Multiple Stable States in Arctic Lakes: Alteration of Mixing Dynamics

1. Frontal Systems Trigger Shifts in Lake Physics that Affect Ecosystem Function Multiple Stable States in Arctic Lakes: Alteration of Mixing Dynamics Sally MacIntyre and Jonathan P. Fram Marine Science Institute, University of California at Santa Barbara Introduction Are Arctic Lakes Transitioning Between or to New Stable States? Arctic Lakes Biological Consequences of Fronts and resulting Low Lake Numbers • Is their heat content increasing? • What is the role of frontal systems in their mixing dynamics? • Consequences of Increased Heating of Lakes • Generally it is assumed that for stratified lakes increased stratification results with concomitant reduction in vertical mixing and transport across the thermocline. • For lakes whose productivity is supported by a flux of nutrients from the hypolimnion, a decrease in primary production may result. • Consequences of Increased Heating of Lakes • For relatively high Kz of 10-5 m2 s-1 • L = 10 m, time scale is 100 days. • L = 4 m, timescale is 18 days. • L = 2 m, timescale is 5 days. • Thus, processes which thicken or thin the thermocline will affect connectivity between the upper and lower waters of a lake and alter the probability of increased primary productivity due to nutrient flux from mixing. • Processes that Induce Mixing • Upper mixed layer: wind and heat loss contribute • Thermocline and hypolimnion: wind which induces internal waves that become unstable resulting in turbulence. Sally, why not include convection? • The Role of Frontal Systems • Increased winds occur with the passage of fronts. • Cold fronts induce heat loss from lakes. • The frequency of transitions between warm and cold fronts will affect heat storage and turbulence production in the upper mixed layer and stratified waters below. • The frequency of fronts systems in this region may be lower during warm than during cool summers. W< 10 Mixing – at ice off and in summer Biotic and abiotic processes Layered communities temporary W > 10 No mixing – at ice off and/or in summer Biotic processes lead to all vertical transports Layered communities develop • Nutrients are mixed from the lake bottom. • Cold Summers - Connectivity between upper and lower water column is high so increased growth may result. • Warm Summers – Connectivity is low. • How Can We Determine Impacts of Climate and Land Use Changes? • Comparative Studies • Lakes scaled by dimensionless indices to determine if dynamical processes are similar • Place in context of regional and large scale atmospheric forcing • What Causes Turbulence? • How does turbulence vary with changing stratification, changing wind forcing, and with lakes of different sizes and over time? • How can we put our understanding of mixing processes into the context of changing weather patterns in the Arctic? • Variations in Seasonal Mixing Dynamics • Winter or Monsoon Mixing – Full water column • Determines annual loading of remineralized nutrients from the deep • Most important for large lakes • Mixing during the stratified period • Determines whether remineralized nutrients returned in ‘summer’ season • Determines whether surface loadings are mixed downwards Summer epilimnion temperature at Toolik lake measured by weekly CTD casts (green) and continuous sensors (red) are consistent and show slight warming. Others have shown significant surface temperature warming in Northern Alaska. Given that the Arctic is warming, we identify warm and cold years at this lake and show how lake temperature and stratification (and thus ecosystem function) may change as the region warms. Chlorophyll in Lake E5, a small unsheltered, fertilized lake near Toolik Lake where non-linear waves also form Cold Year Warm Year Fertilization study is analogous to nutrient loading in a thermokarst lake or from melting permafrost. Nutrients were mixed downwards in the cold year but retained in the upper column in the warm year. Nutrients available to support benthic production In the cold year; pelagic production in the warm year. Chl a is much higher in upper water column in the warm year. Dimensionless Indices – Wedderburn and Lake Numbers Summer epilimnion temperatures are strongly correlated with lake heat content (volume averaged temperature) and stratification (stability). Indicate whether wind forcing is sufficient relative to stratification to induce internal waves and increase shear sufficiently to cause mixing . g is gravitational acceleration, r is water density, h is epilimnion depth, u* is friction due to wind stress, L is distance across the lake,St is the stability number, tw is wind stress, Ao is lake surface area, zt is the distance from the bottom to the center of the metalimnion. zg is the distance from the bottom to the center of volume, and H is lake depth. Cold Year: 2003 Warm Year: 2007 3 D hydrodynamic modeling - Francisco Rueda 1999 and 2004 storm events Transitions Over Time from Differences in Summer Warming LN, W > 10 2 < LN, W < 10 Conclusion During a warm year a strong thermocline develops that is impervious to small cold fronts A front lowers LN and mixes the upper water column Following weeks of cool windy weather, a front lowers LN and mixes the lake top to bottom Shear No shear Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Words words Stream Inflows form metalimnetic intrusions in warm years such as 1999 whose persistence ( days to weeks) depends on the frequency of cold fronts. Introduced waters are more frequently mixed into the mixed layer in cold years with the potential for more of the introduced solutes to exit the lake. In warm years, such as 2007, most winds are mostly diurnal. In cold years, such as 2003, winds are stronger particularly at lower than diurnal frequencies (i.e. fronts). Non-linear internal waves Kz > molecular diffusivity Linear internal waves Kz ~ molecular diffusivity Toolik Lake transitions between Lake Numbers associated with vastly mixing characteristics, so small increases in warming can have large ecological implications. Eddy diffusivity (kz) is related to the vertical flux of nutrients (C). LN, W ~ 1 LN, W << 1 Complete mixing Shear Non-linear internal waves Kz >> molecular diffusivity Thermocline, before wind Acknowledgements Thermocline tilt from wind This project was funded in large part by NSF grants DEB-9726932, -0108572, -0640953, and OCE-9906924 and the Arctic LTER. Data were processed by Arron Layns, Avrey Parson-Field, Bridget Benson, Brice Loose, Chad Helmle, Mary Anne Evans, Rebecca Larson, and Robyn Smyth. Support in the field was provided by Neil Bettez, Chris Crockett, Chris Wallace, Jeremy Meirs, Christie Haupert, Mary Anne Evans, Amanda Field, Cody Johnson, James King, Sandy Roll, Jim Laundre, and George Kling. The project was initially motivated by conversations with George Kling. Site Location: Toolik Lake, Arctic Long Term Ecological Research Program In this 2003 close-up plot of 0.05 oC isotherms below 6 oC, it is apparent that at low Lake Number, instabilities are induced in the internal wave field with associated mixing and potential for vertical fluxes. Toolik Lake 68o37’53”N 149o36’18”W Alaska