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 June-January

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 physical-ecosystem, ecosystem-copepod, etc.

Explore coupled model behaviors, begin hypothesis testing

Ultimate synthesis would be coupled 3D-physical-ecosystem-IBM model over annual cycle

Explore interannual variability, influence of large-space/time scale forcing

Back to Main Report


	Peter Franks (SIO), Changsheng Chen (UMassD), James Pringle, Jeff Runge (UNH), Ted Durbin (URI), Wendy Gentleman (Dalhousie)


	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


	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


	The role of advection

	Population dynamics of zooplankton on GB and the GOM


	Advective supply of Calanus finmarchicus and Pseudocalanus spp. copepodites to GB during January-April and the role of winds


	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


	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?


	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.


	Advective supply and loss of Calanus finmarchicus to GOM basin diapausing populations during June-January


	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


	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)?


	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.


	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


	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?


	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


	Stratification and variability in food supply: the role of food limitation


	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


	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?


	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.


	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.


	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?


	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


	Physical models:
			2D ECOM-si GB model
			3D ECOM-si GOM /GB model
			3D FVCOM GOM /GB
	Particle tracking:
			10⁶ passive particles


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


	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 physical-ecosystem, ecosystem-copepod, etc.
			Explore coupled model behaviors, begin hypothesis testing
	Ultimate synthesis would be coupled 3D-physical-ecosystem-IBM model over annual cycle
			Explore interannual variability, influence of large-space/time scale forcing