High Salinity Sites Research Articles

Changes in gas exchange and chlorophyll fluorescence characteristics of two mangroves and a mangrove associate in response to salinity in the natural environment

Photosynthetic responses of two mangroves and a mangrove associate were investigated in a low salinity site at Beachwood (<12‰) and a high salinity site at Durban Bay (35‰). Measurements were made of CO2 exchange, chlorophyll fluorescence, ion and water relations of Avicennia marina, Bruguiera gymnorrhiza and Hibiscus tiliaceus at the two sites. At Beachwood, CO2 exchange was highest in H. tiliaceus (9.12 µmol m–2s–1) and least in A. marina (3.27 µmol m–2s–1). At the Durban Bay site, CO2 exchange in A. marina was greater than that at Beachwood by 79%, that in H. tiliaceus decreased by 59% while that in B. gymnorrhiza remained unchanged. In all species, photosystem II (PSII) quantum yield decreased with increase in photon flux density (PFD) up to 2,000 µmol m–2s–1. Electron transport rate (ETR) through PSII increased with increase in PFD, reaching saturation at 1000 µmol m–2s–1. At saturating PFD, PSII quantum yield and ETR were higher in the mangroves but lower in H. tiliaceus at the high salinity site. Photochemical and non-photochemical quenching were, however, lower in the mangroves but higher in H. tiliaceus at the high salinity site. Predawn and midday leaf water potential were higher at Beachwood and significantly reduced at Durban Bay, values being highest in H. tiliaceus (–0.4 and –1.8 MPa) and least in A. marina (–2.5 and –3.8 MPa). Leaf concentrations of Na+, Cl–, Ca2+ and Mg2+ in mangroves were significantly higher at Durban Bay than at Beachwood. In H. tiliaceus there were no differences in ion concentrations between sites except for K+, which was significantly higher at Durban Bay.

Low salinities adversely affect photosynthetic performance of the mangrove, Avicennia marina

Photosynthetic performance of the highly salt tolerant mangrove, Avicennia marina, was compared at two sites differing insubstrate soil salinities. Carbon dioxide exchange and chlorophyll fluorescence weremonitored at a high salinity site in Durban Bay (35‰) and at a low salinitysite in Beachwood (< 12‰). Mean CO2 exchange, conductanceand transpiration were consistently higher at the high salinity site. Carbondioxide response curves indicated that carboxylation efficiency was higherand stomatal limitation lower at the Durban Bay site. PSII quantum yield,electron transport rates (ETR) and intrinsic PSII efficiency(Fv/Fm) were significantly higher at the high salinity site.Quenching analysis indicated a higher degree ofphotoinhibition/photoprotection in leaves at the low salinity site. Predawnand midday leaf water potentials were –1.6 and –3.1 MPa at Beachwood,compared to –2.6 and –3.8 MPa, respectively, at Durban Bay. Leafconcentrations of Na+, K+, Ca2+, Mg2+,Cl- and N were significantly higher at Durban Bay. Photosyntheticperformance is apparently impaired at the low salinity site in Beachwood asa result of K+ and N deficiencies in the leaves.

The effect of competition and edaphic conditions on the establishment of halophytes on brine effected soils

In order to test the feasibility of using native halophytes to reclaim brinecontaminated soil, seedlings of five inland halophytes, Atriplexprostrata, Hordeum jubatum, Salicornia europaea, Spergularia marina, and Suaeda calceoliformis, were planted at threedensities on a site near Athens, Ohio which had been contaminated by oilwell brine water. Ten replicates of each density treatment weretransplanted on two distinct areas of high and low salinity in May of 1993. Seedling survivorship, soil moisture, and soil salinity were monitored weeklythroughout the growing season. Biomass production and ion uptake weredetermined for each plant surviving until harvest. Soil analyses wereperformed prior to planting and after harvest to determine overall changesin soil chemistry and to determine the amount of Na+ reductionfrom the soil due to leaching by precipitation during the course of theexperiment. Survival was determined to be density independent for all ofthe species with the exception of S. marina where survival wasfacilitated at high density. Increased salinity negatively affected the survivaland yield of A. prostrata. The remaining species had greater survivalunder high salinity conditions, and density appeared to be the key factorinfluencing yield. Sodium and chloride ions were accumulated in planttissues in much greater amounts than K+, Ca+2or Mg+2. Salicornia europaea plants grown in high densityon the high salinity site accumulated the highest amount of Na+ andH. jubatum grown in low density on the high salinity site accumulatedthe lowest amount of Na+. Soil salinities measured directly from theroot zone were significantly reduced (p<0.05) at the end of thegrowing season when compared to their controls. Atriplex prostrata(high density/low salinity) plots produced the greatest reduction in soilsalinity (15.8%) and S. marina (high density/high salinity) plots hadthe least reduction (1.2%).

