Algal mats reduce eelgrass (Zostera marina L.) growth in mixed and monospecific meadows

- Eelgrass productivity and growth were suppressed by the presence of macroalgal mats.
- Plant richness did not ameliorate algal mat-induced stress in eelgrass.
- Algal mats pose a serious threat to seagrass meadows.

Harmful algal blooms are considered to be one of the biggest threats to benthic vegetation. The amount of annual algal mats has increased due to eutrophication but their effects on perennial vegetation have rarely been tested in field experiments. In regions where eelgrass (Zostera marina) grows in mixed meadows, plant species richness may ameliorate algal-induced stress through facilitative mechanisms and improve eelgrass growth and survival. To test this, we conducted a 3 mo. field experiment at 3.5 m depth in the Baltic Sea, where we applied filamentous brown algae (300 g ww) to eelgrass monocultures and polycultures (Z. marina, Ruppia cirrhosa, Stuckenia pectinata and Potamogeton perfoliatus). The algal stress period lasted for 13 d and plots were sampled after a 5 wk recovery period. In both algal-stressed mono- and polycultures, the presence of algal mats significantly reduced eelgrass shoot and root biomass production and affected the leaf growth negatively. Plant richness did not have strong effects on eelgrass growth and eelgrass in both algal-stressed mono- and polycultures showed significantly reduced productivity. Our results clearly demonstrate that loose-lying algal mats pose a serious threat to seagrass meadows and that plant diversity cannot alone ameliorate negative effects by macroalgal mats.