Spatial and temporal variation in distribution of larval lake whitefish in eastern Lake Ontario: Signs of recovery?

The lake whitefish (Coregonus clupeaformis) is one of the native Lake Ontario fishes that declined severely over the past century. Recent evidence of larval lake whitefish production in a historic spawning area (Chaumont Bay) might signal a recovery of this species in New York waters. We surveyed coastal and open water areas to evaluate densities and estimate total abundance of larval lake whitefish in Chaumont Bay. Other historic spawning areas and embayments with appropriate spawning and nursery habitat were also surveyed, but only a few larvae were found outside of Chaumont Bay. Lake whitefish larvae were found in every embayment sampled within Chaumont Bay, with larval densities of nearly 600/1000 m² in some samples. Greatest abundances occurred in the northern sectors and near the mouth of the bay. Open water densities were generally less than half that of nearshore sites. The total bay-wide estimate for 2005 was approximately 644,000 lake whitefish larvae, but dropped to 230,000–400,000 in 2006 and 2007, respectively. Mean larval growth rates (0.36 mm/day) did not differ by year, but were consistently higher in early May than in late April. Lake whitefish production in Chaumont Bay is encouraging for this species, but the cause and persistence of the decline after 2005 can be determined only by continued monitoring. Other possible bottlenecks of survival may exist at juvenile and adult stages and could significantly affect recruitment dynamics. This species is sensitive to normal climatic fluctuations and increased variability associated with global climatic change could make winter nursery conditions unfavorable for this species.     

What role does stratification play during winter in wind-induced exchange between the multi-depth basins of a large lake (Lake Geneva)?

Detailed studies of wind-driven interbasin exchange in lakes, focusing on the underlying driving forces and how they are affected by stratification, are presently lacking. We therefore investigated how stratification modifies wind-induced exchange between the Petit Lac (PL) (depth 75 m) and Grand Lac (GL) (depth 309 m) basins of Lake Geneva in winter, using field observations, 3D hydrodynamic modeling and particle tracking. Early, weakly-stratified (December) and late, fully-mixed (March) winter conditions in the PL were compared for a typical, strong along-axis wind forcing. During early winter, two-layer exchange develops between the basins, with downwind surface outflow into the GL balanced by intense bottom inflow of deep, cold hypolimnetic GL water into the PL which is enhanced by baroclinic pressure gradients caused by upwelling in the GL. Furthermore, the transversal water-level setup generates barotropic pressure gradients that balance Coriolis force acting on the outflow. This produces unidirectional along-wind epilimnion currents that strengthen interbasin exchange. In late winter, with the thermocline deeper than the PL bottom, upwelling in the GL does not reach the confluence and baroclinicity plays no role, resulting in weaker exchange currents with a depth-veering structure in the upper layers due to Coriolis force. In late winter, interbasin exchange decreases by 50 %, is more local, affects only waters near the confluence, and is more horizontal, with no deepwater upwelling from the GL. Our results suggest that prolonged winter stratification due to global warming will make wind-induced hypolimnetic interbasin-upwelling an increasingly important deepwater renewal process in large multi-depth basin lakes.     

The role of groundwater in loading of nutrients to a restricted bay in a Precambrian Shield lake. Part 1. – Conceptual model and field observations

Previous studies of the nutrient budgets of lakes in Precambrian Shield regions have generally ignored the role of groundwater as a source or sink (influx or efflux) of nutrients. This paper and its companion (Part 2) address this science gap by probing the role of groundwater in the nutrient balance for a restricted bay of a Precambrian Shield lake that has undergone extensive shoreline development. In Part 1, we introduce a conceptual model of the relevant processes, including the role of groundwater. A cottage study site is described, concentrations of nutrients and septic tracers (artificial sweeteners), water levels and other data are reported. This includes data from samples of the septic wastewater plume in groundwater at the cottage, and samples from the bay. The artificial sweetener data indicate seepage of septic plumes to the bay, but significant attenuation of septic-derived nutrients in the subsurface. These data are used for numerical modeling in Part 2.     

