The cyclopoid copepods,Hemicyclops adhaerens andSaphirella sp., in the Damariscotta River estuary,maine, with a note to their possible relationship

During the 14-month collection period,Hemicyclops always appeared as naupliar stages andSaphirella as immature copepodite stages at 3 stations in the middle and lower estuary.Hemicyclops nauplii were abundant during late spring and summer, with a seasonal maximum of 7448/m³ in late spring at the upstream station.Saphirella stages were observed throughout the year, but higher population densities generally occurred in the summer and fall; the highest density was 3413/m³. Both species were usually more abundant upstream than downstream. The larvae may be life stages of a single bottom-dwelling species,Hemicyclops adhaerens.     

Seasonal changes in the depth distributions of interstitial salinities in the Fraser River estuary,British Columbia

The vertical distribution of interstitial salinities to a depth of 6 cm in subtidal estuarine sediments was measured monthly from June 1977 to August 1978 in mud substrates in the Fraser River estuary, British Columbia. Measurements were made at six stations ranging from oligohaline to polyhaline. The results of this study demonstrated that vertical salinity gradients were present, particularly in the mesohaline zone of the estuary, in the sediments and that the magnitude of these gradients changes seasonally.     

Rapid barium removal in the Delaware estuary

Six profiles of dissolved barium covering the entire salinity range of the Delaware River and Bay estuary from March through September 1996 were collected and analyzed. The profiles are similar to one another in both shape and magnitude except for one attribute. A sudden (≤24 days), nearly complete (>90%) removal of dissolved Ba in midestuary occurs in mid-May followed by an 80% recovery in early June. This removal appears to be temporally and spatially coupled to the end of the spring bloom. Based on such episodic behavior, and on recent work with flocculation of diatom exudates, we conclude that the Ba depletion is caused by barite precipitation in the estuary during the late stages of the bloom. This would imply that lower estuary and inner coastal margin sediments associated with eutrophic estuaries receive a seasonal pulse of barite. The suddenness of this event also implies that sedimentary barite is strongly influenced by high productivity events.Comparison of the riverine Ba concentration with the effective riverine end member after desorptive barium release yields an estimated 30–40 nM Ba available from the suspended sediments as they enter the estuary. This estimate is supported by excess barium in unfiltered samples over filtered samples taken from the river and also by calculations done elsewhere.     

Distribution and abundance of ichthyoplankton in the Manicouagan River estuary,a tributary of the lower St. Lawrence estuary

The species composition and relative abundance of ichthyoplankton were investigated during summer 1986 at four stations along the salinity gradient in the Manicouagan River estuary, a tributary of the lower St. Lawrence estuary. Physical characteristics of water masses indicated the presence of a strong saline front (>10‰ per km) delineating the freshwater and marine section of the Manicouagan estuary. The estuary supports a depauperate ichthyoplankton community, including four species of pelagic fish eggs and eight species of fish larvae. Species richness increased with salinity. The ichthyoplankton fauna can be divided into two distinct groups: freshwater and marine. These two groups result initially from spawning preferences exhibited by the different species abundance of freshwater larvae was maximal at the head of the estuary and marine larvae were most abundant at the most saline station. The length frequency distribution suggests that marine larvae are not effectively retained within the estuary. The Manicouagan estuary cannot be considered as a major spawning site nor an important nursery zone for any fish found in this area.     

Seasonal succession and breeding cycles of three species of Acartia (Copepoda: Calanoida) in a maine estuary

Seasonal succession of three species of Acartia in a Maine estuary was examined. The well-defined successional pattern, in which A. tonsa is replaced, by A. clausi in winter and early spring, is well-documented in the middle Atlantic estuaries, but was not observed in this northern New England estuary. Instead, both species increased in their abundance at the same time on most occasions. In general, A. clausi was present throughout the year and produced three generations from March to November. A. longiremis had two or three generations between May and November, and disappeared during late summer. A. tonsa was present from May through December and was most abundant in early fall. A. clausi and A. tonsa were generally most numerous upstream, and A. longiremis downstream. Adult and copepodite V sex ratios varied among species, stations, and seasons. Seasonal patterns of Acartia populations in estuaries of the north Atlantic coast and their possible relationships to environmental factors were also compared and discussed.     

