Recent sedimentary development of Tampa Bay, Florida: A microtidal estuary incised into tertiary platform carbonates

Tampa Bay, a large, microtidal, clastic-filled estuary incised into Tertiary carbonate strata, is the largest estuary on Florida’s west coast. A total of 250 surface sediment samples and 17 cores were collected in Tampa Bay in order to determine the patterns and controlling factors governing the recent infilling and modern sediment distribution, and to examine the results in terms of current models of estuarine sedimentation and development. Surficial sediments in Tampa Bay consist of three facies types, each occurring in a distinct zone: modern terrigenous clastic muds occurring in the upper bay and around the bay periphery; relict, reworked-fluvial, quartz-rich sands occupying the open portion of the middle bay; and modern carbonate-rich, marine-derived sands and gravels occupying the lower bay. Factors controlling sediment distribution include: sediment source and supply rate; bathymetry, which is a function of the antecedent topography; and the winnowing effect of wind-generated waves that prohibits modern accumulation in the shallow middle bay. These factors also play a major role in the recent infilling history of Tampa Bay, which has progressed in four stages during the Holocene sea-level rise. Recently developed models of estuarine sedimentation are based primarily on mesotidal to macrotidal estuaries in terrigenous clastic settings in which sedimentation patterns and infilling history are a result of the relative contribution of marine and fluvial processes. Tampa Bay differs in that it was originally incised into carbonate strata, and neither fluvial or marine processes are interpreted to be major contributors to modern sediment distribution. Tampa Bay, therefore, provides an example of an unusual estuary type, which should be considered in future modeling efforts. *** DIRECT SUPPORT *** A01BY083 00004     

新世纪上海地区相对海平面变化影响因素及预测方法

针对传统相对海平面变化分析方法均将理论海平面变化和地面沉降进行的叠加分析,本文讨论和分析了近20年来全球理论海平面上升速率加快背景下的区域海平面变化速率,利用灰色线性回归组合模型预测上海地区海平面变化趋势。考虑了上海地面沉降发展的新特点,以及长江流域来沙显著减少和河口大型工程建设对上海地区相对海平面变化的影响。在流域来沙量显著减少但来水量变化不大情况下,流域大坝的坝下冲刷使得河槽刷深,河口水位降低,同时长江口深水航道整治工程和促淤围垦工程束狭入海口,使得潮位站水位发生变化,两者的综合效应是目前研究长江口相对海平面变化时必须考虑的重要因素。     

Episodic and Long-Term Sediment Transport Capacity in The Hudson River Estuary

A tidally averaged model of estuarine dynamics is used to estimate sediment transport in the Hudson River estuary over the period 1918 to 2005. In long-term and seasonal means, along-channel gradients in sediment flux depend on the estuarine salinity gradient and along-channel depth profile. Lateral depth variation across the estuary affects the near-bottom baroclinic circulation and consequently the direction of net sediment flux, with generally up-estuary transport in the channel and down-estuary transport on the shoals. Sediment transport capacity in the lower estuary depends largely on river discharge, but is modified by the timing of discharge events with respect to the spring–neap cycle and subtidal fluctuations in sea level. Sediment transport capacity also depends on the duration of high-discharge events relative to the estuarine response time, a factor that varies seasonally with discharge and estuarine length. Sediment fluxes are calculated with the assumption that over long periods, the system approaches morphological equilibrium and sediment accumulation equals sea level rise. The inferred across- and along-channel distributions of sediment erodibility correspond with observations of bed properties. Equilibrium is assumed at long time scales, but at annual to decadal time scales the estuary can develop an excess or deficit of sediment relative to equilibrium. On average, sediment accumulates in the estuary during low- and high-discharge periods and is exported during moderate discharge. During high-discharge periods, maximum export coincides with maximum sediment supply from the watershed, but the nearly cubic discharge dependence of fluvial sediment supply overwhelms the roughly linear increase in estuarine transport capacity. Consequently, sediment accumulates in the estuary during the highest flow conditions. Uncertainty remains in the model, particularly with sediment properties and boundary conditions, but the results clearly indicate variability in the sediment mass balance over long time scales due to discharge events.     

