Shallow Estuary Research Articles

Residual circulation in shallow estuaries: 1. Highly stratified, narrow estuaries

Long‐wave theory and simple turbulence closures have been used to show that three distinct types of circulation (highly stratified, weakly stratified, and partially mixed) arise in narrow, shallow estuaries from the finite amplitude of the tide and the interaction of stratification with vertical mixing. Each type has a different dominant process causing the vertical exchange of salt and fresh water on the flood, and each gives rise to a characteristic residual circulation. The tidal circulation in highly stratified shallow estuaries, described herein, is the result of a finite amplitude internal motion driven by the barotropic tide; shear instabilities at the interface are the major vertical exchange mechanism. The residual circulation is caused primarily by ebb‐flood asymmetry in interface position and thickness. A model based on our theoretical analysis shows that the interface thickness on flood is much less than the depth of flow up to a critical tidal amplitude at which the two‐layer flow is destroyed, and correctly predicts the tidal amplitude of the neap‐spring transition in the Columbia River Estuary. Tidal frequency internal wave motion is not found in weakly stratified and partially mixed estuaries. Because of the reduced tidal shear, shear instabilities are weak or absent, and the residual circulation assumes a very different character, as described in part 2.

Residual circulation in shallow estuaries: 2. Weakly stratified and partially mixed, narrow estuaries

Long‐wave theory is used herein to analyze circulation in weakly stratified and partially mixed estuaries. Unlike the highly stratified systems considered in part 1, the flows considered here have only a minimal tidal‐frequency internal wave component. These estuaries may therefore be modeled as the sum of weakly interacting barotropic and baroclinic modes. The dominant factors driving the residual flow are finite amplitude barotropic effects in weakly stratified estuaries and a combination of barotropic effects and steady horizontal density gradient forcing in partially mixed estuaries. The dominant vertical exchange mechanism in the weakly stratified case is bottom boundary‐induced turbulence, and that in partially mixed estuaries is believed to be random internal wave interactions. A model of the Columbia River Estuary under weakly stratified conditions accurately predicts the observed residual velocity and salinity fields, and the tidal amplitude at which a transition to a highly stratified state takes place. The partially mixed state is unstable in the Columbia River Estuary because the tides and buoyancy input are both too strong.

NH+4 release by non‐growing cultures of the cyanobacterium Mkrocystis firma

The rhythmic changes in NH+4 concentration previously observed in an eutrophic shallow estuary under conditions of growth limitation, NH+4 addition and dominance of cyanobacteria were studied in laboratory cultures of the cyanobacterium Microcystis firma (Breb et Lenorm.) Schmidle, strain Gromov/Len. 398. The rhythm was induced in M. firma by means of carbon limitation and light/dark alternation, and was characterized by a decrease in NH+4 in the growth medium during the first hours of the light phase and an increase in NH+4 in the late light phase lasting until the end of the dark phase. Inhibition of the glutamine synthetase/glutamate synthase pathway [darkness, lack of C skeletons, inhibitors such as L‐methionine‐D,L‐sulfoximine (MSX)] brought about NH+4 release by the cells. From the various processes generating NH+4 intracellularly (active NH+4 uptake. NO−3 reduction, protein and amino acid degradation, photorespiration), NO−3 reduction appears the most important one. This has been concluded from experiments with and without nitrate and/or ammonium in the growth medium and from experiments with 3‐(3.4‐dichlorophenyl)‐1,1‐dimethyl‐urea (DCMU) and MSX. NO−3 was also taken up and reduced in the presence of external ammonium, probably because of the C limitation under otherwise optimal conditions for photosynthesis. It appears, moreover, that glutamine restricts nitrate uptake during the first hours of the light phase but less in subsequent light periods, when carbon limitation might be more severe. The present results may help explain previous observations under natural conditions, provided that nitrate is available in the water samples. Furthermore, this system enables the occurrence of a new, cheaper and more efficient method of photoproduction of ammonium from nitrate.

