Seasonally Hypersaline Estuaries in Mediterranean-climate Regions

Data collected in three Californian estuaries indicate that hypersaline conditions exist during the dry summers typical of a Mediterranean climate. The generalised seasonal and longitudinal hydrographic structures are described and explained. It is argued that this seasonal hypersalinity is common and that it represents a major class of estuaries. The observed accumulation of salt indicates surprisingly long residence times in small basins which have free exchange with the ocean. This semi-isolation of the inner basin leads to a large build-up or severe depletion of nutrients, pollutants and plankton in these systems. Of concern are the trends to increase pollutant loading in the same systems that are experiencing an increase in residence times owing to freshwater extraction in the watershed.     

Spatial and temporal variability of crustal magnetization of a slowly spreading ridge: Mid-Atlantic Ridge (20°–24° N)

The results of the two- and three-dimensional magnetic inversions performed on data located between 20°–24° N on the Mid-Atlantic Ridge indicate the crustal magnetization has decayed exponentially for the last 10 Ma, and that this decay has been fairly symmetric about the ridge axis. After removal of the mean temporal decay, the residual field is characterized by more positive magnetizations at the second-order discontinuities, regardless of initial magnetization direction. A model that involves the preferential emplacement of serpentinized lithologies near the discontinuities is proposed to explain this correlation. The temporal detrending method also indicates that several ridge-parallel depressions located on the flanks of the ridge axis are regions of more positive magnetizations. These bathymetric depressions may mark the locations of detachment faulting that occurred during amagmatic periods of extension. The general symmetry of the crustal magnetization about the ridge axis does not support the occurrence of continuous detachment faulting proposed to correspond to the inner and outer corners of ridge axis discontinuities.     

Photoadaptation of carboxylating enzymes and photosynthesis during a spring bloom

Properties of the light saturation curve of photosynthesis and ribulose-1,5-bisphosphate carboxylase (RuBPC) activity are shown to change qualitatively in a natural population of marine phytoplankton during a spring bloom. Evidence is presented to show that these changes constitute photoadapative responses to increasing irradiance. As irradiance increased during the bloom, both the level of light-saturated photosynthesis (P_m) and the initial slope of the light saturation curve (α = photosynthetic efficiency) increased whether those parameters were normalized to chlorophyll a concentration (P_m^b, α^b) or to cell numbers (P_m^c, α^c). The magnitudes of these changes were such that I_k (= P_m/α, the photoadaptation parameter) did not change, but I_m, the light intensity at which photosynthesis becomes saturated, increased. RuBPC activity, both chlorophyll a (RuBPC^b) and cell number normalized (RuBPC^c), also increased during the bloom. We suggest that these adaptations were achieved by simultaneously increasing the number of photosynthetic units, proportionately decreasing the photosynthetic unit size, and increasing both the concentrations of the enzymes of the dark reactions and possibly also of photosynthetic electron transport components.We also observed diminished levels of photoinhibition in the high light adapted cells late in the bloom and have suggested that this was a consequence of the same suite of physiological changes.In situ carbon fixation per cell increased during the bloom whereas no change occurred in this parameter when normalized to chlorophyll a concentration. Although these photoadaptive responses thus permitted carbon to be fixed in situ more rapidly per cell, at a constant efficiency with respect to investment of energy in the photosynthetic apparatus, they did not result in a change in growth rate. Based on consideratios of the role of time scale in physiological adaptation, however, it is suggested that the observed alterations in photosynthesis with increasing irradiance might permit a cell to more rapidly fill an energy quota for division, possibly an advantage in a mixing environment in which energy is patchily distributed, both spatially and temporalyy.Phosphoenolpyruvate carboxylase activity when normalized to chlorophyll a (PEPC^b) did not change during the bloom while chlorophyll a normalized dark carbon fixation decreased sharply and was quantitatively small compared to PEPC^b. On this basis and considering that RuBPC^b increased during the bloom, it is suggested that, although PEPC may be involved in dark carbon fixation, its most important quantitative role is probably an indirect one in light dependent photosynthesis.We have also considered the relevance of laboratory results on photoadaptation to interpretations of field studies and have suggested that batch culture studies must be treated with caution but that turbidistat and semi-continuous methods provide reasonable simulations of natural conditions.     