The role of groundwater in loading of nutrients to a restricted bay in a Precambrian Shield lake Part 2. – Numerical modeling

Previous modeling of hydrologic and nutrient budgets of lakes in Precambrian Shield regions have generally ignored groundwater as a source of water and supplier or sink of nutrients. This paper and its companion (Part 1) address this science gap by probing the role of groundwater in the nutrient balance for a restricted bay of a Precambrian Shield lake (Lake of the Woods) that experienced extensive shoreline development in the 1990s. Based on field data (Part 1), we applied a 3D reactive transport model (FEFLOW) to simulate the fate of nutrients in a cottage septic plume in groundwater and seepage into the bay. Corresponding simulations are for chloride and septic tracers in the plume, and 1D modeling of the mass balance of one septic tracer (acesulfame) in the bay. Our results suggest that, before they reach the lake, significant portions of the septic plume contaminants are attenuated by various processes, including uptake by transpiring plants. Extrapolating, we estimate that, for annual fluxes to the bay, groundwater seepage contributes ~1–3% of the phosphorus (P) and ~2–5% of the dissolved inorganic nitrogen (DIN). The results suggest that these groundwater nutrient fluxes to the bay are derived mainly from natural background sources. In contrast, cottage septic plumes contribute a quarter of the groundwater flux of DIN to Poplar Bay, while the septic contribution of P to the bay is negligible. We estimate that the largest sources of P and DIN fluxes to Poplar Bay are influx from the open lake and atmospheric deposition.     

Is fish biomass controlled by abiotic or biotic factors? Results of long-term monitoring in a large eutrophic lake

Relationships between biomass and ecological factors including trophic interactions were examined to understand the dynamics of six fish species in Lake Võrtsjärv, a large shallow eutrophic lake located in Estonia (north-eastern Europe). The database contained initially 31 predictive variables that were monitored in situ for nearly forty years. The strongest predictive variables were selected by three parallel approaches: single correlation (Pearson), a multivariate method (Co-inertia analyses), and a machine learning algorithm (Random Forests), followed by a Generalized Least Squares model to determine meaningful relationships with fish biomass. Models with both additive and interactive effects were constructed. The results revealed that the indicators of degraded ecological conditions (high cyanobacteria biomass and their proportion in total phytoplankton, high summer temperature, high nutrient concentration) were negatively correlated to fish biomass. Benthic macroinvertebrates and other biotic predictors (biomass of specific fish prey and predators) were also important contributors to fish biomass dynamics. Together, abiotic and biotic factors explained 40–60% of the variance of fish biomass, depending of the species. Our findings suggest that both abiotic and biotic factors control fish biomass changes in this eutrophic lake.     

Long-term changes in hypolimnetic dissolved oxygen in a large lake: Effects of invasive mussels,eutrophication and climate change on Lake Simcoe, 1980–2012

Lake Simcoe has been influenced by multiple environmental drivers over the past decades, especially by reductions in phosphorus (P) loading, climate change, and invasive species such as dreissenid mussels (DM) which became firmly established in 1996. We examined the cumulative impact of these drivers on the volume-weighted hypolimnetic dissolved oxygen concentration (VWHDO) below 18?m at station K42 in Kempenfelt Bay during ice-free seasons from 1980 to 2012. Hypolimnetic DO depletion began in early spring when thermal stratification was observable but weak and continued throughout the ice-free season until cooling sufficiently lowered water column stability. In comparison to the pre-DM invasion period (1980–1995), mean annual VWHDO_init was 2.4?mg?L^?1 higher in the post-DM period (1996–2012), VWHDO_min was 1.54?mg?L^?1 higher and the mean duration of the depletion period (L) was 16?days longer. Mean DO depletion rate (DR) and temperature adjusted DO depletion rate (DR_adj) were slightly lower (7% and 5%, respectively) after 1996. P controls and DM had a positive effect on VWHDO, presumably by lowering productivity and diverting organic matter away from the hypolimnon. However, longer L apparently offset improvements in VWHDO_min. If lengthening of L associated with regional warming continues, then additional efforts to reduce P loads will be necessary to achieve the goal of maintaining VWHDO_min above the target of 7?mg?O₂?L^?1 throughout the summer and fall.     

