(Halimione) portulacoides is a halophytic, C₃ shrub. It is virtually confined to coastal salt marshes, where it often dominates the vegetation. The aim of this study was to investigate its growth responses to salinity and the extent to which these could be explained by photosynthetic physiology. The responses of young plants to salinity in the range 0–700 mol m⁻³ NaCl were investigated in a glasshouse experiment. The performance of plants was examined using classical growth analysis, measurements of gas exchange (infrared gas analysis), determination of chlorophyll fluorescence characteristics (modulated fluorimeter) and photosynthetic pigment concentrations; total ash, sodium, potassium and nitrogen concentrations, and relative water content were also determined. Plants accumulated Na⁺ approximately in proportion to external salinity. Salt stimulated growth up to an external concentration of 200 mol m⁻³ NaCl and some growth was maintained at higher salinities. The main determinant of growth response to salinity was unit leaf rate. This was itself reflected in rates of CO₂ assimilation, which were not affected by 200 mol m⁻³ but were reduced at higher salinities. Reductions in net photosynthetic rate could be accounted for largely by lower stomatal conductance and intercellular CO₂ concentration. Apart from possible effects of osmotic shock at the beginning of the experiment, salinity did not have any adverse effect on photosystem II (PSII). Neither the quantum efficiency of PSII (Φ_PSII) nor the chlorophyll fluorescence ratio (F_v/F_m) were reduced by salinity, and lower mid-day values recovered by dawn. Mid-day F_v/F_m was in fact depressed more at low external sodium concentration, by the end of the experiment. The growth responses of the hygro-halophyte A. portulacoides to salinity appear largely to depend on changes in its rate of photosynthetic gas exchange. Photosynthesis appears to be limited mainly through stomatal conductance and hence intercellular CO₂ concentration, rather than by effects on PSII; moderate salinity might stimulate carboxylation capacity. This is in contrast to more extreme halophytes, for which an ability to maintain leaf area can partially offset declining rates of carbon assimilation at high salinity.

1 The construction of a raised dike across the coastal saltmarshes of Odiel (Huelva, south-west Spain) in 1977 divided a uniform area of low-lying sediment into two lagoons with very different drainage properties. To the east of the dike, there is rapid drainage into the estuarine channel of the Odiel and Tinto rivers, whereas to the west, the development of sand spits has impeded drainage such that standing water persists long after high tides, despite the slightly higher elevation of this area. 2 Both lagoons have been colonized by isolated clones of Spartina maritima, which have locally enhanced accretion to form domed tussocks (`nucleation'). The diameter of the tussocks was highly correlated with the elevation of their sediment surface, the larger tussocks having been the earlier colonists in a generally accreting system. Spartina in the interior of the tussocks showed reduced tiller density and vigour. 3 Only in the better-drained lagoon have the central, higher areas of the Spartina tussocks been invaded by Arthrocnemum perenne, although seed is freely available in both lagoons. Arthrocnemum formed a sprawling, dense canopy and a superficial, relatively impenetrable root system; it rapidly suppressed the remaining tillers of Spartina, eventually leaving only a fringe around the edge of the tussock. Areas invaded by Arthrocnemum were characterized by a superficial layer (10 cm) of oxidizing sediment $(E_h > + 80 mV)$. Spartina-dominated areas in both lagoons remained highly reducing, even in the surface layers ($E_h < - 150 mV$). 4 In terms of successional mechanisms, primary colonization by Spartina maritima facilitates the invasion by Arthrocnemum, which only becomes established from seed on relatively well-drained, oxidising sediments. The interaction is, however, more complicated than this: declining tiller density within the tussocks prior to invasion by Arthrocnemum is consistent with an inhibition mechanism, and the superior competitive ability of Arthrocnemum, once established, suggests that a tolerance mechanism may also operate

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-

Summary: This account reviews information on all aspects of the biology of Sarcocornia perennis that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history, and conservation. Sarcocornia perennis (Miller) A.J. Scott (Salicornia perennis Miller; Salicornia radicans Sm.; Arthrocnemum [Arthrocnemon]perenne (Miller) Moss) of the European and African coasts is now considered also to include Salicornia virginica L., S. ambigua Michx. and S. gaudichaudiana Moq. of the Americas. Sarcocornia perennis is a spreading, jointed‐stemmed, halophyte that differs from the closely related Salicornia in being a shrubby perennial and in aspects of inflorescence structure. In Britain, it is found mainly on the coasts of south‐east England, on gravelly or sandy foreshores, and well‐drained sediments of tidal salt marshes. Sarcocornia perennis occurs relatively low in the tidal frame but only in well‐drained situations, as seedling establishment and long‐term survival are intolerant of highly reducing surface sediments. It has a role in the early stages of salt‐marsh succession, sometimes as the primary colonist on sandy substrates. In southern Europe, its invasion may be facilitated by the raised clonal tussocks of Spartina maritima, a primary colonist; subsequently, S. perennis dominates the tussocks and supports their rapid radial and upward growth by trapping silt. Pollen flow from the high marsh species Sarcocornia fruticosa may allow invasion by hybrids between the two species. Hybridization between the diploid S. perennis and octaploid S. fruticosa in south‐west Spain has been confirmed by analysis of random amplified DNA (RAPDs). This has also clarified the relationship between the two subspecies present there (the widely distributed ssp. perennis and the more upright, rigid Iberian endemic ssp. alpini (Lag.) Castroviejo). Sarcocornia perennis maintains low water potentials and a positive carbon balance under highly saline conditions. Salinity treatment had no effect on the efficiency of Photosystem II (Fv/Fm). Total germination was also unaffected by salinity up to 2%, although above this it was progressively inhibited; the reduction in germination at seawater concentrations was mainly osmotically enforced dormancy, which would promote dispersal by tides and currents.