Analysis of tidal marsh vegetation patterns in two Georgia estuaries using aerial photography and GIS

Aerial photographs and GIS analysis were used to map the distribution of tidal marsh vegetation along the salinity gradients of the estuaries of the Altamaha and Satilla Rivers in coastal Georgia. Vegetation maps were constructed from 1993 U.S. Geological Survey Digital Orthophoto Quarter Quads, 1∶77,000-scale color infrared photographs taken in 1974 and 1∶24,000-scale black and white photographs taken in 1953, Changes between years were identified using a GIS overlay analysis. Four vegetation classifications were identified and groundtruthed with field surveys: salt marsh (areas containing primarilySpartina alterniflora), brackish marsh (Spartina cynosuroides andS. alterniflora), Juncus (Juncus roemerianus), and fresh marsh (Zizania aquatica, Zizaniopsis miliacae, and others). There was no evidence for an upstream shift in marsh vegetation along the longitudinal axis of either estuary over the time frame of this analysis, which implies there has not been a long-term increase in salinity. Although the inland extent of each marsh zone was further upstream in the Satilla than the Altamaha, they corresponded to similar average high tide salinities in each estuary: areas classified as salt marsh occurred from the mouth up to where average high tide salinity in the water was approximately 15 psu;Juncus ranged from 21 to 1 psu; brackish marsh ranged from 15 to 1 psu; and fresh marsh was upstream of 1 psu. Approximately 63% of the 6,786 ha of tidal marsh vegetation mapped in the Altamaha and 75% of the 10,220 ha mapped in the Satilla remained the same in all 3 yr.Juncus was the dominant classification in the intermediate regions of both estuaries, and shifts between areas classified asJuncus and either brackish or salt marsh constituted the primary vegetation change between 1953 and 1993 (87% of the changes observed in the Altamaha and 95% of those in the Satilla). This analysis suggests that the broad distribution of tidal marsh vegetation along these two estuaries is driven by salinity, but that at the local scale these are dynamic systems with a larger number of factors affecting the frequently changing borders of vegetation patches.     

Inventory and Ventilation Efficiency of Nonnative and Native Phragmites australis (Common Reed) in Tidal Wetlands of the Chesapeake Bay

Nonnative Phragmites is among the most invasive plants in the U.S. Atlantic coast tidal wetlands, whereas the native Phragmites has declined. Native and nonnative patches growing side by side provided an ideal setting for studying mechanisms that enable nonnative Phragmites to be a successful invader. We conducted an inventory followed by genetic analysis and compared differences in growth patterns and ventilation efficiency between adjacent native and nonnative Phragmites stands. Genetic analysis of 212 patches revealed that only 14 were native suggesting that very few native Phragmites populations existed in the study area. Shoot density decreased towards the periphery of native patches, but not in nonnative patches. Ventilation efficiency was 300?% higher per unit area for nonnative than native Phragmites, likely resulting in increased oxidation of the rhizosphere and invasive behavior of nonnative Phragmites. Management of nonnative Phragmites stands should include mechanisms that inhibit pressurized ventilation of shoots.     

Genetic Variation in the Common Reed, <Emphasis Type="Italic">Phragmites australis</Emphasis>, in the Mississippi River Delta Marshes: Evidence for Multiple Introductions

Multiple introductions are believed to play an important role in increasing genetic diversity and adaptability of invasive species, but there are few well-documented examples. The common reed, Phragmites australis, has dramatically increased in tidal wetlands throughout the USA in the past century due primarily to the introduction of a Eurasian lineage. In the Mississippi River “Balize” delta, P. australis is the dominant vegetation where monotypic stands of an introduced form blanket the outer marshes. The delta’s interior marshes, on the other hand, are more vegetatively diverse, serving as important waterfowl foraging habitat. Recent encroachment by various phenotypic forms of P. australis into the interior marshes led to this study examining genetic variation in these stands. Our results revealed four chloroplast DNA haplotypes that also segregate based on microsatellite variation. Three of these are closely related and introduced, but differ relative to time and likely mode of introduction. The “Delta” type (haplotype M1), which is unique to the region and the most common lineage, displays considerable microsatellite diversity. The Eurasian introduced lineage of P. australis (haplotype M), which is invasive elsewhere in North America, is increasing its distribution in the delta. A novel haplotype, AD, was also identified which is phenotypically and genetically similar to haplotype M. Despite the close relatedness, we found no evidence for inter-haplotype gene exchange at the nuclear level, suggesting that intraspecific hybridization is not a contributing factor to these invasions. The site provides a unique opportunity for researchers to understand the dynamics of multiple P. australis invasions.     

