Tidal Transport Research Articles

Tidal movements of East Pacific green turtle Chelonia mydas at a foraging area in Baja California Sur, México

We tracked East Pacific green turtles Chelonia mydas using GPS-VHF telemetry in Estero Banderitas, a tidally-influenced foraging area in Bahia Magdalena, Baja California Sur, Mexico. Tidal currents were measured with a bottom-mounted Acoustic Doppler Profiler (ADP) and the data used to predict tidal current speed and direction at the location and time during which turtles were tracked. Twenty-nine turtles were tracked in the summers of 2000 to 2003. Vagility (mean ± SD; 18.6 ± 11.4 km d -1 ) and speed (0.83 ± 0.47 km h -1 ) of turtles was the greatest so far reported for green turtles at foraging areas. Turtles displayed highly linear movements, and move- ment patterns were circatidal. Vector correlation was used to compare turtle speed and direction with tidal speed and direction. Correlation coefficients were significant for 11 out of 13 tracks, indicating significant linear interdependence between turtles and tides. Speed and direction contributed equally to the correlation. Results indicated a new paradigm for green turtles in feeding areas, where turtles are transported on continual tides that allow them to exploit a patchy and seasonal distribution of algae, their main diet component. This tidal transport is markedly different than Selective Tidal Stream Transport, in which animals use either the ebb or flood tide for transport. Tidal currents may be an accurate indicator of turtle movement in tidal areas, and this transport system has implications for foraging ecology, energetics, and growth.

Sediment Stability and Dispersion at the Black Point Offshore Disposal Site, Saint John Harbour, New Brunswick, Canada

The stability and dispersion of dredged material at an offshore disposal site was studied using integrated multibeam–geophysical surveys, sampling, in situ monitoring, and modelling techniques. The disposed material at the disposal site undergoes significant reworking by strong tidal currents and superimposed waves: 84% of the dumped material, mainly fine-grained sediments, was transported away from the disposal site and only 16%, mainly coarser material, remained near the disposal site centre to form an irregularly shaped spoil mound with ∼1 km radius and approximately 14 m height at the disposal centre. A large slump, 1.4 by 1.4 km in size, developed south of the spoil mound because of the failure of the dumped material. Tidal currents up to 60 cm s−1 were measured and can cause sediment mobilization over more than 50% of a tidal cycle. The total-load transport rate calculated using measured currents can reach 0.16 kg m−1 s−1. Superimposition of 1-year storm waves could enhance this by one to two orders of magnitude. Hydrodynamic and sediment transport modeling indicated that the maximum total transport rate for uniform fine sand at the disposal site can reach 0.024 kg m−1 s−1 under tide-only conditions, in the eastward direction during flooding and the westward direction during ebbing. The net transport averaged over a tidal cycle is relatively small (0.00027 to 0.0027 kg m−1 s−1) and to the southeast. The significant enhancement of sediment transport by storm waves clearly indicates that wave effects should be included in assessing the mobility of dredged material at ocean disposal sites. The majority of the disposed sediment was effectively dispersed by the wave-enhanced tidal transport; therefore the disposal site is deemed suitable for continued disposal of dredged material. Future disposals should be spread to a broader area to limit disposal mound build up.

Estuarine Channel Stability Assessment through Tidal Asymmetry Parameters

Abstract In an estuarine channel the tidal asymmetry, sediment transport, and morphology are interconnected. To maintain a stable channel, we need to take tidal asymmetry into account. A relationship connecting tidal asymmetry and sediment transport rate has been used to evaluate the relative merits of channel modification through numerical simulations. Two criteria have been identified for ensuring the channel stability: (a) maintaining ebb dominance and (b) minimizing the spatial gradient in the net sediment transport function 〈q〉. Using these criteria for a test case, we have evaluated Hooghly estuary (east coast, India), where heavy sedimentation is experienced and a permanent solution is needed. The approach provides better insight into the sedimentation pattern in a semidiurnal tidal process in a shallow estuary with braided channels. Among the factors affecting tidal asymmetry, the relative phase difference (β) between M2 and M4 current constituents has a major influence and is thus responsible for...

