Drift algae, an invasive snail and elevated temperature reduce ecological performance of a warm-temperate seagrass, through additive effects.
Seagrasses are under pressure from multiple concurrent threats, including rising temperatures, invasive species and nutrient-driven algal accumulations. We quantified the abundance of drift algae and the invasive snail Batillaria australis in 3 Halophila ovalis seagrass beds in the Swan River Estuary (Perth, Western Australia), and tested in an aquarium experiment for interactive effects of temperature (21 vs. 27°C), cover of drift algae Chaetomorpha linum (0 vs. 1.4 kg fresh wt m-2) and presence (0 vs. 120 snails m-2) of B. australis on the ecological performance of H. ovalis. The survey showed that drift algae varied considerably between sites and sampling times, and sites experienced average loads of 0.4 to 0.8 kg fresh wt m-2 and extreme loads up to 2.5 kg fresh wt m-2. In contrast, invasive snails were constantly abundant at all sites at all collection times (mean abundance: 489 snails m-2). In the laboratory experiment, drift algae caused increased shoot mortality and leaf loss rate, and suppressed the formation of new nodes. Drift algae also decreased the depth of the sulphide horizon in the sediment and the oxygen concentration in the water column at night, suggesting increased risk of hypoxia. Invasive snails reduced the biomass of H. ovalis leaves and roots, increased leaf plastochrone interval and decreased the depth of the sulphide horizon. Finally, elevated temperature increased leaf loss and reduced leaf biomass, and, in the presence of drift algae, also reduced the length of the 2nd internode. We found relatively few significant higher-order interactions, suggesting a dominance of additive effects of stress. We conclude that temperature, drift algae and invasive snails are already affecting the ecological performance of H. ovalis in Swan River, and that these seagrass beds are likely to come under further pressure if these stressors increase in the future.