Estuarine gradients determining the quality of tidal marsh halophytes as host plants for endophagous insect larvae: Experimental evidence

Phytophagous insects of estuarine tidal marshes which live inside their host plants, are, in contrast to the plants, not directly exposed to the estuarine salinity gradient. Previous field studies, however, have shown that patterns of growth and development ofAgapanthia villosoviridescens larvae, stem-borers of the halophyteAster tripolium, gradually change on tidal marshes along the Westerschelde estuary (HEMMINGA and VAN SOELEN, 1988). In the present study we carried out a laboratory experiment in which we followed growth ofA. villosoviridescens larvae from two different Westerschelde tidal marshes; the larvae either were kept inA. tripolium stems from their own marsh, or they were kept in stems from the other marsh. It was found that larvae from both tidal marshes showed larger weight increases inA. tripolium stems from the least saline marsh. Apparently, differences in host plant quality between stems of the two marshes exist. The results lend support to the hypothesis that growth and development, and distribution and abundance of phytophagous insects in estuarine tidal marshes may be indirectly influenced by estuarine gradients,via the host plant quality which changes along the estuary.     

The performance of the leaf mining microlepidopteran Bucculatrix maritima (Stt.) on the salt marsh halophyte,Aster tripolium (L.), exposed to different salinity conditions

Summary The performance of phytophagous insects is influenced by the nutritional quality of the food plant, which may vary with environmental conditions. Hardly any information exists on food-plant mediated effects of variable soil salinity on the performance of phytophagous insects. Conspicuous differences in salinity levels, however, are found in soils of intertidal wetlands such as salt marshes and mangroves. The growth of larvae of Bucculatrix maritima, a leaf miner of the salt marsh halophyte Aster tripolium, was studied on the host plant along the salinity gradient of the Westerschelde estuary (S.W. Netherlands). In addition, its performance on A. tripolium grown on low or high salinity culture medium was investigated experimentally. Although salinity conditions significantly influenced the chemistry of the host plants, insect performance seemed almost unaffected, although near the mouth of the estuary high environmental salinities may have caused some inhibition of larval growth. The results contrast with our previous studies on the stem-borer Agapanthia villosoviridescens, which showed that growth and development was conspicuously influenced by the changing characteristics of Aster tripolium along the estuarine salinity gradient. The location-dependent qualities of halophytes in an estuary thus appear to have species-specific effects on insect performance. We hypothesize that this phenomenon contributes to the existence of non-identical distribution patterns of phytophagous insects associated with the same halophyte in an estuary.     

Tidal dispersal of salt marsh insect larvae within the Westerschelde estuary

In the Westerschelde estuary, salt marshes are present as isolated patches fringing the estuary. In the present paper tidal transport of stem-boring larvae of Agapanthia villosoviridescens (Coleoptera) from salt marshes of the upper reaches of the Westerschelde estuary to marshes of the lower reaches is demonstrated. The evidence for the origin of the larvae is based on comparisons of growth and development characteristics of larvae found in flood debris belts and resident larval populations. These characteristics are different on the various salt marshes along the Westerschelde, probably as a result of estuarine gradients. Additional evidence for the larval origin comes from the plant composition of the flood debris. The occurrence of upward tidal transport is discussed. Considering the comparatively large area of salt marshes in the upper estuary, tidal dispersal of larvae probably will be dominated by transport in seaward direction. So far, very little is known on the role of tidal currents with regard to the exchange between salt marsh populations. The present results suggest that tidal transport may not only be important for dispersal of aquatic organisms in an estuary, but also for organisms inhabiting the semi-terrestrial estuarine salt marshes.     

Arbuscular mycorrhizal fungi enhance root cadmium and copper accumulation in the roots of the salt marsh plant Aster tripolium L.

