Invasion of tamarisk (<Emphasis Type="Italic">Tamarix</Emphasis> spp.) in a southern California salt marsh

Exotic plants have been demonstrated to be one of the greatest threats to wetlands, as they are capable of altering ecosystem-wide physical and biological properties. One of the most problematic invaders in the western United States has been salt cedar, Tamarix spp., and the impacts of this species in riparian and desert ecosystems have been well-documented. Here we document large populations of tamarisk in the intertidal salt marshes of Tijuana River National Estuarine Research Reserve, a habitat not often considered vulnerable to invasion by tamarisk. Initial research demonstrates that there are multiple species and hybrids of Tamarix invading the estuary and that the potential impact of tamarisk within this salt marsh is significant. This highlights the need for managers and scientists to be aware of the problems associated with tamarisk invasion of coastal marine habitats and to take early and aggressive action to combat any incipient invasion.     

Salt marsh colonization by a rocky shore invader: Balanus glandula Darwin (1854) spreads along the Patagonian coast

Balanus glandula, an east Pacific acorn barnacle from rocky shores, was introduced to Mar del Plata, Argentina more than 40 years ago and has spread over 17 latitudinal degrees southward. Here we report the first record of this species living in a soft-bottom environment colonizing the salt marsh plant species Limonium brasiliense, Spartina densiflora, S. alterniflora and Sarcocornia perennis. In addition, we describe the size frequency distribution, density and spatial distribution of the barnacles colonizing the different plant species. The size frequency distribution of Balanus showed a bimodal pattern in all plants. Barnacles were mostly large in S. densiflora, but small in S. alterniflora, with more balanced distributions of small and large barnacles on S. perennis and L. brasiliense. The highest density of barnacles was observed on S. perennis (x = 35.8 ind/cm², SD = 40.5) and S. alterniflora (x = 33.8 ind/cm², SD = 23), while the lowest on L. brasiliense (x = 1.5 ind/cm², SD = 1.18) and S. densiflora (x = 0.17 ind/cm², SD = 0.09). More than 90% of the barnacles on any given plant were found living. While barnacles colonized only the first few centimeters above the soil surface level in S. alterniflora and L. brasiliense, they reached their highest point on S. perennis. The finding of a rocky shore species successfully colonizing soft-bottom marshes within an invaded region brings new perspectives to discussions in biological invasion ecology, and raises additional considerations for coastal environmental management.     

Crab: snail size-structured interactions and salt marsh predation gradients

We studied size-structured predator-prey interactions between blue crabs (Callinectes sapidus) and marsh periwinkles (Littoraria irrorata) with a combination of field studies, laboratory experiments and individual-based modeling. Size distributions of Littoraria differed among years at the same sites in a salt marsh and could largely be explained by dominance of strong cohorts in the population. At a given site, abundance increased with elevation above tidal datum. Size-selective predation by blue crabs does not appear to be an important regulator of snail size distributions but may have a major effect on local abundance. Laboratory studies indicated that predator-prey interactions between Callinectes and Littoraria are strongly size-dependent. Crabs were generally effective at feeding on periwinkles at size ratios greater than approximately 6 (crab width: snail length). At lower size ratios crabs were far less effective at manipulating the snails, which often survived but with damaged shells. An individual-based model which incorporated information about incidence of snail shell scarring (resulting from non-lethal interactions) and snail density, predicted reduced predation rates and smaller average crab size with distance from the low tide refugium for crabs.     

