Sedimentation stress in a scleractinian coral exposed to terrestrial and marine sediments with contrasting physical, organic and geochemical properties

Terrestrial runoff increases siltation and nutrient availability on coastal coral reefs worldwide. However the factors determining stress in corals when exposed to short-term sedimentation, including the interactions between sediments and nutrients, are little understood. We exposed corals to ten different sediment types at environmentally relevant concentrations (33 to 160 mg DW cm⁻²) and exposure times (12 to 60 h) in laboratory and field experiments. The sediments originated from 2 estuaries, 2 nearshore and one offshore locations and also included ground-up aragonite. For two of these sediments, three grain size fractions were used (silt < 63 μm, fine sand: 63-250 μm, medium sand: 250-500 μm). Sediments were characterised by 19 parameters grouped into “physical”, “organic and nutrient-related” and “geochemical” parameters. Changes in the photosynthetic yield of the coral Montipora peltiformis was measured by pulse-amplitude modulated chlorophyll fluorometry (PAM) as proxy for photophysiological stress from exposure, and to determine rates of recovery. Different sediments exerted greatly contrasting levels of stress in the corals. Our results show that grain size and organic and nutrient-related sediment properties are key factors determining sedimentation stress in corals after short-term exposure. Photophysiological stress was measurable after 36 h of exposure to most of the silt-sized sediments, and coral recovery was incomplete after 48 to 96 h recovery time. The four sandy sediment types caused no measurable stress at the same concentration for the same exposure time. Stress levels were strongly related to the values of organic and nutrient-related parameters in the sediment, weakly related to the physical parameters and unrelated to the geochemical parameters measured. M. peltiformis removed the sandy grain size classes more easily than the silt, and nutrient-poor sediments were removed more easily than nutrient-rich sediments. Anoxia developed on the sediment surfaces of the nutrient-rich silts, which had become slimy and smelled of hydrogen sulphide, suggesting increased bacterial activity. Our finding that silt-sized and nutrient-rich sediments can stress corals after short exposure, while sandy sediments or nutrient-poor silts affect corals to a lesser extent, will help refining predictions of sedimentation threats to coral reefs at given environmental conditions.