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

Larsen A, Egge JK, Nejstgaard JC, Di Capua I, Thyrhaug R, Bratbak G, Thingstad TF - Limnol. Oceanogr. (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Uni Research Environment and Hjort Centre for Marine Ecosystem Dynamics Bergen, Norway.

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.

No MeSH data available.


Related in: MedlinePlus

Observed (Obs.) and modeled (Model) responses for the mesocosm units with glucose (3 × C) and silicate (+Si) added in excess of biological consumption and ammonium as the nitrogen source for the PAME-I (solid lines) and PAME-II (broken lines) experiments. Variables arranged graphically to correspond to the model food web structure in Fig. 1. Model results for the MEDEA experiment (dotted lines) shown for comparison.
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fig08: Observed (Obs.) and modeled (Model) responses for the mesocosm units with glucose (3 × C) and silicate (+Si) added in excess of biological consumption and ammonium as the nitrogen source for the PAME-I (solid lines) and PAME-II (broken lines) experiments. Variables arranged graphically to correspond to the model food web structure in Fig. 1. Model results for the MEDEA experiment (dotted lines) shown for comparison.

Mentions: In terms of the trophic structure of Fig. 1, the contrasting experimental outcomes in units with glucose and silicate in excess can be summarized as a dominance of bacteria (PAME-I) or flagellates (PAME-II), as opposed to the diatom response dominating in the MEDEA experiment (Thingstad et al. 2007). Experimental results analogous to the seven state variables of the model are shown in Fig. 8 and Table3, comparing the the 3C + Si (+NH4) unit in PAME-I and the 3C + NH4 (+Si) unit in PAME-II, both representative of units amended with excess glucose and silicate. Retaining the minimum philosophy used in constructing the original model, the smallest set of modifications we could find to adapt the model to the two PAME experiments consisted of (1) an introduction of ciliate grazing on the small diatoms in PAME-I, accompanied by a corresponding reduction in mesozooplankton clearance rate for diatoms; (2) different initial standing stocks of mesozooplankton (numerical values summarized in Table2). To allow direct comparison between model and experimental data, the set of fixed conversion factors was also expanded (Table2), but these do not affect model dynamics, only conversion from the model's phosphorous units to observed units such as abundances, Chl a, or carbon units. All other parameter values in the model were deliberately retained. With these modifications at the predator level, the large diatom bloom that dominated the model response for the MEDEA experiments is strongly reduced in PAME-I and disappears entirely in PAME-II (Fig. 8). The dominance of a continued bloom of autotrophic flagellates in PAME-II is now reproduced, as is the observed pattern for bacteria with higher abundance and a more dynamical response in PAME-I than in PAME-II (Fig. 8). The difference in the model's intial stock of mesozooplankton disappears at the end of the simulated experimental period, qualitatively in agreement with observations (Fig. 8). In the model, this is rooted in the assumption of a higher copepod clearance rate for ciliates than for diatoms, retained here from the original model. Otherwise, the key to understanding the different response patterns of the two PAME experiments lies in the opposite effect our two predator modifications has on ciliates. Allowing ciliates to feed on the small diatoms stimulates ciliate growth in PAME-I while the increased grazing from an initially higher mesozooplankton stock in PAME-II delays ciliate net increase until late in the experiment when their food has become abundant in the rising flagellate bloom (Fig. 8). The model reflects quite well the differences, both in pattern and level of observed ciliate abundances in PAME-I and PAME-II (Fig. 8; Table3). In the model, an increase in ciliate population induce cascades through two pathways: (1) via a decrease in heterotrophic flagellates into an increase in bacterial abundance, and also (2) through a decrease in autotrophic flagellates into an increase in free phosphate. Therefore, when bacterial growth is P-limited (i.e., C-replete), both abundance and growth rate of bacteria respond positively to an increase in ciliates. With these mechanisms, the model reproduces the observed rapid net growth in bacterial abundance toward the end of the experimental period for PAME-I and the lower and less dynamic bacterial abundance in PAME-II (Fig. 8).


