Semidiurnal Fluctuations Research Articles

Sea Surface Salinity Forecasting with a Comparison Studying Case of GMM-VSG and FB-Prophet Model

With the evolution of artificial intelligence, the utilization of machine learning algorithms for predicting hydrological data has gained popularity in scientific research, especially for the development and operational patterns of marine-related objects in coastal regions. Salinity analysis plays a crucial role in evaluating the resilience and health of marine ecosystems. Traditional numerical models, although accurate, require significant computational resources. Therefore, this study assesses the effectiveness of GMM-VSG proposed by Shanghai University and FB-Prophet created by Meta (Facebook) as rapid alternatives for simulating the nonlinear relationships between salinity and various parameters, like tide-induced free surface elevation, river flows, and wind speed. The algorithms were tested using an eight-year dataset collected at the MAREL buoy at the entrance to bay of Brest. Results indicate that, despite the simplicity of the input data, both algorithms successfully reproduced seasonal and semi-diurnal fluctuations in salinity. This underscores their potential as complementary tools for the ecological monitoring in estuarine environments.

Assessing poroelastic properties of a geothermal reservoir by tidal signal analysis

The gravitational attractions of the solar-system bodies cause tidal phenomena, in response to which the pore pressure of geothermal fields often exhibits diurnal and semidiurnal fluctuations. Such periodic fluctuations can be quantified in relation to the Earth tides and atmospheric loading, which are functions of reservoir poroelastic properties. In this study, the relationship between the poroelastic properties and the tidal effects was formulated and examined graphically. The dependencies of individual properties on the Earth tides and atmospheric loading are different, and clarifying such relationships helps us systematically understand the assessment of poroelastic properties using tidal signals. The analysis method was applied to the actual pressure data acquired at an onshore geothermal field in Japan. The resultant estimates of reservoir properties were reasonable and affirmed the practical applicability of the methodology to geothermal reservoirs. The sensitivity of the property estimations to the prescribed values was examined and constraining the upper bound of Poisson's ratio was found to be important for assessing poroelastic properties. The tidal signal analysis only requires the reservoir pressure transients and barometric pressure without any further well interventions, and thus can provide a safe and cost-effective in situ reservoir property assessment.

A hydrographic method to identify the groundwater net recharge, barometric effect and evapotranspiration from a complicated semidiurnal water table fluctuation

AbstractA hydrographic method was proposed to separate out the hourly scaled groundwater level changes caused by net recharge, barometric effects and evapotranspiration from a semidiurnal water table fluctuation. A characteristic midnight time, with a turning point of the barometric pressure change and high relative air humidity, which meant that neither the barometric effect nor groundwater evapotranspiration occurred, was proposed for determining the net recharge rate rnet. Then, the barometric efficiency fbar was estimated using the other time period without evapotranspiration, and the evapotranspiration rate rETG was finally obtained using the remainder of the water level changes. A case example illustrated that estimation of fbar using the proposed method was more accurate than that using the traditional error analysis method, which may result in a significant underestimation under the condition of the present water level changes. Additionally, the abnormal semidiurnal fluctuations, more specifically, two step‐down fluctuations, which may be a common pattern when the groundwater level is controlled by net recharge, barometric effects and evapotranspiration, can be well understood using the three components separated out. The results also showed that nighttime groundwater evapotranspiration, accounting for an average of 23% of that during the daytime, cannot be ignored.

Effects of nonlinear terms and topography in a storm surge model along the southeastern coast of China: a case study of Typhoon Chan-hom

Based on Finite Volume Coastal Ocean Model (FVCOM), this study constructed a numerical model covering the Bohai Sea, Yellow Sea, and East China Sea. National Centers for Environmental Prediction’s Climate Forecast System Reanalysis (NCEP-CFSR) data were used to drive the model to simulate a large storm surge generated by Typhoon Chan-hom. The model was validated by multiple in situ observations of water levels taken at tidal gauge stations. The effects of the nonlinear terms and topography on the modeling of storm surges were then studied. First, the tide–surge interaction during the storm surge process was analyzed. The results show that the tide–surge interaction can suppress the storm surge at the climax of the astronomical tide and benefit the growth of the storm surge at the ebb of the astronomical tide. The tidal constituents M2, S2 and K1 were added to analyze the influences of the amplitudes and periods of tides on the nonlinear reaction. The results indicate that the nonlinear effect will be enhanced by an increased tidal height; additionally, the nonlinear interaction of semidiurnal tides is more significant than that of diurnal tides. The semidiurnal fluctuation that appears near the peak-value time is also attributed to the tide–surge interaction. Moreover, the tide–surge interaction can be influenced by the water depth and position of the area of interest. According to the numerical results, topography has a certain impact on the storm surge: The peak value of the storm surge will decrease with increasing slope. The existence of the Ryukyu Islands reduces the area influenced by the storm surge on the southeast coast of China but expands the high-value storm surge area.

