Nitrogen and phosphorus limitation in a coastal barrier salt marsh: the implications for vegetation succession

1 A factorial fertilizer experiment was conducted in a 15-year-old coastal barrier salt marsh with a low soil nitrogen content, and in an older 100-year-old marsh with a higher nitrogen content. Plots were fertilized at high and low marsh elevations in both marshes. Nitrogen and phosphorus were applied at low and high concentrations both separately and in combination in each of 3 successive years.
2 Nitrogen limited above-ground plant growth in both young and old salt marshes in all years. Phosphorus limitation of plant growth was apparent in the first year in the young marsh and in the last year in both marshes. In young marshes with low soil organic matter, phosphorus limitation may occur. In addition, phosphorus limitation occurs at both successional stages when a marsh is saturated with nitrogen.
3 Plant species that are typical of nitrogen-rich habitats and late successional stages significantly increased in biomass after fertilization. Limonium vulgare , a low stature species of early and intermediate successional stages, decreased in biomass, whereas the taller Elymus pycnanthus and Artemisia maritima increased. After 3 years of fertilization, plant species composition in a young marsh was similar to the species composition in an unfertilized older marsh. Fertilization of a 100-year-old marsh, however, still resulted in a change in plant species composition, suggesting that succession was still occurring and that, overall, plants in marshes of different age are similar in their response to fertilization.     

The preservation of salt marsh algae by controlled liquid nitrogen freezing.

Five species of benthic marine algae were preserved by controlled liquid nitrogen freezing and storage over periods extending to 1 year. Only a small percent of the algae survived without cryoprotectant. Nannochloris adamsii was an exception; 67% survived after 12 months of storage. Nitzschia acicularis was the best preserved with 5 glycerol as a cryoprotectant, Dimethylsulfoxide was a better cryoprotectant for N. adamsii and Dunaliella quartolecta. Reducing normal brackish salinity (28‰) of the culture medium to one half (14‰) increased the survival percentages for N. acicularis, Cylindrotheca closterium and Phaeodactylum tricornutum. The morphology and physiology of the species tested were unchanged by long storage time in liquid nitrogen.     

Vegetation composition of a maritime salt marsh in Qatar in relation to edaphic features

Abstract. Coverpercentage values for plant species were estimated using the line-intercept method along two belt transects starting at the lower mangrove frontier and ending at the upper height of the salt marsh. Twelve perennial species representing five growth forms were recorded with a total cover ranging between 10 and 50 %, depending on the type of substrate. Principal components analysis of the soil data indicated an array of stands, on the basis of which four vegetation zones couldbe distinguished. The procumbent growth form predominating on the salt marsh is related to high salinity levels. The procumbent vegetation shows a zonation related to the degree of waterlogging. The rhizomatous growth form is found on sites with a low bicarbonate content and saturation percentage levels, while the undershrub- and stoloniferous growth forms occupy sites of low salinity level, linked with intermediate values of the other edaphic factors.     

Salt stress limitation of seedling recruitment in a salt marsh plant community

Summary Seedling recruitment in salt marsh plant communities is generally precluded in dense vegetation by competition from adults, but is also relatively rare in disturbance-generated bare space. We examined the constraints on seedling recruitment in New England salt marsh bare patches. Under typical bare patch conditions seed germination is severely limited by high substrate salinities. We examined the germination requirements of common high marsh plants and found that except for one notably patch-dependent fugitive species, the germination of high marsh plants is strongly inhibited by the high soil salinities routinely encountered in natural bare patches. Watering high marsh soil in the greenhouse to alleviate salt stress resulted in the emergence of up to 600 seedlings/225 cm². The vast majority of this seed bank consisted of Juncus gerardi, the only common high marsh plant with high seed set. We tested the hypothesis that salt stress limits seedling contributions to marsh patch secondary succession in the field. Watering bare patches with fresh water partially alleviated patch soil salinities and dramatically increased both the emergence and survival of seedlings. Our results show that seedling recruitment by high marsh perennial turfs is limited by high soil salinities and that consequently their population dynamics are determined primarily by clonal growth processes. In contrast, populations of patch-dependent fugitive marsh plants which cannot colonize vegetatively are likely governed by spatially and temporally unpredictable windows of low salinities in bare patches.     

