The role of vegetation in the water budget of the Usangu wetlands, Tanzania

The Usangu wetlands were severely degraded over the last twenty years by cattle and the shortage of water due to rice irrigation upstream. The eastern Usangu wetlands that were previously perennial dried out in 2000 and 2002 in the dry season. Following the removal of cattle in 2006 from the eastern Usangu wetlands, perennial wetlands has re-established itself and in 2011 the vegetation had recovered and covered about 95?% of the wetted surface mainly as floating vegetation. These wetlands are the source of water of the Great Ruaha River and the volume of water entering the river has nearly doubled after cattle removal. We suggest that this may be due to the shading effect of the floating vegetation reducing the loss of water through net evaporation to about 0.5?cm?day^?1 as opposed to about 1?cm?day^?1 for open water evaporation in this tropical climate. This suggests the important role of the biology in controlling the water budget. By contrast cattle and rice farms have not been removed from the western Usangu wetlands, located upstream, where the wetlands are now reduced to small areas fringing the rivers. We suggest that the western Usangu wetlands should also be restored in order to further increase flows in the Great Ruaha River. At the same time water governance in the catchments and irrigation areas upstream of Usangu wetlands is also urgently required because present water yields are insufficient to meet the hydroelectric needs of Tanzania, the water users all along the river, as well as the important coastal wetlands associated with the Rufiji Delta during a drought year.     

Soil carbon dynamics and aquatic metabolism of a wet–dry tropics wetland system

Freshwater wetlands are a key component of the global carbon cycle. Wet–dry tropics wetlands function as wet-season carbon sinks and dry-season carbon sources with low aquatic metabolism controlled by predictably seasonal, yet magnitude-variable flow regimes and inundation patterns. However, these dynamics have not been adequately quantified in Australia’s relatively unmodified wet–dry tropics freshwater wetlands. A baseline understanding is required before analysis of land-use or climate change impacts on these aquatic ecosystems can occur. This study characterises geomorphology and sedimentology within a seasonally connected wet–dry tropics freshwater wetland system at Kings Plains, Queensland, Australia, and quantifies soil carbon stocks and wet- and dry-season aquatic metabolism. Soil carbon stocks derived from loss-on-ignition on samples to 1 m depth were 51.5?±?7.8 kg C m^?2, higher than other wet–dry tropics wetlands globally, with potential for long-term retention at greater depths. Gross primary productivity of phytoplankton (GPP) and planktonic respiration (PR) measured through biological oxygen demand bottle experiments in the water column of sediment inundated under laboratory conditions show overall low GPP and PR in both wet- and dry-season samples (all wetland samples were heterotrophic with GPP/PR?<?1). Despite the short-term dominance of aquatic respiration processes leading to net release of carbon in the water column under these conditions, there is appreciable long-term storage of carbon in sediment in the Kings Plains wetlands. This demonstrates the importance of wet–dry-tropics wetland systems as hotspots of carbon sequestration, locally, regionally and globally, and consideration should be given to their conservation and management in this context.

     

Wildlife water utilization and importance of artificial waterholes during dry season at Ruaha National Park, Tanzania

Provision of water to wildlife is crucial during dry season along the Great Ruaha River (GRR) in Ruaha National Park due to mismanagement of water resources upstream. This paper shows that wildlife in the dry areas of the park utilizes effectively the water from natural and artificial waterholes dug in the sandy riverbed of the GRR. Artificial water holes help alleviate the effects of artificial water shortage in the river, and because the location of the artificial water holes varies annually, the impact on the vegetation of aggregating herbivores around water holes was minimized. Water quality was comparable in natural and artificial water holes, and was the highest in holes dug by elephants in the sandy river bed.     

The importance of shading by riparian vegetation and wetlands in fish survival in stagnant water holes,Great Ruaha River,Tanzania

Mass fish mortality occurred in November 2006 in a water hole in the Great Ruaha River, Ruaha National Park, and was due to the lack of shade by riparian vegetation or fringing wetlands. Nearby water holes with shade or fringing wetlands suffered no fish mortality. Despite being vulnerable to intense birds’ predation, fish stayed in the top few centimeters due to hypoxia in deeper water.     

