Transcript Advective supply and loss of

GLOBEC-01: Zooplankton population
dynamics on Georges Bank:
model and data synthesis
Peter Franks (SIO), Changsheng
Chen (UMassD), James Pringle, Jeff
Runge (UNH), Ted Durbin (URI),
Wendy Gentleman (Dalhousie)
Goals
• To improve our mechanistic understanding
of the possible influences of climate
variation on the population dynamics and
production of the target zooplankton species
through its effects on advective transport,
temperature, food availability, and predator
fields
Background: Calanus finmarchicus, Pseudocalanus
moultoni, P. newmani, and Oithona similis
• Calanus is a more opportunistic, highly fecund, broadcast
spawner
• Pseudocalanus and Oithona carry their eggs in egg sacs
(an adaptation thought to reduce egg mortality), and have
lower maximum egg production rates
• C. finmarchicus and Pseudocalanus exhibit different depth
preferences and different susceptibilities to food limitation
and predation
• Also appear to have different source regions, although this
is poorly understood
Questions and Hypotheses
• The role of advection
• Population dynamics of zooplankton on GB
and the GOM
Questions and hypotheses:
The role of advection
• Advective supply of Calanus finmarchicus
and Pseudocalanus spp. copepodites to GB
during January-April and the role of winds
Advection: supply to GB
Background
• Modelling studies suggest that the eastern GOM (strong
influence of SS and/or Slope water) a major source of
near-surface copepods to the NEP
• Western GOM populations supply the crest of GB during
winter wind-driven flows
• These studies used climatological winds - do not capture
variability in 2-15 d band, or interannual variability
Advection: supply to GB
Advection: supply to GB
Questions
• What are the candidate source regions for the three
species?
• How do these change through the season?
• How does physical variability affect these advective
supplies and the relative importance of different advective
pathways?
• Does interannual variability in January-February mean
winds control the origin of copepods transported onto the
bank?
Advection: supply to GB
Hypotheses
• Winter and early spring cross-isobath transport of
copepods is largely caused by locally and event-forced
surface Ekman fluxes.
• Transport paths differ between species and vary seasonally.
• Interannual variability in the source and number of
copepods delivered to GB in January and February will be
directly related to the interannual variability in the winds
over those two months.
• Near-surface copepods will be deposited on GB because of
the reduction in the Ekman velocity caused by the sudden
deepening of the mixed layer there through tidal mixing.
Questions and hypotheses:
The role of advection
• Advective supply and loss of Calanus
finmarchicus to GOM basin
diapausing populations during JuneJanuary
Advection: supply/loss to GOM
Background
• Are GB copepods endogenous to GOM or exogenous (SS,
Slope water)?
• Deep-water circulation affects supply/loss to basins:
• retaining and/or concentrating animals in the basin gyres
• advectively connecting the basin populations residing above the
shallowest closed isobath
• advecting Slope Water animals into the GOM through the NEC
• Exchange of slope and basin copepod populations
profoundly affected by strongly interannually varying
winds
• Turnover of diapausing populations in late summer/fall
Advection: supply/loss to GOM
Questions
• How long will animals in the deep GOM waters remain in
the GOM, i.e. what is the residence time of the deep water?
• To what extent does the deep-water flow move the basin
populations to other basins?
• Do some basins retain diapausers more efficiently than
others?
• How sensitive are the answers to variations in the
circulation (e.g., driven by interannually varying winds,
and ultimately by the NAO)?
Advection: supply/loss to GOM
Hypotheses
• Large-scale geostrophic wind-driven currents will be
strong for isobaths which are not closed.
• Wilkinson and Jordan Basins (which have closed isobaths) will retain
diapausers efficiently, while Georges Basin (which does not have a
closed 200 m isobath) may lose or gain organisms through the NEC to
and from the shelf/slope and the MAB.
• Deep-water circulation may cause some loss of animals out
through the GSC.
• The counterclockwise gyre circulation in the basins may
drive a bottom Ekman current that can concentrate
diapausers in the deep basins.
