Individual-Based modeling of Copepods Geoffrey Cowles Department of Fisheries Oceanography School for Marine Science and Technology University of Massachusetts-Dartmouth May 4, 2008
Outline Copepod Life Processes and Vital Rates Bioenergetics: Growth and Metabolism Mortality Reproduction Behavior (Vertical Distribution / Feeding) ICPBMS [Batchelder and Williams, 1995] [Miller et al., 1998]
Copepod Growth Likely growth is food limited - in situ food concentrations rarely (except during peak bloom) high enough to support metabolic needs. However: They can ingest prey not measure in situ (microzooplankton, detritus) They can locate and benefit from food patches They may be able to alter their physiological condition during times of meager resources
Copepod Feeding Copepods use cephalic appendages to drive feeding currents Perception based on shear of odor signal Shear diffusion much stronger than Fickian Shear detected by antennulary receptor gives time for reorientation At copepod scales, biologically driven flow stronger than average turbulent flow.
Behavior: Diel Vertical Migrations Vertical distributions of Copepods have shown to under go daily (diel) oscillations Theories for the adaptive significant of DVM: Metabolic advantage by migrating to cooler deep layer when not surface feeding. The individual will attain a higher weight for a given amount of food leading to higher overall fecundity. During the day, copepods swim to depth to avoid harmful UV-B exposure. Predator avoidance: Copepods swim to the food rich layer at night to feed and swim to depth during the day to avoid visual predators.
[Fortier et al., 2001]: DVM in Arctic Species
Reverse DVM Based on the fish larvae models, why would copepods exhibit reverse-dvm?
BiModal DVM Anne Sell, U. Hamburg Related to nutritional status, higher lipids, less risk!
[Fortier et al., 2001]: DVM in Arctic Species Clear exhibition of DVM in medium to large Calanoids Metabolic advantage - not likely because temperature is actually inverted and so well mixed that swimming could not offset any metabolic advantage UV-B - not likely, in ice-covered seas, very little UV-B penetrates Predator avoidance - Cod are present under the ice before breakup. Visual predators at a given light level can see larger prey easier, thus, the depth center of mass of zoop during the day should increase with size. This was the case in their work. Timing - the change in light intensity (max at sunrise/sunset) should trigger migration. Observed timing was sunset triggered upward migration, but downward migration began long before sunrise, indicating a strive to minimize surface exposure.
[Batchelder and Williams, 1995]: Bioenergetics Model of M. lucens Examined effects of model assumptions on growth and development of Metridia lucens in the North Atlantic Model driven by temperature from an ocean weather station Model includes temperature effects
[Batchelder and Williams, 1995]: Bioenergetics Model of M. lucens Daily Growth (weight) W t+1 + W t + G where G = A R is net Growth (µgcd 1 ). Assimilation (A = A E I m F 1 F 2 F 3 F 4 ) is based on four components: F 1 =.466 (1.1) T : Temperature based ingestion F 2 : Ingestion dependence on recent feeding F 3 = W 0.7 : Ingestion dependence on weight F 4 = P φ k+p φ : Dependence on food concentration (Ivlev) the assimilation efficiency A E =.7 and a max ingestion rate I m
[Batchelder and Williams, 1995]: Ingestion from feeding history Maximum daily ingestion rates can increase following starvation. Recent feeding λ t below the half-saturation concentration k leads to increased ingestion rate F 2 F 2 = 1.0 [λ t > k] F 2 = 2.0 (λ t /k) [λ t k] where λ t is the prior feeding history λ t = [λ t 1 + (2 P t )]/3 Requires tracking an additional state variable: λ t.
[Batchelder and Williams, 1995]: Metabolism Metabolism R = R a + R b consists of two components and is ultimately dependent on weight, temperature and feeding level active: R a = 0.4A and basal R b = B F 3 [I > 0] R b = B F 3 F 1 [I = 0]
[Batchelder and Williams, 1995]: Mortality Mortality: constant per day based on stage group (naupliar, immature copepodite, adults) Mortality constant per day Mortality rate based on stage group (naupliar, immature copepodite, adults) Implementation: each day choose a uniform random number [0-1] for each individual, if the number is larger than the daily mortality, individual survives. Mortality tuned in the model to produce reasonable numbers ideally, mortality would include a predation component, but data to sparse. Some studies have converted accoustic signals to predation biomass and used that to parameterize time and depth dependent predation rates.
[Batchelder and Williams, 1995]: Reproduction C 5 copepodites transition to adults at 30µgC females begin producing eggs soon after maturation interval between clutches (IBC) 2-7 days (determined in rearing tanks) clutch sizes: 20-40 eggs (rearing tanks) rearing tanks, - put female in, strain for eggs daily IBC fixed for given simulation
[Batchelder and Williams, 1995]: Searching Behavior Selection of feeding depth though to be based on perception of predation, temperature, and food concentrations. Effective Searching Algorithm Each night, individual selects a depth from a uniform list of depths (random) Resamples again from same list. If new location is significantly better (more food), it proceeds to the new depth and repeats the resampling. If new location not better, stay at current depth. Discriminating ability: 50%, that is a 50% better resource required to move to a new depth. Contrast this with the π fitness model we examined for larval fish where they knew to the precision of the computer their potential growth given the functions and data. Without variation in timing and depth of predation, this model will not likely capture the complete dynamics forcing the vertical distributions.
