Hydrologic,vegetation, and substrate characteristics of floating marshes in sediment-rich wetlands of the Mississippi river delta plain,Louisiana, USA

Floating marshes occur over 70% of the western Terrebonne Basin, Louisiana, USA, freshwater coastal wetlands. They are of several types: A free-floating thick-mat (45–60 cm) marsh dominated by Panicum hemitomon and Sagittaria lancifolia; a thick mat marsh dominated by Panicum hemitomon and Sagittaria lancifolia that floats part of the year, but whose vertical floating range is damped compared to adjacent water; and an irregularly-floating thin mat (< 30 cm) dominated by Eleocharis spp. in the spring and Ludwigia leptocarpa and Bidens laevis in the summer and fall. Floating mats must be almost entirely organic in order to be buoyant enough to float. The western Terrebonne wetlands receive large winter/spring supplies of suspended sediments from the Atchafalaya River. Even though sediment concentrations in the adjacent bayou are as high as 100 mg l^–1, the Panicum hemitomon/Sagittaria lancifolia free-floating marsh probably receives no over-surface sediments since it floats continuously. The bulk density data of the damped-floating marsh, however, suggest some mineral sediment input, probably during winter when this marsh is submerged. These two types of floating marsh could not have developed in the present sediment regime of the Atchafalaya River, but as long as they remain floating can continue to exist. Thin floating mats are found in areas receiving the least sediment (<20 mg 1^–1 suspended sediment concentration in adjacent bayous). This low sediment environment probably made possible their formation within the past 20 years. They may represent a transitional stage in mat succession from (1) existing thick-mat floating marsh to a degrading floating marsh, or (2) a floating marsh developing in shallow open water.Corresponding editor: D. Whigham     

The impact of red mud on growth of wetland vegetation and substrate fertility

Greenhouse experiments were conducted to evaluate the impact of red mud, the residue of the Bayer process for extracting alumina from bauxite, on wetland to investigate whether red mud can be used for wetland restoration. Two wetland species,Spartina alterniflora Loisel. andSagittaria lancifolia L., were used to test their response to the following substrate treatments: 100% marsh sediment, 100% red mud, a mixture of 50% marsh sediment and 50% red mud, a mixture of 50% red mud and 50% compost, and neutralized red mud. Each substrate treatment received two fertilization levels, fertilized (N–P–K) and unfertilized. Red mud could support the growth of the salt marsh species,Spartina alterniflora, but not the fresh marsh species.Sagittaria lancifolia. The high Na content and salinity of red mud, even when mixed with marsh sediment or compost, appeared to be the primary cause for the high mortality ofSagittaria in these substrates. Fertilization did not reduce the stress response ofSagittaria to red mud, but significantly increased the growth ofSpartina. Red mud is low in available N and P and required fertilizer or organic matter (e.g., compost) addition to increase fertility. In experiments with both fresh and salt marsh sediments, the concentrations of soluble heavy metals, with the exception of Mn, were not significantly greater for red mud than for marsh sediment. Thus, the release of heavy metals from red mud over the short-term may not pose an environmental concern. However, the mobilization of heavy metals over the long-term and the influence of plants in accumulating metals requires further investigation.     

Restoration of floodplain wetlands: Opening polders along a coastal river in Mediterranean France, Vistre marshes

Restoration of wetlands has become an increasing field of application of ecological research due to mitigation regulations, changes in agricultural practices and an increasing consideration of the role of wetlands in the water cycle. In areas where the history of human use of natural areas is old and intense, restoration projects must not only consider ecological objectives but also social aspects. The Vistre project was developed to answer a social demand of restoring the flood storage function of a riverine wetland, formerly drained and polderized for agriculture. The river is located in the Petite Camargue, southern France, and flows into the sea a few km downstream of the study site. Openings in the dykes, calculated after a preliminary study, partly restored the connection between the polder and the river basin. A monitoring program of flora and fauna was launched to test the hypothesis that the change in hydrological functioning would be sufficient to obtain the desired vegetation and fauna. During the first years of the project, high rainfall and uncontrolled openings of sluices due to difficulties with the local users caused abnormally high water levels. The vegetation changed to hydrophyte-dominated communities and was controlled mainly by the fluctuations in water level. The habitat objective for fish-eating birds was met and a large tree-nesting heron colony established. Solving the social problem and maintenance of the sills should allow most objectives to be reached, although more slowly than expected.     

