Mud bank colonization by opportunistic mangroves: A case study from French Guiana using lidar data

Mud bank colonization by mangroves on the Amazon-influenced coast of French Guiana was studied using light detection and ranging (lidar) data which provide unique information on canopy geometry an sub-canopy topography. The role of topography was assessed through analysis of vegetation characteristics derived from these data. Measurements and analyses of mangrove expansion rates over space and time led to the identification of two distinct colonization processes. The first involves regular step-by-step mangrove expansion to the northwest of the experimental site. The second is qualified as ‘opportunistic’ since it involves a clear relationship between specific ecological characteristics of pioneer Avicennia and mud cracks affecting the mud bank surface and for which probabilities of occurrence were computed from terrain elevations. It is argued from an original analysis of the latter relationship that mud cracks cannot be solely viewed as water stress features that reflect desiccation potentially harmful to plant growth. Indeed, our results tend to demonstrate that they significantly enhance the propensity for mangroves to anchor and take root, thus leading to the colonization of tens of hectares in a few days. The limits and potential of lidar data are discussed with reference to the study of muddy coasts. Finally, the findings of the study are reconsidered within the context of a better understanding of both topography and vegetation characteristics on mangrove-fringed muddy coasts

Glucocorticoids in T cell apoptosis and function

Glucocorticoids (GCs) are a class of steroid hormones which regulate a variety of essential biological functions. The profound anti-inflammatory and immunosuppressive activity of synthetic GCs, combined with their power to induce lymphocyte apoptosis place them among the most commonly prescribed drugs worldwide. Endogenous GCs also exert a wide range of immunomodulatory activities, including the control of T cell homeostasis. Most, if not all of these effects are mediated through the glucocorticoid receptor, a member of the nuclear receptor superfamily. However, the signaling pathways and their cell type specificity remain poorly defined. In this review, we summarize our present knowledge on GC action, the mechanisms employed to induce apoptosis and the currently discussed models of how they may participate in thymocyte development. Although our knowledge in this field has substantially increased during recent years, we are still far from a comprehensive picture of the role that GCs play in T lymphocytes

Estimation of the nesting season of marine turtles from incomplete data : statistical adjustment of a sinusoidal function

Because of logistical and financial constraints, nest counts of marine turtles are often limited in time and space. To overcome this difficulty, we developed a numerical model that fits the seasonal pattern of marine turtles nesting from complete or fragmented datasets. The duration of the main nesting season, the position and amplitude of its maximum as well as the residual number of nests, outside of the main season are obtained numerically by a least square adjustment. For the seven complete time series at our disposal (Dermochelys coriacea and Lepidochelys olivacea turtles, coast of French Guiana), the model reproduces the seasonal pattern with a correlation of r >= 0.97. When applied on a fragmented dataset, the model accuracy depends on the duration and on the temporal distribution of the monitoring (effort equally distributed during the entire season or concentrated on a part of it only). As a result of this study, we clearly advocate a strategy of monitoring distributed all over the nesting season. Following this recommendation, the model estimates the annual number of nests with a median error lower than 10% when considering only 50 days of monitoring

Coastal engineering and large-scale mangrove destruction in Guyana, South America : averting an environmental catastrophe in the making

This short communication highlights potential destabilisation of the muddy coast of Guyana, South America, caused by large-scale mangrove destruction. The stability of the coast of Guyana, which is part of one of the world's most extensive mangrove coasts, depends on large mud banks migrating alongshore from the mouth of the Amazon River and on mangrove colonisation of these banks. Under the pressures of economic development, the coastal zone of Guyana is progressively being transformed into agricultural land and aquaculture estates, protected by coastal dikes. These hard coastal defence structures, constructed in recent years, are less effective in dissipating wave energy than mud banks. They also hinder the various processes involved in the consolidation and subsequent mangrove colonisation of these banks, notably by enclosing mature mangrove forests and preventing propagule transport from these forests to mud banks. If unchecked, the progressive breakdown in the mud-bank and associated mangrove system that has led to progradation of the coastal plain of Guyana over the last 5000 years will result in large-scale coastal erosion that can only be countered by further engineering structures at prohibitive costs. The only coastal defence strategy, sound and viable over the long term, with regards to both environmental conservation and cost, consists in restoring a dynamic mud-bank and mangrove system on this wave-exposed coast

Role of flocculation and settling processes in development of the mangrove-colonized, Amazon-influenced mud-bank coast of South America

