Limnology and Oceanography: Fluids and Environments最新文献

Recent field and experimental studies show that the wakes behind individual patches of aquatic vegetation, as well as the interaction and merger of neighboring wakes, produce zones of diminished velocity that may enhance deposition and encourage patch growth and patch merger. In the present study, these patch-scale biogeomorphic interactions are incorporated into a simple model for vegetated landscape evolution. The initial flow field is solved by using a porous media formulation for hydraulic resistance. The velocity in wake regions is then adjusted to match the wake structure measured in laboratory studies with individual and pairs of vegetation patches. Vegetation is added based on a probabilistic function linked to the velocity field. The simulations explore the influence of initial plant density (ID) and limiting velocity (LV, the velocity above which no plants can grow) on landscape evolution. Three types of stable landforms can occur: full vegetation coverage, channeled, and sparse. By including the influence of wakes, full vegetation coverage can be achieved from initial plant densities as low as 5%. In contrast, simulations that exclude the influence of wakes rarely reach full vegetation coverage, reinforcing the idea that growth within wakes is an important component in vegetated landscape evolution. The model also highlights the role of flow diversion into bare regions (channels) in the promotion of growth within vegetated regions. Finally, sparse landscapes result when the initial plant density is sufficiently low that no wake interactions can occur, so that patch merger cannot occur, emphasizing the importance of the patch interaction length scale.

Predator–prey encounter and capture rates are key drivers of population dynamics of planktonic organisms. Turbulent mixing gives rise to enhanced encounter rates, but high turbulence levels may reduce capture rates. We present an estimate for the optimum turbulence level for a predator that is characterized by a number of parameters, such as its range of interception. A limit at small spatial scales is recovered where classical diffusion competes with turbulent motions. Particular attention is given to the question of turbulence-induced noise signals, which a predator can misinterpret as indicators of prey. Analytical expressions are obtained for the occurrence of these “error signals” in terms of the basic parameters of the problem. The basic hypothesis rests on the assumption that if several such error signals are received within a time needed for the predator to capture prey, then its capacity for capturing prey is reduced or even made impossible. The aim of the study is to present closed general analytical expressions for the capture rate in turbulent environments, where the results contain free parameters that can be used for modeling selected species. Statistical descriptions of the velocity fluctuations on spatial scales in the viscous subrange of turbulence are determined and placed in the context of predator–prey encounter and capture rates. The relevant probability densities are obtained by direct numerical solutions of the Navier–Stokes equation for turbulent conditions. The analysis is given a compact formulation in terms of scaled dimensionless variables.

The transport, deposition, and decomposition of seagrass wrack facilitate significant marine subsidies of material, energy, and organisms to the terrestrial environment. Over the past decade we have improved our understanding of the on-beach decomposition of seagrass wrack and its impact on beach and island communities; however, there is a paucity of research on the transport processes that supply wrack to the beaches. The physical properties of wrack affect its buoyancy and therefore transport, but these properties vary with species, the condition of the wrack when it was formed, the time since the wrack was generated and its ambient environment in the sediment, the water column, at the water surface or on the beach. Understanding how wrack physical properties vary under a range of conditions is needed to predict wrack transport, yet these properties have not previously been reported. We modified classical parameterizations of particle transport to identify knowledge and data gaps for wrack transport processes. We present a preliminary exploration, for Posidonia sinuosa leaves and Amphibolis antarctica stems and leaves, of settling velocities of wrack fragments, critical shear stresses required for their resuspension, bulk physical characteristics of wrack accumulations on beaches (e.g., bulk density, porosity), and physical properties of key wrack components (e.g., tissue density, tensile strength). We also determined how these properties change with drying, aging, and subsequent rewetting.

