Extrinsic Forcing Research Articles

Parameterizing Vertical Mixing Coefficients in the Ocean Surface Boundary Layer Using Neural Networks

AbstractVertical mixing parameterizations in ocean models are formulated on the basis of the physical principles that govern turbulent mixing. However, many parameterizations include ad hoc components that are not well constrained by theory or data. One such component is the eddy diffusivity model, where vertical turbulent fluxes of a quantity are parameterized from a variable eddy diffusion coefficient and the mean vertical gradient of the quantity. In this work, we improve a parameterization of vertical mixing in the ocean surface boundary layer by enhancing its eddy diffusivity model using data‐driven methods, specifically neural networks. The neural networks are designed to take extrinsic and intrinsic forcing parameters as input to predict the eddy diffusivity profile and are trained using output data from a second moment closure turbulent mixing scheme. The modified vertical mixing scheme predicts the eddy diffusivity profile through online inference of neural networks and maintains the conservation principles of the standard ocean model equations, which is particularly important for its targeted use in climate simulations. We describe the development and stable implementation of neural networks in an ocean general circulation model and demonstrate that the enhanced scheme outperforms its predecessor by reducing biases in the mixed‐layer depth and upper ocean stratification. Our results demonstrate the potential for data‐driven physics‐aware parameterizations to improve global climate models.

Valley-scale controls and hydrogeomorphic processes driving channel breakdown in a dynamic floodplain wetland system: The Mara River, Tanzania

The character and behaviour of dryland rivers are driven by complex interactions between extrinsic and intrinsic controls. A long-term perspective is necessary to assess the sensitivity of dryland rivers to extrinsic forcing, such as hydroclimatic changes, and to contextualise the threshold channel responses. The semi-arid Mara River in northern Tanzania is the only perennial river in the UNESCO World Heritage Serengeti-Mara ecosystem and provides water resources for human and wildlife populations. Satellite imagery and discharge modelling were used to assess the valley-scale controls and hydrogeomorphic processes driving the morphological adjustments of the lower Mara River and its floodplain wetland. Both the modern and palaeochannels undergo downstream morphological and hydrological changes as they traverse the broad, alluvial floodplain. In response to marked downstream declines in discharge (from ~300 m3·s−1 to ~45 m3·s−1) and stream power (from ~40 W·m−2 to ~0.5 W·m−2), the modern channel exhibits a nonequilibrium river response resulting in downstream declines in channel size (from ~60 m to ~10 m wide) and eventual channel breakdown in the floodplain wetland. Palaeochannels in the unconfined reaches are typically larger with higher palaeodischarges than the modern channel. While some palaeochannels exhibit similar downstream trends to the modern channel, others may have maintained a continuous channel farther into the wetland. Comparison of the Mara River to other dryland river systems highlights the importance of valley setting and hydroclimate as long-term controls that influence geomorphic river response over time. The lower Mara River represents a nonequilibrium river response to declining discharges related to transmission losses that leads to channel bed aggradation and avulsion, constituting an inherent condition in many dryland alluvial rivers. An improved understanding of the interplay between intrinsic and extrinsic controls is important for the assessment of river character, behaviour, and floodplain wetland development, particularly under future projections of more variable or extreme climate conditions.

Emergence of Broadband Variability in a Marine Plankton Model Under External Forcing

AbstractTemporal variability in plankton community structure and biomass is often driven by environmental fluctuations: nutrient supplies, light, stratification and temperature. But plankton time series also exhibit variability that is not strongly correlated with key physical variables and is distinctly nonlinear in nature. There is evidence, from both laboratory and modeling studies, that oscillations can arise from ecological interactions alone. In the open ocean, it is challenging to establish the roles and relative importance of environmental versus intrinsic processes in generating the observed ecological variability. To explore this competition, we employ a marine plankton model that supports two mechanisms of intrinsic ecological variability operating at distinct frequencies: predator‐prey interactions between zooplankton and phytoplankton, with timescales of weeks, and resource competition that occurs with multiple nutrients phytoplankton species, with timescales of years. The model is forced by imposing variable nutrient input rates. Representing typical open ocean situations, with periods ranging from subseasonal to multi‐annual. We find that intrinsically‐driven variability generally persists in the presence of extrinsic forcing, and that the interaction between the two can produce variability at frequencies that are not characteristic of either source. The intrinsic frequencies are found to be even more energetic when the extrinsic variability is augmented with stochastic noise. We conclude that interactions between intrinsic and extrinsic sources of variability may contribute to the wide range of observed frequencies in phytoplankton time series, and may explain why it is often difficult to relate planktonic variation to environmental variation alone.

