Fig 1.
The geographical location and the views of the Sweepstakes in Big Tub Harbour of Fathom Five National Marine Park near Tobermory, Ontario.
(a) A map of Lake Huron and Georgian Bay, Fathom Five National Marine Park, and Big Tub Harbour. (b) Aerial imagery of Big Tub Harbour. (c) Close up of western end of Big Tub Harbour, showing the two shipwrecks, namely, the Sweepstakes (1867–1885) and located to the south, the City of Grand Rapids (1879–1907). A ring of erosion is visible around the Sweepstake where no aquatic vegetation is growing and sand is exposed. While the Sweepstakes is a fully intact underwater shipwreck, the City of Grand Rapids has only the remains of its broken hull and machinery. (d) A sketch of the Sweepstakes. The prow faces south while the portside faces east towards Georgian Bay. (e) An underwater photograph of portside of the Sweepstakes, with the tripod visible behind diver. (f) An underwater photograph of the prow of the Sweepstakes, the yellow acoustic Doppler profiler (ADP) is visible in background on the bed. Reprinted from SWOOP 2010 imagery under a CC BY license with permission from Parks Canada, original copyright 2010, the Queen’s Printer for Ontario.
Fig 2.
Composition of harbour bed sediments and aquatic vegetation in Big Tub Harbour.
(a) Sediments and (b) aquatic vegetation. The location of the two shipwrecks are shown as silhouettes. The classification provides a coarse sediment structure on the harbour bed and general distribution of submerged aquatic vegetation. Around the two shipwrecks at the head of Big Tub Harbour, the bed is dominated by silty sand and Chara sp. vegetation.
Fig 3.
Bathymetry of Big Tub Harbour and geographical locations of the field instruments relative to the Sweepstakes.
The Sweepstakes wreck (black silhouette) is visible with the prow pointing to the southeast. The numbers correspond to individual instruments as follows: 1: Float 1 (one thermistor), 2: Float 3 (two thermistor loggers), 3: Float 4 (two thermistor loggers), 4: Float 5 (two thermistor loggers), 5: Marine Operations Base (MOB) chain (13 thermistor loggers), 6: HR-ADCP 7: AWAC (also a pressure sensor) 8: ADP and downward looking camera), 9: On shore surface camera. Bathymetry contours in a) and b) are 1 m.
Table 1.
Location, measured water quality parameter, and observation frequency of instruments deployed in Big Tub Harbour, 2015.
Fig 4.
Water temperature time series from 29 June (DOY 180) to 18 August (DOY 230), 2015.
(a) Contour plot of water temperature at MOB (location 5 in Fig 3). Note the strong upwelling signals in the deeper water, the strong daily warming signal near the surface, and the general warming trend as time progresses. (b) Temperature measurements at Float 3 (closest chain to the east of Sweepstakes). (c) Temperature measurements at Float 4. (d) Temperature measurements at Float 5. (e) Temperature measurements at Float 1. Note that the larger fluctuations in temperature at the lower thermistor (at 0.5 m from the harbour bed) are observed in all floats due to cold water upwelling.
Fig 5.
Spectral density diagram for the de-trended bottom temperature (0.5 m from the harbour bed) acquired at float 3.
The record shows significant periods at 12.41 h (semidiurnal) and at 6.15 h. A similar behaviour is seen in all floats in the vicinity of the Sweepstakes shipwreck.
Fig 6.
Continuous Wavelet Transform (CWT) of the temperatures obtained at the bottom thermistors (0.5 m above the harbour bed).
(a) float 1, (b) float 3, (c) float 4, and (d) float 5. CWT expands the time series into time-frequency space. The record shows four distinct upwelling events on DOY 198, 205, 215, and 226 on all temperature records.
Fig 7.
Histogram analyses of bottom currents with the presence and absence of upwelling events.
(a) ADP located at the prow (b) AWAC located at the portside of the shipwreck, and (c) HR-ADCP located at the portside of the shipwreck. The y-axis value is the normalized frequency of speed occurrences between 0–1. Blue color denotes the bottom speeds during non-upwelling events and the red color represents the speeds of the bottom water currents during upwelling periods. The upwelling events are observed on DOY 198, 205, 215, and 226 (See Figs 4 and 6 for visualization of upwelling events).
Table 2.
Bottom (1.5 m from bed) water current speed statistics during upwelling and non-upwelling periods in the vicinity of the Sweepstakes.
Fig 8.
The east-west and north-south velocities up to 1.5 m from the harbour bed.
(a) The east-west velocities extracted by ADP which is located at the prow, (b) The north-south velocities extracted by ADP (c) The east-west velocities extracted by AWAC on the side, and (d) The north-south velocities extracted by AWAC (e) The east-west velocities extracted by HR-ADCP located closest to the side of the shipwreck. (f) The north-south velocities extracted by HR-ADCP. The oscillatory motion in velocity distribution shows a barotropic flow (almost no vertical velocity gradients) in the bottom water column.
Fig 9.
The depth-averaged current speeds up to 1.5m from the harbour bed.
(a) ADP which is located at the prow, (b) AWAC on the side, and (c) HR-ADCP located closest to the side of the shipwreck (note the different scale on y-axis). The mean speed nearest to the shipwreck (HR-ADCP, subplot c) but located on the side was ~1 cm s-1 while the measured mean speed approximately 5 m from the side (AWAC, subplot b) is ~9 cm s-1. Measured mean speed at the prow was ~8 cm s-1. The minimum speed required to initiate suspended load transport is approximately 27 cm s-1.
Fig 10.
Histogram analyses of currents with the presence and absence of boats.
(a) ADP located at the prow (b) AWAC located at the side of the shipwreck, and (c) HR-ADCP located at the side of the shipwreck. Normalized frequency on the y-axis is the probability of occurrences which varies between 0–1. The blue color denotes the velocities at the times that the tour boats were absent while red color represents the water currents at the times that the boats were present. The tour boats were allowed from 9:00 EST to 16:00 EST from 16 June 2015 to 3 September 2015. In almost all cases water velocities are less than predicted erosion threshold of 27 cm s-1, the fastest observed currents are when boats were absent in (a) and (b).
Table 3.
Bottom (1.5 m from bed) water current speed statistics during boat presence and absence periods in the vicinity of the Sweepstakes.
Fig 11.
Spectral density diagram for water pressure perturbations.
Measured at the HR-ADCP on the portside of the wreck (Location 6 in Fig 3). Note that the 4.2-minute period corresponds to the resonant frequency of the harbour.
Fig 12.
Comparison of the amplitude of pressure perturbations with wind and the presence of tour boats for 25 June 2015 to 3 July 2015.
(a) Wind velocity and direction (in azimuthal direction, 0 is north) at Tobermory Airport Weather Station (Hourly Data). (b) Maximum of observed pressure perturbations (obtained using high-pass filtered at 4 minutes). In panel (b), the green background areas indicate the presence of tour boats (based on the camera recorded data).
Fig 13.
Idealized streamline diagram of benthic water currents around the Sweepstakes.
Red five-pointed star estimates the stagnation point, where flow is zero. Blue six-pointed stars indicate location of the current meters in relation to the shipwreck. Solid lines with arrows are representative streamlines. Distance between streamlines represents relative flow velocity, with tighter spacing indicating faster flows. Dotted line represents the centerline. Stagnation point lines on centerline for symmetrical obstruction.