Time-lapse analysis of pressure transients due to ocean tides for estimating CO2 saturation changes

This study proposed and examined a practical technique for analysing CO2 storage responses using offshore pressure transients affected by the ocean tide. The gravitational attractions of the solar-system bodies cause ocean tides, and the pore pressure exhibits diurnal and semidiurnal fluctuations in response to such tidal phenomena. The pressure-fluctuation amplitude is related to the loading efficiency, which is a function of reservoir elastic properties and fluid saturations. Therefore, the loading efficiency can be used to estimate the in situ pore compressibility and the CO2 saturation. Applying the tidal-signal analysis in a time-lapse manner, one may see temporal changes in CO2 saturation and consequently describe the dynamic behavior of sequestered CO2. In the analysis at the offshore CO2 storage site in Tomakomai, Japan, a temporal decrease in CO2 saturation was detected during the shut-in period, which is caused primarily by CO2 migration away from the well. The proposed methodology essentially requires only continuous pressure data, which are routinely available during CO2 storage operations, and thus, can be a cost-effective and labour-saving monitoring technique.

Bathymodiolus growth dynamics in relation to environmental fluctuations in vent habitats

The deep-sea mussel Bathymodiolus thermophilus is a dominant species in the East Pacific Rise (EPR) hydrothermal vent fields. On the EPR volcanically unstable area, this late colonizer reaches high biomass within 4–5 years on new habitats created by lava flows. The environmental conditions and growth rates characterizing the reestablishment of B. thermophilus populations are however largely unknown, leaving unconstrained the role of this foundation species in the ecosystem dynamics. A typical example from the vent field at 9°50’N that was affected by the last massive eruption was the Bio-9 hydrothermal vent site. Here, six years later, a large mussel population had reestablished. The von Bertalanffy growth model estimates the oldest B. thermophilus specimens to be 1.3 year-old in March 2012, consistent with the observation of scarce juveniles among tubeworms in 2010. Younger cohorts were also observed in 2012 but the low number of individuals, relatively to older cohorts, suggests limited survival or growth of new recruits at this site, that could reflect unsuitable habitat conditions. To further explore this asumption, we investigated the relationships between mussel growth dynamics and habitat properties. The approach combined sclerochronology analyses of daily shell growth with continuous habitat monitoring for two mussel assemblages; one from the Bio-9 new settlement and a second from the V-vent site unreached by the lava flow. At both vent sites, semi-diurnal fluctuations of abiotic conditions were recorded using sensors deployed in the mussel bed over 5 to 10 days. These data depict steep transitions from well oxygenated to oxygen-depleted conditions and from alkaline to acidic pH, combined with intermittent sulfide exposure. These semi-diurnal fluctuations exhibited marked changes in amplitude over time, exposing mussels to distinct regimes of abiotic constraints. The V-vent samples allowed growth patterns to be examined at the scale of individual life and compared to long-term records of habitat temperature and oceanographic mooring data in the years following the eruption. Both shell growth and habitat temperature at V-vent varied over the spring-neap tidal cycle and over longer periods of c.a. 60 days. The correlation of growth rate with temperature and, for some individuals, with current velocities supports the idea that tidal forcing impacts growth. Its influence on habitat conditions includes the spring-neap cycle, which is not reflected in current velocities but influences the venting rate. Additionally, it is expected that mesoscale eddies periodically passing across the ridge imprint shell growth through the influence of bottom current on the decimeter-thick mixing interface where mussels thrive. We conclude that diurnal-semidiurnal tidal fluctuations exert major abiotic constraints on B. thermophilus mussels and that low-frequency fluctuations act as significant determinants on growth. Finally, we postulate that the modulation of tidal fluctuations by large-scale hydrodynamic forcing ultimately constrains the capacity of this mussel species to form high biomass aggregations. This study indeed shows that the absence of these strong hydrodynamic drivers would limit the alternance of oxic and sulfidic conditions and significantly affect the growth rate of this species over time.

Can variable pH and low oxygen moderate ocean acidification outcomes for mussel larvae?