Nutrient limitation of the macroalgaEnteromorpha intestinalis collected along a resource gradient in a highly eutrophic estuary

We conducted a laboratory experiment to quantify nutrient (nitrogen and phosphorus) limitation of macroalgae collected along a gradient in water column nutrient availability in Upper Newport Bay estuary, a relatively nutrient-rich system in southern California, United States. We collectedEnteromorpha intestinalis and water for use in the experiment from five sites ranging from the lower end of the estuary to the head. Initial algal tissue N and P concentrations and molar N∶P ratios—as well as water column NO₃ and total Kjeldahl nitrogen (TKN)—increased along a spatial gradient from the lower end toward the head. Water column soluble reactive phosphorus (SRP) varied among sites as well but did not follow a pattem of increasing from the seaward end toward the head. Algae from each site were assigned to one of four experimental treatments: control (C), nitrogen enrichment (+N), phosphorus enrichment (+P), and nitrogen and phosphorus enrichment (+N+P). Each week for 3 wk we replaced the water in each unit with the appropriate treatment water to mimic a poorly flushed estuary. After 3 wk, the degree of nutrient limitation ofE. intestinalis varied spatially with distance from the head of the estuary. Growth ofE. intestinalis collected from several sites increased with N enrichment alone and increased further when P was added in combination with N This indicated that N was limiting and that when N was sufficient, P became limiting. Sites from whichE. intestinalis exhibited nutrient limitation spanned the range of background water column NO₃ (12.9±0.4 to 55.2±2.1 μM) and SRP (0.8±0.0 to 2.9±0.2 μM) concentrations. Algae that were N limited had initial tissue N levels ranging from 1.18±0.03 to 2.81±0.08% dry weight and molar N∶P ratios ranging from 16.75±0.39 to 26.40±1.98.     

Trends in phosphatase activity along a successional gradient of tidal freshwater marshes on the Cooper River South Carolina

Phosphatase activity was measured in sediments from tidal freshwater habitats adjacent to the Cooper River in South Carolina representing different stages of ecological succession. It was found that sediment (0–5 cm) acid phosphatase activity, alkaline phosphatase activity and phosphodiesterase activity increased with increasing successional stage and phytomass. Acid phosphatase activity in creased from 7.5±1.2 (±1 SD) in subtidal sediment from a shallow open water habitat without vegetation to 61.2±4.9 μmol g⁻¹ hr⁻¹ (μmol of p-nitrophenol released per gram of dry sediment per hour) in intertidal sediments colonized by emergent macrophytes, while alkaline phosphatase activity increased from 2.1±0.1 to 19.01±1.5 μmol g⁻¹ hr⁻¹. Phosphodiesterase activity increased from 1.8±0.1 to 20.2±2.0 μmol g⁻¹ hr⁻¹ along the same gradient. Acid phosphatase activity was highly correlated (R²=0.92, P<0.001) with the organic matter content of the sediment. A study of phosphatase kinetics showed that V_max of all phosphatases also increased along the successional gradient. Trends in phosphatase activity and V_max correlated positively with plant biomass and negatively with concentrations of soluble reactive phosphorus in porewater, sediment extractable phosphorus, and total phosphourus. The porewater N∶P atom ratio decreased along the succession gradient from 25.3 in an early stage, open water community to 13.0 in a community dominated by emergent vegetation. The data also show that the distribution of the forms of phosphorus changed with successional stage. The change in distribution and the increased biological demand for phosphorus that paralleled succession were mediated by the activity of phosphatase enzymes.     

