Eloy M. Castellanos

Gas Exchange and Chlorophyll Fluorescence of C3 and C4 Saltmarsh Species

Spartina maritima (Curtis) Fernald, Spartina densiflora Brong, Arthrocnemum perenne (Miller) Moss, and Arthrocnemum fruticosum (L.) Moq are very frequent halophytes on the coasts of SW Europe. The first two are perennial Gramineae with C4 metabolism; the last two are perennial Chenopodiaceae with C3 metabolism. Controlled garden experiments were carried out with the four species to compare their physiological response, i.e., water potential (Ψ), net photosynthetic rate (PN), transpiration rate (E), stomatal conductance (gs), intercellular CO2 concentration (Ci), and chlorophyll fluorescence of photosystem (PS) 2 under saline and non-saline conditions. S. maritima behaves as an osmoconformer species, the other three as osmoregulators. In the four species, PN, E, and gs improved following freshwater irrigation. The variations in PN might be related with biochemical changes (which appear not to affect PS2), but not with significant stomatal fluctuations, which are associated with a lower water use efficiency in the case of Arthrocnemum. The species were segregated into two groups (not depending on their C3 or C4 photosynthetic pathway), in relation with the topographic level of this species in natural conditions: the relative responses of PN in S. maritima and A. perenne were lower than those of S. densiflora and A. fruticosum. The salt-tolerance index supports such segregation. S. densiflora demonstrated the best competitive possibilities against salt-tolerant glycophytes, with its more flexible response in saline or brackish environments, which explains its spreading along the rivers draining into the estuaries of the SW Iberian Peninsula.

Short-term responses to salinity of an invasive cordgrass

Salinity is one of the main chemical factors in salt marshes. Studies focused on the analysis of salinity tolerance of salt marsh plants are very important, since they may help to relate their physiological tolerances with distribution limits in the field. Spartina densiflora is a South America cordgrass, which has started its invasion of the European coastline from the southwestern Iberian Peninsula. In this work, short-term responses in adult tussocks of S. densiflora from southwestern Spain are studied over a wide range of salinity in a greenhouse experiment. Our results point out that S. densiflora has a high tolerance to salinity, showing high growth and net photosynthesis rates from 0.5 to 20 ppt. S. densiflora showed at the lowest salinity (0.5 ppt) high levels of photoinhibition, compensated by higher levels of energy transmission between photosystems. Adaptative mechanisms, as those described previously, would allow it to live in fresh water environments. At the highest salinity (40 ppt), S. densiflora showed a high stress level, reflected in significant decreases in growth, net photosynthesis rate and photochemical efficiency of Photosystem II. These responses support S. densiflora invasion patterns in European estuaries, with low expansion rates along the coastline and faster colonization of brackish marshes and river banks.

Tiller dynamics of Spartina maritima in successional and non-successional Mediterranean salt marsh

Tiller demography was compared in two populations of Spartina maritima present at similar elevations in the coastal saltmarshes of Odiel (Huelva, S.W. Spain). The successional population consisted of colonizing tussocks in a littoral lagoon, and the non-successional population comprised a stable sward that had fringed a major channel for 40 years. At both sites S. maritima was replaced by Arthrocnemum perenne at higher elevation, where sediments were less reducing. Rapid, consistent sediment accretion confirmed the successional nature of the lagoon site but there was little net accretion in the stable sward.Census of permanent quadrats at the successional site chronicled moving concentric ‘waves’ of high tiller density as tussocks expanded. Initially high densities declined after one year to low values at the end of the second year but they had almost recovered after 3 years. The decline represented a combination of reduced numbers of births and increased numbers of deaths. Tiller densities were substantially higher in the stable sward and showed relatively small fluctuations with time. The underlying risk of tiller mortality was similar in the two populations for much of the time but after two years there was increased mortality, mainly associated with flowering, at the successional site; very few tillers flowered in the sward. This mortality contributed to a shift to a younger age structure in the successional population.Data aggregated over consecutive 3-monthly periods were examined for density dependence. None was found in the successional population. In the sward population there was evidence of density-dependent adult and juvenile mortality of tillers, particularly over the first 18 months of the study, when there were compensatory responses to subtle variations in density. The lack of density dependence and relatively low peak density of about 2000 m-2 near to the leading edges of the expanding tussocks at the successional site suggest that tiller placement there was regulated mainly by physiological mechanisms affecting rhizome growth and bud development in well integrated clones.