Natural variation and changing climate in coastal oceans subject meroplanktonic organisms to broad ranges of pH and oxygen ([O2 ]) levels. In controlled-laboratory experiments we explored the interactive effects of pH, [O2 ], and semidiurnal pH fluctuations on the survivorship, development, and size of early life stages of two mytilid mussels, Mytilus californianus and M. galloprovincialis. Survivorship of larvae was unaffected by low pH, low [O2 ], or semidiurnal fluctuations for both mytilid species. Low pH (<7.6) resulted in delayed transition from the trochophore to veliger stage, but this effect of low pH was absent when incorporating semidiurnal fluctuations in both species. Also at low pH, larval shells were smaller and had greater variance; this effect was absent when semidiurnal fluctuations of 0.3units were incorporated at low pH for M. galloprovincialis but not for M. californianus. Low [O2 ] in combination with low pH had no effect on larval development and size, indicating that early life stages of mytilid mussels are largely tolerant to a broad range of [O2 ] reflective of their environment (80-260μmolkg(-1) ). The role of pH variability should be recognized as an important feature in coastal oceans that has the capacity to modulate the effects of ocean acidification on biological responses.

Impact of tides in a baroclinic circulation model of the Adriatic Sea

AbstractThe impact of tides in the circulation of the Adriatic Sea is investigated by means of a nested baroclinic numerical ocean model. Tides are introduced using a modified Flather boundary condition at the open edge of the domain. The results show that tidal amplitudes and phases are reproduced correctly by the baroclinic model and tidal harmonic constants errors are comparable with those resulting from the most consolidated barotropic models. Numerical experiments were conducted to estimate and assess the impact of (i) the modified Flather lateral boundary condition; (ii) tides on temperature, salinity, and stratification structures in the basin; and (iii) tides on mixing and circulation in general. Tides induce a different momentum advective component in the basin, which in turn produces a different distribution of water masses in the basin. Tides impact on mixing and stratification in the River Po region (northwestern Adriatic) and induce semidiurnal fluctuations of salinity and temperature, in all four seasons for the former and summer alone for the latter. A clear presence of internal tides was evidenced in the northern Adriatic Sea basin, corroborating previous findings.

Behavioral rhythms of hydrocarbon seep fauna in relation to internal tides

Behavioral rhythms in marine organisms are generally studied in coastal areas in rela- tion to day-night and tidal cycles. In contrast, the presence of these rhythms in demersal deep-sea fauna remains poorly characterized at present due to technological constraints on direct observation and sampling repeatability. In this study, our aim was to determine the presence of tidally synchro- nized behavioral rhythms of the demersal fauna at a hydrocarbon seep in Sagami Bay (1100 m depth; central Japan) using automated video-image analysis. Time series of visual counts were obtained for eelpouts (zoarcid fishes), red crabs Paralomis multispina, and snails Buccinum soyomaruae. Water pressure data were used as a marker of internal tidal action. By fitting 24-h and 12-h Fourier harmon- ics onto 24-h time series segments, a power content value was obtained as a marker of diurnal and semidiurnal fluctuations within the biological data sets. Rhythms were stronger in eelpouts and were weaker in red crabs and snails. Visual counts of eelpouts peaked during spring phases with an infra- dian periodicity of 13 d. These results are discussed, taking into consideration that animal responses may be of a reactive type (i.e. exogenous) or may be controlled by a biological clock through putative hydrodynamic entrainment. Observed differences in the magnitudes of the rhythms were related to the different modes of species locomotion (i.e. swimming in eelpouts versus walking and crawling in red crabs and snails, respectively) in relation to seabed currents.

Small‐scale sea ice deformation in the Beaufort Sea seasonal ice zone

During spring 2003, the deformation of a single lead in the Beaufort Sea was investigated using four Global Positioning System recording stations. The lead was situated in first‐year ice, in the transition zone between multiyear, and seasonal ice packs. We discuss the opening rate and shear of this lead in the context of weather system forcing of the Beaufort Sea sea ice. It is found that in an opening mode, the lead displays semidiurnal fluctuations in divergence. It was also observed that the lead predominately ridged first‐year ice in closing. The volume of ice ridged was the same order of magnitude as ice growth within the study region during the time period studied. Further work is required to fully understand the implications of the subdiurnal forcing of the ice pack on deformation in the shear zone at the edge of the perennial ice pack in the Beaufort Sea. This study provides information about the temporal and spatial scales that must be resolved in these future investigations.