A shifting ‘river of sand’: The profound response of Australia's Warrego River to Holocene hydroclimatic change

In dryland river catchments, palaeochannels provide some of the key records for understanding the changes in river character and behaviour that can occur in response to different extrinsic and intrinsic controls. Here, we combine modern hydroclimatic datasets, satellite imagery, field surveys, and optically stimulated luminescence (OSL) dating of palaeochannels to investigate the response of the ~850km long Warrego River, located in central eastern Australia, to Holocene hydroclimatic change. The modern Warrego is an intermittent to ephemeral, low energy river (unit stream power, ω, ≤4Wm−2), and is characterised by low sinuosity (‘straight’), narrow (<50m), single-thread, anabranching, and distributary channels that transport fine-grained (dominantly mud and sand) sediment. Marked downstream declines in discharge and stream power mean that many channels are discontinuous, terminating downstream at floodouts or in unchannelled floodplain wetlands before reaching the Darling River. By contrast, OSL ages and palaeohydrological reconstructions show that between ~8 and 5ka, higher energy (~8–10Wm−2), wider (~160m), meandering channels were the characteristic river style. In the Burrenbilla and Coongoola palaeochannels, prevailing La Niña conditions were likely responsible for bankfull discharges between 3 and 59 times those of the modern river, which probably enabled the palaeochannels to maintain perennial flow in continuous courses to the Darling River. After ~5ka, El Niño events became stronger and more frequent, leading to greater catchment aridity and lower, more variable, river discharges. These extrinsic changes resulted in intrinsic adjustments to river character and behaviour, with key geomorphic thresholds being crossed in the cessation of meandering and establishment of the modern single-thread, anabranching, and distributary channels. The Warrego River's dramatic pattern transformation in response to mid- to late Holocene hydroclimatic change has similarities with some other Australian dryland river responses over equivalent timeframes. The pattern transformation demonstrates the Warrego River's sensitivity to extrinsic forcing and represents one of the most profound dryland river changes in the late Quaternary.

Integration of temporal environmental variation by the marine plankton community

Theory and observations suggest that low frequency variation in marine plankton populations, or red noise, may arise through cumulative integration of white noise atmospheric forcing by the ocean and its amplification within food webs. Here, we revisit evidence for the integration of stochastic atmospheric variations by comparing the power spectra of time series of atmospheric and oceanographic conditions to the population dynamics of 150 plankton taxa at Station L4 in the Western English Channel. The power spectra of oceanographic conditions (sea surface temperature, surface nitrate) are redder than those of atmospheric forcing (surface wind stress, net heat fluxes) at Station L4. However, plankton populations have power spectral slopes across trophic levels and body sizes that are redder than atmospheric forcing but whiter than oceanographic conditions. While zooplankton have redder spectral slopes than phytoplankton, there is no significant relationship between power spectral slope and body size or generation length. Using a predator-prey model, we show that the whitening of plankton time series relative to oceanographic conditions arises from noisy plankton bloom dynamics in this strongly seasonal system. The model indicates that, for typical predator-prey interactions, where the predator is on average 10 times longer than the prey, grazing leads to a modest reddening of phytoplankton variability by their larger and longer lived zooplankton consumers. Our findings suggest that, beyond extrinsic forcing by the environment, predator-prey interactions play a role in influencing the power spectra of time series of plankton populations.

