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Effects of whaling on the structure of the Southern Ocean food web: insights on the "krill surplus" from ecosystem modelling.

Surma S, Pakhomov EA, Pitcher TJ - PLoS ONE (2014)

Bottom Line: An additional suite of Ecosim scenarios reflecting several hypothetical trends in Southern Ocean primary productivity were employed to examine the effect of bottom-up forcing on the documented krill biomass trend.Our findings suggest that changes in physical conditions in the Southern Ocean during this time period could have eliminated the ecological effects of rorqual depletion, although the mechanism responsible is currently unknown.The results of this study underscore the need for further research on historical changes in the roles of top-down and bottom-up forcing in structuring the Southern Ocean food web.

View Article: PubMed Central - PubMed

Affiliation: Fisheries Centre, University of British Columbia, Vancouver, BC, Canada.

ABSTRACT
The aim of this study was to examine the ecological plausibility of the "krill surplus" hypothesis and the effects of whaling on the Southern Ocean food web using mass-balance ecosystem modelling. The depletion trajectory and unexploited biomass of each rorqual population in the Antarctic was reconstructed using yearly catch records and a set of species-specific surplus production models. The resulting estimates of the unexploited biomass of Antarctic rorquals were used to construct an Ecopath model of the Southern Ocean food web existing in 1900. The rorqual depletion trajectory was then used in an Ecosim scenario to drive rorqual biomasses and examine the "krill surplus" phenomenon and whaling effects on the food web in the years 1900-2008. An additional suite of Ecosim scenarios reflecting several hypothetical trends in Southern Ocean primary productivity were employed to examine the effect of bottom-up forcing on the documented krill biomass trend. The output of the Ecosim scenarios indicated that while the "krill surplus" hypothesis is a plausible explanation of the biomass trends observed in some penguin and pinniped species in the mid-20th century, the excess krill biomass was most likely eliminated by a rapid decline in primary productivity in the years 1975-1995. Our findings suggest that changes in physical conditions in the Southern Ocean during this time period could have eliminated the ecological effects of rorqual depletion, although the mechanism responsible is currently unknown. Furthermore, a decline in iron bioavailability due to rorqual depletion may have contributed to the rapid decline in overall Southern Ocean productivity during the last quarter of the 20th century. The results of this study underscore the need for further research on historical changes in the roles of top-down and bottom-up forcing in structuring the Southern Ocean food web.

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The time series for primary production in the Southern Ocean used in the three Ecosim scenarios that incorporated bottom-up forcing.
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pone-0114978-g002: The time series for primary production in the Southern Ocean used in the three Ecosim scenarios that incorporated bottom-up forcing.

Mentions: Three separate time series (Fig. 2) were used to force the biomass of primary producers in an additional set of Ecosim scenarios. These were designed to investigate the effects of bottom-up forcing on the total ecosystem response to rorqual depletion (i.e. the “krill surplus” phenomenon). More generally, they were meant to examine the potential relative strengths of top-down and bottom-up effects on the structure of the Southern Ocean food web in the 20th century under various temporal patterns of primary productivity. Therefore, in each scenario, a different time series for primary producers was used in parallel with the rorqual biomass time series described above. In each of the time series, however, the starting value was derived from the primary production required by all consumers (including the unexploited rorqual populations) in 1900. The estimate of this value calculated in the Ecopath model was 120 t/km2. The final value for each time series was taken from the estimate of the total primary production of the Southern Ocean in 2006 obtained by Arrigo et al. [31]. Based on this later publication, the value of ∼104 t/km2 proposed by Arrigo et al. [32] appears to have been an overestimate. For this reason, the total modelled primary productivity was taken to have declined by 58.1% from 1900 to 2010 in all three scenarios, based on the Ecopath estimate for 1900 and Arrigo et al. [31]. This value is comparable with that of 67.4% reported by Boyce et al. [33] based on their analysis of 20th-century trends in Antarctic primary productivity.