Response of underwater light transmittance in the Rhode River estuary to changes in water-quality parameters

A year-long study of incident and underwater light transmittance (400–800 nm) in the Rhode River, Maryland, a tidal tributary to Chesapeake Bay, indicated that light transmittance responded in both intensity and spectral quality to changes in the amount and type of dissolved and suspended materials in the water. At times of relatively clear water, transmittance was similar to that previously reported in the literature for coastal waters. With high concentrations of suspended and dissolved materials in the water, attenuation of irradiance was high in the upper part of the water column and different for the various wave bands, depending on the type of material present. At such times, attenuation was higher in the upper part of the water column under sunny, clear skies than on cloudy days. We believe this to be due to higher concentrations of pigments and suspended particles in the water on sunny days, increasing the scattering and adsorption. A second factor was a lower average cosine on cloudy days, decreasing the effect of scattering on the average path length per meter of depth. High attenuation coefficients in the middle of the spectrum are attributed to accessory pigments. Regression of the diffuse attenuation coefficient on eight water-quality parameters explained up to 93% of the variance in the attenuation coefficient. Chlorophylls a and c and mineral suspensates were the three most important variables for data taken under clear skies. In contrast, under cloudy skies, the three most important variables were different for different wavelengths. Models of irradiance attenuation in turbid estuarine waters require the use of more variables than models for open ocean waters.     

Sedimentation in a fluvially infilling, barrier-bound estuary on a wave-dominated, microtidal coast: the Ouémé River estuary, Benin, west Africa

The Ouémé River estuary is located on the seasonally humid tropical coast of Benin, west Africa. A striking feature of this microtidal estuary is the presence of a large sand barrier bounding a 120 km² circular central basin, Lake Nokoué, that is being infilled by heterogeneous fluvial deposits supplied by a relatively large catchment (50 000 km²). Borehole cores from the lower estuary show basal Pleistocene lowstand alluvial sediments overlain by Holocene transgressive–highstand lagoonal mud and by transgressive to probably early highstand tidal inlet and flood‐tidal delta sand deposited in association with non‐preserved transgressive sand barriers. The change in estuary‐mouth sedimentation from a transgressive barrier‐inlet system to a regressive highstand barrier reflects regional modifications in marine sand supply and in the cross‐barrier tidal flux associated with barrier‐inlet systems. As barrier formation west of the Ouémé River led to an increasingly rectilinear shoreline, the longshore drift cell matured, ensuring voluminous eastward transport of sand from the Volta Delta in Ghana, the major purveyor of sand, to the Ouémé embayment, 200 km east. Concomitantly, the number of tidal inlets, and the tidal flux associated with a hitherto interlinked lagoonal system on this coast, diminished. Complete sealing of Lake Nokoué has produced a large, permanently closed estuary, where tidal intrusion is assured through the interconnected coastal lagoon via an inlet located 60 km east. Since 1885, tides have entered the estuary directly through an artificial outlet cut across the sand barrier. Although precluding the seaward loss of fluvial sediments, permanent estuary‐mouth closure has especially deprived the highstand estuary of marine sand, a potentially important component in estuarine infill on wave‐dominated coasts. In spite of a significant fluvial sediment supply, estuarine infill has been moderate, because of the size of the central basin. Estuarine closure has resulted in two co‐existing highstand sediment suites, with limited admixture, the marine‐derived, estuary‐mouth barrier and upland‐derived back‐barrier sediments. This situation differs from that of mature barrier estuaries characterized by active fluvial‐marine sediment mixing and facies interfingering.     

Dynamics of the turbidity maximum zone in a micro-tidal estuary: Hawkesbury River, Australia

