Variation in egg spawning among subpopulations of sea urchins Strongylocentrotus droebachiensis: a theoretical approach

Abstract

Variation in zygote production, by determining the initial size of the larval pool, can affect the population dynamics of marine invertebrates with a planktonic larval stage. In this paper, we model tem- poral and spatial patterns in egg spawning of the sea urchin Strongylocentrotus droebachiensis in the shal- low 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 Using data from the literature and unpublished sources, we parameterize mathematical models to predict egg spawning for each subpopulation, both on an areal basis and for the entire coast.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 mag- nitude higher in grazing fronts (7.1 〈 10 7 ) than in transitional and post-transitional barrens (5.8 〈 10 6 and 4.4 〈 10 6 , respectively), and 4 to 6 times higher in barrens than in established kelp beds (1.0 〈 10 6 ). Dif- ferences 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 com- bined) on a coastal scale increases ~6-fold from the established kelp bed state (5.6 〈 10 14 ) to the late tran- sition state (3.1 〈 10 15 ) and then drops slightly in the barrens state (2.4 〈 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 (4.4 〈 10 7 , 2.1 〈 10 6 , 1.9 〈 10 6 , and 1.5 〈 10 5 ). Total zygote production (all subpopulations combined) on a coastal scale increases ~16-fold from the established kelp bed state (8.1 〈 10 13 ) to the late transition state (1.3 〈 10 15 ) and then drops slightly in the barrens state (1.1 〈 10 15 ). During most of the transition state, sea urchins in barrens contribute the great- est 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 over- estimated. 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.