Belowground stressors and long-term seagrass declines in a historically degraded seagrass ecosystem after improved water quality

Matthew W Fraser,Gary A Kendrick

doi:10.1038/s41598-017-14044-1

Abstract

Continued seagrass declines in ecosystems with improved water quality may be driven by sediment stressors. One of the most cited examples of a seagrass ecosystem with declines is Cockburn Sound, Western Australia, where 75% of seagrasses (2169 ha) were lost in the 1960s–1980s due to poor water quality. Water quality has subsequently improved in Cockburn Sound, yet shoot density declines continue in some areas. Here, we investigated if sediment stressors (sulfide intrusion and heavy metals) contributed to declining Posidonia sinuosa shoot densities in Cockburn Sound. Seagrass δ34S were depleted at sites with a history of seagrass declines, indicating seagrasses at these sites were under sulfide stress. Heavy metals (Fe, Zn, Mn, Cr, Cu and Cd) in sediments and seagrasses did not show clear patterns with shoot density or biomass, and largely decreased from similar measurements in the late 1970s. However, seagrass cadmium concentrations were negatively correlated to seagrass biomass and shoot density. High cadmium concentrations interfere with sulfur metabolism in terrestrial plants, but impacts on seagrasses remain to be explored. Given that sulfide intrusion can prevent recolonization and drive seagrass declines, management plans in degraded seagrass ecosystems should include management of sediment stressors and water quality to provide comprehensive management.

Highlights

Seagrass declines threaten the substantial ecological and economic services that seagrass ecosystems provide
Posidonia sinuosa shoot density significantly differed across the study locations (F2,33 = 4.8, p = 0.015), with Warnbro Sound (WS) having lower densities than Eastern Banks (EB) and Garden Island (GI) (Table 1)
Shoot density ranged from a minimum of 300 shoots m−2 at WS 3 m to a maximum of 2050 shoots m−2 at GI 2 m

Summary

Introduction

Seagrass declines threaten the substantial ecological and economic services that seagrass ecosystems provide. Seagrass loss has historically been linked to reduced water quality driven by disturbances such as coastal development and nutrient inputs[1]. Reduced light availability limits the capacity of plants to oxygenate surrounding sediments through radial oxygen loss from roots[4], which can increase the susceptibility of seagrasses to sulfide intrusion by limiting the diel cycling of sulfide and iron in the rhizosphere[5]. More negative values indicate a higher contribution of sulfides from sediments (sediment average: −15‰ to −25‰), while a more positive value indicates that sulfate from the water column is the dominant S source for the plant (seawater average: +21‰)[21] Anthropogenic pollutants such as heavy metals have previously been implicated as drivers of seagrass declines, particular in areas adjacent to coastal development[22]. Long-term exposure to metals at slightly elevated concentrations may decrease seagrass health, or lower seagrass resilience to other stressors

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