Seasonal patterns in phytoplankton biomass across the northern and deep Gulf of Mexico: a numerical model study

Fabian A Gomez,Sang-Ki Lee,Frank J Hernandez Jr,Yanyun Liu,Frank E Muller-Karger,John T Lamkin

doi:10.5194/bg-15-3561-2018

Abstract

Abstract. Biogeochemical models that simulate realistic lower-trophic-level dynamics, including the representation of main phytoplankton and zooplankton functional groups, are valuable tools for improving our understanding of natural and anthropogenic disturbances in marine ecosystems. Previous three-dimensional biogeochemical modeling studies in the northern and deep Gulf of Mexico (GoM) have used only one phytoplankton and one zooplankton type. To advance our modeling capability of the GoM ecosystem and to investigate the dominant spatial and seasonal patterns of phytoplankton biomass, we configured a 13-component biogeochemical model that explicitly represents nanophytoplankton, diatoms, micro-, and mesozooplankton. Our model outputs compare reasonably well with observed patterns in chlorophyll, primary production, and nutrients over the Louisiana–Texas shelf and deep GoM region. Our model suggests silica limitation of diatom growth in the deep GoM during winter and near the Mississippi delta during spring. Model nanophytoplankton growth is weakly nutrient limited in the Mississippi delta year-round and strongly nutrient limited in the deep GoM during summer. Our examination of primary production and net phytoplankton growth from the model indicates that the biomass losses, mainly due to zooplankton grazing, play an important role in modulating the simulated seasonal biomass patterns of nanophytoplankton and diatoms. Our analysis further shows that the dominant physical process influencing the local rate of change of model phytoplankton is horizontal advection in the northern shelf and vertical mixing in the deep GoM. This study highlights the need for an integrated analysis of biologically and physically driven biomass fluxes to better understand phytoplankton biomass phenologies in the GoM.

Highlights

The Gulf of Mexico (GoM) is characterized by large spatial differences in plankton productivity and biomass, ranging from the oligotrophic Loop Current to the highly productive northern shelf
The greatest chlorophyll concentration values are within the MS river delta, and the lowest values within the region influenced by the Loop Current
We found that the seasonal patterns in model phytoplankton biomass are largely determined by small imbalances between biologically and physically driven fluxes, the latter mainly associated with horizontal advection in the Louisiana–Texas shelf and turbulent vertical diffusion in the deep GoM

Summary

Introduction

The Gulf of Mexico (GoM) is characterized by large spatial differences in plankton productivity and biomass, ranging from the oligotrophic Loop Current to the highly productive northern shelf Productivity in this last region is strongly influenced by river runoff. The large plankton production and vertical stratification driven by the MS-A river system discharge promote the development of a hypoxic bottom layer a few meters thick off Louisiana and Texas during summer (Obenour et al, 2013). This hypoxic layer can negatively impact metabolism and growth of fish and invertebrates (Rosas et al, 1998; Craig and Crowder, 2005), and disturb species distribution and composi-. In the oligotrophic deep GoM, the spatiotemporal patterns in phytoplankton biomass are mainly associated with seasonal changes in thermal stratification and mesoscale ocean dynamics (e.g., Muller-Karger et al, 2015)

Results

Discussion

Conclusion