Deltaic and estuarine environments and their Late Quaternary dynamics on the Sunda and Sahul shelves

Deltaic and estuarine environments have been, and continue to be, some of the most rapidly changing environments. Those associated with the Sunda shelf generally receive large volumes of sediment and were characterised by a diverse and productive vegetation before much of it was cleared and converted for agriculture, silviculture or urban development. By contrast estuaries in northern Australia receive far less sediment supply, and record a much less modified pattern of landform change during the Holocene. Three periods of change are discussed: first, the long-term geological development and response of deltaic–estuarine plains to eustatic cycles of sea-level change, particularly postglacial sea-level rise to present; second, Holocene development of deltaic–estuarine environments, dominated by patterns of coastal progradation and distributary migration, under relatively stable sea level; and third, the impact of human modifications. These observations provide a framework within which response of the deltaic–estuarine environments to future, anticipated environmental change can be assessed.     

Sea-level fluctuations and the geometric variability of tide-dominated sandbodies

This paper presents examples of various large tidal sandbodies from the Eocene Roda Sandstone in the southern Pyrenees and the Late Pleistocene and Early Holocene in the East China Sea. An attempt is made to summarize the geometric variability of these large tidal sandbodies in relation to the sediment supply and tidal discharge of the depositional system. Transverse sand bars were developed in low-sinuosity, high-gradient channels with high influxes of coarse sediments and water from fluvial systems. Tidal point bars were formed in meandering low-gradient estuarine channel where tidal influence was stronger and sediment was finer than those of the transverse sand bar. A tidal delta complex was built up at the estuary mouth with an abundant sediment supply and an increased tidal discharge. Tidal sand ridges were formed when relict fluvial or deltaic sands were eroded and reworked by strong tidal currents during subsequent sea-level rise.

Since the sediment supply and the tidal discharge of the depositional system were closely related to the eustatic sea-level change and basin subsidence, i.e. the relative sea-level change, special attention will be given to the relationship between geometric variability of tidal sandbodies and the sequence stratigraphic framework in which various sandbodies occurred. Three orders of eustatic sea-level fluctuations can be recognized. The third-order eustatic sea-level cycle, together with basin subsidence, controlled the development of systems tracts and the occurrence of different tidal sandbodies, such as estuary and tidal flat facies during the late stage of a LSW systems tract (type 1 sequence) or a SM systems tract (type 2 sequence); tidal point bar facies, tidal delta facies or tidal sand-ridge facies during a TR systems tract; estuary facies during an early HS systems tract; and fluvial sand bar facies in a late HS systems tract and the early stage of a SM or LSW systems tract. There are also the fourth-order and fifth-order eustatic fluctuations, which are superimposed on the third-order eustatic changes and have important control on the build-up, abandonment and preservation of composite and single tidal sandbodies, respectively.

Since the deposition of tidal sandbodies is very sensitive to eustatic sea-level changes, recognition of various tidal sandbodies is important in sequence stratigraphy analyses of sedimentary basins and in the facies prediction of clastic sediments in basin modelling.     

Late Holocene Marsh Expansion in Southern San Francisco Bay, California

Currently, the largest tidal wetlands restoration project on the US Pacific Coast is being planned and implemented in southern San Francisco Bay; however, knowledge of baseline conditions of salt marsh extent in the region prior to European settlement is limited. Here, analysis of 24 sediment cores collected from ten intact southern San Francisco Bay tidal marshes were used to reconstruct spatio-temporal patterns of marsh expansion to provide historic context for current restoration efforts. A process-based marsh elevation simulation model was used to identify interactions between sediment supply, sea-level rise, and marsh formation rates. A distinct age gradient was found: expansion of marshes in the central portion of southern San Francisco Bay dated to 500 to 1500 calendar years before present, while expansion of marshes in southernmost San Francisco Bay dated to 200 to 700 calendar years before present. Thus, much of the tidal marsh area mapped by US Coast Survey during the 1853–1857 period were in fact not primeval tidal marshes that had persisted for millennia but were recently formed landscapes. Marsh expansion increased during the Little Ice Age, when freshwater inflow and sediment influx were higher than during the previous millennium, and also during settlement, when land use changes, such as introduction of livestock, increased watershed erosion, and sediment delivery.     