Vortex and Biogeochemical Dynamics for the Hypoxia Formation Within the Coastal Transition Zone off the Pearl River Estuary

AbstractSatellite‐derived chlorophyll data, in situ measurements, and time series buoy monitoring revealed the persistent and deteriorating seasonal eutrophication and hypoxia with two distinct centers in the coastal transition zone (CTZ) between the Pearl River Estuary and the adjacent continental shelf off Hong Kong. We used a process‐oriented coupled physical‐biogeochemical numerical model to investigate the intrinsic physical and biogeochemical dynamics under the extrinsic forcing of wind, river discharge, and tides for the hypoxia formation in the CTZ. We found that the convergence induced by cyclonic vortices in the CTZ, which were formed by the buoyancy‐driven currents and wind‐driven shelf currents, created the stable water column with weak mixing and long residence time and accumulated nutrients and organic matter for the eutrophication and hypoxia development. The biophysical responses to spring‐neap cycle further demonstrated the coherent linkages of oceanic currents, vortex formation, and convergence of organic matter within the CTZ and indicated the critical role of mixing in the dissolved oxygen (DO) budgets during the spring‐neap tidal cycle. The study found that biogeochemical substances and processes are necessary conditions for eutrophication and hypoxia formation, and when these necessary conditions are combined with the favorable hydrodynamic conditions, they form a sufficient condition for eutrophication and hypoxia formation in the CTZ.

Tree Ring Reconstructions of Stemwood Biomass Indicate Increases in the Growth Rate of Black Spruce Trees Across Boreal Forests of Canada

AbstractThe claim that changes in atmospheric composition and climate have enhanced the growth rate of trees is prevalent in science, yet it is not supported by many recent tree ring studies. In this study, we analyzed historical time trends in stemwood biomass growth derived from black spruce (Picea mariana (Mill.) BSP) trees at 248 plots across Canada. The sample consisted of trees that were live and dominant at the time of sampling (LDS). Observations of stemwood biomass of LDS trees at a reference age of 75 years (Bsw75,LDS) increased by 154 to 321% over 1901–2001 depending on the method of trend estimation. Simulations from a calibrated individual‐based Growth and Yield model—forced with varying degrees of hypothetical trend in tree growth—were used to estimate the proportion of trend that could be attributed to intrinsic factors, including artefacts introduced by sampling from LDS trees instead of from the population of trees. Imposing no time trend in simulated tree growth, stemwood biomass of 75‐year‐old LDS trees (Bsw75,LDS,Model) increased by 63% (41 to 85% CI). We conclude that the remaining variation in growth of LDS trees (154 to 321% minus 63% = 91 to 258%) can be attributed to net extrinsic forcing. The scaling relationship between LDS and population trees further suggested that stemwood biomass growth of the population (Gsw75,POP) increased by 47 to 82%. By accounting for both natural dynamics and artefacts of the sampling design in estimation of net intrinsic forcing, we gained confidence that growth rate of black spruce trees across Canada increased significantly over 1901–2001. While growth enhancement is consistent with beneficial effects of increasing levels of reactive nitrogen, carbon dioxide, and warming, there remains uncertainty in the degree that applied procedures fully account for known sampling artefacts.

HyCReWW: A Hybrid Coral Reef Wave and Water level metamodel

Wave-induced flooding is a major coastal hazard on tropical islands fronted by coral reefs. The variability of shape, size, and physical characteristics of the reefs across the globe make it difficult to obtain a parameterization of wave run-up, which is needed for risk assessments. Therefore, we developed the HyCReWW metamodel to predict wave run-up under a wide range of reef morphometric and offshore forcing characteristics. Due to the complexity and high dimensionality of the problem, we assumed an idealized one-dimensional reef profile, characterized by seven primary parameters. XBeach Non-Hydrostatic was chosen to create the synthetic dataset, and Radial Basis Functions implemented in MATLAB® were chosen for interpolation. Results demonstrate the applicability of the metamodel to obtain fast and accurate results of wave run-up for a large range of intrinsic reef morphologic and extrinsic hydrodynamic forcing parameters, offering a useful tool for risk management and early warning systems.