Relationship between lake trout spawning,embryonic survival,and currents: A case of bet hedging in the face of environmental stochasticity?

Lake trout, Salvelinus namaycush, spawning in the Great Lakes occurs primarily on cobble substrate at relatively shallow water depths that can experience strong water currents. Strong currents may limit embryonic survival by damaging or displacing eggs, but may also reduce the accumulation of fine material and limit foraging by potential egg predators. To better understand the importance of currents, we evaluated the role of currents in spawning habitat selection, egg density and survival, and egg predator density at a spawning reef in Lake Champlain (USA). Most spawning occurred one week after the largest storm event associated with the strongest currents and greatest upwelling. Highest spawning activity was associated with a relatively shallow part of the reef that had the highest current velocity and greatest potential for egg displacement. Within the interstices, the survival of naturally deposited eggs was unrelated to the concurrent loss of artificial eggs. We propose that the reproductive strategy of spawning on shallow areas of a reef that have the highest current velocity and high potential for egg loss represents a type of bet hedging to optimize survival of those embryos that remain within interstices. This strategy may have evolved in response to environmental stochasticity that resulted in higher egg survival.     

Do wood fibers from a pulp mill affect the distribution of total and methyl mercury in river sediments?

The St. Lawrence River near Cornwall Ontario is affected by industrial mercury contamination of sediments and biota. It has been suggested that pulp and paper mill effluents may stimulate bacterial mercury methylation in these sediments, leading to contamination of aquatic biota. To test this hypothesis, we examined sediment–porewater dynamics of total mercury (THg) and methyl mercury (MeHg) at a site with high concentrations of wood fibers from a pulp and paper mill effluent and a nearby reference site with low wood fiber content. Dissolved phase THg (THg_diss) and MeHg (MeHg_diss) in porewater profiles showed that 38 ± 30.9% (SD) of THg in porewaters was in the methylated form regardless of wood fiber content. MeHg_diss and THg_diss concentrations were homogeneous between porewater and overlying water, indicating (a) that there is minimal net diffusion of MeHg_diss and THg_diss and (b) that redox-dependent processes such as sulfate reduction and Fe reduction were not associated with MeHg_diss distribution in these sediment profiles. MeHg and THg in solid phase showed coincident subsurface peaks at depths > 40 cm suggesting either that historical deposits of MeHg on particles (MeHg_(p)) are preserved in deep sediments, or that Hg methylation is active in deep sediments.     

Trends in the distribution and abundance of Hexagenia spp. in Saginaw Bay,Lake Huron, 1954–2012: Moving towards recovery?

Since at least the 1940s, multiple anthropogenic disturbances to the Laurentian Great Lakes have had detrimental effects on benthic habitats and biota including decimating the environmentally sensitive burrowing mayfly genus Hexagenia around the mid-1950s. While remediation efforts have facilitated recovery of some populations, benthic surveys in Saginaw Bay, Lake Huron in the last 50 years have only occasionally discovered Hexagenia nymphs. Recently, adult Hexagenia swarms have been reported near the bay; therefore, we corroborated the local presence of Hexagenia adults and evaluated the current status of Saginaw Bay Hexagenia nymphs. We quantified adults during mayfly emergence events in 2010 at three Tawas City, Michigan, USA area locations, and found > 17 Hexagenia/m²/site. We quantified nymphs from Ponar grab samples collected at 57 sites in Saginaw Bay between 2009 and 2012, and found 1.5 nymphs/m² overall with nymphs present at 15.8% of sites sampled, their greatest documented distribution in Saginaw Bay since 1956. Additionally, we mapped bay sediment composition and related sampling site abiotic conditions with both Hexagenia presence and abundance using Zero-Inflated Poisson regression. Model results indicate that the probability of observed Hexagenia absence being true absence is positively related to both sediment sandiness and surficial dissolved oxygen concentration while Hexagenia abundance is greatest where surficial temperatures are ~ 18.6 °C and is also related to sediment sand content. The documentation of nearby adults and in bay nymphs may indicate the beginning of a Hexagenia return to Saginaw Bay, and, therefore, a possible improvement of the ecosystem's benthic health.     