Impact of surface water extraction on water quality and ecological integrity in Arusha National Park,Tanzania

Surface water has been extracted from Arusha National Park (ANP) to meet human demand for over 30 years; however, there has been no evaluation of the impact of extraction on surface water quality, budget or ecological integrity. A reduction in water availability and flow is likely to also have impacts on the distribution and space use of large mammals. To assess the surface water quality and budget, 30 water sources were measured for three months over the dry and wet seasons. Nearly 70% of water is extracted, with the complete extraction of surface water common during the dry and early wet seasons. However, extraction did not lead to a decrease in downstream water quality, but wetland plant diversity was highest in areas with no surface water extraction. Extraction also influences large mammal space use: abundance along seven transects was typically higher upstream of extraction sites, especially in the case of large herbivores. Impacts of extraction therefore include the disconnection of streams, changes in space use of large mammals, decreases in plant diversity and changes in species composition of the riparian wetlands. We therefore recommend that monitoring and evaluation of extraction as well as sustainable water use practices be introduced as a matter of urgency.     

Net landscape carbon balance of a tropical savanna: Relative importance of fire and aquatic export in offsetting terrestrial production

The magnitude of the terrestrial carbon (C) sink may be overestimated globally due to the difficulty of accounting for all C losses across heterogeneous landscapes. More complete assessments of net landscape C balances (NLCB) are needed that integrate both emissions by fire and transfer to aquatic systems, two key loss pathways of terrestrial C. These pathways can be particularly significant in the wet–dry tropics, where fire plays a fundamental part in ecosystems and where intense rainfall and seasonal flooding can result in considerable aquatic C export (ΣF_aq). Here, we determined the NLCB of a lowland catchment (~140 km²) in tropical Australia over 2 years by evaluating net terrestrial productivity (NEP), fire‐related C emissions and ΣF_aq (comprising both downstream transport and gaseous evasion) for the two main landscape components, that is, savanna woodland and seasonal wetlands. We found that the catchment was a large C sink (NLCB 334 Mg C km^?2 year^?1), and that savanna and wetland areas contributed 84% and 16% to this sink, respectively. Annually, fire emissions (?56 Mg C km^?2 year^?1) and ΣF_aq (?28 Mg C km^?2 year^?1) reduced NEP by 13% and 7%, respectively. Savanna burning shifted the catchment to a net C source for several months during the dry season, while ΣF_aq significantly offset NEP during the wet season, with a disproportionate contribution by single major monsoonal events—up to 39% of annual ΣF_aq was exported in one event. We hypothesize that wetter and hotter conditions in the wet–dry tropics in the future will increase ΣF_aq and fire emissions, potentially further reducing the current C sink in the region. More long‐term studies are needed to upscale this first NLCB estimate to less productive, yet hydrologically dynamic regions of the wet–dry tropics where our result indicating a significant C sink may not hold.     

The influence of flow and season upon leaf-litter breakdown in monsoonal Hong Kong streams

We investigated the effects of water extraction and flow regime on leaf litter breakdown in 10 Hong Kong streams experiencing varying degrees of extraction: five with severe extraction (>85% downstream flow removed), four with moderate extraction (20–85% removed), and one without extraction (~0% removed). Breakdown rates, macroinvertebrate assemblages and nutrients were compared in upstream (reference) reaches and downstream reaches with reduced flow in each stream during high-flow (wet season) and low-flow (dry season) conditions to determine whether responses to flow were influenced by Hong Kong’s monsoonal climate. Litter breakdown was slower in downstream reaches during the wet season, and discharge was the main determinant of breakdown rate in both reaches across the 10 study streams. In contrast, inter-reach differences in breakdown during the dry season were small and unrelated to flow. Instead, breakdown rates among streams were influenced by the abundance of detritivorous shredders and chironomids (Chironominae), as well as phosphate concentrations, indicating the importance of biological processing of litter under low-flow conditions. Although litter breakdown rates appear to offer a promising functional indicator of flow-reduction impacts during the wet season, the pattern seen reflected the physical effects of flow rather than any underlying biological processes, and the lack of response to flow reduction during the dry season was notable.     