Reconstruction of ancient sea conditions with an example from the Swiss Molasse

Ancient sea conditions can be estimated from the grain size, spacing and steepness of preserved ripple-marks. The element of greatest uncertainty in such reconstructions is the relationship between near-bed orbital diameter of water particles and the ripple spacing. This relationship is simple for vortex ripples of high steepness but is problematical for the low-steepness forms known as post-vortex, rolling-grain or anorbital ripples.

The existence field for wave ripples is between the threshold velocity for sediment movement and the onset of sheet flow, most low-steepness forms occurring close to the bed planation threshold. A range of maximum period of formative waves can be obtained using combinations of orbital diameter and orbital velocity, assuming linear wave theory to be a reasonable approximation.

Probable wave heights, wave lengths and water depths can be investigated using the transformation of wave parameters in shallowing waters and the constraints on wave dimensions provided by the wave-breaking condition. Given reasonable estimates of wave height, crude estimates of wave power allow a comparison of ancient wave-influenced sequences with modern counterparts.

Wave ripple-marks preserved in the Upper Marine Molasse of western Switzerland have been investigated. Results, which are in agreement with regional geology, suggest deposition in a seaway of approximately 100 km width, where moderate period waves (T = 3–6 s) were generated. The depositional facies belts were adjusted to the prevailing waves, tides and fluvial outflows.     

A note on the effects of a thin visco-elastic mud layer on small amplitude water-wave propagation

In this note we investigated the effects of a thin visco-elastic mud layer on wave propagation. Within the framework of linear water-wave theory, analytical solutions are obtained for damping rate, dispersion relation between wave frequency and wave number, and velocity components in the water column and mud layer. The wave attenuation rate reaches a maximum value when the mud layer thickness is about the same as the mud boundary layer thickness. Heavier mud has a weaker effect on the wave damping. However, the wave attenuation rate does not always decrease as the elastic shear modulus increases. In the range of small values for elastic shear modulus, the wave attenuation can be amplified quite significantly. The current solutions are compared with experimental data with different wave conditions and mud properties. In general, good agreements are observed.     

The impact of mesoscale eddies on plankton dynamics in the upper ocean

A coupled QuasiGeostrophic mixed-layer ECOsystem model (QGECO) is used to investigate the impact of the underlying mesoscale eddy field on the spatial and temporal scales of biological production and on overall rates of primary productivity. The model exhibits temporal trends in the biological and physical fields similar to those observed in the North Atlantic; i.e. the mixed layer shallows in spring causing a rapid increase in phytoplankton concentrations and a corresponding decline in nutrient levels. Heterogeneity is produced in the mixed layer through Ekman pumping velocities resulting from the interaction of windstress and surface currents. This variability impacts on biological production in two ways. Firstly, spatial variations in the depth of the mixed layer affect the photosynthetically active radiation (PAR) availability and hence production rates, and secondly, eddy enhanced exchange between the surface water and those at depth bring additional nutrients into the euphotic zone. These processes result in significant spatial and temporal heterogeneity in the ecosystem distributions.Investigation of the spatial heterogeneity of the biological system finds variability to be significantly greater than that of the mixed layer. The relationship between the eddy field and the ecosystem is investigated. The structure and correlation of the biogeochernical fields change with time. The biological fields are found to have a shorter horizontal scale, but whiter spectrum than the underlying eddy field.Overwinter conditions are found to have a profound effect on the variability, size and timing of the following spring bloom event. Variations in the nitrate levels are primarily responsible for the variability in the biological system in the first year. In subsequent years the variation in the overwintering population is found to be dominant.     

Errors in recent ocean tide models: possible origin and cause

Recently, the TOPEX/POSEIDON Science Working Team has recommended the FES95.2.1 and CSR3.0 ocean tide models for reprocessing the TOPEX/POSEIDON Geophysical Data Records. Without doubt, the performance of these models, especially in the deep oceans, is excellent. However, from a comparison of these hydrodynamically consistent models with the purely empirical DW3.2 and DEOS96.1 models, it appears that FES95.2.1 and CSR3.0 are affected by basin boundary related errors which are caused by the basin-wise solution procedure of the FES ocean tide model series. In their turn, the empirical DW3.2 and DEOS96.1 models seem to suffer from significant errors in the Antarctic seas due to the seasonal growth and decay of Antarctic sea ice. Also, bathymetry-induced differences were found between the hydrodynamically consistent models and the empirical models. Concerning these differences, TOPEX/POSEIDON and ERS-1 crossover statistics unfortunately do not provide conclusive results on which models are in error.