Belowground decomposition of mangrove roots in Florida coastal everglades

Mangrove root decomposition rates were measured by distributing mesh bags containing fine root material across six sites with different soil fertility and hydroperiod to compare ambient differences to substrate quality. Roots from a site with lower soil phosphorus concentration were used as a reference and compared to ambient roots at five other sites with increased phosphorus concentration. Four mesh bags of each root type (ambient versus reference), separated into four 10-cm replicate intervals, were buried up to 42 cm depth at each site and incubated for 250 d (initiation in May 2004). Mass loss of ambient mangrove roots was significant at all study sites and ranged from 17% to 54%; there was no significant difference with depth at any one site. Reference decomposition constants (−k) ranged from 0.0012 to 0.0018 d⁻¹ among Taylor Slough sites compared to 0.0023–0.0028 d⁻¹ among Shark River sites, indicating slower decomposition rates associated with lower soil phosphorous and longer flood duration. Reference roots had similar decomposition rates as ambient roots in four of the six sites, and there were no significant correlations between indices of root substrate quality and decomposition rates. Among these distinct landscape gradients of south Florida mangroves, soil environmental conditions have a greater effect on belowground root decomposition than root substrate quality.     

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.     

Evaluating the progress of restored cordgrass (Spartina foliosa) marshes: Belowground biomass and tissue nitrogen

We report the first data on belowground tissue mass and nitrogen (N) concentration forSpartina foliosa in southern California, assessing one natural and two constructed marshes on San Diego Bay. Biomass at the natural marsh was low compared to that of otherSpartina spp., but higher than values reported forS. foliosa in northern California. In sandy constructed marshes planted 5 and 10 years before this study,S. foliosa had lower belowground tissue N, lower N crop (%N×biomass), and shallower roots than in the adjacent natural marsh. We took advantage of a 2-yr, large-scale fertilization project being performed in the older constructed marsh and examined biomass and N storage after N additions. Although there was a trend toward N accumulation with fertilization, N crop remained at approximately 50% of natural marsh levels, unlike the large aboveground responses to N addition in our previous studies. Lower belowground reserves help to explain poor aerial growth in the created marshes and suggest the need for finer sediments (with greater potential for holding and supplying nutrients) to sustain (S. foliosa. While fine sediments are beginning to accumulate on the surface of the created marshes, vertical accretion is more likely to shift the plant community toward other species than to enhanceS. foliosa growth. We suggest salvaging and importing fine, organic marsh sediments or providing organic amendments to establish proper substrate conditions. Overexcavating and allowing fine sediments to accumulate remains an option, although the time scale is unpredictable due to the stochasticity of accretion events.     

Damage to cordgrass by scale insects in a constructed salt marsh: Effects of nitrogen additions

Because tall cordgrass (Spartina foliosa) is needed for nesting by the endangered light-footed, clapper rail, managers of constructed salt marshes in southern California are proposing large-scale nitrogen fertilization to improve cordgrass growth. How this might affect an existing infestation of scale insects (Haliaspis spartina) and the degree of damage these insects cause to their cordgrass hosts was unknown. We explored the effects of timing and duration of fertilization onHaliaspis damage to cordgrass, as well as the timing ofHaliaspis dispersal, in a constructed marsh at Sweetwater Marsh National Wildlife Refuge in San Diego Bay, California. Fertilization did not result in increasedHaliaspis abundance. After a large dispersal pulse in late May,Haliaspis establishment in the long-term fertilized plots was greater than in the controls; however, this trend reversed in August, when many more stems in the control plots were infested with large numbers ofHaliaspis. Since adultHaliaspis cannot leave a feeding site, losses of individuals in the fertilized plots were apparently due to mortality, perhaps resulting from mechanical or chemical changes in the fertilized plants or increased predation. Late in the growing season, plots fertilized with 10 applications of urea over 20 wk had the lowest meanHaliaspis abundance. Plots fertilized only in March, April, June, or August did not differ from controls in meanHaliaspis abundance.Haliaspis was never abundant in the fertilized or control plots in the adjacent natural marsh. This study suggests that fertilization, of constructed salt marshes in San Diego Bay may proceed without concern that furtherHaliaspis outbreaks will be facilitated.     