It is known that vegetation plays an important role in the retention of heavy metals in salt marshes by taking up and accumulating the metals. In this study, we investigated whether arbuscular mycorrhizal fungi (AMF) increase Cd and Cu uptake and accumulation in the root system of the salt marsh species Aster tripolium L., and whether indigenous AMF isolated from polluted salt marshes have higher capacity to resist and alleviate metal stress in A. tripolium than isolates of the same species originated from non-polluted sites. Plants inoculated with Glomus geosporum, either isolated from a polluted salt marsh site (PL isolate) or from a non-polluted site (NP isolate), and non-mycorrhizal (NM) plants were compared in a pot experiment at four different Cd and Cu concentrations. Cd had no effect in root colonization, whereas high concentrations of Cu decreased colonization level in plants inoculated with the NP isolate. AM colonization did not increase plant dry weight or P concentration but influenced root Cd and Cu concentrations. Inoculation with PL and NP isolates enhanced root Cd and Cu concentrations, especially at highest metal addition levels, as compared to NM plants, without increasing shoot Cd and Cu concentrations. There was no evidence of intraspecific variation in the effects between AMF isolated from polluted and non-polluted sites, since there were no differences between plants inoculated with PL or NP isolate in any of the tested plant variables. The results of this study showed that AMF enhance metal accumulation in the root system of A. tripolium, suggesting a contribution of AMF to the sink of metals within vegetation in the salt marshes.     

Role of biotic interactions and physical factors in determining the distribution of marsh species along an estuarine salinity gradient

Understanding the mechanisms that shape plant distribution patterns is a major goal in ecology. We investigated the role of biotic interactions (competition and facilitation) and abiotic factors in creating horizontal plant zonation along salinity gradients in the Elbe estuary. We conducted reciprocal transplant experiments with four dominant species from salt and tidal freshwater marshes at two tidal elevations. Ten individuals of each species were transplanted as sods to the opposing marsh type and within their native marsh (two sites each). Transplants were placed at the centre of 9‐m² plots along a line parallel to the river bank. In order to disentangle abiotic and biotic influences, we set up plots with and without neighbouring vegetation, resulting in five replicates per site. Freshwater species (Bolboschoenus maritimus and Phragmites australis) transplanted to salt marshes performed poorly regardless of whether neighbouring vegetation was present or not, although 50–70% of the transplants did survive. Growth of Phragmites transplants was impaired also by competition in freshwater marshes. Salt marsh species (Spartina anglica and Puccinellia maritima) had extremely low biomass when transplanted to freshwater marshes and 80–100% died in the presence of neighbours. Without neighbours, biomass of salt marsh species in freshwater marshes was similar to or higher than that in salt marshes. Our results indicate that salt marsh species are precluded from freshwater marshes by competition, whereas freshwater species are excluded from salt marshes by physical stress. Thus, our study provides the first experimental evidence from a European estuary for the general theory that species boundaries along environmental gradients are determined by physical factors towards the harsh end and by competitive ability towards the benign end of the gradient. We generally found no significant impact of competition in salt marshes, indicating a shift in the importance of competition along the estuarine gradient.     

Impact of different sheep grazing intensities on salt marsh vegetation in northern Germany

Abstract. Mainland salt marshes in Schleswig-Holstein (northern Germany) have been grazed intensively by sheep for several decades. In 1988 experimental sites were established in the lower and middle salt marsh of Sönke-Nissen-Koog and subjected to different grazing intensities. From 1989 to 1993 the impact of sheep grazing on the composition and structure of the salt marsh vegetation was studied through the yearly analysis of permanent plots, vegetation mapping and measurements of the vegetation height. The intensively grazed site (10 sheep/ha) is covered by a short monotonous Puccinellia maritima sward with Salicornia europaea and Suaeda maritima. Halimione portulacoides and Aster tripolium, especially flowering plants, are rare. On the sites with 1.5 and 3 sheep/ha Puccinellia maritima remained dominant. The population density of Salicornia europaea decreases after reduction of the grazing intensity, whereas Suaeda maritima finds optimal growing conditions. Stands of Halimione portulacoides and flowering Aster tripolium plants are rare near the sea dike but their cover and size increases further away from the dike. In the plot with 1.5 sheep/ha the height of the vegetation increases along the gradient from the dike towards the tidal flats, due to local differences in actual grazing intensity. When grazing is stopped, Puccinellia maritima is successively replaced by Festuca rubra, Halimione portulacoides and Aster tripolium. High variability of vegetation height indicates structural diversity. Patches of higher and lower vegetation correspond with the distribution pattern of different plant species. In terms of nature conservation cessation of grazing is recommended.     