Nutrient Budget in the Seasonal Wetland of the Okavango Delta, Botswana

The nutrient (P and N species) and chloride budgets were investigated in a representative floodplain in the seasonal wetlands of the Okavango Delta, Botswana. A variety of sources of nutrients in the surface water were considered, namely ion species coming with the floodwater, those generated from dry floodplain soils and those from water-soluble dust deposition (both local and long-range sources). Concentrations of total-nitrogen and chloride in surface water were below 1 mg l⁻¹. Total-phosphorus concentrations were 0.05 mg l⁻¹, reflecting the oligotrophic character of the system. Dust deposition rates were highest for chloride at 2.44 g m⁻² year⁻¹ followed by 0.79 g m⁻² year⁻¹ for total-N, 0.40 g m⁻² year⁻¹ for ammonia and only 0.02 g m⁻² year⁻¹ for total-P, respectively. Chloride was derived primarily from long-range transport, while N and P species were of more local origin. Dissolution rates for these ions combined were calculated to be 3.9 g m⁻² for the flooded area in the 1999 season and thus all dry deposits must be re-dissolved. The accumulation of dust deposits on dry surfaces and their subsequent dissolution causes 2–5 times higher concentrations of nitrogen, phosphorus and chloride with the onset of the flood, thus boosting the nutrient stock in the crucial phase of the onset of flooding. Chloride dissolved from dry soil surfaces and dust contributed approximately 40% to the overall floodplain budget. Although contributions from the soil surface and dust to the nitrogen and phosphorus pools of the floodplain are less prominent (with 10% of total), they nonetheless represent a significant source of nutrients in the entire system. Extrapolation to annually flooded swamps (10,000 km²) indicates a maximum contribution of 40% for total-nitrogen and 60% for total-phosphorus from dust deposition on wet or dry surfaces to the nutrient pool of the water body.     

Decomposition rates and phosphorus concentrations of purple Loosestrife (Lythrum salicaria) and Cattail (Typha spp.) in fourteen Minnesota wetlands

Purple Loosestrife is rapidly displacing native vegetation in North American wetlands. Associated changes in wetland plant communities are well understood. Effects of Loosestrife invasion on nutrient cycling and decomposition rates in affected wetlands are unknown, though potentially of significance to wetland function. We used litter bag methods to quantify decomposition rates and phosphorus concentrations of purple Loosestrife (Lythrum salicaria) and native cattails (Typha spp.) in fourteen Minnesota wetlands. A 170-day study that began in autumn modeled decomposition of Loosestrife leaves. Loosestrife stems andTypha shoots that had overwintered and fragmented were measured in a 280- day study that began in spring. In general, Loosestrife leaves decomposed most rapidly of the three;Typha shoots decomposed faster than Loosestrife stems. Significant decay coefficients (k-values) were determined by F-testing single exponential model regressions of different vegetation types in the fourteen wetlands. Significant decay coefficients were:k = 2.5 × 10⁻³ and 4.32 × 10⁻³ for all Loosestrife leaves (170 d);k = 7.2 × 10⁻⁴ and 1.11 × 10⁻³ for overwintered Loosestrife stems (280-d) andk = 7.9 × 10⁻⁴, 1.42 × 10⁻³ and 2.24 × 10⁻³ for overwinteredTypha shoots (280-d). Phosphorus concentrations of plant tissue showed an initial leaching followed by stabilization or increase probably associated with microbial growth. Loosestrife leaves had twice the phosphorus concentration of Loosestrife stems andTypha shoots. Our results indicate that conversion of wetland vegetation from cattails to Loosestrife may result in significant change in wetland function by altering timing of litter input and downstream phosphorus loads. Conversion of a riverine, flow- through wetland fromTypha to Loosestrife may effectively accelerate eutrophication of downstream water bodies. Impacts of Loosestrife invasion must be considered when wetlands are managed for wildlife or for improvement of downstream water quality.     

