Estuarine Discharge Research Articles

The effects of mixing and spreading on density in near-field river plumes

Abstract Water mass modification in surface-trapped, near-field river plumes is examined using a 1.5-layer reduced gravity model and a three-dimensional numerical model. Solutions to the layer model are shown to be qualitatively similar to previous observations and three-dimensional simulations of near-field plumes. Analytic analysis of the layer model demonstrates how the near-field plume is controlled by the competing processes of mixing and spreading. The two models are then used to explore the parameter space dependence of density changes within the near-field plume and their associated cross-shore length scales. Both the magnitude of density changes and their length scales are proportional to either estuarine discharge or fresh water discharge; density changes are also inversely proportional to the estuary mouth width. One surprising feature of the parameter space solutions is that the density of water exiting the near-field plume, a measure of the net dilution of the entire near-field plume, is shown to be inversely proportional to local mixing rates. This is because when local mixing is lower, the influence of plume spreading becomes greater; this spreading accelerates the plume, requiring more net mixing to bring the plume back to subcritical flow.

Distribution and vertical dynamics of planktonic communities at Sofala Bank, Mozambique

Coastal ecosystem processes are largely influenced by the interaction of different factors operating at various temporal and spatial scales, specifically those responsible for primary production patterns that modulate zooplankton and subsequent trophic levels. Hydrological processes, such as tidal cycles and coastal currents, nutrients availability, phytoplankton groups (studied through algal pigment signatures analysed by HPLC), and zooplankton abundance and distribution were investigated at the Sofala Bank (Mozambique), with special emphasis on their horizontal distribution and vertical dynamics (48 h). Horizontal distribution has shown inshore–offshore gradients in all analysed parameters, as well as inshore waters intrusion probably related to Zambezi River delta runoff. Tidal currents were responsible for major hydrological vertical variations and for horizontal and vertical advection of phytoplankton biomass in the surface and deepest layers, respectively. Nutrient concentrations were typical from oligotrophic regions, and nutrient ratios were strongly influenced by depleted nitrate + nitrite concentrations, indicating low estuarine discharges typical from the dry season. The very low N:P ratio obtained suggests strong nitrogen limitation to phytoplankton communities, supporting the low phytoplankton abundance observed. Both phytoplankton pigments and zooplankton were found mainly near the bottom (40 m depth), despite the latter displayed vertical migrations triggered by light variations. Phytoplankton community was dominated by microflagellates, specifically prymnesiophyceans, and behaved as a whole, except Cyanobacteria that displayed vertical distribution movements different from other phytoplankton groups, being mainly concentrated at mid-water column depths (10–20 m). This investigation enhances physico-chemical phenomena and their importance determining the planktonic communities vertical dynamics at Sofala Bank, a tropical coastal ecosystem of the Western Indian Ocean where planktonic dynamics are still poorly described and understood.

The effects of mixing and spreading on density in near-field river plumes

Water mass modification in surface-trapped, near-field river plumes is examined using a 1.5-layer reduced gravity model and a three-dimensional numerical model. Solutions to the layer model are shown to be qualitatively similar to previous observations and three-dimensional simulations of near-field plumes. Analytic analysis of the layer model demonstrates how the near-field plume is controlled by the competing processes of mixing and spreading. The two models are then used to explore the parameter space dependence of density changes within the near-field plume and their associated cross-shore length scales. Both the magnitude of density changes and their length scales are proportional to either estuarine discharge or fresh water discharge; density changes are also inversely proportional to the estuary mouth width. One surprising feature of the parameter space solutions is that the density of water exiting the near-field plume, a measure of the net dilution of the entire near-field plume, is shown to be inversely proportional to local mixing rates. This is because when local mixing is lower, the influence of plume spreading becomes greater; this spreading accelerates the plume, requiring more net mixing to bring the plume back to subcritical flow.