Growth strategies of four salt marsh plants on Mankyung River estuary in Korea

The emergence of seedlings, the length of roots and shoots, and the biomass of four dominant plant species and shore height were measured to investigate the growth strategy of these plants on the salt marsh of Mankyung River estuary. Four salt marsh plants showed a distinctive zonation, for example, Suaeda japonica was predominantly spread around the low salt marsh, Atriplex gmelini and Aster tripolium were in the middle, and S. asparagoides was in the upper part of the marsh. In terms of emergence of seedlings, S. japonica appeared first followed by A. gmelini, S. asparagoides, and A. tripolium. The growth strategies of halophytes were as follows: S. japonica germinated earlier than the other halophytes so that its root grew rapidly and extensively at the beginning of growth. This species adopted a continuous germination strategy, allowing growth whenever favorable conditions were provided. A. gmelini germinated later than S. japonica, as a quasi-simultaneous germination type, it showed the highest germination rate within the shortest time. Aster tripolium germinated later than any other halophyte. Since this species exhibited characteristics between the continuous germination type and the quasi-simultaneous germination type, it did not show a very high germination rate. Instead, it showed continuous germination and consistent growth of both above-ground and underground parts. Suaeda asparagoides showed an especially high emergence rate at the beginning of its growth. However, the high density retarded its growth until the middle stage. Its roots extended longer than the other halophytes, allowing it to grow well in the dry conditions of the upper marsh.     

Decomposition in estuarine salt marshes: the effect of soil salinity and soil water content

The relation between decomposition rates and soil salinity and moisture conditions in tidal marshes of the Westerschelde estuary was investigated. In the first part of the study, these soil factors were experimentally manipulated in field plots which were either screened from rainwater or which received an additional weekly supply of freshwater from April to September 1989. These treatments had no clear effect on soil salinities and moisture conditions in a low marsh site. Decomposition rates of Spartina anglica leaves (kept in litterbags in the plots) also did not differ between treatments. In screened plots of a middle marsh site, decomposition rate of Elymus pycnanthus leaves decreased significantly. The effect of the experimental treatments on soil moisture content was variable, but comparatively high soil salinity values (up to 61.3) were consistently found in these plots. It is suggested that the elevated salinity levels induced the decrease in decomposition rate.In the second part of the study, cellulolytic decomposition, measured by loss of tensile strength of strips of cotton test cloth, was investigated in relation to a non-manipulated range of soil salinities (3.8–24.2), by exposing the strips in a series of tidal marshes along the salt gradient of the Westerschelde estuary. No correlation between decomposition rate and soil salinity was found. In addition, no relation was found between decomposition rate and soil water content. The results of both parts of this study lead us to the hypothesis that rate limitation of decomposition in estuarine tidal marsh soils is found at high soil salinities only.     

Effects of temperature,salinity and cold stratification on seed germination in halophytes

Salt tolerance of halophytes corresponds with the habitat requirement of the species. It is an important factor during the germination phase and it can determine successful establishment. This paper presents the effects of alternating temperature–light regimes (4/8°C, 10/20°C, 20/32°C; 12 h dark: 12 h light) and different salinity levels (0, 200, 400, 600 mmol l²¹ NaCl) on seed germination of five halophytes, Halimione pedunculata, Bupleurum tenuissimum, Aster tripolium, Triglochin maritimum and Armeria maritima. The five species differ with respect to family and life‐form and spatially correspond to a decreasing salt gradient (i.e. distance from salt water, with H. pedunculata being the most tolerant and A. maritima being the least). Armeria maritima, A. tripolium and T. maritimum seeds were additionally subjected to a cold stratification experiment. The results showed that Halimione pedunculata, an annual therophyte of year‐round heavily saline habitats, was dormant under all experimental conditions. Bupleurum tenuissimum, a species typical to sites of varying salinity prone to leaching during spring and autumn rainfall, germinated best under cold and warm temperatures, but only under non‐saline conditions. Aster tripolium and T. maritimum, close neighbours in salt marshes, showed very similar germination behaviour: seeds of both species tolerated high levels of salinity and germinated best in summer temperatures during periods of highest soil salinity, and germination was significantly promoted by cold. Armeria maritima, a species usually found on the marginal fringes of saline habitats, germinated only under low salt levels and maximum germination was under cold (spring) and warm (autumn) temperatures, with no significant effect of cold stratification.     