The effect of environmental factors on floating fresh marsh end-of-season biomass

We used both stepwise and quantile regression to determine the sources of environmental variation that explained the observed inter-annual variation in end-of-season freshwater floating marsh aboveground biomass over an 18-year period. The vegetation at our study site had high species diversity with an average of 20 species recorded from 10 0.25 m⁻² plots. However, Panicum hemitomon was clearly the dominant contributing 74% of the total biomass. Only three other species (Solidago sempervirens, Vigna luteola, and Thelypteris palustris) were so common that they were sampled in all years. We expected that the most important factors controlling interannual variation in aboveground biomass are temperature and nitrogen availability. We also expected that nitrogen availability to the plants is affected by water movement through and under the mat driven by precipitation (lower N), evaporation (transportation of higher N waters to roots), and local runoff (higher N). Stepwise regression analysis indicated that P. hemitomon average biomass was negatively related to average water level and positively related to maximum water level and had a curvilinear response with TKN. Using quantile regression the best fit for P. hemitomon maximum-biomass with two parameters was obtained using hot days (positive relationship) and maximum water level (negative relationship). Both analytical methods showed maximum water level (negative relationship) and cold front passage (positive relationship) to be the environmental parameters that best explained interannual variation in S. sempervirens biomass. V. luteola biomass was positively related to temperature. Stepwise regression added chloride concentration as an additional positive parameter explaining V. luteola biomass, while quantile regression identified nitrogen as an important positive parameter. Both analytical methods identified pH, TKN, and water level as environmental parameters that were negatively correlated with T. palustris biomass. The overall negative effect of water level on all species was unexpected in this floating mat system. We initially assumed that higher water levels were due to higher runoff which should have a positive effect on biomass. However, higher water levels may also be related to a higher retention time in this fresh-water tidal system, which decreases water exchange and nutrient replenishment.     

Effect of experimental phosphorus enrichment on oligotrophic tropical marshes in Belize,Central America

Sedges from genus Eleocharisdominate extensive wetlands in the sugar cane growing areas of the Caribbean. Correlative data suggest that macrophytes in these wetlands are phosphorus limited. To determine effects of increased P input that can be expected, e.g. from agricultural runoff, a common sugar cane fertilizer was applied to representative plots in four marl-based and four peat-based marshes. The plots were located in the proximity of patches of Typha domingensis, which has been reported to be able to outcompete Eleocharis under nutrient rich conditions. Responses to the fertilizer treatment were documented as changes in: Plant height, density, biomass, net primary production, nutrient resorption, decomposition, plant and soil nutrient concentrations, percent cover of cyanobacterial mats, and potential colonization by Typha. Additions of phosphorus significantly increased plant density and height and, consequently, the aboveground net primary production. Phosphorus resorption efficiency following senescence was independent of fertilizer addition in Eleocharis but decreased in Typha from the fertilized plots. Phosphorus resorption proficiency was lower in fertilized plots for both Typha and Eleocharis. Decomposition of litter and cellulose assays was significantly faster in fertilized plots. No spontaneous establishment of Typha occurred in the fertilized plots, but survival of transplanted Typha was higher in fertilized plots than in controls. Increased plant density in fertilized plots led to elimination of a key component of these ecosystems, the nitrogen fixing cyanobacterial mats.     