Lacunal allocation and gas transport capacity in the salt marsh grass Spartina alterniflora

Summary Lacunal allocation as the fraction of the total cross sectional area of leaves, stem bases, rhizomes, and roots was determined in both tall and short growth forms of Spartina alterniflora collected from natural monospecific stands. The results indicate that in both growth forms lacunal allocation is greater in stem bases and rhizomes than in leaves and roots and that tall form plants allocate more of their stem and rhizome to lacunae than short form plants.Measurements made in natural stands of Spartina alterniflora suggest that total lacunal area of the stem base increases with increasing stem diameter and that stem diameter increases with increasing plant height and above-ground biomass. However, the fraction of cross section allocated to lacunae was relatively constant and increased only with the formation of a central lacuna.Experimental manipulations of surface and subsurface water exchange were carried out to test the influence of flooding regime on aerenchyma formation. No significant differences in lacunal allocation were detected between plants grown in flooded (reduced) and drained (oxidized) sediments in either laboratory or field experiments. While aerenchyma formation in Spartina alterniflora may be an adaptation to soil waterlogging/anoxia, our results suggest that lacunal formation is maximized as a normal part of development with allocation constrained structurally by the size of plants in highly organic New England and Mid-Atlantic marshes.The cross sectional area of aerenchyma for gas transport was found to be related to the growth of Spartina alterniflora with stands of short form Spartina alterniflora exhibiting a lower specific gas transport capacity (lacunal area per unit below ground biomass) than tall form plants despite having a similar below-ground biomass supported by a 10 fold higher culm density. The increased specific gas transport capacity in tall vs. short plants may provide a new mechanism to explain the better aeration, higher nutrient uptake rates and lower frequency of anaerobic respiration in roots of tall vs. short Spartina alterniflora.     

Oxygen loss from Spartina alterniflora and its relationship to salt marsh oxygen balance

Spartina alterniflora has been reported to lose significant amounts of oxygen to its rhizosphere with potentially important effects on salt-marsh biogeochemical cycling and plant productivity. The potential significance of this oxidative pathway was evaluated using laboratory split-chamber experiments to quantify oxygen loss from intact root systems under a wide variety of pre-treatment and incubation conditions including antibiotics to inhibit microbial respiration. The aerenchyma system of S. alterniflora was found to transport O₂, N₂, Ar, and CH₄ from above-ground sources to its below-ground roots and rhizomes. While non-respiratory gases were observed to move from the lacunae to water bathing the root systems, net O₂ loss did not occur; instead oxygen present outside of the roots/rhizomes was consumed. Net oxygen loss was found when resistance to gas transport was reduced in the lacunae-rhizosphere pathway by placing the root systems in a gas phase and when plant respiration was significantly reduced. Root system respiration appeared to be the major variable in the plant oxygen balance. When root and rhizome respiration was inhibited using poisons or lowered by cooling, the oxygen deficit was greatly reduced and oxygen loss was indicated. The effect of seasonal temperature changes on root system oxygen deficit presents a possible explanation as to why Spartina produces root systems with respiration rates that cannot be supported by gas transport. Overall, while oxygen loss from individual plant roots is likely, integrating measured root system oxygen loss with geochemical data indicates that the mass amount of oxygen lost from S. alterniflora root systems is small compared to the total oxygen balance of vegetated salt marsh sediments.     

The fishes associated with three intertidal salt marsh creeks in a temperate southern African estuary

This study compares the fish assemblages in a range of intertidal salt marsh creeks in the Kariega Estuary, South Africa, as well as highlighting any differences between the ichthyofaunal structure of the creeks and adjacent Zostera bed habitats. The superimposition of environmental variables on the creek biotic groupings (60% similarity level) indicated consistent relationships between both the creek water depth and mouth area with the major creek fish clusters. Water temperature, salinity and turbidity did not display any pattern which could explain the disparity between the ichthyofaunal groupings. The results also indicated that intertidal creeks form a unique littoral habitat within the Kariega Estuary, with ichthyofaunal compositions very different to those found in nearby eelgrass beds. Furthermore, intertidal creeks in the Kariega system appear to be similar to creeks found elsewhere in the world in that their fish assemblages are dominated by 0+ juveniles, have a conspicuous marine transient and estuarine resident component, and very few piscivorous representatives. These results also give weight to the hypothesis that southern African salt marsh creek habitats serve as temporary refuges to juvenile fishes, a role that has also been proposed in other parts of the world.     