Rootstock and salinity effects on rates of berry maturation, ion accumulation and colour development in Shiraz grapes

Shiraz grapevines on either their own roots, or on the rootstocks Ramsey, 1103 Paulsen, 140 Ruggeri or 101-14, were grown at two separate sites within the Murray-Darling viticultural region with similar irrigation regimes but with an irrigation water salinity of either 0.43 dS/m (low salinity site) or 2.3 dS/m (high salinity site). Rootstock effects on grape berry development, ion concentrations, soluble solids and acidity were followed during one season. Wines were also made and compared using spectral analysis and sensory evaluation. Rootstock effects that were common across both sites were (1) a close relationship between K+ and soluble solids accumulation in developing grape berries which commenced at the onset of veraison and was indicative of a link between K+ and sucrose transport in the phloem, and (2), higher wine K+, pH and colour hue for all rootstocks with one exception, namely 101-14 at high salinity where 101-14 responded similarly to own roots. Juice K+, pH and loss of K+ from juice during winemaking were highest for grapes from the high salinity site. Mean berry weight was smaller and the range in berry size across rootstocks was narrower at the saline site. The narrower range in berry sizes may have contributed to fewer rootstock effects on wine spectral characteristics at high salinity. There was no effect of rootstock on CO2 assimilation rate or stomatal conductance at either site, although intrinsic leaf-based water-use efficiency measured as A/g was 50% higher at the saline site. All treatments exhibited berry shrivel at maturity, but the extent was smaller at high salinity. Slower development of berry colour during veraison was observed on some rootstocks, for example 101-14, and while unrelated to canopy size per se, a higher leaf-to-fruit ratio for 101-14 may have been a factor. Slower berry colour development during veraison had no bearing on the colour density of wine made from the harvested grapes.

Controls on soil cellulose decomposition along a salinity gradient in a Phragmites australis wetland in Denmark

Although soil organic matter decomposition is an important process determining nutrient transformations and availability in wetland ecosystems, few studies have attempted to assess which environmental factors are most important in controlling spatial differences in decomposition rates found along environmental gradients. Relative soil decomposition was determined in a Phragmites australis Cav. Trin ex Steudel dominated wetland in northern Jutland, Denmark along a natural salinity gradient, where nutrients, soil moisture, temperature and salinity among other factors also varied. Our objective was to identify which edaphic factors most limited rates of relative soil decomposition, as evaluated by measuring cellulose decomposition with the cotton strip technique. Replicate cotton strips were placed at seven marsh sites along the salinity gradient, and soil and interstitial water samples were collected and analyzed for major macro- and micronutrients (NH 4–N, NO 3–N, P, PO 4, K, Mg, Ca, Na, S, Fe, Mn, Zn, Cu, Mo, B, Si), pH, Eh, conductivity, temperature, and soluble sulfides. Cellulose decomposition, expressed as cotton tensile strength loss (CTSL) per day, decreased with increasing salinity, except at the highest salinity site where a significant increase occurred. Mean CTSL values, averaged for each marsh site, varied 3-fold from 1.8 to 5.5% loss per day. Principal component and multiple regression analyses were used to prioritize the importance of the various factors that might control this spatial difference in CTSL rates. Although soil conductivity (salinity) accounted for the large percentage (45%) of the variation in the environmental data, soil fertility- and soil reduction-associated variables explained the greatest percentage (56%) of the spatial variation in cellulose decomposition. Univariate correlation analyses supported the conclusion that soil fertility, primarily inorganic nitrogen and phosphorus, is the major environmental factor determining soil cellulose decomposition rates along this salinity gradient.