Modelling tidal energetics of the Columbia River Estuary

The Columbia River Estuary is shallow and has a mixed diurnal and semidiurnal tide, strong riverflow, broad tidal flats and variable channel cross-section. It is weakly non-linear with respect to tidal forcing, as measured by a ratio of tidal amplitude to depth. These features, common to many shallow estuaries, were incorporated into a one-dimensional harmonic transport model. The harmonic method utilizes the fact that nearly all of the energy in the system is in a few fundamental tidal frequencies, the first overtides thereof and the mean flow. This allows the representation of the flow as a series of harmonic components whose spatial variability is determined by the model. The model provides a qualitative explantation for and accurate quantitative predictions of along-channel variations in tidal properties in terms of the momentum balance. Near the mouth of the estuary, the boundary shear stress, the pressure gradient and the acceleration terms are all important in the force balance, and the tide behaves like a damped oscillator. Far upriver the pressure gradient is primarily balanced by friction at the bed and tidal wave propagation can be described as a diffusion process. Changes in the channel width in mid-estuary cause partial reflection of the tidal wave and a maximum in tidal range. This partial reflection decreases the tidal energy flux in the upstream direction. The distribution of the time averaged energy fluxes was also determined from the model. The analysis shows that there are two regions of high energy dissipation. Energy is supplied near the mouth of the estuary by the divergence of the landward flux of tidal potential energy and far upriver by the divergence of the seaward flux of fluvial potential energy. These two regions of high dissipation are separated by a region of low dissipation, the energy flux divergence minimum. This spatial division of the estuary is mirrored in the system biology and geology. It is likely that the energy flux divergence minimum is a common feature of shallow estuaries with strong riverflow.

Hydrography and Circulation of the Chubut River Estuary (Argentina)

The hydrography and circulation of the Chubut River were investigated under exceptionally low river discharge. The frontal zone formed by the entrance of the tide in the estuary may be observed as far as 4.5 km from the mouth, showing that the salt intrusion due to tidal effects reaches further inland than during normal river discharge. Based on the classification of Hansen and Rattray (1966), the estuary corresponds to Type 1 with some vertical stratification observed on the seaward side of the frontal zone. A lateral salinity gradient was found, which was not the result of Coriolis force. The general morphology of the estuary and the consequent secondary circulation due to meanders and interchannel bars may explain the lateral variation. Wind effect is a major component of the circulation and mixing of this shallow estuary.

Adaptive Changes in Hematologic and Plasma Chemical Constituents in Captive Beluga Whales, Delphinapterus leucas

Forty-two beluga whales, Delphinapterus leucas, were captured in the Seal and Churchill River estuaries in western Hudson Bay during July, 1985 and 1987. Blood samples were drawn from each whale, and analyzed for cellular elements, electrolytes, metabolites, enzymes, proteins, and adrenocortical hormones. Most of the whales were released immediately after sampling; six were maintained in holding facilities for 10 k during 1985. Blood samples drawn during the early stages of acclimation to captivity, and at irregular intervals thereafter, revealed the variety of metabolic adjustments that accompanied the transition to captivity. The stress and exertion of capture resulted in increased levels of aldosterone, Cortisol, glucose, iron, potassium, and the enzymes creatine kinase, aspartate aminotransferase, alanine aminotransferase, and γ-glutamyltranspeptidase. Acute changes in leucocytes included lymphopenia, eosinopenia, and mild neutrophilia. Most of these indices normalized within the first week in captivity. Progressive changes were noted in triglycerides and creatinine, reflecting the whales' altered diet and caloric intake. A steady decline in red cell mass was indicative of reduced demands on oxygen carrying capacity, and provided a clue to the significance of low hematocrits reported for whales sampled after several weeks in shallow estuaries.

Effect of temperature on nitrogen-limited growth rate and chemical composition of Ulva curvata (Ulvales: Chlorophyta)

Ulva curvata (Kutz.) de Toni growing in shallow estuaries experiences a highly variable supply of dissolved inorganic N, which can limit growth rates. The effects of N supply variability and annual temperature and light variation on growth rates and chemical composition were assessed in plants grown in outdoor tanks supplied with running seawater and either pulsed or continuous N additions for 8 d. Environmental variables were measured every 12 h, growth rates every 2 d, and plant nitrogenous constituents every 4 d. The experiment was repeated 7 times over the course of a year. The slopes of growth rate-tissue N and growth rate-N supply plots increased with temperature and varied at periods at least as short as 2 d. Temperature explained 44% of variation in growth rate, and temperature, light, and N supply together explained 53% of variation in growth rate. N-limitation occurred only above 20°C, the approximate temperature for the maximum growth rate. Growth rate and tissue N were not affected by N supply frequency. Tissue N, ribulose-bisphosphate carboxylase activity, and chlorophyll contents were correlated with each other and with environmental variables but not with growth rate. U. curvata thus does not maintain balanced growth in the strict sense, but rather stores nutrients supplied as pulses for use when they become growth-limiting. Since the relative importance of any single growth-limiting factor is highly dynamic, assessing the importance of multiple limiting factors requires longterm, high-frequency sampling of principal limiting factors and seaweed growth rates.