Variation with depth and season in metal sulfides in salt marsh soils

Seasonal monitoring of metal sulfides was carried out in four soils ofthe Ría de Ortigueira salt marshes. Soils from the high salt marsh (withsuboxic redox conditions at the surface), had low concentrations of ironsulfides (AVS and pyrite fraction) and thus a low degree of trace metalpyritization (DTMP) in surface layers (0–10 cm), butconcentrations of metals associated with the pyrite fraction increasedconsiderably at depth (27.5 cm). In the low salt marsh soils (withanoxic conditions at the surface) maximum concentrations of metal sulfides werefound in the surface layers of soils colonized by Spartina maritima. These results are explained by the double effectexerted by roots in strongly reduced soils. On the one hand, they stimulate theactivity of sulfate-reducing bacteria and on the other, they favour the partialoxidation of the soil, thus generating polysulfides with which Fe²⁺immediately precipitates as pyrite, whereas in the deepest, permanently anoxiclayers, pyrite must be formed in a reaction in which FeS is an intermediate, asfollows: FeS + H₂S FeS₂ + H₂.Concentrations of metal sulfides also varied greatly with the season, with twopatterns being distinguished. In soils colonized by S. maritima in both high and low salt marshes, the lowestconcentrations were found in summer. At this time of the year there is a netloss of metal sulfides throughout the profile, presumably due to physiologicalactivity of plants (evapotranspiration and release of oxygen from roots). Incontrast, maximum concentrations of AVS and pyritic metals were found in thesummer in the low salt marsh soils not colonized by vascular plants (creekbottom). In this case, the higher temperatures increased the activity ofsulfur-reducing bacteria leading to synthesis and accumulation of metalsulfidesin the soil.     

The salt marsh vegetation on the East Adriatic coast

Phytosociological characteristics and a rate of endangerment of the salt marsh vegetation in central and southern parts of the east Adriatic coast were studied. A total of 73 vegetation relevés were made and elaborated according to the standard procedures of the Braun-Blanquet method. Numerical analysis of the relevés and life forms was performed. Two associations of the class Arthrocnemetea-Puccinellio festuciformis-Arthrocnemetum fruticosi and Limonio-Artemisietum coerulescentis were recorded, together with the association Juncetum maritimo-acuti of the class Juncetea maritimi. These associations show no significant differentiation with regard to their floristic composition, but only in terms of particular species cover and frequency rates, as well as by life forms and microecological habitat conditions. These are specific habitats, rarely found in the studied area, and highly threatened by human activity (tourism, agriculture).     

Nitrogen resorption from senescing leaves of three salt marsh plant species

Seasonalvariation in leaf nitrogen of mature green and senescent leaves and nitrogenresorption efficiency in three plants (Spartina maritima, Halimioneportulacoides and Arthrocnemum perenne) of aTagus estuary salt marsh are reported. Total nitrogen concentrations in greenand senescent leaves were higher during winter (December and March). Soilinorganic nitrogen availability showed an opposite pattern with higherconcentrations during summer (June and September) when total leaf biomass washigher. Nitrogen resorption efficiency ranged between 31 and 76% andH. portulacoides was the plant that better minimizednitrogen loss by this process. Nitrogen resorption occurred mainly from thesoluble protein pool, although other fractions must have been broken down duringthe resorption process. No significant seasonal variation in nitrogen resorptionefficiency and no relation to leaf total nitrogen or soil nitrogen availabilitywere found. This suggests that the efficiency of the resorption process is notdetermined by the plant nitrogen status nor by the availability of the nutrientin the soil. Nevertheless, resorption from senescing leaves may play animportant role in the nitrogen dynamics of salt marsh plants and reduce thenitrogen requirements for plant growth.     