Hydrographic characterization of two tidal creeks with implications for watershed land use, flushing times, and benthic production

Many coastal ecosystems are undergoing anthropogenic stress from large increases in population and urbanization. In many regions changes in freshwater and material inputs to the coastal zone are altering the biogeochemical and biological capacities of ecosystems. Despite increased watershed inputs, large tidal volumes and flushing indicative of macrotidal estuaries can modulate the fate of introduced materials masking some of the symptoms of eutrophication. The Land Use Coastal Ecosystem Study (LU-CES) examined linkages between land use and environmental properties of Malind and Okatee Creeks in South Carolina from 2001 to 2004. The objectives of this particular study were to assess the hydrography of the two macrotidal creek ecosystems, explore differences in dissolved oxygen (DO), and develop a better understanding of the variations in primary and benthic secondary production in southeastern creek ecosystems. Depth, pH, salinity, and DO were reduced and more variable in Malind Creek than in Okatee Creek, although both creeks had strong semidiurnal frequencies in salinity time signatures. While time series analyses of DO saturation in Malind Creek revealed a dominant semidiurnal pattern, Okatee Creek had a distinctly diel DO pattern. The strongly semidiurnal fluctuations in DO and reduced flushing time indicated that biological processes were not fast enough to influence DO in Malind Creek. The Okatee Creek system had a much greater storage volume, a wider marsh, and a dominant 25-h DO frequency. These attributes contributed to an estimated 8-10 times more phytoplankton-based carbon in Okatee Creek and twice the annual benthic production. As expected from their proximity to the upland, low surface area, and high organic content, both ecosystems were net heterotrophic. This fundamental understanding of tidal creek hydrography is being used to help define linkages among differential watershed land uses, flushing characteristics, and levels of biological production in coastal ecosystems of the southeastern United States.

Atmospheric forcing of the Oregon coastal ocean during the 2001 upwelling season

Meteorological conditions during an intensive oceanographic observational program in May through August 2001 along the central Oregon coast are described and related to larger‐scale and longer‐term conditions. Southward wind stresses of 0.05−0.1 N m−2 occurred roughly 75% of the time, with a sustained period of dominantly southward stress from mid‐June through July. Wind variations were correlated with variations in the large‐scale Aleutian Low and North Pacific High pressure centers; correlations with the continental Thermal Low were small. Intraseasonal oscillations in alongshore wind stress (periods near 20 days) correlated with the north‐south position of the jet stream. These stress oscillations drove 20 day oscillations in upper ocean temperature, with a lag of roughly 5 days for maximum correlation and amplitudes near 4°C. The sum of sensible and latent air‐sea heat fluxes was generally into the atmosphere through June, then weakly into the ocean thereafter, with fluctuations on synoptic timescales. Semidiurnal fluctuations in surface air temperature were observed at two northern moorings, apparently forced indirectly by nonlinear internal ocean tides. The diurnal cycle of wind stress was similar for both southward and northward wind conditions, with the diurnal alongshore fluctuation southward in the evening and northward in the morning. During southward winds the marine atmospheric boundary layer (MABL) was typically defined clearly by a strong temperature inversion, and a shallow stable internal boundary layer often formed within the MABL over cool upwelled waters, with surface air temperature roughly 1°C lower inshore than offshore. During northward winds, essentially no low‐level temperature stratification was observed.

Particulate trace metal and major element distributions over consecutive tidal cycles in Port Jackson Estuary, Australia

Particulate trace metal (Cu, Cr, Ni, Pb and Zn) and major element (Fe, Mn and Al) concentrations have been determined following intensive sampling over two consecutive spring tidal cycles in the 'turbidity maximum zone' (TMZ) of the Port Jackson estuary, Australia. Salinity, temperature, pH, dissolved oxygen, suspended particulate matter (SPM) and chlorophyll a were also determined. A three-factor analysis of variance was used to test temporal variability in concentrations of particulate trace metals and major elements as a result of tidal oscillation. Estuarine master variables, such as temperature and pH, varied within a narrow range; nevertheless, the tidal signal was clear for surface and bottom waters. In surface water, no variance was detected in SPM concentrations between consecutive tidal cycles or between tidal stages (i.e. flood, ebb and slack water). In bottom water, however, SPM concentrations were significantly higher (P≤0.05) at flood tide than at slack high water and ebb tide. Concentrations of particulate trace metals and major elements in surface water do not display significant variability between tidal cycles or stages. Nevertheless, differences within each tidal stage were significant (P≤0.05) for all elements. In bottom water, only particulate Fe and Al exhibited significant differences (P≤0.05) between tidal cycles, whereas particulate Ni was the only trace element that presented significant differences (P≤0.05) between tidal stages, following the distribution of SPM, with highest concentrations at flood tide. Among the metals studied, significant variation was found at all three temporal scales examined (i.e. from hours to consecutive tidal cycles), although the patterns of variation were different for each metal. The semi-diurnal fluctuation of SPM and particulate trace metal concentrations during spring tides is interpreted as a resuspension–deposition cycle caused by cyclical oscillations of bottom currents. The results are discussed in the context of the implications of tidal cycle influence on the geochemistry and cycling of particulate trace metals in the Port Jackson estuary.

