Freshwater requirements of a semi-arid supratidal and floodplain salt marsh

When rivers are impounded, the reduction in downstream flow can produce important and often adverse effects, especially in the estuarine environment. One or more dams have been proposed for the Olifants River system in the Western Cape, South Africa. This estuary has an extensive area of salt marsh that was examined to see whether it required occasional flooding with freshwater to wash out accumulated salts. The dominant salt marsh species,Sarcocornia pillansii, occurred in supratidal and floodplain areas where the water table was shallowest, the soil moisture highest, and the soil electrical conductivity lowest. Aerial photographs and simulated runoff data showed that no flood had covered the floodplain during the previous 80 years. The data indicate that salt marsh plants use saline groundwater during the dry months of the year in order to survive, but use the short season winter rainfall period with low salinity conditions to grow and reproduce. This study demonstrated that live roots ofS. pillansii reached the water table during the dry season. Tissue and soil water potentials, the relationship between vegetation cover, depth to the water table, and electrical conductivity of the groundwater support the conclusion that saline groundwater is the only source of water during the drier months of the year. Freshwater flooding of the river in winter may be important because it covers the supratidal area with less saline water and reduces the depth to the water table on the floodplain. This makes the groundwater more accessible to the halophytes growing on the floodplain.     

Effects of increased inundation and wrack deposition on a high salt marsh plant community

Salt marshes respond to both slowly increasing tidal inundation with sea level rise and abrupt disturbances, such as storm-induced wrack deposition. The effects of inundation pattern and wrack deposition have been studied independently but not in combination. We manipulated inundation of tidal creek water and wrack presence individually and in combination, in two neighboring communities within a Virginia high salt marsh during 1994 and 1995. The effects of these manipulations were assessed by measurements of aboveground plant biomass. Altered inundation by itself produced little response in the various categories of plant biomass measured. Wrack deposition affected all species (i.e., Juncus roemerianus, Spartina patens, and Distichlis spicata) showing a significant reduction in aboveground biomass, as expected. Recovery after wrack deposition was dependent on the species. S. patens and D. spicata recovered from wrack deposition within one growing season, while J. roemerianus did not. Because the effects of wrack deposition greatly exceeded those of experimentally increased inundation, the possible interactions between the two were masked. Increased inundation may have inhibited the colonization of bare areas by some species after the removal of wrack from an area, although statistical significance at α=0.01 was not reached. Our results confirm that wrack deposition can cause the redistribution of species within the high marsh community. Altered inundation may have a greater effect on the re-establishment of the plant community after wrack deposition than it does without wrack deposition.     

Productivity of algal epiphytes in a Georgia salt marsh: effect of inundation frequency and implications for total marsh productivity

Primary production by algal epiphytes of dead Spartina alterniflora shoots in a Georgia salt marsh was measured using the ¹⁴C technique. A 2³ factorial design was used to quantify the effects of light intensity and inundation frequency (stem height) on carbon fixation at two sites along a salt marsh creek. Algae inundated daily fixed carbon more rapidly than those which had dried for several days, but this may have been the results of greater biomass on more frequently immersed stems. This result corroborates studies showing desiccation is not always a severe stress for intertidal algae. Similarity of epiphyte algal productivity to that of salt marsh benthic diatoms suggests that, given adequate substrate, the epiphytes may be an important source of primary production during some seasons of the year.     

Interactive effects of animal disturbance and elevation on vegetation of a tidal freshwater marsh

We studied interactions between animal disturbance (geese, carp, and muskrat) and elevation in a field experiment in tidal freshwater marshes of the Patuxent River, Maryland, United States. Vegetation changes were recorded in fenced and unfenced plots in high and low marsh community types for 2 yr using measurements of areal cover and within-plot frequency (which were averaged to create a dominance index), Leaf Area Index (LAI), and aboveground biomass. We related light environment to differences in vegetation using below-canopy measurements of Photosynthetically Active Radiation (PAR). In the low marsh, total cover of all species, cover of annual species, biomass, and LAI were significantly higher in plots fenced to exclude animals (exclosures) than in unfenced plots (fenced/unfenced total cover=76/40%, annual cover=45/10%, biomass=936/352 g m^?2, LAI=3.3/1.4). PAR was significantly lower in fenced than unfenced plots (fenced/unfenced=115/442 μmol s-1 m^?2). Despite the strong effect of fencing on biomass, species richness per plot (i.e., the number of species per plot, or species density) was not affected significantly by fencing in the low marsh. Most of the observed differences in cover, biomass, LAI, and PAR were due to variation in the abundance of the herbaceous annual speciesBidens laevis (dominance index fenced/unfenced=45/10%) andZizania aquatica (30/12%). In the high marsh community, fencing had only minor effects on plant community composition and did not significantly affect species richness, cover, biomass, PAR, or LAI. Our results show that animals can dramatically affect low marsh vegetation, primarily via physical disturbance or herbivory of shallowly rooted seedlings of annual species.     