The salt marshes of the Westerschelde and their role in the estuarine ecosystem

The salt marshes along the Westerschelde estuary have been influenced by various human activities of which reclamation has been a major cause for the loss of salt marsh area. The salinity gradient in the aquatic system is also mirrored in the vegetation of the salt marshes.The role of the salt marshes for the estuary as a whole is manifold but a major importance is their function as a sink for anthropogenic substances.The possible role as a carbon and mineral source for the estuary is discussed in this paper. It is estimated that the total area of salt marsh adds about 8% to the organic matter input in the estuary while the nutrient input may be as high as 25%.Communication nr. 403 of the Delta Institute, Yerseke.     

High occurrence of rare inland halophytes on post‐mining sites in western Ukraine

Rare inland halophytic vegetation including the associations Salicornietum prostratae, Spergulario marginatae‐Suaedetum prostratae, Puccinellietum limosae, com. Puccinellietum limosae – a variant of Aster tripolium subsp. pannonicus and Astero pannonici‐Bolboschoenetum compacti have been found on 3 localities in the Drohobych region of western Ukraine. These are isolated (‘island’) localities, almost unknown until today. Halophytes grew there on salted sites in the vicinity of the old salt industry. Species of halophytes previously not known from this region were discovered: Aster tripolium subsp. pannonicus, Bolboschoenus maritimus, Gypsophila paniculata, Limonium gmelinii, Lotus tenuis, Puccinellia distans, Sonchus palustris, Suaeda prostrata, Trifolium fragiferum var. bonannii, Triglochin maritima, Typha laxmannii and Zannichellia palustris subsp. pedicellata. The occurence of these plants strictly depends on the presence of salt in the soil and they are endangered by shrinkage of salted sites following the termination of the salt industry in Drohobych region.     

How do Mycorrhizas Affect C and N Relationships in Flooded Aster tripolium Plants?

The mycorrhizal associations established between plants and fungi have multiple effects on plant growth, directly affecting stress tolerance. This work aimed to explore arbuscular mycorrhizal (AM) effects on carbon and nitrogen relationships of Aster tripolium L. and consequently on its flooding tolerance. Mycorrhizal and non-mycorrhizal juvenile plants were submitted to non-flooding and tidal flooding conditions for 56 d. Tidal flooding reduced biomass, but the presence of mycorrhiza had an ameliorating effect. The AM symbioses seem to have, like flooding, a stressful effect on A. tripolium at an early stage of plant development. However, once the plant was established, an improvement of growth performance of plants with mycorrhiza under flooding conditions was observed. The better tolerance of AM plants to flooding was mediated through an improvement of the osmotic adjustment of the plant tissues (higher concentrations of soluble sugars and proline) and through the increment of nitrogen acquisition in tidal-flooded plants.     

Stomatal responses to sodium ions in Aster tripolium: a new hypothesis to explain salinity regulation in above-ground tissues

A study has been made of the ionic relations of stomata of Aster tripolium L., a maritime halophyte which colonizes coastal saltmarshes. The results obtained allow us to add this species to the growing list for which an involvement of K⁺ transport in stomatal movements has been demonstrated. However, an additional and ecologically important characteristic was found: there was a suppression of stomatal opening by increasing NaCl concentrations. A new hypothesis is offered of the mechanism for controlling salt and water relations in A. tripolium, a species which does not possess glands or other means of excreting salt. It is suggested that when the capacity of the tissues to accumulate salt in cell vacuoles is exceeded, the concentration of Na⁺ ions in the apoplast around the guard cells begins to rise. This causes partial stomatal closure, reduces transpiration and increases water-use-efficiency. Therefore, the flow of salt into the leaves is reduced but growth (and the manufacture of the new photosynthates required to support it) can continue. Aster tripolium can be added to the small list of known species which readily yield isolated epidermis suitable for detailed stomatal studies. Throughout this study, we have compared its stomatal physiology with C. communis, which has been thoroughly investigated in the past.     

