Cross‐shelf thermal variability in southern Lake Michigan during the stratified periods

Abstract

[1] Results from a field experiment in southern Lake Michigan are used to quantify the cross-shelf nearshore variability in Great Lakes temperatures during the stratified season. The experiment was conducted along the Indiana coast of southern Lake Michigan, with temperature and velocity moorings arranged in a cross-shelf transect that extended to approximately 20 km from shore (40 m depth). The field site is noteworthy because of its location at the end of a major axis of an elliptical Great Lake, the relatively mild bathymetric slope, and local shoreline orientation that is perpendicular relative to the dominant summer winds. Measurements demonstrate that the location of the thermocline-bottom intersection is highly variable, causing a wide zone of extreme thermal variability in the nearshore region with time scales of variability ranging from hours to months. Near-inertial internal Poincaré waves are shown to cause large thermocline excursions but primarily only during periods of elevated activity. Several full upwelling events were observed, but in general, they were brief, lasting only 1-2 days, and had very limited spatial extent (2.5 km or less). Nonetheless, the offshore extent of the upwelling front was shown to be reasonably estimated with a simple estimate of the cross-shelf transport caused by alongshore wind events. A persistent feature that determined the zone of elevated thermal variability (the thermocline-shelf intersection point) was the strongly tilted thermocline, which resulted in the thermocline being located very close to shore. No evidence was found to support the hypothesis that internal Kelvin waves affect thermal variability at the study location.

Dreissenid metabolism and ecosystem-scale effects as revealed by oxygen consumption

“…Following a three-hour acclimation exposure to the experimental temperature, mussels were transferred into triplicate 175 mL chambers and filled with temperature specific water from the aerated 4 L bottles. While three hours is a relatively short acclimation period, this time scale is similar to observations of swift temperature changes in the Lake Michigan nearshore zone (Troy et al, 2012).…”

“…If profundal mussels had been allowed a longer acclimation period, their respiration response to increasing temperatures may have been similar to the shallow morph response. Shallow morph mussels' greater temperature slope, particularly under short acclimation times, suggests adaptability to changing temperature conditions, an advantage in the nearshore where bottom temperatures can quickly change (Troy et al, 2012). These mussels appear to be metabolically active even at low temperatures, necessitating higher food maintenance requirements met by the relatively food-rich nearshore waters.…”

Modeling the effect of invasive quagga mussels on the spring phytoplankton bloom in Lake Michigan

“…Currents during the stratified period were dominated by oscillating currents at a near-inertial period (18 h at the latitude of the ADCP stations), a phenomenon that has been described by others (Beletsky et al, 2006) and is caused by internal Poincaré waves supported by the stratified water column (Troy et al, 2012). Simulation of bottom currents is specifically of interest due to their role in particle resuspension, formation of the benthic nepheloid layer (Hawley, 2004), and potential transport of food particles to benthic filter feeders.…”

Small eddies observed in Lake Superior using SAR and sea surface temperature imagery

“…Bottom slope directly contributes to the gradient strength, which is strongest where bottom slope is steepest (Bennington et al, 2010). Variability of nearshore temperature structure and circulation in stratified basins is due to wind driven motions of the thermocline including internal waves and upwelling and downwelling (Austin and Lentz, 2002;Troy et al, 2012).…”