Transport of nutrients and organic matter in a mangrove swamp

Field observations of the tidal transport of nutrients and organic matter in a mangrove swamp on Ishigaki Island, Okinawa, Japan revealed that groundwater flow from the swamp plays a significant role in increasing concentration of dissolved inorganic phosphorous and organic matter in a major channel. In contrast, dissolved inorganic nitrogen is transported from the mangrove swamps via surface water flow in the small channels of the swamp.

Simulation of cumulative effects of nearshore restoration projects on estuarine hydrodynamics

Many nearshore restoration projects are currently underway at coastal locations where human influence and development have disrupted natural habitat and coastal ecological systems. The objectives of these projects in general are to restore the lost estuarine functions to the tidal marshland. Often these projects are conducted with little understanding of the potential effects of other nearby projects within the ecosystem, and similarly, it is easy to neglect the effect of the local project on the larger estuarine scale. In this paper, a modeling study is presented to evaluate the hydrodynamic responses of multiple restoration projects and their cumulative effect in the Snohomish River estuary in Washington, USA. The concept of absolute mean tidal transport is introduced and used to measure the cumulative effect of the proposed restoration projects on the estuarine hydrodynamics. The results show that the hydrodynamic responses due to multiple restoration projects are additive in the estuary, and the effect is nonlinear. The hydrodynamic response under restoration conditions depends on the size of the restoration area and the geometric configuration of the existing river channels. Within a complex braided estuary such as the Snohomish, the influence of a specific restoration project is not only experienced locally, but also found to significantly affect tidal transport in all distributary branches within the system.

Seasonal and vertical variations in emergence behaviors of Neomysis americana

Emergence patterns were observed in the macrotidal Damariscotta River estuary in Maine in fall 2005 and summer 2006. High temporal and vertical resolution was achieved with two multifrequency echo sounders sampling from 265 kHz to 3 MHz. Time‐series waterfall plots and spectral analysis revealed that emergence patterns observed at 265‐420 kHz (frequencies that detect animals larger than mesozooplankton) followed combinations of diel and tidal rhythms that differed with both depth in the water column and season. The dominant emergent species at the study site, Neomysis americana, showed nocturnal emergence in summer. Toward the end of its emergence season in fall, however, its dominant rhythm shifted to semidiurnal (12.4‐h period). The timing of major emergence in the fall coincided with low slack tides near the surface but with peak flood speed near the bottom, providing plausible mechanisms of retention and selective tidal stream transport within the estuary. Our results confirm earlier suggestions from net sampling that emergence in this species is best conceived as a broadening of the vertical spread of the population distribution rather than a migration of the bulk of the population far off the bottom: Most individuals spent most of their time near the bottom, even during emergence. Prior acoustic work that did not resolve the lowermost water column reported emergent mysids to overwhelm the holoplankton in biovolume, but it, too, seriously underestimated mysid abundance because it failed to resolve this high concentration of animals in the lowermost water column. In the mesozooplankton size category, emergence reached surface waters only during low slack tides and systematically avoided times of the fastest currents. This study highlights the importance of high temporal and spatial sampling resolution in detecting and understanding components of emergence and points away from simple models of diel vertical migration toward state‐dependent, individual‐based models of habitat utilization.

