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

Zhengui Wang,Richard Dugdale,Frances Wilkerson,Yinglong J Zhang,Huijie Xue,Fei Chai,Xiao Hua Wang

doi:10.3389/fmars.2021.633707

Abstract

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–2year–1over shoals to 160 g[C] m–2year–1in the deep channel. Primary productivity in northern SFB is influenced by euphotic zone depth (Ze). Our results show thatZein shallow water regions (&lt;2 m) is mainly determined by water depth, whileZein deep water regions is controlled by SPM concentration. As a result,Zehas 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 parameterFlightfor “averaged light limitation” in the euphotic zone. The averaged chlorophyll-a concentration in the euphotic zone andFlightshare 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.

Highlights

IntroductionA healthy ecosystem in SFB is important to San Francisco Bay the millions of people populated around it and to the resident birds and fishes (Warnock and Takekawa, 1995; Novick and Senn, 2014)
San Francisco Bay (SFB) is the largest estuary along the US west coast
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

Summary

Introduction

A healthy ecosystem in SFB is important to San Francisco Bay the millions of people populated around it and to the resident birds and fishes (Warnock and Takekawa, 1995; Novick and Senn, 2014). SFB includes a well-mixed lagoon in the south, a central embayment connected to the ocean, and a partially mixed estuary of two sub-embayments (San Pablo Bay and Suisun Bay) in the north (Cloern, 1987; Liu et al, 2018). Nutrient load from the WWTPs to the northern SFB is mainly in the form of NH4 (about 34,300 kg day−1), while nutrient loads associated with river discharge is mainly in the form of NO3 (about 10,400 kg day−1) (Novick and Senn, 2014). Despite the large nutrient input, the primary production in SFB is low (Cloern and Jassby, 2012; Dugdale et al, 2016) and nutrients are exported into the ocean (Wang et al, 2020), except during some episodic phytoplankton bloom events in response to climate anomalies (Cloern et al, 2005, 2010)

Methods

Results

Discussion

Conclusion