Biogeochemical response of organic-rich freshwater marshes in the Louisiana delta plain to chronic river water influx

To help evaluate effects of Mississippi River inputs to sustainability of coastal Louisiana ecosystems, we compared porewater and substrate quality of organic-rich Panicum hemitomon freshwater marshes inundated by river water annually for more than 30 years (Penchant basin, PB) or not during the same time (Barataria basin, BB). In the marshes receiving river water the soil environment was more reduced, the organic substrate was more decomposed and accumulated more sulfur. The porewater dissolved ammonium and orthophosphate concentrations were an order of magnitude higher and sulfide and alkalinity concentrations were more than twice as high in PB compared with BB marshes. The pH was higher and dissolved iron concentrations were more than an order of magnitude lower in PB marshes than in BB marshes. The influx of nutrient-rich river water did not enhance end-of-year above-ground standing biomass or vertical accretion rates of the shallow substrate. The differences in porewater chemistry and substrate quality are reasonably linked to the long-term influx of river water through biogeochemical processes and transformations involving alkalinity, nitrate and sulfate. The key factor is the continual replenishment of alkalinity, nitrate and sulfate via overland flow during high river stage each year for several weeks to more than 6 months. This leads to a reducing soil environment, pooling of the phytotoxin sulfide and inorganic nutrients in porewater, and internally generated alkalinity. Organic matter decomposition is enhanced under these conditions and root mats degraded. The more decomposed root mat makes these marshes more susceptible to erosion during infrequent high-energy events (for example hurricanes) and regular low-energy events, such as tides and the passage of weather fronts. Our findings were unexpected and, if generally applicable, suggest that river diversions may not be the beneficial mitigating agent of wetland restoration and conservation that they are anticipated to be.     

Characterization of the indigenous PAH-degrading bacteria of <Emphasis Type="Italic">Spartina</Emphasis> dominated salt marshes in the New York/New Jersey Harbor

The aerobic polyaromatic hydrocarbon (PAH) degrading microbial communities of two petroleum-impacted Spartina-dominated salt marshes in the New York/New Jersey Harbor were examined using a combination of microbiological, molecular and chemical techniques. Microbial isolation studies resulted in the identification of 48 aromatic hydrocarbon-degrading bacterial strains from both vegetated and non-vegetated marsh sediments. The majority of the isolates were from the genera Paenibacillus and Pseudomonas. Radiotracer studies using ¹⁴C-phenanthrene and ¹⁴C-pyrene were used to measure the PAH-mineralization activity in salt marsh sediments. The results suggested a trend towards increased PAH mineralization in vegetated sediments relative to non-vegetated sediments. This trend was supported by the enumeration of PAH-degrading bacteria in non-vegetated and vegetated sediment using a Most Probable Numbers (MPN) technique, which demonstrated that PAH-degrading bacteria existed in non-vegetated and vegetated sediments at levels ranging from 10² to 10⁵ cells/g sediment respectively. No difference between microbial communities present in vegetated versus non-vegetated sediments was found using terminal restriction fragment length polymorphism (of the 16S rRNA gene) or phospholipid fatty acid analysis. These studies provide information on the specific members and activity of the PAH-degrading aerobic bacterial communities present in Spartina-dominated salt marshes in the New York/New Jersey Harbor estuary.     

Comparison of created and natural freshwater emergent wetlands in Connecticut (USA)

Five three- to four-year old created palustrine/emergent wetland sites were compared with five nearby natural wetlands of comparable size and type. Hydrologic, soil and vegetation data were compiled over a nearly two-year period (1988-90). Created sites, which were located along major highways, exhibited more open water, greater water depth, and greater fluctuation in water depth than natural wetlands. Typical wetland soils exhibiting mottling and organic accumulation were wanting in created sites as compared with natural sites. Typha latifolia (common cattail) was the characteristic emergent vegetation at created sites, whereas a more diverse mosaic of emergent wetland species was often associated with Typha at the natural sites. Species richness was slightly higher in created (22–45) vs. natural (20–39) wetlands, but the mean difference (33 vs. 30) was not significant. Nearly half (44%) of the 54 wetland taxa found at the various study sites were more frequently recorded at created than natural wetlands. The presence of mycorrhizae in roots of Typha angustifolia (narrow-leaved cattail) and Phragmites australis (common reed) was greater at created than natural wetlands, which may be related to differential nutrient availability. Wildlife use at all sites ranged from occasional to rare, with more sightings of different species in the natural (39) than created (29) wetlands. The presence of P. australis and introduced Lythrum salicaria (purple loosestrife) may pose a threat to future species richness at the created sites. One created site has permanent flow-through hydrology, and its vegetation and wildlife somewhat mimic a natural wetland; however, the presence of P. australis and its potential spread pose an uncertain future for this site. This study suggests the possibility of creating small palustrine/emergent wetlands having certain functions associated with natural wetlands, such as flood water storage, sediment accretion and wildlife habitat. It is premature to evaluate fully the outcome of these wetland creation efforts. A decade or more is needed, emphasizing the importance of long term monitoring and the need to establish demonstration areas.     