Chironomid grazing on benthic microalgae in a Louisiana salt marsh

Field experiments were conducted to examine spatial and temporal variation in chironomid (predominantlyTanypus clavatus) abundance, and their trophic relationship with benthic microalgae. High performance liquid chromatography (HPLC) analysis of chironomid gut pigments indicated that diatoms comprised the bulk of the microalgae ingested by chironomids.¹⁴C-feeding studies were used to obtain quantitative estimates of chironomid, copepod, ostracod, and nematode grazing on benthic microalgae. Daily consumption of standing microalgal biomass by chironomids ranged from 0.12% (January) to 125% (May), but was highly variable. There were no significant diel, temporal (over the scale of months), or spatial patterns in individual chironomid grazing rates. There was significant temporal variation in the proportion of microalgal biomass consumed by the total meiofaunal assemblage, and highest grazing impacts occurred in May, coincident with high abundances of chironomids, harpacticoid copepods, and ostracods. The grazing impact of chironomids was comparable to or greater than that of other known grazers of microalgae (copepods, ostracods). Functional-response experiments performed in the laboratory revealed that chironomid ingestion rates increased with increasing food availability over short (1 to 2 h) time scales. Field data did not indicate a functional response to food availability over longer (mo) time scales, possibly because of other environmental incluences. Gut residence time (determined using fluorescently labeled beads) changed with variable feeding rates, which were in turn a function of variable food availability. Chironomid larvae have the ability to consume significant fraction of the microphytobenthos in absolute terms, and relative to other meiofauna, indicating that they are an important component of the salt marsh food web.     

Differential effects of salinity and soil saturation on native and exotic plants of a coastal salt marsh

In many southern California salt marshes, increased freshwater inflows have promoted the establishment of exotic plant species. A comparative study showed that a native, perennial, high marsh dominant,Salicornia subterminalis, and an invasive, exotic annual grass,Polypogon monspeliensis, responded differently to soil salinity and saturation.Salicornia subterminalis seeds and young plants were more salt tolerant, and the native grew best at high salinities (23 g 1^?1 and 34 g 1^?1) in greenhouse experiments. In contrast, the exotic had reduced growth at high salinities relative to nonsaline controls. The native,S. subterminalis, grew poorly as the duration of soil saturation increased from 2 wk to 32 wk, butP. monspeliensis grew equally well for all durations tested. The response ofS. subterminalis andP. monspeliensis to increased salinity indicated that salt applications might be used to protect native vegetation in salt marshes where salt-sensitive exotics are a problem. A field experiment verified that a salt application of 850 g m^?2 mo^?1 for 3 mo was sufficient to control the exotic, while not noticeably affecting the native. Thus, salt applications may be a practical method for controllingP. monspeliensis invasions in areas receiving urban runoff or other unwanted freshwater inflows.     

Halophyte recruitment in a salt marsh restoration site

In restored salt marshes, seedling recruitment can be limited where large areas of soil are exposed and physical conditions are harsh. On a 0.7-ha excavated marsh plain, we studied recruitment as a function of abiotic (elevation) and biotic factors in 2 × 2 m plots planted with 0, 1, 3, or 6 species from the pool of 8 native halophytes. The random draws of 3-species and 6-species assemblages produced approximately equal numbers of plants per species for the experiment as a whole, yet only three species recruited> 10 seedlings per plot.Salicornia virginica andSalicornia bigelovii each produced> 15,000 seedlings in 1998, andSuaeda esteroa produced> 2,500 seedlings in 1999. For these 3 species, seedling recruitment increased with elevation in 1998, but this trend weakened in 1999, when species richness affected recruitment (fewer seedlings in more species-rich plots). Abiotic effects preceded biotic interactions in determining seedling recruitment patterns early in the development of the salt marsh. Effects of species richness appeared to be scale-dependent in that having all species present in the site likely enhanced overall recruitment (all species had 2 or more seedlings), while plantings of 6 species in a 2 × 2 m plot reduced seedling density.S. virginica was the only species that increased its presence and relative cover in the experimental site over the 4-yr study. Protocols for planting southern California salt marsh restoration sites could omit this species, but all others probably need to be introduced to restore diverse vegetation.     