The Guianas coast downdrift (northwestward) of the Amazon River mouth is characterized by the periodic migration of mud banks that originate from the Amazon. The characteristic sizes of these banks as well as their rates of alongshore migration have been estimated from remote sensing. However, the physical mechanisms leading to their displacement are not yet fully understood. The present work is aimed at investigating a number of micro-scale processes involved in the coastal and estuarine dynamics of fine sediments, and expected to occur during the migration of mud banks. The relative magnitudes of flocculation, hindered settling and consolidation have been determined. Sampling of coastal mud was carried out during a field survey in 2001 that focussed on the fluid mud layer in the leading edge of a mud bank in French Guiana. Settling column experiments were conducted under quiescent conditions for various mean sediment concentrations in the range of 2.5-110 g.l(-1), which is typical of mud bank concentrations. The time dependent vertical profiles of suspended sediment concentration were monitored using an optical settling tank equipped with 32 pre-calibrated optical sensors. The corresponding settling velocities were deduced from the equation of the conservation of mass. The results show that the timescales of hindered settling and consolidation processes are much larger than the timescales of mixing mechanisms such as tides or propagating waves. The individual floc settling velocities are too small to counterbalance the turbulent mixing induced by breaking waves. Hindered settling, favoured by wave action and by the high background suspended sediment concentrations, is thus an overarching process in wave-driven mud bank migration. This pervasive hindered settling regime is characteristic of the wave-exposed outer and leading edges of mud banks where active mobilization of mud assures mud bank migration. As a result, consolidation is theoretically precluded. The experiments pointed out, however, that additional flocculation and differential settling should enhance sedimentation during slack water and under low wave conditions. We deduce from this that enhanced settling in the inner,subtidal-intertidal, parts of leading edges of banks during such conditions is important in the temporary sedimentation that generates gel-like fluid mud patches and mud bars. These low-energy inner, leading parts of banks form an accreted substrate colonized by mangroves, which further contribute to enhanced settling and rapid sedimentation

A Laboratory Study of Dilute Suspension Mud Floc Characteristics in an Oscillatory Diffusive Turbulent Flow

International audienceTo further the understanding of mud flocculation the COSINUS project funded a series of laboratory experiments whereby dilute mud suspensions were sheared (between 3.7 s-1 - about 20 s-1) with nominal concentrations ranging from 200-600 mg l-1, within a Plexiglas tank, using an oscillating grid. The floc properties were then examined using the LabSFLOC instrument. Results showed that for low concentrations of natural Tamar estuary (UK) mud exposed to high shear, the largest flocs were slightly less than the Kolmogorov eddy size of about 220 μm. The high shear resulted in all flocs having settling velocities of only 0.6 mm s-1. Lowering the shear for the same mud improved flocculation, and raised the macrofloc settling velocity to 1.8 mm s-1. This translated into macroflocs constituting 64% of the floc mass, and 80% of the mass settling flux. In contrast, low concentrations of natural Gironde mud displayed significantly faster settling macroflocs at each shear increment than Tamar mud, but the largest flocs were of the same order as the Tamar mud flocs. The suspended matter distribution showed the macroflocs only constituted 20-30% of the particulate mass. However, their faster settling rate transformed the low macrofloc mass into 62% of the settling flux. The removal of organic matter from the Gironde mud resulted in few flocs exceeding 125 μm in diameter

A Laboratory Study of Dilute Suspension Mud Floc Characteristics in an Oscillatory Diffusive Turbulent Flow

International audienceTo further the understanding of mud flocculation the COSINUS project funded a series of laboratory experiments whereby dilute mud suspensions were sheared (between 3.7 s-1 - about 20 s-1) with nominal concentrations ranging from 200-600 mg l-1, within a Plexiglas tank, using an oscillating grid. The floc properties were then examined using the LabSFLOC instrument. Results showed that for low concentrations of natural Tamar estuary (UK) mud exposed to high shear, the largest flocs were slightly less than the Kolmogorov eddy size of about 220 μm. The high shear resulted in all flocs having settling velocities of only 0.6 mm s-1. Lowering the shear for the same mud improved flocculation, and raised the macrofloc settling velocity to 1.8 mm s-1. This translated into macroflocs constituting 64% of the floc mass, and 80% of the mass settling flux. In contrast, low concentrations of natural Gironde mud displayed significantly faster settling macroflocs at each shear increment than Tamar mud, but the largest flocs were of the same order as the Tamar mud flocs. The suspended matter distribution showed the macroflocs only constituted 20-30% of the particulate mass. However, their faster settling rate transformed the low macrofloc mass into 62% of the settling flux. The removal of organic matter from the Gironde mud resulted in few flocs exceeding 125 μm in diameter

