Krill population cycles
Alexey B Ryabov, André M de Roos,
Bettina Meyer, So Kawaguchi,
Bernd Blasius
Why does krill abundance oscillate?
Atkinson et al. 2004
Sea ice and summer conditions are the main drivers of krill abundance
Effects of summer chlorophyll
Survival of new cohorts was associated with anomalies in summer chlorophyll
level or in primary production
Sabe et al. 2014,
Nat Commun
Steinberg et al.
2015, Deep Sea Res
Sea ice duration, ENSO index
Ross et al. 2014, MEPS
Recruitment success has been linked to the
El Niño-Southern Oscillation (ENSO) cycle
The krill population dynamics is driven by food availability
But the food availability can be affected either by external factors (climate variability) or
by grazing (if the population is resource limited)
Too strong consumption can reduce reproduction or maturation
An ontogenetic asymmetry (e.g., a difference in weight specific food assimilation
between larvae and adults) can lead to oscillations in abundance in a stage-structured
population
Persson & de Roos, Ecology 94
h
Quetin & Ross 2003, 2014
The cycle period is around 5-6 years with 2 successive years of good
recruitment and 3-4 years without recruitment
The appearance of a new strong cohort is synchronized with extinction
of an old strong cohort
Juvenile, ind/m3, next year
Biomass, mg DW/m 3
Total
Females in repr. cycle
1
10
0
10
Data
y=-0.027x + 0.076
0.25
0.2
0.15
0.1
0.05
-1
10
1990
0
1995
2000
Year
2005
2010
0
1
2
3
4
3
Female in repr. cycle, mg DW/m
There is a negative effect of krill biomass and on krill recruitment, which implies
resource limitation of the whole population
Model
• The model captures the effects of
seasonality on reproduction and
ontogenetic development of krill.
• Growth and fertility are
proportional to a difference
between ingestion and
maintenance rates
• In summer: all feeding stages
compete for phytoplankton
• In winter: Adults can starve, larvae
need to feed on ice algae, because
larvae have a significant starvation
mortality
• In the model, population
cycles can occur even in the
absence of interannual
variability in phytoplankton
production
• two successive years of
successful recruitment
followed by 3-4 years of
unsuccessful recruitment
• a new cohort appears when
an old strong cohort dies
• the negative effect of krill
biomass on the juvenile
abundance one year later
The mechanism of the cycles
During autumn the phytoplankton
concentrations and duration of the starvation
periods are strongly sensitive to krill biomass
density.
An abundant krill population (adults and/or
larvae) depletes phytoplankton and leads to a
long starvation period of larvae during
autumn.
A small population has a smaller impact on
phytoplankton, and phytoplankton
concentrations are sufficient for larvae to
survive.
The mechanism of the cycles
The cyclic changes in the biomass
lead to cyclic changes in mortality.
Large biomass -> large starvation
mortality -> small absolute
recruitment -> a decrease on total
biomass.
Small biomass -> small starvation
mortality -> over reproduction->
Large biomass -> ….
Effects of climate variability
Power spectrum
The six year oscillation cycle is retained
in the model with among-year random
variations in algal productivity
Effects of climate variability
The correlation between
summer chlorophyll
level and krill abundance
next summer increases
with increasing
perturbation level.
Effects of climate variability
Synchronization of two uncoupled population by
climate (Moran effect)
The correlation between two separate populations increases with the level of environmental
disturbances, leading ultimately to a complete synchronization of two uncoupled populations
• Krill population cycles can be induced by competition between different cohorts
for food
• The model can naturally explain the presence of
‐ two successive years of successful recruitment followed by 3-4 years of
unsuccessful recruitment
‐ a new strong cohort appears when an old strong cohort becomes extinct
‐ total krill biomass has a negative effect on the juvenile abundance one year
later
• External climatological factors can modulate the cycle phase and duration and
also can synchronize the cycles over large scales
It is important to take into account
the effect of consumers on food
availability, especially for the
periods when the food is depleted
Extinction
Stationary
Why does krill abundance oscillate?
Fielding et al. 2014
Only surface temperature with 5 month lag
was significantly correlated with krill
density. The effect of SAM and ENSO was
not significant
Frequency
Frequency
Frequency
Frequency
Frequency
Frequency
Quetin et al 2003
3
3
1991
2
0
0
3
1
20
40
60
80
1992
2
20
40
60
80
1993
20
40
60
80
1994
80
1998
0
0
3
20
40
60
80
1999
0
0
3
20
40
60
80
2000
2
1
1
20
40
60
80
1995
2
0
0
3
20
40
60
80
2001
2
1
1
20
40
60
80
1996
2
0
0
3
20
40
60
80
2002
2
1
0
0
60
1
2
0
0
3
40
2
1
0
0
3
20
1
2
0
0
3
0
0
3
2
1
0
0
3
1997
2
1
1
20
40
60
Length, mm
80
0
0
20
40
60
Length, mm
80
Length distributions
3
3
Frequency
2003
2006
2
2
1
1
0
0
20
40
60
80
3
0
0
20
40
60
3
Frequency
2004
2007
2
2
1
1
0
0
20
40
60
80
3
0
0
20
40
60
Frequency
Quetin et al 2014
80
3
2005
2008
2
2
1
1
0
0
80
20
40
60
Length, mm
80
0
0
20
40
60
Length, mm
80
Modeling equations
Change =
Growth
− Mortality
𝜕𝑐 𝑡, 𝑤
𝜕𝑔 𝑅, 𝑤 𝑐 𝑡, 𝑤
=−
− 𝑑 𝑅, 𝑤 𝑐 𝑡, 𝑤
𝜕𝑡
𝜕𝑤
𝑤 dry weight, 𝑡 time
𝑐 𝑡, 𝑤 density of the cohort (individuals/(m3 mg))
𝑔 𝑅, 𝑤 is daily growth rate (mg/day)
𝑑 𝑅, 𝑤 mortality (year-1).
Reproduction
𝑤𝑚𝑎𝑥
𝑔 𝑅, 𝑤𝑒𝑔𝑔 𝑐 𝑡, 𝑤𝑒𝑔𝑔 = 𝑆 න
𝜇 𝑅, 𝑤 𝑐 𝑡, 𝑤 𝑑𝑤
𝑤𝑟𝑒𝑝𝑟
Resource
dynamics
𝑤𝑚𝑎𝑥
𝑑𝑅
= 𝐷(𝐺(𝑡) – 𝑅) − න
𝐼(𝑅, 𝑤)𝑐(𝑡, 𝑤)𝑑𝑤 ;
𝑑𝑡
𝑤𝐿𝑎𝑟𝑣