Spatially explicit population responses of crayfish Procambarus alleni to potential shifts in vegetation distribution in the marl marshes of Everglades National Park,USA

Hydropattern disturbance has had wide-ranging impacts on wetland communities of the Florida Everglades, especially on the habitats and the aquatic biota of the seasonally flooded marl marshes. We used the Everglades crayfish Procambarus alleni as a model to study the associations among hydrology, vegetation distribution, and population dynamics to assess the potential impacts of hydrological changes on the aquatic faunal community in Everglades National Park. To classify benthic habitats as sources or sinks for the crayfish population, we quantified vegetation community structure using GIS maps in which dominant vegetation types were weighted by local hydroperiod (length of inundation). Regression analysis showed that this habitat classification was associated with crayfish density distribution. We then used a spatially explicit, stage-structured population model to describe crayfish population fluctuations under current environmental conditions and to simulate the potential population-level responses to habitat changes that might occur following hydrological restoration. In habitat that was initially saturated with crayfish, the crayfish population size declined under current environmental conditions and then stabilized at about 13% of the initial density over a 50-year period. A 4-month increase in hydroperiod was then simulated by converting shorter-hydroperiod Muhlenbergia-dominated marsh habitat to longer-hydroperiod Cladium-dominated marshes. The model predicted a rapid 7-fold increase in crayfish density following the simulated habitat restoration. This indicated that several functional effects may result from the restoration of historical hydropatterns in marl marshes: (1) the areal extent of habitat sinks will be reduced to isolated patches, whereas the spatial distribution of aquatic source habitats will expand; (2) crayfish population size will increase and persist over time; (3) the minimum threshold needed to increase secondary aquatic productivity may be a 7-month hydroperiod over 90% of the marl marsh landscape. Restoration of historical hydropatterns could thus have cascading positive effects throughout the Everglades aquatic food web.     

Evolution of complex life cycles of amphibians: bridging the gap between metapopulation dynamics and life history evolution

Current evolutionary models for amphibian life cycles reflect tradeoffs in size-specific growth and mortality rates between the aquatic and terrestrial stages. A limitation of these models is that they do not incorporate evolutionary phenomena that are associated with metapopulation structure. In this work I address components of the evolution of complex life cycles (CLCs) that are tied to the metapopulation dynamics of amphibians that use seasonal wetlands that vary in hydroperiod. In particular, I describe how selection for the minimum length of the larval period affects metapopulation viability and the selection/migration equilibrium. Selection to increase the minimum length of the larval period functionally reduces the number of viable breeding sites on the landscape, increases the average distance between neighboring sites, and increases the risk of metapopulation extinction. Within a metapopulation, asymmetric gene flow between populations that are adapted to different hydroperiods tends to swamp local selection for long larval periods at sites with long hydroperiods. The evolutionary stability of CLCs of many species with metapopulation structure may reflect the fact that extremely small metamorphs cannot survive on land, while lineages with long larval periods incur a high risk of metapopulation extinction. I encourage theorists to more carefully consider how life history traits and metapopulation viability are related for these and other taxa.     

Partitioning beta‐diversity through different pond hydroperiod lengths reveals predominance of nestedness in assemblages of immature odonates

Patterns of freshwater invertebrate assemblage structure in the transition from permanent to non‐permanent lentic habitats are well described in the literature. However, the effects of small changes in the hydroperiod of non‐permanent ponds on invertebrate assemblage structure remain less studied, especially on β‐diversity. Thus, we tested the effects of different pond hydroperiod lengths on the assemblage structure of immature odonates, in terms of both α‐ and β‐diversity. Small high‐altitude ponds with different hydroperiod lengths (assigned to ‘short’, ‘medium’ and ‘long’ hydroperiods) were sampled in southern Brazil between 2013 and 2014. Based on the hypothesis that shorter hydroperiods filter constituents of lentic fauna, i.e. that long‐living species cannot inhabit shorter‐hydroperiod ponds, we expected to find higher α‐ and β‐diversity in longer hydroperiods, as well as predominance of the nestedness component in β‐diversity. Restricted occurrence of some genera and higher α‐diversity of immature odonate assemblages was detected in long‐hydroperiod ponds. Within‐hydroperiod β‐diversity values did not vary among hydroperiods, because the occasional occurrence of some genera with high dispersal ability of adults in short‐hydroperiod ponds yielded similar values of the β‐diversity among hydroperiods. Partitioning of β‐diversity among hydroperiods revealed a significant higher contribution of the nestedness component rather than turnover. This pattern is explained by the occurrence of some generalist genera across the whole gradient of hydroperiod, as a subset of fauna in longer‐hydroperiod ponds. Thus, our results suggest that reduction in hydroperiod length, if occurring in the future climate change, would favor habitat‐generalist taxa in lentic ecosystems.     