Nitrogen fixation and nitrogen limitation of primary production along a natural marsh chronosequence

We experimentally tested the conditions where heterospecific attraction is more likely to occur. The heterospecific attraction hypothesis predicts that colonizing or migrant individuals use the presence of resident species as a cue for profitable breeding sites. In other words, increasing resident densities will result in increased migrant densities until the costs of interspecific competition override the benefits of heterospecific attraction. The experiment consisted of a reference and a manipulation year. In the reference year, resident titmice were permitted to breed at intermediate densities whilst in the manipulation year, resident densities were manipulated in nine study plots. Three treatments were performed as low, intermediate and high resident densities and migrant density responses were measured in both years. Relative between-year migrant and resident densities were analyzed by regression analysis. Migrant foliage gleaning guild densities responded linearly and positively, as did densities of habitat generalists, in particular Chaffinch ( Fringilla coelebs),. The ground-foraging guild did not show a response. This study provides support for predictions of the heterospecific attraction hypothesis and suggests that information on habitat quality with reference to both food availability and safe breeding sites are important in using heterospecifics as cues. Based on Chaffinch response data, artificially increased resident densities were not high enough for competitive effects between residents and migrants to decrease heterospecific attraction. It seems unlikely that in northern environments natural resident densities will reach high levels where competitive effects would occur, therefore heterospecific attraction will always be beneficial. This study again shows the importance of heterospecific attraction in migrant habitat selection and as a process promoting species diversity in northern breeding bird assemblages.     

Effects of regular salt marsh haying on marsh plants, algae, invertebrates and birds at Plum Island Sound, Massachusetts

The haying of salt marshes, a traditional activity since colonial times in New England, still occurs in about 400 ha of marsh in the Plum Island Sound estuary in northeastern Massachusetts. We took advantage of this haying activity to investigate how the periodic large-scale removal of aboveground biomass affects a number of marsh processes. Hayed marshes were no different from adjacent reference marshes in plant species density (species per area) and end-of-year aboveground biomass, but did differ in vegetation composition. Spartina patens was more abundant in hayed marshes than S. alterniflora, and the reverse was true in reference marshes. The differences in relative covers of these plant species were not associated with any differences between hayed and reference marshes in the elevations of the marsh platform. Instead it suggested that S. patens was more tolerant of haying than S. alterniflora. Spartina patens had higher stem densities in hayed marshes than it did in reference marshes, suggesting that periodic cutting stimulated tillering of this species. Although we predicted that haying would stimulate benthic chlorophyll production by opening up the canopy, we found differences to be inconsistent, possibly due to the relatively rapid regrowth of S. patens and to grazing by invertebrates on the algae. The pulmonate snail, Melampus bidendatus was depleted in its δ¹³C content in the hayed marsh compared to the reference, suggesting a diet shift to benthic algae in hayed marshes. The stable isotope ratios of a number of other consumer species were not affected by haying activity. Migratory shorebirds cue in to recently hayed marshes and may contribute to short term declines in some invertebrate species, however, the number of taxa per unit area of marsh surface invertebrates and their overall abundances were unaffected by haying over the long term. Haying had no impact on nutrient concentrations in creeks just downstream from hayed plots, but the sediments of hayed marshes were lower in total N and P compared to references. In sum, haying appeared to affect plant species composition but had only short-term affects on consumer organisms. This contrasts with many grassland ecosystems, where an intermediate level of disturbance, such as by grazing, increases species diversity and may stimulate productivity. From a management perspective, periodic mowing could be a way to maintain S. patens habitats and the suite of species with which they are associated.     

White phosphorus poisoning of waterfowl in an Alaskan salt marsh.