Atmospheric Deposition to the Turkey Lakes Watershed: Temporal Variations and Characteristics

We investigated the atmospheric concentrations and deposition fluxes of major ions to the Turkey Lakes Watershed (TLW) between 1980 and 1996. During that time, daily SO₄ ²⁻ concentrations in precipitation decreased markedly, while NO₃ ⁻, NH₄ ⁺, and H⁺ concentrations remained roughly constant. It appears that precipitation acidity did not decrease in spite of declining SO₄ ²⁻ concentrations due to a concurrent and counterbalancing decrease in the concentrations of Ca²⁺, Mg²⁺, and K⁺ in precipitation. The reasons for the decline in base cations are unknown, but this decline is probably related to decreasing emissions of soil-derived particles from agricultural, industrial, and road sources. A similar situation was seen during the same period in other parts of Canada, the eastern United States, and Europe. Wet, dry, and total (wet + dry) deposition fluxes of sulphur (S) and nitrogen (N) were estimated annually for the years 1980–96. The 17-year mean annual total (wet + dry) deposition of S to the watershed was estimated at 38.5 mmol m⁻² y⁻¹ (range 24.3–50.3). Total S deposition decreased by 35% from the early 1980s (1982–84) to the mid-1990s (1994–96), a decline consistent with the 23% decline in annual SO₂ emissions in eastern North America during the same period. In contrast, the annual total (wet + dry) deposition of oxidized N ranged from 39.8 to 60.4 mmol m⁻² y⁻¹, with a 15-year mean of 50.1 mmol m⁻² y⁻¹ and a net increase of 10% between the early 1980s (1983–85) and the mid-1990s (1994–96). This is in keeping with a 10% increase in NO_x emissions in eastern North America during the same period. For both S and N (oxidized), wet deposition dominated over dry deposition as the major mechanism for atmospheric input to the watershed. Annually, wet deposition accounted for approximately two-thirds of the total atmospheric deposition of both S and N. Dry S deposition was due more to gaseous SO₂ deposition (two-thirds of dry S deposition) than to particulate SO₄ ²⁻ deposition (one-third of dry S deposition). Dry deposition of oxidized N, however, was dominated (95%) by gaseous HNO₃ deposition, with minimal input from particulate NO₃ ⁻ deposition. Compared to several selected watershed/forest sites in Canada, the United States, and Europe, the estimated total deposition of S and N at the TLW was relatively high during the measurement period. Received 5 October 1999; accepted 1 March 2001.     

Effect of Watering and Soil Moisture on Mercury Emissions from Soils

This paper presents data from experiments that measured Mercury (Hg) flux as a function of water addition and subsequent soil drying, and maintenance of soil water content over time utilizing small dynamic gas exchange chambers and large mesocosms. When soil surfaces were dry and water was added at an amount less than that necessary to saturate the soil an immediate large (relative to dry soil flux) release of Hg occurred. Diel Hg emissions from soils, unenriched (0.02 μg g⁻¹) and enriched (3 μg g⁻¹) in Hg and wet below saturation, were significantly elevated above that occurring from dry soils (2–5 times depending on soil water content) for weeks to months. Enhancement of emissions from wet soils in direct sunlight were greater than that from soils shaded or in the dark suggesting that a synergism exists between soil moisture and light. When soils were watered to saturation Hg emissions were suppressed or remained the same depending on the degree of saturation. It is hypothesized that the addition of soil water in amounts less than that necessary to saturate the soil surface results in an immediate release of elemental Hg from soil surface as the more polar water molecule out competes Hg for binding sites. As the water moves into the soil, Hg adsorbed to soil particles is desorbed into soil gas and dissolved in the soil water. The process of evaporation facilitates movement of Hg as mass flow to the soil surface where it is made available for subsequent release. The latter is hypothesized to be an important process by which Hg is recharged at the soil–air interface.     