Drought, but not salinity, determines the apparent effectiveness of halophytes colonized by arbuscular mycorrhizal fungi

The halophytes Plantago maritima, Aster tripolium, Artemisia santonicum, Puccinellia limosa, Festuca pseudovina and Lepidium crassifolium from two different saline soils of the Hungarian steppe were examined for colonization by arbuscular mycorrhizal fungi (AMF). The salt aster (A. tripolium) and the sea plantain (P. maritima) were examined more thoroughly by recording root colonization parameters, the salt content in the soil and monthly precipitations in 2001 and 2002. Mycorrhizal colonization was maximal in late spring to early summer and had a second peak later in the autumn. Arbuscule formation and overall mycorrhizal colonization appeared to be inversely correlated with the intensity of rainfall at the investigated sites. The results suggest that, in addition to seasonality, drought may play an important role in governing mycorrhizal activity in saline habitats. In greenhouse experiments, conditions in which AMF could overcome the inhibitory effects of sodium chloride on establishing plant–mycorrhizal symbiosis were not met.     

Contrasting Functional Performance of Juvenile Salmon Habitat in Recovering Wetlands of the Salmon River Estuary, Oregon, U.S.A.

For an estuarine restoration project to be successful it must reverse anthropogenic effects and restore lost ecosystem functions. Restoration projects that aim to rehabilitate endangered species populations make project success even more important, because if misjudged damage to already weakened populations may result. Determining project success depends on our ability to assess the functional state or “performance” and the trajectory of ecosystem development. Mature system structure is often the desired “end point” of restoration and is assumed to provide maximum benefit for target species; however, few studies have measured linkages between structure and function and possible benefits available from early recovery stages. The Salmon River estuary, Oregon, U.S.A., offers a unique opportunity to simultaneously evaluate several estuarine restoration projects and the response of the marsh community while making comparisons with a concurring undiked portion of the estuary. Dikes installed in three locations in the estuary during the early 1960s were removed in 1978, 1987, and 1996, creating a “space‐for‐time substitution” chronosequence. Analysis of the marsh community responses enables us to use the development state of the three recovering marshes to determine a trajectory of estuarine recovery over 23 years and to make comparisons with a reference marsh. We assessed the rate and pattern of juvenile salmon habitat development in terms of fish density, available prey resources, and diet composition of wild juvenile Oncorhynchus tshawytscha (chinook salmon). Results from the outmigration of 1998 and 1999 show differences in fish densities, prey resources, and diet composition among the four sites. Peaks in chinook salmon densities were greatest in the reference site in 1998 and in the youngest (1996) site in 1999. The 1996 marsh had higher densities of chironomids (insects; average 864/m²) and lower densities of amphipods (crustaceans; average 8/m³) when compared with the other sites. Fauna differences were reflected in the diets of juvenile chinook with those occupying the 1978 and 1996 marshes based on insects (especially chironomids), whereas those from the 1987 and reference marshes were based on crustaceans (especially amphipods). Tracking the development of recovering emergent marsh ecosystems in the Salmon River estuary reveals significant fish and invertebrate response in the first 2 to 3 years after marsh restoration. This pulse of productivity in newly restored systems is part of the trajectory of development and indicates some level of early functionality and the efficacy of restoring estuarine marshes for juvenile salmon habitat. However, to truly know the benefits consumers experience in recovering systems requires further analysis that we will present in forthcoming publications.     