The cause of the yearly death of an estimated 1,000 to 2,000 migrating dabbling ducks (Anas spp.) and 10 to 50 swans (Cygnus buccinator and C. columbianus) has remained a mystery for the last ten years in Eagle River Flats (ERF), a 1,000 ha estuarine salt marsh near Anchorage, Alaska, used for artillery training by the U.S. Army. We have gathered evidence that the cause of this mortality is the highly toxic, incendiary munition white phosphorus (P4). The symptoms of poisoning we observed in wild ducks included lethargy, repeated drinking, and head shaking and rolling. Death was preceded by convulsions. Farm-reared mallards dosed with white phosphorus showed nearly identical behavioral symptoms to those of wild ducks that became sick in ERF. White phosphorus does not occur in nature but was found in both the sediments where dabbling ducks and swans feed and in the gizzards of all carcasses collected in ERF. We hypothesize that feeding waterfowl are ingesting small particles of the highly toxic, incendiary munition P4 stored in the bottom anoxic sediments of shallow salt marsh ponds.     

Food limitation of a Delaware salt marsh population of the mummichog,Fundulus heteroclitus (L.)

Summary The relationship of the mummichog, Fundulus heteroclitus, to its food supply was examined using 8 m x 20 m enclosures under field conditions. Density effects were examined by stocking these enclosures with fish at densities ranging from 1/8 to four times the natural population density. Mean growth rate in each pen was estimated from measurements on individually marked fish. Fish enclosed at normal density grew at the same rate as fish from the natural population, indicating that caging had minimal effects on growth rate. When enclosed below natural density, mummichogs had a growth rate that was 2–3 times higher than that of the natural population. Fish enclosed at four times normal density had a negative mean growth rate, a higher mortality rate than fish enclosed at natural density, and no egg production.In further experiments, food was added to the same type of enclosure at fish densities ranging from one-half to four times natural density. Food additions had a positive effect on growth rate at all densities. Food additions also caused egg production of fish enclosed at the highest density to increase to a rate equivalent with the natural population. We conclude that the total food supply may regulate the maximum size of the Canary Creek mummichog population by affecting the individual growth rate, mortality rate, and fecundity.     

Photosynthetic and growth responses of three freshwater algae to phosphorus limitation and daylength

1. Three common species of freshwater phytoplankton, the diatom Nitzschia sp., green alga Sphaerocystis schroeteri and cyanobacterium Phormidium luridum, were grown under contrasting daylengths [18 : 6 h light : dark cycles (LD) versus 6 : 18 h LD] and phosphorus (P) regimes (P‐sufficient versus 1 μm P). The rates of growth and photosynthesis, as well as growth efficiencies and pigment concentrations, were compared among treatments. 2. The growth and photosynthetic parameters of the three species depended on both P status and daylength in a species‐specific way. The responses to P limitation depended on daylength and, conversely, the responses to daylength depended on P status. 3. Growth rates and the maximum rates of photosynthesis (P_max) of all species decreased under P limitation under both light regimes. However, the decrease of P_max because of P limitation was greater under long daylength. The P_max of the green alga S. schroeteri decreased the most (ca. sixfold) under P limitation compared with the other two species. The photosynthesis saturation parameter I_k also decreased under P limitation; the decline was significant in Nitzschia and Sphaerocystis. P‐limitation significantly increased photoinhibition (β) in Nitzschia and Sphaerocystis, but not in Phormidium. The excess photochemical capacity (the ratio of the maximum photosynthesis rate to the photosynthesis rate at the growth irradiance), characterising the ability to utilise fluctuating light, was significantly lower under P limitation. 4. The growth efficiency (growth rate normalised to daylength) declined with increasing daylength in all species. Under short daylength the cyanobacterium Phormidium had the lowest growth efficiency of the three species. 5. The cellular chlorophyll a concentration in both Nitzschia and Sphaerocystis was significantly higher under short daylength, but only under P‐sufficient conditions. In Nitzschia, under short daylength, P‐limitation significantly decreased cellular chlorophyll concentration. In contrast, P‐limitation increased cellular chlorophyll concentration in Sphaerocystis, but under long daylength only. The ratio of chlorophyll a to b in the green alga also declined under short daylength and under P‐limited conditions.     