Bed sediment, velocity and turbidity data are presented from a large (145 km long), generally well-mixed, micro-tidal estuary in south-eastern Australia. The percentage of mud in the bed sediments reaches a maximum in a relatively narrow zone centred ≈30–40 km from the estuary mouth. Regular tidal resuspension of these bed sediments produces a turbidity maximum (TM) zone in the same location. The maximum recorded depth-averaged turbidity was 90 FTU and the maximum near-bed turbidity was 228 FTU. These values correspond to suspended particulate matter (SPM) concentrations of roughly 86 and 219 mg l^?1, respectively. Neither of the two existing theories that describe the development and location of the TM zone in the extensively studied meso- and macro-tidal estuaries of northern Europe (namely, gravitational circulation and tidal asymmetry) provide a complete explanation for the location of the TM zone in the Hawkesbury River. Two important factors distinguish the Hawkesbury from these other estuaries: (1) the fresh water discharge rate and supply of sediment to the estuary head is very low for most of the time, and (2) suspension concentrations derived from tidal stirring of the bed sediments are comparatively low. The first factor means that sediment delivery to the estuary is largely restricted to short-lived, large-magnitude, fluvial flood events. During these events the estuary becomes partially mixed and it is hypothesized that the resulting gravitational circulation focuses mud deposition at the flood-determined salt intrusion limit (some 35 km seaward of the typical salt intrusion limit). The second factor means that easily entrained high concentration suspensions (or fluid muds), typical of meso- and macro-tidal estuaries, are absent. Maintenance of the TM zone during low-flow periods is due to an erosion-lag process, together with a local divergence in tidal velocity residuals, which prevent the TM zone from becoming diffused along the estuary axis.     

Subtidal sea level variability in a shallow Mississippi River Deltaic estuary, Louisiana

The relative roles of river, atmospheric, and tidal forcings on estuarine sea level variability are examined in Breton Sound, a shallow (0.7 m) deltaic estuary situated in an interdistributary basin on the Mississippi River deltaic plain. The deltaic landscape contains vegetated marshes, tidal flats, circuitous channels, and other features that frictionally dissipate waves propagating through the system. Direct forcing by local wind stress over the surface of the estuary is minimal, owing to the lack of significant fetch due to landscape features of the estuary. Atmospheric forcing occurs almost entirely through remote forcing, where alongshore winds facilitate estuary-shelf exchange through coastal Ekman convergence. The highly frictional nature of the deltaic landscape causes the estuary to act as a low-pass filter to remote atmospheric forcing, where high-frequency, coastally-induced fluctuations are significantly damped, and the damping increases with distance from the estuary mouth. During spring, when substantial quantities of controlled Mississippi River inputs (?q = 62 m³ s^-1) are discharged into the estuary, upper estuary subtidal sea levels are forced by a combination of river and remote atmospheric forcings, while river effects are less clear downestuary. During autumn (?q = 7 m³ s^-1) sea level variability throughout the estuary is governed entirely by coastal variations at the marine boundary. A frequency-dependent analytical model, previously used to describe sea level dynamics forced by local wind stress and coastal forcing in deeper, less frictional systems, is applied in the shallow Breton Sound estuary. In contrast to deeper systems where coastally-induced fluctuations exhibit little or no frictional attenuation inside the estuary, these fluctuations in the shallow Breton Sound estuary show strong frequency dependent amplitude reductions that extend well into the subtidal frequency spectrum.     

Seasonal changes in the nekton community of the Suwannee River estuary and the potential impacts of freshwater withdrawal

In Florida, issues related to alterations of estuarine salinity caused by freshwater withdrawal have recently gained increasing attention. We examined nekton community structure in the Suwannee River estuary (1997–2000) and investigated the relationship between environmental factors and the abundance of fisheries resources. We compared nekton community structure and environmental factors seasonally and annually using multidimensional scaling (MDS) ordination and cluster analysis and observed a strong seasonal pattern. This pattern was consistent among years and closely paralleled those for temperature and river discharge. Representative species for cold seasons includedLeiostomus xanthurus andLagodon rhomboides, and those for warm seasons includedMembras martinica andAnchoa hepsetus. Species that contributed most to the dissimilarity in community structures between wet and dry seasons were abundant and generally preferred lower salinity (e.g.,L. xanthurus, Eucinostomus spp., andMenidia spp.). A period of low freshwater inflow during the latter portion of our study coincided with both decreases and increases in the abundances of some dominant and some economically important species. We have established a baseline which will assist in measuring the effects of long-term changes in freshwater input on the nekton communities of the Suwannee River estuary, but our ability to predict these effects is still limited.     