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.     

Growth dynamics of cordgrass,Spartina alterniflora loisel., on control and sewage sludge fertilized plots in a Georgia salt marsh

Seasonal plant growth dynamics were followed for a year in undisturbed plots of tall and short formSpartina alterniflora Loisel. and in plots of short formS. alterniflora which were enriched with sewage sludge at a rate of 100 g dry sludge m^?2wk^?1, corresponding to a nitrogen enrichment of 2 g N m^?2wk^?1. Monthly determinations of aboveground live and dead biomass, density of live stems, the ratio of number of young shoots to total number of shoots, and belowground mass of macro-organic matter to a depth of 30 cm were made for each area. Sludge fertilization increased the live biomass of the short formS. alterniflora by up to 150% of the control live biomass, but had little effect on the dead biomass, stem density, or proportion of young shoots. There was a trend of increased amount of belowground macro-organic matter in fertilized compared to control plots during the last 6 months of the study. In all areas, there was a marked decrease in the proportion of young shoots from winter to early summer, followed by a rapid increase in the percent of young shoots from late summer to fall. Sampling of plots 7 and 20 months after termination of sludge enrichment showed higher plant biomass and % N content in surface soils, but no difference in N content of live plant tissue, in fertilized compared to unfertilized marsh. After 20 months, about half of the sludge nitrogen remaining in the soils of the fertilized plots had disappeared.     

The potential role of roots and rhizomes in structuring salt-marsh benthic communities

The density of the Carolina marsh clam,Polymesoda caroliniana (Bosc), was determined in three adjacent tidal marsh communities which differed only in plant species composition. Clam density was inversely related to the density (biomass) of plant roots and rhizomes in sediments and directly related to density of plant stems (numbers). Clam abundance was not related to the basal area of plant stems. Each plant community contained clams of various ages from juvenile to adult indicating continued recruitment and survival. These data suggest thatP. caroliniana is most abundant inJuncus roemerianus marshes because there are fewer roots and rhizomes (mean of 2.5 kg m^?2) to hamper burrwing as compared toSpartina alterniflora andcynosuroides (5.1 and 6.3 kg m^?2, respectively) dominated marshes. Salinity, floding frequency, distance from flooding water, and sediment type were essentially constant among the three plant communities. Root/rhizome density should be collected along with other environmental parameters during studies of benthic organisms on marshes because it potentially limits the occurrence or abundance of some species.     

Plant growth and productivity along complex gradients in a Pacific northwest brackish intertidal marsh

Environmental characteristics were measured and recorded in the Skagit Marsh, a brackish intertidal marsh on Puget Sound, Washington. Four transects were placed perpendicular to a known gradient of increasing salinity which began with fresh water at the bank of one of the outlets of the Skagit River and reached a surface water salinity of 22‰ at a point alongshore 5 km north of the outlet. The environmental characteristics which were measured varied along gradients (soil texture, organic carbon in fines, soil column temperature, free soil water salinity) or had a patchy distribution (soil redox potential, soil macro-organic matter). Growth and production vary across the marsh. The maximum aboveground standing crop (1,742 g m^?2 dry weight) was measured at a site with 0–4‰ free soil water salinity, dominated by the sedgeCarex lyngbyei. In more saline areas (8–12‰), the bulrushScirpus americanus was dominant and standing crop values dropped to a third of the maximum. Species performance varied in a complex manner as did the environment.C. lyngbyei had diminished growth and decreased standing crop in areas where salinity was higher.S. americanus was equally productive in low elevation, high salinity sites and in high elevation, low salinity sites. An increase in shoot density for dominant species occurred in saline areas as individual shoot weights and leaf areas decreased. Because species responded differently, environmental variation was magnified in the population and community responses of the marsh vegetation.     