Seasonal flooding,nitrogen mineralization and nitrogen utilization in a prairie marsh

Flooding can be an important control of nitrogen (N) biogeochemistry in wetland ecosystems. In North American prairie marshes, spring flooding is a dominant feature of the physical environment that increases emergent plant production and could influence N cycling. I investigated how spring flooding affects N availability and plant N utilization in whitetop (Scolochloa festucacea) marshes in Manitoba, Canada by comparing experimentally spring-flooded marsh inside an impoundment with adjacent nonflooded marsh. The spring-flooded marsh had net N mineralization rates up to 4 times greater than nonflooded marsh. Total growing season net N mineralization was 124 kg N ha^–1 in the spring-flooded marsh compared with 62 kg N ha^–1 in the nonflooded marsh. Summer water level drawdown in the spring-flooded marsh decreased net N mineralization rates. Net nitrification rates increased in the nonflooded marsh following a lowering of the water table during mid summer. Growing season net nitrification was 33 kg N ha^–1 in the nonflooded marsh but < 1 kg N ha^–1 in the spring-flooded marsh. Added NO₃^–1 induced nitrate reductase (NRA) activity in whitetop grown in pot culture. Field-collected plants showed higher NRA in the nonflooded marsh. Nitrate comprised 40% of total plant N uptake in the nonflooded marsh but <1% of total N uptake in the spring-flooded marsh. Higher plant N demand caused by higher whitetop production in the spring-flooded marsh approximately balanced greater net N mineralization. A close association between the presence of spring flooding and net N mineralization and net nitrification rates indicated that modifications to prairie marshes that change the pattern of spring inundation will lead to rapid and significant changes in marsh N cycling patterns.     

Plant nitrogen and phosphorus limitation in 98 North American grassland soils

The availability of nutrients is a critical determinant of ecological dynamics in grasslands, but the relationships between soil resource availability and nutrient limitation across ecosystems are not clear. To better understand how soil nutrient availability determines nutrient limitation in vegetation, we grew the same species of grass (Schizachyrium scoparium) in 98 North American grassland soils and fertilized them factorially with nitrogen (N) and phosphorus (P). On average adding N, P, and the two nutrients together increased biomass relative to unfertilized plants by 81%, 22%, and 131%, respectively. Plants grown on low-P soils were not primarily limited by P. Instead, these plants were colimited by N and P, while plants grown on high-P soils were primarily limited by N and only secondarily limited by P. Limitation was not predicted by total soil N. The preponderance of colimitation between N and P on low-P soils suggests that low P availability alters the N cycle to constrain supplies to plants such that N and P are made available in proportion to their demand by plants.     

Physiological ecology of acetylene reduction (nitrogen fixation) in a Delaware salt marsh

The effects of several fixed nitrogen compounds on acetylene reduction activity (nitrogen fixation) of surface sediments from a Delaware salt marsh were studied. Ammonia addition caused little decrease in activity early in the summer but resulted in a considerable decrease (85–95%) in activity late in the summer and early in the fall. Nitrate caused a near complete suppression of activity at all times. Other compounds such as glutamate, urea, and yeast extract caused a slight increase in activity in tallSpartina sediments and caused more than a 2.5-fold increase in shortSpartina sediments. There was a lag period (1–2 days) before the commencement of in vitro acetylene reduction activity during the spring and early summer, but this lag period was not present in the late summer. The addition of chloramphenicol to samples from a shortSpartina zone caused decreases in activity similar to those obtained with ammonia, whereas chlorate amendments yielded results which, when compared on an electron basis, were comparable to those obtained with nitrate. These results indicated that the observed lag period may be the result of a physiological response to the in situ levels of ammonia and/or nitrate. It is suggested here that in situ nitrogenase activity may be controlled by two processes: (a) repression and derepression of nitrogenase synthesis mediated by the levels of ammonia, and (b) competition for reducing power (electrons) and energy (ATP) between the processes of nitrate reduction and nitrogen fixation.     