δC, δN, carbon, nitrogen and phosphorus as indicators of plant ecophysiology and organic matter pathways in Everglades deep slough, Florida, USA

Historically, the Florida Everglades was characterized by a corrugated landscape of shorter hydroperiod, elevated sawgrass (Cladium jamaicense) ridges and longer hydroperiod, deep water slough communities. Drainage and compartmentalization of the Everglades have fundamentally altered this pattern, and sawgrass ridge communities have expanded at the expense of deep water slough communities throughout much of the landscape. In this study we provide a simple isotopic and nutrient characterization of major components of the slough ecosystem to elucidate physiological and nutrient differences among species and to suggest pathways for organic matter decomposition that contribute to peat development in deep water sloughs. We examined carbon (C) and nitrogen (N) isotopes and C, N and phosphorus (P) concentrations of the floating-leaved macrophytes Nymphaea odorata and Nymphoides aquatica, the emergent macrophyte Eleocharis elongata, and the submerged species Utricularia foliosa and Utricularia purpurea, as well as soil and flocculent material from the southern Water Conservation Area 3-A. Flocculent material and soils had the highest N content (4.5 ± 0.2%) and U. foliosa and N. odorata had the highest P content (0.13 ± 0.01% to 0.12 ± 0.01%). The range for δ¹⁵N average ± SE values was 5.81 ± 0.29‰ (U. foliosa) to −1.84 ± 0.63‰ (N. odorata), while the range for δ¹³C values was −23.83 ± 0.12‰ (N. odorata) to −29.28 ± 0.34‰ (U. purpurea). Differences of up to 10‰ in C isotopic values of U. foliosa and N. odorata suggest fundamental physiological differences between these species. Along a degradation continuum, enrichment of ¹³C and ¹⁵N and extent of decomposition was negatively related to phosphorus concentrations. A two end-member ¹³C mixing model suggested that Utricularia species were the primary organic source for flocculent materials, whereas organic matter derived from root decomposition of N. odorata contributed to the progressively enriched δ¹³C values found with depth in soils. These results illustrate the fundamentally important roles of Nymphaea and Utricularia species in ecosystem dynamics of deep water sloughs.     

Vegetation death and rapid loss of surface elevation in two contrasting Mississippi delta salt marshes: The role of sedimentation, autocompaction and sea-level rise

From 1990 to 2004, we carried out a study on accretionary dynamics and wetland loss in salt marshes surrounding two small ponds in the Mississippi delta; Old Oyster Bayou (OB), a sediment-rich area near the mouth of the Atchafalaya River and Bayou Chitigue (BC), a sediment-poor area about 70 km to the east. The OB site was stable, while most of the marsh at BC disappeared within a few years. Measurements were made of short-term sedimentation, vertical accretion, change in marsh surface elevation, pond wave activity, and marsh soil characteristics. The OB marsh was about 10 cm higher than BC; the extremes of the elevation range for Spartina alterniflora in Louisiana. Vertical accretion and short-term sedimentation were about twice as high at BC than at OB, but the OB marsh captured nearly all sediments deposited, while the BC marsh captured <30%. The OB and BC sites flooded about 15% and 85% of the time, respectively. Marsh loss at BC was not due to wave erosion. The mineral content of deposited sediments was higher at OB. Exposure and desiccation of the marsh surface at OB increased the efficiency that deposited sediments were incorporated into the marsh soil, and displaced the marsh surface upward by biological processes like root growth, while also reducing shallow compaction. Once vegetation dies, there is a loss of soil volume due to loss of root turgor and oxidation of root organic matter, which leads to elevation collapse. Revegetation cannot occur because of the low elevation and weak soil strength. The changes in elevation at both marsh sites are punctuated, occurring in steps that can either increase or decrease elevation. When a marsh is low as at BC, a step down can result in an irreversible change. At this point, the option is not restoration but creating a new marsh with massive sediment input either from the river or via dredging.     