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

Larsen A, Egge JK, Nejstgaard JC, Di Capua I, Thyrhaug R, Bratbak G, Thingstad TF - Limnol. Oceanogr. (2015)

Observed (Obs.) and modeled (Model) responses for the mesocosm units with glucose (3 × C) and silicate (+Si) added in excess of biological consumption and ammonium as the nitrogen source for the PAME-I (solid lines) and PAME-II (broken lines) experiments. Variables arranged graphically to correspond to the model food web structure in Fig. 1. Model results for the MEDEA experiment (dotted lines) shown for comparison.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4459036&req=5

fig08: Observed (Obs.) and modeled (Model) responses for the mesocosm units with glucose (3 × C) and silicate (+Si) added in excess of biological consumption and ammonium as the nitrogen source for the PAME-I (solid lines) and PAME-II (broken lines) experiments. Variables arranged graphically to correspond to the model food web structure in Fig. 1. Model results for the MEDEA experiment (dotted lines) shown for comparison.
Mentions: In terms of the trophic structure of Fig. 1, the contrasting experimental outcomes in units with glucose and silicate in excess can be summarized as a dominance of bacteria (PAME-I) or flagellates (PAME-II), as opposed to the diatom response dominating in the MEDEA experiment (Thingstad et al. 2007). Experimental results analogous to the seven state variables of the model are shown in Fig. 8 and Table3, comparing the the 3C + Si (+NH4) unit in PAME-I and the 3C + NH4 (+Si) unit in PAME-II, both representative of units amended with excess glucose and silicate. Retaining the minimum philosophy used in constructing the original model, the smallest set of modifications we could find to adapt the model to the two PAME experiments consisted of (1) an introduction of ciliate grazing on the small diatoms in PAME-I, accompanied by a corresponding reduction in mesozooplankton clearance rate for diatoms; (2) different initial standing stocks of mesozooplankton (numerical values summarized in Table2). To allow direct comparison between model and experimental data, the set of fixed conversion factors was also expanded (Table2), but these do not affect model dynamics, only conversion from the model's phosphorous units to observed units such as abundances, Chl a, or carbon units. All other parameter values in the model were deliberately retained. With these modifications at the predator level, the large diatom bloom that dominated the model response for the MEDEA experiments is strongly reduced in PAME-I and disappears entirely in PAME-II (Fig. 8). The dominance of a continued bloom of autotrophic flagellates in PAME-II is now reproduced, as is the observed pattern for bacteria with higher abundance and a more dynamical response in PAME-I than in PAME-II (Fig. 8). The difference in the model's intial stock of mesozooplankton disappears at the end of the simulated experimental period, qualitatively in agreement with observations (Fig. 8). In the model, this is rooted in the assumption of a higher copepod clearance rate for ciliates than for diatoms, retained here from the original model. Otherwise, the key to understanding the different response patterns of the two PAME experiments lies in the opposite effect our two predator modifications has on ciliates. Allowing ciliates to feed on the small diatoms stimulates ciliate growth in PAME-I while the increased grazing from an initially higher mesozooplankton stock in PAME-II delays ciliate net increase until late in the experiment when their food has become abundant in the rising flagellate bloom (Fig. 8). The model reflects quite well the differences, both in pattern and level of observed ciliate abundances in PAME-I and PAME-II (Fig. 8; Table3). In the model, an increase in ciliate population induce cascades through two pathways: (1) via a decrease in heterotrophic flagellates into an increase in bacterial abundance, and also (2) through a decrease in autotrophic flagellates into an increase in free phosphate. Therefore, when bacterial growth is P-limited (i.e., C-replete), both abundance and growth rate of bacteria respond positively to an increase in ciliates. With these mechanisms, the model reproduces the observed rapid net growth in bacterial abundance toward the end of the experimental period for PAME-I and the lower and less dynamic bacterial abundance in PAME-II (Fig. 8).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Uni Research Environment and Hjort Centre for Marine Ecosystem Dynamics Bergen, Norway.

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.

No MeSH data available.


Related in: MedlinePlus