|dc.description.abstract||While seagrasses and their associated macrofaunal assemblages naturally endure eutrophication events, nutrient enrichment as a result of anthropogenic activities has subjected them to prolonged levels of elevated nutrients. The impacts of elevated nutrient concentrations vary between studies, with sediment composition, seagrass metrics, and the macrofauna that live within them often implicated as mitigating the observed responses. How meadows and their associated macrofaunal community assemblages respond individually has been the focus of many studies to date; though few examine both variables in tandem while also considering the role of the surrounding sediment. Here I attempt to discern the role that sediment characteristics have in determining how meadows and macrofauna respond to elevated nutrient levels within the Tauranga Harbour, New Zealand.
The sediment pore water ammonium (NH4+) concentration was elevated using a slow release fertiliser at a dose hypothesised to stimulate a response, but not mass death (200 N g/m2) within seagrass meadows. This was conducted over 60 days during the seagrass growing season at six sites that varied across a gradient of sediment mud content. Different seagrass metrics were measured as attributes of seagrass meadows, the amalgamation of which was referred to as seagrass health. The overall community composition of macrofaunal assemblages was measured, with species richness and abundance examined as aspects.
Treatment effects were observed in all attributes of seagrass health at the sandiest site, which lost most of its elevated NH4+ concentration, and thus at this site sediment characteristics were deemed responsible for how seagrass attributes responded. Two muddy sites also exhibited a response in seagrass cover, and no correlations between sediment composition and responses in seagrass health were apparent. As such, it was concluded that sediment characteristics alone could not be used to predict whether meadows would respond to nutrient enrichment.
None of the sites which exhibited a response in seagrass metrics also did so in macrofaunal community assemblage. The macrofaunal assemblages of two sites, one sandy and one muddy, responded to enrichment, while also decreasing in both species richness and abundance. The sandy site lost its elevated nutrient concentration due to the combination of low mud content and fine particle size, though the thick cover of its meadows acted as a buffer to seagrass degradation. The muddy site had the highest mud content of all sites, and as such retained its sediment pore water NH4+ concentration in many of its treatment plots. The sensitivity of the polychaete worm Heteromastus filiformis to elevated NH4+ and bioturbation by the bivalve Linucula hartvigiana contributed to responses in assemblages seen at both sites. As these sites were from different sediment classes, mud content alone could not be used to predict whether macrofaunal assemblages would respond to enrichment.
The combination and interaction of site traits may explain why community assemblages responded to treatment, despite different sediment characteristics and attributes of seagrass health. As such, sediment characteristics, seagrass metrics, and the composition of macrofaunal assemblages may work in tandem to denote the magnitude and type of response evocated in community assemblages.||