Boundary layer instability beneath periodic internal solitary waves

We investigated the stability of the bottom boundary layer (BBL) beneath periodic internal solitary waves (ISWs) of depression over a flat bottom through two-dimensional direct numerical simulations. We explored the effects of variation in wave Reynolds number $Re_{ISW}$ and wave period on the nature of the instability, and energy production in the separated BBL. The instability characteristics and rate of vortex shedding of the BBL were strongly dependent on $Re_{ISW}$. The BBL was laminar and convectively unstable at $Re_{ISW}$ 90 and 300, respectively. At $Re_{ISW}=300$, the convective wave packet was periodically amplified by each successive ISW, until vortex-shedding occurred. This implies noise-amplification behavior and suggests that the discrepancies in the critical $Re_{ISW}$, for vortex shedding between lab and different numerical simulations, are due to differences in background seed noise. Instability energy decreased under the front shoulder of the ISW, analogous to flow relaminarization under a favourable pressure gradient. At larger $Re_{ISW}=900$, the BBL was initially convectively unstable, and then the instability tracked with the ISW, characteristic of global instability, regardless of the ISW periodicity. The simulated initial convective instability at both $Re_{ISW}$ 300 and 900 is in agreement with local linear stability analysis which predicts that the instability group speed is always lower than the ISW celerity. Increased free-stream perturbations and larger $Re_{ISW}$ shift the location of vortex shedding (and enhanced bed shear stress) closer to the ISW trough, thereby potentially changing the location of maximum sediment resuspension from the ISW, in agreement with field observations at higher $Re_{ISW}$

Factors affecting the development and dynamics of hypoxia in a large shallow stratified lake: Hourly to seasonal patterns

[1] The examination of hypoxia in the hypolimnion of large lakes traditionally focuses on the assessment of its spatial and temporal extent and its effect on water quality. In Lake Erie, hypoxia typically occurs between July and October in the central basin; however, there is considerable interannual variability both spatially and temporally. The processes driving this interannual variability as well as the small-scale time variation in oxygen depletion (e.g., −0.7 to +0.3 mg L−1 d−1) were examined in a field study conducted in the western part of the central basin of Lake Erie in 2008 and 2009. Data were obtained from a spatial array of moorings as well as sampling cruises that examined the physical and biological conditions needed to investigate the dynamics of the oxygen depletion and create a vertical oxygen budget. The flux of oxygen through the thermocline to the hypolimnion was a significant source of oxygen equivalent to ∼18% of the total oxygen depletion in the hypolimnion over the stratified period. The total oxygen depletion in the hypolimnion was due to equivalent amounts of hypolimnetic oxygen demand due to respiration in the water column and flux of oxygen to the bottom due to sediment oxygen demand. This latter finding was strongly dependent on hypolimnion thickness in Lake Erie, which also appeared to be an important parameter driving the rate of oxygen depletion by controlling the vertical volumetric fluxes and hence the competition between vertical flux and community respiration in the hypolimnion of shallow lakes

Impacts of hydrodynamics and benthic communities on phytoplankton distributions in a large, dreissenid-colonized lake (Lake Simcoe, Ontario, Canada)

We quantified the vertical and horizontal variation of chlorophyll a (Chl-a) to test how benthic filter feeders (dreissenid mussels), rooted macrophytes, and hydrodynamics influence phytoplankton distributions in a large lake (Lake Simcoe, Canada). Water column Chl-a did not differ significantly among sites of different depth, distance offshore, or rooted macrophyte biomass, but among the nearshore sites (7.5–10 m deep) it was higher where mussel biomass was greater. This counterintuitive result may be explained by wind-driven horizontal circulation during our specific study periods together with the patchy distribution of the mussels in the lake. Information on mixing depths, vertical eddy diffusivity, and mussel biomass was used to predict when and where the grazing pressure of mussels would likely deplete near-bottom phytoplankton. Chl-a depletion was frequently predicted at sites with moderate or high mussel biomass and sufficient thermal stratification to impede vertical mixing but never at sites without thermal stratification. Observations were consistent with predictions in most cases. The results suggested that mussels at depths of 7.5–15 m (a depth range of generally high mussel biomass in the lake) may frequently suffer food limitation due to near-bottom depletion during the early and middle stratified season. A deep Chl-a maximum was documented and may be important for mussel nutrition at such times

