Limnology and Oceanography: Fluids and Environments最新文献

We applied simulated rainfall to a salt marsh surface to mimic the effects of a low-tide summer storm, and we collected runoff and sediment transported to the downslope end of the 1- ×  2-m plot. Sediment concentration (SC), discharge, and rainfall rate were assessed in the field, and sediment metal contents were analyzed in the lab (Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Ag, Cd, Al, Sn, Hg, Tl, and Pb). SC peaked at 6100 mg L^–1 after 3 min and remained within a quasi-steady range for 16 min. Time series of metal contents were highly variable, but most were elevated relative to the background content by a factor of 4–4000. Up-scaling the plot results indicates that a single storm may mobilize 4.8–8.4 tonnes km^–2 min^–1 of sediment, or up to 96–168 tonnes yr^–1. We used the observed metal contents on the order of 0.1–200 μg g^–1 to estimate the annual loading of heavy metals that occurs in the larger subtidal channels, fed by intertidal creeks draining the salt marsh. This study highlights the importance of low-tide salt marsh rainfall-runoff processes as an intraestuarine material cycling process and shows that rainfall-entrained sediment is a potentially large nonpoint source of metals to benthic and aquatic ecosystems.

Scaling is an integral component of ecology and earth science. To date, the ability to determine the importance of air–water gas exchange across large spatial scales is hampered partly by our ability to scale the gas transfer velocity and stream hydraulics. Here we report on a metadata analysis of 563 direct gas tracer release experiments that examines scaling laws for the gas transfer velocity. We found that the gas transfer velocity scales with the product of stream slope and velocity, which is in alignment with theory on stream energy dissipation. In addition to providing equations that predict the gas transfer velocity based on stream hydraulics, we used our hydraulic data set to report a new set of hydraulic exponents and coefficients that allow the prediction of stream width, depth, and velocity based on discharge. Finally, we report a new table of gas Schmidt number dependencies to allow researchers to estimate a gas transfer velocity using our equation for many gasses of interest.

Simple transport models predict that the distance organisms drift downstream in rivers is determined by their settling velocity (w_s), the release height (z_r), and the stream velocity (U). Unfortunately, empirical evidence is lacking on whether and how factors such as w_s affect mussel larvae dispersion in rivers under natural turbulent conditions. The main goal of this study was to examine how U and w_s affect the transport of freshwater unionid mussel larvae (glochidia) in a turbulent reach of the Grand River, Ontario, Canada. Glochidia of Actinonaias ligamentina and Lampsilis fasciola, which had a 2.5-fold difference in their w_s (0.9 ± 0.02 [mean ±  SE] and 2.2 ± 0.02 mm s^− 1, respectively), were released and captured in a series of drift nets downstream. Larval dispersion in rivers appeared to be strongly affected by hydrodynamic conditions. The results indicated that glochidia are dispersed more rapidly with increased U. This is likely due to increased turbulence and lateral and vertical mixing, which were consistent with the predictions of a 3-dimensional advection–diffusion model. The decline of glochidia with distance was well described with an inverse power function, but only on days when the average U measured at 40% water depth was >40 cm s^− 1. In contrast to the predictions of simple transport models, the observed downstream transport did not differ significantly between glochidia with different w_s. Further studies are needed to better understand the effect of differences in w_s and z_r under different hydrodynamic conditions, which may also be important for other dispersal phenomena.

Positively buoyant organisms such as the macroalga Sargassum and the cyanobacterium Trichodesmium often form surface accumulations visible in satellite imagery that have lateral scale separation of tens of kilometers and cannot be explained by Langmuir circulation. Here we discuss the accumulation of floating materials in the ocean in presence of meso- and submesoscale activity. Using high-resolution simulations of the ocean mesoscale in both idealized (a 3-dimensional box where coherent eddies are forced by small-scale winds) and realistic domains (western Gulf of Mexico) where extensive concentrations of floating Sargassum have been recorded in satellite images, we show that the distribution of tracers at the ocean surface departs rapidly from that observed a few tens of meters below it. Such distribution does not resemble that observed for passive tracers in quasi-geostrophic turbulence. The strong divergence and convergence zones generated at the surface by ageostrophic processes in the submesoscale range are responsible for the creation of areas where the floating material accumulates. Floating particles are expelled from the core of mesoscale eddies and concentrate in convergence regions in patterns comparable to the ones observed through the satellite images. In light of those results, Sargassum and/or Trichodesmium may provide a useful proxy to track convergence/divergence processes resulting from ageostrophic processes at the ocean surface.