The influence of subsurface hydrology on nutrient supply and smooth cordgrass (Spartina alterniflora) production in a developing barrier island marsh

The supply of nutrients from surface and subsurface water flow into the root zone was measured in a developing barrier island marsh in Virginia. We hypothesize that high production of tall-formSpartina alterniflora in the lower intertidal zone is due to a greater nitrogen input supplied by a larger subsurface flux. Individual nitrogen inputs to the tall-form and short-formS. alterniflora root zones were calculated from water flow rates into the root zone and the nutrient concentration corresponding to the source of the flow. Total dissolved inorganic nitrogen (DIN) input (as ammonium and nitrate) was then calculated using a summation of the hourly nutrient inputs to the root zone over the entire tidal cycle based on hydrologic and nutrient data collected throughout the growing season (April–August) of 1993 and 1994. Additionally, horizontal water flow into the lower intertidal marsh was reduced experimentally to determine its effects on nutrient input and plant growth. Total ammonium (NH₄ ⁺) input to the tall-formS. alterniflora root zone (168 μmoles 6 h^?1) was significantly greater relative to the short-form (45 μmoles 6 h^?1) during flood tide. Total NH₄ ⁺ input was not significantly different between growth forms during ebb tide, and total nitrate (NO₃ ^?) and total DIN input were not significantly different between growth forms during either tidal stage. During tidal flooding, vertical flow from below the root zone accounted for 71% and horizontal flow from the adjacent mudflat accounted for 19% of the total NH₄ ⁺ input to the tall-formS. alterniflora root zone. Infiltration of flooding water accounted for 15% more of the total NO₃ ^? input relative to the total NH₄ ⁺ input at both zones on flood tide. During ebb tide, vertical flow from below the root zone still accounted for the majority of NH₄ ⁺ and NO₃ ^? input to both growth forms. After vertical flow, horizontal subsurface flow from upgradient accounted for the next largest percentages of NH₄ ⁺ and NO₃ ^? input to both growth forms during ebb tide. After 2 yr of interrupted subsurface horizontal flow to the tall-formS. alterniflora root zone, height and nitrogen content of leaf tissue of treatment plants were only slightly, but significantly, lower than control plants. The results suggest that a dynamic supply of DIN (as influenced by subsurface water flows) is a more accurate depiction of nutrient supply to macrophytes in this developing marsh, relative to standing stock nutrient concentrations. The dynamic subsurface supply of DIN may play a role in spatial patterns of abovegroundS. alterniflora production, but determination of additional nitrogen inputs and the role of belowground production on nitrogen demand need to also be considered.     

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.     

A method for quantifying living roots of Spartina (cordgrass) and Juncus (needlerush)

Quantifying living roots in a marsh is a necessary but difficult task in wetland research. The two main difficulties usually encountered are distinguishing living from dead roots and processing a dense mat of fine roots. We found that living roots of salt-marsh plants release much more dissolved organic carbon (DOC) in boiling water than dead roots. Based on the finding, we developed a DOC procedure to quantify living roots of Spartina alterniflora and Juncus roemerianus. The DOC released in boiling water is a function of root activity, and the amount released can be used to calculate the living root biomass of a sample. The amount of living roots determined by the DOC method correlated well with the amount of living roots determined by the manual, sorting method (r² = 0.78, p<0.01). The DOC method is more objective, precise, and much less tedious than the manual sorting method.     

