Assessing the effect of oil spills on the dynamics of the microbial plankton community using a NPZD model

J González,E Fernández,F.G Figueiras

doi:10.1016/j.ecss.2021.107734

Abstract

A mesocosms approach was used to assess the seasonal variability and the effect of oil spills on the structure and dynamics of marine microbial plankton communities in a coastal upwelling system. To this aim, four experiments were conducted during the main characteristic periods of the seasonal cycle in the Ría de Vigo (NW of Spain): spring phytoplankton bloom, summer stratification, autumn upwelling and winter mixing. In each of these experiments, enclosed communities in control and oiled bags were monitored and key variables and rates were measured during 9 days. Temporal changes in community structure and function were interpreted applying a simple NPZD (Nutrients-Phytoplankton-Zooplankton-Detritus) model in which the strategy was to maintain invariant as many model parameters as possible. The analysis of those parameters that were modified to accurately simulate the time evolution of the microbial communities in the mesocosms, provided important information about the main processes influencing the plankton community structure and dynamics in the bags as well as about the effect of oil. This modelling approach allowed assessing the dynamics of the four communities just varying phytoplankton maximum growth rates, bacterial growth efficiencies, microzooplankton grazing rates and phytoplankton sinking rates. Comparison of parameters in control and oiled bags in each period allowed to infer the negligible effect of oil spills on the structure and dynamics of the microbial plankton communities in this coastal upwelling system.

Highlights

Hydrocarbon inputs to the ocean vary between 470,000 and 8.7 million tons per year, with an annual average input of 1.3 million tons (NRC, 2003)
The dominance of diatoms in these two experiments is consistent with the high relative contribution of > 20 μm phytoplankton both in terms of Chl a and primary production (Figs. 3 and 4), which agrees with previous observations in the Ría de Vigo (Figueiras et al, 1991; Fernandez et al, 1991; Lorenzo et al, 2004; Nogueira and Figueiras, 2005), and other coastal upwelling systems as California (Estrada et al, 1979), Benguela (Pitcher et al, 1991) or Perú (Smith et al, 1983; Hare et al, 2005)
The use of a simple NPZD model as an interpretative tool allowed us to infer the main factors responsible for the differences observed in the microbial plankton communities of the Ría de Vigo in four seasons

Summary

Introduction

Hydrocarbon inputs to the ocean vary between 470,000 and 8.7 million tons per year, with an annual average input of 1.3 million tons (NRC, 2003). These studies reported a negative direct effect of oil, some investigations occasionally reported an apparent positive effect on phytoplankton (Vargo et al, 1982; Car man et al, 1997; Nayar et al, 2005; Klotz et al, 2018) This apparent contradiction can be explained by differences in sensitivity to oil among species (Ostgaard et al, 1984; Peterson et al, 2003; Gonzalez et al, 2009), which may cause changes in the competitiveness and/or in tro phic interactions within the microbial community. Differences in sensitivity among species can be responsible for changes in

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