Effects of predation on Zostera marina L seed abundance

Predator effects on Zostera marina L. seed abundance were studied in the York River, VA, USA, using enclosure and exclosure caging experiments. Seeds were placed in cages in two concurrent experiments. The first experiment was a predator exclosure experiment to test the effects of excluding predators, using a full predator exclosure cage, a partial exclosure top-only cage, a partial exclosure side-only cage and uncaged plots. The second experiment was a predator enclosure experiment, using two highly abundant macro-benthic predators in the Chesapeake Bay: the decapod crustacean Callinectes sapidus Rathbun and the sciaenid fish Micropogonias undulatus L. Additionally, two-week long trials of sequentially protected and exposed seeds were also performed. Replicate treatment plots were sampled by removing the top 5-10 cm of the sediment surface with a suction sampler and still viable seeds in each plot were counted. Full exclosure cages contained significantly higher numbers of seeds than the uncaged or partial caged treatments. Seed abundances in the C. sapidus enclosure cages were significantly less than the full exclusion cage, but not significantly different than the uncaged treatments. Seed abundances in the M. undulatus cages were not significantly different than the full exclusion cage. The least number of seeds were found in the uncaged and partial cage treatments. Results of the sequentially protected and exposed trials were similar to results from the one-week uncaged treatments. These experiments suggest that seed predation can affect the abundance of Z. marina seeds, possibly causing up to 65% of the seed losses observed in these experiments, Results suggest that seed predation has the potential to be an important force governing the sexual reproductive success and propagation of eelgrass beds and that the degree of seed loss via predation may be related to predator and primary food abundances.