| dc.description.abstract | Variation in zygote production, by determining the initial size of the larvalpool, can aff ect
the population dynamics of marine invertebrates with a planktonic larval stage. In this paper, we
model temporal and spatial patterns in egg spawning of the sea urchin Strongylocentrotus
droebachiensis in the shallow subtidal zone along the Atlantic coast of Nova Scotia. In this region,
population outbreaks of sea urchins have caused major transitions from kelp beds to barrens (areas
devoid of fleshy macroalgae). We define 3 community states and associated subpopulations of sea
urchins: (1) the established kelp bed state, where sea urchins are in low density and small; (2) the
transition state, with sea urchins in kelp beds, newly created (transitional) barrens, and grazing
fronts (dense aggregations of large sea urchins along the edges of kelp beds); and (3) the barrens
state after fronts have dispersed and intermediate densities of moderately sized sea urchins remain
in post-transitional barrens. Using data from the literature and unpublished sources, we
parameterize mathematical models to predict egg spawning for each subpopulation, both on an areal
basis and for the entire coast. The predicted number of eggs spawned per m(2) is 1 order of
magnitude higher in grazing fronts (7.1 x 10(7)) than in transitional and post-transitional barrens
(5.8 x 10(6) and 4.4 x 10(6), respectively), and 4 to 6 times higher in barrens than in established
kelp beds (1.0 x 10(6)). Differences among subpopulations in the number of eggs spawned are directly
related to differences in adult density, female gonad output, and body size. The total number of
eggs spawned (all subpopulations combined) on a coastal scale increases similar to6-fold from the
established kelp bed state (5.6 x 10(14)) to the late transition state (3.1 x 10(15)) and then drops
slightly in the barrens state (2.4 x 10(15)). During most of the transition state, sea urchins in
barrens spawn the greatest number of eggs. Based on published values, we estimate that fertilization
rates are highest in grazing fronts (62 %), intermediate in transitional and post-transitional
barrens (36 and 43 %,respectively) and lowest in kelp beds (15 %), resulting in up to 2 orders of
magnitude differences in the number of zygotes produced in these subpopulations 14.4 x 10(7), 2.1 x
10(6), 1.9 x 10(6), and 1.5 x 10(5)). Total zygote production tall subpopulations combined) on a
coastal scale increases similar to 16-fold from the established kelp bed state (8.1 x 10(13)) to the
late transition state (1.3 x 10(15)) and then drops slightly in the barrens state (1.1 x 10(15)).
During most of the transition state, sea urchins in barrens contribute the greatest number of
zygotes to the total zygote pool, with those in grazing fronts contributing up to 44 % of all
zygotes. Sensitivity analysis and evaluation of model assumptions indicate that our predictions
should be correct in a relative sense (i.e. comparing among subpopulations), although absolute
numbers may be overestimated. A disparity between temporal patterns of estimated zygote production
and observed settlement rates of S, droebachiensis in the shallow subtidal zone suggests that zygote
production interacts with other factors to determine settlement rates of this species. | en_US |