Nutrient inputs to the Choptank River estuary: Implications for watershed management

Degraded water quality due to water column availability of nitrogen and phosphorus to algal species has been identified as the primary cause of the decline of submersed aquatic vegetation in Chesapeake Bay and its subestuaries. Determining the relative impacts of various nutrient delivery pathways on estuarine water quality is critical for developing effective strategies for reducing anthropogenic nutrient inputs to estuarine waters. This study investigated temporal and spatial patterns of nutrient inputs along an 80-km transect in the Choptank River, a coastal plain tributary and subestuary of Chesapeake Bay, from 1986 through 1991. The study period encompassed a wide range in freshwater discharge conditions that resulted in major changes in estuarine water quality. Watershed nitrogen loads to the Choptank River estuary are dominated by diffuse-source inputs, and are highly correlated to freshwater discharge volume. in years of below-average freshwater discharge, reduced nitrogen availability results in improved water quality throughout most of the Choptank River. Diffuse-source inputs are highly enriched in nitrogen relative to phosphorus, but point-source inputs of phosphorus from sewage treatment plants in the upper estuary reduce this imbalance, particularly during summer periods of low freshwater discharge. Diffuse-source nitrogen inputs result primarily from the discharge of groundwater contaminated by nitrate. Contamination is attributable to agricultural practices in the drainage basin where agricultural land use predominates. Groundwater discharge provides base flow to perennial streams in the upper regions of the watershed and seeps directly into tidal waters. Diffuse-source phosphorus inputs are highly episodic, occurring primarily via overland flow during storm events. Major reductions in diffuse-source nitrogen inputs under current landuse conditions will require modification of agricultural practices in the drainage basin to reduce entry rates of nitrate into shallow groundwater. Rates of subsurface nitrate delivery to tidal waters are generally lower from poorly-drained versus well-drained regions of the watershed, suggesting greater potential reductions of diffuse-source nitrogen loads per unit effort in the well-drained region of the watershed. Reductions in diffuse-source phosphorus loads will require long-term management of phosphorus levels in upper soil horizons. *** DIRECT SUPPORT *** A01BY074 00021     

Discharge diversion in the Patía River delta,the Colombian Pacific: Geomorphic and ecological consequences for mangrove ecosystems

In the Patía River delta, the best-developed delta on the western margin of South America, a major water diversion started in 1972. The diversion of the Patía flow to the Sanquianga River, the latter a small stream draining internal lakes from the Pacific lowlands, shifted the active delta plain from the south to the north and changed the northern estuarine system into an active delta plain. The Sanquianga Mangrove National Park, a mangrove reserve measuring 800 km², lies in this former estuary, where major hydrologic and sedimentation changes are occurring. Overall, major environmental consequences of this discharge diversion in terms of geomorphic changes along distributary channels and ecological impacts on mangrove ecosystems are evidenced by: (1) distributary channel accretion by operating processes such as sedimentation, overbank flow, increasing width of levees, sedimentation in crevasses, interdistributary channel fill, and colonization of pioneer mangrove; (2) freshening conditions in the Sanquianga distributary channel, a hydrologic change that has shifted the upper estuarine region (salinity <1%) downstream; (3) downstream advance of freshwater vegetation, which is invading channel banks in the lower and mixing estuarine zones; (4) die-off of approximately 5200 ha of mangrove near the delta apex at Bocas de Satinga, where the highest sediment accumulation rates occur; and (5) recurrent periods of mangrove defoliation due to a worm plague. Further analyses indicate strong mangrove erosion along transgressive barrier islands on the former delta plain. Here tectonic-induced subsidence, relative sea-level rise, and sediment starving conditions due to the channel diversion, are the main causes of the observed retreating conditions of mangrove communities. Our data also indicate that the Patía River has the highest sediment load (27 × 10⁶ t yr⁻¹) and basin-wide sediment yield (1500 t km⁻² yr⁻¹) on the west coast of South America. Erosion rates from the Patía catchment have been more pronounced during the decades of 1970–1980 and 1990–2000, as a result of land degradation and deforestation. The high sediment and freshwater inputs into the mangrove ecosystem create additional stress (both at ongoing background levels and, occasionally, at dramatic levels), which may periodically push local environmental parameters beyond the thresholds for mangrove survival. The future environmental state of the Sanquianga Mangrove National Reserve deserves more scientific and governmental attention.     

