Abstract
The suspension feeding bivalve Austrovenus stutchburyi is a key species on intertidal sandflats in New Zealand, affecting the appearance and functioning of these systems, but is susceptible to several environmental stressors including sedimentation. Previous studies into the effect of this species on ecosystem function have been restricted in space and time, limiting our ability to infer the effect of habitat change on functioning. We examined the effect of Austrovenus on benthic primary production and nutrient dynamics at two sites, one sandy, the other composed of muddy-sand to determine whether sedimentary environment alters this key species' role. At each site we established large (16 m2) plots of two types, Austrovenus addition and removal. In winter and summer we deployed light and dark benthic chambers to quantify oxygen and nutrient fluxes and measured sediment denitrification enzyme activity to assess denitrification potential. Rates of gross primary production (GPP) and ammonium uptake were significantly increased when Austrovenus was added, relative to removed, at the sandy site (GPP, 1.5 times greater in winter and summer; ammonium uptake, 8 times greater in summer; 3-factor analysis of variance (ANOVA), p<0.05). Denitrification potential was also elevated in Austrovenus addition plots at the sandy site in summer (by 1.6 times, p<0.1). In contrast, there was no effect of Austrovenus treatment on any of these variables at the muddy-sand site, and overall rates tended to be lower at the muddy-sand site, relative to the sandy site (e.g. GPP was 2.1 to 3.4 times lower in winter and summer, respectively, p<0.001). Our results suggest that the positive effects of Austrovenus on system productivity and denitrification potential is limited at a muddy-sand site compared to a sandy site, and reveal the importance of considering sedimentary environment when examining the effect of key species on ecosystem function.
Highlights
Estuaries are highly productive ecosystems that play a major role in biogeochemical cycles, but are subject to multiple stressors that will likely be exacerbated by climate change and expanding human habitation of coastal areas [1,2,3]
Austrovenus density in +AS plots ranged from c. 500 to 1000 ind. m22 at the sandy site and from c. 1800 to 2500 ind. m22 at the muddy-sand site (Figure 2)
Densities in +AS plots were equivalent to planned densities, i.e. mean Austrovenus density in +AS plots was significantly lower at the sandy site (700 ind. m22) compared to the muddy-sand site (2000 ind. m22, p,0.001)
Summary
Estuaries are highly productive ecosystems that play a major role in biogeochemical cycles, but are subject to multiple stressors that will likely be exacerbated by climate change and expanding human habitation of coastal areas [1,2,3]. The effects of contaminants, invasive species, coastal alteration and development might be restricted to estuaries near large population centres, enhanced sedimentation rates threaten many estuaries, even when there have been only moderate levels of catchment development [1]. In many cases it has been shown in estuarine systems that certain key species, rather than biodiversity per se, can have a disproportionate effect on indicators of ecosystem functioning such as nutrient cycling and productivity Habitat change may not necessarily cause species loss but might more subtly affect ecosystem function by alteration of a species’ functional role. The influence of this species on biogeochemical exchange and microbial communities is likely to differ between habitat types [16]. Most studies to date are restricted temporally and spatially making it difficult to understand the effects of habitat change on a key species’ influence on ecosystem function. As sedimentation alters estuarine habitats by increasing sediment mud content we used a site with muddy-sand sediments as a proxy for habitat change, to compare with a site comprising only sandy sediment
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