Internal Waves in Monterey Submarine Canyon

Velocity, temperature, and salinity profile surveying in Monterey Submarine Canyon during spring tide reveals an internal wave field almost an order of magnitude more energetic than that in the open ocean. Semidiurnal fluctuations and their harmonics dominate, near-inertial motions are absent. The ratio of horizontal kinetic to available potential energy is less than one in much of the canyon, inconsistent with hydrostatic internal waves. The excess potential energy may be due to isopycnal displacements induced by barotropic tide flow over the sloping bottom. Removal of the expected barotropic contribution raises the energy ratio to 2.04–2.10, in line with the semidiurnal internal wave value of 2.13. Finescale shear and strain are also elevated. Finescale parameterizations for turbulent eddy diffusivities, which have proven successful in the open ocean, underestimate upper-canyon microstructure estimates of 100 × 10−4 m2 s−1 by a factor of 30. Energy fluxes and near-bottom velocities are strongly steered by the sinuous canyon topography. A vertically integrated influx of 5 kW m−1 at the mouth diminishes to ±1 kW m−1 toward the shallow end of the canyon. Both sinks and sources of internal wave energy are indicated by energy-flux convergences and divergences along the canyon axis. Along-axis energy-flux convergences are consistent with microstructure dissipation rates ϵ. The high diapycnal eddy diffusivities may drive strong nutrient fluxes to enhance bioproductivity.

Some features of tidal motion near the Guinean shelf

We have determined the energetic and spatial characteristics of baroclinic and barotropic tidal semi-diurnal fluctuations using the current temperature and velocity data collected by 12 self-contained buoy stations (SCBS) near the shelf of the Republic of Guinea at depths ranging from 50 to 1000 m. Intensive baroclinic motion was observed to prevail near the bottom within the depth range 100–1000 m. The energy of long waves largely propagated from the bottom to the surface, whereas the energy of short waves was transferred in the opposite direction.

Tidal fluctuations as seen in ECMWF data

Diurnal and semi-diurnal fluctuations in the troposphere and lower stratosphere during the 1986 summer and 1986/87 winter, deduced from data of the European Centre for Medium Range Weather Forecasts, are documented. In general, the tidal fluctuations are consistent with the results of previous investigators based on station data. It thus suggests the adequacy of the data generated by the analysis-forecast system for presenting the global structure of tidal fluctuations in the troposphere. The semi-diurnal fluctuations in ECMWF data are consistent with the predictions of tidal theory. The diurnal fluctuations, which are more sensitive to local factors such as the sea-land contrast and topography, exhibit a complicated geographical dependence. It appears that the current tidal theory, by neglecting the lower boundary effects and zonally varying factors, is incomplete for explaining the diurnal tidal motions in the troposphere.

Suspended sediment fluctuations in the Tagus estuary on semi-diurnal and fortnightly time scales

Nine multi-ship synoptic surveys of the distribution of suspended sediment, each survey including the distribution at both low and high tide, were carried out over a 12-month period in the mesotidal Tagus estuary in Portugal. Additional measurements of the semi-diurnal fluctuations of suspended sediment concentration and current strength were made at fixed stations during a neap and a spring tide. During the study period, the river discharge of water and suspended sediment remained below the mean annual discharge and did not show a pronounced seasonal fluctuation. A turbidity maximum, defined as an area with suspended sediment concentrations greater than 50 mg l −1, was absent during neap tides (1·3-m amplitude), but appeared and grew in both extent and turbidity as the tidal amplitude increased. The turbidity maximum was fully developed during spring tides (> 3-m amplitude) with concentrations greater than 50 mg l −1 throughout the entire estuary. Maximum concentrations, reaching as much as 1000 mg l −1 during spring tides, were always found in the inner shallow bay region of the estuary. In contrast to the salinity distribution, which fluctuated between partly stratified during neap tides and well mixed during spring tides, the vertical distribution of suspended matter in the turbidity maximum zone was always stratified with the highest concentrations near the bottom. The semi-diurnal fluctuation of the suspended sediment concentration was negligible during neap tides, but attained magnitudes during spring tides that were comparable to the fortnightly fluctuation. The fluctuation in suspended matter concentration is interpreted as a fortnightly erosion-sedimentation cycle, caused by a cyclic variation in the strength of the bottom currents. Superimposed on this fortnightly cycle is a semi-diurnal cycle. The amount of material involved in these cycles is equivalent to one year's input of suspended sediment by the Tagus river during normal discharge conditions.