The relative importance of wetland size and hydroperiod for amphibians in southern New Hampshire,USA

In the United States, the regulatory approach to wetland protection has a traditional focus on size as a primary criterion, with large wetlands gaining significantly more protection. Small, isolated wetlands have received less protection; however, these wetlands play a significant role in the maintenance of biodiversity of many taxonomic groups, including amphibians. An important question for directing conservation and management efforts for amphibians is whether size is a useful criterion for regulatory decisions. Because hydroperiod has an important influence on amphibian composition in wetlands, I conducted a study to examine the relative influence of wetland size and hydroperiod on amphibian occurrence. I sampled 103 wetlands in southern New Hampshire in 1998 and 1999 using dipnet sampling to document the presence of larval amphibians. Wetlands were placed into one of three hydroperiod categories; short (<4 months), intermediate (4–11 months), or long (permanent) based on field observations of drying pattern. Wetland size was determined from digitized national wetland inventory (NWI) maps (most wetlands) or measured in the field. I examined patterns of amphibian species richness and individual species occurrence using generalized linear models. Wetland size ranged from 0.01 to 3.27 ha. Overall, species richness was significantly influenced by hydroperiod (χ² = 18.6, p <0.001), but not size (χ² = 1.4, p = 0.24). Examination within hydroperiod categories revealed several significant relationships with wetland size. Species richness was related to wetland size in wetlands with short and intermediate hydroperiods, but not wetlands with long hydroperiods. Wetland size does not appear to be a useful sole criterion for determining wetland functional value for amphibians; assessments of functions of seasonally inundated wetlands for amphibians would benefit from examination of hydroperiod.     

No detectable role for predators mediating effects of aquatic habitat size and permanence on populations and communities of container‐dwelling mosquitoes

1. General theory from aquatic ecology predicts that smaller aquatic habitats have shorter hydroperiods favouring species that are better resource competitors and complete development quickly. Larger habitats are predicted to have longer hydroperiods enabling longer‐lived predators to persist. Habitats with long hydroperiods and predators are predicted to favour slower‐developing, predator‐resistant species, rather than competitive species. 2. In a field experiment, habitat size and hydroperiod were manipulated independently in water‐filled containers, to test these hypotheses about processes structuring aquatic communities. Human‐made containers were used that are dominated by mosquitoes that vary in desiccation resistance, competitive ability, and predation resistance. 3. Habitat size and drying had significant effects on abundances of larvae of the common species in these communities. There was sorting of species by habitat size and by drying, with species that are better competitors relatively more abundant in smaller, more ephemeral habitats, and predator‐resistant, slower‐developing species relatively more abundant in larger or permanently flooded habitats. There were no detectable effects of habitat size or drying on the dominant predator. 4. Habitat size and its interaction with drying affected inputs of eggs to containers. Habitat size also affected relative abundances of the two dominant species in the egg population. 5. Although habitat size and hydroperiod significantly affected composition of these communities, these impacts did not appear to be mediated through effects on predator abundance. Species‐specific differences in habitat size and drying regime preferences, and habitat‐dependent larval performance appear to be the main forces shaping these communities.     

