Research

3) What are the fundamental processes in the estuaries of the region, and how do they influence vulnerability to coastal land use and nutrient loading?

The estuaries of the Sea of Cortez and the Yaqui region are vastly under-studied, and there is reason to believe that knowledge gained from estuaries in other regions of the world are not applicable to these semi-desert sub-tropical systems. The systems of the Yaqui Basin are undergoing substantial human impact through nutrient loading (from upland agriculture, urban growth and the livestock sector) as well as coastal development, but little is known about the consequences for these systems.

The potential response of the coastal estuaries depends both on the physical characteristics of the water bodies and interactions between physical and biological processes. As a first step towards understanding the functioning of the estuaries of the Yaqui Valley region, we propose to carry out hydrographic measurements in two estuaries (Tobari and Yavarros). Because little work has been done on these systems, our task is one of characterizing basic hydrodynamic and biogeochemical processes.

System Characterization. The potential response of the coastal estuaries to nutrient loading and eutrophication of coastal environments depends both on the physical characteristics of the water bodies and interactions between physical and biological processes. Because little work has been done on these systems, our task is one of characterizing basic hydrodynamic processes, such as tides, wind-driven flows and buoyancy (freshwater/evaporation) driven flows, and understanding basic biogeochemical characteristics as well.

This activity entails both short, intensive field experiments designed to map spatial variability and longer fixed deployments to characterize temporal variability. Salinity and velocity will be mapped a several transects throughout the two estuaries using a small boat equipped with a CTD (an instrument that measures salinity and temperature as a function of depth as it falls through the water column) and an ADCP (Acoustic Dopper Current Profiler - an instrument that measures profiles of water velocity using the Doppler shift of sound it emits).

These measurements will be done twice for roughly two weeks (capturing one fortnightly spring-neap tidal cycle) each time, once during low flow conditions and once during high flow conditions. These transect measurements will be supplemented by simultaneous collection of time series of salinity, temperature, turbidity, and pressure at three locations (mouth and internal) in the estuary using self-contained loggers. At one of the internal locations we will deploy loggers both on the surface and near the bottom to look for water column stratification. These time series data will be used to understand the context of the daily transects and, in the future, to develop numerical circulation models which can be used to model phytoplankton and nutrient dynamics.

This work will be done by CECARENA-ITESM in collaboration with Prof. Monismith and PhD. student Mike Beman. Part of the collaboration will involve Ms. Esther Cruz-Colin spending several months visiting the Environmental Fluid Mechanics Laboratory at Stanford to work on data analysis and to explore implementation of a numerical circulation model of the Tobari.