gophen 2000, Vymazal and Richardson 1995). Because of these limitations, especially with regard to P, algae contribute to the removal of P from the water column and alter P cycling in these systems. Algae will often take up P and deposit it to the sediment where it becomes buried (Vaithiyanathan and Richardson
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1998). In addition, layers of algae at the sediment water interface can slow phosphorus transport from sediments to the water column (Hansson 1989). In some specific cases, increased photosynthesis of nutrient enriched algal communities raises pH, which, in calcium-rich systems, can lead to precipitation of phosphate complexes and deposition and subsequent burial of phosphorus (Brenner et al. 2006). Large increases in dissolved oxygen due to algal and plant photosynthesis can affect respiration of higher trophic level organisms (e.g., fish) as well as P deposition (Dodds 2003). These types of responses are variable and can depend on any number of factors. Self shading and water temperature, for instance, have been shown to reduce periphyton biomass by causing reductions in algal metabolic activity (Dodds 2003). Hydrologic regime can influence the effect of nutrients on algae assemblages. Gell et al. (2002) found that nutrient enrichment along a hydrologically-induced nutrient gradient caused shifts in algal assemblages to more eutrophic species. Evaporation of water and subsequent concentration of nutrients resulted in further changes of assemblages toward nutrient and saline tolerant species. Sediment can also have an influence on algal processing of nutrients. Ortega-Mayagoitia et al. (2003) found that in a eutrophic wetland, the slow growing algae common to wetlands of the study region, were displaced by other types of faster growing phytoplankton. These faster growing phytoplankton communities varied depending on whether sediment was present in the system. Most likely, these responses were due to the influence of sediment on nutrient diffusion and subsequent effects on nutrient availability. In general, algal communities respond as do other organisms to nutrient enrichment – production may increase but natural species are usually replaced by those better able to exploit the nutrient rich conditions. Murkin et al. (1991) compared two naturally similar wetlands that differed only by an anthropogenic source of nutrient input to the one wetland. The wetland with nutrient inputs had higher N and P levels throughout the year and phytoplankton and epiphytic periphyton biomass were also much higher. The naturally oligotrophic wetland showed N and P deficiency throughout the year. In experimental additions of P and N to oligotrophic wetlands, Murkin et al. (1994) and Campeau et al. (1994) found that phytoplankton, epiphytic periphyton and metaphyton species demonstrated significant increases in biomass and changes in community composition during the treatments. The Everglades are unique, naturally oligotrophic wetlands (low productivity). They are naturally low in phosphorus and their algal communities have shown significant changes in response to phosphorus inputs. McCormick and Odell (1996) noted a shift in the composition of periphyton mats located in two different wetland areas with different phosphorus levels. In the phosphorus limited region periphyton mats were composed of cyanobacteria and large numbers of epiphytic diatoms, common to the oligotrophic everglades. In the region with slightly elevated phosphorus, periphyton mats were composed primarily of green algae, with reduced numbers of diatom epiphytes. This type of change represents a shift in community structure caused by changes in the environment that favor more eutrophic algal species. Other studies have shown similar responses, in which certain periphyton species are displaced by other species as phosphorus inputs increase (Raschke 1993, McCormick et al. 1996, Pan et al. 2000).