PREDICTING SURVIVAL OF GRASS SHRIMP (PALAEMONETES PUGIO) EXPOSED TO NAPHTHALENE, FLUORENE, AND DIBENZOTHIOPHENE

The composition and persistence of dissolved polycyclic aromatic hydrocarbons (PAHs) released to the water column during oil spills are altered by weathering, tidal transport, and addition of dispersants. Conventional toxicity effect metrics, such as the median lethal concentration (LC50), are inaccurate predictors of mortality from all toxicant exposure duration/concentration combinations likely to occur during spills. In contrast, survival models can predict the proportions of animals dying as a consequence of exposures differing in duration and intensity. Extending previous work with ethylnaphthalene, dimethylnaphthalene, and phenanthrene, survival time models were developed that include exposure duration and concentration to predict time to death for grass shrimp (Palaemonetes pugio). Two additional PAHs (naphthalene and fluorene) and a heterocyclic aromatic hydrocarbon (dibenzothiophene) were evaluated for the present study. Preliminary explorations of these models confirmed that quantitative structure- activity regression models were possible for predicting survival model parameters from compound characteristics. Conventional 48-h LC50s also were calculated for the compounds and combined with published LC50s to predict relative PAH toxicity to P. pugio based on octanol-water partitioning.

Relationships between individual behaviour and morphometry under different experimental conditions of temperature and feeding in glass eels (Anguilla anguilla)

After Anguilla anguilla larvae reach the European coast, metamorphosing glass eels exhibit an estuarine migration phase and can potentially colonize the continental area. Associated behaviours to upstream movement in estuary and river basin differ strongly: passive tidal transport in estuary, active swimming beyond the upstream tidal limit. Moreover, the migratory behaviour may shift towards a density-dependent dispersal beyond this limit. A positive relationship has previously been established between glass eels’ body condition and migratory behaviour in estuary and also higher in the river basin. An experiment was settled to test for the density-dependent versus migratory behaviour under controlled conditions. The relationships between some behaviours (swimming, grouping, feeding, and aggressiveness) and body condition was investigated at the individual level. Two controlled factors were crossed, leading to four combinations of high and low levels of food and temperature. The high level of food led to a lesser loss in body condition. Swimming activity was positively related to initial body condition and loss in body condition, but these two variables were not related to aggressiveness. We conclude that the density-dependent dispersal hypothesis was not reinforced by these present results.

A Model Study of the Estuarine Turbidity Maximum along the Main Channel of the Upper Chesapeake Bay

A three-dimensional, intratidal sediment transport model is developed for the estuarine turbidity maximum (ETM) in the upper Chesapeake Bay. The model considers three particle size classes, including the fine class mostly in suspension in the water column, the medium class alternately suspended and deposited by tidal currents, and the coarse size suspended only during the times of relatively high energy events. Based on the results of a box model, depth-limited erosion with continuous deposition is employed for the medium and coarse classes by varying the critical shear stress for erosion as a function of eroded mass. For the fine class, mutually exclusive erosion and deposition is employed with a small constant value for the critical shear stresses for erosion and deposition to assure quick erosion of recently deposited fine particles but without allowing further erosion of consolidated bed sediments. The model is run to simulate the annual condition in 1996, and the model generally gives a reasonable reproduction of the observed characteristics of the ETM relative to the salt limit and tidal phase. The model results for 1996 are analyzed to study the characteristics of the ETM along the main channel of the upper bay in intertidal and intratidal time scales. Under a low flow condition, local erosion/deposition and bottom horizontal flux convergence are the main processes responsible for the formation of the ETM, with the settling flux confining the ETM to the bottom water. Under a high flow condition, a distinctive ETM is formed by strong convergence of the downstream flux of sediments eroded from the upstream of the null zone and the upstream flux of sediments settled at the downstream of the null zone. Intratidal variation of the ETM is mainly controlled by erosion and the tidal transport of eroded sediments for a low flow condition. Under the direct influence of a high flow event, the ETM is mainly formed by erosion during ebbing tidal current strengthened by large freshwater discharge and by convergence of ebbing freshwater discharge and flooding tidal current. During the rebounding stage of a high flow event, intratidal variations are mainly controlled by tidal asymmetry caused by the interaction between tidal currents, gravitational circulation, and stratification.

