Light Regulation of Phytoplankton Growth in San Francisco Bay Studied Using a 3D Sediment Transport Model

In San Francisco Bay (SFB), light availability is largely determined by the concentration of suspended particulate matter (SPM) in the water column. SPM exhibits substantial variation with time, depth, and location. To study how SPM influences light and phytoplankton growth, we coupled a sediment transport model with a hydrodynamic model and a biogeochemical model. The coupled models were used to simulate conditions for the year of 2011 with a focus on northern SFB. For comparison, two simulations were conducted with ecosystem processes driven by SPM concentrations supplied by the sediment transport model and by applying a constant SPM concentration of 20 mg l–1. The sediment transport model successfully reproduced the general pattern of SPM variation in northern SFB, which improved the chlorophyll-a simulation resulting from the biogeochemical model, with vertically integrated primary productivity varying greatly, from 40 g[C] m–2 year–1 over shoals to 160 g[C] m–2 year–1 in the deep channel. Primary productivity in northern SFB is influenced by euphotic zone depth (Ze). Our results show that Ze in shallow water regions (<2 m) is mainly determined by water depth, while Ze in deep water regions is controlled by SPM concentration. As a result, Ze has low (high) values in shallow (deep) water regions. Large (small) differences in primary productivity exist between the two simulations in deep (shallow) water regions. Furthermore, we defined a new parameter Flight for “averaged light limitation” in the euphotic zone. The averaged chlorophyll-a concentration in the euphotic zone and Flight share a similar distribution such that both have high (low) values in shallow (deep) water regions. Our study demonstrates that light is a critical factor in regulating the phytoplankton growth in northern SFB, and a sediment transport model improves simulation of light availability in the water column.

SEDIMENT TRANSPORT FIELD DATA AND NUMERICAL MODELING STUDY TO SUPPORT DREDGE PIT INFILL RATE ESTIMATES

Site specific bedload and suspended sediment transport data collected at two test pit locations over a four-day period during April 2015 were analyzed to calibrate a numerical sediment transport model of Cook Inlet, AK. The field data campaign was designed to collect suspended load and bedload field measurements and was carried out in two phases. During Phase 1, both suspended load and bedload measurements were taken at approximately 55 ft water depth. The suspended sediment concentration was observed to be nearly uniform over the water column. Laboratory analysis showed the suspended sediment had an effective grain size of approximately 0.03 mm with 0.005 mm within a 95percent confidence interval. During Phase 2, hydrodynamic, suspended load and bedload measurements were collected over four tidal cycles in the surfzone. A two-dimensional sediment transport model was developed to simulate sediment transport infill rates at the dredged areas of the Project site. The model was calibrated by comparing measured suspended load measurements made at two offshore locations. Calibration results showed that the suspended load transport rate, which is the dominant sediment transport regime in the area, can be predicted accurately at the project site. Based on the calibrated sediment transport model, preliminary annual sediment infill rates were estimated to lie between 1.1 to 1.6 ft/yr at offshore and nearshore locations, respectively, for the presently observed and measured conditions.