Drought effects on photosynthesis of the mangrove, Avicennia germinans, under contrasting salinities

Drought effects on leaf photosynthesis of A. germinans growing under two contrasting salinities were studied in a Venezuelan fringe mangrove. During both wet and dry seasons, severe chronic-photoinhibition at predawn was not observed but strong down regulation occurred at midday during both seasons. Carbon assimilation rates (A, μmol CO2 m−2 s−1) declined during the dry season from 11.9±1.8 to 7.0±1.5 and from 9.6±2.0 to 4.7±2.5 in plants from low and high salinity sites, respectively. Changes in carbon assimilation per unit of chlorophyll (A/Chl, mmol CO2 mol−1 Chl) were from 31.6±4.7 to 20.5±4.3 and from 21.9±4.7 to 15.2±8.2 in the low and high salinity plants, respectively. Therefore, neither changes in Chl nor seasonal differences in photoprotective down regulation could account fully for the decrease in leaf photosynthesis during drought. A reduction in CO2 diffusion due to lowered stomatal conductance was not large enough to explain such a dramatic effect of drought on leaf photosynthesis. Stomatal response could be mitigated by the capability of A. germinans for osmotic adjustment under high salinity and/or drought. However, this intracellular salt accumulation may reduce carbon assimilation capacity further by decreasing the metabolism of leaf cells, increasing dark respiration and/or photorespiration.

Patterns of litter production across a salinity gradient in a Pterocarpus officinalis tropical wetland

Historically, Pterocarpus officinalisJacq. (Leguminoseae) dominated freshwater wetlands in the coastal plains of Puerto Rico, but deforestation has reduced its distribution to small patches adjacent to mangrove forests in areas of higher salinity. The objective of this study was to determine how a gradient in soil salinity affected litter, flower, and fruit production in a Pterocarpus officinalis.Three 100 m2 plots were established in each of three sites along a salinity gradient: pasture/Pterocarpus edge (low salinity, mean salinity at 60 cm–9.7 g Kg−1), Pterocarpus forest (intermediate salinity, 11.5 g Kg−1) and a Pterocarpus/mangrove ecotone (high salinity, 15.0 g Kg−1). Across this gradient, P. officinalis accounted for 100% of the relative basal area in the low and intermediate sites and 43% in the high salinity site which was domimated by Laguncularia racemosa. The basal area of P. officinalis decreased along the gradient from 73.5 m2 ha−1 in the low salinity site to 42.0 m2 ha−1 in the high salinity site. Litterfall was sampled on average every 23 days in 45 0.25 m2 traps (5 traps per plot) for two years. Annual litterfall for the forest was 11.9 Mg ha−1 yr−1. Peaks in litterfall were associated with high precipitation in May 1995 and tropical storms in September 1995. Leaf fall of P. officinalis was significantly higher in the low salinity site (4.8 Mg ha−1 yr−1) than the high salinity site (1.8 Mg ha−1 yr−1), but total stand litterfall was greatest in the area of high salinity due to the greater contribution of L. racemosa. Pterocarpus flower and fruit production was approximately 10 times greater in low and intermediate salinity sites in comparison with the high salinity site. An increase in global temperature, will lead to higher sea level and higher soil salinity in costal wetlands. To conserve this wetland forest type it is critical to expand the distribution into areas of lower salinity where this species occurred historically.

PHOSPHOGLUCONATE DEHYDROGENASE POLYMORPHISM AND SALINITY IN THE WHITE SANDS PUPFISH.

