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

Abstract

Seagrasses continue to decline at an alarming rate throughout the planet's temperate regions. After a decline recolonization or restoration starts from small patches of single shoots which then propagate vegetatively. We investigated the effects of plant density within a patch and nutrient resources on growth and survival of eelgrass (Zostera marina L.), the dominant seagrass species in the northern temperate zone. We created small (0.5 m2) eelgrass patches by planting single shoots in circular plots at high (20 cm) and low (40 cm distance between shoots) density. In a factorial design, the sediment was nutrient-enriched (1) through biodeposition of transplanted mussels (Mytilus edulis L.) (2) by a slow-release NPK-fertilizer or (3) not enriched. The experiment was run over 1 growth period at a relatively nutrient-poor site (<30 μmol NH4+l-1 porewater) in the Baltic Sea. Mussels increased NH4+ concentrations and the fertilizer increased both NH4+ and PO43- in the sediment porewater and the overlying water column, but this had only limited effects on eelgrass shoot growth rates and increase in shoot density, 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 positive effects on shoot growth, areal expansion of patches and increase in shoot density. These results suggest that nutrient availability is not a major factor in eelgrass patch colonization or survival in the Baltic. Positive interactions among eelgrass shoots appear to be more important than competitive processes, during the early stages of recolonization.