Microbial nitrogen removal in a developing suburban estuary along the South Carolina coast

The conversion of undisturbed coastal regions to commercial and suburban developments may pose a threat to surface and groundwater quality by introducing nitrate-nitrogen (NO₃ ^?-N) from runoff of land-applied wastewater and fertilizers. Microbial denitrification is an important NO₃ ^?-N removal mechanism in coastal sediments. The objective of this study was to compare denitrification and nitrate conversion rates in coastal sediments from a golf course, suburban site, undeveloped marsh, and nonmarsh area near rapidly developing Hilton Head Island, South Carolina. Nitrous oxide was measured using gas chromatography and nitrate and ammonium concentrations were measured using a flow injection autoanalyzer in microcosms spiked, with 50 μg NO₃ ^?-N gdw^?1. The two marsh sites had the greatest ammonium production, which was correlated with fine sediment particle size and higher background sediment nitrate and surface water sulfate concentrations. The golf course swale had greatest denitrification rates, which were correlated with higher total carbon and organic nitrogen in sediments. Nitrate was consumed in golf course sediments to a greater extent than in the undeveloped marsh and upland freshwater sites, suggesting that the undeveloped sites and receiving estuaries may be more susceptible to nitrate contamination than the golf course swale and marsh under nonstorm conditions. Construction of swales and vegetated buffers using sediments with high organic carbon content as best management practices may aid in removing nitrate and other contaminants from runoff prior to its transport to the receiving marsh and estuary.     

Nutrient limitation in the lower Housatonic River estuary

One of the most serious threats to freshwater and marine ecosystems is high rates of anthropogenic nutrient loading, particularly nitrogen (N) and phophorus (P). One of the major freshwater sources of nutrients to Long Island Sound (LIS) is the Housatonic River (HR). Current management plans that call for reducing N inputs without reducing P inputs may change the N: P ratio in the water column and the pattern of algal nutrient limitation and species composition in the tidal portion of the river. To assess the current pattern of algal nutrient limitation in the HR estuary, nutrient bioassays were conducted in spring, summer, and fall at 5 sites throughout the tidal portion and adjacent LIS. Diatoms were a dominant taxon at all sites throughout the sampling period. Other seasonally important taxa include cyanobacteria, cryptophytes, and euglenoids. Phytoplankton in LIS were always strongly N limited and were co-limited by P in spring. During low flow (summer), phytoplankton in the lower HR estuary were N limited. Phytoplankton in the middle reaches showed no evidence of N or P limitation and were likely limited by other factors. In spring, phytoplankton in the upper HR estuary were P limited. Periods of N or P limitation were better correlated with periods of lower concentrations of nitrate or phosphate than with differences in N: P ratio. These results suggest that decreases in N concentration could increase the prevalence of N limitation throughout the estuary that in turn may reduce phytoplankton biomass and alter species composition of the phytoplankton.     

Evaporation, precipitation, and associated salinity changes at a humid, subtropical estuary

The distilling effect of evaporation and the diluting effect of precipitation on salinity at two estuarine sites in the humid subtropical setting of the Indian River Lagoon, Florida, were evaluated based on daily evaporation computed with an energy-budget method and measured precipitation. Despite the larger magnitude of evaporation (about 1,58 mm yr⁻¹) compared to precipitation (about 1,180 mm yr⁻¹) between February 2002 and January 2004, the variability of monthly precipitation induced salinity changes was more than twice the variability of evaporation induced changes. Use of a constant, mean value of evaporation, along with measured values of daily precipitation, were sufficient to produce simulated salinity changes that contained little monthly (root-mean-square error = 0.33‰ mo⁻¹ and 0.52‰ mo⁻¹ at the two sites) or cumulative error (<1‰ yr⁻¹) compared to simulations that used computed daily values of evaporation. This result indicates that measuring the temporal variability in evaporation may not be critical to simulation of salinity within the lagoon. Comparison of evaporation and precipitation induced salinity changes with measured salinity changes indicates that evaporation and precipitation explained only 4% of the changes in salinity within a flow-through area of the lagoon; surface water and ocean inflows probably accounted for most of the variability in salinity at this site. Evaporation and precipitation induced salinity changes explained 61% of the variability in salinity at a flow-restricted part of the lagoon.     

