A simulation study of the feedback of phytoplankton on thermal structure via light extinction

Abstract

Summary 1. A simulation study of the feedback of phytoplankton biomass on temperature stratification in the large, monomictic Lake Constance was undertaken. Phytoplankton biomass affects the light extinction coefficient (LEC) of the water and, in turn, the vertical distribution of short‐wave radiation, which shapes the temperature stratification in the lake. 2. A sensitivity study of the variation in LEC using the hydrodynamic model DYRESM showed that a high LEC is associated with stronger stratification, shallower thermoclines, higher surface temperatures and reduced heat content during the heating phase of the lake. During the cooling phase, a shallower thermocline at high LEC leads to a faster decrease in water temperature so that during autumn, a high LEC is associated with lower surface temperatures. Thermal structure was particularly sensitive to changes in LEC when its value was below 0.5 m−1. 3. When LEC is simulated dynamically with the coupled hydrodynamic–ecological model DYRESM‐CAEDYM, its value is a function of phytoplankton dynamics that change vertically and temporally. Comparing simulations with and without dynamic LEC (i.e. with and without phytoplankton dynamics) produced a complex picture: during the vegetation period, we often found a warmer surface layer and colder water beneath in the simulations with dynamic LEC, as expected from the higher LEC when phytoplankton is abundant. However, since phytoplankton biomass (as LEC) fluctuates and because of occasional cooling phases, the patterns were comparatively weak and not consistent over the whole growing season. 4. The most obvious patterns emerged by comparing simulations of oligotrophic and eutrophic conditions. In the eutrophic state, with its higher LEC, stratification was stronger and characterized by higher surface water temperatures, a shallower thermocline and colder water temperatures between 5 and 10 m depth. 5. Statistical analysis of long‐term data of water temperatures in Lake Constance, corrected for external forcing by air temperature, revealed a significant tendency towards warmer temperatures at 7.5 and 10 m depths with decreasing LECs over the course of reoligotrophication. This finding is consistent with our model results.

Satellite-derived light extinction coefficient and its impact on thermal structure simulations in a 1-D lake model

“…It was also shown that the model is very sensitive to variations in K d when the value is less than 0.5 m −1 . This finding is in agreement with the study of Rinke et al (2010) and the recent study of Heiskanen et al (2015) who determined that the impact of seasonal variations of K d on the simulated thermal structure is small, for a lake with K d values larger than 0.5 m −1 . The studies suggested that the response of 1-D lake models to K d variations is nonlinear.…”

“…However, the sensitivity increased rapidly for K d values less than our Min (0.58 m −1 ). This result supported the study of Rinke et al (2010) that the thermal structure of lakes is particularly sensitive to changes in K d when its value is below 0.5 m −1 . More recently, Heiskanen et al (2015) confirmed the critical threshold of K d (ca.…”

Modelling the plankton groups of the deep, peri-alpine Lake Bourget

“…Remaining phytoplankton were separated by size, forming 'small' and 'large' algal groups, given the importance of size in determining light and light utilization traits, among others (Finkel et al, 2010). There exist modelling studies where taxonomic classes were parameterized directly as functional groups (e.g., Jöhnk et al, 2008;Carraro et al, 2012), but categorization based on size, thereby combining di erent taxonomic classes within a given functional group like in this study is not uncommon (e.g., Mieleitner and Reichert, 2008;Gal et al, 2009;Rinke et al, 2010).…”

Parameter sensitivity analysis of a 1-D cold region lake model for land-surface schemes

“…From the point of view of atmospheric feedbacks, ecosystem function is more than just ontologically relevant and is a controlling factor for heat exchange. Previous studies illustrate the feedback between phytoplankton and thermal structure, via light extinction modulation (Tilzer, 1983(Tilzer, , 1988Mazumder et al, 1990;Rinke et al, 2010). Thermal stratification modulates oxygen concentrations and therefore ecosystem function (Elçi, 2008).…”