Estimates of phosphorus entrainment in Lake Mendota: a comparison of one‐dimensional and three‐dimensional approaches

Abstract

Entrainment of phosphorus across the thermocline can be an important nutrient source for phytoplankton in stratified lakes. In eutrophic stratified lakes, seasonal entrainment can be responsible for delayed recovery following a decrease in external phosphorus load. We compared seasonal estimates of entrainment derived from single-and multilocation thermocline migration approaches. Entrainment estimates from these methods were similar. A sampling approach based on a single centralized location produced whole-lake entrainment estimates in general agreement with the mean value from the multilocation approach (the single-location approach reasonably reproduced patterns at a weekly and seasonal time scale). In this study, it was not essential to account for spatial variation in estimating annual rates of entrainment. We also estimated entrainment based on a threedimensional (3-D) hydrodynamic model over a short period of significant thermocline migration. The 3-D model was most useful in exposing spatial and temporal variation in temperature and phosphorus profiles that are otherwise difficult to observe. Spatial variation in phosphorus profiles was associated with upwelling of metalimnetic water represented by the 3-D model. These transient dynamics, though a relatively small portion of an annual phosphorus budget, may supply nutrients to epilimnetic phytoplankton during periods of nutrient limitation. There is a need for further research that combines 3-D hydrodynamic modeling with field collection of biological and chemical data.

Trends and abrupt changes in 104 years of ice cover and water temperature in a dimictic lake in response to air temperature, wind speed, and water clarity drivers

“…The mean depth is 12.7 m, and maximum depth is 25.3 m. For detailed bathymetry refer to Kamarainen et al (2009).…”

“…Thus, increases in air temperature may increase the epilimnetichypolimnetic temperature gradient and decrease the extent of summer algal blooms in the Lake Mendota where internal loading of phosphorus has been shown to be significant to its summer phosphorus budget (Lathrop et al, 1998;Soranno et al, 1997). Additionally, phosphorus entrainment predominately occurs during periods of high wind (Kamarainen et al, 2009;Stauffer and Armstrong, 1986). Algal blooms have been associated with increases in epilimnetic phosphorus due to these high-wind episodes (Robarts et al, 1998;Soranno et al, 1997;Stauffer and Lee, 1973).…”

Spatial heterogeneity strongly affects estimates of ecosystem metabolism in two north temperate lakes

“…Indeed, pelagic zones have been shown to exhibit spatial heterogeneity in plankton populations, chlorophyll a, and nutrient concentrations (Pinel-Alloul and Pont 1991;Kamarainen et al 2009;Mackay et al 2011), all of which could translate into heterogeneity in metabolism estimates. However, sites with occasional GPP or R estimates that do not make sense biologically (e.g., ''negative'' respiration, or primary production in the dark) point to the possibility that at least some of the variation is driven by factors that are not biological, including the likely possibility of advection of water masses past the sensor.…”

Importance of climate change-physical forcing on the increase of cyanobacterial blooms in a small, stratified lake

“…The amount of P entrained in the epilimnion from the metalimnion when the thermocline deepened 1m d -1 (Soranno et al, 1997) was calculated by the volume of water transported (estimated from bathymetric data), multiplied by the average P-concentration at the thermocline depth (Kamarainen et al, 2009).…”