Environmental gradients,plant distribution,and species richness in arctic salt marsh near Prudhoe Bay,Alaska

Spatial patterns of plant cover and species composition in arctic salt marsh and salt affected tundra near Prudhoe Bay, Alaska reflect gradients in elevation, soil conductivity, and soil concentrations of the ions prevalent in seawater. Soil conductivity and soil concentrations of Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄ ⁼ and Cl^– were significantly related to site elevation, decreasing as elevation increased. Vascular plant species richness increased significantly as soil conductivity and soil ion concentrations decreased, and site elevation increased. Puccinellia phryganodes was the only species present in low elevation sites with low plant cover, high soil conductivity and high soil concentrations of Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄ ⁼ and Cl^–. Mid-gradient sites were dominated by Carex subspathaceae. Plant cover at these sites was greater than at lower elevation sites, but bare ground was still present. Higher elevation sites had the lowest concentrations of soil ions and the lowest soil conductivities. These sites had little bare ground, contained as many as 16 species, and were dominated by Dupontia fischeri and Eriophorum angustifolium. Ordinations indicated that a complex topographic gradient related most closely to elevation and site distance from the coast best explains variation in the vegetation cover. Irregular deposition along the coastline partially or completely buried three sites in peat or sand up to 20 cm deep. Such rapid changes in plant cover and species composition contributes to the community patch mosaic typical of these marshes. Results suggest an individualistic response of plant species to the environmental gradients in salt marsh and salt affected tundra and are indicative of successional models developed in other marginal arctic environments.     

Root and rhizome distribution as an indicator of upper salt marsh wetland limits

Four wetland plant species, Deschampsia cespitosa, Distichlis spicata, Grindelia integrifolia, and Salicornia virginica, were tested to determine the effect of soil moisture on the vertical distribution of roots and rhizomes. In an 8-month greenhouse experiment involving plants grown from seeds and rhizomes the occurrence of more than 65% of the root and rhizome mass in the upper 10 cm of soil was indicative of saturated conditions. Roots and rhizomes were more evenly distributed under field capacity and low moisture conditions. The percent of total root and rhizome biomass in the upper 10 cm of a 35 cm core may be useful in determining whether or not a plant is experiencing saturated soil conditions, a criterion often used for defining wetlands. Limited field studies support the conclusion of the greenhouse study.     

Decomposition of Spartina anglica,Elytrigia pungens and Halimione portulacoides in a Dutch salt marsh in association with faunal and habitat influences

Decomposition of Spartina anglica, Elytrigia pungens and Halimione portulacoides was studied for 20.5 months in situ in two habitats on a salt marsh in The SW Netherlands. Litter bags of three different mesh sizes were used to exclude meio- and/or macrofauna. The middle-marsh habitat was flooded more frequently than the plant-debris habitat in the highest marsh zone. Decomposition of the three species followed an exponential pattern of decay: instantaneous decay rates varied from 0.0026 to 0.0054 per day. Decay rates were significantly influenced by habitat factors and fauna, while there was a significant interaction between plant species and habitat. In case of a significant meio- and/or macrofauna effect, this became noticeable 12–16 weeks after the start of decomposition and resulted in a difference of 5–10% ash-free dry weight remaining after 20.5 months. Nematodes were the dominant microfaunal group in the plant litter. Densities were influenced by habitat conditions but not by resource quality, season and meio- and/or macrofauna. Only initial C/N and C/P ratios were correlated with differences in decomposition rates between the plant species. During the later stages of decomposition N and P concentrations of the plant litter were higher in the plant-debris habitat than in the middle-marsh habitat, probably as a result of fluctuating detritivores densities. The course of the decomposition process differed per plant species and per habitat. The results of this study underline the importance of knowledge of long-term decomposition rates.     

Saturnospora ahearnii,a new salt marsh yeast from Louisiana

Saturnospora ahearnii, a new, nonhyphal saturn-spored yeast, is described. The type strain (NRRL Y-7555, CBS 6121) of this species was isolated from the rhizosphere of marsh grass in Louisiana. Comparisons of nuclear DNA complementarity showedS. ahearnii to be closely related toS. saitoi, a species presently known only from Japan.     