Seismic behaviour of the North Anatolian Fault beneath the Sea of Marmara (NW Turkey): implications for earthquake recurrence times and future seismic hazard

Possible long-term seismic behaviour of the Northern strand of the North Anatolian Fault Zone, between western extreme of the 1999 İzmit rupture and the Aegean Sea, after 400 AD is studied by examining the historical seismicity, the submarine fault mapping and the paleoseismological studies of the recent scientific efforts. The long-term seismic behaviour is discussed through two possible seismicity models devised from M _S ≥ 7.0 historical earthquakes. The estimated return period of years of the fault segments for M1 and M2 seismic models along with their standard deviations are as follows: F4 segment 255 ± 60 and 258 ± 12; F5 segment 258 ± 60 and 258 ± 53; F6 segment 258 ± 60 and 258 ± 53; F7 segment 286 ± 103 and 286 ± 90; F8 segment 286 ± 90 and 286 ± 36. As the latest ruptures on the submarine segments have been reported to be during the 1754–1766 earthquake sequence, and the 1912 mainshock rupture has been evidenced to extend almost all over the western part of the Sea of Marmara, our results imply imminent seismic hazard and, considering the mean recurrence time, a large earthquake to strike the eastern part of the Sea of Marmara in the next two decades.     

Environmental Determinants of Emergent Macrophyte Vegetation in Pacific Northwest Estuarine Tidal Wetlands

We investigated whether within wetland environmental conditions or surrounding land cover measured at multiple scales were more influential in structuring regional vegetation patterns in estuarine tidal wetlands in the Pacific Northwest, USA. Surrounding land cover was characterized at the 100, 250, and 1,000 m, and watershed buffer scales. Vegetation communities were characterized by high species richness, lack of monotypic zonation, and paucity of invasive species. The number of species per site ranged between 4 and 20 (mean?±?standard deviation?=?10.2?±?3.1). Sites supported a high richness (mean richness of native species 8.7?±?2.8) and abundance of native macrophytes (mean relative abundance 85 %?±?19 %). Vegetation assemblages were dominated by a mix of grasses, sedges, and herbs with Sarcocornia pacifica and Distichlis spicata being common at sites in the oceanic zone of the estuary and Carex lyngbyei and Agrostis stolonifera being common at the fresher sites throughout the study area. The vegetation community was most strongly correlated with salinity and land cover within close proximity to the study site and less so with land cover variables at the watershed scale. Total species richness and richness of native species were negatively correlated with the amount of wetland in the buffer at all scales, while abundance of invasive species was significantly correlated to within wetland factors, including salinity and dissolved phosphorus concentrations. Landscape factors related to anthropogenic disturbances were only important at the 100-m buffer scale, with anthropogenic disturbances further from the wetland not being influential in shaping the vegetation assemblage. Our research suggests that the traditional paradigms of tidal wetland vegetation structure and environmental determinants developed in east coast US tidal wetlands might not hold true for Pacific Northwest wetlands due to their unique chemical and physical factors, necessitating further detailed study of these systems.     

Tidal nitrogen exchanges across a freshwater wetland succession gradient in the upper Cooper River,South Carolina

Tidal freshwater sections of the Cooper River Estuary (South Carolina) include extensive wetlands, which were formerly impounded for rice culture during the 1,700s and 1,800s. Most of these former rice fields are now open to tidal exchange and have developed into productive wetlands that vary in bottom topography, tidal hydrography and vegetation dominants. The purpose of this project was to quantify nitrogen (N) transport via tidal exchange between the main estuarine channel and representative wetland types and to relate exchange patterns to the succession of vegetation dominants. We examined N concentration and mass exchange at the main tidal inlets for the three representative wetland types (submerged aquatic vegetation [SAV], floating leaf vegetation, and intertidal emergent marsh) over 18-21 tidal cycles (July 1998–August 2000). Nitrate + nitrite concentrations were significantly lower during ebb flow at all study sites, suggesting potential patterns of uptake by all wetland types. The magnitude of nitrate decline during ebb flow was negatively correlated with oxygen concentration, reflecting the potential importance of denitrification and nitrate reduction within hypoxic wetland waters and sediments. The net tidal exchange of nitrate + nitrite was particularly consistent for the intertidal emergent marsh, where flow-weighted ebb concentrations were usually 18–40% lower than during flood tides. Seasonal patterns for the emergent marsh indicated higher rates of nitrate + nitrite uptake during the spring and summer (> 400 μmol N m^-2 tide^-1) with an annual mean uptake of 248 ± 162 μmol m^–2 tide^–1. The emergent marsh also removed ammonium through most of the year (207 ± 109 μmol m^–2 tide^–1), and exported dissolved organic nitrogen (DON) in the fall (1,690 ± 793 μmol m^–2 tide^–1), suggesting an approximate annual balance between the dissolved inorganic N uptake and DON export. The other wetland types (SAV and floating leaf vegetation) were less consistent in magnitude and direction of N exchange. Since the emergent marsh site had the highest bottom elevation and the highest relative cover of intertidal habitat, these results suggest that the nature of N exchange between the estuarine waters and bordering wetlands is affected by wetland morphometry, tidal hydrography, and corresponding vegetation dominants. With the recent diversion of river discharge, water levels in the upper Cooper estuary have dropped more than 10 cm, leading to a succession of wetland communities from subtidal habitats toward more intertidal habitats. Results of this study suggest that current trends of wetland succession in the upper Cooper River may result in higher rates of system-wide inorganic N removal and DON inputs by the growing distributions of intertidal emergent marshes.     