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McCormick et al. (2001) found essentially three different periphyton assemblages at different levels of phosphorus. Although compositional changes occurred in response to P additions, the overall biomass decreased leading the authors to conclude that P enrichment in this type of setting may affect overall wetland function by reducing the dominance of periphyton, altering the food quality of the periphyton for herbivores, and changing the nutrient storage functions of the wetland. The rates at which phosphorus enrichment affect periphyton appears to be related to the amount of P, however, McCormick et al. (2001) suggest that the length of loading can be influential and that small inputs over a longer period of time can have similar ecological effects to larger inputs over a short time period. Cyanobacteria (blue-green algae), algae that reside at the soil/water interface or in the water column where they form dense mats with other algae, can be affected both directly and directly by nutrient enrichment. Cyanobacteria (blue-green algae) are able to flourish in harsh, often nutrient poor environments in part due to their ability to fix nitrogen. Species richness of cyanobacterial assemblages has been shown to decrease dramatically as a result of eutrophication (Rejmankova et al. 2004). Rejmankova and Komarkova (2000, 2005) found that additions of phosphorus to phosphorus limited wetlands caused increased production; however, high levels of phosphorus and nitrogen resulted in disintegration of cyanobacterial mats and shading due to growth of large amounts of phytoplankton or macrophytes (Rejmankova 2001). Displacement of cyanobacteria by filamentous green algae has been a common response to phosphorus enrichment in the Everglades (McCormick et al. 1996, 1998). Loss of this component of the ecosystem has been shown to have further ramifications for overall functioning of these ecosystems. Cyanobacterial mats represent an important substrate for invertebrates and can be a major component in some wetland ecosystem functioning (Rejmankova et al. 2004). Aerobic conditions are maintained by these periphyton mats through respiration and photosynthesis. Phosphorus storage (Otsuki and Wetzel 1972), nitrogen fixation (Craft and Richardson 1993), and soil formation (Craft and Richardson 1998) are additional documented functions of cyanobacterial mats in un-enriched portions of the everglades that can be disturbed by nutrient enrichment and subsequent growth of various eutrophic periphyton. 3.3 Plants Because of the prevalence and high visibility of plants in wetland systems, they have been studied more than any other wetland biota and their responses to nutrient enrichment are well-documented. As with other biological communities, the response of wetland plants to nutrient enrichment can be variable and depends not only on the type and amount of nutrient enrichment, but the natural background conditions found in wetlands including resident biota, as well as water, soil, and hydrologic characteristics. In general, however, nutrient enrichment results in increased plant production and biomass, decreased plant diversity, and shifts in community structure and composition (Svengsouk and Mitsch 2000, Daoust and Childers 2003, Rickey and Anderson 2004, Vojtıskova et al. 2003). The ways in which resident plant species respond to nutrient enrichment, as well as the types of invasive species and the ways in which these species
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exploit the nutrient enriched environment, dictate the plant community composition and structure of eutrophic wetlands. Nutrient enrichment is essentially a change to the natural environment – an alteration of the conditions that native plants and other organisms have adapted to. Changes to the nutrient status of a wetland result in the establishment of plants that take advantage of increased nutrient availability. Plant species that were rare or absent may be better suited to live there and become dominant. When nutrient enrichment occurs, large, fast growing species flourish and outcompete native species adapted to low fertility conditions. In a Midwestern meadow wetland, Green and Galatowitsch (2002) found that native vegetation was adversely affected in several ways by nitrate addition. As nitrate levels increased, reed canary grass proliferated and suppressed various types of native plants. Similarly, Grevilliot et al. 1998 found that nutrient enriched areas favored the growth of competitive, fast growing species to the detriment of low or slow-growing species resulting in a decline in plant species diversity as the natural community was displaced by a few dominant species. In an anthropogenically influenced bog in Vermont, undisturbed areas of the wetland were distinctly different than disturbed areas (Gustafson and Wang 2002). Nitrogen and phosphorus soil levels were higher in the disturbed areas and only one of the nine native plant varieties was found. Although diversity decreased with nutrient enrichment, overall plant vegetative cover was substantially higher in the nutrient enriched areas due to high productivity of the invasive eutrophic species. The ways in which wetland plants take up, process, and store nutrients are highly variable and are dependent on plant physiology, as well as background environmental conditions and the nature of nutrient inputs (Bedford et al. 1999). In a low productivity, P-limited fen, Boeye et al. (1997) found that N addition had no effect on the plant community. Being P-limited, when P was added, production increased. Additions of N at this point increased the effect of the P additions, suggesting that once P was no longer limiting the N was able to be used by the plants. In a more productive, N-limited fen