Air–water gas exchange governs fluxes of gas into and out of aquatic ecosystems. Knowing this flux is necessary to calculate gas budgets (i.e., O₂) to estimate whole-ecosystem metabolism and basin-scale carbon budgets. Empirical data on rates of gas exchange for streams, estuaries, and oceans are readily available. However, there are few data from large rivers and no data from whitewater rapids. We measured gas transfer velocity in the Colorado River, Grand Canyon, as decline in O₂ saturation deficit, 7 times in a 28-km segment spanning 7 rapids. The O₂ saturation deficit exists because of hypolimnetic discharge from Glen Canyon Dam, located 25 km upriver from Lees Ferry. Gas transfer velocity (k₆₀₀) increased with slope of the immediate reach. k₆₀₀ was < 10 cm h^− 1 in flat reaches, while k₆₀₀ for the steepest rapid ranged 3600–7700 cm h^− 1, an extremely high value of k₆₀₀. Using the rate of gas exchange per unit length of water surface elevation (K_drop, m^− 1), segment-integrated k₆₀₀ varied between 74 and 101 cm h^− 1. Using K_drop we scaled k₆₀₀ to the remainder of the Colorado River in Grand Canyon. At the scale corresponding to the segment length where 80% of the O₂ exchanged with the atmosphere (mean length = 26.1 km), k₆₀₀ varied 4.5-fold between 56 and 272 cm h^− 1 with a mean of 113 cm h^− 1. Gas transfer velocity for the Colorado River was higher than those from other aquatic ecosystems because of large rapids. Our approach of scaling k₆₀₀ based on K_drop allows comparing gas transfer velocity across rivers with spatially heterogeneous morphology.

Nonfeeding postlarval pueruli of spiny lobsters migrate tens of kilometers across the continental shelf to settle in coastal waters. A model that analyzes hydrodynamic forces during swimming in the puerulus of Jasus edwardsii is described. The model calculates the speed at which forward propulsion balances drag. Calculated speeds agree with observed puerulus behavior. The computed mechanical work is converted to metabolic energy consumption using an assumed efficiency. Values concur with reported estimates of the utilization of lipid energy reserves in pueruli. For biochemical energy reserves reported for pueruli collected 20 km off the east coast of New Zealand, the model suggests that this distance and durations of 5 days active swimming are the approximate limits to endurance. Sustained swimming exceeding 15 cm s^− 1 will likely exhaust energy reserves before an animal can reach the coast, whereas swimming at less than 5–7 cm s^− 1 is inefficient because of the overhead of nonswimming, inactive metabolism. Successful onshore migration of this species is potentially limited by the animals' energy reserves. Reduced energy reserves at the outset due to prior poor feeding, or delays encountered en route due to unfavorable currents, could lead to exceeding the stored reserves of the pueruli, and death. Potentially, relatively small shifts in coastal ocean climate conditions could generate marked changes in recruitment to important spiny lobster fisheries, as has recently been observed in many coastal populations of spiny lobsters.

In streams, hydrodynamic forces may influence food web structure by limiting the spatial distribution or diversity of primary consumers. To examine the spatial relationships between organisms and physical drivers, we measured distributions of local bed shear stress (τ_w), periphyton, and herbivorous invertebrates (larvae of the mayfly Epeorus longimanus (Eaton)) on experimentally deployed, submerged stones (diameters ranging from 22 to 33 cm) in a mountain stream in British Columbia. In general, τ_w increased from the upstream to the downstream portion of stones, where there was an abrupt decrease in τ_w due to flow separation. Periphyton density was significantly related to stone surface roughness and topography (i.e., more algae on rougher, higher areas of the substrate). The high-shear regions of the upper, exposed surfaces of the stones were inhabited by high densities of Epeorus larvae (up to 1500 larvae m^− 2); larvae migrated diurnally, with most larvae moving to the underside of stones during the day. Larval density was positively related to stone surface roughness and topography and to a lesser extent with periphyton and τ_w, whereas larvae avoided regions of flow separation. Experimental reversal of the orientation of stones with respect to flow direction indicated that Epeorus larval positioning was a proximate response to near-bed flows, rather than biotic factors such as food availability or predation. Whereas hydrodynamic factors influenced the microdistribution of these primary consumers, the spatial relationship with shear stress was much more complex than anticipated.

