| dc.description | Along the Atlantic coast of Nova Scotia, population outbreaks of herbivorous sea urchins Strongylocentrotus droebachiensis have caused major transitions in the shallow subtidal ecosystem from kelp beds to barrens (areas devoid of fleshy macroalgae). This thesis examines the effects of diet on the reproductive ecology of S. droebachiensis, and the influence of reproduction on the population dynamics of this species and hence the dynamics of the ecosystem. Using gonad index and histological methods, I demonstrated that S. droebachiensis has an annual reproductive cycle with spawning in March/April. A field study at two sites showed that sea urchins in kelp beds and grazing fronts (high-density aggregations at the borders of kelp beds) consumed a higher quality diet and had a higher gonad index than those in barrens. Dietary differences did not influence the quality of gonads. Analysis of size-at-age data indicated that diet had a small effect on adult growth rates, which were marginally higher in kelp beds and grazing fronts than barrens at one site. Juvenile growth rates showed the same pattern among habitats but differences were more pronounced. I measured the effects of food ration and feeding regime on reproductive maturation and growth of juveniles in a 22-month laboratory experiment. Sea urchins fed a high ration of kelp, with or without a protein (mussel flesh) supplement, and most of those fed a low ration of only kelp, were reproductively mature after 10 months. In contrast, sea urchins fed only coralline algae remained immature at the end of the experiment, and had lower survival rates than those in kelp-fed treatments. Growth rate and gonad index were high in sea urchins fed kelp and mussels, intermediate in those fed a high ration of kelp, and low in those fed a low ration of kelp. These results suggest that juveniles in kelp beds, because of a better diet, have a greater reproductive value than those in barrens. I used larvae produced by adults from this experiment to investigate the effects of parental nutritional condition and larval food (phytoplankton) ration on larval traits in a follow-on experiment. Larval food ration had a strong positive effect on the rates of development and metamorphosis; in contrast, parental nutrition had little effect on these variables. However, my results suggest that when planktonic food is abundant, larvae of well-nourished adults in kelp beds may metamorphose sooner than those of poorly nourished adults in barrens. Using models of fertilization kinetics and egg production-based on data from this study and the literature, I predicted that sea urchins in barrens make the largest contribution to the overall zygote pool during the transition from kelp beds to barrens. Model results also suggest that temporal variation in zygote production alone cannot explain sea urchin outbreaks off Nova Scotia. | en_US |