Occurrence of the endangered shortnose sturgeon,Acipenser brevirostrum,in the Delaware River estuary

All available capture records of the endangered shortnose sturgeon,Acipenser brevirostrum, for the Delaware River Estuary from the early nineteeth century to the present were compiled. During 1817 through 1913 some 1,949 captures were reported, most as a bycatch of the shad gill net fishery. No documented captures during 1913 to 1954 were reported in the literature. Thirty-seven shortnose sturgeon were reported captured from 1954 through 1979, mostly incidental to fishery and ecological studies. Most specimens were taken in the upper tidal freshwater portion of the estuary (rkm 200–214). Seasonal-spatial distribution appeared similar to that observed for northern shortnose sturgeon populations. Taxonomic data obtained from seven specimens generally agreed with those from other drainages.     

Flushing of dense, hypoxic water from a cavity of the Swan River estuary, Western Australia

Flushing of dense water from cavities of the upper reaches of the Swan River estuary in Western Australia was investigated using measured salinity and dissolved oxygen profiles and a two-dimensional, laterally averaged hydrodynamic model (TISAT). Seasonal flushing of dense, hypoxic bottom waters from a relatively deep site took place over ∼3 days at the onset of winter in 1994. Model simulations of the purging of this dense water did not correspond closely with changes in the densimetric Froude number. Purging, expressed as depth of the halocline as a fraction of the total cavity depth, occurred when the simulated mean horizontal velocity at 2 m depth (top of cavity) changed from negative to strongly positive, indicating arrest of upstream flow and continuous downstream flow. This corresponded to freshwater discharge of about 50 m³ s⁻¹. Oxygen depletion of bottom waters was closely related to stratification. Oxygen dynamics at the onset of winter river flow was analysed using an exponential decay model, assuning that there was no net inflow or outflow across the halocline and thus no vertical transport of oxygen during a period of strong stratification. The rate constant for oxygen decay at Ron Courtney Island (RCI) was estimated to be 0.232 d⁻¹ for this period. Bottom waters at RCI declined to less than 1 mg 1⁻¹ prior to complete flushing through increased river flows. This study provided in sights to how freshwater flows may be allocated to maintain suitable oxygen levels in the bottom waters of estuarine cavities.     

Phosphorus distribution in sediments of the Delaware River estuary

In order to determine the primary factors related to the accumulation of phosphorus in estuarine sediments, a study of phosphorus fractions in sediments of the Delaware River Estuary was undertaken. A correlation matrix between the phosphorus fractions, determined by serial extraction, and 14 sediment variables was computed. Total phosphorus and total inorganic phosphorus in the sediment-phosphorus reservoir decreases with increasing salinity. This variation is correlated with decreasing iron oxyhydroxide content in the sediment. Neither clay content nor calcium carbonate content appear to be significantly correlated with variation in total inorganic phosphorus content in the fine-grained sediments of this estuary. Although calcium phosphate is concluded to be a major constituent of the sediment-phosphorus reservoir, there was no evidence found that it is authigenic in this environment.     

A model study of turbidity maxima in the York River estuary,Virginia

A three-dimensional numerical model is used to investigate the mechanisms that contribute to the formation of the turbidity maxima in the York River, Virginia (U.S.). The model reproduces the basic features in both salinity and total suspended sediments (TSS) fields for three different patterns. Both the prominent estuary turbidity maximum (ETM) and the newly discovered secondary turbidity maximum (STM) are simulated when river discharge is relatively low. At higher river inflow, the two turbidity maxima move closer to each other. During very high river discharge event, only the prominent turbidity maximum is simulated. Diagnostic model studies also suggest that bottom resuspension is an important source of TSS in both the ETM and the STM, and confirm the observed association between the turbidity maxima and the stratification patterns in the York River estuary. The ETM is usually located near the head of salt intrusion and the STM is often associated with a transition zone between upriver well mixed and downriver more stratified water columns. Analysis of the model results from the diagnostic studies indicates that the location of the ETM is well associated with the null point of bottom residual flow. Convergent bottom residual flow, as well as tidal asymmetry, is the most important mechanisms that contribute to the formation of the STM. the STM often exists in a region with landward decrease of bottom residual flow and net landward sediment flux due to tidal asymmetry. The channel depth of this region usually decreases sharply upriver. As channel depth decreases, vertical mixing increases and hence the water column is better mixed landward of the STM.