Organic matter fluxes and marsh stability in a rapidly submerging estuarine marsh

We studied organic matter cycling in two Gulf Coast tidal, nonsaline marsh sites where subsidence causes marine intrusion and rapid submergence, which mimics increased sea-level rise. The sites experienced equally rapid submergence but different degrees of marine intrusion. Vegetation was hummocked and much of the marsh lacked rooted vegetation. Aboveground standing crop and production, as measured by sequential harvesting, were low relative to other Gulf CoastSpartina patens marshes. Soil bulk density was lower than reported for healthyS. alterniflora growth but that may be unimportant at the current, moderate sulfate levels. Belowground production, as measured by sequential harvesting, was extremely fast within hummocks, but much of the marsh received little or no belowground inputs. Aboveground production was slower at the more saline site (681 g m^?2 yr^?1) than at the less saline site (1,252 g m^?2 yr^?1). Belowground production over the entire marsh surface averaged 1,401 g m^?2 yr^?1 at the less saline site and 585 g m^?2 yr^?1 at the more saline site. Respiration, as measured by CO₂ emissions in the field and corrected for CH₄ emissions, was slower at the less saline site (956 g m^?2 yr^?1) than at the more saline site (1,438 g m^?2 yr^?1), reflecting greater contributions byS. alterniflora at the more saline site which is known to decompose more rapidly thanS. patens. Burial of organic matter was faster at the less saline site (796 g m^?2 yr^?1) than at the more saline site (434 g m^?2, yr^?1), likely in response to faster production and slower decomposition at the less saline site. Thus vertical accretion was faster at the less saline site (1.3 cm yr^?1) than at the more saline site (0.85 cm yr^?1); slower vertical accretion increased flooding at the more saline site. More organic matter was available for export at the less saline site (1,377 g m^?2 yr^?1) than at the more saline site (98 g m^?2 yr^?1). These data indicated that organic matter production decreased and burial increased in response to greenhouse-like conditions brought on by subsidence. *** DIRECT SUPPORT *** A01BY069 00016     

Microalgal productivity, community composition, and pelagic food web dynamics in a subtropical, turbid salt marsh isolated from freshwater inflow

Carbon entering the food web originating from microalgal productivity may be as important to salt marsh consumers as carbon originating from vascular plant production. The objective of this study was to further our understanding of the role played by microalgae in salt marshes. We focused on microalgal productivity, community dynamics, and pelagic food web linkages. Across three consecutive springs (2001–2003), we sampled the upper Nueces Delta in southeast Texas, United States; a shallow, turbid system of ponds and elevated vegetated areas stressed by low freshwater inflow and salinities ranging from brackish (11) to hypersaline (300). Despite high turbidity and low external nutrient loadings, microalgal productivity was on the order of that reported for vascular plants. Primary productivity in surface waters ranged from 0 to 2.02 g C m⁻² d⁻¹ and was usually higher than primary productivity associated with the benthos, which ranged from 0 to 1.14 g C m⁻² d⁻¹. This was likely due to high amounts of wind-driven resuspended sediment limiting production at greater depths. Most of the water column microalgal biovolume seemed to originate from the benthos and was comprised mostly of pennate diatoms. But true phytoplankton taxa were also observed, which included cryptomonads, chlorophyhtes dinoflagellates, and cyanobacteria. Succession from r-selected to K-selected taxa with the progression of spring, a common phenomena in aquatic systems, was not observed. Codominance by both potentially edible and less edible taxa was found. This was likely due to decreased grazing pressure on r-selected taxa as salinity conditions became unfavorable for grazers. In addition to a decoupled food web, reduced primary and net productivity, community respiration, and microalgal and zooplankton population densities were all observed at extreme salinities. Our findings suggest that a more accurate paradigm of salt marsh functioning within the landscape must account for microalgal productivity as well as production by vascular plants. Because the value of microalgal productivity to higher trophic levels is taxa specific, the factors that govern microalgal community structure and dynamics must also be accounted for. In the case for the Nueces Delta, these factors included wind mixing and increasing salinities.     