A Paleoecological History of the Late Precolonial and Postcolonial Mesohaline Chesapeake Bay Food Web

Geochemical (total nitrogen, total organic carbon, total phosphorus, total sulfur, and carbon and nitrogen stable isotopes) and selected biotic (diatom, foraminifera, polychaete) indicators preserved in two estuarine sediment cores from the mesohaline Chesapeake Bay provide a history of alterations in the food web associated with land-use change. One core from the mouth of the Chester River (CR) (collected in 2000) represents a 1,000-year record. The second core (collected in 1999), from the Chesapeake Bay’s main stem opposite the Choptank River (MD), represents a 500-year record. As European settlers converted a primarily forested landscape to agriculture, sedimentation rates increased, water clarity decreased, salinity decreased in some areas, and the estuarine food web changed into a predominantly planktonic system. Representatives of the benthic macrofaunal community (foraminifera and the polychaetes Nereis spp.) were affected by local changes before there were widespread landscape alterations. Nitrogen stable isotope records indicated that land-use changes affected nitrogen cycling beginning in the early 1700s. Extreme changes were evident in the mid-nineteenth century following widespread deforestation and since the mid-twentieth century reflecting heightened eutrophication as development increased in the Chesapeake Bay watershed. Results also demonstrate how paleoecological records vary due to the degree of terrestrial inputs of freshwater runoff and nutrients at core locations within the Chesapeake Bay.     

Tectonic Subsidence of California Estuaries Increases Forecasts of Relative Sea-Level Rise

Even along the generally uplifting coast of the Pacific US, local geologic structures can cause subsidence. In this study, we quantify Holocene-averaged subsidence rates in four estuaries (Carpinteria Slough, Goleta Slough, Campus Lagoon, and Morro Bay) along the southern and central California coast by comparing radiocarbon-dated estuarine material to a regional sea-level curve. Holocene-averaged rates of vertical motion range from subsidence of 1.4?±?2.4, 1.2±0.4, and 0.4?±?0.3 mm/year in Morro Bay, Carpinteria Slough, and Goleta Slough, respectively, to possible uplift in Campus Lagoon (?0.1?±?0.9 mm/year). The calculated rates of subsidence are of the same magnitude as rates of relative sea-level rise experienced over the late Holocene and effectively double the ongoing rates of relative sea-level rise experienced over the last five decades on other parts of the coast. The difference in rates of vertical motion among these four estuaries is attributed to their geological settings. Estuaries developed in subsiding geological structures such as synclines and fault-bounded basins are subsiding at much higher rates than those developed within flooded river valleys incised into marine terraces. Restoration projects accounting for future sea-level rise must consider the geologic setting of the estuaries and, if applicable, include subsidence in future sea-level rise scenarios, even along the tectonically uplifting US Pacific Coast.     

