Interaction of substrate muddiness and low irradiance on seagrass: A mesocosm study of Zostera muelleri

•Substrate muddiness interacts with low irradiance to affect seagrass performance.•Seagrass growing in muddy substrates requires a higher irradiance for persistence.•Substrate light interaction should be considered for habitat suitability assessment. Seagrass meadows are important estuarine habitats, and in recent decades, have suffered global declines. Fine sediment pollution is recognised as a major cause of decline, usually attributed to the combined effects of reduction of photosynthetically available radiation (PAR) and burial. However, intruded fine sediment affects the seagrass rhizosphere and this interacts with reduced irradiance to affect seagrass performance. We undertook a 2 × 2 factorial mesocosm experiment, to examine the interaction between substrate muddiness and irradiance on seagrass growth and survival over a six-week period. The seagrass Zostera muelleri was grown on two substrates from the same estuary: (1) an inner estuary substrate with high mud content (42 %) from a location where seagrass formerly grew; and (2) an outer estuary substrate with moderate mud content (20 %) from a location at which seagrass persists. Two irradiance levels were used: (1) low (6.3 mol quanta m−² d−¹) and (2) very low (2.3 mol quanta m−² d−¹) both above a published compensation irradiance (Ec) of 1.9 mol quanta m−² d−¹. Belowground biomass and rhizome growth were significantly reduced by substrate muddiness but not detectably affected by irradiance. Shoot growth, was reduced by both reduced irradiance and increased muddiness, with a significant interaction. We conclude that muddification of substrates imposes an increased irradiance requirement for Z. muelleri to cope with adverse rhizosphere conditions, which should be taken into account when planning seagrass conservation and restoration interventions.