Seasonal and spatial hydrological variability drives aquatic biodiversity in a flood‐pulsed,sub‐tropical wetland

1. Flood‐pulsed wetlands make vital contributions to local and global biodiversity. However, the patterns and controls of spatial and temporal variation in aquatic biodiversity in flood‐pulsed wetlands are not well understood. We analysed the relationship between variation in hydrological regime and the patterns of aquatic biodiversity in a large pristine flood‐pulsed wetland, the Okavango Delta, Botswana. 2. Surveys of water chemistry, diatoms and macroinvertebrates were conducted over the seasonal phases of the flood pulse. Hydrological variables of flood frequency and hydroperiod class were collated from 16 years of satellite images. Multivariate regression trees and generalised least squares regression were used to determine the chief controls of community composition and taxon richness. 3. Hydroperiod class, phase of the flood and conductivity explained 32% and 43% of the variation in diatom and invertebrate taxon richness, respectively. There was a negative relationship between hydroperiod class and invertebrate taxon richness on the rising, peak and receding flood, whereas at low flood there was no significant relationship. Multivariate regression tree analysis revealed hydroperiod class, phase of the flood and conductivity as the dominant forces shaping invertebrate and diatom community composition. 4. Seasonal and spatial variation in hydrological conditions are the principal drivers of variation in aquatic biodiversity in flood‐pulsed wetlands. In pristine flood‐pulsed wetlands, increased productivity caused by the arrival of the flood waters appears to override disturbance and connectivity in shaping taxon richness and community composition. Thus, these data suggest that the maintenance of a rich mosaic of habitats covering a broad range of hydroperiod is the key to preserving aquatic biodiversity and natural ecosystem function in flood‐pulsed wetlands.     

Macroinvertebrate community structure across a wetland hydroperiod gradient in southern New Hampshire,USA

We conducted a field study to examine the influence of hydroperiod and concomitant changes in abiotic (wetland size, pH, conductivity, dissolved oxygen and water temperature) and biotic (predatory fish presence) characteristics on macroinvertebrate communities in isolated wetlands in southern New Hampshire. Invertebrates were sampled using dipnet sweeps in 42 wetlands with short (<4 months), intermediate (4–11 months) or long (permanent) hydroperiods in 1998 and 1999. We found that invertebrate genera richness, and to a lesser degree abundance, increased linearly along the hydrological gradient, and in response to temperature and dissolved oxygen. Relative abundance of genera also differed markedly with respect to hydroperiod. Most notably, invertebrate communities changed from Acilius-dominated communities to Notonecta-dominated communities. Invertebrate relative abundances in permanent wetlands also differed with respect to the occurrence of predatory fish. Some genera (e.g., Libellula, and Dytiscus) were more likely to occur in permanent wetlands without fish, whereas other genera (e.g., Buena, and Basiaeshna) were more likely to occur in wetlands with predatory fish. Because aquatic invertebrate communities differed markedly with respect to wetland hydroperiod, and in relation to the occurrence of predatory fish, it is essential to retain a diversity of wetlands in the landscape to ensure the long-term persistence of aquatic invertebrate biodiversity.     

Untangling food‐web structure in an ephemeral ecosystem

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Hindcasting Historical Breeding Conditions for an Endangered Salamander in Ephemeral Wetlands of the Southeastern USA: Implications of Climate Change

The hydroperiod of ephemeral wetlands is often the most important characteristic determining amphibian breeding success, especially for species with long development times. In mesic and wet pine flatwoods of the southeastern United States, ephemeral wetlands were a common landscape feature. Reticulated flatwoods salamanders (Ambystoma bishopi), a federally endangered species, depend exclusively on ephemeral wetlands and require at least 11 weeks to successfully metamorphose into terrestrial adults. We empirically modeled hydroperiod of 17 A. bishopi breeding wetlands by combining downscaled historical climate-model data with a recent 9-year record (2006–2014) of observed water levels. Empirical models were subsequently used to reconstruct wetland hydrologic conditions from 1896–2014 using the downscaled historical climate datasets. Reconstructed hydroperiods for the 17 wetlands were highly variable through time but were frequently unfavorable for A. bishopi reproduction (e.g., only 61% of years, using a conservative estimate of development time [12 weeks], were conducive to larval development and metamorphosis). Using change-point analysis, we identified significant shifts in average hydroperiod over the last century in all 17 wetlands. Mean hydroperiods were shorter in recent years than at any other point since 1896, and thus less suitable for A. bishopi reproduction. We suggest that climate change will continue to impact the reproductive success of flatwoods salamanders and other ephemeral wetland breeders by reducing the number of years these wetlands have suitable hydroperiods. Consequently, we emphasize the importance of conservation and management for mitigating other forms of habitat degradation, especially maintenance of high quality breeding sites where reproduction can occur during appropriate environmental conditions.     