A method to assess the freshwater inflow requirements of estuaries and application to the Mtata estuary, South Africa

The National Water Act (Act 36 of 1998) in South Africa recognizes basic human water requirements as well as the need to sustain the country's freshwater and estuarine ecosystems in a healthy condition for present as well as future generations. In this Act, provision is made for a water reserve to be estimated prior to the authorization of water use (e.g., for agriculture, large volume residential and industrial uses) through licensing. This reserve is the water required to satisfy basic human needs (i.e., 25 1 person^?1 d^?1) and to protect aquatic ecosystems to ensure present and future sustainable use of the resource. This led the Departments of Water Affairs and Forestry and estuarine scientists throughout South Africa to develop a method to determine the freshwater inflow requirements of estuaries. The method includes documenting the geographical boundaries of the estuary and determining estuarine health by comparing the present state of the estuary with a predicted reference condition with the use of an Estuarine Health Index. The importance of the estuary as an ecosystem is taken from a national rating system and together with the present health is used to set an Ecological Reserve Category for the estuary. This category represents the level of protections afforded to an estuary. Freshwater is then reserved to maintain the estuary in that Ecological Reserve Category. The Reserve, the quantity and quality of freshwater required for the estuary, is determined using an approach where realistic future river runoff scenarios are assessed, together with data for present state and reference conditions, to evaluate the extent to which abiotic and biotic conditions within an estuary are likely to vary with changes in river inflow. Results from these evaluations are used to select an acceptable river flow scenario that represents the highest reduction in freshwater inflow that will still protect the aquatic ecosystem of the estuary and keep it in the desired Ecological Reserve Category. The application of the Reserve methodology to the Mtata estuary is described.     

The role of sediment denitrification in reducing groundwater-derived nitrate inputs to Nauset Marsh estuary, Cape Cod, Massachusetts

The Nauset Marsh estuary is the most extensive (9.45 km²) and least disturbed salt marsh/estuarine system within the Cape Cod National Seashore, even though much of the 19 km² watershed area of the estuary is developed for residential or commercial purposes. Because all of the Nauset watershed is serviced by on-site individual sewage disposal systems, there is concern over the potential impact of groundwater-derived nutrients passing from these systems to the shallow receiving waters of the estuary. The purpose of this study was to determine whether denitrification (the bacterial conversion of nitrate to gaseous nitrogen) in estuarine sediments could effectively remove the nitrate from contaminated groundwater before it passed from the watershed to the estuary. Rates of denitrification were measured both in situ and in sediment cores, in areas of active groundwater discharge, in relatively pristine locations, and in areas situated down-gradient of moderate to heavily developed regions of the watershed. Denitrification rates for 47 sediment cores taken over an annual cycle at 5 stations ranged from non-detectable to 47 μmol N₂ m⁻² h⁻. Mean denitrification rates were positively correlated with sediment organic content, and varied seasonally due to changes in sediment organic content and to the effect of water temperatures on sediment oxygen penetration depths. There was no correlation between observed denitrification rates and corresponding nitrate concentrations in groundwater. A comparison of in situ denitrification rates (supported by groundwater nitrate) with denitrification rates observed in sediment cores (supported by remineralized nitrate) showed that groundwater-driven denitrification rates were small, and not in excess of denitrification rates supported by remineralized nitrate. Most of the denitrification in Nauset sediments was apparently fueled by remineralized nitrate through coupled nitrification/denitrification. Denitrification did not contribute significantly to the direct loss of nitrate from incoming groundwater at Nauset Marsh estuary. Groundwater flow was rapid, and much of it occurred in freshwater springs and seeps through very coarse, sandy, well-oxygenated sediments of limited organic content. There was little opportunity for denitrification to occur during groundwater passage through these sediments. These results have important management implications because they suggest that the majority of nitrogen from contaminated groundwater crosses the sediment/water interface and arrives at Nauset Estuary, where it is available to primary producers. Preliminary budget calculations suggest that while denitrification was not an effective mechanism for the direct removal of nitrate in contaminated groundwater flowing to Nauset Marsh estuary, it may contribute to significant nitrogen losses from the estuary itself.     