The phosphogluconate dehydrogenase (Pgdh) locus is the only polymorphic allozyme locus observed among 37 loci examined in all four populations of a New Mexico state Endangered species, the White Sands pupfish (Cyprinodon tularosa). We report evidence suggesting that this polymorphism may be associated with salinity. Salinity levels vary widely within and between habitats occupied by White Sands pupfish. The frequency of the Pgdh100 allozyme was correlated with salinity but not with temperature. Frequency of Pgdh100 differed between low (3.76 parts per thousand (ppt)) and high (9.23 ppt) salinity sites at Malpais Spring despite no obvious barriers to fish movement. Frequencies of Pgdh100 in two introduced populations differed from that of the presumptive founding stock and correlated with salinity in the current habitats.

Growth and Mortality of North Carolina-Heritage Oysters, Crassostrea virginica, in North Carolina and in Chesapeake Bay

ABSTRACT Populations of the Eastern oyster, Crassostrea virginica, in Chesapeake Bay have been greatly reduced over the last decade by two protozoan parasites. One of these, Perkinsus marinus, has been especially harmful, sometimes killing 70-80% of harvestable oysters in a given area. However, while P. marinus has caused substantial mortalities in the Chesapeake, oysters in states farther south and along the Gulf coast have not experienced such intense effects. This may be due to differences in P. marinus abundance and virulence at various sites or to differences in the tolerance of C. virginica stocks or to a combination of these and other factors. During the period March 1992 to May 1993, growth, intensity of infection with P. marinus, and survival of native North Carolina oysters (NC) held under standard tray culture conditions at several sites in both regions (Wye River, Maryland; Mobjack Bay, Virginia; Pamlico River, North Carolina; and Bogue Banks, North Carolina) were evaluated. At the Chesapeake Bay sites (Wye River and Mobjack Bay) the performance of the NC strain to native Chesapeake Bay oysters (CB) spawned synchronously were compared. Both NC and CB oyster spat were introduced at average lengths of 20-22 mm in March 1992. P. marinus infection reached 100% in oysters held at all high and moderate salinity sites by July 1992, at which time the CB strain held in Chesapeake Bay ceased to grow. Shortly after growth ceased, CB oysters experienced an accelerated mortality that rapidly progressed to 100%. Meanwhile, the native NC strain held at high and moderate salinity sites in both regions also experienced mortality, but at a much reduced rate of only 20-30%. High levels of P. marinus infection were accompanied by continued growth of the NC strain. When mortality did occur in the NC strain, it was associated with a much higher intensity of P. marinus than the infection intensity correlated with death of CB oysters. At the low salinity site, P. marinus infection persisted at low intensity throughout the latter portion of the culture period but was not associated with mortality of either strain. The trends in growth and disease tolerance for the two strains suggest that performance of oysters is related to genetic makeup and to other factors such as acclimatization and virulence off! marinus.

Salinity effects on the leaf water relations components and ion accumulation patterns in Avicennia germinans (L.) L. seedlings.

Physiological traits involved in leaf water relations were evaluated in Avicennia germinans (L.) L. seedlings growing at different salinities in the field. Analysis of pressure-volume (P-V) curves and sap osmometry were combined to evaluate osmotic adjustment and cell elasticity, and the contribution of accumulated inorganic ions to osmotic potential was estimated. Seedlings growing in soils with interstitial water salinity above that of normal sea water showed a modification of the relationship between water potential and relative water content. Thus, their leaf osmotic potential at maximum turgor (Ψπ(max)) and at zero turgor (Ψπ(0)) was 1.41 and 1.82 MPa lower respectively, than that of the seedlings from the low salinity site. Volumetric moduli of elasticity () were between 17 and 23 MPa. Thus, ɛ was about 6 MPa lower in high-salinity plants indicating that their cells were slightly more elastic. Ionic concentration analysis showed that Σ [anions] and Σ [cations] were higher in the high-salinity site (22-35%) while the water content per unit dry mass was only 12-17% lower. Reduction in water content was insufficient to explain the increase in ion concentration. Ion concentration explained 73 and 66% of the osmotic potential estimated by P-V curves for leaves from low- and high-salinity sites, respectively. In conclusion, this study provided evidence that leaves of A. germinans seedlings adapt to hypersaline soils by increasing solute concentration by 52% and cell elasticity by 26%. Both processes allow leaf water uptake and turgor maintenance over a large range of soil water potential.