Flow paths of water and sediment in a tidal marsh: Relations with marsh developmental stage and tidal inundation height

This study provides new insights in the relative role of tidal creeks and the marsh edge in supplying water and sediments to and from tidal marshes for a wide range of tidal inundation cycles with different high water levels and for marsh zones of different developmental stage. Net import or export of water and its constituents (sediments, nutrients, pollutants) to or from tidal marshes has been traditionally estimated based on discharge measurements through a tidal creek. Complementary to this traditional calculation of water and sediment balances based on creek fluxes, we present novel methods to calculate water balances based on digital elevation modeling and sediment balances based on spatial modeling of surface sedimentation measurements. In contrast with spatial interpolation, the presented approach of spatial modeling accounts for the spatial scales at which sedimentation rates vary within tidal marshes. This study shows that for an old, high marsh platform, dissected by a well-developed creek network with adjoining levees and basins, flow paths are different for tidal inundation cycles with different high water levels: during shallow inundation cycles (high water level <0.2 m above the creek banks) almost all water is supplied via the creek system, while during higher inundation cycles (high water level >0.2 m) the percentage of water directly supplied via the marsh edge increases with increasing high water level. This flow pattern is in accordance with the observed decrease in sedimentation rates with increasing distance from creeks and from the marsh edge. On a young, low marsh, characterized by a gently seaward sloping topography, material exchange does not take place predominantly via creeks but the marsh is progressively flooded starting from the marsh edge. As a consequence, the spatial sedimentation pattern is most related to elevation differences and distance from the marsh edge. Our results imply that the traditional measurement of tidal creek fluxes may lead in many cases to incorrect estimations of net sediment or nutrient budgets.     

Effects of nitrogen addition and salt grass (Distichlis spicata) upon high salt marsh vegetation in Northern California, USA

In the salt marshes of Tomales Bay, California, where grazing by cattle increases the input of nitrogen to the marsh (either directly or indirectly as runoff from within the salt marsh watershed), high salt marsh vegetation is dominated byDistichlis spicata and is less diverse than marshes without excess nutrients. Using a field experiment, I investigated the role of soil fertility on the plant community of the high salt marsh. I hypothesized that when soil fertility is increased by nitrogen addition plant productivity will increase, as indicated by height, biomass, and cover, and competitive exclusion, byD. spicata, will lead to a reduction in species richness and evenness, especially where the initial density ofDistichlis is high (from transplanting). After two growing seasons, biweekly nitrogen addition to the high salt marsh led to increased plant biomass and cover. Diversity was not reduced, and space preemption byDistichlis-transplants did not confer a competitive advantage. Although the dominant species thrived (e.g.,Salicornia virginica, D. spicata, Triglochin concinna) they did not displace subdominant species and decrease diversity. The vegetation response in this high salt marsh system does not support the hypothesis that as biomass and cover (indicators of productivity) increase in response to increased nitrogen, competitive exclusion will occur and diversity will decrease.     

Methane release from soils of a Georgia salt marsh

A seasonal study of methane release from marsh soils to the atmosphere indicates that ebullition is a significant process varying both seasonally and spatially. Release rates are higher during summer months than winter months and ebullition is greatest in the short Spartina alterniflora marshes and least in the tall S. alterniflora marshes. The annual amounts of methane released in the short and tall marshes are 53.1 and 0.4 gm^?3 which represents a loss of 8.8 and 0.002% of the net carbon fixation in the two respective marsh types.In vitro experimentation shows that methane production is sensitive to changes in temperature and addition of H₂ and CO₂.     

Ammonium and phosphate dynamics in a Virginia salt marsh

Experimental chambers were used in a Virginia salt marsh to partition the tidal flux of dissolved nutrients occurring at the marsh surface and in the water column. On five dates from June to October 1989, six replicate chambers in the short Spartina alterniflora zone were monitored over complete tidal cycles. When reservoir water, used to simulate tidal flooding in the chambers, was initially low in dissolved nutrients, the marsh surface was a source of both ammonium and phosphate to the water column. Calculations of the physical processes of diffusion and advection could not account for total nutrient release from the marsh surface. We hypothesize the primary source of nutrients was organic matter mineralization in surface sediments, which released nutrients into the flooding water column. Assimilation (uptake) of phosphate measured in water-column incubation experiments was nearly equal to phosphate released from the marsh surface. Surface release of ammonium, however, was somewhat greater than water-column uptake. In this salt marsh, benthic production and release of ammonium and phosphate is comparable in magnitude to pelagic consumption, thereby yielding only a small “net” transfer of these nutrients to the estuary.     