Phenology ofLemanea fucina (Rhodophyta) in a Rhode Island river,USA

In a fourth-order river in Rhode Island, USA, cover, frequency of occurrence, length and node number of the red algaLemanea fucina were greatest in July and August, at which time current velocity and water depth were minimum and temperature was maximum. Population abundance and plant size were lowest in fall-early winter. The length of reproductive region and carpospore frequency were also highest in June to August, whereas percent plants with open tips was least in July and highest in February. It appears that growth and reproduction are confined to a period from April to August, after which there is thallus deterioration and carpospore release; between September and March remnants of this population remain.Lemanea plants are epiphyte-free from September to January. Subsequently, they are colonized by caddisflies and then the red algaAudouinella violacea; by August, 100% of theLemanea plants are covered byAudouinella.     

Mayflies,stoneflies, and caddisflies of streams and marshes of Indiana Dunes National Lakeshore,USA

United States National Parks have protected natural communities for one hundred years. Indiana Dunes National Lakeshore (INDU) is a park unit along the southern boundary of Lake Michigan in Indiana, USA. An inventory of 19 sites, consisting of a seep, 12 streams, four marshes, a bog, and a fen were examined for mayflies (Ephemeroptera), stoneflies (Plecoptera), and caddisflies (Trichoptera) (EPT taxa). Volunteers and authors collect 35 ultraviolet light traps during summer 2013 and supplementary benthic and adult sampling added species not attracted by lights or that were only present in colder months. Seventy-eight EPT species were recovered: 12 mayflies, two stoneflies, and 64 caddisflies. The EPT richness found at INDU was a low proportion of the number of species known from Indiana: caddisflies contributed only 32.7% of known state fauna, mayflies and stoneflies contributed 8.4% and 2.3%, respectively. Site EPT richness ranged from one for a seep to 34 for an 8 m-wide stream. Richness in streams generally increased with stream size. Seven new state records and rare species are reported. The number of EPT species at INDU is slightly larger than that found at Isle Royale National Park in 2013, and the community composition and evenness between orders were different.     

Primary producers and nutrient loading in Silver Springs,FL, USA

The characteristics and dynamics of primary producer communities of Silver Springs was examined to compare with that observed by Odum [Odum, H.T., 1957. Trophic structure and productivity of Silver Springs, Florida. Ecol. Monogr. 27, 55–112.] as a means of evaluating the impacts of changes that have occurred over time. The Silver Springs ecosystem is considered an ecosystem at risk, where nitrate levels have more than doubled over the past 50 years. The spatial and temporal abundance and distribution of above-sediment primary producers in Silver Springs, FL, USA, was estimated on a system-wide basis using a GIS platform. The results of study suggest that while the Sagittaria component of Silver Springs has remained relatively stable, epiphyte and benthic algal mat community biomass has expanded, particularly benthic forms, like Lyngbya. However, we argue for caution in weighing the significance of long-term comparisons of system-wide biomass in light of considerable spatial heterogeneity in aquatic primary producer communities.     

Development of floating rafts after the rewetting of cut-over bogs: the importance of peat quality

The usual method of restoring cut-over bogs is to rewet the peat surface, but this often leads to the remaining peat layers being deeply inundated. For Sphagnum-dominated vegetation to develop at deeply inundated locations, it is important for floating rafts of buoyant residual peat to develop. In this study, the chemical and physical characteristics of buoyant and inundated peat collected from rewetted cut-over bog were compared. In general, buoyant peat was poorly humified; high methane (CH₄) production rates (2 µmol g ^–1 DW day ^–1) were important to ensure buoyancy. Although the peat water CH₄ concentrations increased with depth, the CH₄ production rates were higher in the uppermost peat layers. High CH₄ production rates were related positively with P concentrations and negatively with lignin concentrations. The pH to bulk density ratio (0.05) also appeared to be a good indicator of CH₄ production rates, providing an easy and cheap way to measure the variable for restoration practitioners. Our results indicated that analysing certain simple characteristics of the residual peat can greatly improve the success of the rewetting measures taken in cut-over bogs. If the analysis reveals that the residual peat is unsuitable for floating raft formation, deep inundation is inappropriate unless suitable peat from other locations can be introduced.     