Periodicity in Stem Growth and Litterfall in Tidal Freshwater Forested Wetlands: Influence of Salinity and Drought on Nitrogen Recycling

Many tidally influenced freshwater forested wetlands (tidal swamps) along the south Atlantic coast of the USA are currently undergoing dieback and decline. Salinity often drives conversion of tidal swamps to marsh, especially under conditions of regional drought. During this change, alterations in nitrogen (N) uptake from dominant vegetation or timing of N recycling from the canopy during annual litter senescence may help to facilitate marsh encroachment by providing for greater bioavailable N with small increases in salinity. To monitor these changes along with shifts in stand productivity, we established sites along two tidal swamp landscape transects on the lower reaches of the Waccamaw River (South Carolina) and Savannah River (Georgia) representing freshwater (≤0.1 psu), low oligohaline (1.1–1.6 psu), and high oligohaline (2.6–4.1 psu) stands; the latter stands have active marsh encroachment. Aboveground tree productivity was monitored on all sites through monthly litterfall collection and dendrometer band measurements from 2005 to 2009. Litterfall samples were pooled by season and analyzed for total N and carbon (C). On average between the two rivers, freshwater, low oligohaline, and high oligohaline tidal swamps returned 8,126, 3,831, and 1,471 mg N?m^?2 year^?1, respectively, to the forest floor through litterfall, with differences related to total litterfall volume rather than foliar N concentrations. High oligohaline sites were most inconsistent in patterns of foliar N concentrations and N loading from the canopy. Leaf N content generally decreased and foliar C/N generally increased with salinization (excepting one site), with all sites being fairly inefficient in resorbing N from leaves prior to senescence. Stands with higher salinity also had greater flood frequency and duration, lower basal area increments, lower tree densities, higher numbers of dead or dying trees, and much reduced leaf litter fall (103 vs. 624 g?m^?2 year^?1) over the five study years. Our data suggest that alternative processes, such as the rate of decomposition and potential for N mineralization, on tidal swamp sites undergoing salinity-induced state change may be more important for controlling N biogeochemical cycling in soils than differences among sites in N loading via litterfall.     

An investigation of salt marsh dieback in Georgia using field transplants

In 2001 and 2002, Georgia salt marshes experienced a dieback event that, affected more than 800 ha throughout the coastal zone. The dieback event was unprecedented in the state and affected bothSpartina alterniflora andJuncus roemerianus. A transplant study was conducted from May to October 2003 to determine if healthy plants could survive in dieback areas. Transplants were carried out at two locations on the Georgia coast in areas ofS. alterniflora dieback along the banks of tidal creeks, an area ofS. alterniflora dieback in the mid marsh, and aJ. roemerianus dieback, area in the mid marsh. Transplant survival was nearly 100% and growth (measured as increases in the height of the 5 tallest stems and the number of stems per experimental pot) was observed in both healthy (control) and dieback areas.J. roemerianus grew more slowly thanS. alterniflora, with no, observed increase in stem height and an average 38% increase in stem density as compared to an average 57% increase in stem height and 137% increase in stem density inS. alterniflora. Differences in growth were inconsistent but in most cases no significant differences were observed between healthy and dieback areas. Soil characteristics measured over the course of the experiment were generally comparable between healthy and dieback areas (redox potential averaged 69±123 [SD] across all observations at all sites, pH averaged 6.7 ± 0.3 and salinity averaged 24.9±4.4), but porewater ammonium (NH₄) concentration was often higher in dieback areas (overall mean NH₄ concentration, was 138±127 μM in dieback areas versus 33±40 μM in healthy areas). These results suggest that the cause of dieback was no, longer present at the time of this study and that transplants are a possibility for restoring affected areas.     

