Giovanni Coco

Forecasting the limits of resilience: integrating empirical research with theory

Despite the increasing evidence of drastic and profound changes in many ecosystems, often referred to as regime shifts, we have little ability to understand the processes that provide insurance against such change (resilience). Modelling studies have suggested that increased variance may foreshadow a regime shift, but this requires long-term data and knowledge of the functional links between key processes. Field-based research and ground-truthing is an essential part of the heuristic that marries theoretical and empirical research, but experimental studies of resilience are lagging behind theory, management and policy requirements. Empirically, ecological resilience must be understood in terms of community dynamics and the potential for small shifts in environmental forcing to break the feedbacks that support resilience. Here, we integrate recent theory and empirical data to identify ways we might define and understand potential thresholds in the resilience of nature, and thus the potential for regime shifts, by focusing on the roles of strong and weak interactions, linkages in meta-communities, and positive feedbacks between these and environmental drivers. The challenge to theoretical and field ecologists is to make the shift from hindsight to a more predictive science that is able to assist in the implementation of ecosystem-based management.

A mechanism for the generation of wave-driven rhythmic patterns in the surf zone

The coupling between topographic irregularities and wave-driven mean water motion in the surf zone is examined. This coupling occurs because the topographic perturbations produce excess gradients in the wave radiation stress that cause a steady circulation. This circulation, in turn, creates a sediment transport pattern that can reinforce the bottom disturbance and may thereby lead to the growth of large-scale bed forms. To investigate this coupling mechanism, the linearized stability problem with an originally plane sloping beach and normal wave incidence is solved in two different cases. First, the breaking line is considered to be fixed, and second, the perturbations in water depth that produce a displacement of the breaker line are accounted for. The first case shows that the basic topography can be unstable with respect to two different modes: a giant cusp pattern with shore-attached transverse bars that extend across the whole surf zone and a crescentic pattern with alternate shoals and pools at both sides of the breaking line showing a mirroring effect. In the second case, the varying breaker line may have a strong influence on the circulation. This is clear for the giant cusp topography whose growth is totally inhibited. In contrast, the morphology and the growth of the crescentic pattern remains almost unchanged.

Review of wave‐driven sediment resuspension and transport in estuaries

Waves are fundamentally important to the physical and biological functioning of estuaries. Understanding and predicting contaminant transport, development of sedimentary structures, geomorphological response to changes in external forcings such as rising sea level, and response of estuarine ecosystems to contaminant stressors require understanding of the relative roles of wave- and current-driven sediment transport. We review wave-driven sediment resuspension and transport in estuaries, including generation of bed shear stress by waves, initiation of sediment motion by waves, and the ways waves modulate, add to, and interact with sediment transport driven by currents. A key characteristic of the wave-induced force on the seabed is extreme spatial and temporal variations; simple analytical models are revealing of the way such patterns develop. Statistical methods have been widely applied to predict wave resuspension of intertidal-flat bed sediments, and physically based predictors of resuspension developed from open-coast studies appear to also apply to short-period estuarine waves. There is ample experimental evidence to conclude that over the long term, waves erode and tidal currents accrete intertidal flats. Waves indirectly add to the formation of fluid mud by adding to the estuarine pool of fine sediment, and waves may fluidize subtidal seabeds, changing bed erodibility. Models have been used to explore the dynamic balance between sediment transport by waves and by currents and have revealed the key control of waves on estuarine morphology. Estuarine intertidal flats are excellent natural laboratories that offer opportunities for working on a number of fundamental problems in sediment transport.

Self-organization mechanisms for the formation on nearshore crescentic and transverse sand bars

The formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow-water equations and on a simplified sediment transport parameterization. By assuming normally approaching waves and a saturated surf zone, rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed. Two types of bedforms appear: one is a crescentic bar pattern centred around the breakpoint and the other, herein modelled for the first time, is a transverse bar pattern. The feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations. Basically, the waves stir up the sediment and keep it in suspension with a certain cross-shore distribution of depth-averaged concentration. Then, a current flowing with (against) the gradient of sediment concentration produces erosion (deposition). It is shown that inside the surf zone, these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms. Numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro-morphodynamic input conditions to which of the patterns develops. It is suggested that crescentic bar growth would be favoured by high-energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment. In intermediate conditions mixed patterns may occur.

Investigation of a self-organization model for beach cusp formation and development

A recent numerical investigation of “self-organization” [Werner and Fink, 1993] suggests that the feedback process between currents and sediment response can result in “self-organized” patterns and can be used to predict beach cusp formation and spacing. A similar model based on self-organization is tested here in order to understand the processes occurring during beach cusp formation and development, to evaluate the sensitivity toward the parameters used, and to examine how the model might relate to field observations. Results obtained confirm the validity of the self-organization approach and its capacity to predict beach cusp spacing, with values in fair agreement with the available field measurements, with most of the input parameters primarily affecting the rate of the process rather than the final spacing. However, changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior. Cusp spacing tends to change with time, and cusp regularity shows large long-term variations. Cusps are found to be accretionary in the swash zone, and in agreement with most observations, mean flows are horn divergent over developed topography. Simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed. Results indicate that preexisting large-amplitude cusps are destroyed if their spacing is substantially different from that expected under self-organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach. These findings support the hypothesis that self-organization is a robust mechanism for beach cusp formation.

Feedbacks between bivalve density, flow, and suspended sediment concentration on patch stable states.