Effects of flooding, salinity and herbivory on coastal plant communities, Louisiana, United States.

Flooding and salinity stress are predicted to increase in coastal Louisiana as relative sea level rise (RSLR) continues in the Gulf of Mexico region. Although wetland plant species are adapted to these stressors, questions persist as to how marshes may respond to changed abiotic variables caused by RSLR, and how herbivory by native and non-native mammals may affect this response. The effects of altered flooding and salinity on coastal marsh communities were examined in two field experiments that simultaneously manipulated herbivore pressure. Marsh sods subjected to increased or decreased flooding (by lowering or raising sods, respectively), and increased or decreased salinity (by reciprocally transplanting sods between a brackish and fresh marsh), were monitored inside and outside mammalian herbivore exclosures for three growing seasons. Increased flooding stress reduced species numbers and biomass; alleviating flooding stress did not significantly alter species numbers while community biomass increased. Increased salinity reduced species numbers and biomass, more so if herbivores were present. Decreasing salinity had an unexpected effect: herbivores selectively consumed plants transplanted from the higher-salinity site. In plots protected from herbivory, decreased salinity had little effect on species numbers or biomass, but community composition changed. Overall, herbivore pressure further reduced species richness and biomass under conditions of increased flooding and increased salinity, supporting other findings that coastal marsh species can tolerate increasingly stressful conditions unless another factor, e.g., herbivory, is also present. Also, species dropped out of more stressful treatments much faster than they were added when stresses were alleviated, likely due to restrictions on dispersal. The rate at which plant communities will shift as a result of changed abiotic variables will determine if marshes remain viable when subjected to RSLR.

Relating Benthic Infaunal Community Structure to Environmental Variables in Estuaries Using Nonmetric Multidimensional Scaling and Similarity Analysis

In 1994, 19 stations were sampled (2 replicates/station) with Young grabs in association with the EMAP-Estuaries Carolian Province Base Monitoring in Florida. A total of 295 unique benthic infauna taxa and 9647 individuals were identified and enumerated. Environmental data (bottom-water quality, sediment grain size, sediment metals, and organics) and benthic community data were analyzed using hierarchical agglomerative cluster analysis and ordination via nonmetric multidimensional scaling. Bray-Curtis similarities and Euclidean distance were used as the distance measures for biotic and abiotic data, respectively. Multivariate analyses were complemented by examining incremental contributions of benthic taxa to similarity values using a percentage similarity technique. A low-salinity site in a tributary to the St. Johns River had benthic communities uniquely different from those of moderate-to high-salinity sites. A diverse assemblage of polychaetes, gastropods, bivalves, amphipods, sipunculans, and phoronids was consistently associated with relatively unimpacted sites in the Indian River Lagoon. Infaunal community structure in the northern portion of the study area was influenced by the nearby Atlantic Ocean. Community shifts in association with latitudinal gradients and concentrations of sediment metals and organics were apparent. The nonparametric multivariate techniques used in this study were particularly effective at delineating and defining fine-scale community differences.

A comparative study of two sympatric species within the genus Callinectes: osmoregulation, long-term acclimation to salinity and the effects of salinity on growth and moulting