Mangrove zonation in the dry life zone of the Gulf of Fonseca, Honduras

The structural attributes and zonation patterns of mangrove forests in a dry environmental setting were evaluated in relation to gradients of soil resources, soil regulators, and hydroperiod between October 2000 and August 2001 in the Gulf of Fonseca, Honduras. Transects perpendicular to the edge of tidal channels were established at four mangrove sites, each sectioned into three mangrove zones (fringe, transition, and scrub) based upon distinct tree-height gradients. Porewater sulfide concentrations among zones were normally below levels of detection (<0.03 mM) and mean redox values ranged from 163.4±9.9 to −42.4±15.8 mv, indicating slightly reducing conditions in all zones. Mean porewater NO_x ⁻ and PO₄ ³⁻ concentrations were not significantly different among zones and ranged from 3.3±0.5 to 4.5±0.4 μM and from 0.05 ±0.02 to 0.18±0.04 μM, respectively. Soil nitrogen concentrations were 1–3 mg g⁻¹ in all zones, and mean nitrogen:phosphorus atomic ratios were <20 (range: 12.9–14.9), indicative of fertile wetland soils. Mean basal areas were significantly different among mangrove zones (fringe > transition) and were coupled with differences in height within species and zones.Avicennia germinans occupied areas with higher elevations associated with higher salinities ranging from 80 to 140 g kg⁻¹ in the dry season.Rhizophora mangle dominated lower elevations where salinities ranged from 38 to 57 g kg⁻¹ in both the dry and wet seasons. Spatial analysis on mangrove seedlings along transects confirmed that seedling distribution along the intertidal profile was statistically correlated with conspecific adults, indicating that survival and growth of seedlings in different intertidal locations are closely matched to canopy membership. The sharp transition (<100 m) in forest structure and distribution of mangrove species along the intertidal zone of subhumid coastal environments demonstrate common zonation patterns where species distribution and species-specific habitat suitability are mediated closely with tolerance to soil regulators (high soil salinity) rather than resource availability.     

Long-Term Effects of Tidal Exclusion on Salt Marsh Plain Species at Estero de Punta Banda,Baja California

Disturbance is an important factor influencing plant species composition and diversity. We addressed changes in plant composition and soil characteristics in Estero de Punta Banda, Baja California, Mexico following 22 years of disturbance by tidal exclusion. Currently, sediments in the non-tidal site are dry, 26 ± 1% moisture, and hypersaline, 143 ± 12; while those at the tidal marsh are wet, 36.2 ± 1% moisture, with 40.3 ± 2.6 salinity. The non-tidal site has lost seven species including annuals, short- and long-lived perennials, ephemerals, and parasites. Current dominants are the perennials Batis maritima and Sarcocornia pacifica. Average species richness at the non-tidal site is 4.4 ± 0.32 vs.10 ± 0.18 species per square meter at the tidal site. Average species diversity index is lower at the diked area. The general biodiversity loss that results from tidal exclusion in arid estuaries, contrasts with the species-rich communities that develop in diked humid-climate estuaries.     