Longshore Coherence of Currents on the Southern California Shelf During the Summer

Observations of horizontal currents and temperature made along a mid-shelf isobath during the summer on the Southern California shelf are used to determine longshore coherent length scales. The observations are characterized by weak atmospheric forcing and strong density stratification of the water column. For periods longer than a day, coherent longshore length scales of currents are around 25 km, while coherent length scales of temperature were longer than the instrumental array (57.5 km). For periods ranging from a day to an hour, near-surface motions were dominated by diurnal fluctuations which were coherent over lengths of 5 km, whereas near-bottom motions, dominated by semidiurnal fluctuations, showed evidence of a signal propagating up coast at phase speeds of order 1 m s−1. The observed scales of motion in different frequency bands are used to estimate the dispersive properties of the shelf motions. The dispersion coefficient is found to vary with scale, as the current variance associated with larger, lower-frequency motions becomes available to dispersion, and this variation is compared to the behavior of dispersion coefficients deduced by Okubo (1971).

Zooplankton sample variability in a tidal estuary: An interpretative model1

A series of 347 vertical zooplankton hauls was made at a 175‐h anchor station in the upper St. Lawrence estuary, Québec, and current meter profiles were recorded. The 95% confidence interval of a single observation in most of the copepod time series was about 25–400% of the mean. Temporal variability consisted for the most part of semidiurnal fluctuations superimposed on a long term trend. Other periodic components were also evident. Most components were related to the advection, at discrete frequencies, of longitudinal zooplankton gradients of different magnitude and sign. The relationship was generalized by a mathematical model which shows that a distinct component of sample variability can be found at all frequencies of the current tidal constituents, provided that a longitudinal gradient is present. The relative magnitude of the variability components, for a constant gradient slope in both time and space, is identical to that of the amplitude/frequency product of the current tidal constituents. Other components not explained by the model can be related to periodic vertical displacements of the pycnocline caused by the interaction of an internal tide with a sill in the sampling area.

Respiratory and osmoregulatory responses of the hermit crab, Clibanarius vittatus (Bosc), to salinity changes

Intertidal hermit crabs were stepwise acclimated to 10, 20, and 30‰ salinity ( S) and 21 ± 1 °C. Hemolymph osmolality, sodium, chloride, and magnesium were isosmotic (isoionic) to ambient sea water at 30‰ and hyperosmotic (hyperionic) at 20 and 10‰ S, while hemolymph potassium was significantly hyperionic in all acclimation salinities. Total body water did not differ significantly at any acclimation salinity. Oxygen uptake rates were higher in summer-than winter-adapted crabs. No salinity effect on oxygen consumption occurred in winter-adapted individuals. Summer-adapted, 30‰ acclimated crabs had a significantly lower oxygen consumption rate than those acclimated 10 and 20‰ S. Crabs exposed to 30 10 30‰ and 10 30 10‰ semidiurnal (12 h) and diurnal (24.8 h) fluctuating salinity regimes showed variable osmoregulatory and respiratory responses. Hemolymph osmolality followed the osmolality of the fluctuating ambient sea water in all cases, but was regulated hyperosmotically. Hemolymph sodium, chloride, and magnesium concentrations were similar to hemolymph osmolality changes. Sodium levels fluctuated the least. Hemolymph potassium was regulated hyperionically during all fluctuation patters, but corresponded to sea water potassium only under diurnal conditions. The osmoregulatory ability of Clibanarius vittatus (Bosc) resembles that reported for several euryhaline brachyuran species. The time course of normalized oxygen consumption rate changed inversely with salinity under semidiurnal and diurnal 10 30 10‰ S fluctuations. Patterns of 30 10 30‰ S cycles had no effect on oxygen consumption rate time course changes. The average hourly oxygen consumption rates during both semidiurnal fluctuations were significantly lower than respective control rates, but no statistical difference was observed under diurnal conditions.