Ontogenetic and individual diet variation in amphibian larvae across an environmental gradient

1. Variation among individuals within size or age classes can have profound effects on community dynamics and food‐web structure. We investigated the potential influence of habitat disturbance on intrapopulation niche variation. 2. Amphibians occupy a range of lentic habitats from short‐hydroperiod intermittent ponds to long‐hydroperiod permanent ponds. We quantified ontogenetic diet variation and individual specialisation in wood frog tadpoles (Lithobates sylvaticus) and blue‐spotted salamander larvae (Ambystoma laterale) to investigate the influence of hydroperiod on population niche width across a natural hydroperiod gradient using stable isotope and gut content analyses. In one of the few tests using larval forms, we tested the niche variation hypothesis, which predicts that populations with larger niche widths also have increased individual variation. 3. Our results support the niche variation hypothesis, indicating that more generalised populations exhibit higher within‐individual diet variation. We report gradual changes in the relative importance of diet items, decreased dietary overlap and increased trophic position in L. sylvaticus throughout development. A. laterale became more enriched in δ¹³C and increased in δ¹⁵N throughout its larval period. We did not find a relationship between hydroperiod and niche parameters, indicating that niches are conserved across heterogeneous habitats. In contrast to most documented cases, we estimated low levels of individual specialisation in amphibian larvae. 4. Amphibians are an important link between aquatic and terrestrial ecosystems, whereby diet shifts can influence food‐web structure by altering energy flow pathways and the trophic position of higher consumers, ultimately changing food‐chain length.     

Effects of variable hydroperiods and water level fluctuations on denitrification capacity, nitrate removal, and benthic-microbial community structure in constructed wetlands

The hydrologic character of wetlands is one of the attributes by which they are defined. There are, however, conflicting reports about the detrimental versus beneficial responses of wetland systems to water level fluctuations and variable hydroperiods. We conducted water level and hydroperiod fluctuation studies in full-scale experimental wetlands in order to determine the effects of hydraulic operation on wetland performance (in terms of nutrient removal), and benthic-bacterial community function (in terms of denitrification potential, DNP) and structure (via terminal restriction fragment length polymorphisms, T-RFLP). In our comparison, detention time was the controlling factor for nitrate removal at the system level. However, widely fluctuating water levels and variable hydroperiods did not diminish either the nitrate removal capacity of the experimental wetlands, or the size or composition of benthic-bacterial communities relative to the more stable water level systems. Rather, significant differences in denitrification potential rates, bacterial cell densities, and benthic community structure were a function of sampling location within the experimental wetlands regardless of hydraulic operation. The results of this study support the need for reconsidering the hydraulic criteria for wetland delineation.     