Trade-wind waves and mud dynamics on the French Guiana coast, South America : Input from ERA-40 wave data and field investigations

The South American coast between Brazil and Venezuela is affected by longshore migrating mud banks derived from the fine-grained Amazon sediment discharge. Onshore mud migration prevails over shallow 'bank' areas alternating alongshore with deeper 'inter-bank' areas. The transport on the inner shelf, and attachment to the shoreline, of this migrating mud has been attributed mainly to wind waves. However, the lack of in situ data on waves hampers understanding of the relationship between waves and mud dynamics. A 44-yr record (1960-2004) of the ERA-40 wave dataset generated by the European Centre for Medium-Range Weather Forecasts (ECMWF) was used, in conjunction with field investigations in French Guiana, to define both event-scale and longer-term patterns of mud mobilisation induced by waves. The ratio H-0(3)/T-2, combining wave height H and period T, and the angle of wave incidence alpha, were singled out as the most relevant parameters for describing wave forcing. Typical 'bank' and 'inter-bank' profiles and corresponding mud densities, and a 3-month record of changes in the thickness of the fluid mud layer in an estuarine navigation channel were monitored by echo-sounding from October 2002 to January 2003. An 80-day record of bed-level changes in the intertidal zone was obtained from August to November 2004 using a pressure transducer. The results on the wave regime of French Guiana confirm a distinctly seasonal pattern, and highlight an increase in H-0(3)/T-2 over the 44-yr period related to an increase in trade-wind velocities determined from corresponding trends in Atlantic wind pseudo-stress off the South American coast. Wave forcing over bank areas leads to the liquefaction of a 1-3 m-thick layer of mud that is transported onshore (and alongshore by the longshore component of wave energy). The episodic nature of high wave energy events generally results in the formation of mud bar features from the shoreward mobilisation of gel-like fluid mud. The effect of waves on mud is particularly marked following long periods of low energy, and especially at the onset of the high wave energy season (October to May), when even moderate wave energy events can lead to significant mobilisation of mud. Significant phases of increased wave energy are attended by higher long-term (annual) rates of longshore mud bank migration but the correlation is rather poor between the wave forcing parameter H-0(3)/T-2 and migration rates because stronger wave forcing is generally associated with low angles of wave incidence. This suggests a complementary role of other hydrodynamic mechanisms, such as geostrophic and tidal currents, in longshore mud bank migration

An experimental investigation of floc characteristics in a diffusive turbulent flow

The work presented aimed at investigating the influence of the physical processes effecting mud flocculation in a grid stirred experiment. Two natural muds from the Gironde estuary (organic and another with the organic content removed) and mud from the Tamar estuary (organic) were used to examine the influence of the biological and chemical compositions on interparticular collision efficiency (i.e. stickiness). For every experiment, the mean sediment concentration were deduced from extracted water samples and by optical methods. The turbulent shearing was estimated from high frequency measurements of the velocity profile. Floc size and settling velocity were obtained by high resolution video observational techniques. The study showed that particles are advected with the large scale turbulent eddies from low to high turbulent shearing regions. It does not imply an instantaneous response and floc properties remained predominantly homogeneous in the water column of the experimental tank. From the measurements made, it is therefore possible to conclude that time scales associated to the turbulent transport are much smaller than the ones associated to flocculation processes. As sediment concentrations increased up to 8 g.l -1 , the increase in particle collision frequency enhanced the flocculation process to such a high degree that only a small percentage of the total floc population fell within the microfloc (< 120 µm) size range. Flocculation is especially significant for trials with organic muds and the effect of floc break-up by inter-particular collisions was not observed, at all shear and concentration levels, during our study. The Gironde mud with the organic components neutralised produced only a minor number of flocs, at all shear and suspended concentration combinations, which exceeded the pre-sieving mesh size of 125 µm. This highlights the important role organic matter plays in the flocculation of natural cohesive sediments. This paper compares the distribution of size and settling velocity of macroflocs and microflocs and mean floc properties for the different sediment types under the various experimental concentration and shear ranges. The findings highlight how floc properties change in response to their ambient environment. ADDITIONAL INDEX WORDS: cohesive sediments, floc size, settling velocity, turbulent shear, grid stirred experimen