Apparent facilitation of an invasive mealybug by an invasive ant

Summary In the southeast United States, the invasive ant Solenopsis invicta is known to derive important carbohydrate (honeydew) resources from mealybugs utilizing grasses. Most important appears to be an invasive mealybug, Antonina graminis. We studied whether this mealybug and a similar native species also benefit from association with S. invicta. We found that mealybug occurrence increases significantly with increasing proximity to S. invicta mounds, suggesting that mealybugs benefit as well. Mutual benefits derived by S. invicta and A. graminis are consistent with a hypothesis proposing that associations among invasive species can be important in their success at introduced locations.     

Distinct life histories of peracarid crustaceans in a Ria Formosa salt marsh (S. Portugal)

Pitfall traps were exposed between February 1993 andSeptember 1994 every 4th to 6th week near the highwater mark in a Ria Formosa salt marsh (Portugal).With only a few exceptions, peracarid crustaceansaccounted for more than 95% of the individualscaught. Three amphipod species (Orchestiagammarellus(Pallas), Orchestia mediterraneaCosta, Talorchestia deshayesii(Audouin)) and3 isopod species (Tylos ponticus(Arcangeli),Halophiloscia couchi(Kinahan) and Porcellio lamellatus(Uljanin) Budde-Lund) wereidentified. In general, these species were most activefrom spring to autumn.Data collected included information on growth rate,life expectancy and timing of reproduction. Theamphipods displayed more opportunistic life-historypatterns with high growth rates, reproductive activityduring most of the year, early sexual maturity andrelatively short life expectancies. The isopods weremore persistent with slower growth rates, morerestricted reproductive periods and longer lifeexpectancies.There is no indication in the literature that most ofthese species are particularly common in salt marshes.In addition, peracarid crustaceans are not mentionedas dominant taxa in most studies of salt marsh fauna.We suggest that the peracarids play an important rolein the degradation of organic matter in salt marshesand hypothesize that the high numbers of peracaridsfound in Ria Formosa is due to the high contributionof leaf shedding plants.     

Prescribed fire and cutting as tools for reducing woody plant succession in a created salt marsh

This paper reports on efforts to reduce woody successional growth by the native shrub Iva frutescens L. in a created salt marsh by using prescribed fire and cutting. Experimental treatments included a winter burn, cutting plants at ground level, and a combination burn-and-cut treatment, with replicate plots of each. Iva frutescens proved to be extremely hardy, with zero mortality following the cutting, burning, or combination treatment; similar levels of regrowth were observed for all treatments. Individual shrub response, however, was found to be related to initial plant size, ground water level and salinity, and two fire characteristics (total heating >60°C and total heat index >60°C). Fire severity, sediment nutrient concentrations, and other abiotic factors had no observable effects.     

Mobility of metals in salt marsh sediments colonised by <Emphasis Type="Italic">Spartina maritima</Emphasis> (Tagus estuary,Portugal)

Chemical associations of Zn, Pb, Cu, Co and Cd were determined using a sequential extraction procedure in sediments colonised by S. maritima in three salt marshes within the Tagus estuary: Rosário, Corroios and Pancas. Concentrations of these metals were also analysed in above- and belowground parts of Spartina maritima, as well as in sediments colonised by the plant. The highest metal concentrations in sediments were found in the marshes near the industrial and urban areas, whereas metal concentrations in plants were not significantly different among sites. This was thought to be a consequence of differences observed in metal bioavailability: Metals in Pancas, the least polluted location, were largely associated to easily accessible fractions for plant uptake, probably as a result of low organic matter content and high sandy fraction in sediments. S. maritima was able to induce the concentration of metals between its roots in the three salt marshes. The results obtained in this study indicate that S. maritima could be useful to induce phytostabilisation of metals in sediments, although the effectiveness to modify chemical associations is highly dependent on existing sediment parameters, and thus different results could be obtained depending on site characteristics. Guest editors: J. Davenport, G. Burnell, T. Cross, M. Emmerson, R. McAllen, R. Ramsay & E. Rogan Challenges to Marine Ecosystems     

Effects of grazing and inundation on pasture quality and seed production in a salt marsh