Numerical experiments on the long-term morphodynamics of the Colorado River Delta

The interaction among tidal currents, sediment transport, and long-term changes of the sea bottom in the Colorado River Delta have been investigated applying a two dimensional nonlinear hydrodynamic-numerical finite differences model. The system was forced by the dominant M 2 tidal component at the open boundary. We carried out calculations to study the morphodynamics of the actual bathymetry caused by the bedload sediment transport. To investigate the origin of actual morphological features, we performed experiments using a smoothed bathymetry, in which the islands Montague, Gore, and Pelicano were eliminated. Under the imposed tidal hydrodynamics, the results indicate that the bedload transport contributes significantly in the genesis of sandbanks and in the formation and maintaining of the Montague and Gore Islands.

Validation of a water quality model for the Ria de Aveiro lagoon, Portugal

The main purpose of this work is to validate a water quality model for the Ria de Aveiro, in order to better use it as a predictive tool in the study of the main water quality processes in the lagoon. Ria de Aveiro is a very shallow lagoon, dominated by tidal currents, presenting a very complex network structure, composed by several sub-systems and dominated by tidal asymmetries, resulting in areas of ebb and flood dominance and weak gravitational circulation. Tidal currents are the main forcing in the dissolved oxygen distribution ( DO). The DO concentrations inside the main areas of the lagoon remain quite high, around 8 mg O 2/L, during great part of the year. The model sensitivity analysis shows that the ocean remains the main source of oxygen as well as the main factor controlling the DO distribution throughout the main lagoon areas, due to the exchanges between the ocean and the lagoon. This exchanges results from the good connection between them, which assure a tidal transport of oxygen and a constant renewal of the dissolved oxygen within the lagoon. As sea waters flows into the main lagoon channels, it increases, progressively, the oxygen concentration of the lagoon inner areas, to levels very comparable to the ocean level. This feature is responsible for the relatively high values of the DO concentrations found at the lagoon main areas, (varying within the range of 8 to 9 mg O 2/L). The occurrence of some episodic hypoxia situation at the very far and shallow areas, namely during summer and neap tides, when the oxygen consumption related processes are dominant, is nevertheless possible. Indeed, at the intertidal areas and at areas situated at the far end of the lagoon, under the influence of weak tidal currents, the dissolved oxygen concentrations may become very low, if the phytoplankton concentration and metabolism are low. Furthermore, as the depth is reduced at those areas, if the air temperature increase, the DO lost to the atmosphere is enhanced. The sensitivity analysis shows, as well, that the primary production and the phytoplankton respiration are the main factors affecting the oxygen budget in the water column, where changes in the primary production, resulting from changes of the phytoplankton abundance or metabolism, may influence the oxygen concentrations in the water column. This study shows that the model is able to realistically simulate the water quality of the lagoon constituting, therefore, a tool for the study and management of the Ria de Aveiro ecosystem.

Simulating storm waves in the Irish Sea

A third-generation wave model (PRO-WAM) has been applied to the North Atlantic and to a nested Irish Sea domain. The accuracy of the model was assessed by hindcasting storms that occurred during 1–15 January 2005 when the wind speed reached 25 m/s and wave heights of 12 m were recorded off the west coast of Ireland. The agreement between the model and the data from five offshore wave buoys was excellent at four of the five locations. An analysis of the relationship between wave height and period at the fifth site suggests that the buoy had malfunctioned. Two rapid-calculation methods for estimating a wave field were evaluated. The first method involved computing the time-invariant waves for a range of fixed wind speeds and directions then creating a look-up table for each grid cell. The second method used formulae derived from the JONSWAP spectrum. Of the two methods, the look-up table technique provided the better results. The PRO-WAM model is run operationally each day and the results stored in a data archive. The output is suitable for specifying the offshore boundary conditions for near-shore, high-resolution wave models, which could be interfaced to tidal and sediment transport models.

