Relationships of nitrogen loadings,residential development,and physical characteristics with plant structure in New England salt marshes

We examined the vascular plant species richness and the extent, density, and height ofSpartina species of ten Narragansett Bay, Rhode Island (United States) fringe salt marshes which had a wide range of residential land development and N-loadings associated with their watersheds. Significant inverse relationships of tallS. alterniflora with species richness and with the extent and density ofS. patens and shortS. alterniflora were observed. Extent and density ofS. patens and extent of shortS. alterniflora were positively and significantly related with plant species richness. Marsh elevation and area did not significantly correlate with plant structure. Flood tide height significantly and inversely correlated withS. patens, but did not significantly relate toS. alterniflora or plant species richness. Marsh width significantly and positively correlated with plant species richness andS. patens and inversely correlated with tallS. alterniflora. Significant inverse relationships were observed for N-load, % residential development, and slope withS. patens, shortS. alterniflora, and species richness, and significant positive relationships with tallS. alterniflora. The marsh slope and width were significantly correlated with N-load and residential development that made it difficult to determine to what extent anthropogenic stressors were contributing to the variation in the plant structure among the marshes. At five marhes with similar slopes, there were significant inverse relationships of N-load withS. patens (density and extent) and a positive relationship with tallS. alterniflora (extent). Although there were no significant relationships of slope with the plant metrics among the five sites, other physical factors, such as the flood tide height and marsh width, significantly correlated with the extent and density ofSpartina species. Significant relationships of N-load with plant structure (albeit confounded by the effect of the physical characteristics) support the hypothesis of competitive displacement of dominant marsh plants under elevated nitrogen. It is likely that the varying plant structure in New England marshes is a response to a combination of natural factors and multiple anthropogenic stressors (e.g., eutrophication and sea level rise).     

Exchange of microbial biomass between aSpartina alterniflora marsh and the adjacent tidal creek

Salt marsh systems of the southeastern United States are characterized by extensive stands ofSpartina alterniflora. These marshes may influence the concentrations of material suspended in flooding and receding tidal waters. The ability of aSpartina alterniflora-dominated marsh to influence the concentration of suspended microbial biomass was investigated through the use of a 142-m long flume. The flume extended through stands of tall-, medium-, and short-heightSpartina. Water passing through the tallSpartina lost a considerable portion of microbial biomass. Initial samples from medium-heightSpartina were collected from water that had already passed through the tall grass. These samples contained 20 to 70% less microbial biomass than did water entering the tallSpartina. Calculations of mass transport suggest that the tallSpartina zone of marsh acts as a sink for microbial biomass while the short-heightSpartina tends to export biomass (to the tallSpartina zone). The marsh as a whole acts as a sink for microbial biomass. Transport estimates from 32 individual tide cycles were modeled to obtain an annual estimate of transport. As a consequence of high variability among individual transport estimates, no annual transport estimate could be distingushed from a net-zero transport.     

The influence of morphology in determining the decomposition of two salt marsh macrophytes

Annual decomposition rates of Spartina alterniflora height forms and Juncus roemerianus were determined in situ in three North Carolina salt marshes using the litter bag method. The decomposition of Spartina was significantly influenced by size, i.e., height form, with the taller plants which had greater amounts of stem tissue, being more resistant to decay. Instantaneous decay rates for short and medium Spartina were not significantly different at any site, but they were both significantly greater than that of the tall form at two of the three study sites. Juncus decomposed more slowly than Spartina during the first 8 months, but had decomposed as completely as all three height forms of Spartina at two of the study sites by the end of the 13-month study period. Constant submergence appeared to inhibit decomposition since there was twice as much undecomposed plant material remaining in bags placed in tidal creeks as in those on the marsh surface.     