We explore the role of oceanic dispersal in setting patterns of phytoplankton diversity, with emphasis on the role of mesoscale turbulence, using numerical simulations that resolve mesoscale eddies and a diverse set of phytoplankton types. The model suggests that dispersal of phytoplankton by oceanic transport processes increases phytoplankton diversity at the local scale of O(10–100) km (α-diversity), extends the range of many phytoplankton types, and decreases the ability of rare types to persist in isolated areas. As a consequence, phytoplanktonic assemblages are modified and diversity decreases at the regional scale of O(1000) km (γ-diversity). By progressively accounting for different classes of motion, we show that the increase of α-diversity ensues from vertical mixing of the organisms, dispersal by mean lateral currents, and in slightly larger proportion, dispersal due to eddies. With the progressive inclusion of mechanisms of dispersal, the community becomes dominated by a smaller number of types but with larger degree of coexistence, in larger home range areas. From a resource competition perspective, physical transport can reduce the effective concentration R* of a limiting resource R, thus allowing more types to become equally fit. In addition, mixing of nearby populations allows coexistence of types with unequal fitness. The simulations suggest that mesoscale turbulence plays a particular role, concomitantly providing a means for different phytoplankton types to achieve comparable fitness and extending the exclusion time scale for less competitive types.

Exchange and retention of coastal waters modulate dispersal of marine larvae, affecting marine ecosystem dynamics. A hypothesis was put forward in the 1980s describing the coastal upwelling front as a “tattered curtain” that retains larvae. This front was envisioned to be broken up by squirts and eddies, hitting the coast under upwelling relaxation events. Here we revise this hypothesis by using an idealized ocean model of an eastern boundary upwelling current, and an idealized particle/larvae model appropriate for shelf-spawning benthic species. Modeled larval settlement patterns were controlled by retention in the core of the upwelling jet, bounded by regions of high-velocity shear on the flanks of the jet. Squirts, filaments, poleward-moving eddies, and meanders modulated settlement patterns locally, while dense packets moved equatorward within the upwelling jet. Correlation between settlement (i.e., particles 20–40 d old <10 km from shore) and wind was low for a lagged wind product (r=0.33) and moderate for a 20-d integrated wind product (r=0.62). We determined that it is not upwelling relaxation but sustained, moderate upwelling that can result in a highly retentive jet that entrains larvae and acts as a barrier to cross-shelf transport; however, the amount of retention is highly variable. Settlement was low after strong, persistent upwelling completely tattered the jet. Jet cores in general should act as important retentive transport barriers across diverse coastal systems, a view supported by dynamical theory, modeling studies, and larval recruitment observations.

We examined the effect of fine-scale fluid turbulence on phytoplankton community structure in an idealized, size-structured community model. It has been shown that turbulence can enhance nutrient transport toward a cell, particularly for larger cells in highly turbulent conditions. Our model suggests that under weak grazing pressure the effect of this mechanism on relative phytoplankton fitness and community structure is negligible. Under these conditions, the high nutrient affinity of small cells dominates relative fitness and allows them to outcompete larger cells. In contrast, when grazing pressure is strong, the turbulent enhancement of nutrient uptake and fitness for larger cells can become ecologically significant. Here, increasing turbulence broadens the size range of coexisting phytoplankton and increases the size of the dominant cell type at equilibrium. We also estimate and map open ocean turbulent dissipation rates as a function of climatological surface wind stresses. The turbulent enhancement of nutrient uptake is most likely to be ecologically significant in regions with low nutrient levels, strong grazing pressure, and relatively high turbulence, such as in windier portions of the subtropical gyre or post-bloom conditions at higher latitudes. In these regions, turbulence may help sustain larger cell populations through otherwise unfavorable environmental conditions.

This study examines the effects of internal seiches on heat transport through the sediment-water interface, and the internal seiche-related temperature and oxygen fluctuations above the sediment, in the littoral zone of a stratified lake. High-resolution temperature profiles were taken within the upper sediment, accompanied by temperature and oxygen measurements within the overlying water. Heat transport in the upper sediment alternated between diffusion and convection at the periodicity of the internal seiches, with the strongest oscillations at a period of 2.4 h. During long-duration events (>30 min) of seiche-driven cooling of the sediment surface, the thermal instability extended as much as 9 cm down into the sediment, followed by free convective transport in the upper sediment. The vertical convective heat fluxes were close to those of Rayleigh–Bénard convection for pure fluid flow. The convective heat fluxes were, on average, three times higher than the diffusive heat fluxes, and the maximum convective heat fluxes of 50–100 W m⁻² were 10–20 times higher than the maximum diffusive heat fluxes. Internal seiches caused advective oxygen fluctuations above the sediment that can potentially reinforce the effect of convection on biochemical processes within the lake sediments. Periodic temperature and oxygen variations due to internal seiching can cover ∼10% of the sediment area, depending on seasonal stratification and lake morphometry. In these areas, convection intensifies the transport of heat, nutrients, and oxygen through the sediment surface and represents an important feature of the ecology of lakes.