Short-term environmental variability and phytoplankton abundance in a shallow tidal estuary

Fixed-point sampling of a shallow tidal estuary was performed hourly for 14 d in summer of 1982 and again in winter of 1983. This sampling regime was of statistically appropriate duration to allow characterization of the variability between periods of 2 to 96 h by spectral analysis of the time-series. The project (PULSE) took place in the Newport River Estuary, located behind the Outer Banks of North Carolina, USA. In all, twentyeight parameters were monitored, encompassing the meteorology, hydrology, water chemistry and phytoplankton-production physiology. Although the annual cycle was monitored, only the winter and summer seasons are compared here, i.e., the lowest water temperatures with the highest water temperatures. The physics, chemistry and biology of the estuary at the hourly scale were highly variable and non-random. The estuary is riverine in winter; growth-limiting nitrogen is supplied as nitrate (NO 3 - ) and ammonium (NH 4 + ) by runoff from the drainage basin. In summer, the estuary is lagoonal; nitrogen is supplied as NH 4 + by biological regeneration. Chlorophyll a biomass varies primarily at the ∼4 d period in winter and at the diel period in summer. Although finely tuned to environmental variability, phytoplankton abundances were at equilibrium insofar as daily chlorophyll production was balanced by losses, i.e., grazing, export and deposition. Most important, high-frequency processes, here periods at the scale of cell-division times, can be very important in phytoplankton ecology.

The influence of diel cycle, tidal direction and trawl alignment on beam trawl catches in an equatorial estuary

Using a 3 m beam trawl, catch variation in the fish and prawn assemblage of the Labu estuary, Papua New Guinea was investigated during July and December 1981 and September 1984. About six times more organisms, three and a half times the biomass and twice as many species were caught per trawl at night than during the day. The greatest number of species, individuals and biomass were caught just after dusk. With the exception ofSecutor ruconius, there was no significant difference in the catches trawled at ebbing and flooding tides. Over twice as many individuals ofS. ruconius were caught during ebbing tides. Alignment of trawl direction in relation to the tidal current did not have a significant effect on the fish catch, but affected the prawn (Metapenaeus demani) catch. It is postulated that most species are not carried to and fro by tidal currents, but maintain their position in the estuary with respect to the substratum. Only prawns appeared to be carried by the currents. This study indicates that the most representative trawl results in terms of species diversity and abundance in shallow estuaries may be obtained at night and, that in estuaries with a small tidal range, tidal direction and velocity have little influence on catch. It also indicates that analyses of demersal nekton assemblages from daytime only estuarine trawl surveys must be approached with caution.

An endogenous annual clock in the toxic marine dinoflagellate Gonyaulax tamarensis.

Blooms of the toxic dinoflagellate Gonyaulax tamarensis (synonyms Protogonyaulax tamarensis and Alexandrium tamarense) cause outbreaks of paralytic shellfish poisoning (PSP) in coastal waters throughout the world. In the Gulf of Maine, episodes occur between April and November, a seasonality due in part to life-cycle alternations between motile, vegetative cells and resting cysts which overwinter in bottom sediments. Newly formed cysts have a mandatory 2-6 month dormancy period during which germination is not possible, but once mature, the resting state will continue if temperatures are unfavourable or oxygen is unavailable. We now report another factor controlling germination of cysts of G. tamarensis from deep coastal waters--an endogenous annual clock that can override an otherwise favourable environment for germination. Similar annual variability in germination has not been observed for cysts of this species from shallow estuaries. These results represent the first conclusive demonstration of an endogenous circannual rhythm in a marine plant. They are evolutionarily and ecologically significant because an endogenous annual clock can lead to the release of motile cells into deep and relatively invariant bottom waters at those times when temperature and light at the surface are suitable for growth. In shallow waters where seasonal variability is large and extends to bottom sediments, a strategy similar to that of the seeds of terrestrial plants would be more appropriate, namely a direct coupling between germination and the external environment.

