Gerald S. Miller

Studies of Large-Scale Currents in Lake Erie, 1979–80☆

Abstract Currents and water temperatures were recorded at a large-scale grid of fixed moorings in Lake Erie from May 1979 through June 1980. Currents measured in the lower half of the central basin water column were mostly return flows (beneath the surface wind drift) driven by the surface pressure gradient. Often observed was a complex system of Lake Erie circulation gyres as predicted by models. Another common occurrence was for one of the central basin gyres to become dominant and envelop the whole basin in either uniform clockwise or anticlockwise flow. It is not fully certain why one of the circulation cells grows as opposed to the others, but the curl of the wind stress had influence. The currents were more barotropic than predicted by full Ekman layer current models. Tidal-like currents driven by the longitudinal seiches of Lake Erie dominate the island-filled passages between the western and central Lake Erie basins, with currents across the whole island chain very closely in phase. Processes of hypolimnion volume entrainment are suggested from the central basin temperature recordings. Large volume water exchanges between the central and eastern basins occurred after the water mass in the vicinity of the shallow ridge that separates them had become unstratified. These and other topics are discussed as the large data set generated from the experiment is explored.

Hatching, Dispersal, and Bathymetric Distribution of Age-0 Wild Lake Trout at the Gull Island Shoal Complex, Lake Superior

We studied age-0 lake trout (Salvelinus namaycush) associated with spawning and nursery areas of the Gull Island Shoal complex in western Lake Superior. Post-emergent age-0 lake trout were captured on rocky spawning substrate with a 3-m beam trawl and at the nursery area with a bottom trawl from June to September 1990 and June to August 1991. Catch data suggested that age-0 lake trout move distances of 7–11 km to the nursery area over a 3-month period. Water currents, measured at Gull Island Shoal, may be a part of the transport mechanism. Examination of daily-growth increments on the sagittae and back-calculation from the date of capture revealed that most fish hatched between 6 June and 19 July in 1990 and between 30 April and 30 May in 1991. The duration of the hatch was 100 days in 1990 and 120 days in 1991, and the estimated incubation period is about 7 months for lake trout eggs at this site. Similar hatch-date distributions of age-0 captured on different sampling dates suggested that natural mortality was low.

Currents and Temperatures in Green Bay, Lake Michigan☆

Abstract Current velocities and water temperatures were measured in the four main passages between Green Bay and Lake Michigan and at several sites within the bay during summer and fall 1977. Monthly resultant currents indicate there is anticlockwise circulation in the bay during dominant southwesterly wind and a reversal of this pattern during episodes of northeasterly wind. It is common for two layers to flow through the mouth of the bay in opposite directions during the stratified season. Cold hypolimnetic lake water entering through the mouth and extending far into the bay maintains stratification and promotes flushing. The effects of resonance of forced and free long wave disturbances are prominent in current records; these oscillations are coherent and in phase across the mouth.

Observation of Ekman Veering at the Bottom of Lake Michigan

An experiment in 100 m of water off the east coast of Lake Michigan during 1984 provided continuous current velocity recordings at four levels located from 1 m to 9 m above the lake bottom and also at 50 m. Near-inertial-period current oscillations were prominent and superimposed on longer-period velocity variations. Current speed at 1 m elevation varied from less than the threshold speed of the Savonious rotor sensor to 13 cm s−1. Profiles of current speed were close to logarithmic during many episodes of measurable currents over a 4-month-long recording interval. Over 50% of hourly-averaged profiles were in the nearly-logarithmic category (R > 0.987) during the month of September 1984. Veering averaging 11° was observed between low-pass-filtered current velocities measured at elevations of 1 m and at 9 m. Boundary layer theories developed from ocean studies are surveyed and the Lake Michigan data are examined from that perspective.

Mysis Vertical Migration in Grand Traverse Bay, Lake Michigan, Observed by an Acoustic Doppler Current Profiler

The acoustic return signal from Acoustic Doppler Current Profilers (ADCP) moored in Traverse Bay during a 90-day summer period showed a rapid 5-10 dB increase about 1/2 hour after sunset and a similar decrease 1/2 hour before sunrise. The pattern is characteristic of zooplankton diel vertical migration, most likely Mysis relicta. These are the first reported observations of freshwater invertibrate migrations using ADCP backscatter. A 15–20 m thick sound scattering layer also persisted throughout the summer. This layer, constrained between the 6°C and 10°C isotherms, generally followed the internal thermocline fluctuations. These backscatter data demonstrate that determining characteristics of diel migration, and monitoring zooplankton temporal and spatial variability are possible using ADCPs.

Low-Frequency Water Volume Transport Through the Midsection of Green Bay, Lake Michigan, Calculated From Current and Temperature Observations

Abstract Moorings with current meters and temperature recorders were deployed along a mid-bay transect on either side of Chambers Island in Green Bay from September 1988 to October 1989. Daily transport estimates calculated from the current, temperature, and wind data, show temporal and spatial variability. Net summer transport during stratification west of Chambers Island was out of the lower bay (350 m 3 s −1 ) in the epilimnion, into the lower bay (900 m 3 s −l ) in the hypolimnion, and out of the lower bay (320 m 3 s −1 ) through the passage east of the island. The residence time for the lower bay is reduced to less than 1 year using the above transport compared to a water balance estimate of over 3 years. During the mid-September to May isothermal period, a horizontal transport gradient existed. Under the solid ice cover of winter, transport variability was significantly reduced and was uniformly out of the lower bay (500 m 3 s −1 ). Fluctuations in daily transport were often large (up to 2 × 10 4 m 3 s −1 ). The temporal and spatial variability of the water volume transport suggests that material mass fluxes between lower and upper Green Bay will be similarly dependent.