The quick and the dead: larval mortality due to turbulent tidal transport

Marine populations are typically connected over greater spatial scales than their terrestrial counterparts due to many species having a highly dispersive, planktonic larval phase. However, high levels of larval mortality in the plankton may reduce connectivity between populations. The effect of turbulence on larval mortality was investigated under natural conditions in a field experiment. Larvae were collected before and after being subjected to turbulent tidal flow from a marine reserve, with differential mortality being observed between taxa. Thin-shelled veligers of gastropods and bivalves showed significantly increased mortality, while barnacle nauplii and cyprids, bryozoan cyphonaute larvae and polychaete trochophores showed no effect of turbulent tidal transport. Where appropriate, marine reserve design should account for the reduced connectivity between populations associated with turbulent tidal transport between reserve and adjacent areas.

Current and thermohaline characteristics of the Arabian Sea during January 1998

Based on a ship survey during January 1998, the characteristics of the flow, the thermohaline properties and the volume transport of the Arabian Sea are discussed. A strong westward flow exists between 10.5°N and 11°N, part of which turns to the south as the Somali current near the coast at about 10°N and the rest turns north. At the passage between the African continent and the Socotra Island, the northern branch separates into two flows: the left one enters the passage and the right one flows eastward along the southern slope of the island. Off the island the flow separates once more, most of it meandering northeast and a small fraction flowing southeast. Volume transport calculation suggests that the tidal transport is one or two orders of magnitude smaller than the total transport in this region and it becomes more important near the coast. The average velocity of the flow in the upper layer (0–150 m) is about 20 cm s−1, with a maximum of 53 cm s−1 appearing east of the Socotra Island, and the subsurface layer (200–800 m) has an average velocity of 8.6 cm s−1; the velocity becomes smaller at greater depths. The depth of the seasonal thermocline is about 100 m, above which there is a layer with well mixed temperature and dissolved oxygen. High-salinity and oxygen-rich water appears near the surface of the northern Arabian Sea; a salinity maximum and oxygen minimum at 100 m depth along 8°N testifies the subduction of surface water from the northern Arabian Sea. Waters from the Red Sea and the Persian Gulf also influence the salinity of the area.

Particle deposition in the pulmonary region of the human lung: Multiple breath aerosol transport and deposition

Comprehensive knowledge of aerosol deposition in the lung during multiple breathing cycles is essential to understanding the long term adverse effects of environmental particulate pollution as well as various therapeutic strategies for aerosolized drug delivery. In this work, a simple semi-empirical model for whole lung aerosol bolus dispersion and deposition developed in an accompanying study [Park, S.S. and Wexler, A.S., 2007. Particle deposition in the pulmonary region of the human lung: A semi-empirical model of single breath transport and deposition. Journal of Aerosol Science 38(2), 228–245] was used to estimate regional particulate dosimetry during multiple breaths. To further validate the transport and deposition model of Park and Wexler [2007. Particle deposition in the pulmonary region of the human lung: A semi-empirical model of single breath transport and deposition. Journal of Aerosol Science 38(2), 228–245], the washin and washout experiments of Davies and coworkers were simulated; predictions compared well to observations. Typical models of pulmonary particle deposition simulated transport to these distal airways by a flow-through approximation where particle-laden air is assumed to flow into the airways and out the alveoli, but resting tidal volumes do not transport particles to the distal pulmonary airways in a single breath. By simulating tidal transport and deposition over a series of breath, we find that the concentration of retained particles as a function of lung depth increases with each tidal cycle and these particles penetrate deeper with succeeding breaths. The retained particle concentration increases more slowly with each breath, so that after the 8th breath, the concentration distribution within the lung attains a steady state. Comparison with observed data and previous model predictions is made in terms of total and generational deposition fractions at steady state. After accounting for the retained fractions, the total predicted deposition fraction was similar to the experimental data while other previous model predictions underestimated it. Predicted deposition fraction per generation showed similar patterns to other model simulations yet higher deposition fractions in the more proximal pulmonary regions. This is a result of the enhanced alveolar deposition in the first half of the acinar generations due to alveoli expansion and contraction.