Primary production and seasonal aspects of emergent plants in a tidal freshwater marsh

Seasonal changes in aboveground plant biomass, cover, and frequency were monitored in Sweet Hall Marsh, a tidal freshwater marsh located on the Pamunkey River, Virginia, during the 1974 growing season.Peltandra virginica accumulated the most biomass, 423.40 g per m², followed byLeersia oryzoides at 67.75 g per m². Annual net community production was estimated to be 775.74 g per m² by using a multiple-harvest technique. Comparisons with other studies revealed that production was somewhat low for tidal freshwater marshes but mostly higher than production in Virginia brackish and saline wetlands. Measurements revealed an annual succession of plant species from spring to fall. The pattern observed was early dominance byPeltandra followed by a rise in importance ofPolygonum spp.,Impatients capensis andLeersia.     

Algal mat productivity: Comparisons in a salt marsh

This paper documents the role of salt marsh algal mats in the productivity of a southern California tidal wetland. The productivity of the mats, which are composed of filamentous bluegreen and green algae and diatoms, varies both temporally and spatially in relation to tidal inundation and overstory vegetation. The estimates of net primary productivity (NPP) were highest under the canopy ofJaumea carnosa (Less.) Gray (341 g C m^?2 yr^?1) at low elevation. Elsewhere, NPP appeared to be limited by low light (276 g C m^?2 yr^?1 underSpartina foliosa Trin.) and desiccation (185 g C m^?2 yr^?1 underBatis martima L. and 253 g C m^?2 yr^?1 underMonanthochloe littoralis Engelm). Algal NPP was from 0.8 to 1.4 times that of the vascular plant overstory NPP. It is hypothesized that the arid environment of southern California and resulting hypersaline soils reduce vascular plant cover, which leads to high algal productivity.     

Shifting nutrient limitation and eutrophication effects in marsh vegetation across estuarine salinity gradients

In light of widespread coastal eutrophication, identifying which nutrients limit vegetation and the community consequences when limitation is relaxed is critical to maintaining the health of estuarine marshes. Studies in temperate salt marshes have generally identified nitrogen (N) as the primary limiting nutrient for marsh vegetation, but the limiting nutrient in low salinity tidal marshes is unknown. I use a 3-yr nutrient addition experiment in mid elevation,Spartina patens dominated marshes that vary in salinity along two estuaries in southern Maine to examine variation in nutrient effects. Nutrient limitation shifted across estuarine salinity gradients; salt and brackish marsh vegetation was N limited, while oligohaline marsh vegetation was co-limited by N and phosphorus (P). Plant tissue analysis ofS. patens showed plants in the highest salinity marshes had the greatest percent N, despite N limitation, suggesting that N limitation in salt marshes is partially driven by a high demand for N to aid in salinity tolerance. Fertilization had little effect on species composition in monospecificS. patents stands of salt and brackish marshes, but N+P treatments in species-rich oligohaline marshes significantly altered community composition, favoring dominance by high aboveground producing plants. Eutrophication by both N and P has the potential to greatly reduce the characteristic high diversity of oligohaline marshes. Inputs of both nutrients in coastal watersheds must be managed to protect the diversity and functioning of the full range of estuarine marshes.     

Autocompaction of shallow silty salt marsh clay

Empirically derived algorithms are suggested by means of which it is possible to evaluate relations between physical properties in silty salt marsh clay. A relation between loss on ignition, sand content and bulk dry density in the topmost 5 cm was found to explain 84% of the variation of the latter. The bulk dry density in the topmost ≈ 0.5 m was found to vary as a logarithmic function of depth below the surface controlled by two constants directly dependent on the bulk dry density in the topmost 5 cm. Integration of this function illustrates that mass depth (MSD_z) down core can be directly related to the bulk dry density of the surface layer. For sediments with a surface bulk dry density between 400 kg m^? 3 and 800 kg m^? 3 the porosity was found to vary (+/?10%) with MSD_z as another logarithmic function. The results allow for an evaluation of the use of marker horizons in the topmost layers. Right after such a layer has been spread on the surface, sedimentation measured in length per time (accretion), will be significantly larger than for subsequent periods. Using marker horizons, it is therefore not trivial to know the level of the marker, as deeper located markers will indicate successively smaller accretion rates with the same sediment input.     

