Contrasting response to nutrient manipulation in A rctic mesocosms are reproduced by a minimum microbial food web model

Abstract

A minimum mathematical model of the marine pelagic microbial food web has previously shown to be able to reproduce central aspects of observed system response to different bottom-up manipulations in a mesocosm experiment Microbial Ecosystem Dynamics (MEDEA) in Danish waters. In this study, we apply this model to two mesocosm experiments (Polar Aquatic Microbial Ecology (PAME)-I and PAME-II) conducted at the Arctic location Kongsfjorden, Svalbard. The different responses of the microbial community to similar nutrient manipulation in the three mesocosm experiments may be described as diatom-dominated (MEDEA), bacteria-dominated (PAME-I), and flagellated-dominated (PAME-II). When allowing ciliates to be able to feed on small diatoms, the model describing the diatom-dominated MEDEA experiment give a bacteria-dominated response as observed in PAME I in which the diatom community comprised almost exclusively small-sized cells. Introducing a high initial mesozooplankton stock as observed in PAME-II, the model gives a flagellate-dominated response in accordance with the observed response also of this experiment. The ability of the model originally developed for temperate waters to reproduce population dynamics in a 10°C colder Arctic fjord, does not support the existence of important shifts in population balances over this temperature range. Rather, it suggests a quite resilient microbial food web when adapted to in situ temperature. The sensitivity of the model response to its mesozooplankton component suggests, however, that the seasonal vertical migration of Arctic copepods may be a strong forcing factor on Arctic microbial food webs.

Simple models combining competition, defence and resource availability have broad implications in pelagic microbial food webs

“…c) situation, glucose addition led to a dynamic, glucose‐sensitive, bacterial response and disappearance of the diatoms (Larsen et al . ), a result seemingly in contrast with the Danish experiment. This difference in diatom‐bacteria balance was attributed to the difference in diatom community composition: large filamentous diatoms in Denmark fitting the minimum model of Fig.…”

“…Despite all the biological detail still ignored in this description, we found this minimum model able to reproduce the contrasting results of two Arctic mesocosm experiments (Larsen et al . ). With the two experiments done in early and late Arctic summer, their timing corresponded to periods before and after copepod diapause, and therefore to high‐ and low‐copepod abundance in the surface layer, respectively.…”

“…b and c (Larsen et al . ) responded, under comparable experimental perturbations, differently at the level of bacterial and viral community composition (Sandaa et al . ).…”

“…An experimental illustration of this is how the two Arctic mesocosm experiments with food web structures as represented in Fig. 4b and c (Larsen et al 2015) responded, under comparable experimental perturbations, differently at the level of bacterial and viral community composition (Sandaa et al 2017). In analogy with the Mediterranean case, this was interpreted using the simple pentagon of Fig.…”

Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms

“…Nutrients were added in the evening to avoid contamination of the morning sampling, and began the day after the bags were filled (on the evening of August 3) and 4.5 days before the first day of sampling. Previous study evidenced that such a time lag allows the phytoplankton communities to differentiate depending on nutrient additions (Gismervik et al, 2002;Sommer et al, 2004;Larsen et al, 2015).…”