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For more information about this project, contact Spencer Gardner, spengard@umich.edu

 

Stages of Stratification

In large lakes, shallow nearshore waters warm more rapidly than deeper offshore waters. Differences in the thermal density of nearshore and offshore water result in horizontal stratification (i.e., separation), a common feature of large lakes characterized by the spring coastal thermal bar. As temperatures increase in early summer, the thermal bar extends offshore. Ultimately, the system then transitions to vertical stratification across the entire lake.

The timing of thermal bar formulation and subsequent summer vertical stratification is directly influenced by climatic conditions, including preceding winter and spring temperature and wind-driven mixing.

Large Lake Water Currents

Water currents in large lakes are almost entirely wind-driven but can also change based on seasonal trends in thermal density stratification.

Due to Coriolis force, wind-induced friction in the northern hemisphere directs water currents 90 degrees F to the right of prevailing winds (and to the left in the southern hemisphere). The formulation of the spring thermal bar limits water currents moving nearshore to offshore, instead facilitating predominately alongshore currents and retention of material nearshore.

Summer stratification hinders vertical mixing of the water column but promotes water currents moving between nearshore and offshore. Consistently strong patterns in wind can induce relatively short-lived coastal upwelling events, which disrupt vertical stratification.

There is uncertainty about the potential impact of climate change on seasonal trends in the wind direction and magnitude. However, altered wind patterns, coupled with warmer temperatures, could affect water circulation and patterns of upwelling and downwelling.

Ice Loss and Lake Warming

Decreasing ice cover potentially results in high winter evaporation, owing to the prolonged exposure of relatively warm waters to cold winter winds. Without winter ice, wind-driven mixing of the water column destabilizes lakes’ physical, chemical, and biological processes. As a result, this increases winter light penetration, water temperatures, and nutrient availability. 

Future ice cover in large lakes is expected to continue decreasing due to increased warming.

 

Mechanisms of Fish Recruitment

Larval fish emerge from eggs at small sizes with underdeveloped swimming abilities that leave them largely at the mercy of lake water currents. Water currents may transport larval fish to favorable temperatures and concentrations of prey, or may transport larvae to unfavorable conditions and unfavorable concentrations of prey, 

Annual climate-driven variability of water currents and the timing of larval emergence affect thermal exposure and prey availability, influencing growth and survival. Synchronized larval emergence with favorable conditions likely facilitate strong recruitment. Increased climatic variability may result in more frequent mismatches between larval emergence and favorable water currents and environmental conditions, leading to consistently poor recruitment. 

Understanding Larval Recruitment in Large Lakes Pt. 1

Simulating the historic and potential future physical and biological processes of Lake Michigan, we evaluated how past and future water currents, thermal conditions, and zooplankton prey have generated and may lead to variable recruitment in two important Lake Michigan fish species, alewife (Alosa pseudoharengus) and yellow perch (Perca flavescens).

 

Understanding Larval Recruitment in Large Lakes Pt. 2

Larval transport patterns were strongly influenced by thermal conditions, with the probability of larvae being transported offshore increasing seasonally and occurring earlier in warm years. Simulated trends in historic recruitment strength (strong vs. weak years) followed observed recruitment patterns. 

Future years were characterized by declines in zooplankton that may result in more frequent mismatches between the emergence of larval fish and zooplankton prey. Simulation indicated that historically favorable sites (i.e., those where larvae displayed high growth and survival) may shift northward with increasing temperatures.

References

Please view PDF file for details on references.