Juvenile blue crabs, Callinectes sapidus Rathbun, and lesser blue crabs, C. similis Williams, were exposed to a range of salinities for 67 days to determine if salinity exerted species specific differential effects on growth and moulting. Growth was measured in terms of carapace width, wet weight, dry weight and ash-free dry weight. Growth rates of the two species (% increase in dry weight/day) were differentially affected by salinity. C. similis juveniles exposed to 5‰ grew significantly slower than those exposed to 10‰, with those at 30‰ exhibiting intermediate growth rates. Salinity had no effect on growth rates of C. sapidus by any measure of weight or carapace width. C. sapidus grew at faster rates than C. similis at low salinity as measured by wet and dry weight, and at all treatments as measured by carapace width. Growth per moult (wet weight) of C. similis was greater than that of C. sapidus. However, intermoult periods of C. similis were longer and exhibited a more pronounced effect of salinity than those of C. sapidus. Feeding rates and hemolymph osmolalities were measured at the beginning and end of the 67 day exposure period. Weight-adjusted feeding rate of C. sapidus increased significantly after 67 days exposure to low salinity, whereas that of C. similis decreased significantly. Each species exhibited a decline in hemolymph osmolality at low salinities by the end of the exposure period. In order to determine if either species exhibits an ontogenetic shift in ability to regulate hemolymph osmolality, juvenile and adult C. sapidus and C. similis were collected and exposed to a range of salinities for measurement of hemolymph osmolalities. These crabs were collected and exposed separately from those used in the 67 day exposure. Adult C. sapidus maintained higher hemolymph osmolalities than juveniles when exposed to low salinities (≤25‰). Hemolymph osmolalities of adult C. similis exposed to low salinity varied with salinity of collection site. Those from a high salinity site (30‰) exhibited hemolymph osmolalities no different than juveniles when exposed to salinities of 2.5 and 10‰. Those from a lower salinity site (22‰) exhibited greater hemolymph osmolalities than juveniles, osmoregulating at levels insignificantly different from adult C. sapidus. Results of this study indicate that although previously published studies may have overestimated the effects of low salinity on C. similis relative to it`s more euryhaline congener C. sapidus, effects of salinity alone are probably sufficient to limit this species' distribution to waters of 10‰ or greater.

Growth and Survival of Polygonum aviculare L. at a Brine-contaminated Site in Southeastern Ohio

-Populations of Polygonum aviculare from low-salinity and high-salinity sites at a brine spill location in Ohio were studied to determine the effects of edaphic factors and intraspecific competition on seed germination, growth and survival. The effects of salinity and plant density on the growth, reproductive output and survival of P aviculare were investigated in the field. An additional study was performed in incubators to determine the effect of thermoperiod and salinity on seed germination. Seed germination was inhibited by increasing salinity and a thermoperiod of 35 Gday/25 Gnight. High summer temperatures and increased soil salinity may explain why viable seeds remained in the soil after the spring germination period in both highand low-salt plots. Soil salinity was consistently higher in the high-salinity plots than in the low-salinity plots during the growing season, while soil moisture levels did not differ (P < 0.05) between them. Low-salinity plots had higher plant densities and lower mortality than high-salinity plots. Survival was density-dependent in the low-salinity plots, but regulated primarily by salinity stress in the high-salinity plots. Biomass production was lower in the high-salinity than in low-salinity plots. Both intraspecific competition and salinity stress reduced growth and survival of P aviculare.

Nutrient relations of the halophytic shrub, Sarcobatus vermiculatus, along a soil salinity gradient

Recent water level declines of a saline and alkaline lake (Mono Lake, California, USA) have exposed large areas of former lake bottom substrates that have been sparsely colonized by the halophytic shrub, Sarcobatus vermiculatus. To increase understanding of the interaction of salt and nutrient relations in halophytes we investigated spatial and seasonal patterns of leaf elemental composition and growth for Sarcobatus along an extreme salinity gradient using four sites varying in age since exposure. Soil and groundwater Na and EC increased from non-saline at the site farthest from the Lake to highly saline at the site closest to the Lake. Leaf and stem length growth declined along this same transect. In June, when shoot growth rates were maximal, leaf Na paralleled substrate availability and reached a high of 130 g kg−1 leaf dry weight near the Lake. Through the remainder of the growing season, plants on the lower salinity site continued to accumulate increasing amounts of leaf Na, whereas plants at the highest salinity site reached a plateau earlier in the season. The similarity of high leaf Na at all sites by the end of the season indicates a dominance of Na nutrition over the entire gradient, and a possible upper bound of Na accumulation and tolerance for this species. In June, Sarcobatus leaf N, P were higher at sites further from the Lake and did not correspond to soil N (total) and P that were highest near the Lake. Leaf K, Ca, and Mg in June also did not match substrate availability, and were low enough to indicate marginal sufficiency for growth. However, the increasing substrate Na did not interfere with K, Ca and Mg uptake based on selectivity coefficients (e.g. (leaf K/leaf Na)/ (soil K/soil Na)) that did not decline with increasing substrate salinity. Uptake and transport to leaves, of K and Ca relative to Na, remained proportional to what was in the soil across the entire gradient of soil salinity. Uptake and transport of Mg relative to Na increased dramatically in inverse proportion to declining soil availability near the Lake, indicating that Mg may be a critical limiting factor for tolerance of high salinity sites. The key determinants of Sarcobatus growth and ecological dominance along this extreme salinity gradient (non-saline to highly saline) are the propensity to accumulate large amounts of leaf Na over the entire gradient, and the ability to maintain adequate uptake of N, P, K, Ca, and Mg under highly variable substrate combinations of nutrients and Na.

