Forecasting impacts of changing detrital biodiversity on Australia's estuarine ecosystems

A project undertaken at Macquarie University and supervised by M Bishop

Annually, the world’s estuarine and coastal areas contribute more than US$12.6 trillion worth of ecosystem goods and services, more than twice that provided by the world’s forests. Much of this value is derived from the rich microbial and faunal communities of soft sediments, which play an important role in global carbon and geochemical cycling and support food webs that include commercially-important fisheries, threatened shore birds and marine mammals. Our recent work has identified substantial changes to the detrital inputs (non living organic material) into soft-sediment systems in Australian estuaries since 1950 from predominantly seagrass dominated towards more labile micro- and macroalgal sources. These long term changes to detrital composition may cause sediment anoxia, decreased biodiversity, productivity and ecosystem function.

To assess whether soft sediments in urbanised Australian estuaries are heading towards severe anoxia, reduced biodiversity and decreased production of heavily populated coasts in other parts of the world, we:

  1. Investigated causal links between detrital identity and diversity, microphytobenthos, faunal assemblages and secondary production.
  2. Determined the combined impacts of detrital enrichment and elevated nutrient loads on soft-sediment communities, microphytobenthos and productivity.
  3. Forecast impacts of predicted changes in detrital composition, nutrient enrichments and predatory fish, birds and crabs on soft sediment community structure, primary and secondary production and probability of sediment anoxia.

Most food web models treat the diverse sources of detritus that fuel coastal systems as a homogeneous resource. We showed that detrital sources vary markedly in their value to consumers (Bishop et al. 2010), and when mixed they produce non-additive effects that are not easily predicted from their individual components (Bishop & Kelaher 2008). Consequently, the replacement of native with invasive detrital sources can have large impacts on food-webs (Bishop & Kelaher in review). Similarly, decline and local extirpation of macrophytes can impact nearby soft-sediment habitats dependent on the material as an organic matter subsidy (Bishop et al. 2010).

Impacts of invasive species to detrital food webs

We assessed how in highly modified estuaries the replacement of seagrasses with the invasive alga Caulerpa taxifolia influences the detrital food web of mudflats that are dependent on spatial subsidies of organic material from submerged aquatic vegetation. Following enrichment of sediment plots with either Caulerpa taxifolia or one of the native seagrasses Posidonia australis or Zostera capricorni, the abundance of surface-dwelling microalgae was greater in plots receiving the non-native alga than either of the two seagrasses. This enhancement of primary productivity in plots receiving C. taxifolia did not translate to greater macroinvertebrate abundances. By 4 months after enrichment, there were fewer macroinvertebrates in these plots with the high loading of C. taxifolia than in any other treatment (Fig. 4). We argue that this effect of C. taxifolia at high loading is due to deterrence of invertebrates by caulerpenynes. Our results demonstrate that replacement of native with invasive litter sources can have large impacts on detrital pathways of nutrition.

Cross-habitat impacts of species decline

Human activities are changing the distribution and abundance of rocky reef macroalgae that supply detritus to sedimentary habitats. The once abundant canopy-forming macroalga, Phyllospora comosa, is now locally extinct from 70 km of coastline around urbanised Sydney, Australia. Reefs are now dominated by the smaller species Sargassum spp., with lesser amounts of the kelp Ecklonia radiata. Using manipulative field experiments we assessed impacts to soft-sediment communities of: (1) replacing P. comosa with Sargassum sp. or E. radiata; and (2) varying the ratio of these inputs. Moderate dosing of estuarine sediments with P. comosa increased the abundance and species richness of macroinvertebrates (Fig. 5). Sargassum sp. and E. radiata each increased macroinvertebrate species richness relative to controls, but neither was able to sustain the densities of fauna supported by P. comosa (Fig. 5). Detrital mixtures dominated by P. comosa supported similar macroinvertebrate communities to monocultures of the alga, but mixtures with less than one third P. comosa were not functionally equivalent. Thus, even P. comosa population declines will have cascading impacts on estuarine sediments that are apparent long before the species becomes locally extinct. As populations of canopy-forming macroalgae decline in other parts of the world, similar impacts to subsidised soft-sediment ecosystems may be expected.

Impacts of nutrient enrichment on soft-sediment communities

Our experimental enrichments of estuarine sediments with detritus and Osmocote fertilizer demonstrated that impacts of nutrient enrichment are dependent on its source (algae vs fertilizer). Enrichment of sediments with fertilizer had strong positive effects on primary productivity of algae in surface sediments that were not replicated by addition of labile or refractory detritus. Predators dampened the effects of nutrient enrichment on sediment communities. In systems with abundant predators, moderate nutrient enrichment of sediments impacted bacterial communities and primary production, but these impacts did not propagate up to macroinvertebrates. When predators were removed from this system (such as in the instance of overharvest of fisheries), moderate nutrient enrichment led to sediment anoxia and collapse of macroinvertebrate communities. Nutrient impacts were exacerbated by the decomposition of algal blooms, which rob sediments of oxygen.

This work is significant because it focuses on soft sediment eutrophication caused by excessive detritus, which is one of the most severe widespread impacts of coastal ecosystems, and the cause of loss of fisheries resources and ecosystem function. As coastal populations increase, this knowledge will be invaluable for maintaining water quality criteria that ensure the sustainability of estuarine soft-sediment communities and the food webs and ecosystem functions that they support.

Publications (as of June 2010)

Bishop MJ, Coleman MA, Kelaher BP (2010) Cross-habitat impacts of species decline: response of estuarine sediment communities to changing detrital resources. Oecologia 163:517–525

Bishop MJ, Kelaher BP (2008). Non-additive, identity-dependent effects of detrital species mixing on soft-sediment communities. Oikos 117:531-542.

Captions to Figures

Figure !. Botany Bay is highly developed estuary showing symptoms of excessive nutrient enrichment.

Figure 2. Experiments in Botany Bay investigating the links between detrital impacts and food availability for fish and wading birds.

Figure 3. Tidally-driven field work in Botany Bay often requires very early starts.

Figure 4. Experimental enrichment of 0.25 m2 sediment plots with 60 g (patterned bars) of dried detritus from the invasive alga Caulerpa taxifolia (C, white bars) reduced abundances (mean + SE, n = 7) of macroinvertebrates below those of plots receiving 30 g (plain bars) or 60 g (patterned bars) of dried detritus from the native seagrasses Posidonia australis (P, dark bars) and Zostera capricorni (Z, light grey bars).

Figure 5. Experimental enrichment of 0.25 m2 sediment plots with either 30 g (plain bars), 60 g (lightly patterned bars) or 90 g (densely patterned bars) of dried detritus revealed that neither Ecklonia radiata (E, light grey bars) or Sargassum sp. (S, dark grey bars) were able to support the density (Mean + SE, n = 7) of macroinvertebrates sustained by the Phyllosopora comosa (P, white bars), now locally extinct from the Sydney Region.
Adapted from Bishop et al. 2010.

 

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