The role of lesser snow geese as nitrogen processors in a sub-arctic salt marsh

Summary Ammonia volatilization losses from faeces of Lesser Snow Geese were measured during the summer of 1987 on the salt-marsh flats at La Pérouse Bay. Amounts of ammonia volatilized increased with increasing ambient temperature, and ranged from 1.0 to 15.1 mg N per 100 mg of nitrogen present as soluble ammonium ions at the start of the 8-h experiment. Using estimates of faecal deposition reported previously, the annual loss via volatilization was estimated at 0.08 g N m^-2, or 7.9% of the nitrogen present in goose faeces. Percent change in soluble ammonium ions in fresh faeces after 8 h ranged from -51.1% to +41.1%, indicating that net mineralization of organic nitrogen occurred in some of the faeces. Microbial respiration of fresh goose faeces increased exponentially with temperature. However, variable rates of net mineralization per unit rate of respiration indicated that the substrate quality affected microbial immobilization and thus net nitrogen mineralization. In feeding experiments, captive goslings grazed different types of vegetation, each with distinctive nutritional qualities. Forage quality had significant effects on goose feeding behavior and subsequent rates of nitrogen mineralization in fresh faeces. Net nitrogen mineralization rates in faeces from geese which grazed the three vegetation types ranged from 1.31 to 4.97 mg NH₄⁺–N g_DW^-1 24 h^-1. Because plant growth in this salt marsh is nitrogen-limited, where swards are grazed, mineralization of organic faecal nitrogen represents an essential link in the maintenance of the flow of nitrogen into the sediments and the sustained growth of vegetation at a time when most required by the geese.     

Microclimate and substrate quality controls on nitrogen mineralization in a New England high salt marsh

New England high salt marsh primary productivity is limited by N, but variation in plant N availability across salt marsh vegetation zones has not been quantified. To investigate this, we measured in situ net N mineralization rates throughout the growing season in three zones of a Maine high salt marsh, Juncus gerardii, Spartina patens, and mixed perennial forb. We also measured microclimate factors (soil temperatures and moistures) and substrate quality parameters (soil organic matter, soil total nitrogen, soil C:N ratio) to see if either related to differences in net N mineralization. To determine the relative importance of substrate quality and microclimate, we measured N mineralization of the different soil types in the laboratory, holding microclimate parameters constant. We also investigated the relative importance of microclimate and substrate statistically, with principal components analysis and multiple regression. In situ net N mineralization rates were significantly higher in the forb zone than in graminoid zones, but graminoid zone N mineralization rates did not vary significantly from each other. Soil temperatures, moistures, carbon, and nitrogen were all significantly higher in the forb zone than in graminoid zones, but C:N ratio did not vary significantly across zones. Principal components analysis and multiple regression revealed that microclimate was a more significant predictor of total N mineralized over the course of the growing season than was substrate quality. In contrast, when microclimate conditions were held constant, forb zone N mineralization was still significantly higher than that of graminoid zones, suggesting that substrate quality does exert some control on this process. Thus, both microclimate and substrate quality appear to influence N mineralization rates across vegetation zones of this Maine salt marsh.     

Changes in nitrogen and phosphorus cycling suggest a transition to phosphorus limitation with the stand development of larch plantations

Plant and Soil - The changes of nutrient limitation status for tree growth across a plantation chronosequence have great implications for plantation management. The underlying mechanisms for such a...     

Modeling productivity of a salt marsh

A model of primary productivity in a salt marsh is developed and compared to a regression analysis study of data showing dependence of growth on growing season, mean tidal height, and average monthly temperatures for several grass species.