Dispersion of the salt-marsh periwinkle Littoraria irrorata: Effects of water level,size, and season

This paper documents horizontal and vertical dispersion patterns of a Texas population of the saltmarsh periwinkle, Littoraria irrorata, over a 15-month period. The study was conducted within a tidal marsh on the Anahuac National Wildlife Refuge in Galveston Bay. Two mark-recapture experiments demonstrated that L. irrorata rarely move more than 2 m from their release point over long periods of time and do not home to individual Spartina plants. Adult L. irrorata forage farther away from the base of Spartina stalks at low tide than do juvenile snails. Remaining near the plant base may decrease both temperature and desiccation stress on juveniles. During warm months, L. irrorata climb grass stalks with tidal inundation and forage on the substratum at low tide. Snails are inactive and aggregate in detrital debris at the base of Spartina clumps during the winter. *** DIRECT SUPPORT *** A01BY058 00016     

The effect of sewage sludge on salt-marsh denitrifying bacteria

Sewage sludge was applied biweekly to the soil surface of a shortSpartina alterniflora marsh in order to evaluate the marsh’s ability to assimilate the sludge nitrogen. After nine months there was a significant decrease of the denitrification potential in the first 15cm of the soil profile. In laboratory experiments the sludge was shown to have an immediate inhibitory effect on potential soil denitrification rates indicating that the decreasedin situ potential was probably a result of direct toxicity of the sludge on this bacterial respiratory process.     

Impact of Fertilization on a Salt Marsh Food Web in Georgia

We examined the response of a salt marsh food web to increases in nutrients at 19 coastal sites in Georgia. Fertilization increased the nitrogen content of the two dominant plants, Spartina alterniflora and Juncus roemerianus, indicating that added nutrients were available to and taken up by both species. Fertilization increased Spartina cover, height, and biomass and Juncus height, but led to decreases in Juncus cover and biomass. Fertilization increased abundances of herbivores (grasshoppers) and herbivore damage, but had little effect on decomposers (fungi), and no effect on detritivores (snails). In the laboratory, herbivores and detritivores did not show a feeding preference for fertilized versus control plants of either species, nor did detritivores grow more rapidly on fertilized versus control plants, suggesting that changes in herbivore abundance in the field were driven more by plant size or appearance than by plant nutritional quality. Community patterns in control plots varied predictably among sites (i.e., 17 of 20 regression models examining variation in biological variables across sites were significant), but variation in the effects of fertilization across sites could not be easily predicted (i.e., only 6 of 20 models were significant). Natural variation among sites was typically similar or greater than impacts of fertilization when both were assessed using the coefficient of variation. Overall, these results suggest that eutrophication of salt marshes is likely to have stronger impacts on plants and herbivores than on decomposers and detritivores, and that impacts at any particular site might be hard to distinguish from natural variation among sites.     

Comparison of production computation methods in a Southeastern North CarolinaSpartina alterniflora salt marsh

Tagging studies ofSpartina alterniflora Loisel showed no significant differences in stem longevity of short, medium, and tall height forms. Mean stem longevity was 7.9 months, and the experimental turnover rate was 1.5 crops per yr. Five methods to measure productivity (peak standing crop, Milner and Hughes, Smalley, Wiegert and Evans, and Lomnicki, et al.) yielded annual net aerial primary production (NAPP) estimates ranging from 214 to 1,038 g dry wt per m² per yr in a stand of shortSpartina. Turnover rates were computed for each of the methods by dividing the respective production value by the peak standing crop (242 g dry wt per m² per yr). Each computed turnover rate was compared with the experimental value of 1.5 crops per yr to ultimately determine that the methods of peak standing crop, Milner and Hughes, and Smalley were underestimates and that the Wiegert and Evans method was an overestimate of NAPP in tidal marsh systems. Based on its calculated turnover rate of 1.9 crops per yr, a modified Lomnicki, et al. method provided the best NAPP estimate (454 g dry wt per m² per yr).     