Controls on salt marsh accretion: A test in salt marshes of Eastern Canada

We used¹³⁷Cs-dating to determine vertical accretion rates of 15 salt marshes on the Bay of Fundy, the Gulf of St. Lawrence, and the Atlantic coast of Nova Scotia. Accretion rates are compared to a number of factors assumed to influence vertical marsh accretion: rates of relative sea-level rise, climatic parameters (average daily temperatures and degree days) and latitude (related to insolation and day length), sediment characteristics (organic matter inventory, bulk, mineral, and organic matter density), distance of the core site from the nearest source of tidal waters, and the tidal range. Uniques to our study is a consideration of climatic parameters and latitude, which should influence organic matter production, and thus vertical accretion rates. Significant predictors of accretion rates (in order of importance) were found to be organic matter inventory, distance from a creek, and range of mean tides. Contrary to conclusions from previous studies, we found that accretion rates decreased with increasing tidal range, probably because we considered a wider span of tidal ranges, from micro- to macrotidal. Although four marshes with low organic matter inventories also show a deficit in accretion with respect to relative sea-level rise, organic matter is not limiting in two-thirds of the marshes studied, despite shorter growing seasons.     

The ecological role of zooplankton in a Long Island salt marsh

The role of zooplankton in a salt marsh ecosystem was studied in Flax Pond, Old Field, N. Y., a 30-hectare tidal pond adjacent to Long Island Sound. Various marine crustaceans accounted for over 95% of the zooplankton caloric biomass in all but three months, in which ctenophores (Mnemiopsis leidyi) accounted for about 20%. Mass balance analysis of crustacean biomass showed a seasonal trend with increased “consumption” by the marsh from July to November. Analysis of groups (or species) for all months showed total numbers were reduced by passing through, or interacting with, the marsh. The most abundant group for each sample date also was significantly reduced from outflowing waters for all months, as were the group copepodids and miscellaneous calanoids from July to November. The energy requirements for the crustacean zooplankton community could have been supplied amply by the estimated standing crop of phytoplankton in the marsh. Phytoplankton net primary production was low, but it was ample to satisfy crustacean energy needs in all months. Crustaceans and phytoplankton alone were not enough to support estimated ctenophore nutrition requirements in summer. Therefore, detritus may also have been an important ctenophore food source.     

The effect of multiple stressors on salt marsh end-of-season biomass

It is becoming more apparent that commonly used statistical methods (e.g. analysis of variance and regression) are not the best methods for estimating limiting relationships or stressor effects. A major challenge of estimating the effects associated with a measured subset of limiting factors is to account for the effects of unmeasured factors in an ecologically realistic matter. We used quantile regression to elucidate multiple stressor effects on end-of-season biomass data from two salt marsh sites in coastal Louisiana collected for 18 yr. Stressor effects evaluated based on available data were flooding, salinity air temperature, cloud cover, precipitation deficit, grazing by muskrat, and surface water nitrogen and phosphorus. Precipitation deficit combined with surface water nitrogen provided the best two-parameter model to explain variation in the peak biomass with different slopes and intercepts for the two study sites. Precipitation deficit, cloud cover, and temperature were significantly correlated with each other. Surface water nitrogen was significantly correlated with surface water phosphorus and muskrat density. The site with the larger duration of flooding showed reduced peak biomass, when cloud cover and surface water nitrogen were optimal. Variation in the relatively low salinity occurring in our study area did not explain any of the variation inSpartina alterniflora biomass.     

Early diagenesis of fatty acids and hydrocarbons in a salt marsh environment

In a salt marsh environment, plant-produced fatty acids and aliphatic hydrocarbons undergo significant modification upon being deposited in sediment. The major changes include alteration of the distribution pattern of straight chain components, increase in the concentration of branched components and decrease in the concentration of unsaturated components. Changes are similar in both oxidizing and reducing environments. Carbon isotope measurements indicate that there is little horizontal movement of sediment organics. Spartina alterniflora has a δC¹³ = ? 12.5% (vs NBS 20) and Juncus romerianus has a δC¹³ = ?23.2%. Sediment organic matter taken from cores up to 60 cm in depth retains a δC¹³ value similar to the plant species growing at the core site. Microbial metabolism appears to be responsible for the observed in situ changes in sediment lipids. The in situ changes did not result in significant alteration of δC¹³ values.