Do grazers respond to or control food quality? Cross-scale analysis of algivorous fish in littoral Lake Tanganyika

Food quality determines the growth rate of primary consumers and ecosystem trophic efficiencies, but it is not clear whether variation in primary consumer densities control, or is controlled by, variation in food quality. We quantified variation in the density and condition of an abundant algae-eating cichlid, Tropheus brichardi, with respect to the quality and productivity of algal biofilms within and across rocky coastal sites in Lake Tanganyika, East Africa. Adjacent land use and sediment deposition in the littoral zone varied widely among sites. Tropheus brichardi maximized both caloric and phosphorus intake at the local scale by aggregating in shallow habitats: algivore density decreased with depth, tracking attached algae productivity (rETRMAX) remarkably well (r2 = 0.84, P = 0.00033). In contrast, algivore density was unrelated to among-site variation in algal productivity. Rather, there was significant increase in algal quality (r2 = 0.44, P = 0.011) and decrease in algal biomass (r2 = 0.53, P = 0.0068) with T. brichardi density across sites, consistent with strong top-down control of primary producers. The amount of inorganic sediment on rock surfaces was the strongest predictor of among-site variation in algivore density (r2 = 0.69, P = 0.00096), and algivore gut length increased with sedimentation (r2 = 0.36, P = 0.034). These patterns indicate extrinsic and top-down forcing of algal food quality and quantity across coastal landscapes, combined with adaptive habitat selection by fish at the local scale. Factors that degrade food quality by decreasing algal nutrient content or diluting the resource with indigestible material are likely to depress grazer densities, potentially dampening top-down control in high-light, low-nutrient aquatic ecosystems.

Chaotic Excitation and Tidal Damping in the GJ 876 System

Abstract The M-dwarf GJ 876 is the closest known star to harbor a multi-planetary system. With three outer planets locked in a chaotic Laplace-type resonance and an appreciably eccentric short-period super-Earth, this system represents a unique exposition of extrasolar planetary dynamics. A key question that concerns the long-term evolution of this system, and the fate of close-in planets in general, is how the significant eccentricity of the inner-most planet is maintained against tidal circularization on timescales comparable to the age of the universe. Here, we employ stochastic secular perturbation theory and N-body simulations to show that the orbit of the inner-most planet is shaped by a delicate balance between extrinsic chaotic forcing and tidal dissipation. As such, the planet’s orbital eccentricity represents an indirect measure of its tidal quality factor. Based on the system’s present-day architecture, we estimate that the extrasolar super-Earth GJ 876 d has a tidal Q ∼ 104–105, a value characteristic of solar system gas giants.

Episodic modes of operation in hydrothermal gold systems: Part II. A model for gold deposition

Abstract Coupling between the physical processes intrinsic to a hydrothermal system can lead to episodic and chaotic behaviour. Such behaviour includes variations in both space and time of the temperature, fluid pressure and activity of H 2 S, which result in the deposition of alteration mineral assemblages, zoned pyrite and gold; these variations are multifractal. In particular, the coupling of deformation and simultaneous endothermic and exothermic reactions with fluid flow leads to the highly localized deposition of gold. We discuss the physical and chemical mechanisms for such episodic and localization behaviour and explore the non-linear dynamic reasons why such mechanisms are recorded in the multifractal paragenetic sequence and deformation history. The synchronization of intrinsic episodicity as described here and extrinsic forcing induced by episodic seismicity provides another mechanism for enhancing the yield of gold deposition processes and hence the grade of orogenic gold deposits.