Site conditions, not parental phenotype, determine the height ofSpartina foliosa

The effect of parental growth form on ramets introduced to new sites has broad implications for ecological restoration. We asked whether transplants ofSpartina foliosa need to come from tall parents in order to produce tall stands for nesting by the light-footed clapper rail, one of southern California’s many endangered, salt-marsh-dependent species. Tall and shortS. foliosa transplants, collected from local salt marshes, were grown in a common garden and in microcosms, with different results. In the common garden, which had hypersaline soil, offspring of tall versus short clones did not differ, but they did respond to soil variations among blocks. In blocks with less sandy soil, stems were taller and up to 5 times more numerous, and patch sizes were up to 3 times larger. After two years in low-salinity microcosms, transplants receiving nitrogen additions produced more stems and over twice the total stem length of controls, regardless of parental height form. A slight parental effect was seen in this benign environment; the maximum height of tall-form transplants was 15 cm taller than that of short-form transplants, but the effect did not persist through year 2. Results indicate that both height forms ofS. foliosa grew better (taller and more dense) with less environmental stress (lower salinity, more nutrients). We conclude that environmental differences are more important than parental height form in determiningS. foliosa growth. This means that, for restoration purposes,S. foliosa ramets can and should be collected from short clones, rather than from existing, prime nesting habitat.     

Productivity of cordgrass,Spartina alterniflora,estimated from live standing crops,mortality, and leaf shedding in a Virginia salt marsh

Net annual productivity of tall and medium form cordgrass,Spartina alterniflora, was estimated by a new clip sampling method in a sloping foreshore salt marsh at Wallops Island, Virginia. This method measured live standing crops only, to avoid problems of measuring dead biomass inherent in other methods. Losses from live standing crops by shoot mortality and by leaf shedding were estimated from these measurements and added separately to production of live tillers and of live culms. This allowed quantification of various components of production.Spartina tillering in different zones of the marsh produced 62 to 211 g dry weight per m² per yr. Tiller mortality removed 37 to 106 g per m² per yr from live standing crops. Culms produced 348 to 1,132 g per m² before flowering and die-back. Culm mortality removed 28 to 246 g per m² before flowering. Leaf shedding removed an additional 83 g per m² in tall formSpartina. Altogether, net annual productivity These estimates are much higher than previous estimates of productivity and standing crops inSpartina marshes nearby.     

Relative effects ofLittoraria irrorata andProkelisia marginata onSpartina alterniflora

Spartina alterniflora salt marshes along the southeastern United States are some of the most productive and well studied ecosystems in the world. The role of physicochemical forces in regulatingSpartina growth is well understood, while the importance of grazers remains less clear. Recent studies have shown that the abundant marsh periwinkle,Littoraria irrorata, can exert strong control overSpartina through its grazing activities, but relatively little is known about its relative effects in comparison to other marsh plant consumers. To test the relative importance of snail and insect consumers onSpartina biomass, we conducted a 7-mo field experiment testing top-down regulation ofSpartina with all combinations ofL. irrorata (removed, control, c. 215 periwinkles m⁻²) andSpartina planthopper,Prokelisia marginata (removed, control). Snail removal resulted in a 50% increase inSpartina biomass while removal of planthoppers had no detectable effect. Planthopper density also increased by 50% when snails were excluded. In this South Carolina marsh,L. irrorata exerts a stronger top-down control ofSpartina thanP. marginata. These results indicate trophic cascade regulation ofSpartina salt marsh is more likely to occur through the predator(s)-Littoraria-plant interaction than through the predator(s)-Prokelisia-plant relationship.     

Sources and transformations of organic matter in surface soils and sediments from a tidal estuary (North Inlet, South Carolina, USA)

Surface soil and sediment samples collected along a forest-brackish marsh-salt marsh transect in a southeastern U.S. estuary were separated into three different fractions (sand, macro-organic matter, and humus) based on size and density. Elemental, stable carbon isotope, and lignin analyses of these samples reveal important contrasts in the quantity, composition, and sources of organic matter, between forest and marsh sites. Elevated nitrogen contents in humus samples suggest nitrogen incorporation during humification is most extensive in forest soils relative to the marsh sites. The lignin compositions of the macro-organic and humus samples reflect the predominant type of vegetation at each site. Lignin phenol ratios indicate that woody and nonwoody litter from, gymnosperm and angiosperms trees (pines and oaks) is the major source of vascular plant-derived organic matter in the forest site and that angiosperm, grasses (Juncus andSpartina) are the major sources of lignin at the marsh sites. The phenol distributions also reveal that oxidative degradation of lignin is most extensive in the forest and brackish marsh zones whereas little lignin decay occurs in the salt marsh samples. In forest soils, most organic matter originates from highly altered forest vegetation while at the brackish marsh site organic matter is a mixture of degradedJuncus materials and microbial/algal remains. Organic matter in the salt marsh appears to be composed of a more complex mixture of sources, including degradedSpartina detritus as well as algal and microbial inputs. Microbial methane oxidation appears to be an important process and a source of¹³C depleted organic carbon in subsurface sediments at this site.     

