Coupled physical and biogeochemical dynamics in shallow aquatic systems: Observations, theory and models

Date of Completion

January 2007


Biogeochemistry|Biology, Limnology




Shallow (<3m) water bodies are ubiquitous, receive considerable loads of carbon and nutrients and their internal processes can exert significant control on the downstream biogeochemical fluxes. These shallow systems are characterized by tight coupling between biogeochemical and physical dynamics. This work investigates these coupled dynamics through observation, theory and model results. Observations were made in Mirror Lake (mean depth=0.7m) at the University of Connecticut, using a novel system for high temporal and spatial resolution sampling with real-time data processing and display.^ Diurnal stratification/destratification dynamics affect biogeochemical cycles by controlling the rates of vertical transport and mixing. Geochemically significant daytime stratification can be predicted with a single parameter based on the product of water depth and the diffuse attenuation coefficient of visible light. Nighttime destratification is controlled by surface cooling, particularly under light winds. Using the potential energy anomaly as an integrated measure of water column stratification, diurnal stratification/destratification dynamics can be modeled as a linear function of the heat fluxes across the air-water interface, water depth and the diffuse light attenuation coefficient. ^ Diurnal stratification/destratification dynamics can be quantified by the vertical eddy diffusivity (Kz). Using the Princeton Ocean Model (POM), a three-dimensional ocean circulation model adapted to shallow inland water bodies, it is shown that Kz varies over four orders of magnitude in response to diurnal stratification dynamics, and that transitions from low Kz (quiescent conditions) to high Kz (turbulent mixing) occur over very small time and space scales.^ A one-dimensional biogeochemical model of dissolved oxygen and carbon dynamics for Mirror Lake was coupled to the Kz results from POM. The results indicate that aerobic and anaerobic decomposition of organic carbon occurs primarily in the water column, and the pond is thus under strong hydrological control. A "snow-globe" model for the physical transport of particle-associated bacteria is used to explain observations of the coupled physical and biogeochemical dynamics in Mirror Lake. This mode of coupled dynamics is likely to be significant for all shallow aquatic systems. ^