Temporal development of salt marsh value for nekton and epifauna: utilization of dredged material marshes in Galveston Bay,Texas, USA

Densities of nekton and other fauna were measured inthree created salt marshes to examine habitatdevelopment rate. All three marshes were located onPelican Spit in Galveston Bay, Texas, USA and werecreated on dredged material from the Gulf IntracoastalWaterway. The youngest marsh was planted on 1-mcenters in July of 1992. At the time sampling wasinitiated in fall 1992, the marshes were 9, 5, andless than 1 year in age; sampling continued in thefall and spring through spring 1994. Animaldensities were measured within the vegetation at twoelevations using an enclosure sampler. In the fall of1992, 4 months following the planting of the 92Marsh,densities of most marsh organisms were lower in thismarsh compared with the older two marshes. Significantly lower densities were observed fordominant crustaceans (including three species of grassshrimps, two species of commercially-important penaeidshrimps, thinstripe hermit crabs Clibanarius vittatus,and juvenile blue crabs Callinectes sapidus), adominant fish (Gobionellus boleosoma), and thedominant mollusc (Littoraria irrorata). By the fallof 1993, however, densities of most nekton specieswere similar among the three created salt marshes. Incontrast, reduced densities of less mobile epifauna(C. vittatusand L. irrorata) persisted in the 92Marshthroughout the 2 years of sampling. The patterns ofnekton utilization exhibited in these marshes suggestthat the 92Marsh reached its maximum habitat supportfunction for these animals in less than 1 year. Comparisons of the older marshes with natural marshesin the bay system, however, suggest that all three ofthese created marshes are functioning at lower levelsthan natural marshes in terms of supporting productionof commercially important fishery species such aspenaeid shrimps and C. sapidus.     

Differential growth of crayfish Procambarus alleni in relation to hydrological conditions in marl prairie wetlands of Everglades National Park, USA

Environmental conditions influence crustacean growth by affecting molt intervals and incremental increases in length and weight. In the seasonally-flooded marl prairie wetlands of eastern Everglades National Park, U.S.A., hydropattern exerts considerable influence on aquatic primary productivity, and so may influence the availability of food resources for higher trophic levels. The seasonal hydroperiod has been drastically altered by anthropogenic factors, but the impacts on the aquatic community are not well known. We studied whether differences in growth of crayfish Procambarus alleni could be detected in habitats with different hydroperiods. We first described growth patterns based on incremental increases in length and weight of crayfish on a high protein diet in the laboratory. Regression analyses indicated that growth patterns in males and females were similar. Although the intermolt period increased with age, the proportional increases in length and weight were similar through successive molts. The relationship between length and weight of crayfish was best described by a power equation for allometric growth. We then compared growth curves for crayfish subpopulations from different areas of the marl prairie. In habitats with the longest hydroperiods, crayfish weight-at-size was not significantly different from that in laboratory crayfish on the high protein diet. However, weight gain per unit increase in length in short hydroperiod sites was significantly less than in long hydroperiod sites or in the laboratory. These results indicate that crayfish productivity may be associated with hydroperiod in these stressed wetlands, and this may contribute to observed source-sink population regulation.     