Zooplankton swimming near the substratum experience boundary layer flow that is characterized by steep velocity gradients and turbulence. How do small swimming organisms navigate flows at this interface to forage and interact with mates? To address this question, we collected field measurements of the swimming behavior of the marine ostracod Paravargula trifax near complex living substrata, which were exposed to two conditions: slow “ambient flow” and faster “experimental flow.” Ostracod trajectories and background flow were recorded simultaneously using a self-contained underwater velocimetry apparatus (SCUVA). Particle image velocimetry (DPIV) produced instantaneous velocity vector fields in which the ostracods were swimming. Mean velocities, local shear stresses, turbulence intensity, and boundary shear velocity (u_*) were greater in the experimental flow treatment. In slow ambient flow (u_rms = 0.39 ± 0.13 [mean ± SD] cm s ^− 1), ostracod swimming tracks were more tortuous and swimming angles corrected for background flow were randomly distributed compared with tracks in faster flow (u_rms = 3.49 ± 0.50 cm s ^− 1), indicating decreased maneuverability in rapidly flowing, turbulent water. Modeled, passive neutrally buoyant particles moved at substantially slower speeds, and their tracks were less tortuous than those of the ostracods, thus illustrating the importance of behavior as well as environmental flow in determining ostracod trajectories. Frequencies of encounters by ostracods with the benthos and with other ostracods were not different between treatments. However, in the experimental flow treatment, interactions with other ostracods occurred more frequently in the boundary layer than in the free stream, suggesting that microhabitats in the boundary layer may allow for enhanced mating encounters.

Larval responses to hydromechanical cues potentially have important effects on larval dispersal and settlement. This study examined the behavior of mussel larvae (Mytilus edulis) in laboratory-generated turbulence representative of nearshore currents. We video recorded the behavior of early- and late-stage veligers in a grid-stirred tank at five turbulence levels under light and dark conditions. Water velocities and kinetic energy dissipation rates were measured using particle image velocimetry and acoustic Doppler velocimetry. We characterized the vertical velocity distributions for sinking, hovering, and swimming modes in still water and calculated the average larval behavioral velocity in turbulence. In still water, young larvae had more positive (upward) velocities than old larvae, and both stages had more positive velocities in light than in dark. In turbulence, the mean larval vertical velocity varied from positive at low dissipation rates to negative at dissipation rates above a threshold of 8.3 × 10 ^− 2 cm² s ^− 3. At this threshold, the Kolmogorov length scale (η = 590 μm) was two to three times the mean larval shell lengths (171–256 μm), implying that turbulence is detectable even by larvae that are smaller than the smallest eddies. Responses to turbulence were unaffected by larval age or light conditions and contributed substantial behavioral variation. By sinking in strong turbulence, mussel larvae could increase their flux to the bed in energetic coastal flows, particularly over rough substrates like mussel beds. The response to turbulence by early-stage larvae will also affect their dispersal and may help larvae remain near coastal populations.

Worldwide, small lakes ( < 1 km²) are numerically dominant, yet the potential for interaction between physical and ecological processes therein has been largely ignored. High-frequency time series of the thermal and current structures in a small dimictic lake (Lake Bromont, Quebec) revealed the occurrence of recurrent internal waves during the summer of 2007. Amplitudes and frequencies of the internal wave modes were characterized, along with wind and stratification conditions, during two focal periods of 5 days at the beginning and the end of the summer. Owing to a resonance with the daily wind, the second and third vertical mode seiches dominated over the first mode, which was observed only during larger wind events. Although the lake is small (0.41 km²) and shallow (mean depth of 4 m), the response of the thermal structure of the lake to wind forcing was very similar to that of alpine and other deep lakes. The phytoplankton community was controlled by the contrasting gradients of light and nutrients. Consequently, metalimnetic communities of cyanobacteria exposed to the recurrent internal waves, which occurred throughout the summer, formed the dominant phytoplankton biomass in the lake. The regular vertical excursion of the metalimnion influenced both light availability and nutrient fluxes and most likely contributed to an enhanced algal biomass.