Contribution of Do?ana Wetlands to Carbon Sequestration

Inland and transitional aquatic systems play an important role in global carbon (C) cycling. Yet, the C dynamics of wetlands and floodplains are poorly defined and field data is scarce. Air-water fluxes in the wetlands of Doñana Natural Area (SW Spain) were examined by measuring alkalinity, pH and other physiochemical parameters in a range of water bodies during 2010–2011. Areal fluxes were calculated and, using remote sensing, an estimate of the contribution of aquatic habitats to gaseous transport was derived. Semi-permanent ponds adjacent to the large Guadalquivir estuary acted as mild sinks, whilst temporal wetlands were strong sources of (−0.8 and 36.3 ). Fluxes in semi-permanent streams and ponds changed seasonally; acting as sources in spring-winter and mild sinks in autumn (16.7 and −1.2 ). Overall, Doñana''s water bodies were a net annual source of (5.2 ). Up–scaling clarified the overwhelming contribution of seasonal flooding and allochthonous organic matter inputs in determining regional air-water gaseous transport (13.1 ). Nevertheless, this estimate is about 6 times < local marsh net primary production, suggesting the system acts as an annual net sink. Initial indications suggest longer hydroperiods may favour autochthonous C capture by phytoplankton. Direct anthropogenic impacts have reduced the hydroperiod in Doñana and this maybe exacerbated by climate change (less rainfall and more evaporation), suggesting potential for the modification of C sequestration.     

Influences of riparian logging on plants and invertebrates in small,depressional wetlands of Georgia,U.S.A.

We studied 12 small, seasonally flooded, depressional wetlands on the Atlantic Coastal Plain of Georgia, U.S.A. Each wetland was embedded in stands of managed plantation pine. The pine trees surrounding each wetland had been harvested and replanted beginning in 1997 (2 sites), 1995 (2 sites), 1993 (1 site), 1988 (2 sites), 1984 (2 sites) or 1975 (3 sites). Regressions of various environmental variables with harvest histories indicated that those wetlands surrounded by smaller trees had greater light levels, water temperatures, pH, herbaceous plant cover and biomass, terrestrial invertebrate diversities and numbers, and water flea numbers, and lower water electrical conductivities and aquatic oligochaete numbers than those wetlands surrounded by more mature trees. Detected variations in hydroperiod, water depth, dissolved oxygen levels, sediment inputs, macrophyte diversity, periphyton biomass and densities of most aquatic invertebrates were not clearly correlated with past histories of peripheral tree harvest. This study suggests that harvesting trees around small wetlands initiates physical and ecological changes within the embedded habitats and that changes can persist for up to 15 years.     

Seasonal movements of crayfish in a fluctuating wetland: implications for restoring wading bird populations

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Impacts of hydroperiod on growth and survival of larval amphibians in temporary ponds of Central Pennsylvania,USA

The effects of variable hydroperiod (three levels) and initial density of amphibians (two levels) on survival, growth rate, and time to and mass at metamorphosis were studied for wood frogs (Rana sylvatica), Jefferson salamanders (Ambystoma jeffersonianum), and spotted salamanders (A. maculatum). Experiments were carried out in 260-1 mesocosms set up outdoors in a forest. These pond simulations were designed to mimic conditions that occur in palustrine temporary wetlands in central Pennsylvania. No animals reached metamorphosis in the short hydroperiod (56 days). However a greater proportion (66%) of tadpoles of R. sylvatica survived to the end of the 56-day, treatment than the 84- or 158-day treatments (29 and 14%, respectively), from which all survivors metamorphosed. In contrast, neither of the salamanders metamorphosed by 84 days; survival to metamorphosis at 158 days was 15% for A. jeffersonianum and 10% for A. maculatum. Average instantaneous growth rates for A. jeffersonianum decreased with each increase in hydroperiod. Growth of R. sylvatica was greater in the 56-day hydroperiod than in hydroperiods of 84 or 158 days. Initial amphibian density had no effect on growth or survival of any species. It appears that salamander larvae were predatory on tadpoles, since survival of R. sylvatica was negatively correlated with survival of A. jeffersonianum in 84-day treatments and with growth of A. maculatum in 158-day treatments.     