Effect of estuarine flow on ocean circulation using a coupled coastal-bay estuarine model: an application to the 1999 Orissa cyclone

A coupled coastal-bay estuarine numerical model is described and applied to investigate the combination of wind-estuarine driven circulation off the Orissa coast. The model is based on coupling of a 2-dimensional estuarine model with a 3-dimensional coastal-bay model. The models are linked through the elevation at the interface. Using the coupled model, the numerical experiments are carried out to elicit the dynamical linking between the estuarine outflow and the coastal ocean to simulate the ensuing adjoining coastal circulation. During the southwest monsoon, it is noticed that the estuarine discharge from the northern head-bay river system and the river systems that join the Bay of Bengal along the Orissa coast would sufficiently modify the coastal circulation along the coast. Numerical experiments are also carried for the model simulation of surges generated by the 1999 Orissa cyclone. It is shown that the estuarine system would influence significantly on surge development and associated inundation through the rivers.

Algal growth potential as an indicator of eutrophication degree in coastal areas under sewage disposal influence

Algal growth potential was used to quantify the degree of eutrophication of three coastal regions in São Paulo, Brazil which are subject to sewage disposal. Surface water was collected in Praia Grande, Santos, Guarujá and São Sebastião during two surveys (low tourist season—October 1997 and high tourist season—March 1998). Water was filtered and used in chlorophyll-a and nutrient analyses and in bioassays. Samples were inoculated with the diatom Phaeodactylum tricornutum Bohlin, and then subdivided into seven replicates. During a ten days experiment, in vivo fluorescence was performed and algal growth potential was calculated. Values were expressed as absolute in vivo fluorescence, as well as with a ratio of in vivo fluorescence of sample in relation to the fluorescence of control sea water for each treatment which defines the trophic status. The results of these bioassays showed a growing degree of eutrophication among regions: Praia Grande ∼ São Sebastião → Guarujá → Santos. It also identified sewage disposals and estuarine discharges as the main sources causing eutrophication. This bioassay technique is an important tool for environmental pre-monitoring studies, giving useful results in a short time.

Seasonal discharge of estuarine freshwater to the western Chukchi Sea shelf identified in stable isotope profiles of mollusk shells

Oxygen and carbon isotope profiles of bivalve mollusks (Serripes groenlandicus and Macoma calcarea) record distinct changes of water mass properties in the Bering Strait region associated with the seasonal discharge of estuarine freshwater from Kolyuchin Bay into the western Chukchi Sea. Cycles in the δ18O profiles are correlated with shell growth bands and interpreted as annual; maximum δ18O values corresponding to coldest water temperatures coincide with dark internal bands in the shell that mark slower growth during winter. A 0.5 ‰ offset between the baseline winter δ18O values of two specimens (HX68 and HX65), at different distances from the estuarine discharge, is attributed to a difference in seawater δ18O between the two sites and indicates that the influence of the estuarine water persists through much of the year. The isotope profiles of specimen AK55, collected north of Bering Strait in the Chukchi Sea, reflect the Bering Shelf‐Anadyr Water flowing from south of Bering Strait. Comparison of the δ13C profiles shows that HX68, collected closer to Kolyuchin Bay, has high‐amplitude negative δ13C excursions associated with incorporation of dissolved inorganic carbon that is isotopically depleted in 13C, probably from terrestrial runoff. The significant positive correlation between δ18O and δ13C values in these specimens suggests that the properties of the ambient water masses are reflected in the stable isotope profiles. These results show that the stable isotope profiles of mollusk shells can serve as proxy indicators of water mass properties, particularly during runoff events in Arctic coastal regions that are logistically difficult to sample directly.