Temporal and spatial variation of an aquatic invertebrate community subjected to avian predation at the Odiel salt pans (SW Spain)

We studied the temporal variation in abundance and distribution of inverte- brates in the water column in both traditional and industrial salt pans in the Odiel marshes, south-west Spain, in 2001. We selected 12 ponds that were representative of different salinities and stages of salt production. Every two months, invertebrates were sampled within the 0 - 20 cm depth range used by foraging waders. We identified 40 taxa, including 30 aquatic and eight terrestrial metazoan invertebrates. Aquatic inverte- brate taxa richness and diversity decreased significantly with increasing pond salinity, whereas total biomass showed a non-significant increase. Overall, Artemia partheno- genetica constituted 67.6 % of invertebrates by number, and 95.5 % of the biomass. The copepod Cletocamptus retrogressus represented 31.1 % of invertebrates by num- ber, and 0.6 % of the biomass. Invertebrate biomass and dominance of A. parthenogen- etica was highest in September and lowest in November. There was significant spatial and temporal variation in abundance for all aquatic taxa, and for a given pond and month, the depth, distance to shoreline and fetch (wind effects) all had important par- tial effects on invertebrate distribution. Ordination methods showed a strong relation- ship between community structure and water chemistry (salinity, pH and redox poten- tial), with the salinity gradient being the most important. There was a clear separation between low salinity ponds with fish and submerged macrophytes and other ponds. The more intensively managed industrial salines held lower densities and biomass of invertebrates than traditional salines, perhaps owing to greater protection from wind or greater inputs of detritus. The beetle Ochthebius corrugatus was abundant in the tradi-

Seasonal variation in the diet of Redshank Tringa totanus in the Odiel Marshes, southwest Spain: a comparison of faecal and pellet analysis

Capsule Redshank diet from southern Europe during migration shows spatial and seasonal variations. Aims To assess seasonal variation in Redshank diet at a major passage site, and to compare data derived from analysing pellets or faeces. Methods At the Odiel Marshes in 2001, pellets from spring migration (39), autumn migration (121) and midwinter (15) were analysed, together with faecal samples from autumn (84). Results The abundance of different invertebrate groups in pellets varied between seasons. In spring, Chironomus salinarius pupae and larvae dominated by volume, followed by Ephydridae larvae and the beetle Paracymus aenus. Polychaetes and molluscs dominated in autumn, and isopods in midwinter. In autumn, chironomid larvae, Mesembryanthemum nodiflorum seeds and Artemia cysts were relatively more abundant in faeces, whereas polychaetes, isopods, molluscs and cestode cysticercoids were more abundant in pellets. Harder and/or larger items were thus relatively more abundant in pellets than faeces. Pellet analysis gave more emphasis to mudflat prey, and faeces to saltpan prey. Conclusion Pellet and faecal analysis give different results for wader diet, and it is useful to combine the two methods. However, they show significant correlations both in diet range and rank abundance of prey items. Redshank diet shows much seasonal and spatial variation in southern Europe.

CLONAL GROWTH AND TILLER DEMOGRAPHY OF THE INVADER CORDGRASS SPARTINA DENSIFLORA BRONGN. AT TWO CONTRASTING HABITATS IN SW EUROPEAN SALT MARSHES

Salt marshes offer a valuable opportunity to study the effects of environment on the clonal growth of species able to live throughout a wide vertical range on the tidal frame. Spartina densiflora is a species native to South America that is invading marshes in SW Europe, NW Africa, and SW North America, where its populations are found from low to high topographic elevations, altering the composition of plant communities. The aim of this study was to increase our knowledge regarding the competitive ability of S. densiflora, analyzing its clonal growth and its ramet demography during 25 months in two expanding populations at a low and a high marsh. Four clonal characteristics were recorded in both populations: (1) dense occupation of available space inside tussocks; (2) lack of a dormant period and high tiller production rates and growth; (3) evidence of physiological integration between ramets; and (4) high rates of sexual reproduction. Furthermore, S. densiflora developed different strategies of clonal growth in contrasting habitats. Tussocks in the low marsh had lower tiller longevity and higher tiller density, natality, and mortality rates, showing faster ramet turnover than in the high marsh. These clonal growth traits would facilitate S. densiflora persistence under the effects of catastrophic events. Spartina densiflora clonal growth traits indicate strong adaptability to different environmental conditions and strong competitive ability, since its tiller dynamics enable it to invade occupied space, and it effectively colonizes new safe-sites by sexual reproduction. These observations help us to understand how this invader has become the most abundant plant in many estuaries of the SW Iberian Peninsula.