[Batchelder and Williams, 1995]: Results Growth of 25 individuals: Left: no searching behavior, Right: seaching behavior
[Batchelder and Williams, 1995]: Conclusions Foraging decreased development time Ability to acclimate to low food levels decreased development time Time of high mortality, individuals that can take advantage of heterogeneous environments will be more successful. Time of low mortality, rapid growth not as critical. Food-searching increased inter-annual variability Hunger acclimation reduced inter-annual variability
[Miller et al., 1998]: A coupled bio-physical model Examined advective potential, timing, and development of Calanus finmarchicus Late Winter: Overwintering C.f. (Gen0) ascend from deep basins of GoM Females moult and produce clutches of eggs (Gen1) Eggs hatch and nauplii develop over 2-3 months G1 develop through spring and can produced Gen2 (two generations on Bank simultaneously) Late summer, Calanus C5 (and some G4, C6F) descend to depth and enter diapause for the next six months Repeat.
[Miller et al., 1998]: Key Questions 1 What are the source areas for C. finmarchicus on GB 2 Are C. finmarchicus resting stocks in GoM repopulated by: individuals remaining near the basins, or individuals from GB, or from upstream. 3 Is there an advection cause behind the great concentration of C. finmarchicus at the north end of the Great South Channel in spring? 4 Does the hotspot of Calanus nauplli on the NE peak (believed to support cod/haddock spawning) have an advective explanation?
Calanus finmarchicus: Why study In terms of biomass, C.f. is the dominant copepod in the Gulf of Maine / GB Other important species: Pseudocalanus, Centropages spp., Oithona similis
Calanus finmarchicus: 13 Stages Egg (non feeding) Nauplii (6) Copepodite (5) Adult (can reproduce)
[Miller et al., 1998]: Physical Model Used the bimonthly averaged fields from Quoddy Individuals advected in 25-m surface layer Used a horizontal random walk of 1000 m 2 d 1 This is a 2-D problem (using velocities from 25m vertical average) Individuals exit diapause from Wilkinson, Georges, Jordan, and Emerald (on SS) basins
[Miller et al., 1998]: Domain
[Miller et al., 1998]: Model Mechanics From Diapause to Moult State Vector for Resting C5 1 or 0 Alive or Dead 0 to 10 Inactive to Moult-Ready X Latitude Y Longitude N Element Number Start from 0, on winter solstice, probability of emerging from diapause increase 0.01 per day. After diapause, 10 day delay to moulting (adult) Randomly subsample stock of matured G 0 females to give 500 total.
[Miller et al., 1998]: Model Mechanics Using subsampled females, rerun time from solstice. On maturation, assign the following vector G 0 females 1 or 0 Alive or Dead Variable Age Since Maturation, Days 0 to 1 Clutch Readiness Faction X Latitude Y Longitude Z Depth N Element Number Clutch Readiness CR assigned using T-dependent Clutch interval data (higher temps, shorter interval). CR = 1, clutch of 50 eggs produced, CR reset to 0 Each egg assigned a new vector
[Miller et al., 1998]: Model Mechanics Each egg of G 1 and later generations assigned following vector: Nauplius/Copepodites 1 or 0 Alive or Dead 1 to 12 Stage 0 to 1 Moult Fraction X Latitude Y Longitude Z Depth N Element Number Eggs stage 1, Nauplii 2-7, Copepodites 8-12 Each hour, 1/24 of inverse of stage duration in days added to moult fraction (MF) MF hits 1, stage is updated, MF reset to 0 Mortality applied using daily stage-based survivorship rates - sets first vector component
[Miller et al., 1998]: Model Mechanics When G 1 or G 2 completes C5 development, it either matures or enters diapause (based on rough estimates from field data) Mature/Diapause Ratio G1 50:50 G2 10:90 Male/Female ratio at maturation = 50/50 Generally 10 5 G 2 individuals tracked
[Miller et al., 1998]: No Advection Long duration of C5 makes it dominant day 80-120 Day 100 splits the generations In the field, split occurs nearly a month later (food limits?) 250 females produces 8000 resting C5 from G1 22500 resting C5 in G2
[Batchelder and Williams, 1995]: Dispersal
[Miller et al., 1998]: Abundance on GB Georges is earliest contributor GB G2 from Georges Basin is reduced (by advection) G1 from Scotian Shelf low numbers
[Miller et al., 1998]: Self Recruitment Only Wilkinson is self-sustaining. Remaining basins not self-stocking Must be upstream flux to basins
[Miller et al., 1998]: Conclusions Simulation confirms that Bank is restocked Flux of Calanus from outside GoM (SS) also possible Direct flux from SS possible across the NEC Wilkinson is significantly restocking, with some individuals circumnavigating GB once (G1) or twice (G2) before diapause. Other basins must be restocked from upstream (St. Lawrence, labrador, etc) Hot Spot 1 (GSC where the right whales feed) is full of copepods preparing for diapause Hot Spot 2 (nauplii on NE peak available for cod/haddock larvae), model shows these likely are G1 nauplii from G0 females from George Basin. Mortality is tuned to give a ratio of 100:1 of resting stocks in July and C5+adults in early january.
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