Rhythmic swimming activity in Anguilla anguilla glass eels: Synchronisation to water current reversal under laboratory conditions

Synchronisation of swimming activity to water current reversal every 6.2 h was tested in the European glass eel ( Anguilla anguilla L.). When presented with a change in water current direction, glass eels exhibited rhythmic patterns of activity with a period close to the tidal one. Glass eels began to swim with the current and then alternated between positive and negative rheotaxis after each change in the water current direction. Results are discussed in relation to the flood tidal transport theory. Following synchronisation to current reversal, glass eels subjected to constant conditions displayed a weak rhythmic activity suggesting that locomotor behaviour might, in the wild, synchronise to several environmental cues related to the tide. Results obtained with different densities also suggest that social cues might improve the synchronisation.

Predicting survival of grass shrimp (Palaemonetes pugio) during ethylnaphthalene, dimethylnaphthalene, and phenanthrene exposures differing in concentration and duration

Both exposure duration and concentration determine the lethal consequences of polycyclic aromatic hydrocarbons (PAHs) released during oil spills. Many factors, such as weathering, tidal transport, and addition of surfactants, can change the composition of individual dissolved compounds and the duration over which an individual is exposed. Conventional toxicity testing methods produce effect metrics, such as the median lethal concentration (LC50), that are not applicable to predicting mortality at all toxicant exposure durations that are likely to occur during a spill. In the present study, survival time models were developed that explicitly include toxicant exposure duration and concentration to predict time-to-death for grass shrimp (Palaemonetes pugio) exposed to three PAHs (1-ethylnaphthalene, 2,6-dimethylnaphthalene, and phenanthrene) commonly found in the water-soluble fraction derived from oil. Conventional 48-h LC50s also were calculated for the compounds (ethylnaphthalene, 295 microg/L; dimethylnaphthalene, 500 microg/L; and phenanthrene, 360 microg/L). In contrast to LC50s, survival models and associated response surfaces can be used to predict the proportions of shrimp that will die at various times throughout the exposure period.

Cross-shelf transport of pink shrimp larvae: interactions of tidal currents, larval vertical migrations and internal tides

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 345:167-184 (2007) - DOI: https://doi.org/10.3354/meps06916 Cross-shelf transport of pink shrimp larvae: interactions of tidal currents, larval vertical migrations and internal tides Maria M. Criales1,*, Joan A. Browder2, Christopher N. K. Mooers1, Michael B. Robblee3, Hernando Cardenas1, Thomas L. Jackson2 1Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA 2NOAA Fisheries, Southeast Fisheries Science Center, 75 Virginia Beach Drive, Miami, Florida 33149, USA 3US Geological Survey, Center for Water and Restoration Studies, 3110 SW 9th Avenue, Ft Lauderdale, Florida 33315, USA *Email: mcriales@rsmas.miami.edu ABSTRACT: Transport and behavior of pink shrimp Farfantepenaeus duorarum larvae were investigated on the southwestern Florida (SWF) shelf of the Gulf of Mexico between the Dry Tortugas spawning grounds and Florida Bay nursery grounds. Stratified plankton samples and hydrographic data were collected at 2 h intervals at 3 stations located on a cross-shelf transect. At the Marquesas station, midway between Dry Tortugas and Florida Bay, internal tides were recognized by anomalously cool water, a shallow thermocline with strong density gradients, strong current shear, and a high concentration of pink shrimp larvae at the shallow thermocline. Low Richardson numbers occurred at the pycnocline depth, indicating vertical shear instability and possible turbulent transport from the lower to the upper layer where myses and postlarvae were concentrated. Analysis of vertically stratified plankton suggested that larvae perform vertical migrations and the specific behavior changes ontogenetically; protozoeae were found deeper than myses, and myses deeper than postlarvae. Relative concentrations of protozoea in the upper, middle and bottom layers were consistent with a diel vertical migration, whereas that of postlarvae and myses were consistent with the semidiurnal tides in phase with the flood tide. Postlarvae, the shallowest dwellers that migrate with a semidiurnal periodicity, experienced the largest net onshore flux and larval concentrations were highly correlated with the cross-shelf current. These results provide the first evidence of an onshore tidal transport (a type of selective tidal stream transport, STST), in decapod larvae migrating in continental shelf waters offshore, ca. 100 km from the coast and at a depth of 20 m, while approaching the coastal nursery grounds. Longer time series would be necessary to establish whether internal tides play any role in the larval onshore transport of this species and determine if the STST is the dominant onshore transport mechanism. KEY WORDS: Selective tidal transport · Pink shrimp larval behavior · Internal tides · Farfantepenaeus duorarum Full text in pdf format PreviousNextCite this article as: Criales MM, Browder JA, Mooers CNK, Robblee MB, Cardenas H, Jackson TL (2007) Cross-shelf transport of pink shrimp larvae: interactions of tidal currents, larval vertical migrations and internal tides. Mar Ecol Prog Ser 345:167-184. https://doi.org/10.3354/meps06916 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 345. Online publication date: September 13, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Tidal exchange, bivalve grazing, and patterns of primary production in Willapa Bay, Washington, USA

