Effect of seed density and sediment nutrient heterogeneity on recruitment and early patch growth in the seagrass Cymodocea nodosa

Two mesocosm experiments were performed to examine the effects of seed density and small-scale nutrient heterogeneity on seedling recruitment and early development of patches of Cymodocea nodosa. In the first experiment, seeds were sown at 4 densities selected after determining mean natural density of seeds in the field, and the emergence and survival rates of seedlings were monitored for 1 yr. In the second experiment, seedlings were grown individually for 2 yr, either on substrate made heterogeneous or homogeneous with the addition of a fertiliser or on unfertilised substrate. The emergence rate and proportional survival of seedlings decreased significantly with increasing seed density. Seedlings grown on fertilised substrates were able to form patches, while those grown on unfertilised substrate failed to branch, providing experimental evidence of the role of nutrient limitation as a constraint for patch initiation. The size of seed-generated patches was significantly larger (in terms of total length of the rhizome network and number of shoots) on heterogeneous substrate than on homogeneous substrate at the same overall level of nutrient supply. Apparently, no preferential proliferation of shoots and branches occurred in the nutrient-rich half of the heterogeneous substrate. The results support our hypothesis that negative density dependence at the seed-seedling stage plays an important role in regulating population size. They also reveal that seedlings experiencing nutrient heterogeneity are capable of exploiting resources more efficiently than those in homogeneous environments, but are probably unable to escape from unfavourable patches or to select favourable ones, at least at the scale examined here.