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

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.

Figure 1

Mean δ³⁴S for seagrasses in Cockburn and Warnbro Sound. (A) Leaf, (B) Rhizome and (C) Root δ³⁴S of seagrasses across three regions in Cockburn and Warnbro Sound. Colour of bars represent depth of sites (white = 2 m, light grey = 3 m, dark grey = 5 m, black = 7 m). Each bar represents the mean δ³⁴S at each site, with error bars representing 1 standard error (n = 3). Mean (D) Leaf, (E) Rhizome and (F) Root δ³⁴S grouped by long-term trends in shoot density at each site. White bars represent sites with a history of shoot declines, while grey bars are from sites with no shoot declines. Each bar represents the mean δ³⁴S, with error bars representing 1 standard error (n = 18).

Figure 2

Scatterplots showing relationship between (A) rhizome [Fe] and root δ³⁴S and (B) rhizome [Fe] and rhizome δ³⁴S. Dashed lines represent linear model, while grey outline shows 95% confidence intervals. White dots denote replicates from sites with a history of seagrass declines, while black dots denote replicates from sites with steady seagrass trends.

Figure 3

Scatterplots showing relationship rhizome cadmium concentrations and shoot densities. Dashed line represents linear model, grey outline shows 95% confidence interval. White dots denote replicates from sites with a history of seagrass declines, while black dots denote replicates from sites with steady seagrass trends.

Figure 4

Map showing study area. GI = Garden Island, WS = Warnbro Sound, EB = Eastern Banks. Red icons denote sites with a history of seagrass declines, while green icons represent sites with steady shoot density trends. Map was created with QGIS version 2.8.2 (Open Source Geospatial Foundation Project, http://qgis.osgeo.org).