Mössbauer study of synthetic Mg0.22Fe0.78SiO3 clinopyroxene

Summary  Transmission M?ssbauer spectra of synthetic Ca-free P2₁/c Mg_0.22Fe_0.78SiO₃ clinopyroxene were collected at temperatures in the range 4.2 to 745 K and in an external magnetic field of 60 kOe at 180 K. The magnetic order-disorder transition temperature was determined by M?ssbauer thermoscanning to be 21 ± 3 K. Above this temperature, all M?ssbauer spectra consist of a superposition of two doublets, respectively produced by Fe²⁺ ions at an almost regular octahedral M1 site and at a more distorted octahedral M2 site. The temperature variation of the Fe²⁺ center shifts were analyzed using the Debye model for the lattice vibrations. The characteristic M?ssbauer temperatures were found to be 356 K ± 35 K for M1 and 333 K ± 25 K for M2. From the external field (60 kOe) M?ssbauer spectrum recorded at 180 K, the principal component V _zz of the electric field gradient (EFG) was determined to be positive for both sites but precise values for the magnitudes of the asymmetry parameters η of the EFG could not be determined. The temperature variations of the M1 and M2 quadrupole splittings ΔE _Q(T ) are consistent with the higher distortion of the M2 octahedra. Using the crystal-field model to interpret ΔE _Q(T ), the energy gaps δ₁ and δ₂ of the first excited electronic states within the ⁵D orbital term were estimated to be 410 ± 50 cm⁻¹ and 730 ± 50 cm⁻¹ for M1, and δ₁ = 1050 ± 75 cm⁻¹ for M2. Received May 29, 2000;/revised version accepted July 13, 2001     

Compositions and sources of extractable organic matter in Mesopotamian marshland surface sediments of Iraq: II. Polar compounds

The concentrations of polar organic compounds including n-alkanoic acids, n-alkanols, steroids and triterpenoids were determined in extracts of shallow sediments from the Mesopotamian marshlands of Iraq. The sediments were collected by a stainless steel sediment corer, extracted with a dichloromethane and methanol mixture (3:1 v:v) by ultrasonic agitation and then analyzed by gas chromatography–mass spectrometric (GC–MS). The analysis results showed that the n-alkanoic acids ranged from C₈ to C₂₀ with concentrations of 7.8 ± 1.2 μg/g sample, whereas the concentrations of n-alkanols, which ranged from C₁₂ to C₃₉ were from 28.6 ± 4.3 to 121.7 ± 18.3 μg/g sample. The steroids and triterpenoids included stenols, stanols, stenones, stanones, tetrahymanol, tetrahymanone and extended ββ-hopanes. The total concentrations of steroids and triterpenoids ranged from 26.8 ± 4.1 to 174.6 ± 26.2 μg/g and from 0.74 ± 0.11 to 11.2 ± 1.7 μg/g sample, respectively. The major sources of these lipids were from natural vegetation, microbial (plankton) residues and bacteria in the sediments, with some contribution from anthropogenic sources (livestock, sewage and petroleum). Further studies of these wetlands are needed to characterize the input rate, transformation and diagenesis of the organic matter and to assess its various sources.     

Denitrification and the stoichiometry of nutrient regeneration in Waquoit Bay, Massachusetts

To determine the removal of regenerated nitrogen by estuarine sediments, we compared sediment N₂ fluxes to the stoichiometry of nutrient and O₂ fluxes in cores collected in the Childs River, Cape Cod, Massachusetts. The difference between the annual PO₄ ³⁻ (0.2 mol P m⁻² yr⁻¹) and NH₄ ⁺ (1.6 mol N m⁻² yr⁻¹) flux and the Redfield N∶P ratio of 16 suggested an annual deficit of 1.5 mol N m⁻² yr⁻¹. Denitrification predicted from O₂∶NH₄ ⁺ flux ratios and measured as N₂ flux suggested a nitrogen sink of roughly the same magnitude (1.4 mol N m⁻² yr⁻¹). Denitrification accounted for low N∶P ratios of benthic flux and removed 32–37% of nitrogen inputs entering the relatively highly nutrient loaded Childs River, despite a relatively brief residence time for freshwater in this system. Uptake of bottom water nitrate could only supply a fraction of the observed N₂ flux. Removal of regenerated nitrogen by denitrification in this system appears to vary seasonally. Denitrification efficiency was inversely correlated with oxygen and ammonium flux and was lowest in summer. We investigated the effect of organic matter on denitrification by simulating phytoplankton deposition to cores incubated in the lab and by deploying chambers on bare and macroaglae covered sediments in the field. Organic matter addition to sediments increased N₂ flux and did not alter denitrification efficiency. Increased N₂ flux co-varied with O₂ and NH₄ ⁺ fluxes. N₂ flux (261±60 μmol m⁻² h⁻¹) was lower in chambers deployed on macroalgal beds than deployed on bare sediments (458±70 μmol m⁻² h⁻¹), and O₂ uptake rate was higher in chambers deployed on macroalgal beds (14.6±2.2 mmol m⁻² h⁻¹) than on bare sediments (9.6±1.5 mmol m⁻² h⁻¹). Macroalgal cover, which can retain nitrogen in the system, is a link between nutrient loading and denitrification. Decreased denitrification due to increasing macroalgal cover could create a positive feedback because decreasing denitrification would increase nitrogen availability and could increase macroalgae cover.     