Delving deeper: metabolic processes in the metalimnion of stratified lakes

Many lakes exhibit seasonal stratification, during which they develop strong thermal and chemical gradients. An expansion of depth-integrated monitoring programs has provided insight into the importance of organic carbon processing that occurs below the upper mixed layer. However, the chemical and physical drivers of metabolism and metabolic coupling remain unresolved, especially in the metalimnion. In this depth zone, sharp gradients in key resources such as light and temperature co-occur with dynamic physical conditions that influence metabolic processes directly and simultaneously hamper the accurate tracing of biological activity. We evaluated the drivers of metalimnetic metabolism and its associated uncertainty across 10 stratified lakes in Europe and North America. We hypothesized that the metalimnion would contribute highly to whole-lake functioning in clear oligotrophic lakes, and that metabolic rates would be highly variable in unstable polymictic lakes. Depth-integrated rates of gross primary production (GPP) and ecosystem respiration (ER) were modelled from diel dissolved oxygen curves using a Bayesian approach. Metabolic estimates were more uncertain below the epilimnion, but uncertainty was not consistently related to lake morphology or mixing regime. Metalimnetic rates exhibited high day-to-day variability in all trophic states, with the metalimnetic contribution to daily whole-lake GPP and ER ranging from 0% to 87% and<1% to 92%, respectively. Nonetheless, the metalimnion of low-nutrient lakes contributed strongly to whole-lake metabolism on average, driven by a col- linear combination of highlight, low surface-water phosphorous concentration and high metalimnetic volume. Consequently, a single-sensor approach does not necessarily reflect whole-ecosystem carbon dynamics in stratified lakes

A framework for ensemble modelling of climate change impacts on lakes worldwide : the ISIMIP Lake Sector

Empirical evidence demonstrates that lakes and reservoirs are warming across the globe. Consequently, there is an increased need to project future changes in lake thermal structure and resulting changes in lake biogeochemistry in order to plan for the likely impacts. Previous studies of the impacts of climate change on lakes have often relied on a single model forced with limited scenario-driven projections of future climate for a relatively small number of lakes. As a result, our understanding of the effects of climate change on lakes is fragmentary, based on scattered studies using different data sources and modelling protocols, and mainly focused on individual lakes or lake regions. This has precluded identification of the main impacts of climate change on lakes at global and regional scales and has likely contributed to the lack of lake water quality considerations in policy-relevant documents, such as the Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC). Here, we describe a simulation protocol developed by the Lake Sector of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) for simulating climate change impacts on lakes using an ensemble of lake models and climate change scenarios for ISIMIP phases 2 and 3. The protocol prescribes lake simulations driven by climate forcing from gridded observations and different Earth system models under various representative greenhouse gas concentration pathways (RCPs), all consistently bias-corrected on a 0.5 degrees x 0.5 degrees global grid. In ISIMIP phase 2, 11 lake models were forced with these data to project the thermal structure of 62 well-studied lakes where data were available for calibration under historical conditions, and using uncalibrated models for 17 500 lakes defined for all global grid cells containing lakes. In ISIMIP phase 3, this approach was expanded to consider more lakes, more models, and more processes. The ISIMIP Lake Sector is the largest international effort to project future water temperature, thermal structure, and ice phenology of lakes at local and global scales and paves the way for future simulations of the impacts of climate change on water quality and biogeochemistry in lakes.Peer reviewe

Effects of cooling and internal wave motions on gas transfer coefficients in a boreal lake

Peer reviewe

Activity-dependent degeneration of axotomized neuromuscular synapses in Wld(S) mice

AbstractActivity and disuse of synapses are thought to influence progression of several neurodegenerative diseases in which synaptic degeneration is an early sign. Here we tested whether stimulation or disuse renders neuromuscular synapses more or less vulnerable to degeneration, using axotomy as a robust trigger. We took advantage of the slow synaptic degeneration phenotype of axotomized neuromuscular junctions in flexor digitorum brevis (FDB) and deep lumbrical (DL) muscles of Wallerian degeneration-Slow (WldS) mutant mice. First, we maintained ex vivo FDB and DL nerve-muscle explants at 32°C for up to 48h. About 90% of fibers from WldS mice remained innervated, compared with about 36% in wild-type muscles at the 24-h checkpoint. Periodic high-frequency nerve stimulation (100Hz: 1s/100s) reduced synaptic protection in WldS preparations by about 50%. This effect was abolished in reduced Ca2+ solutions. Next, we assayed FDB and DL innervation after 7days of complete tetrodotoxin (TTX)-block of sciatic nerve conduction in vivo, followed by tibial nerve axotomy. Five days later, only about 9% of motor endplates remained innervated in the paralyzed muscles, compared with about 50% in 5day-axotomized muscles from saline-control-treated WldS mice with no conditioning nerve block. Finally, we gave mice access to running wheels for up to 4weeks prior to axotomy. Surprisingly, exercising WldS mice ad libitum for 4weeks increased about twofold the amount of subsequent axotomy-induced synaptic degeneration. Together, the data suggest that vulnerability of mature neuromuscular synapses to axotomy, a potent neurodegenerative trigger, may be enhanced bimodally, either by disuse or by hyperactivity