Holocene Environmental Change and River-Mouth Sedimentation in the Baie des Anges, French Riviera

River mouths on the steep, high-relief coast of the French Riviera exhibit thick sequences of Holocene marine, estuarine, deltaic, and river channel-floodplain sediments that overlie basal fluvial Pleistocene gravel. Gravel is uncommon in most of the early to middle Holocene aggradational-progradational marine, estuarine, deltaic sediments, despite an ample supply from rock units in the steep adjoining uplands. River-mouth gravel is common only in late Holocene river channels and in barrier beaches perched on finer-grained nearshore sediments. Neither downslope grain-size fining on alluvial fans nor sediment stacking patterns during sea-level (base-level) rise readily account for the lack of early to middle Holocene gravel in the river-mouth sediment wedges. Holocene sea-level rise led to the storage of fine-grained sediments in shallow marine, estuarine, and deltaic environments in the present coastal zone. We infer that humid temperate conditions, a dense forest cover, landscape stabilization, and a regular quiescent river flow regime associated with the Atlantic climatic optimum limited gravel supply in the adjoining catchments and gravel entrainment downstream during the early Holocene. Sea-level stabilization in the middle and late Holocene coincided with a marked change in bioclimatic conditions toward the present Mediterranean-type regime, which is characterized by a less dense forest cover, soil erosion, and episodic catastrophic floods. The late Holocene was thus a time of downstream bedload channel aggradation, fine-grained floodplain and paludal sedimentation, and seaward flushing of clasts leading to the formation and consolidation of the gravel barrier beaches that bound the rivermouths and embayments.     

A Modeling Study of the Impacts of Mississippi River Diversion and Sea-Level Rise on Water Quality of a Deltaic Estuary

Freshwater and sediment management in estuaries affects water quality, particularly in deltaic estuaries. Furthermore, climate change-induced sea-level rise (SLR) and land subsidence also affect estuarine water quality by changing salinity, circulation, stratification, sedimentation, erosion, residence time, and other physical and ecological processes. However, little is known about how the magnitudes and spatial and temporal patterns in estuarine water quality variables will change in response to freshwater and sediment management in the context of future SLR. In this study, we applied the Delft3D model that couples hydrodynamics and water quality processes to examine the spatial and temporal variations of salinity, total suspended solids, and chlorophyll-α concentration in response to small (142 m³ s^?1) and large (7080 m³ s^?1) Mississippi River (MR) diversions under low (0.38 m) and high (1.44 m) relative SLR (RSLR = eustatic SLR + subsidence) scenarios in the Breton Sound Estuary, Louisiana, USA. The hydrodynamics and water quality model were calibrated and validated via field observations at multiple stations across the estuary. Model results indicate that the large MR diversion would significantly affect the magnitude and spatial and temporal patterns of the studied water quality variables across the entire estuary, whereas the small diversion tends to influence water quality only in small areas near the diversion. RSLR would also play a significant role on the spatial heterogeneity in estuary water quality by acting as an opposite force to river diversions; however, RSLR plays a greater role than the small-scale diversion on the magnitude and spatial pattern of the water quality parameters in this deltaic estuary.     

Eutrophication and carbon sources in Chesapeake Bay over the last 2700 yr: human impacts in context

To compare natural variability and trends in a developed estuary with human-influenced patterns, stable isotope ratios (δ¹³C and δ¹⁵N) were measured in sediments from five piston cores collected in Chesapeake Bay. Mixing of terrestrial and algal carbon sources primarily controls patterns of δ¹³C_org profiles, so this proxy shows changes in estuary productivity and in delivery of terrestrial carbon to the bay. Analyses of δ¹⁵N show periods when oxygen depletion allowed intense denitrification and nutrient recycling to develop in the seasonally stratified water column, in addition to recycling taking place in surficial sediments. These conditions produced ¹⁵N-enriched (heavy) nitrogen in algal biomass, and ultimately in sediment. A pronounced increasing trend in δ¹⁵N of +4‰ started in about A.D. 1750 to 1800 at all core sites, indicating greater eutrophication in the bay and summer oxygen depletion since that time. The timing of the change correlates with the advent of widespread land clearing and tillage in the watershed, and associated increases in erosion and sedimentation. Isotope data show that the region has experienced up to 13 wet-dry cycles in the last 2700 yr. Relative sea-level rise and basin infilling have produced a net freshening trend overprinted with cyclic climatic variability. Isotope data also constrain the relative position of the spring productivity maximum in Chesapeake Bay and distinguish local anomalies from sustained changes impacting large regions of the bay. This approach to reconstructing environmental history should be generally applicable to studies of other estuaries and coastal embayments impacted by watershed development.     

