Modeling silicate–nitrate–ammonium co-limitation of algal growth and the importance of bacterial remineralization based on an experimental Arctic coastal spring bloom culture study

Tobias R Vonnahme,Rolf Gradinger,Ulrike Dietrich,Svein Kristiansen,Dick Van Oevelen,H Rodger Harvey,Silke Thoms,Martial Leroy,Christoph Völker

doi:10.5194/bg-18-1719-2021

Abstract

Abstract. Arctic coastal ecosystems are rapidly changing due to climate warming. This makes modeling their productivity crucially important to better understand future changes. System primary production in these systems is highest during the pronounced spring bloom, typically dominated by diatoms. Eventually the spring blooms terminate due to silicon or nitrogen limitation. Bacteria can play an important role for extending bloom duration and total CO2 fixation through ammonium regeneration. Current ecosystem models often simplify the effects of nutrient co-limitations on algal physiology and cellular ratios and simplify nutrient regeneration. These simplifications may lead to underestimations of primary production. Detailed biochemistry- and cell-based models can represent these dynamics but are difficult to tune in the environment. We performed a cultivation experiment that showed typical spring bloom dynamics, such as extended algal growth via bacterial ammonium remineralization, reduced algal growth and inhibited chlorophyll synthesis under silicate limitation, and gradually reduced nitrogen assimilation and chlorophyll synthesis under nitrogen limitation. We developed a simplified dynamic model to represent these processes. Overall, model complexity in terms of the number of parameters is comparable to the phytoplankton growth and nutrient biogeochemistry formulations in common ecosystem models used in the Arctic while improving the representation of nutrient-co-limitation-related processes. Such model enhancements that now incorporate increased nutrient inputs and higher mineralization rates in a warmer climate will improve future predictions in this vulnerable system.

Highlights

Marine phytoplankton are responsible for half of the CO2 fixation on Earth (Field et al, 1998; Westberry et al, 2008)
We extended the Geider et al (1998) model (G98) model to represent the role of silicate limitation, bacterial regeneration of ammonium, and different kinetics for ammonium and nitrate uptake (EXT) and fitted it to the BAC+ experiment while retaining the parameter values previously estimated for G98
The experimental incubations represented typical spring bloom dynamics for coastal Arctic systems, including an initial exponential growth phase terminated by N and Si limitation (Hypothesis III) and the potential for an extended growth period via regenerated production

Summary

Introduction

Marine phytoplankton are responsible for half of the CO2 fixation on Earth (Field et al, 1998; Westberry et al, 2008). In high-latitude oceans, diatoms are an important group contributing 20 %–40 % of the global CO2 fixation (Nelson et al, 1995; Uitz et al, 2010). Marine primary production can be bottom-up limited by light and/or nutrients like nitrogen (N), phosphorous (P), silicon (Si), and iron (Fe). Their availability is affected by pronounced geographical and seasonal variations (Eilertsen et al, 1989; Loebl et al, 2009; Iversen and Seuthe, 2011; Moore et al, 2013). Arctic coasts are one of the fastest changing systems due to climate change. Modeling their dynamics is difficult but crucial for predictions of primary production with climate change

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