Effects of whaling on the structure of the Southern Ocean food web: insights on the "krill surplus" from ecosystem modelling.

Surma S, Pakhomov EA, Pitcher TJ - PLoS ONE (2014)

The time series for primary production in the Southern Ocean used in the three Ecosim scenarios that incorporated bottom-up forcing.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4269391&req=5

pone-0114978-g002: The time series for primary production in the Southern Ocean used in the three Ecosim scenarios that incorporated bottom-up forcing.
Mentions: Three separate time series (Fig. 2) were used to force the biomass of primary producers in an additional set of Ecosim scenarios. These were designed to investigate the effects of bottom-up forcing on the total ecosystem response to rorqual depletion (i.e. the “krill surplus” phenomenon). More generally, they were meant to examine the potential relative strengths of top-down and bottom-up effects on the structure of the Southern Ocean food web in the 20th century under various temporal patterns of primary productivity. Therefore, in each scenario, a different time series for primary producers was used in parallel with the rorqual biomass time series described above. In each of the time series, however, the starting value was derived from the primary production required by all consumers (including the unexploited rorqual populations) in 1900. The estimate of this value calculated in the Ecopath model was 120 t/km2. The final value for each time series was taken from the estimate of the total primary production of the Southern Ocean in 2006 obtained by Arrigo et al. [31]. Based on this later publication, the value of ∼104 t/km2 proposed by Arrigo et al. [32] appears to have been an overestimate. For this reason, the total modelled primary productivity was taken to have declined by 58.1% from 1900 to 2010 in all three scenarios, based on the Ecopath estimate for 1900 and Arrigo et al. [31]. This value is comparable with that of 67.4% reported by Boyce et al. [33] based on their analysis of 20th-century trends in Antarctic primary productivity.

Bottom Line: An additional suite of Ecosim scenarios reflecting several hypothetical trends in Southern Ocean primary productivity were employed to examine the effect of bottom-up forcing on the documented krill biomass trend.Our findings suggest that changes in physical conditions in the Southern Ocean during this time period could have eliminated the ecological effects of rorqual depletion, although the mechanism responsible is currently unknown.The results of this study underscore the need for further research on historical changes in the roles of top-down and bottom-up forcing in structuring the Southern Ocean food web.

View Article: PubMed Central - PubMed

Affiliation: Fisheries Centre, University of British Columbia, Vancouver, BC, Canada.

ABSTRACT
The aim of this study was to examine the ecological plausibility of the "krill surplus" hypothesis and the effects of whaling on the Southern Ocean food web using mass-balance ecosystem modelling. The depletion trajectory and unexploited biomass of each rorqual population in the Antarctic was reconstructed using yearly catch records and a set of species-specific surplus production models. The resulting estimates of the unexploited biomass of Antarctic rorquals were used to construct an Ecopath model of the Southern Ocean food web existing in 1900. The rorqual depletion trajectory was then used in an Ecosim scenario to drive rorqual biomasses and examine the "krill surplus" phenomenon and whaling effects on the food web in the years 1900-2008. An additional suite of Ecosim scenarios reflecting several hypothetical trends in Southern Ocean primary productivity were employed to examine the effect of bottom-up forcing on the documented krill biomass trend. The output of the Ecosim scenarios indicated that while the "krill surplus" hypothesis is a plausible explanation of the biomass trends observed in some penguin and pinniped species in the mid-20th century, the excess krill biomass was most likely eliminated by a rapid decline in primary productivity in the years 1975-1995. Our findings suggest that changes in physical conditions in the Southern Ocean during this time period could have eliminated the ecological effects of rorqual depletion, although the mechanism responsible is currently unknown. Furthermore, a decline in iron bioavailability due to rorqual depletion may have contributed to the rapid decline in overall Southern Ocean productivity during the last quarter of the 20th century. The results of this study underscore the need for further research on historical changes in the roles of top-down and bottom-up forcing in structuring the Southern Ocean food web.

Show MeSH
Related in: MedlinePlus