Effects of filamentous macroalgae mats on growth and survival of eelgrass, Zostera marina, seedlings

A laboratory experiment was conducted to assess the effect of filamentous algae mats on the performance of seedlings of the eelgrass, Zostera marina. The seedlings were covered by three levels (3, 6 and 9 cm) of natural (Chaetomorpha linum) and imitation algae mats and it was hypothesised that the effects of the natural algae on seedling growth may be more severe because of the metabolic demands of the algae. Results show that coverage by both C. linum and imitation algae significantly reduced seedling growth and increased allocation of resources to above ground tissues. No clear effects of algae type on seedling performance were found and since there were no differences in oxygen or sulphide concentrations between the treatments, we attribute the reductions in seedling growth to algal shading. In a comparison with other studies it is shown that lower turbulence, higher temperature, organically enriched sediments and longer duration of exposure than applied here may result in oxygen depletion thereby intensifying the negative effects of light attenuation by algal mats on seedling performance.

► Performance of eelgrass, Zostera marina, seedlings under macroalgae mats.
► Shading versus poor biogeochemical conditions.
► Shading by algal mats reduces growth of seedlings.
► Physical parameters control the formation of reduced biogeochemical conditions within mats.