Environmental flow allocations in monsoonal Hong Kong

1. Hong Kong streams are subject to aggressive water extractions but the downstream water needs of ecosystems – i.e. environmental flow (e‐flow) requirements – have not yet been addressed. This study investigated hydro‐ecological relationships that could be used to establish e‐flow allocations for streams in monsoonal Hong Kong. 2. Data were collected during the wet and dry seasons from 10 unpolluted streams experiencing a gradient of flow reductions (c. 0–98%). Relationships between flow conditions (percentage discharge reduction and absolute discharge volume) and responses of macroinvertebrate composition and periphyton condition were established for each season. 3. Declines in richness of Ephemeroptera and abundance of hydropsychid caddisflies, as well as increases in the proportion of predators, were linearly related to percentage discharge reduction during both seasons. Relationships were also recorded for eight other macroinvertebrate richness or compositional metrics during the dry season only. Relationships between macroinvertebrate assemblage attributes and absolute discharge volume across downstream reaches were also evident. Periphyton was relatively insensitive to flow reductions and did not provide useful hydro‐ecological relationships, although declines in autotrophic index were related to percentage discharge reduction during the dry season. 4. Using hydro‐ecological relationships established for macroinvertebrates, two levels of e‐flow were proposed: a ‘threshold’ intended to maintain near‐natural conditions and a ‘degradation limit’ that allowed no more than 25% of the maximum indicator response to flow reduction. Calculated threshold e‐flows required downstream allocation of ≥74% of natural flows; degradation limit e‐flows were ≥12% (wet) and ≥27% (dry). The discharge needed to maintain threshold conditions was 30–105 L s^?1 (wet) and 5–14 L s^?1 (dry), with degradation limit e‐flows of 19–57 L s^?1 (wet) and 3–6 L s^?1 (dry), relative to natural mean discharges of 77–303 L s^?1 (wet) and 3–18 L s^?1 (dry). 5. The proposed e‐flow allocations are indicative only, and significant obstacles to implementation have yet to be surmounted. Any such implementation requires monitoring of outcomes in order to refine the allocations and inform adaptive flow management for Hong Kong streams.     

River flow influence on the fish community of the Tagus estuary (Portugal)

The influence of river flow on the fish community was assessed for the Tagus estuary (Portugal), based on sampling surveys carried out between 1979 and 2002. Four estuarine areas were sampled using similar fishing gear and effort in all the years considered in this study (1978–1980; 1995–1997; and 2001–2002). According to river freshwater flow values, sampling years were classified as wet (mean value of 714 m³ s⁻¹, sd = 110 m³ s⁻¹) or dry (mean value of 164 m³ s⁻¹, sd = 19m³ s⁻¹). Species richness varied between 22 and 39 according to the year, but no significant differences were related to river flow. The number of species per ecological guild was also similar in wet and dry years. Fish assemblage was dominated by marine occasional, estuarine resident and marine-estuarine opportunist species that represented near 90% of all fish species. The highest densities were represented by estuarine resident species. Fish density in dry and wet years differed significantly (mean density of 10.51 individuals 1,000 m⁻² and 3.62 individuals 1,000 m⁻², respectively), and the major differences were registered for estuarine resident, marine-estuarine opportunist and catadromous species. These differences probably reflected the estuarine habitat availability and also differences in fish densities in some estuarine areas under different flow conditions. The multivariate ordination analyses performed outlined both seasonal and spatial variation trends in fish distribution and abundance. The estuarine longitudinal gradient and its relationship with species distribution were less evident in dry years. Relationships between species abundance and river flow were different according to species, which is probably due to different needs in the timing and magnitude of river flow. Electronic supplementary material  The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Composition of phytobenthos in “lab-lab”, a periphyton-based extensive aquaculture technology for milkfish in brackishwater ponds during dry and wet seasons

The study compared the species composition of phytoperiphyton (“lab-lab”) present in ponds when gradually filled with water weekly to depths of 5, 10, 15 and 30 cm between the wet and dry seasons, for one month before the stocking of fish was studied. This was done during the dry season (March–April, 2003) and wet season (June–July, 2002). Periphyton was allowed to grow on 24 artificial substrates set at equal distances in a 1000 m² pond. “Lab-lab” that colonized the artificial substrates and that on the pond surrounding the substrates were scraped off from a measured surface area. Simultaneously, water was collected for the analysis of physical, chemical and biological parameters. Sampling was done bi-weekly coinciding with 2 and 7 days submergence at a desired depth before adjusting the water level. The major algae consisted of the diatoms (Bacilliarophyta), the blue green algae (Cyanobacteria), and the green algae (Chlorophyta). The diatoms were dominant during the dry season while the cyanobacteria dominated during the wet season. Twenty eight genera were observed during the dry season and 25 genera were noted in the wet season. Variation in genera and density that were observed every sampling period, was influenced by environmental conditions and the incoming water. The total algal density ranged from 100.7 × 10⁸ – 855.1 × 10⁸ and to 24.7 × 10⁸ – 83.9 × 10⁸ organisms.m⁻² during the dry and wet seasons, respectively. Presented at the 6th Meeting of the Asian Pacific Society of Applied Phycology, Manila, Philippines.     

