Do nutrient availability and plant density limit seagrass colonization in the Baltic Sea?

Seagrasses continue to decline at an alarming rate throughout the planet's temperate regions. After a decline recolonization or restoration starts fro m small patches of single shoots which then propagate vegetatively. We inve stigated the effects of plant density within a patch and nutrient resources on growth and survival of eelgrass (Zostera marina L.), the dominant seagr ass species in the northern temperate zone. We created small (0.5 m(2)) eel grass patches by planting single shoots in circular plots at high (20 cm) a nd low (40 cm distance between shoots) density. In a factorial design, the sediment was nutrient-enriched (1) through biodeposition of transplanted mu ssels (Mytilus edulis L.) (2) by a slow-release NPK-fertilizer or (3) not e nriched. The experiment was run over 1 growth period at a relatively nutrie nt-poor site (<30 mu mol NH4+ l(-1) porewater) in the Baltic Sea. Mussels i ncreased NH4+ concentrations and the fertilizer increased both NH4+ and PO4 3- in the sediment porewater and the overlying water column, but this had o nly limited effects on eelgrass shoot growth rates and increase in shoot de nsity, which were high overall (up to 75 mm shoot(-1) d(-1), doubling shoot density every 3 mo). In contrast, increased plant density had clear positi ve effects on shoot growth, areal expansion of patches and increase in shoo t density. These results suggest that nutrient availability is not a major factor in eelgrass patch colonization or survival in the Baltic. Positive i nteractions among eelgrass shoots appear to be more important than competit ive processes, during the early stages of recolonization.