investigated in the same study, P fertilization had little effect on plant productivity. In two oligotrophic everglades (P-limited) systems, P additions resulted in significant but slightly different responses (Daoust and Childers 2004). In one system (wet-prairie) that received P inputs, macrophytes had a higher turnover rate than previously and showed a reduced reliance on internal stores of P. In the other system (sawgrass marsh), in response to P inputs, the predominant macrophyte initially stored P inputs by increasing below-ground biomass. After a period of time, P allocation was shifted to aboveground tissue without increasing leaf turnover rates, as occurred in the wet-prairie system. These examples, illustrate the variability of nutrient responses due to physiological variation in plant species and differences in the ways in which nutrients are processed. Adaptations that have allowed certain plant species to thrive in oligotrophic environments include slow growth, low photosynthetic rates, and high root to shoot biomass ratios (Chapin 1980). These adaptations allow these species to conserve and store nutrient reserves so that they are able to survive on the limited supply of nutrients in low fertility settings. The adaptations of eutrophic species are opposite those of oligotrophic species. High growth and photosynthesis rates, low root to shoot ratios and high absorption
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capabilities allow these species to exploit nutrient rich environments. These species use the abundant supply of nutrients to grow large and reproduce quickly so they can compete with other highly productive species in the eutrophic environment. Nutrient storage is minimal because it is less necessary given the supply rate, and nutrients are used directly for tissue production. McJannet et al. (1995) studied the variation in response of plant species from infertile and fertile habitats to nutrient enrichment. They found that ruderal species, which typically colonize disturbed, high nutrient areas, had low nutrient tissue content. They attributed this to the high growth rate of these species – nutrients absorbed from the soil were immediately used for tissue production, as opposed to being stored in tissue. This response is presumably a strategy that allows these species to flourish in enriched areas where the natural species are at a disadvantage due to the modified setting and their inability to adapt to it. In response to nutrient enrichment, the species from infertile environments stored extra nutrients in their tissue (as evidenced by high nutrient tissue levels). This response is suited to nutrient-poor systems, where storage of nutrients is essential to long term survival; however, in enriched settings this response may result in reduced productivity and survival of these species. Neatrour et al. (2005) also identified physical differences between plants colonizing high versus low fertility wetlands. Fine root biomass was lower for plants living in the high fertility wetland than in the low fertility one. Additionally, fine root biomass was shown to be inversely correlated with nutrient levels. Pauli et al. (2002) fertilized oligotrophic fens over two years and found variable responses among plant species. A habitat specialist species produced less above and below ground biomass and appeared as if continued fertilization would lead to its exclusion by species better able to exploit the nutrient enriched environment. Conversely, a generalist species in response to nutrient enrichment increased shoot to root ratio and was able to keep pace with the productivity of other plants. Other studies have also shown that fine root biomass is lower in high fertility systems (Vogt et al. 1987) and the opposite has been shown for low fertility systems (Nadelhoffer 2000). These observations further confirm the idea that plants may become dominant or subordinate with nutrient enrichment due to physiological adaptations that support or prevent uptake and use of high levels of nutrients. Studies on competition among plant species in different environments have confirmed that different plant species possess competitive adaptations that make them better suited for particular environments (e.g., eutrophic wetlands) than other species. It has been shown that different species respond variably to competition (Keddy et al. 1998) (which can be initiated through nutrient enrichment and subsequent invasion of eutrophic species); therefore, depending on the native species in a wetland, the response to nutrient enrichment and propagation of invasive species can be highly variable. Keddy et al. (1994) found that physiological traits of a plant species influence its competitive performance. Responses essentially involve ways in which the plant species re-allocates resources in an attempt to adapt to the altered environment and compete with the invasive species, which are suited to exploit the eutrophic environment. Common responses are increased upward growth (Larcher 1995) (an attempt to reach the top of the canopy),
