Investigating water movements around a shallow shipwreck in Big Tub Harbour of Lake Huron: Implications for managing and preserving underwater shipwrecks.

The Sweepstakes, in Fathom Five National Marine Park, is Ontario's most iconic shipwreck with over 100,000 visitors each summer. Continued exposure to water currents has directly and indirectly affected the integrity of the wreck and resulted in management interventions including efforts to stabilize the wreck and control vessel activity (both duration and speed). Despite these efforts, a scour ring is present in the sediment around the Sweepstakes, raising concerns regarding the prolonged stability of the wreck. An extensive series of field measurements were made during the summer of 2015 with the aim of differentiating between natural hydrological processes present at this site and human-derived water movements during the summer visitor season. There is a high-degree of natural current variability from processes as diverse as wind-induced surface gravity waves, internal gravity waves, and diurnal flows due to differential heating. Our results show that summer circulation driven by internal gravity waves derived from upwelling, surface waves, and differential heating was insignificant with respect to sediment resuspension and thus unlikely to produce the observed scour around the shipwreck. Scour is most likely caused by energetic winter storms, which should be a focus of future studies. While vessel induced currents were detectable at the shipwreck, they were no larger than the normal summer hydrodynamic variability, thus suggesting that management efforts continue to protect the site generally.

Biofilm growth on buoyant microplastics leads to changes in settling rates: Implications for microplastic retention in the Great Lakes.

Buoyant microplastic pollution disperses widely from sources via strong wind-driven water currents in lakes and oceans. This ability for dispersal depends critically upon the particle's density, which can change over time due to microbial growth (biofilm). This study quantifies biofilm-induced sinking rates of irregularly-shaped polypropylene granules (~125-2000 µm) via ex-situ experiments emulating a Great Lakes freshwater environment. Biofilm development increases particle density and lowers microplastic rise velocities, eventually causing sinking. We observed sinking for 100% of small and intermediate microplastics, and 95% of large microplastics. Under constant environmental conditions, sinking onset was observed sooner for smaller particles (~125-212 µm, 18 days) than for larger particles (~1000-2000 µm, 50 days). Differences in settling onset would lead to size-fractionation of particle sedimentation, whereby smaller particles are deposited closer to their sources relative to larger particles. Our study demonstrates a novel mechanism by which buoyant microplastics can selectively sink from the lake surface.

Telemetry-Determined Habitat Use Informs Multi-Species Habitat Management in an Urban Harbour.

Widespread human development has led to impairment of freshwater coastal wetlands and embayments, which provide critical and unique habitat for many freshwater fish species. This is particularly evident in the Laurentian Great Lakes, where such habitats have been severely altered over the last century as a result of industrial activities, urbanization, dredging and infilling. In Toronto Harbour, extensive restoration efforts have been directed towards improving the amount and quality of aquatic habitat, especially for fishes. To evaluate the effectiveness of this restoration work, use of the restored area by both target species and the fish community as a whole must be assessed. Individuals from four species (Common Carp, Largemouth Bass, Northern Pike and Yellow Perch) were tagged and tracked continuously for 1 year using an acoustic telemetry array in Toronto Harbour area of Lake Ontario. Daily site fidelity was estimated using a mixed-effects logistic regression model. Daily site fidelity was influenced by habitat restoration and its interactions with species and body size, as well as season and its interactions with species and body size. Daily site fidelity was higher in restored sites compared to non-restored sites for Yellow Perch and Northern Pike, but lower for Largemouth Bass and Common Carp. For all species, daily site fidelity estimates were highest during the summer and lowest during autumn. The approach used here has merit for evaluating restoration success and informing future habitat management activities. Creating diverse habitats that serve multiple functions and species are more desirable than single-function-oriented or single-species-oriented designs.

The interaction of large amplitude internal seiches with a shallow sloping lakebed: observations of benthic turbulence in Lake Simcoe, Ontario, Canada.

Observations of the interactions of large amplitude internal seiches with the sloping boundary of Lake Simcoe, Canada show a pronounced asymmetry between up- and downwelling. Data were obtained during a 42-day period in late summer with an ADCP and an array of four thermistor chains located in a 5 km line at the depths where the thermocline intersects the shallow slope of the lakebed. The thermocline is located at depths of 12-14 m during the strongly stratified period of late summer. During periods of strong westerly winds the thermocline is deflected as much as 8 m vertically and interacts directly with the lakebed at depth between 14-18 m. When the thermocline was rising at the boundary, the stratification resembles a turbulent bore that propagates up the sloping lakebed with a speed of 0.05-0.15 m s(-1) and a Froude number close to unity. There were strong temperature overturns associated with the abrupt changes in temperature across the bore. Based on the size of overturns in the near bed stratification, we show that the inferred turbulent diffusivity varies by up to two orders of magnitude between up- and downwellings. When the thermocline was rising, estimates of turbulent diffusivity were high with KZ ∼10(-4) m(2)s(-1), whereas during downwelling events the near-bed stratification was greatly increased and the turbulence was reduced. This asymmetry is consistent with previous field observations and underlines the importance of shear-induced convection in benthic bottom boundary layers of stratified lakes.

The dilution and dispersion of ballast water discharged into Goderich Harbor.

Observations are presented on dilution and dispersion rates of ballast water discharged under normal operational conditions at the semi-enclosed port of Goderich, Ontario. The ballast water was tagged with Rhodamine-WT dye and microscopic magnetically-attractive tracer particles. Maximum concentrations of dye immediately after discharge were diluted to 1-5% of initial ballast tank concentrations, and within 3 days had decreased to less than 0.1% of initial concentrations. Inside the harbor, there was 10-20% of the ballast water still present after 2 days, consistent with a flushing rate of 0.8-1.15 day(-1). Magnetic particles were collected up to 7.5km outside the harbor after one day, consistent with a dilution factor of order 10(5) outside the harbor. The results of this study are discussed in the context of ballast water discharge standards proposed by the International Maritime Organization to minimize the introduction of aquatic nonindigenous species through ships' ballast water and sediments.