Factors Affecting <Emphasis Type="Italic">Pisidium amnicum</Emphasis> (Müller, 1774; Bivalvia: Sphaeriidae) Distribution in the River Minho Estuary: Consequences for its Conservation

The bivalve Pisidium amnicum (Müller 1774) is a common species in several European freshwater ecosystems. However, few Iberian watersheds are colonized by this species, and the River Minho estuary is possibly the Iberian aquatic ecosystem with the larger population. In October 2004–2007, investigations on spatial and temporal variations in P. amnicum abundance and biomass were carried out at 16 sites along the River Minho tidal freshwater wetlands. Mean abundance and biomass per site ranged from 0 to 750 ind m⁻² and 0 to 7.42 g AFDW m⁻², respectively. A clear decrease in the spatial distribution, abundance, and biomass was observed during the 4-year assessment. Furthermore, a stepwise multiple regression model showed that organic matter and conductivity explained 50.2% of the variation in P. amnicum abundance (R ² = 0.502, F _{[2, 15]} = 7.569, p = 0.005). Ecological knowledge is essential to the implementation of future conservation plans for P. amnicum, and the results of this study are of paramount importance to identify habitats that should be protected in order to preserve this species and provide scientific reference that may be useful in the development of management and/or restoration plans.     

Phytoplankton Biomass and Composition in a River-Dominated Estuary During Two Summers of Contrasting River Discharge

Estuaries located in the northern Gulf of Mexico are expected to experience reduced river discharge due to increasing demand for freshwater and predicted periods of declining precipitation. Changes in freshwater and nutrient input might impact estuarine higher trophic level productivity through changes in phytoplankton quantity and quality. Phytoplankton biomass and composition were examined in Apalachicola Bay, Florida during two summers of contrasting river discharge. The <20 μm autotrophs were the main component (92?±?3 %; n?=?14) of phytoplankton biomass in lower (<25 psu) salinity waters. In these lower salinity waters containing higher dissolved inorganic nutrients, phycocyanin containing cyanobacteria made the greatest contribution to phytoplankton biomass (69?±?3 %; n?=?14) followed by <20 μm eukaryotes (19?±?1 %; n?=?14), and phycoerythrin containing cyanobacteria (4?±?1 %; n?=?14). In waters with salinity from 25 to 35 psu that were located within or in close proximity to the estuary, >20 μm diatoms were an increasingly (20 to 70 %) larger component of phytoplankton biomass. Lower summer river discharges that lead to an areal contraction of lower (5–25 psu) salinity waters composed of higher phytoplankton biomass dominated by small (<20 μm) autotrophs will lead to a concomitant areal expansion of higher (>25 psu) salinity waters composed of relatively lower phytoplankton biomass and a higher percent contribution by >20 μm diatoms. A reduction in summer river discharge that leads to such a change in quantity and quality of estuarine phytoplankton available will result in a reduction in estuarine zooplankton productivity and possibly the productivity of higher trophic levels.     

Rhizome growth dynamics of native and exotic haplotypes ofPhragmites australis (Common reed)