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foraging behavior (Campbell and Grime 1989)(growth and expansion toward available resources), and a persistence response that is used by plants adapted to low fertility conditions (Boutin and Keddy 1993). Keddy et al. (1994) also found environmental conditions influenced the ability of plant species to tolerate the presence of neighboring species. This observation implies that certain plant species are better able to exploit particular conditions found in a wetland and can impact the ability of neighboring species to survive the existing conditions, an observation consistent with resource ratio theory (Tilman 1982). The hydrologic regime of a wetland, as described previously, can also influence exposure and availability of nutrients. In a study of a single peatland, species richness of study quadrats was inversely correlated with P and N levels (Drexler and Bedford 2002). The areas of this wetland with the lowest species richness were closest to a nearby farm and received the most direct flows of ground and surface flow from this farm. Conversely, the high species richness areas were located farther away from the farm and were positioned directly along ground-water flow paths of springs. Johnson and Rejmankova (2005) also found a pattern of displacement of natural plant species by a single dominant type that was related to soil P levels and distance from nearby farming activities. Similarly, in an everglades study, Doren et al. (1996) found that cattails, had displaced sawgrass, in areas closest to canal flow structures, the source of nutrient inputs. Sawgrass and other natural plant species were in greater abundance the farther away from the canal structures. The cattail flourishes in wetland soils high in P and in these situations has been shown to outcompete common everglades species such as sawgrass and spikerush that are adapted to low P soils (Newman et al., 1996). Soil P is a key factor influencing the vegetation patterns; nutrient content in the water column has less of an influence (Doren et al. 1996). In portions of the everglades, water quality has improved and nutrient additions have declined; however, because soil P content remains high in some areas due to years of P inputs and deposition, cattail populations continue to flourish (Childers et al. 2003). 3.4 Invertebrates Invertebrates are important links between different wetland trophic levels (Mitsch and Gosselink 2000). They process detrital material, returning nutrients to the system. They also serve as a food source for higher trophic levels such as fish, birds, and some mammals. As an integrated component of the wetland ecosystem, invertebrate communities can be affected by nutrient enrichment in a number of ways. Nutrient induced water and soil chemistry changes can alter microbial, algal, and plant communities that serve as invertebrate food sources and habitat. Because all components of the wetland ecosystem are interconnected, changes in these trophic levels would likely have effects on the invertebrate community. Although the relationship of invertebrates and nutrient enrichment has not been as welldocumented as those of algae and plants, the general response patterns are similar. Eutrophication, either directly or indirectly, alters invertebrate habitat (detritus, macrophytes) and food sources (algae, macrophytes, detritus) and, therefore, affects the
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types of invertebrate taxa that are able to flourish in the wetland. In an Everglades study, McMormick et al. (2004) described macroinvertebrate community composition along a gradient of eutrophication. Plant life along this gradient ranged from natural sawgrass and slough wet-prairie species in the reference (i.e., naturally oligotrophic areas) to cattails in the most highly enriched areas. Algal communities also vary along this type of gradient with cyanobacterial mats and associated diatoms being replaced by green algae (Rejmankova et al. 2004). Macroinvertebrate abundance significantly increased with nutrient enrichment; however, diversity was consistent along the gradient. Composition of the invertebrate assemblage, however, changed in response to nutrient enrichment. The abundance of insects decreased with enrichment. Chironomids and oligochaetes were abundant along the gradient, however, the species within these groups changed substantially with eutrophication. These changes in the invertebrate community composition were attributed to the parallel changes in algal and plant communities that serve as habitat and food sources for invertebrates. The particular invertebrate taxa found at different nutrient levels are presumably those that are most capable of using the available algal and plant food sources and habitats. Similarly, Spieles and Mitsch (2003) found that shifts from autochthonous to allochthonous organic waste, resulted in significant increases in invertebrate standing crop. Vegetation is the main habitat for wetland invertebrates. It serves as shelter from predators for some invertebrate taxa; and acts as a hunting ground for other predators. Nutrient additions that alter the composition of the plant community, thus, could be expected to alter invertebrate community dynamics through effects on vegetative habitat and predatory risk. Hornung and Foote (2006) found that invertebrate biomass, as well as community composition, were related to the abundance and types of macrophytes. Herbivorous invertebrate taxa were more plentiful in areas with a wide variety of macrophyte species; conversely, predatory invertebrates were more prevalent in areas with simpler plant architecture, similar to nutrient enriched settings. de Szalay and Resh (1996) also found that alterations to plant community composition influenced the colonization patterns of invertebrate species. Tolonen et al. (2003) observed that large species of invertebrates that are often preyed upon by fish, were most concentrated in dense macrophyte beds, whereas smaller invertebrates such as chironomids were found in more open water. Nutrient enrichment, which has been shown to have significant effects on wetland plant community abundance and diversity, could potentially cause a shift in invertebrate assemblage composition similar to the variations shown in the aforementioned studies. A common food source of invertebrates is dead or dying macrophytes. Increased abundance and changes in the types of plant detrital material, as can occur in response to nutrient enrichment (Daoust and Childers 2004), can affect invertebrate communities. Menendez (2005) found that gastropods played a significant role in the processing of dead plant material and that invertebrate density increased in response to increased detrital material. In this study, it was also found that grazing by invertebrates caused immobilization of nitrogen and phosphorus in the detritus. It was hypothesized that the grazing effect of gastropods caused a reduction in diatoms that colonize the detritus in this system and favored bacterial colonization of decomposing leaves. Other studies