We explore the role of biophysical feedbacks occurring at the patch scale (spatial scale of tens of meters) that influence bivalve physiological condition and affect patch stability by developing a numerical model for the pinnid bivalve, Atrina zelandica, in cohesive sediments. Simulated feedbacks involve bivalve density, flow conditions (assumed to be primarily influenced by local water depth and peak current speed), suspended sediment concentration (evaluated through a balance between background concentration, deposition, and erosion), and changes in the physiology of Atrina derived from empirical study. The model demonstrates that high bivalve density can lead to skimming flow and to a concomitant decrease in resuspension that will affect suspended sediment concentration over the patch directly feeding back on bivalve physiology. Consequently, for a given flow and background suspended sediment load, the stability of a patch directly depends on the size and density of bivalves in the patch. Although under a range of conditions patch stability is ensured independently of bivalve density, simulations clearly indicate that sudden changes in bivalve density or suspended sediment concentration can substantially affect patch structure and lead to different stable states. The model highlights the role of interactions between organisms, flow, and broader scale environmental conditions in providing a mechanistic explanation for the patchy occurrence of benthic suspension feeders.

The role of tides in beach cusp development

[1] Field measurements of morphology and swash flow during three episodes of beach cusp development indicate that tides modulate the height and cross-shore position of beach cusps. During rising tide, beach cusp height decreases as embayments accrete more than horns and the cross-shore extent of beach cusps decreases. During falling tide, beach cusp height increases as embayments erode more than horns and cross-shore extent increases. A numerical model for beach cusp formation based on self-organization, extended to include the effects of morphological smoothing seaward of the swash front and infiltration into the beach, reproduces the observed spacing, position, and tidal modulation. During rising tide, water particles simulating swash infiltrate, preferentially in embayments, causing enhanced deposition. During falling tide, exfiltration of water particles combined with diversion of swash from horns causes enhanced erosion in embayments. Smoothing of beach morphology in the swash zone seaward of the swash front and in the shallow surf zone accounts for most of the observed tidal modulation, even in the absence of infiltration and exfiltration. Despite the qualitative, and in some cases quantitative, agreement of the model and measurements, the model fails to reproduce observed large deviations of horn orientation from shore normal, some aspects of beach cusp shape, and deviations from the basic tidal modulation, possibly because of the simplified parameterization of cross-shore sediment transport and the neglect of the effects of sea surface gradients on flow. INDEX TERMS: 4546 Oceanography: Physical: Nearshore processes; 4255 Oceanography: General: Numerical modeling; 3220 Mathematical Geophysics: Nonlinear dynamics; KEYWORDS: beach cusp, tides, infiltration, exfiltration, erosion

Regularity and randomness in the formation of beach cusps

Abstract Controversy still exists on whether rhythmic features such as beach cusps are the result of the presence of standing edge wave motions in the hydrodynamics or if they are the result of self-organising processes. The compatibility between the two mechanisms is here investigated through the use of a numerical model simulating the formation and development of beach cusps. Simulations characterised by different forcing conditions have been performed. A series of ‘random’ simulations, each individual run simply differing from the others in the seed used in the random number generator, showed the typical self-organisation behaviour with the features appearing at different locations and even with slightly different spacing. ‘Regular’ series of simulations have been run by changing the wavelength of the template superimposed by the hydrodynamic forcing and the number of cycles when the template was present. Results surprisingly indicate that even a minimum number of cycles with a regular forcing can deterministically induce the final shoreline configuration. Furthermore, if the forcing template has the same wavelength as the one resulting from purely random simulations, growth rates are much faster than those obtained with random conditions. Implications for the kind of field measurements necessary to discern which of the two mechanisms is responsible for beach cusp formation have also been considered.

Modeling the morphodynamic response of tidal embayments to sea-level rise

Sea-level rise has a strong influence on tidal systems, and a major focus of climate change effect studies is to predict the future state of these environmental systems. Here, we used a model to simulate the morphological evolution of tidal embayments and to explore their response to a rising sea level. The model was first used to reproduce the formation of channels and intertidal flats under a stable mean water level in an idealised and initially unchannelled tidal basin. A gradual rise in sea level was imposed once a well-developed channel network had formed. Simulations were conducted with different sea-level rise rates and tidal ranges. Sea-level rise forced headward erosion of the tidal channels, driving a landward expansion of the channel network and channel development in the previously non-inundated part of the basin. Simultaneously, an increase in channel drainage width in the lower part of the basin occurred and a decrease in the overall fraction of the basin occupied by channels could be observed. Sea-level rise thus altered important characteristics of the tidal channel network. Some intertidal areas were maintained despite a rising sea level. However, the size, shape, and location of the intertidal areas changed. In addition, sea-level rise affected the exchange of sediment between the different morphological elements. A shift from exporting to importing sediment as well as a reinforcement of the existing sediment export was observed for the simulations performed here. Sediment erosion in the inlet and the offshore transport of sediment was enhanced, resulting in the expansion of the ebb-tidal delta. Our model results further emphasise that tidal embayments can exhibit contrasting responses to sea-level rise.

Complex positive connections between functional groups are revealed by neural network analysis of ecological time series.

The relationships between functional linkages within communities and community dynamics are fundamental to biodiversity‐stability relationships. By teasing apart the hidden layers within artificial neural networks (ANNs), we developed webs defining how functional groups influence each others temporal dynamics. ANNs were based on 15 years of bimonthly monitoring of macrobenthic communities on three intertidal sandflats in Manukau Harbor (New Zealand). Sites differed in web topology and diversity, with the site dominated by one functional group exhibiting only a few strong links, the lowest α‐, β‐, and γ‐diversity, and the highest temporal stability in α‐diversity. However, positive interactions between functional groups, nonconcordant with harborwide or site‐specific environmental variables, always dominated the interaction webs. The increased number of links we observed with increased temporal variation of species richness within functional groups and overall diversity supports the insurance hypothesis. While our findings suggest that there may be no consistent model characterizing the topology of temporal interactions between functional groups, decreasing diversity is likely to decouple interactions between functional groups and decrease ecosystem functionality.