Rapid expansion of the cane toad (Bufo marinus) invasion front in tropical Australia

Abstract Cane toads (Bufo marinus) are large toxic anurans that have spread through much of tropical Australia since their introduction in 1935. Our surveys of the location of the toad invasion front in 2001 to 2005, and radiotracking of toads at the front near Darwin in 2005, reveal much faster westwards expansion than was recorded in earlier stages of toad invasion through Queensland. Since reaching the wet‐dry tropics of the Northern Territory, the toads have progressed an average of approximately 55 km year⁻¹ (mean rate of advance 264 m night⁻¹ along a frequently monitored 55‐km road transect during the wet season of 2004–2005). Radiotracking suggests that this displacement is due to rapid locomotion by free‐ranging toads rather than human‐assisted dispersal; individual toads frequently moved >200 m in a single night. One radiotracked toad moved >21 800 m in a 30‐day period; the fastest rate of movement yet recorded for any anuran. Daily displacements of radiotracked toads varied with time and local weather conditions, and were highest early in the wet season on warm, wet and windy nights. The accelerated rate of expansion of the front may reflect either, or both: (i) evolved changes in toads or (ii) that toads have now entered an environment more favourable to spread. This accelerated rate of expansion means that toads will reach the Western Australian border and their maximal range in northern Australia sooner than previously predicted.     

Macrophyte productivity and community development in created freshwater wetlands under experimental hydrological conditions

Macrophyte biomass production and species richness were monitored from 1988 through 1991 in four freshwater wetlands constructed on the floodpain of the Des Plaines River, Lake County, Illinois, USA. The wetlands were constructed in 1988 and pumping of river water began in 1989 under two differentd hydrologic regimes: two wetlands received high water inflow (equivalent to 40 cm wk⁻¹ of water depth) and two received low flow (11 cm wk⁻¹). Biomass production showed no relationship to the hydrologic inflows after two years of experimentation, with both the highest and lowest production occuring in low flow wetlands. Rates of primary production increased between 1990 and 1991 under low flow conditions and decreased under high flow conditions, primarily as a result of the initial composition of the plant community. The change from dry conditions in 1988 to flooded conditions in 1989 altered the species composition in each wetland to include almost 100% wetland-adapted species. Similarity in species composition among the four wetlands diverged from 1988 to 1989 as the plant community adjusted to flooded conditions and then converged in both 1990 and 1991 as the wetlands developed.     

Distribution of arbuscular mycorrhizal fungi in stands of the wetland grass Panicum hemitomon along a wide hydrologic gradient

Although wetland plant species usually aggregate into zones that correspond with their water depth/dryness tolerances, it is not known whether associated arbuscular mycorrhizal (AM) fungi show a similar zonation. We assessed the distribution of AM fungi in two similar depressional wetlands dominated by the semi-aquatic grass Panicum hemitomon by sampling soil in plots along dry-to-wet gradients that spanned 80 cm in relative elevation, and identifying/counting viable AM fungal spores. We found that eight of nine AM fungal species were common to both of the wetlands. Within each wetland, there were significant differences in species composition related to relative water depth. The zonation patterns were not identical between wetlands but revealed that certain species were relegated to the drier portions of the gradient in both. No species were relegated to only the wet portions of the wetlands; those that dominated there were also present in the drier areas. Our data show that water depth is an important factor determining the distribution of the AM fungi, even when, as in our study wetlands, the host plant remains constant along a dry-to-wet gradient. This suggests that the fungi are not physiologically equivalent in their tolerance to wetland conditions. Received: 9 October 1998 / Accepted: 24 February 1999     

Tropical wetlands: seasonal hydrologic pulsing,carbon sequestration,and methane emissions