Nutrient biogeochemistry in an upwelling-influenced estuary of the Pacific northwest (Tillamook Bay,Oregon, USA)

Tillamook Bay, Oregon, is a drowned river estuary that receives freshwater input from 5 rivers and exchanges ocean water through a single channel. Similar to other western United States estuaries, the bay exhibits a strong seasonal change in river discharge in which there is a pronounced winter maximum and summer minimum in precipitation and runoff. The behavior of major inorganic nutrients (phosphorus, nitrogen, and silica) within the watershed is examined over seasonal cycles and under a range of river discharge conditions for October 1997–December 1999. Monthly and seasonal sampling stations include transects extending from the mouth of each river to the mouth of the estuary as well as 6–10 sites upstream along each of the 5 major rivers. Few studies have examined nutrient cycling in Pacific Northwest estuaries. This study evaluates the distributions of inorganic nutrients to understand the net processes occurring within this estuary. Based upon this approach, we hypothesize that nutrient behavior in the Tillamook Bay estuary can be explained by two dominant factors: freshwater flushing time and biological uptake and regeneration. Superimposed on these two processes is seasonal variability in nutrient concentrations of coastal waters via upwelling. Freshwater flushing time determines the amount of time for the uptake of nutrients by phytoplankton, for exchange with suspended particles, and for interaction with the sediments. Seasonal coastal upwelling controls the timing and extent of oceanic delivery of nutrients to the estuary. We suggest that benthic regeneration of nutrients is also an important process within the estuary occurring seasonally according to the flushing characteristics of the estuary. Silicic acid, nitrate, and NH₄ ⁺ supply to the bay appears to be dominated by riverine input. PO₄ ⁻³ supply is dominated by river input during periods of high river flow (winter months) with oceanic input via upwelling and tidal exchange important during other times (spring, summer, and fall months). Departures from conservative mixing indicate that internal estuarine sources of dissolved inorganic phosphorus and nitrogen are also significant over an annual cycle.     

Influence of an Observed Decadal Decline in Wind Speed on Turbidity in the San Francisco Estuary

Turbidity is an important habitat component in estuaries for many fishes and affects a range of other ecological functions. Decadal timescale declines in turbidity have been observed in the San Francisco Estuary (Estuary), with the declines generally attributed to a reduction in sediment supply to the Estuary and changes to the erodible sediment pool in the Estuary. However, we analyzed hourly wind data from 1995 through 2015 and found statistically significant declines of 13 to 48% in wind speed around the Estuary. This study applied a 3-D hydrodynamic, wave, and sediment transport model to evaluate the effects of the observed decrease in wind speed on turbidity in the Estuary. The reduction in wind speed over the past 20 years was predicted to result in a decrease in turbidity of 14 to 55% in Suisun Bay from October through January. These results highlight that the observed declines in both wind speed and sediment supply over the past 20 years have resulted in reduced turbidity in the San Francisco Estuary from October through January. This decline in turbidity in Suisun Bay potentially has negative effects on habitat for fish like the endangered Delta Smelt which are more commonly caught in relatively turbid water.     