During a six-year period, changes in the composition of dominant plant species of importance to foraging birds in a salt marsh on the Swedish west coast were followed inside and outside exclosures to document effects of grazing on herbage quality and seed production. Since marshes provide an important habitat for foraging geese and ducks, it was of interest to determine how cattle grazing would affect herbage production inAgrostis stolonifera andPuccinellia maritima and seed and root-tuber production inScirpus maritimus. Measurements of cover and height in permanent plots revealed that a wetter weather type favouredAgrostis, probably through reduced salinity, at the expense ofPuccinellia, which was the most favoured food of both cattle and birds.Agrostis out-competedPuccinellia when grazing pressure was low. Seed production inScirpus maritimus was reduced by cattle grazing, particularly whenPhragmites australis formed part of the vegetation. In the absence of cattle grazing, both herbage- and seed producing plants were gradually reduced, andPhragmites increased. Since high herbage consumption and high seed production are mutually exclusive, grazing rotation in combination with mowing is suggested as a management strategy.     

Decomposition and nutrient release in halophytes of a Mediterranean salt marsh

This study dealt with the decomposition and nutrient release from the halophytes Atriplex portulacoides, Arthrocnemum macrostachyum, Limoniastrum monopetalum, and Spartina densiflora, the dominant species in the Castro Marim salt marsh, Portugal. Environmental effects on decomposition were also assessed. The study was carried out for one year using the in situ litterbag technique. S. densiflora showed a lower decomposition rate (k = 0.003 day⁻¹) than the other study species (k = 0.005-0.009). Study species showed similar decomposition patterns, that is, the weight loss mostly occurred during the autumn-winter period (study beginning in November). This indicates that temperature in this period did not hamper the decomposition process. The decomposition rate was positively affected by the initial N concentration (r² = 0.87, P < 0.05) and negatively by the C:N ratio (r² = 0.86, P < 0.05) in decomposing materials. At the end of the study, S. densiflora and L. monopetalum, the species with lower initial N concentrations, retained much higher proportion of initial N (89-109%) than the others (5-14%). Also, S. densiflora with the lowest P concentration retained higher proportion of initial P (48%) than the others (5-20%). Release of K and Mg were also slower from S. densiflora and was associated with their initial low concentration in this species. The lowest Mn release was observed from A. macrostachyum and also in relation to the lowest initial concentration. Our study supports the hypothesis that decomposition patterns of marsh species are mostly associated with differences regarding their morphology and chemical composition. Given the higher resistance of S. densiflora to decomposition, its progressive spreading may result in accumulation of organic detritus overtime in invaded salt marshes.     

Nitrogen addition does not reduce belowground competition in a salt marsh clonal plant community in Mississippi (USA)

Previous studies have suggested that belowground competition for nutrients influences plant zonation in salt marshes. In this study, I tested the hypothesis that competition for nitrogen structured a clonal plant community in a nitrogen-limited salt marsh in coastal Mississippi, USA. In contrast to most previous field studies that have investigated mechanisms of competition, I examined clonal growth responses of established genets of a nitrogen-demanding low-intertidal species (Spartina alterniflora) to nitrogen addition and the removal of a nitrogen-conserving high-intertidal species (Juncus roemerianus). Nitrogen addition stimulated clonal invasion of the Juncus zone by Spartina but did not reduce the significant competitive effects of Juncus on Spartina. Simulated Juncus shade did not reduce invasion of the Juncus zone by Spartina, indicating that belowground competition reduced clonal invasion. In the last year of the study, the border shifted unexpectedly towards the Spartina zone, resulting in competitive displacement of Spartina by Juncus. Nitrogen addition did not prevent or slow this displacement, further contradicting the nitrogen competition hypothesis. Although growth rates were much more strongly limited by nitrogen in Spartina than in Juncus, nitrogen addition did not cause the displacement of Juncus by Spartina after three growing seasons. I conclude that zonation of Spartina and Juncus is maintained by preemption of space and greater tolerance of low nitrogen supplies by Juncus in the high marsh. These results contrast sharply with findings of reduced belowground competition with nutrient addition in previous studies and highlight the important role of nutrient-mediated competition for space between clonal plants.