General circulation of Lake Superior: Mean, variability, and trends from 1979 to 2006

110

To understand carbon and oxygen dynamics in sediments with deep oxygen penetration, we investigated eight locations (160-318-m depth) throughout Lake Superior. Despite the 2-4 weight percent organic carbon content, oxygen penetrated into the sediment by 3.5 to . 12 cm at all locations. Such deep penetration is explained by low sedimentation rates (0.01-0.04 cm yr 21 ), high solubility of oxygen in freshwater, and a shallow (, 2 cm) bioturbation zone. In response mainly to oxygen variations in the bottom waters, the sediment oxygen penetration varied seasonally by as much as several centimeters, suggesting that temporal variability in deeply oxygenated sediments may be greater than previously acknowledged. The oxygen uptake rates (4.4-7.7 mmol m 22 d 21 , average 6.1 mmol m 22 d 21 ) and carbon mineralization efficiency (, 90% of deposited carbon) were similar to those in marine hemipelagic and pelagic sediments of comparable sedimentation rates. The reactivity of organic carbon was found to decrease with age similarly to the power-law documented in marine environments. The burial flux of carbon into the deep sediment (0.7 mmol m 22 d 21 ) was 2.5% of the previously estimated primary production. Maximum volume-specific carbon degradation rates were 0.3-1.5 mmol cm 23 d 21 ; bioturbation coefficient near the sediment surface was 3-8 cm 2 yr 21 . These results indicate that carbon cycling in large freshwater systems conforms to many of the same trends as in marine systems.

Section: Discussionmentioning

confidence: 99%

Carbon mineralization and oxygen dynamics in sediments with deep oxygen penetration, Lake Superior

Crowe

Miklesh

et al. 2012

110

“…Water temperatures and wind speeds in Lake Superior have increased in the past decade, and ice coverage has decreased during the same period in the lake (Austin and Colman 2007;Bennington et al 2010). The increase in water temperature coupled with longer duration of the stratified season could increase the role of microbial processes in converting recently synthesized POC into DOC (Jiao et al 2010).…”

Section: Date Of Sample Collectionmentioning

confidence: 99%

Radiocarbon and stable carbon isotopic insights into provenance and cycling of carbon in Lake Superior

Zigah

Minor

Werne

et al. 2011

This study applies radiocarbon and stable carbon isotopic distributions to investigate carbon sources and cycling within Lake Superior. We report the radiocarbon (D 14 C) and stable carbon isotope (d 13 C) values and the carbon concentrations within dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in the lake's western basin water column. Samples were taken during spring mixing and late-summer thermal stratification over a 2-yr period (2007)(2008)(2009). Distinct processes operating in the surface (photosynthesis) and deep waters (sediment resuspension and pore-water intrusion) control the relative contribution of modern and ancient DOC and POC in the water column. The terrigenous carbon input to the open lake POC varied from 13% 6 4% during late summer stratification to 9% 6 3% during spring mixing, with most of the terrestrial carbon being 14 C-enriched (modern). The DIC reservoir cycles rapidly, with a bulk D 14 C DIC value that records atmospheric radiocarbon levels from 3 yr prior to sampling. The DOC pool recycles on a longer time scale than does the DIC, with a DOC residence time of # 60 yr. The suspended POC was in most cases older than co-occurring DOC, most likely as a result of resuspension of lake sediments.

“…As far as more modern data are concerned, compilations of year-round bathythermograph survey data were considered by both Bennett (1978) and Assel (1986), demonstrating the dimictic nature of Lake Superior and establishing the magnitude of the lake's heat budget. Currently, regular monitoring efforts of Superior's open waters include conductivity-temperature-depth (CTD) surveys conducted by the Environmental Protection Agency (EPA) in the late spring and summer since 1992, data from which have been used by modeling groups to ground-truth models (Bennington et al 2010;White et al 2012), and National Data Buoy Center surface-water temperature data, which have been used to study inter-annual variability in summer water temperatures (Austin and Colman 2007). However, neither of these data sets provides any winter temperature data.…”

mentioning

confidence: 99%

Winter thermal structure of Lake Superior

Titze

Austin

2014

Moorings equipped with thermistors that span the water column were deployed at up to seven locations throughout Lake Superior from October 2005 through May 2013. This year-round, multi-year, multi-location, full water-column record of the thermal structure reveals significant inter-annual and spatial variability in Lake Superior's winter heat content, thermocline depth, and phenology. There is a stark contrast in thermal structure between the cold, icy winter of 2009, during which strong negative stratification formed, and the much warmer winter of 2012, during which the stratification was much weaker. Significant inter-annual and spatial variability was also observed in ice cover, which influenced heat content. Ice cover significantly inhibits heat flux between the lake and the atmosphere, and spatial variability in ice cover translates into spatial variability in end-of-winter heat content. This is found to be preserved through the spring warming season, and is strongly correlated with variability in the timing of the onset of summer stratification, with regions that have warmer end-of-winter water columns stratifying earlier than regions with colder end-of-winter water-columns. Observed spring warming rates appear to depend not on mooring depth, but on regionally averaged depth; this suggests the existence of a lateral mixing mechanism.