This paper summarizes the importance of climate on tropical wetlands. Regional hydrology and carbon dynamics in many of these wetlands could shift with dramatic changes in these major carbon storages if the inter-tropical convergence zone (ITCZ) were to change in its annual patterns. The importance of seasonal pulsing hydrology on many tropical wetlands, which can be caused by watershed activities, orographic features, or monsoonal pulses from the ITCZ, is illustrated by both annual and 30-year patterns of hydrology in the Okavango Delta in southern Africa. Current studies on carbon biogeochemistry in Central America are attempting to determine the rates of carbon sequestration in tropical wetlands compared to temperate wetlands and the effects of hydrologic conditions on methane generation in these wetlands. Using the same field and lab techniques, we estimated that a humid tropical wetland in Costa Rica accumulated 255 g C m⁻² year⁻¹ in the past 42 years, 80% more than a similar temperate wetland in Ohio that accumulated 142 g C m⁻² year⁻¹ over the same period. Methane emissions averaged 1,080 mg-C m⁻² day⁻¹ in a seasonally pulsed wetland in western Costa Rica, a rate higher than methane emission rates measured over the same period from humid tropic wetlands in eastern Costa Rica (120–278 mg-C m⁻² day⁻¹). Tropical wetlands are often tuned to seasonal pulses of water caused by the seasonal movement of the ITCZ and are the most likely to be have higher fire frequency and changed methane emissions and carbon oxidation if the ITCZ were to change even slightly.     

Water-level fluctuations in North American prairie wetlands

Oscillatory water-level fluctuations are reversible changes in water levels around a long-term mean. Long-term water-level studies in wetlands in the prairie pothole region of North America and proxy data (e.g., tree rings) for water levels in this region indicate that oscillatory water-level fluctuations have occurred for thousands of years. Because there has been no standard set of terms to describe oscillatory water-level fluctuations, some terminology is proposed that is based on previous work on riverine wetlands. Changes in prairie wetland vegetation caused by oscillating water levels are called wet–dry cycles. Field studies indicate that two kinds of vegetation change are common during wet–dry cycles, fluctuations and successions. Fluctuations are changes in the relative abundance of species between the wet and dry phases of the cycles. They occur whenever the range of water levels during a cycle is small (ca. 50 cm), as in seasonal wetlands. Succesions are changes in species composition. They occur wherever the range of water levels is large (ca. 1.5–2.0 m), as in semi-permanent wetlands. During successions, high water levels during the wet phase can typically eliminate most emergent species and low or no water during the dry phase allows emergent species to become re-established from seed and terrestrial annuals to dominate the vegetation. Experimental studies at the ecosystem- and species-level have confirmed observations made during field studies of semi-permanent wetlands, e.g., that water depth tolerance is the primary determinant of distribution of emergent species. Both qualitative and quantitative assembly-rule models of wet–dry cycles have been developed. When adequate data are available, the latest quantitative models can accurately predict changes in composition and distribution of emergent vegetation in semi-permanent wetlands during all or parts of a wet–dry cycle.     

Evaluation of nutrient retention in four restored Danish riparian wetlands

During the last 15–20 years, re-establishment of freshwater riparian wetlands and remeandering of streams and rivers have been used as a tool to mitigate nutrient load in downstream recipients in Denmark. The results obtained on monitoring four different streams and wetland restoration projects are compared with respect to hydrology, i.e. flow pattern and discharge of ground or surface water, retention of phosphorus (P), and removal of nitrogen (N). Furthermore, the monitoring strategies applied for quantifying the post-restoration nutrient retention are evaluated. The four wetland restoration projects are the Brede River restoration (including river valley groundwater flow, remeandering and inundation), Lyngbygaards River restoration (groundwater flow, irrigation with drainage water, inundation with river water and remeandering), Egeskov fen (fen re-establishment and stream remeandering) and Egebjerg Meadows (fen restoration and hydrological reconnection to Store Hansted River). Retention of phosphorus varied between 0.13 and 10 kg P ha⁻¹ year⁻¹, while the removal of nitrogen varied between 52 and 337 kg N ha⁻¹ year⁻¹. The monitoring strategy chosen was not optimal at all sites and would have benefitted from a knowledge on local hydrology and water balances in the area to be restored before planning for the final monitoring design. Furthermore, the outcome concerning P retention would have benefitted from a more frequent sampling strategy.