Particle Dynamics and Trace Metal Reactivity in Estuarine Plumes

Iron, Mn, Cu, Pb and Zn have been determined in suspended particulate matter (SPM) collected in the estuarine plume regions of the Humber (during winter, spring and summer) and Thames (winter only). Metal concentrations (w/w) were found to increase with SPM concentration and could be defined in terms of the mixing of an ambient, slow settling population, with variable proportions of a diluent population. The end-members of the particle mixing series are fine material derived from coastal erosion, which is modified seasonally by biological production, and contaminated estuarine material which is contained within the estuarine discharge or derived from local resuspension of reworked deposits by tidal currents and wave activity. Iron-normalized metal concentrations exhibited an inverse relationship with SPM concentration in the Humber region and regression analyses enabled seasonal changes in end-member compositions to be evaluated. Since the metal:Fe ratios of the ambient population did not accord with those of local cliff samples, additional sources of metal were proposed whose importance to particle composition increases with a reduction in SPM concentration. Qualitatively, the seasonal variation of end-member compositions was consistent with (i) the coupling between redox processes occurring in the bed sediment and adsorption of metals (Mn, Cu, Zn) released from the pore waters onto ambient and diluent suspended particles in the overlying water column, and (ii) adsorption of metal (Pb) by ambient suspended particles from an extraneous (atmospheric) source. In the Thames plume, an increase in Fe-normalized metal concentrations with increasing particle concentration resulted from the mixing of end-member particles and the effects of additional metal from an internal or extraneous source were less clear, possibly because of metal desorption from suspended particles traversing the salinity gradient in the outer estuary. The processes described in this study regulate the internal cycling of trace metals in estuarine plume regions and the export of metals to neighbouring shelf sea environments.

Coastal Oceanography Applications of Digital Image Data from Marine Radar

A marine radar attached to a digital image capture system has been adapted for deployment from shore sites. Optimum operating parameters have been identified in relation to the resolution and sampling capabilities of both the radar and the digitization system. Special software enables individual images to be geographically located so that several images can be averaged to reduce speckle and to eliminate features that fluctuate at high frequency. Test deployments at several coastal sites in the south of England confirm that the system is capable of making unique measurements for a variety of oceanographic applications in the coastal zone. The spatial distribution of wave crests approaching the shore can be measured, and by averaging over time, the location of the breaker zone can be established. Although the precise mechanisms for backscatter of grazing incidence radar are not fully understood, it is clear that the radar can also detect the surface signatures of estuarine fronts and wastewater discharge plumes. The interaction between tidal flow and bathymetric features can also be detected by the surface roughness signature. The radar is particularly useful for monitoring the gradual evolution of processes over a tidal cycle.

Observations of a pulsed buoyancy current downstream of Chesapeake Bay

A plume of low‐salinity water was observed along the North Carolina coast 100 km south of the mouth of Chesapeake Bay during the Coastal Ocean Processes Pilot field program conducted from August through October 1994. The presence of the plume was episodic, occurring every 3 to 8 days. The timing was related to wind patterns, which influence both the delivery of estuarine discharge to the shelf and the plume's passage down the coast. When not affected by local winds, the low‐salinity water was confined to within 7–9 km of the coast and was about 8 m deep. Downwelling winds narrow and deepen the plume, whereas upwelling winds cause it to thin and spread offshore, eventually detaching from the coast. The low‐salinity plume propagated along the coast at speeds comparable to linear internal wave phase speed, except when strong downwelling wind conditions caused the plume to be in contact with the bottom. The observed propagation speeds are faster than those predicted by previous numerical modeling efforts. The plumes slowed with distance from the source, as mixing with ambient shelf water reduced the density contrast. This buoyancy source was balanced by an alongshore current with a southward velocity of 0.3 to 0.7 m s−1, bounded by a region of high horizontal velocity shear at the offshore salinity front. The currents observed in the nose of the plume are consistent with properties of an internal gravity current under rotation.

Seasonal variability in microplanktonic biomasses in the Gironde dilution plume (Bay of Biscay): relative importance of bacteria