Mapping multiple disease resistance genes using a barley mapping population evaluated in Peru, Mexico, and the USA

We used a well-characterized barley mapping population (BCD 47 × Baronesse) to determine if barley stripe rust (BSR) resistance quantitative trait loci (QTL) mapped in Mexico and the USA were effective against a reported new race in Peru. Essentially the same resistance QTL were detected using data from each of the three environments, indicating that these resistance alleles are effective against the spectrum of naturally occurring races at these sites. In addition to the mapping population, we evaluated a germplasm array consisting of lines with different numbers of mapped BSR resistance alleles. A higher BSR disease severity on CI10587, which has a single qualitative resistance gene, in Peru versus Mexico suggests there are differences in pathogen virulence between the two locations. Confirmation of a new race in Peru will require characterization using a standard set of differentials, an experiment that is underway. The highest levels of resistance in Peru were observed when the qualitative resistance gene was pyramided with quantitative resistance alleles. We also used the mapping population to locate QTL conferring resistance to barley leaf rust and barley powdery mildew. For mildew, we identified resistance QTL under field conditions in Peru that are distinct from the Mla resistance that we mapped using specific isolates under controlled conditions. These results demonstrate the long-term utility of a reference mapping population and a well-characterized germplasm array for locating and validating genes conferring quantitative and qualitative resistance to multiple pathogens.     

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.     

Decline in zoobenthos densities in the profundal sediments of Lake Mendota (Wisconsin,USA)

High densities of zoobenthos inhabited Lake Mendota's profundal zone in the early 1900s through the mid-1940s. Chaoborus punctipennis was the most abundant organism during the winter, along with moderate densities of Chironomus spp., Pisidium sp., oligochaetes, and Procladius sp. By the early 1950s, Chaoborus punctipennis densities had declined to 10% of former levels, while Chironomus increased significantly. However, by the mid-1960s, Chaoborus, Chironomus, and Pisidium densities had decreased to very low population levels. By 1987–89, Pisidium was no longer found. Zoobenthos that had not decreased from earlier surveys were oligochaetes and Procladius, although further sampling of oligochaetes is needed to confirm current densities. These organisms are the most tolerant of severe anoxia.Four possible reasons for this decline were evaluated: (a) decline in food availability, (b) increase in fish predation, (c) use of toxic insecticides in the drainage basin, and (d) changes in the profundal sediment environment. Based on literature information and long-term data for Lake Mendota, a change in the profundal sediment environment is the most likely explanation for the decline in the less-tolerant zoobenthos species. Although the duration and extent of anoxia in the hypolimnion have not changed since the early 1900s, hypolimnetic ammonia and hydrogen sulfide concentrations apparently have increased as Mendota became more eutrophic after the mid-1940s. However, further study is needed to determine if these higher concentrations or other factors were responsible for the dramatic decline in lake Mendota's profundal zoobenthos.     

The influence of vegetation on erosion and accretion in salt marshes of the Oosterschelde,The Netherlands

The total root strength of two plant species (Spartina anglica and Limonium vulgare) is related to salt marsh cliff erosion in the Krabbenkreek (Oosterschelde). A ranking order in cliff stability is predicted on the basis of these root strength calculations. It turns out that the S. anglica root system is more effective in reducing lateral cliff erosion than the root system of L. vulgare. Also the establishment of S. anglica by germination of seeds is studied in relation to the erosion/deposition rates at an accretion site in the Krabbenkreek. The percentage of seeds washed away depends on the mobility of the superficial sediment which increases with decreasing height above N.A.P. (Dutch Ordnance Level). Above 0.90 m + N.A.P. a germination of 20% is measured, but seedlings survive the winter period only in the zone where a patchy vegetation already exists. It is concluded that generative spread of S. anglica in the Krabbenkreek is not very likely under the present hydrodynamic conditions.Nomenclature follows Heukels & van Ooststroom (1977).Acknowledgement: The research presented herein was carried out at the Delta Department, Environmental Division of the Ministry of Transport and Public Works, Middelburg. This work forms part of the research on salt-marsh ecosystems of the Oosterschelde. Assistance with sowing and sampling was given by the Field Survey section of the Environmental Division. Analyses of root weight and root diameter were carried out by the Institute of Soil Fertility in Haren by order of the Environmental Division. I gratefully acknowledge Ir A. de Jager and J. Floris of this institute for putting a special sampling auger at my disposal. G. den Hartog, P. van Vessem and Drs P. M. Schoot of the State University of Utrecht are thanked for their assistance during the study. Special thanks are due to Drs J. M. Roels for this critical editorial review of this paper.