压实作用下广西典型红树林区沉积速率及海平面上升对红树林迁移效应的制衡

气候变化造成的海平面上升是迫使红树林向陆迁移的主要驱动力, 而其自身通过捕沙促淤不同程度的减缓了海平面上升速率的影响。基于广西典型红树林区8根短柱的210Pb测年和含水率分析, 以考虑/未考虑沉积物压实作用为研究情景, 通过对比研究红树林区潮滩地表高程抬升速率和相对海平面上升速率的大小关系, 揭示当前海平面上升对广西红树林向陆/向海迁移的驱动机制。研究发现:未考虑压实作用下的沉积速率约是考虑压实作用下沉积速率的1.00~1.34倍(平均1.12倍), 压实作用明显;压实沉积速率介于0.16~0.78 cm/a, 其底层压实沉积速率与潮滩地表高程抬升速率相等。压实作用下, 英罗湾和丹兜海红树林区的地表高程抬升速率小于相对海平面上升速率;与未考虑压实作用得到的结论相悖。由于广西红树林海岸大都建有防波堤, 限制了红树林向陆的迁移;表明英罗湾和丹兜海的红树林正面临海平面上升的威胁。压实作用校正与否对地表高程抬升速率与相对海平面上升速率相当的区域尤为重要。     

Impact of Channel Deepening on Tidal and Gravitational Circulation in a Highly Engineered Estuarine Basin

Deepening of estuarine channels is a common practice to ensure navigation. Here, we investigate whether such deepening impacts physical processes such as the strength of the estuarine exchange flow, the horizontal salinity gradient, and tidal dynamics. We analyze recent and historical hydrodynamic observations in Newark Bay, New Jersey, to assess the effect of channel deepening on tides, circulation, and salinity. The Bay’s navigational channel has undergone significant deepening, from 3 to 10 m in the nineteenth century to ~16 m today. Observations presented here include sea-level data from the nineteenth, twentieth, and twenty-first century, and moored Doppler current data and bottom salinity measurements made over the past 20 years. Results show a doubling of the estuarine exchange flow, a slight increase in salinity and in the horizontal salinity gradient, a decrease in tidal current amplitude, and a spatially variable change in the tidal range. The doubling of the exchange flow is consistent with the Hansen and Rattray scaling provided that the horizontal salinity gradient is unable to fully adjust landward because the dredging is limited to a short reach of the estuary. However, uncertainty in channel depth leaves open the possibility that the exchange flow is also augmented by an increase in the horizontal salinity gradient and/or a reduction in vertical mixing. Nevertheless, results demonstrate that a relatively small (15%) increase in depth appears to have doubled the exchange flow. We believe that this result is relevant to other systems where dredging is limited to a short reach of an estuary.     

Long-term estuarine variability and associated biological response

Corpus Christi Bay, one of seven major Texas estuaries, is characterized by low freshwater inflow, small tidal flushing, low annual rainfall, and high evaporation rates. Minimal exchange of water makes this estuary sensitive to episodic environmental variation caused by sudden surges of freshwater from flooding rains or hurricanes. It is suggested that this episodic variability stimulates estuarine production. For the last 11 years, detailed data have been collected on benthic community structure, primary and secondary productivity, and sediment nutrient regeneration which are combined with other information, such as fishery yields, into a reconstructed long-term data set. During this same period significant environmental changes in the estuary have been documented. In 1979 the lowest salinity recorded over the 11-year record was related to a short-term, high intensity rainfall. The benthos responded with abundance and biomass levels far greater than any other year during the study interval. Correlated with increased benthic production were large increases in shrimp yields. During more subtle changes with respect to freshwater input in 1981, significant alterations in primary productivity were quantified. Primary, secondary, and tertiary carbon production estimates derived from the reconstructed long-term data base indicated the benthos as a major link between primary producers and other consumers. Carbon flow from primary producers, however, appeared inadequate to support benthic production. Nutrient recycling was judged to provide more than 90% of nitrogen needed to support phytoplankton production and was considered a major factor influencing ecosystem function. The matching of biological responses to significant environmental changes in this estuary provided insight into ecosystem function and stressed the importance of short-term variability. Although recycling was identified as a major source of nutrients supporting primary production, it was concluded that episodic environmental change from freshwater input provided a much needed stimulus to productivity. These episodic changes replaced materials lost through recycling and sustained productivity over the long term.