Seasonal and spatial distribution of the Carolina marsh clamPolymesoda caroliniana (Bosc) in a Mississippi tidal marsh

The abundance of the marsh clamPolymesoda caroliniana at four stations on a Mississippi tidal marsh was followed from February 1977 through April 1978. The stations varied with respect to inundation time and vegetative cover. The largest population was found in an area flooded 12% of the year (136 individuals per m²) and in an area flooded 3.2% of the year (126 individuals per m²). However, the latter area contained no large clams (>25.5 mm). Smaller populations were found in the upper reaches of a tidal creek draining the marsh (68 individuals per m²) and in an area flooded 5.8% of the year (73 individuals per m²). The average abundance of the clams on the marsh was 100 per m². At least three recruitment periods were apparent. In laboratory studies, clams larger than 15 mm survived longer than 30 days without water. Clams smaller than 12 mm survived less than 6 days without water. Several decapods (Callinectes sapidus, Uca longisignalis, andSesarma reticulatum) consumed clams smaller than 10 mm but were unable to eat larger clams under laboratory conditions.Polymesoda caroliniana may be important to the functioning of the marsh by removing large quantities of particulate matter from flooding water.     

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.     

Nutrient and metal accumulation in a freshwater tidal marsh

Stratigraphic records from sediment cores collected in a freshwater tidal marsh and in the estuary upstream and downstream from the marsh were used to determine the accumulation of nutrients and trace metals over long time periods. Analysis of pollen and seeds show that the high marsh has formed only within the past 100 yr, following increased sedimentation rates in the area. Variations in nutrient and trace metal accumulations over several decades show that pollutants from agricultural runoff and wastewater discharge are stored to a greater extent in high-marsh than in low-marsh sediments. Greater accumulation rates in the high marsh are probably related to its greater sedimentary organic carbon concentration.     

The value of salt marsh edge vs interior as a habitat for fish and decapod crustaceans in a Louisiana tidal marsh

Flume nets of various lengths and a 3-m seine were used to sample the fishes and macrocrustaceans using a flooded Louisiana salt marsh and the adjacent tidal creek. The experiment allowed for species-specific comparisons of the flooded marsh at the creek edge versus the interior. Of the 37,667 organisms collected in flume nets from January through November 1989, 89% were decapods (nine species) and 11% were fish (29 species). An additional 18,539 organisms (75% decapods and 25% fish) were collected from concurrent seine samples taken from July through November. Comparison of catches among different flume lengths and low tide versus high tide seine collections revealed distinct patterns of marsh habitat utilization. Densities of most organisms were highest within 3 m of the water’s edge, but significant numbers of marsh-resident fish species used the interior marshes. The edge marshes appeared to be used by both transient and resident species; however, the interior marshes were used primarily by marsh-resident species (Cyprinodontiformes andPalaemonetes sp.) that are excellent food sources for adult transient-species. Four zonations of marsh use are described for transients, residents, and rare species.     

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.     

Changing elevation,accretion, and tidal marsh plant assemblages in a South San Francisco Bay tidal marsh

Analyses of organic content, pollen, and the carbon-isotopic composition of a 3.5-m sediment core collected from a subsided tidal marsh located in South San Francisco Bay, California, have provided a 500-yr record of sediment accretion and vegetation change before, during, and after a rapid 1 m increase in sea level. Core chronology was established using¹⁴C dating of fossil plant material, the first appearance of pollen types produced by plants not native to California, and changes in lead concentrations coincident with anthropogenic contamination. Prior to the mid 19th century, rates of sediment accretion were between 1 and 4 mm yr⁻¹; sediment accretion accelerated to an average of 22 mm yr⁻¹ following the initiation of subsidence. Changes in tidal marsh vegetation also accompanied this depositional change. Vegetation shifted from a high to low marsh assemblage, as indicated by a larger percentage of grass pollen, rhizomes ofSpartina foliosa, and a strong C₄ signal. Between 1980 and 2001, Triangle marsh again developed high marsh vegetation, as indicated by higher percentages of the Amaranthaceane pollen type, seed deposition, includingSalicornia spp., and more negative carbon isotopic ratios.