Short-term sedimentation in Tagus estuary,Portugal: the influence of salt marsh plants

Short-term sediment deposition was studied at four salt marsh areas in the Tagus estuary. In areas covered with Sarcocornia perennis, Sarcocornia fruticosa, Halimione portulacoides and Spartina maritima and also in the non-vegetated areas, sedimentation was measured as the monthly accumulation of sediments on nylon filters anchored on the soil surface, from August 2000 to May 2001. Our experiments were used also to determine the influence of the different plant species in vertical accretion rates. Short-term sedimentation rates (from 2.8 to 272.3 g m⁻² d⁻¹) did show significant differences when the four salt marshes studied in the Tagus estuary were compared to each others. Salt marshes closer to the sediment sources had higher sedimentation rates. Our results suggest that the salt marsh type and surface cover may provide small-scale variations in sedimentation and also that sediment deposition values do change according to the position of the different plant species within the salt marsh. Sedimentation is an essential factor in salt marsh vertical accretion studies and our investigation may provide support to help forecast the adaptative response of the Tagus estuary wetlands to future sea level rise.     

Notes On The Occurrence Of Clathrus Cancellatus,Tournf., In Argyllshire

Background: Estuaries are characterised by salinity gradients and regular flooding events. These environmental factors form stress gradients, along which species composition changes.

Aims: Analyse and compare patterns of plant species diversity along the estuarine salinity and flooding gradients of the Elbe and Connecticut Rivers.

Methods: Vegetation was sampled at three elevations (low, mid, high) in five sites of each marsh type (fresh, brackish, salt) in both estuaries. Patterns of species density (SD) and evenness (E) along the gradients were analysed and compared between the two estuaries with three-factor ANOVAs.

Results: The regional species pool was 33% higher for the Connecticut than for the Elbe. SD of fresh marshes (19 ± 2.2) was more than twice in the Connecticut than in the Elbe. We found an overall increase in SD from low to high elevation and from salt to freshwater marshes in both estuaries. However, SD and E were strongly depressed at intermediate elevations in the Elbe fresh and brackish marshes.

Conclusions: Although diversity patterns in the two estuaries show overall similarities, patterns of SD and E differ, when particular elevational zones and marsh types are compared. We hypothesise this to be due to evolutionary and historical influences on the regional species pools, shaping the impact of local biotic and abiotic processes.     

A new species of Pemphigus Hartig (Hornoptera: Aphidoidea) from Western Europe

A new aphid species, Pemphigus trehernei, found on the roots of Aster tripolium L. in Western Europe is described. The gall, fundatrix and alate fundatrigenia were produced artificially on Populus nigra L. var. italica Duroi. The life cycle and host range of the species were investigated.     

Potential effects of physiological plastic responses to salinity on population networks of the estuarine crab <Emphasis Type="Italic">Chasmagnathus granulata</Emphasis>

Chasmagnathus granulata is a South American crab occurring in estuarine salt marshes of the Brazilian, Uruguayan and Argentine coasts. Life history is characterized by an export strategy of its larval stages. I reviewed information on experimental manipulation of salinity during embryonic and larval development (pre- and posthatching salinities), and on habitat characteristics of C. granulata in order to determine potential effects of larval response to salinity in the field and to suggest consequences for the population structure. Local populations are spread over coastal areas with different physical characteristics. Benthic phases occupy estuaries characterized by different patterns of salinity variation, and release larvae to coastal waters characterized by strong salinity gradients. The zoea 1 of C. granulata showed a strong acclimatory response to low salinity. This response operated only during the first weeks of development (during zoeae 1 and 2) since subsequent larval survival at low posthatching salinities was consistently low. Larvae developing at low salinity frequently followed a developmental pathway with five instead of four zoeal stages. The ability to acclimate and the variability in larval development (i.e. the existence of alternative developmental pathways) could be interpreted as a strategy to buffer environmental variability at spatial scales of local or population networks. Early survivorship and production of larvae may be relatively high across a rather wide range of variability in salinity (5–32‰). Plastic responses to low salinity would therefore contribute to maintain a certain degree of population connectivity and persistence regardless of habitat heterogeneity. Electronic Publication     

The effect of lead contamination of soils and air on its accumulation in pollen

Summary Concentrations of heavy metals in the honey, derived fromAster tripolium was the starting-point for this investigation. Lead content of pollen inAster tripolium andTaraxacum officinale is mainly due to airborne lead. Accumulation of other heavy metals such as zinc and copper in pollen occur mainly via a translocation process from roots to flowers.