Environmental determination of shoot height in populations of the cordgrass Spartina maritima.

Spartina species tend to exhibit a range of phenotypes, often with short and tall growth forms. Such differences have been attributed variously to environmentally induced phenotypic plasticity and genetic diferentiation between populations. This work examines the basis of height variation inSpartina maritima (Curtis) Fernald at Odiel salt marshes, southwest Spain. Populations from sites with lower sediment redox potentials tended to have significantly taller shoots. Thirty-four natural populations with an 8-fold range of shoot height were transplanted to a common environment on an unvegetated, intertidal plain and shoot height was measured annually for 3 yr. There was a striking convergence in height across populations after transplantation and the change in height in each year of a population was linearly related to its initial height. Most populations grew taller after transplantation, suggesting environmental limitation in their natural habitats. Populations that were originally tall tended to become shorter. The change in shoot height was negatively related to the difference in surface sediment redox potential between their natural sites and the common transplant site. Hypoxic sediments may stimulate stem growth, resulting in improved photosynthetic gas exchange and internal aeration of roots and rhizomes. Although height variation inS. maritima appears mainly to be a result of phenotypic plasticity, a genetic component cannot be ruled out. This study emphasizes the importance of long-term studies, preferably longer than turnover time of shoot populations. The highly plastic growth form ofS. maritima allows it to colonize a wide range of habitats in environmentally heterogeneous salt marshes.

Causes and consequences of salt-marsh erosion in an Atlantic estuary in SW Spain

This study reports on the quantification of horizontal erosion by undermining of slopes in the atlantic mesotidal salt marshes of Odiel, SW Spain, and analyses its causes and consequences. Horizontal erosion has produced considerable losses of salt marsh area, including zones of mature salt marsh. Human pressure, such as from water-borne traffic or the exploitation of the slopes for the capture of bait, increases the natural erosion processes. The role of vegetation in protecting the slopes against erosion is studied. Channel banks covered with plants, many of which belong to species with long-living, above-ground creeping stems, were less eroded than those without vegetation cover. The enormous volume of sediments moved (ca. 7000m3 in one year) could contribute to the silting-up of the navigable channels of the estuary, so that continual dredging is necessary to allow access to shipping. These sediments are highly contaminated, and dredging exposes them more directly to the trophic network of the estuary. There is a considerable loss of natural resources. Finally, the integrated management of this coastal ecosystem is discussed.

Salinity alleviates zinc toxicity in the saltmarsh zinc-accumulator Juncus acutus

The potential importance of Juncus acutus for remediation of Zn-contaminated lands has been recognized, because of its Zn tolerance and capacity to accumulate Zn. Since it is also a halophyte, the extent to which salinity influences its Zn tolerance requires investigation. A factorial greenhouse experiment was designed to assess the effect of NaCl supply (0 and 85 mM NaCl) on the growth, photosynthetic physiology and tissue ions concentrations of plants exposed to 0, 30 and 100 mM Zn. Our results indicated that NaCl supplementation alleviated the effects of Zn toxicity on growth, as Zn at 100 mM reduced relative growth rate (RGR) by 60% in the absence of NaCl but by only 34% in plants treated also with NaCl. This effect was linked to a reduction in Zn tissue concentrations, as well as to overall protective effects on various stages in the photosynthetic pathway. Thus, at 85 mM NaCl plants were able to maintain higher net photosynthesis (AN) than in the absence of added NaCl, although there were no differences in stomatal conductance (gs). This contributed to preserving the trade-off between CO2 acquisition and water loss, as indicated by higher intrinsic water use efficiency (iWUE). Hence, AN differences were ascribed to limitation in the RuBisCO carboxylation, manifested as higher intercellular CO2 concentration (Ci), together with dysfunction of PSII photochemistry (in term of light harvest and energy excess dissipation), as indicated by higher chronic photoinhibition percentages and variations in the photosynthetic pigment profiles in presence of Zn under non-saline conditions.