The determination and prediction of longitudinal dispersion coefficients in a narrow, shallow estuary

Measurements of velocity and salinity for parts of a flood and an ebb tide in the Conwy estuary have been used to evaluate the time-dependent longitudinal dispersion coefficients due to vertical and transverse oscillatory shear effects. The values have been compared with semi-analytical predictive formulae. Simple empirical expressions are suggested for relating the dispersion coefficients to hydraulic parameters for the conditions observed.

Salinity intrusion in the Eastmain River estuary following a major reduction of freshwater input

On July 19, 1980, 80% of the runoff of Eastmain River, a subarctic shallow estuary, was diverted into the La Grande River for hydroelectric development. Consequent to the diversion, the estuary, which had been mostly salt‐free, was subjected to a gradual saltwater intrusion along a 10‐km section in its lower reaches. The adjustment of the salinity regime to a new quasi‐steady state took place over a period of about 40 days. The circulation field reached a new quasi‐equilibrium within 8 days. The 2‐month interval following the river diversion, termed the transition period, was analyzed in terms of both time and space modifications of the circulation and salinity fields. A one‐dimensional finite difference explicit scheme numerical model was applied to the estuary as a complement to observational data. Good agreement was obtained between the model results and the low‐frequency current meter observations of salinity and longitudinal velocity. These results showed that the salinity intrusion was primarily governed by tidal dispersion.

Ecology and Bioproduction in the Microphytobenthos of the Chain of Shallow Inlets (Boddens) South of the Darss‐Zingst Peninsula (Southern Baltic Sea)

AbstractThe significance of epipsammic microphytobenthos bioproduction in a shallow estuary was investigated by the oxygen method. Bioproduction reached a maximum in May or June. A close correlation was found between the gross production rate and algal biomass at station K. The biomass and production rates were distinctly lower at station B than at station K as station B was more exposed. Circadian variations and depth profiles of the gross production rate correlated best with irradiation intensity. Mean gross production from April to October was 1761 O2.m−2 and 75 g C·m−2.

Phosphatase activities in the waters of a shallow estuary, Western Australia

Phosphatase activities in a large, shallow estuary in south western Western Australia have been assessed. Differences were found in enzyme activities in the two sections of the estuary. There were seasonal and diurnal variations as well. Phosphatase activities were most closely correlated with chlorophyll and organic nitrogen concentrations. In diurnal studies, the rates of change of phosphatase activity corresponded with the rates of change in orthophosphate. The role of phosphatase activity in regeneration of esterified phosphorus in the estuary is discussed.

A study of non-linear tidal propagation in shallow inlet/estuarine systems Part II: Theory

The generation of tidal asymmetries is clarified via numerical integration of the one-dimensional equations for channel geometries characteristic of shallow estuaries. Channels without tidal flats develop a time asymmetry characterized by a longer falling than rising tide. This behavior is enhanced by strong friction and large channel cross-sectional area variability over a tidal cycle. Resulting tidal currents have a shorter, intense flood and a longer, weak ebb (flood-dominant). Addition of tidal flats to the channels can produce a longer rising tide and stronger ebb currents (ebb-dominant), if the area of tidal flats is large enough to overcome the effects of time-variable channel geometry. Weaker friction with flats can also produce this asymmetry. Despite the physical complexity of these systems, essential features of estuarine tidal response can be recovered from one-dimensional models. Shallow estuaries are shown to have a system response leading to stable, uniform senses of tidal asymmetry (either flood- or ebb-dominated, due to phase-locking of forced tidal constituents), with down-channel development in magnitude of asymmetry. These concepts are illustrated by modeling idealized representations of tidal channels at Nauset Inlet, MA, and Wachapreague Inlet, VA, which have flood- and ebb-dominance, respectively.