Identification of microdetritus derived fromSpartina and its occurrence in the water column and intertidal sediments of Cumberland Basin,Bay of Fundy

A cellulose-specific staining procedure employing Herzberg’s chlor-zinc-iodide solution was developed to aid in the identification of microdetritus derived fromSpartina alterniflora and tested on samples of suspended and sedimented particulate matter collected in the Cumberland Basin at the head of the Bay of Fundy. Not all of the particles reacting positively to the stain could have been identified as originating fromSpartina on the basis of morphology alone, and the stain improved speed and confidence in identification even when particles could be identified morphologically.Spartina dominated particles greater than 100 μm while most of the smaller particles were amorphous aggregates of uncertain origin. In April 1985, at the start of the salt-marsh growing season, the average concentration of suspended microdetritus derived fromSpartina in Cumberland Basin surface water was 129 mg C m^?3 or 0.3–2.7% of the particulate organic carbon. The average concentration in intertidal sediments was 0.036 mg C g^?1 or 0.2–0.9% of the sediment organic carbon. Summing all reservoirs in the sediment (to a depth of 1 cm) and water column, the total amount of detritus originating fromSpartina in Cumberland Basin is 10–24% of the estimated annual net primary production of low marshS. alterniflora.     

Nutrient limitation of the macroalga,Penicillus capitatus,associated with subtropical seagrass meadows in Bermuda

Nutrient limitation of the rhizophytic macroalgaPenicillus capitatus found associated with subtropical seagrass meadows in Bermuda was determined from enrichment assays and subsequent tissue analyses. The photosynthetic response ofP. capitatus to additions of inorganic nitrogen (N) or phosphorus (P), measured as oxygen evolution in closed incubation chambers, increased significantly in both the 16 h and 6 d experiments only with nitrogen enrichment. The average photosynthetic response for all treatments was virtually identical in the two experiments, indicating that there was not a significant time lag in nutrient uptake and that the short term (16 h) assay accurately reflected the longer term (6 d) photosynthetic response to nutrient enrichment. Average tissue nitrogen levels for the nitrogen-treated algae were 29% higher than the phosphorus-treated algae and 18% greater than the controls, corroborating the results from the photosynthesis assay.P. capitatus may acquire nutrients directly from sediment sources via rhizoid holdfasts. Ratios of total dissolved nitrogen (TN) to total dissolved phosphorus (TP) in pore water at 10 and 20 cm depths (6.1 and 4.5, respectively) indicate a nitrogen-limited nutrient pool. These low pore water TN:TP ratios may be a function of a limited sorptive capacity of the calcium carbonate sediments for phosphate, anthropogenic nutrient inputs, or high rates of denitrification, all of which would induce N rather than P limitation in these carbonate-rich sediments.     

Short-term effects of salinity reduction and drainage on salt-marsh biogeochemical cycling andSpartina (cordgrass production

To assess the biogeochemical effects of tidal restrictions on salt-marsh sulfur cycling and plant growth, cores of short-formSpartina alterniflora peat were desalinated and kept either waterlogged or drained in greenhouse microcosms. Changes in netSpartina production, and porewater and solid phase chemistry of treated cores were compared to natural conditions in the field collection site over a 21-mo period. Net production among treatments increased significantly in drained and waterlogged peat compared to field conditions during the first growing season. Constantly high sulfide in waterlogged cores accompanied reduced plant growth. Aeration invigorated growth in draimed cores but led to oxidization of sulfide minerals and to lowered pH. During the second growing season, growth declined in the drained treatment, probably because of acidification and decreased dissolved inorganic nitrogen. Results are pertinent to the success of current wetland protection and restoration activities in the coastal zone.     