A Bayesian‐Based System to Assess Wave‐Driven Flooding Hazards on Coral Reef‐Lined Coasts

AbstractMany low‐elevation, coral reef‐lined, tropical coasts are vulnerable to the effects of climate change, sea level rise, and wave‐induced flooding. The considerable morphological diversity of these coasts and the variability of the hydrodynamic forcing that they are exposed to make predicting wave‐induced flooding a challenge. A process‐based wave‐resolving hydrodynamic model (XBeach Non‐Hydrostatic, “XBNH”) was used to create a large synthetic database for use in a “Bayesian Estimator for Wave Attack in Reef Environments” (BEWARE), relating incident hydrodynamics and coral reef geomorphology to coastal flooding hazards on reef‐lined coasts. Building on previous work, BEWARE improves system understanding of reef hydrodynamics by examining the intrinsic reef and extrinsic forcing factors controlling runup and flooding on reef‐lined coasts. The Bayesian estimator has high predictive skill for the XBNH model outputs that are flooding indicators, and was validated for a number of available field cases. It was found that, in order to accurately predict flooding hazards, water depth over the reef flat, incident wave conditions, and reef flat width are the most essential factors, whereas other factors such as beach slope and bed friction due to the presence or absence of corals are less important. BEWARE is a potentially powerful tool for use in early warning systems or risk assessment studies, and can be used to make projections about how wave‐induced flooding on coral reef‐lined coasts may change due to climate change.

What do we mean by regime shift? Distinguishing between extrinsic and intrinsic forcing in paleoecological data

What do we mean by regime shift? Distinguishing between extrinsic and intrinsic forcing in paleoecological data

Increasing net ecosystem biomass production of Canada's boreal and temperate forests despite decline in dry climates

AbstractRepeated measurements of tree biomass at field plots describe recovery from disturbances, sampling artifacts, and potential effects of environmental change on forest ecosystems. Challenges in differentiating between intrinsic and extrinsic sources of variation, both in theory and in practice, continue to confound claims of an anthropogenic carbon sink in forest biomass. Here we analyzed observations at 10,307 plots across southern ecozones of Canada to investigate temporal trends in stand‐level biomass growth (G), biomass loss due to mortality (M), and net ecosystem biomass production (NEBP) of intact stands. Net extrinsic forcing (Fex) was expressed by the collective dependence of biomass fluxes on climate anomalies, nitrogen deposition (N), and atmospheric carbon dioxide concentration (C). Inferences drawn directly from linear mixed‐effects model coefficients only reflect the static behavior of the model specifically at field plot locations. We, therefore, defined a dynamic landscape‐scale net extrinsic forcing (Fex′), which additionally accounted for potential negative feedback responses to anthropogenic growth enhancement. Simulations were performed over 1501–2012 to estimateFex′. Overall,Fex′ was positive, suggesting that environmental changes drove a 90% increase in NEBP. The increase in NEBP was confined to wet regions, while the biomass sink in dry regions decreased, suggesting that large expanses of northern forests, historically located near the boundary between wet and dry climates, may be at high risk of decline under continued increases in evaporative demand. These results have important implications for the greenhouse gas balance of Canada's forest sector.

Thermal mapping of a pāhoehoe lava flow, Kīlauea Volcano

Pāhoehoe lava flows are a major component of Hawaiian eruptive activity, and an important part of basaltic volcanism worldwide. In recent years, pāhoehoe lava has destroyed homes and threatened parts of Hawai'i with inundation and disruption. In this study, we use oblique helicopter-borne thermal images to create high spatial resolution (~1m) georeferenced thermal maps of the active pāhoehoe flow on Kīlauea Volcano's East Rift Zone. Thermal maps were created on 27days during 2014–2016 in the course of operational monitoring, encompassing a phase of activity that threatened the town of Pāhoa. Our results illustrate and reinforce how pāhoehoe flows are multicomponent systems consisting of the vent, master tube, distributary tubes, and surface breakouts. The thermal maps accurately depict the distribution and character of pāhoehoe breakouts through time, and also delineate the subsurface lava tube. Surface breakouts were distributed widely across the pāhoehoe flow, with significant portions concurrently active well upslope of the flow front, often concentrated in clusters of activity that evolved through time. Gradual changes to surface breakout distribution and migration relate to intrinsic processes in the flow, including the slow evolution of the distributary tube system. Abrupt disruptions to this system, and the creation of new breakouts (and associated hazards), were triggered by extrinsic forcing—namely fluctuations in lava supply rate at the vent which disrupted the master lava tube. Although the total area of a pāhoehoe flow has been suggested to relate to effusion rate, our results show that changes in the proportion of expansion vs. overplating can complicate this relationship. By modifying existing techniques, we estimate time-averaged discharge rates for the flow during 2014–2016 generally in the range of 1–2m3s−1 (mean: 1.3±0.4m3s−1)—less than half of Kīlauea's typical eruption rate on the East Rift Zone and suggestive of a weak eruptive regime during 2014–2016. We caution, however, that this discharge rate approach requires further independent corroboration. The thermal maps provide the first synoptic characterization of pāhoehoe flow activity at high spatial resolution, essential both for operational hazard assessment and fundamental understanding of pāhoehoe behavior.