Environmental setting of San Francisco Bay

San Francisco Bay, the largest bay on the California coast, is a broad, shallow, turbid estuary comprising two geographically and hydrologically distinct subestuaries: the northern reach lying between the connection to the Pacific Ocean at the Golden Gate and the confluence of the Sacramento-San Joaquin River system, and the southern reach (herein called South Bay) between the Golden Gate and the southern terminus of the bay. The northern reach is a partially mixed estuary dominated by seasonally varying river inflow, and the South Bay is a tidally oscillating lagoon-type estuary. Freshwater inflows, highest during winter, generate strong estuarine circulation and largely determine water residence times. They also bring large volumes of dissolved and particulate materials to the estuary. Tidal currents, generated by mixed semidiurnal and diurnal tides, mix the water column and, together with river inflow and basin geometry, determine circulation patterns. Winds, which are strongest during summer and during winter storms, exert stress on the bay’s water surface, thereby creating large waves that resuspend sediment from the shallow bay bottom and, together with the tidal currents, contribute markedly to the transport of water masses throughout the shallow estuary.

Calibration studies concerning a one-dimensional numerical tidal model with particular reference to resistance coefficients

Since the hydrodynamics of flow in most shallow estuaries is dominated by the boundary resistance, the selection of appropriate values for the roughness coefficients in any numerical tidal model is important. The paper describes studies involving the intra-tidal variation of resistance coefficients in twelve reaches of the Conwy estuary in North Wales. Resistance coefficients are evaluated for six water level data sets covering the neap to spring tidal variation by cubature based and parameter estimation techniques. Results from these two inverse methods are shown to agree well despite somewhat different schematizations for the estuary. A direct check on the intra-tidal variation of resistance coefficients at one particular reach is made possible by specifically gathered field data described in an earlier paper by Knight. In general the resistance coefficients exhibit three basic trends—a strong stage dependence, a flow directional dependence due to flood or ebb dominated bed forms, and a high variability with location along the estuary, especially where sand banks are oxposed at low water. The optimized resistance coefficients are then used in a one-dimensional numerical tidal model in order to ascertain the effect of particular modes of calibration. Estuary mean errors in water levels are of the order of 2,5 and 16 cm for the three calibrations tested.

Lagrangian Transport Model for Estuaries

A two‐dimensional (in a vertical plane), nonlinear model is used to analyze tide‐induced mean currents in shallow estuaries forced with tides of significant amplitude. The model is first verified by comparing its results with known analytical solutions of Lagrangian currents in a uniform and homogeneous estuary. The Lagrangian velocity fields are derived from the computed mean Eulerian and Stokes velocities in the estuary. A set of model experiments is performed to investigate the influences of tidal forcing, density gradients, and topographic variations on the mean circulation in tidal channels. It is shown that the tide‐induced transport opposes density‐induced transport in tidal rivers. Model sensitivity analysis indicates that for mean estuarine circulation studies, it is essential to include the coupled effects of density and tides active in the domain of interest.

Physics of shallow estuaries and bays

The objective of the symposium is to promote the exchange of information on recent developments in estuarine physics between physical oceanographers and coastal engineers. Emphasis will be on shallow estuaries and bays (well‐mixed and partially mixed) and reef lagoons.The symposium stresses results of field and laboratory measurements and the formulation of the governing equations and boundary conditions as opposed to numerical techniques. Both well‐mixed and partially mixed estuaries are included. The following topics will be covered: (1) large‐scale transport processes (mixing); (2) tide and wind‐induced residual flow; (3) small‐scale turbulence; (4) suspended sediment transport; and (5) exchange between ocean and estuary/bay.

Diurnal variation of nitrogen cycling in coastal, marine sediments

Denitrification was studied in the sediment of a shallow, Danish estuary in the spring and summer of 1982 and 1983. The acetylene blockage technique was used in undisturbed cores to estimate the in-situ activity of the process. A declining rate of denitrification was observed at the onset of the growth season when the allochthonous input of nitrate levelled off; the denitrification decreased from 5.1 mmol N m-2 d-1 in the early spring (April) to 0.3 mmol N m-2 d-1 in the early summer (June). The activity in the uppermost centimeter comprised more than 70% of the total. In this surface zone, a diurnal pulse of oxygen production from photosynthesis of benthic microalgae was found to affect the denitrification; the later had a distinct minimum in the day when the oxygen production was high and inhibitory. The subsequent, nocturnal stimulation of the denitrification was typically a factor of three or more in the investigated periods. We propose that oxygen is an important control factor for the daily, in-situ denitrification in shallow estuaries where light is adequate to reach the sediment. The availability of nitrate seems more important as a control factor for the seasonal variation of the process.