Agricultural use of sewage sludge under sub-humid Mediterranean conditions: effect on growth,yield, and metal content of a forage plant

A field experiment was carried out in order to investigate the effects of sewage sludge application on the growth and yield components of triticale (X Triticosecale Wittmack). Five treatments were compared: a control (C) without application of sludge or nitrogen fertilization; a mineral fertilization treatment (MF) applied as ammonium nitrate; and three sewage sludge treatments (SS), 6, 12, and 18 t ha^?1, applied 15 days prior to triticale sowing. The main results showed that SS application improved plant growth by increasing leaf area index, tillering capacity, accumulated aboveground dry matter, and plant height of triticale. As a result, 18 t ha^?1 of SS could be recommended the suitable dose for triticale, where dry matter production was more than twofold above the control value. No toxic effects arising from the heavy metals in triticale plants were observed. The Cu concentration was the only trace element that increased in the straw tissues with sludge application, although the values recorded were below critical environmental thresholds. Furthermore, growth and yield responses of triticale to all SS rates are comparable even sometimes more important than those for mineral fertilizer.     

Blue crab,<Emphasis Type="Italic">Callinectes sapidus</Emphasis>, response to the invasive common reed,<Emphasis Type="Italic">Phragmites australis</Emphasis>: Abundance,size, sex ratio,and molting frequency

Much effort has been directed recently at restoring marshes, by the removal of the invasive common reed,Phragmites australis, yet it is not clear how fish and invertebrates have responded either to the invasion ofPhragmites or to marsh restoration. The blue crab,Callinectes sapidus, uses marsh habitats during much of its benthic life. We investigated the response of blue crabs toPhragmites invasion and restoration efforts by comparing crab abundance (catch per unit effort), mean size and size frequency distribution, sex ratio, and molting of crabs in three physically similar areas differing in marsh vegetation;Spartina-dominated,Phragmites-dominated, and a treated area (Phragmites removed and now dominated bySpartina) in one marsh in the upper portion of Delaware Bay. Field sampling occurred monthly (April to November) from 1999 to 2001 using replicate daytime otter trawls in large marsh creeks. Crabs were categorized by carapace width into recruits (<30 mm), juveniles (30–115 mm), and adults (>115 mm). Juveniles dominated the system, representing 69.4% of all crabs. Similar monthly increases in mean size and molting patterns during the growing season (May–August) occurred inSpartina (natural and treated sites) andPhragmites sites suggesting that, subtidal habitats, used for molting, in these areas do not differ. More juveniles in the feeding molt stage (i.e., intermolt) than in other molt stages and more recruits predominantly in the feeding molt stage than adults were inSpartina, suggesting differences in the marsh surfaces used as feeding habitats withSpartina being preferred. Sex ratios of each life history stage were skewed towards males, but this was related to the low salinity of Alloway Creek, rather than marsh surface vegetation. Our results suggest that marsh surface vegetation influences the way blue crabs use marsh surface habitats, thus restoration efforts focusing on changing vegetation type may have a positive influence on blue crabs.     

Effects of common reed (<Emphasis Type="Italic">Phragmites australis</Emphasis>) expansions on nitrogen dynamics of tidal marshes of the northeastern U.S.