Bacterial and phytoplankton biomasses were estimated in a four-year study along a saline gradient (the dilution plume of the Gironde estuary on the Aquitanian continental shelf), by measuring bacterial abundance and mean cell volumes (epifluorescence counts) and chlorophyll- a concentrations (in vitro fluorescence). The spatial and temporal distribution of phytoplankton enabled us to determine the seasonal succession of algal blooms, sprouting in the marine waters of the inner shelf in early spring (picoplanktonic forms), reaching their annual maximum (10 μg L −1) in plume waters in advanced spring, and moving to estuarine waters in the summer (> 20 μm forms). Bacterial biomasses followed mainly a decreasing dilution gradient, especially in winter and early spring, with maximum estuarine values reaching 10 7 cell mL −1 (more than 30 % of small attached cells and more than 60 % of free small cocci), in accordance with estuarine discharges into the inner shelf zone. Moreover, plume waters showed an additionnal peak of bacterial numbers and mean cell volumes in early summer, with increasing proportions of larger rod- and vibrio-shaped bacteria. A clear morphometrical succession was followed in marine waters, from a majority of attached and coccal cells in winter, to maximal proportions of larger vibrio-shaped bacteria, inducing a maximal spring level of total abundance (2 × 10 6 cell mL −1) and high mean cell volumes (0.044 μm 3), coinciding with the marine vernal bloom. Bacterial estimated biomass represented, consequently, a wide range of proportions compared to phytoplankton: only when a good correlation was found between them, bacterial-C/phytoplankton-C ratio was close to 1 4 , a correlation that improved in warmer periods, throughout different water types. Due to the relavitely narrow range of chlorophyll- a values, we could not find a high predictive correlation when pooling all the 4-year data, a relationship that has been outlined in important reviews by pooling together different aquatic systems. We noticed, however, that in the particular environment of our study (high inputs of particulate detritic matter) bacterial-C usually dominated the microbial pool (specially in estuarine and plume waters and in light-limited conditions). This dominance, added to the frequent dominance of small-sized phytoplankton, leads to an enhanced “microbial loop” activity, defining a “maintenance system” (oligotrophic conditions), which alternates with some eutrophic periods.

Sources and transport of land-derived particulate and dissolved organic matter in the Gulf of Mexico (Texas shelf/slope): The use of ligninphenols and loliolides as biomarkers

The fate and transport of terrestrial organic matter across the continental margin in the Gulf of Mexico was studied in 1992 and 1993 using chemical biomarkers. Lignin-phenols were utilized as biomarkers for terrestrial inputs and indicated that much of the terrestrial organic matter inputs were deposited on the shelf/break and slope. The lignin-phenol concentrations (normalized to carbon) in POC, HMW DOC, and sediments in slope waters were considerably higher than at other open ocean sites studied previously. The dominant mechanism for transport of terrestrially-derived POC and HMW DOC across the shelf and slope was hypothesized to be advection of riverine and estuarine discharges through benthic nepheloid layers. Based on loliolide concentrations in the water column, we believe that lateral transport of these materials at the shelf/break (through extensions of benthic nepheloid layers) may have been an important mechanism for the injection of terrestrially-derived organic matter into deep slope waters.

Facies, internal structures and sequences of modern Gironde-derived muds on the Aquitaine inner shelf, France

Seventy cores from the Aquitaine continental shelf were examined using radiographic and grain-size techniques in order to describe the sedimentary structures of the muddy deposits, and to evaluate their depositional processes. Four lithofacies are identified in this fine-grained deposit: (a) homogeneous silty sand, (b) interbedded homogeneous mud and sand, (c) silty-clayey mud, and (d) mottled mud. They show a logical pattern in relationship to the water depth and the distance from the coast. Primary structures are present particularly in the landward and central portion of the mud fields, where the sediment is organized into sequences with a sharp-based erosional contact, overlain by a fining-up succession (centimetre to decimetre scale). The beginning of each of these is characteristic of a high-energy storm event, which is common on this shelf. The settling of suspended fine sediment corresponds to the flood estuarine discharge during quiet periods. Primary sedimentary structures decrease in the distal area where the muddy sediment is frequently reworked by infauna. Finally, primary structures and their preservation depend on the relative magnitudes of surface waves, storms, infaunal mixing and fluvial sediment deposition rates (i.e. floods).

