Model Simulations of a Mesocosm Experiment Investigating the Response of a Low Nutrient Low Chlorophyll (LNLC) Marine Ecosystem to Atmospheric Deposition Events

Abstract

Atmospheric deposition of nitrogen and phosphorus represents an important source of nutrients, enhancing the marine productivity in oligotrophic areas, e.g., the Mediterranean. A comprehensive biogeochemical model (ERSEM) was setup and customized to simulate a mesocosm experiment, where dissolved inorganic nitrogen and phosphorus by means of atmospheric dust (single addition/SA and repetitive addition/RA in three successive doses) was added in controlled tanks and compared with a control (blank), all with Cretan Sea (Eastern Mediterranean) water. Observations on almost all components of the pelagic ecosystem in a ten-day period allowed investigating the effect of atmospheric deposition and the pathways of the added nutrients. The model was able to reasonably capture the observed variability of different ecosystem components and reproduce the main features of the experiment. An enhancement of primary production and phytoplankton biomass with added nutrients was simulated, in agreement with observations. A significant increase of bacterial production was also reproduced, while the model underestimated the observed increase and variability in bacterial biomass, but this deviation could be partly removed considering a lower carbon conversion factor from cell abundance data. A slightly stronger overall response was simulated with the single dust addition, compared to the repetitive that showed a few days delay. The simulated carbon pathways indicated that nutrient additions did not modify the microbial food web structure, but just increased its trophic status. Changes in model assumptions and parameter set that were necessary to reproduce the observed variability in the mesocosm experiment were discussed through a series of sensitivity simulations. Bacterial production was assumed to be mostly affected by the in situ produced labile organic matter, while it was further stimulated by the addition of inorganic nutrients, adopting a function of external nutrient concentrations for bacteria nutrient limitation. The effective increase in phytoplankton nutrient uptake rate was Tsiaras et al.Mesocosm Simulations of Atmospheric Deposition necessary, in order to reproduce the observed primary production, under such low nutrient concentrations, as also the increase of the grazers growth rate. The model was thus tuned to better work under very low nutrient concentrations, such as those found in the Eastern Mediterranean.

An integrated open-coastal biogeochemistry, ecosystem and biodiversity observatory of the eastern Mediterranean – the Cretan Sea component of the POSEIDON system

“…mesocosms; see Sect. 4.8) for the analysis and modelling of specific processes, such as the microbial functioning and the effect of the atmospheric deposition (Christodoulaki et al, 2013;Tsiaras et al, 2017), or for the assimilation algorithms of sea colour data in BGC models (Kalaroni et al, 2016).…”

“…However, many biological variables expressed in biomass units are usually derived from the initial data with some conversion factor assumed. These conversions introduce biases that have to be carefully considered by biologists and modellers (Flynn, 2005;Frangoulis et al, 2010;Tsiaras et al, 2017). In the case of the Cretan Sea biogeochemical-ecosystem model, the choice of appropriate conversion factors from literature, that can be considered, is made by the data providers (biologists) in communication with modellers and is revised regularly.…”

“…Mixotrophs field data were the most particular case, since they were split in four subgroups, creating an autotrophic and a heterotrophic subgroup to express mixotrophy (not represented in the model), which were subdivided again based on cell size (> 20 and < 20 µm). A model application using such field data grouping/splitting for validation can be found in Tsiaras et al (2017). Planktonologists and the modellers of the observatory agreed that there is no rule of thumb, and modelfield data correspondences should be adjusted according to the season and region.…”

Saharan dust deposition initiates successional patterns among marine microbes in the Western Atlantic

“…In general, evidence shows that dust addition can stimulate both heterotrophic and autotrophic communities for some period of time before returning to baseline conditions (e.g., Lekunberri et al ; Marín et al ; Pitta et al ). These shifts can have significant effects on carbon cycling, by causing the trophic state to shift from net autotrophy to net heterotrophy (Astrahan et al ; Tsiaras et al ).…”

Challenges for Sustained Observing and Forecasting Systems in the Mediterranean Sea

“…In terms of general calibration, validation activities are applied to the operational models as data from the observatory are used in conjunction with experiments (e.g., mesocosms) for the analysis and modeling of specific processes (e.g. Tsiaras et al, 2017) and assimilation algorithms of sea color data are tested and validated in biogeochemical models (Kalaroni et al, 2016).…”