Several recent studies indicate that the replacement of extant species withPhragmites australis can alter the size of nitrogen (N) pools and fluxes within tidal marshes. Some common effects ofP. australis expansion are increased standing stocks of N, greater differentiation of N concentrations between plant tissues (high N leaves and low N stems), and slower whole-plant decay rates than competing species (e.g.,Spartina, Typha spp.). Some of the greater differences between marsh types involveP. australis effects on extractable and porewater pools of dissolved inorganic nitrogen (DIN) and N mineralization rates. Brackish and salt marshes show higher concentrations of DIN in porewater beneathSpartina spp. relative toP. australis, but this is not observed in freshwater tidal marshes whenP. australis is compared withTypha spp. or mixed plant assemblages. With few studies of concurrent N fluxes, the net effect ofP. australis on marsh N budgets is difficult to quantify for single sites and even more so between sites. The magnitude and direction of impacts ofP. australis on N cycles appears to be system-specific, driven more by the system and species being invaded than byP. australis itself. WhereP. australis is found to affect N pools and fluxes, we suggest these alterations result from increased biomass (both aboveground and belowground) and increased allocation of that biomass to recalcitrant stems. Because N pools are commonly greater inP. australis than in most other communities (due to plant and litter uptake), one of the most critical questions remaining is “From where is the extra N inP. australis communities coming?” It is important to determine if the source of the new N is imported (e.g., anthropogenic) or internallyproduced (e.g., fixed, remineralized organic matter). In order to estimate net impacts ofP. australis on marsh N budgets, we suggest that further research be focused on the N source that supports high standing stocks of N inP. australis biomass (external input versus internal cycling) and the relative rates of N loss from different marshes (burial versus subsurface flow versus denitrification).     

Filtration and oxygen consumption in mussels,Mytilus edulis,with and without pea crabs,Pinnotheres maculatus

Filtration rates and oxygen consumption rates were measured in mussels (Mytilus edulis) with and without pea crabs (Pinnotheres maculatus). Noninfested mussels had a significantly higher rate of oxygen consumption per hour (0.578 ml±0.012) than did infested mussels (0.352 ml±0.012). There was no significant effect of pea crab size on mussel respiration. Filtration rates of infested mussels were significantly lower than those of uninfested mussels. Assimilation efficiency was not significantly affected by pea crab infestation. The relationship between body size and oxygen consumption inP. maculatus is given by the following equation: {ie264-1} W^0.626, where {ie264-2} is oxygen uptake (ml h^?1), and W is dry weight (g). There was no difference between the sexes. It is concluded that the decreased oxygen consumption observed in infested mussels is not due to limitation of oxygen availability, but rather reflects a real metabolic response to the presence of the symbiont and the concomittant deprivation of food to the host. The effect is probably reversible, that is, damage can be compensated for after the symbiont has vacated the mussel, depending upon the period of infestation. Our results indicate that the mussels infested by pea crabs may be at an energetic disadvantage relative to mussels without pea crabs.     

Methane release from a brackish intertidal salt-marsh embayment of Chesapeake Bay,Maryland

Gaseous methane loss from a brackish, intertidal salt marsh sediment was measured in April, June, August, and October 1977. Twenty-four sediment cores were taken on each date. Annual loss of methane carbon from the mud flats was 10.7 g CH₄?C per m² per year, a value closer to freshwater values than marine systems. Loss of methane fromSpartina peat was low.     

Effects of grazing by feral horses,clipping, trampling,and burning on a Georgia salt marsh

Responses ofSpartina alterniflora marsh to combinations of feral horse grazing, clipping, simulated trampling, and a late winter burn were studied on Cumberland Island National Seashore, Georgia. Replicated 200-m² plots were established and sampled bimonthly from July 1983 to November 1984. Clipping and trampling each reduced peak aboveground biomass by 20% in 1983 and 50% (clipping) and 55% (trampling) in 1984. A March burn reduced peak aboveground biomass by 35% in 1984. Trampling and burning earch reduced net aboveground primary production (NAPP) by 35%, but clipping did not reduce NAPP. Standing stocks of live rhizomes were correlated with aboveground biomass and were reduced with experimental treatments. Abundance of the periwinkle snail (Littorina irrorata) was also reduced. Horse grazing had a substantial impact on standing stocks and NAPP ofSpartina, but grazing was not uniform throughout the marsh. Moderately grazed plots had NAPP reduced by 25% compared to ungrazed plots. Heavily grazed plots had extremely low NAPP, and abovegroundSpartina never exceeded 40 g m^?2 dry mass compared to 360 g m^?2 within exclosures.