Willapa Bay, Washington, USA, is a shallow, coastal-plain, upwelling-influenced estuary where Pacific oysters Crassostrea gigas are intensively cultivated. CTD transect data show that in the long-term average over the May to September growing season, Willapa Bay is a sink for oceanic phytoplankton, not a net exporter: as the tidal circulation stirs ocean water into the estuary, chlorophyll concentration declines by 30 to 60% relative to a hypothetical dilution of the ocean end-member. A 3D circulation model (General Estuarine Transport Model, GETM) was modified to include a phyto- plankton-like tracer subject to variable intertidal benthic grazing rates. The grazing rate that best reproduces the along-estuary phytoplankton biomass profile agrees, within confidence limits, with an estimate of cultivated-oyster filtration from literature values. Oysters and other intertidal benthic grazers may thus be the primary cause of the net loss of phytoplankton within the estuary in summer. These grazers appear to be within a small factor of their carrying capacity: as bay-total filtration capacity is increased in the model, the chlorophyll intrusion shortens and food intake per individual grazer declines. Nevertheless, only 8 to 15% of the net tidal supply of oceanic phytoplankton is consumed within the estuary. Even in this well-flushed system, the small-scale structure of tidal transport—rather than total oceanic supply—controls overall food availability for the benthos.

Tidal transport and dispersal of marine toxic microalgae in a shallow, temperate coastal lagoon

A numerical model was used to predict toxic microalgal transport and dispersion in Ria de Aveiro in Portugal. A previously developed Lagrangean particle tracking model coupled to a calibrated two-dimensional hydrodynamic model of Ria de Aveiro was used. Microalgae were regarded as passive particles and the methodology used allowed the determination of their trajectories, as induced by the tidal currents predicted by the hydrodynamic model. The model assumes Ria de Aveiro as vertically homogeneous and does not take into account the vertical distribution patterns of microalgae. Simulations were carried out during extreme spring and neap tides, with microalgal released at the mouth of the lagoon at the local flood. The maximum and minimum areas affected during the occurrence of toxic microalgal blooms were estimated to evaluate the suitability of the distribution of the sampling stations included in the local monitoring program. It was found that the tidal currents greatly determine the microalgal horizontal distribution and dispersal in the lagoon. The results confirmed that the locations of water and bivalve shellfish sampling stations, postulated by INIAP/IPIMAR, in the context of the local harmful algal bloom (HAB) program, were appropriate, although some possible refinements were identified.