The distribution,density, and biomass of the zebra mussel (Dreissena polymorpha) on natural substrates in Lake Winnipeg 2017–2019

The distribution, density, biomass and size-structure of the zebra mussel (Dreissena polymorpha) population in Lake Winnipeg were examined between 2017 and 2019. Zebra mussels have colonized most of the available hard substrate in the south basin and Narrows region, but colonization of the north basin remains low at present, even on suitable substrate. Numerical densities and shell free biomass peaked at 5530 ± 953 m^?2 and 64.7 ± 57.9 g shell free dry mass m^?2 respectively. The distribution appeared to be strongly limited by substrate type and availability, with further limitations on the distribution imposed by physical disturbance in shallow waters and unsuitable substrate in deeper areas of the lake. Zebra mussels <1 year old dominated the populations, and individuals >18 mm were exceedingly rare. Poor recruitment was observed at sites along the eastern side of the south basin compared to elsewhere in the lake. The proximate causes of these differences in colonization success and recruitment are not clear, but may be in part due to heterogeneous patterns of key physico-chemical environmental conditions such as calcium concentrations required for successful development of juvenile mussels and colder water temperatures in the north basin. This study provides a baseline of information on which to track further expansion of zebra mussels in Lake Winnipeg and assist efforts to develop an understanding of how zebra mussels may affect the ecology of Lake Winnipeg.     

What role for law in achieving transboundary drainage basin security?--the development and testing of the Legal Assessment Model (LAM) for transboundary watercourse states.

The beneficial use of the world's transboundary waters raises difficult issues for drainage basin security on most parts of the globe. International law provides that each transboundary watercourse State is entitled to, and obliged to ensure, an "equitable and reasonable use" of these shared waters. The IWLRI developed and tested a Legal Assessment Model (LAM) through the work of interdisciplinary teams working in three different transboundary situations--China (upstream), Mozambique (downstream) and Palestine (shared groundwater). The LAM provides a tool for transboundary watercourse States to use in the preparation of their national water strategy for use at the national and international levels. The model should now be tested at the basin level, with a view to assisting to accomplish the peaceful and rational use of transboundary waters in line with the governing rule of international law and thereby to facilitate the overall policy objective of drainage basin security.     

The Migratory Behaviour and Fallback Rate of Landlocked Atlantic Salmon (Salmo salar) in a Regulated River: does Timing Matter?

The behavior of early (June–July) and late (August–September) migrating, adult Atlantic salmon, in The River Klarälven, Sweden, was analyzed using radio telemetry. River Klarälven is a regulated river without functioning fishways, instead upstream migrating salmon are trapped and trucked past eight hydropower plants before released back to the river. We distinguished two parts of the spawning migration, that is, one part being the migration from the place where the fish was released to the spawning grounds. The other part was a holding phase on the spawning grounds with little or no movements before spawning. The late salmon spent less of their total time on holding, 36.2%, and more on migration, 63.8%, compared with early migrating salmon, which distributed their time rather evenly between migration, 47.5%, and holding, 52.5%. In total, early salmon used 30% more time migrating and 156% more time holding than late salmon. Some Atlantic salmon (Salmo salar) fell back over the hydropower plant after release and got excluded from spawning. The fallback rates of transported, tagged spawners were higher in the early than in the late group in both years. The fallback rate in 2012 was 42.8% of the early group and 15.1% in the late. In 2013, there were 51.7 % fallbacks in the early group and 3.4% in the late. The salmon fell back on average 9 days after being released in 2012 and 16 days in 2013. A high mean daily discharge on the day of release increased the probability of becoming a fallback. Copyright © 2016 John Wiley & Sons, Ltd.