Microgeographic life history variation in an alpine caddisfly: plasticity in response to seasonal time constraints

1. Temporally constrained environments, such as habitats with short growth seasons or short hydroperiods, cause potentially strong selection on life histories. Depending on the predictability of these events and the extent of spatial and temporal heterogeneity, local populations could become adapted either via a fixed phenotype or via life history plasticity in response to these environmental cues. 2. We used a common garden experiment to investigate microgeographic variation in life history responses to combined changes in photoperiod (ambient/late) and hydroperiod (constant/drying) time constraint cues in an alpine caddisfly (Trichoptera). We compared six populations (three permanent/three temporary streams) originating from a small, alpine floodplain and which spanned an expected gradient in growth period duration (GPD) with distance from glaciers. 3. We made two main predictions in relation to locally varying selection pressures: (i) populations nearest glaciers (shorter GPD and strongest time constraints) should have the fastest development rates and (ii) populations from permanent streams should be less able to respond to drying hydroperiods than populations from temporary streams. 4. All populations and both sexes accelerated development in response to late season photoperiod cues. However, only permanent stream populations showed an increase in development time with increasing GPD, suggesting that other factors were influencing populations in temporary streams. 5. Permanent stream populations showed countergradient variation (genetic and environmental influences were in opposition) in development time, and under‐compensation of growth rates resulted in a converse Bergmann cline in body size (smaller body size along gradients of declining season length). The extent of plasticity in response to hydroperiod, and the combined effects of both time constraints, differed between populations and sexes, but were not consistent among populations. 6. Taken together, our results suggest adaptive plasticity in response to season length. The lack of a predictable pattern in response to hydroperiod may be due to gene flow or weak selection. We conclude that spatially structured populations can strongly differ in phenotypic plasticity even at microgeographic scales.     

Invertebrate community patterns in Mediterranean temporary wetlands along hydroperiod and salinity gradients

1. Temporary aquatic habitats often are inhabited by a unique fauna and flora and contribute significantly to regional diversity. Temporary wetlands around the world are disappearing rapidly. The individual and interacting impacts of factors influencing community structure and dynamics in temporary wetlands are not always well known.
2. Camargue wetlands are mainly characterized by variable salinity and hydroperiod. The individual and combined impacts of these local factors, together with regional variables, on invertebrate communities remain unknown. We therefore characterized and sampled invertebrates in 30 temporary wetlands along salinity and hydroperiod gradients in the Camargue (Southern France) 3, 5 and 7 months after inundation.
3. Over the three sampling occasions, a total of 17 cladoceran species and 49 macroinvertebrate taxa were identified. Hydroperiod and salinity were the most important variables explaining variation in taxonomic composition and can be considered key factors shaping the invertebrate communities in Camargue wetlands. The impact on taxon richness was significantly positive for hydroperiod but significantly negative for salinity. Regional factors had no significant effect on the structure of the studied invertebrate communities, suggesting that dispersal was not limiting and that species sorting was the most important structuring process.
4. The results of this study suggest that the combined and interacting effects of salinization and hydrological modification of Mediterranean temporary wetlands (due to water management, climate change, etc.) can result in reduced diversity in large numbers of Mediterranean wetlands and induce a considerable decline in regional diversity of aquatic invertebrates.     

Diapause in copepods (Crustacea) from ephemeral habitats with different hydroperiods in Everglades National Park (Florida,U.S.A.)

Water management practices in the Everglades have severely stressed the natural system, particularly by reducing the hydroperiods of much of the region. During the dry season of 1999, we investigated the influence of hydroperiod on the species composition and dormancy patterns of freshwater copepod communities in seasonal wetlands of Everglades National Park, Florida, U.S.A. The habitats were characterized by an annual dry season, from December through June. We sampled at two locations: the Long Pine Key area of the Rocky Glades region (short hydroperiod, ca. 4–5 months), and western Taylor Slough (intermediate hydroperiod, ca. 8–10 months). Both areas have experienced a reduction in natural hydroperiods and an increase in the frequency of dry-down. We collected weekly plankton samples from Rocky Glades solution holes to assess the potential species pool of copepods. To document the taxa capable of surviving dry-down by resting, we performed three immersion trials in which we rehydrated, in laboratory aquaria, sediment patches from solution holes and surface soils from all stations. Only a subset of the planktonic species collected emerged from the dried sediments. The cyclopoids Microcyclops rubellus and Paracyclops poppei were dominant. This is the first record of diapause for P. poppei. Species distributions from the different hydroperiod soil patches indicated that more diapausing species occurred at the sites that dried for shorter periods. Emerging individuals of M. rubellus and P. poppei were mainly ovigerous females, demonstrating a resting strategy seldom before recorded. The cyclopoid Diacyclops nearcticushad not been previously reported to diapause, but they emerged from the dried sediments in our trials. Our collections included six new records for Florida: Diacyclops nearcticus, Megacyclops latipes, Orthocyclops modestus, Elaphoidella marjoryae, Bryocamptus sp. and Bryocamptuscf. newyorkensis. Paracyclops poppei, Macrocyclops fuscus and Arctodiaptomus floridanus are new records for Everglades National Park. Clearly, diapause is an important strategy for the persistence of copepods in short-hydroperiod wetlands. The duration of the dry period appears to be inversely related to the number of species that emerge from diapause.     