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(Polunin 1982, Lee 1990) have found increased bacterial activity and decomposition of litter in response to invertebrate grazing on algae colonizing the litter. Battle and Golladay (2001) found that invertebrate density and community composition varied among different wetland areas depending on variations in microbial processing of detrital material. Microbial activity, as well as plant detrital composition, is also affected by nutrient enrichment, and thus, indirect effects would also be expected on invertebrates in these cases. Although less studied, invertebrate communities also feed on living macrophytes (Newman 1991). Furthermore, the macrophyte taxa on which particular herbivorous invertebrate taxa feed are very specific. For instance, Painter and McCabe (1988) found that a particular macrophyte was nearly eliminated due to herbivory by a lepidopteran taxon. Aquatic beetles have been shown to cause significant declines in water lily populations due to direct herbivory of these plants (Otto and Wallace 1989). Therefore, it would be expected that changes to the macrophyte community due to nutrient enrichment could affect the invertebrate taxa that selectively feed on these macrophytes. In addition to trophic effects, nutrient enrichment can have other more directly harmful effects on invertebrates. Bacterial outbreaks caused by nutrient enrichment can be lethal to invertebrates and the extent of bacterial infestation has been shown to be indicative of the level of nutrient enrichment (Lemly and King 2000). Dissolved oxygen levels can become severely depressed in wetlands due to nutrient enrichment. McCormick et al. (2004) noted that many of the invertebrate taxa found at highly enriched sites were adapted to tolerate extended periods of low dissolved oxygen. Davis (1975) noted that even sub-lethal levels of dissolved oxygen can eliminate invertebrate species through effects on reproduction or feeding. Modeling by Spieles and Mitsch (2003) found that increases in the abundance of allochthonous material, which could occur in response to nutrient enrichment, resulted in reduced diel dissolved oxygen and an increase in the abundance of hypoxia tolerant invertebrates. 3.5 Vertebrates Certain varieties of fish, amphibians, birds, and mammals flourish in wetlands and, just as with all the other trophic levels, are affected by nutrient enrichment. As expected, a common response of wetland vertebrates to nutrient enrichment is dependent on the effects of nutrient enrichment on food sources and habitat. For instance, algae and invertebrates are common food sources for fish – alterations in the abundance or makeup of these communities, as commonly occurs in response to nutrient enrichment, alters food availability to particular fish species (Sierszen et al 2004). This occurrence can then alter the competitive vigor of the fish species and cause changes in fish species abundance (Tolonen et al. 2003). Rader and Richardson (1994) found that several small fish species (e.g., Gambusia) were two to three times more abundant in enriched portions of the everglades than in unenriched areas. Reduced dissolved oxygen in wetlands due to nutrient enrichment can directly affect fish respiration and survival rate (McComb and Davis 1993), or force fish into more oxygenated areas (e.g., the water surface) where they are more easily preyed upon (Fisher and Willis 2000)
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This general process could occur in any of the other vertebrate groups that colonize wetlands. Certain aquatic bird species, for instance, will use aquatic vegetation for habitat and nesting. The western grebe uses extensive reed beds for nesting. Nutrient enrichment has been shown to cause broad scale changes in the types of aquatic vegetation colonizing a wetland. Changes of this nature in a grebe nesting location would potentially eliminate or reduce suitable nesting habitat. Duck species are known to be wide ranging in their feeding habits. Some species are omnivores but feed only on insects during egg laying (Gammonley and Laubhan 2002). Changes in the insect community can affect dietary needs for egg production (Hornung and Foote 2006). Wetland mammals such as beavers and muskrats rely on plants and trees for habitat and can affect the wetland ecosystem through harvesting of these. Because of their reliance on wetland vegetation for critical nesting materials, nutrient effects on the abundance and composition of these materials would potentially affect mammal colonization of wetlands. These types of nutrient effects apply to amphibians, as well. For example, certain plant species are used as attachment sites for amphibian eggs and serve as cover for larvae (Beebee 1996) – reduced abundance of these species due to nutrient enrichment could affect amphibian reproductive success. 