Diel vertical migrations (DVMs) of many plankton species, including single-celled protists, are well documented in the field and form a core component of many large-scale numerical models of plankton transport and ecology. However, the sparse quantitative data available describing motility behaviors of individual protists have frequently indicated that motility exhibits only short-term correlation on the order of a few seconds or hundreds of micrometers, resembling diffusive transport at larger scales—a result incompatible with DVM, which requires ballistic (straight-line) motion. We interrogated an extensive set of three-dimensional protistan movement trajectories in an effort to identify spatial and temporal correlation scales. Whereas the horizontal components of movement were diffusive, the vertical component remained highly correlated (i.e., nonrandom) for nearly all species for the duration of observation (up to 120 s and 6.1 mm) and in the absence of any environmental cues besides gravity. These persistent motility patterns may have been obscured in some previous studies due to the use of restrictive containers, dimensionally lumped, isotropic analyses, and/or an observation bias, inherent to observing free-swimming organisms with stationary cameras, which we accounted for in this study. Extrapolated over a 12-h period, conservative estimates of vertical travel ranges for the protists observed here would be 3–10 m, while diffusive horizontal motion would result in about 10 cm of travel at most. Hence, these extended observations of phylogenetically diverse swimming protists, coupled with a quantitative analysis that accounts for anisotropy in the data, illustrate the small-scale mechanistic underpinnings of DVM.

Boundary currents have been recognized as potential drivers of spatial heterogeneity in the ocean because of their role in physical transport and influence on large-scale coastal processes. In this study, we used particle tracking methods in a data-assimilating eddy-resolving ocean circulation model to determine the effect of multiple boundary currents on connectivity around temperate Australia during the austral winter. Results demonstrated that oceanographic connectivity was asymmetric around Australia, having greater eastward trajectories due to more favorable ocean boundary currents during this season. We validated connectivity patterns with genetic data from an ecologically important species, the kelp, Ecklonia radiata, which has greater genetic similarity between the west and south coasts of Australia, compared with the east coast, likely due to predominantly eastward propagule dispersal. Boundary current circulation was a coarse predictor of kelp genetic connectivity on multigeneration time scales, and the nature of these relationships varied among the three boundary current systems according to mean current strength.

Zooplankton are not uniformly distributed in space but are patchy at multiple scales as a result of interactions between their directed motion and both large- and small-scale water currents. Using field data from July and August 2009 and 2010 we report observations of the relationship between enhanced small-scale spatial variability in zooplankton and the presence of internal waves in the weakly stratified epilimnion of Lake Opeongo, Ontario. To quantify this physical–biological coupling, we compared the variance of isotherm displacement and gradient Richardson number (Ri_g) measured using moored sensors, with the small-scale spatial distributions of zooplankton measured using an optical plankton counter towed along linear transects intersecting the moorings. For the smallest size ranges of zooplankton (284–450 μm) we found that spatial variability was statistically greatest at intermediate Ri_g (0.25<Ri_g<1), whereas no such relationship existed for the two larger zooplankton size classes. The highest values of variability of isotherm displacement were also found at intermediate Ri_g. Direct comparison between isotherm variability and spatial variability of zooplankton also demonstrated this proportional relationship. Comparisons of 2010 temperature transects with the moored temperature data set suggest typical and consistent wave frequency of 4 × 10⁻⁴ − 2.5 × 10⁻³ Hz and wavelength of 50–500 m. Vertical velocities estimated from wave characteristics were faster than swimming speeds of small zooplankton, essentially rendering them passive. This is consistent with our observations that increased variability in the distribution of small-bodied zooplankton at intermediate values of Ri_g are linked to increased variability of isotherm displacement at those values of Ri_g.