Carbon cycle responses of semi-arid ecosystems to positive asymmetry in rainfall.

Recent evidence shows that warm semi-arid ecosystems are playing a disproportionate role in the interannual variability and greening trend of the global carbon cycle given their mean lower productivity when compared with other biomes (Ahlström etal. 2015 Science, 348, 895). Using multiple observations (land-atmosphere fluxes, biomass, streamflow and remotely sensed vegetation cover) and two state-of-the-art biospheric models, we show that climate variability and extremes lead to positive or negative responses in the biosphere, depending on vegetation type. We find Australia to be a global hot spot for variability, with semi-arid ecosystems in that country exhibiting increased carbon uptake due to both asymmetry in the interannual distribution of rainfall (extrinsic forcing), and asymmetry in the response of gross primary production (GPP) to rainfall change (intrinsic response). The latter is attributable to the pulse-response behaviour of the drought-adapted biota of these systems, a response that is estimated to be as much as half of that from the CO2 fertilization effect during 1990-2013. Mesic ecosystems, lacking drought-adapted species, did not show an intrinsic asymmetric response. Our findings suggest that a future more variable climate will induce large but contrasting ecosystem responses, differing among biomes globally, independent of changes in mean precipitation alone. The most significant changes are occurring in the extensive arid and semi-arid regions, and we suggest that the reported increased carbon uptake in response to asymmetric responses might be contributing to the observed greening trends there.

Numerical modeling of intrinsically and extrinsically forced seasonal circulation in the China Seas: A kinematic study

AbstractWe developed a new three‐dimensional, high‐resolution ocean circulation model for the entire China Seas (CS) region. The model considered the linked physics associated with the western boundary current, monsoonal wind, and tidal forcings, and topography in both the CS and the adjacent oceans. From this well‐validated model, we derived new insights into the three‐dimensional seasonal circulation of the CS in response to the intrinsic forcing of monsoonal winds and extrinsic forcing of flow exchange with adjacent oceans through the straits and over the slope around the periphery of the CS. Besides the East Asian monsoon forcing, we found that the extrinsic forcings interact coherently with each other and with the interior circulation to jointly shape the CS circulation. Specifically, we revealed rotating layered circulation in the CS. The circulation in the South China Sea has a vertical cyclonic‐anticyclonic‐cyclonic pattern in the upper‐middle‐lower layers, which we relate to the inflow‐outflow‐inflow transport in those layers in the Luzon Strait. The circulation in the East China Sea (ECS) is characterized by a vertically variable cyclonically rotating flow, and the circulation in the Yellow Sea (YS) is represented by a cyclonic movement in the upper layer and an anticyclonic movement in the lower layer. We attribute the cross‐shelf variation of the along‐shelf current to the ECS circulation pattern, while the vertically variable intrusive current at the central trough, together with the seasonally varied west and east coastal currents, shape the two‐layer circulation in the YS.