Salinity Responses in Two Populations of Viviparous Rhizophora mangle L. Seedlings

Viviparous seedlings of the red mangrove, Rhizophora mangle L., were collected from adult trees growing in areas of low (59'oo) and high (360oo) surface water salinities, and planted as groups in large rectangular pots and treated with seawater corresponding to the two natural concentrations. Among individuals collected from the same site, growth was significantly enhanced in the low (5%oo) versus high (360oo) salinity treatment. Stem and leaf development exhibited larger differences between high and low salinities than did root growth. Propagules showed no increased capacity for growth when treated with salinities similar to the parent tree environments. Seedlings from the high salinity site exhibited faster growth than those from the low salinity site when treated with low salinity water. However, differences between sites were insignificant in the high salinity treatment. Results are discussed in the context of vivipary as a preconditioning mechanism for salt-tolerance and the general effects of salinity on physiological and possible genetic aspects of early development.

Defense Responses of Hemocytes Withdrawn from Crassostrea virginica Infected with Perkinsus marinus

Hatchery-produced, unparasitized oysters ( Crassostrea virginica) were transplanted to high and low salinity sites in the Chesapeake Bay in May 1992. These locations included Mobjack Bay, Virginia (MB, ∼20%), the Choptank River, Maryland (CR, ∼14%), and the Wye River, Maryland (WR, ∼8%); the salinities subsequently varied with rainfall in the region of each site. The protozoan parasite Perkinsus marinas rapidly infected the oysters at each site. Oysters were periodically sampled and individually assayed for intensity of P. marinus infection, total circulating hemocyte count (THC) and zymosan-iuduced chemiluminescence (CL) activity of the hemocytes. The CL assay measures the production of reactive oxygen intermediates (ROIs) thought to mediate antimicrobial activity of the hemocytes. The MB oysters were 100% infected and showed a high intensity of infection by October, 1992; during the following year, high levels of infection were reached rapidly and the population was completely decimated by mid-July, 1993. Disease intensity and progression were slower in oysters from the low salinity sites (CR and WR). The THC and zymosan-stimulated CL responses of hemocytes from moderately or heavily infected oysters were significantly higher than those of uninfected or lightly infected oysters. The increased number of hemocytes from oysters with advanced infections probably represents a general protective response to the presence of infectious agents in tbe hemolymph. Hemocytes withdrawn from oysters with heavy P. marinus infections have markedly increased CL activity after phagocytosis of zymosan particles. Statistical analysis of the data showed that this activated condition was strongly associated with the level of P. marinus infection of the host, but not influenced by the ambient salinity of the oysters' habitat. However, phagocytosis of P. marinus merozoites by hemocytes fails to trigger CL, regardless of the infection intensity of the oysters from which the hemocytes were withdrawn. These data suggest that ingestion of microorganisms may nonspecifically enhance hemocytic defense responsiveness, as measured by their increased production of ROIs upon subsequent phagocytic stimulation by zymosan. However, not all microbes can initiate responses from these activated hemocytes; this may partially explain the virtual inability of C. virginica hemocytes to control P. marinus infections by intracellular cidal mechanisms.