Connecting carbon and nitrogen storage in rural wetland soil to groundwater abstraction for urban water supply

We investigated whether groundwater abstraction for urban water supply diminishes the storage of carbon (C), nitrogen (N), and organic matter in the soil of rural wetlands. Wetland soil organic matter (SOM) benefits air and water quality by sequestering large masses of C and N. Yet, the accumulation of wetland SOM depends on soil inundation, so we hypothesized that groundwater abstraction would diminish stocks of SOM, C, and N in wetland soils. Predictions of this hypothesis were tested in two types of subtropical, depressional‐basin wetland: forested swamps and herbaceous‐vegetation marshes. In west‐central Florida, >650 ML groundwater day^?1 are abstracted for use primarily in the Tampa Bay metropolis. At higher abstraction volumes, water tables were lower and wetlands had shorter hydroperiods (less time inundated). In turn, wetlands with shorter hydroperiods had 50–60% less SOM, C, and N per kg soil. In swamps, SOM loss caused soil bulk density to double, so areal soil C and N storage per m² through 30.5 cm depth was diminished by 25–30% in short‐hydroperiod swamps. In herbaceous‐vegetation marshes, short hydroperiods caused a sharper decline in N than in C. Soil organic matter, C, and N pools were not correlated with soil texture or with wetland draining‐reflooding frequency. Many years of shortened hydroperiod were probably required to diminish soil organic matter, C, and N pools by the magnitudes we observed. This diminution might have occurred decades ago, but could be maintained contemporarily by the failure each year of chronically drained soils to retain new organic matter inputs. In sum, our study attributes the contraction of hydroperiod and loss of soil organic matter, C, and N from rural wetlands to groundwater abstraction performed largely for urban water supply, revealing teleconnections between rural ecosystem change and urban resource demand.     

Assessing the net effect of anthropogenic disturbance on aquatic communities in wetlands: community structure relative to distance from canals

Anthropogenic alterations of natural hydrology are common in wetlands and often increase water permanence, converting ephemeral habitats into permanent ones. Since aquatic organisms segregate strongly along hydroperiod gradients, added water permanence caused by canals can dramatically change the structure of aquatic communities. We examined the impact of canals on the abundance and structure of wetland communities in South Florida, USA. We sampled fishes and macroinvertebrates from marsh transects originating at canals in the central and southern Everglades. Density of all aquatic organisms sampled increased in the immediate proximity of canals, but was accompanied by few compositional changes based on analysis of relative abundance. Large fish ( >8 cm), small fish ( <8 cm) and macroinvertebrates ( >5 mm) increased in density within 5 m of canals. This pattern was most pronounced in the dry season, suggesting that canals may serve as dry-down refugia. Increases in aquatic animal density closely matched gradients of phosphorus enrichment that decreased with distance from canals. Thus, the most apparent impact of canals on adjacent marsh communities was as conduits for nutrients that stimulated local productivity; any impact of their role as sources of increased sources of predators was not apparent. The effect of predation close to canals was overcompensated by increased secondary productivity and/or immigration toward areas adjacent to canals in the dry season. Alternatively, the consumptive effect of predatory fishes using canals as dry-season refuges is very small or spread over the expanse of marshes with open access to canals. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.