Autogenic avulsion, channelization and backfilling dynamics of debris‐flow fans

AbstractAlluvial fans develop their semi‐conical shape by quasi‐cyclic avulsions of their geomorphologically active sector from a fixed fan apex. On debris‐flow fans, these quasi‐cyclic avulsions are poorly understood, partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas. In this study, debris‐flow fans were experimentally created under constant extrinsic forcing, and autogenic sequences of backfilling, avulsion and channelization were observed. Backfilling, avulsion and channelization were gradual processes that required multiple successive debris‐flow events. Debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia. In the channelization phase, debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed, resulting in longer run‐out and deeper channels. Backfilling commenced when debris flows reached their maximum possible length and channel depth, as defined by channel slope and debris‐flow volume and composition, after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated. The terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low‐gradient zone forcing deposition. Consequently, the next debris flow was shorter which led to more in‐channel sedimentation, causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout. This topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas. Debris‐flow avulsion cycles are governed by the same large‐scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans, although the detailed processes are unique to debris‐flow fans. This novel result provides a basis for modelling of debris‐flow fans with applications in hazards and stratigraphy.

Environmental Disturbances Decrease the Variability of Microbial Populations within Periphyton.

A central pursuit of microbial ecology is to accurately model changes in microbial community composition in response to environmental factors. This goal requires a thorough understanding of the drivers of variability in microbial populations. However, most microbial ecology studies focus on the effects of environmental factors on mean population abundances, rather than on population variability. Here, we imposed several experimental disturbances upon periphyton communities and analyzed the variability of populations within disturbed communities compared with those in undisturbed communities. We analyzed both the bacterial and the diatom communities in the periphyton under nine different disturbance regimes, including regimes that contained multiple disturbances. We found several similarities in the responses of the two communities to disturbance; all significant treatment effects showed that populations became less variable as the result of environmental disturbances. Furthermore, multiple disturbances to these communities were often interactive, meaning that the effects of two disturbances could not have been predicted from studying single disturbances in isolation. These results suggest that environmental factors had repeatable effects on populations within microbial communities, thereby creating communities that were more similar as a result of disturbances. These experiments add to the predictive framework of microbial ecology by quantifying variability in microbial populations and by demonstrating that disturbances can place consistent constraints on the abundance of microbial populations. Although models will never be fully predictive due to stochastic forces, these results indicate that environmental stressors may increase the ability of models to capture microbial community dynamics because of their consistent effects on microbial populations. IMPORTANCE There are many reasons why microbial community composition is difficult to model. For example, the high diversity and high rate of change of these communities make it challenging to identify causes of community turnover. Furthermore, the processes that shape community composition can be either deterministic, which cause communities to converge upon similar compositions, or stochastic, which increase variability in community composition. However, modeling microbial community composition is possible only if microbes show repeatable responses to extrinsic forcing. In this study, we hypothesized that environmental stress acts as a deterministic force that shapes microbial community composition. Other studies have investigated if disturbances can alter microbial community composition, but relatively few studies ask about the repeatability of the effects of disturbances. Mechanistic models implicitly assume that communities show consistent responses to stressors; here, we define and quantify microbial variability to test this assumption. Author Video: An author video summary of this article is available.

On Eddy-Mixed Longshore Currents: Video Observation and 3D Modeling off Grand Popo Beach, Benin

ABSTRACT Marchesiello, P.; Almar; R.; Benshila; R.; Larnier, S.; Castelle, B., and McWilliams, J.C., 2016. Morphological Change near Artificial Reefs as a Beach Erosion Countermeasure. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). J. Coast. Res., Special Issue, No. 75, pp. 408–412. Coconut Creek (Florida), ISSN 0749-0208. The link between intrinsic shear-flow instability of longshore currents and their mean cross-shore profiles has long been suggested. Yet, recent investigations give increasing credit to extrinsic wave forcing mechanisms, downplaying the role of shear waves on the observed variability. Our results for Grand Popo beach, Benin, provide an original attempt to map mean longshore currents forced by an oblique swell. A 3D model investigation shows that their broad mean flow and the frequency band of eddy variability are consistent with shear instability, conversion of mean to eddy kinetic ...