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: 10 6 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