Kenyan coral reef-associated gastropod assemblages: distribution and diversity patterns

A survey of Kenya's shallow water (<2 m) coral reef-associated prosobranch fauna was undertaken to determine patterns of distribution, density, diversity and species richness, and the possible role of other reef fauna and human utilization on these patterns. The sample assemblage of 135 species from 25 families is similar to other Indian Ocean regions with no apparent endemism or subregional faunal affinities. Species richness, determined by species-individual relationships, has been reduced by approximately 45% since the Pleistocene. Northern Kenya, typified by small coral islands experiencing river and estuarine discharges had low densities and species richness and high species variability. This is attributable to the interrelated factors of river discharge, small reefs and reduced predator refuge. Southern Kenya's more expansive fringing reef has a denser and richer fauna but appears less species rich than Tanzania. Variation within reefs suggests similarities in diversity between reef lagoons, flats and edges, but lagoons had lower densities than reef flat or edge sites. This is attributable to greater predation rates within lagoons. Species composition between reef locations was variable but differed for comparisons between reef lagoons and reef flats. The population densities of thirty commercially collected species were compared between shelled and unshelled reefs. Only two commercial strombids, Lambis truncata and L. chiragra, had lower densities within shelled compared to unshelled reefs. Within six southern Kenvan reef lagoons, total gastropod densities were negatively correlated with the Balistidae (triggerfish) and total fish densities and positively with sea urchin densities. The removal of balistids through fishing appears to lead to co-occurring population increases in gastropod and sea urchin populations which, in most instances, appears to negate the effect of shell collecting.

Larval transport and its association with recruitment of blue crabs to Chesapeake Bay

The blue crab ( Calinectes sapidus) harvest in Chesapeake Bay has undergone large yearly fluctuations, creating hardships in the fishing industry and uncertainties in its management. It has previously been suggested that part of the fluctuation may be due to environmental influences during a sensitive period in their life history when blue crab larvae are planktonic outside the bay. During this period, they reside principally in the neuston where wind forced transport has the maximum influence. It is shown, through vector/scalar correlations of wind stress with harvest, that approximately 36% of the harvest variation can be accounted for by the wind patterns during the months from June through September. The influence of alongshore sea level slope and cummulative estuarine discharge (both relating to transport through pressure gradient forcing) on harvest were investigated, but the results were negligible, or ambiguous at best.

Dispersal Pathways for Particle-Associated Pollutants

Particle-associated pollutants (totaling 10(7) metric tons per year) are introduced into the New York Bight by ocean dumping, estuarine discharge, sewage outfalls, eolian transport, and shipping waste and spillage. Oceanic and estuarine circulation processes dilute and transport the particles by a natural dispersal system that also tends to be highly distributive; particle-associated pollutants apparently seek the same sinks in the Hudson River shelf valley and intracoastal weltlands, regardless of their point of introduction.

Spatial Distribution Patterns of Planktonic Fish Eggs in Lower Mobile Bay, Alabama

Abstract A 12-month survey of planktonic fish eggs in lower Mobile Bay yielded 110 samples containing over 105 eggs. Fourteen taxa were represented, of which eight were abundant enough for analysis. By normal and inverse schemes of numerical classification, two distinct assemblages (station groups), coastal and estuarine, were found. The coastal assemblage was influenced by high-salinity tidal intrusions, whereas the estuarine assemblage was influenced by low-salinity estuarine water and river discharge. The spawning location of adult fish, circulation patterns, and salinity gradients (due to river discharge) were responsible for delineating the two assemblages. There also were two species associations (species groups). Eggs of Anchoa mitchilli, Sciaenidae, and Symphurus sp. were ubiquitous in both the estuarine and coastal assemblages. The eggs of Harengula jaguana , Anchoa hepsetus, Anchoa sp., Chloroscombrus chrysurus, and Trinectes maculatus were found predominantly in the coastal assemblage. Month-to...

Ocean Currents along a Nearshore Frontal Zone on the Continental Shelf of the Southeastern United States

Abstract River and estuarine discharges mix to form a frontal zone along a 400 km length of the coast of the south-eastern United States. The frontal width extends from the coast outward 10–20 km. Two ships, anchored across the frontal zone during autumn, obtained simultaneous hourly profiles of currents and density for, five consecutive tidal cycles. The frontal zone contained a baroclinic coastal current flowing southward. The flow was strongly convergent; more water entered on the coastal side than exited on the seaward side. Much of the inflow was apparently turned southward. Thus, the frontal zone acted as a dynamic barrier that inhibited the advection of mixed river discharge farther offshore. A thermal wind relationship suitably predicted vertical shear through the frontal zone. Bottom friction seemed to play only a secondary role. Data suggest that the baroclinic coastal current was modified by the presence of a barotropic current seaward of the frontal zone. The force responsible for the barotrop...