Phragmites australis (common reed), a clonal grass, has expanded from a minor component of the mid-Atlantic wetlands to a dominant species. It has been suggested that invasive populations ofPhragmites are an exotic haplotype responsible for the dramatic increase in the distribution of the species. We used field observations and measurements and a greenhouse assay to compare native (haplotype F) and exotic (haplotype M) populations, growing adjacent to one another in a brackish marsh near Odessa, Delaware. In the marsh, shoots of the exotic strain emerged from the rhizomes earlier than those of the native and by March there was an order of magnitude more new shoots of the exotic strain than the native. In August, the exotic strain was 30% taller than the native, had twice the leaf biomass, and twice the total biomass. Nine of ten morphological and biomass characteristics measured differed significantly between the native and exotic strains. A greenhouse assay was conducted by planting rhizomes collected in March in shallow trays and growing them for 70 d followed by shoot harvest (Harvest 1). Rhizomes were measured, replanted, and grown for 35 d after which time they were measured and shoots were harvested (Harvest 2). At Harvest 1, shoot height was approximately 80% greater in the exotic strain, shoot biomass was three times higher, aboveground to belowground biomass ratio was twice as high, and rhizome internode length was 50% greater in the exotic strain than the native. These traits, in addition to number of shoots, were also greater in the exotic strain at Harvest 2. The number of rhizome buds at Harvest 1 was three times greater in the native than in the exotic strain. The greater number of rhizome buds in the native would seem to be an advantage, but it did not result in more shoot production. Buds were maintained in an inactive state that does not allow this strain to compete well in a wetland environment inhabited by a more efficient spreader. The earlier emergence of new shoots from the rhizomes, the greater aboveground structure, the greater rhizome internode length, and the quick transition of rhizome buds to shoot or rhizome explain in part the exotic strain's advantage over the native and the mechanisms for its invasive nature.     

<Emphasis Type="Italic">Phragmites australis</Emphasis> invasion and expansion in tidal wetlands: Interactions among salinity,sulfide, and hydrology

Through their physiological effects on ion, oxygen, and carbon balance, respectively, salinity, sulfide, and prolonged flooding combine to constrain the invasion and spread ofPhragmites in tidal wetlands. Initial sites of vigorous invasion by seed germination and growth from rhizome fragments appear limited to sections of marsh where salinity is <10‰, sulfide concentrations are less than 0.1 mM, and flooding frequency is less than 10%. In polyhaline tidal wetlands the invasion sites include the upland fringe and some high marsh creek banks. The zones of potential invasion tend to be larger in marshes occupying lower-salinity portions of estuaries and in marshes that have been altered hydrologically. Owing to clonal integration and a positive feedback loop of growth-induced modification of edaphic soil conditions, however, a greater total area of wetland is susceptible toPhragmites expansion away from sites of establishment. Mature clones have been reported growing in different marshes with salinity up to 45‰, sulfide concentration up to 1.75 mM, and flooding frequency up to 100%. ForPhragmites establishment and expansion in tidal marshes, windows of opportunity open with microtopographic enhancement of subsurface drainage patterns, marsh-wide depression of flooding and salinity regimes, and variation in sea level driven by global warming and lunar nodal cycles. To avoidPhragmites monocultures, tidal wetland creation, restoration, and management must be considered within the context of these different scales of plant-environment interaction.     

Evolution of the velocity ellipsoids in the thin disk of the Galaxy and the radial migration of stars

Data from the revised Geneva-Copenhagen catalog are used to study the influence of radial migration of stars on the age dependences of parameters of the velocity ellipsoids for nearby stars in the thin disk of the Galaxy, assuming that the mean radii of the stellar orbits remain constant. It is demonstrated that precisely the radial migration of stars, together with the negative metallicity gradient in the thin disk, are responsible for the observed negative correlation between the metallicities and angular momenta of nearby stars, while the angular momenta of stars that were born at the same Galactocentric distances do not depend on either age or metallicity. The velocity components of the Sun relative to the Local Standard of Rest derived using data for stars born at the solar Galactocentric distance are (U _⊙, V _⊙, W _⊙) _LSR = (5.1 ± 0.4, 7.9 ± 0.5, 7.7 ± 0.2) km/s. The two coordinates of the apex of the solar motion remain equal to 〈l _⊙〉 = 70° ± 7° and 〈b _⊙〉 = 41° ± 2°, within the errors. The indices for the power-law age dependences of them ajor, middle, and minor semi-axes become 0.26±0.04, 0.32±0.03, and 0.07±0.03, respectively. As a result, with age, the velocity ellipsoid for thin-disk stars born at the solar Galactocentric distance increases only in the plane of the disk, remaining virtually constant in the perpendicular direction. Its shape remains far from equilibrium, and the direction of the major axis does not change with age: the ellipsoid vertex deviation remains constant and equal to zero within the errors (〈L〉 = 0.7° ± 0.6°, 〈B〉 = 1.9° ± 1.1°). Such a small increase in the velocity dispersion perpendicular to the Galactic plane with age can probably be explained by “heating” of the stellar system purely by spiral density waves, without a contribution from giant molecular clouds.