4. Ecosystem functional responses As demonstrated in the preceding discussion of wetland trophic levels, nutrient enrichment affects all aspects of wetland ecosystems. Changes in primary production and respiration rates, as well as changes to the primary producer communities themselves, are often some of the first noticeable responses of a wetland to nutrient enrichment. In oligotrophic wetlands, dominant algae and plant species are those that are adapted to low nutrient environments, often storing and conserving nutrients. As nutrients enter such a system, algae and plant species possessing adaptations that allow them to gain a competitive advantage by efficiently using the now abundant nutrients outcompete native taxa. Although oligotrophic species are highly efficient at thriving in low nutrient settings, their responses to nutrient inputs often do not improve their competitive fitness in the new environment. Because of inadequate competitiveness under nutrient enrichment, oligotrophic species are often extirpated by eutrophic species that possess more suitable adaptations. McCormick et al. (2001), for instance, found that phosphorus enrichment caused increased metaphyton and epiphyton biomass-specific productivity, as well as increased respiration rates. They also found that the oligotrophic cyanobacterial mats, native to un-enriched portions of the wetland, were replaced by different cyanobacteria and algae species better-suited to the eutrophic environment. Increased primary production and the subsequent changes in water and soil chemistry, as well as the changes to primary producer community structure and composition, has direct effects on secondary production. As secondary production is directly dependent on primary production, it too is affected by eutrophication. Plants, algae, and primary producer detritus serve as food, as well as habitat, for invertebrates, fish, birds, amphibians, and mammals. Secondary producers exhibit strong preferences for certain types of food and shelter; changes in these
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resources, as is often the case with nutrient enrichment, result in changes in secondary producer biomass and production. Replacement of existing vegetation by dense Typha stands in marshes of Belize has been shown to result in substantial changes to invertebrate habitat (Johnson and Rejmankova 2005). These Typha stands provide better breeding habitat for a species of mosquito known to be an efficient malaria vector (Rejmankova 1998). The vegetation found in unenriched marshes of this region is, generally, colonized by a different mosquito species that is a less efficient malaria vector. Nutrient-induced increases in primary production can also result in changes to key aspects of soil and water chemistry critical to secondary producer survival. Increased photosynthesis and subsequent, respiration can cause decreases in dissolved oxygen that affects survival of species intolerant of low DO (McComb and Davis 1993). Nutrient enrichment can affect the rate of decomposition by bacterial and fungal communities through direct effects on biochemical reactions. Nutrients increase microbial biomass and lower the C:N and C:P contents of detritus, increasing its palatability and decay rate (Dodds 2002). Indirect effects of nutrient inputs on decomposition rates can also occur through influences on the amount and types of detrital material available for consumption by microbes. Gsell et al. (2004), for instance, observed increased bacterial activity in response to carbon amendments, similar to the increase in detritus that can occur in response to nutrient enrichment. Biogeochemical reactions are also affected. For example, the rate of denitrification, which uses nitrate as the oxidizing agent (electron acceptor) to metabolize organic carbon, increases in response to nutrient enrichment resulting in increased carbon breakdown and production of nitrogen gas (Mitsch and Gosselink 2000). Eriksson and Andersson (1999) found that the activity of nitrifying bacteria differed substantially between detritus of different macrophyte species and concluded that variability in nitrification activity within wetland ecosystems is related to the vegetative community composition. 5. Nutrient effects on wetland uses In addition to their use as filters for polluted waters, wetlands are used by humans for drinking water, aquatic life use (as described in the USEPA Clean Water Act), and recreational activities such as wildlife viewing, fishing, and swimming. Nutrient enrichment can negatively affect these uses through a variety of means. For example, drinking water uses can be threatened by nutrient enrichment through excess nitrate as well as proliferation of eutrophic plant and algal taxa that contribute to taste and odor problems or produce toxic compounds. Aquatic life uses are threatened by nutrient enrichment through ammonium toxicity, effects on water chemistry (e.g., dissolved oxygen), proliferation of nuisance/nonnative taxa, and loss of habitat (e.g., by proliferation of plant/algal biomass). Lastly, recreation is threatened by nutrient enrichment through the loss of habitat for recreationally targeted taxa and changes in the desirability of water contact caused by proliferation of biomass and/or nuisance taxa. 6. Conclusions
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Although responses to nutrient enrichment within wetland ecosystems are wide-ranging from changes in biochemical cycles to broad alterations in biological community composition, the key idea is that nutrient enrichment represents a major change to the overall environment. Starting at the point of time in which enrichment occurs, the system as a whole – microbial, algal, plant, invertebrate, and vertebrate assemblages – will respond in ways consistent with the changing environment. Species that are best able to exploit the existing conditions and outcompete other species will be those that flourish and species that are unable to adapt to the new conditions will be displaced resulting in different wetland ecosystems before and after nutrient enrichment.
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