Questions and hypotheses:
The role of advection
• Role of advection for copepod populations
on GB
Advection: supply/loss to GB
Background
• Fronts have implications for the relative importance of
local vs. exchange processes, and the environmental
conditions experienced by the plankton on GB
• Animals on the crest are generally retained on GB
(Gentleman, 2000), and experience high food and
predation levels
• Animals on the lower-food SF are generally advected off
GB in winter-spring, but may be advected northward, and
possibly even back to the NEP in late spring
Advection: supply/loss to GB
Questions
• How do the time scales of advection change with
interannual and/or event-level variations in the physical
flow?
• Will inclusion of physical variability influence copepod
loss rates more than incorporation of the details of
swimming behaviors of copepod life stages?
Advection: supply/loss to GB
Hypotheses
• Inclusion of physical variability will have a greater effect
on copepod loss rates from GB and on different regions of
the bank than incorporation of the details of behavior
• Particles with certain behaviors may be retained on GB
more than passive particles, however most of the loss will
be caused by variability in physical forcing
Questions and hypotheses
Population dynamics
• Stratification and variability in food supply:
the role of food limitation
Population dynamics:stratification and food
Background
• Food limitation period of Calanus egg production varies
from year to year
• Regional timing of blooms varies in space, and type of
food resource varies in space and time
• Copepod developmental rates correlated with chlorophyll,
but chlorophyll likely a proxy for other food sources
Population dynamics: stratification and food
Questions
• How does interannual variability in heat fluxes and
horizontal freshwater fluxes modify the onset of
stratification and subsequent primary production in the
GOM and SF?
• What is the relationship between stratification and the
strength and timing of copepod food limitation?
• How does the timing and location of the winter bloom over
the GOM affect the population structure of copepods
coming onto GB?
• Can food limitation and the absence of deep resting stage
explain why Pseudocalanus are not observed over the
Central GOM?
Population dynamics: stratification and food
Hypotheses
• Changes in abundance and size-class structure of the
plankton are caused by changes in stratification.
• Timing of blooms over GOM and GB controlled by
surface turbulence/cooling vs. solar heating/advection of
buoyant SS water.
• Early winter bloom over GOM leads to enhanced copepod
abundance on GB.
• Low total food on the SF in April is a recurrent but
predictably variable feature, arising from a combination of
changing stratification levels and increased grazing
pressure by copepods.
Questions and hypotheses
Population dynamics
• Mortality and invertebrate predation
Population dynamics:mortality/predation
Background
• Calanus mortality varies spatially and temporally on GB;
losses due to mortality > advective losses
• Invertebrate predators include Centropages, Metridia,
Temora, Sagitta, and Pleurobrachia
• High consumption of all copepod life stages by the hydroid
Clytia gracilis, particularly on the crest; predator
populations peak there in April-May
• Cannibalistic feeding by C. finmarchicus may lead to
density- dependent mortality.
Population dynamics:mortality/predation
Questions
• How much of the heterogeneity of observed trends in
abundance of the target species on GB can be explained by
differential mortality?
• What is the relationship between mortality rate and
predator abundance?
• What are the mechanisms that cause all regions to exhibit
low naupliar abundances in April-May?
Population dynamics:mortality/predation
Hypotheses
• Variation in mortality rate is an important source of
variation in abundance of the target copepod species.
• This variation is linked to climate by its influence on
advection of females and late copepodite stages from the
GOM.
• Mortality of Calanus egg and naupliar stages is an
important loss of prey for fish larvae feeding on the SF
Tools
• Physical models:
• 2D ECOM-si GB model
• 3D ECOM-si GOM /GB model
• 3D FVCOM GOM /GB
• Particle tracking:
• 106 passive particles
Tools
• Biological models:
• Ecosystem models (NPZ, mass-stratified models)
• Copepod population dynamics (stage-structured
IBM)
– food limitation effects on different aspects of the vital
rates
– individual variability in development and reproduction
– age-within-stage-dependent mortalities
Approach
• Concentrate on 1995, 1998, 1999 (most complete data sets)
• Begin working in parallel - physical models/particle
tracking, ecosystem models, copepod models
• Perform idealized studies
• Collate data
• Subsequently begin coupling models - 3D physicalecosystem, ecosystem-copepod, etc.
• Explore coupled model behaviors, begin hypothesis testing
• Ultimate synthesis would be coupled 3D-physicalecosystem-IBM model over annual cycle
• Explore interannual variability, influence of large-space/time scale
forcing