Distribution of sedimentary phosphorus in Gulf of Mexico estuaries

Sediment samples were collected from all the major bays and estuaries along the US Gulf of Mexico Coastline in 1986–1988 by the NOAA NS & T Program (NOAA (1989), The National Status and Trends Program for marine environmental quality; NOAA/OMA, Rockville, MD, USA). These have been analyzed for total phosphorus as well as leachable Al-P, Ca-P and Fe-P. Total phosphorus concentration ranges from 1·8 to 45·9 μmol/g and averages 11·3 μmol/g. Higher values were found in Louisiana estuaries than in those in Texas but the highest values occur in the phosphate rock mining region of southern Florida. Ca-P is the most abundant leachable species and is higher in samples from higher salinity sites. Previous unpublished data by one of the authors (D.A.) on solid phase and pore water phosphate from 18 cores collected on the Mississippi River Delta, are reported for the first time here. Low dissolved phosphate (< 5 μM/liter) was found at the surface of several cores but values at 1 m depth were mostly much higher (up to 145 μM/liter). Pore water phosphate gave a significant positive correlation with solid phase inorganic phosphorus, but a poor correlation with solid organic phosphorus, showing the importance of inorganic reactions in controlling dissolved PO 4 in pore water.

Recruitment and Growth of the Eastern Oyster, Crassostrea virginica, in North Carolina

The effects of location, salinity, and depth on recruitment and growth of the eastern oyster Crassostrea virginica in Pamlico and Core sounds, North Carolina, were investigated from 1988 to 1990. We measured length and density of spat settling on oyster cultch deployed at deep (∼3 m) and shallow (∼1 m) depths at six sites in areas with low salinity and six sites in areas with high salinity. These data were compared with similar data taken at some of these sites by the North Carolina Division of Marine Fisheries since 1981 as part of their cultch planting program. Recruitment was generally greater in the high salinity sites, compared to the low salinity sites. Recruitment was less at shallow depths compared to deeper depths. In all three years the highest recruitment occurred in August and September, corresponding to the months of maximum water temperature. Recruitment was highly variable in space and time, but appeared to diminish from 1988 to 1990. Recruitment was reduced by sedimentation and a variety of sessile organisms. All sites appeared to have a similar potential for growth.

Production ecology of Ruppia maritima L. s.l. and Halodule wrightii Aschers, in two subtropical estuaries

Seasonal growth and production dynamics of Ruppia maritima L. s.l. were compared over a 3-year period in two south Texas estuaries characterized by different salinity and N regimes as a result of freshwater inputs. Measurements of shoot production in the Guadalupe Estuary (0–25‰ salinity) and the Nueces Estuary (32–38‰ salinity) revealed no major differences in the magnitude of growth, but the plant populations differed in the seasonality of growth, the time of flowering, and the persistence of an over-wintering population. During the period of rapid shoot development, from March to August, leaf elongation rates usually ranged from 2 to 4 mm·day −1 (0.04–0.08 mg dry wt·mg shoot −1·day −1), although peak growth rates of up to 8 mm·day −1 were also recorded. In both estuaries, R. maritima was a strict opportunist, yearly colonizing bare areas and completing its entire growth cycle in 4 months. Overwintering populations of R. maritima existed at the lower nutrient and higher salinity site in Nueces Estuary but not at the high nutrient, low salinity site in Guadalupe Estuary. Halodule wrightii Aschers, was absent from Guadalupe Estuary, but in the Nueces Estuary, H. wrightii maintained large and persistent overwintering populations characterized by sustained year-round growth and larger below-ground root and rhizome fractions of total biomass (50–85%) compared to R. maritima (20–70%). The high nutrient regimes associated with large inputs of freshwater in the Guadalupe Estuary appear to have little beneficial effect on the growth dynamics of R. maritima. Instead, significant reductions in underwater light may be most important for growth. Fouling by algal epiphytes, higher riverine inflow, and local physiographic differences in wave exposure appear to be the primary factors regulating light levels and thus the proliferation and year-round persistence of R. maritima.