Analysis and interpretation of TIROS-N AVHRR infrared imagery, western Gulf of Mexico

The early morning (0300 local time) 5 December 1978 TIROS-N overpass across the Gulf of Mexico coincided with the mild cold-air outbreak of 4–6 December. In the dry cloud-free region behind the cold front, the channel 4 data (thermal infrared 10.5–11.5 μm) of the TIROS-N Advanced Very High Resolution Radiometer (AVHRR) provided excellent radiometric data on the sea-surface temperature patterns. An AVHRR data calibration program has been developed to convert the thermal data to temperature in degrees Celsius and data from visible radiation channels into percent albedo. Digital interactive analysis of the calibrated channel 4 data in the cloud-free region behind the front revealed a series of oceanographic features in the sea-surface temperature patterns. Temperature differences of 6–8°C occurred between the inner shelf and deep Gulf waters. Estuarine discharge plumes 2°C cooler than shelf waters and a large cyclonic eddy with isothermal core are discernible in the enhanced 0.25°C thermal resolution, 1.1-km spatial resolution imagery. The radiometric sea-surface temperatures are 2°C below surface measurements made by two NOAA data buoys anchored along the 26°N parallel in the deep Gulf. Under these atmospheric and oceanic conditions, the satellite data available from the digital archives of the National Climatic Center are a valuable record of continental shelf and near-surface deep-ocean processes.

South China Sea: Present and potential coastal area resource use conflicts

Abstract Coastal areas of the South China Sea exhibit broad geomorphological, climatic, biological and physical oceanographic similarities. Conflicts between coastal area resource uses in this physical marine region are becoming apparent. These conflicts result from a combination of increasing coastal area population concentration and traditional dependence on the coastal area for livelihood, food, trade and waste disposal, recently supplemented by recreational, industrial and non-renewable resource extraction activities. This paper reviews the general present status of major competing coastal area resource uses and their conflicts, and delineates present and potential geographic areas of concern. Extractive industries, industrial water use, and estuarine discharge conflict with fisheries, tourism/recreation, and human health. Geographic areas of concern include the region's major coastal urban concentrations, the Malacca Straits, the Bight of Bangkok and the greater Gulf of Tailand. To ameliorate conflict resolution, governmental philosophies geared to Western models of growth development should also incorporate the concept of planned growth, i.e., optimum allocation of coastal area resource use designed to minimize conflict, so recently, and thus painfully, adopted in the West.

Burrowing, feeding, and spatial distribution of the school prawn Metapenaeus macleayi (Haswell) in the Hunter river region, Australia

The influence of the food content and the particle size of the substratum on the distribution and relative abundance of Metapenaeus macleayi (Haswell) has been investigated by periodic trawl sampling for prawns and laboratory studies of their food, feeding, and burrowing behaviour. M. macleayi are opportunistic omnivores; they pick up material from the bottom with their chelipeds and convey it to the mouthparts where edible matter is sorted and ingested. They burrow into the sediment with their pereopods and pleopods, and are usually totally buried beneath the surface. A respiratory water current enters a tube formed by the antennal scales and the antennules, flows over the gills and then out of the carapace; this current is regularly reversed with increased strength, presumably to carry away de-oxygenated water. The results of experiments on substratum preference with adequately nourished juveniles suggest that the particle size of the sediment is more important than the food in the substratum in determining the distribution; the apparent preference of juveniles for a fine sandy substratum both in the laboratory and in the natural habitat may be attributed to the minimum threshold velocity of the sand particles. Adults are most abundant in turbid coastal waters arising from estuarine discharge and here the size of the sediment particles appears to be less important than the food content of the substratum in determining the distribution. The mangrove and reed swamps in the Hunter region play an important rôle in the food requirements of juvenile and adult prawns.