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We undertook a long-term (27 mo) field experiment to test if a chronic increase in water column nutrients could cause a decline in 2 temperate Australian seagrasses and if this decline could be linked to nutrient-mediated changes in epiphytes. Two seagrasses, Amphibolis antarctica and Posidonia sinuosa, were exposed to minor increases (~2 to 5×) in nutrient (N, P) concentrations utilising slow-release fertiliser over a 15 mo period at a shallow (~2 m depth), oligo trophic marine site in Gulf St Vincent, South Australia. Fertiliser had a significant detrimental effect on biomass, density, and canopy height in both seagrasses. Moreover, the seagrass biomass reductions coincided with increased epiphyte loads and changes in epiphyte composition. After a 12 mo recovery period, epiphyte loads in the fertiliser treatments had returned to levels comparable to the control, but the fertiliser-treated seagrasses had not recovered. While the precise mechanism of seagrass decline is still unclear, our results have demonstrated that (under certain circumstances) chronic, yet minor, increases in water column nutrient concentrations can cause the slow decline of Amphibolis and Posidonia spp. Furthermore, while future management decisions regarding anthropogenic nutrient discharges into seagrass ecosystems should be assessed on a case-by-case basis, our results and those of other workers investigating large-scale losses of Amphibolis and Posidonia in southern Australia indicate that extreme caution must be applied where these seagrasses occur in shallow, sheltered oligo trophic marine environments.

Nutrient exposure causes epiphytic changes and coincident declines in two temperate Australian seagrasses