Computing the critical time window

The critical time window is time period between (1) the day of the year that dissolved oxygen (DO) begins to be depleted from the deep waters of a lake and (2) the day of the year that mixed layer water temperatures are low enough for cool- and cold-water fish to persist. On this page, the following parameters can be specified that define the critical time window for the lake of interest.

• Critical temperature - The maximum allowable temperature for cool- or cold-water fish in the lake of interest.
• Lake maximum depth - The maximum depth of the lake of interest.
• Latitude and Longitude - The geographic location of the lake of interest.

The inset map shows the selected location as a filled yellow circle. Approximately 100 lakes sampled in the NLA in the vicinity of the selected location are shown as open red circles. Sampled dimictic lakes from the data set are shown as filled black circles.

• Elevation - The elevation of the lake of interest.

• Lake area - The surface area of the lake of interest.

• Spring turnover day - Spring turnover day is the day of the year that the lake water column is isothermal and fully mixed, and this day can be estimated as the day when surface water temperatures are 4 C. Spring turnover day always precedes the first day of stratification and is more directly relevant to the model for oxygen depletion because it established the day of maximum deep water DO concentrations. The open circles show estimates of spring turnover day for each of the modeled lakes as a function of the day that mean air temperature is 4 C (AIR4). AIR4 for the lake of interest is shown as the vertical red line segment. The resulting prediction for the spring turnover day for the selected location is shown as the horizontal line segment.

• Mixed layer water temperature - Estimated mixed layer water temperature for the lake of interest is shown as the line and the gray shading. Open circles show the temperatures for the lakes near the selected location (shown in red on the inset map). The red line segment shows the critical temperature selected by the user, and the vertical red line segment shows the predicted day that water temperature will decrease below this threshold.

A numerical summary of the two dates that define the critial time window is shown in the lower right corner.

Computing chlorophyll criteria

Computing a chlorophyll criterion requires data several additional aspects of the lake of interest:

• DOC- The average dissolved organic carbon concentration (DOC) in the lake.
• Thermocline depth - The thermocline depth. The specified thermocline depth is combined with the lake maximum depth specified on Critical time window tab to compute the depth of the lake below the thermocline.
• Refugia depth - The minimum depth of the cool water refugia that will provide sufficient habitat for fish throughout the year. This minimum depth is used to compute a mean DO target for the lake.
• Dissolved oxygen threshold - The threshold dissolved oxygen concentration for the species of interest.
• Certainty level - The certainty level specifies the range (gray area) within which the criterion value is located with the specified probability. The level is quantified as the credible interval, a measure of model uncertainty that is similar to a confidence interval. For example, if a certainty level of 75% is selected, there is a 75% chance that the criterion is within the bounded region. The criterion magnitude is computed as the intersection of the lower bound of the gray area and the targeted threshold (shown as red line segments).

To provide insight into the degree with which the characteristics specified for the lake of interest are within the range of conditions sampled by the NLA, the distribution of values for DOC and depth below the thermocline sampled in the NLA is shown on the left plot. Approximately 15 samples with values similar to those specified for the lake of interest are highlighted in red (denoted as “Nearest neighbors” in the plot legend).

The final plot shows the volumetric oxygen demand (VOD) as a function of chlorophyll. All available data are shown as open circles, and the samples identified as being most similar to the lake of interest are highlighted in red. The VOD necessary to maintain the desired fish refugia is shown as a horizontal red line segment, and the chlorophyll criterion is shown as the vertical line segment.

A numerical summary of the chlorophyll criterion is shown in the lower right corner.