Reanalysis of vertical mixing in mesocosm experiments: PeECE III and KOSMOS 2013

Sabine Mathesius,Heiner Dietze,Markus Schartau,Julia Getzlaff,Andreas Oschlies

doi:10.5194/essd-12-1775-2020

Sabine Mathesius, Heiner Dietze + Show 3 more

Open Access

https://doi.org/10.5194/essd-12-1775-2020

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Abstract

Abstract. Controlled manipulation of environmental conditions within large enclosures in the ocean, so-called pelagic mesocosms, has become a standard method to explore potential responses of marine plankton communities to anthropogenic change. Among the challenges of interpreting mesocosm data is the often uncertain role of vertical mixing, which usually is not observed directly. To account for mixing nonetheless, two pragmatic assumptions are common: either that the water column is homogeneously mixed or that it is divided into two water bodies with a horizontal barrier inhibiting turbulent exchange. In this study, we present a model-based reanalysis of vertical turbulent diffusion in the mesocosm experiments PeECE III and KOSMOS 2013. Our diffusivity estimates indicate intermittent mixing events along with stagnating periods and yield simulated temperature and salinity profiles that are consistent with the observations. Here, we provide the respective diffusivities as a comprehensive data product in the Network Common Data Format (NetCDF). This data product will help to guide forthcoming model studies that aim at deepening our understanding of biogeochemical processes in the PeECE III and KOSMOS 2013 mesocosms, such as the CO2-related changes in marine carbon export. In addition, we make our model code available, providing an adjustable tool to simulate vertical mixing in any other pelagic mesocosm. The data product and the model code are available at https://doi.org/10.1594/PANGAEA.905311 (Mathesius et al., 2019).

Highlights

In a rapidly changing world (Steffen et al, 2011, 2018; Stocker et al, 2014), a better understanding of anthropogenic pressures on marine ecosystems and biogeochemical cycles is needed, so that major risks can be identified in advance and mitigated
4.1 Observed and simulated profiles of temperature and salinity. For both mesocosm experiments that we investigated in this study, PeECE III and KOSMOS 2013, our results show that vertical mixing patterns are very heterogeneous in time and space
This study sets out to simulate the physical processes in the mesocosm experiments PeECE III and KOSMOS 2013 (Schulz et al, 2008; Bach et al, 2016), with a focus on vertical mixing

Summary

Introduction

In a rapidly changing world (Steffen et al, 2011, 2018; Stocker et al, 2014), a better understanding of anthropogenic pressures on marine ecosystems and biogeochemical cycles is needed, so that major risks can be identified in advance and mitigated. The response of marine plankton communities to anthropogenic change is often explored in so-called pelagic mesocosms, large polyethylene bags, either drifting or mounted in coastal waters, usually 2 m in diameter and up to 20 m long. Mesocosms are big enough to prevent many “bottle effects” that are existent in laboratory experiments, while at the same time they are small enough to allow for a cost-efficient controlled manipulation of environmental conditions and frequent monitoring of species composition and biogeochemistry. They provide unique insights into possible responses of planktonic ecosystems to anthropogenic pressures that cannot be obtained from laboratory studies.

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