Rates of denitrification (isotope pairing) and nitrogen fixation (acetylene reduction) were simultaneously measured in three temperate, intertidal Zostera muelleri meadows and adjacent non‐vegetated tidal flats within Western Port, Australia. Net daily nitrogen fluxes ranged from −276 (net denitrification) to 520 μmol N/m2 d (net nitrogen fixation), and were generally positive, with only two instances of net negative fluxes. The highest fluxes were observed at the sites with the lowest water column nitrate concentrations. No significant differences in net nitrogen fluxes were found between vegetated and non‐vegetated sediments (p = 0.213). Nitrogen fixation was generally the dominant process occurring, which was stimulated in the presence of vegetation except at the most marine‐influenced site, where nitrogen fixation in non‐vegetated sediments was higher. Nitrogen fixation rates in non‐vegetated sediments were highly correlated to cyanobacterial cell counts (although no mats were present). Rates were ∼65 μmol N/m2 d at 0 cell counts, suggesting a basal rate driven within the sediment. Additional slurry experiments confirmed significant rates of nitrogen fixation within the sediment, which were stimulated by sucrose and terminated by nitrate (p < 0.05), strongly suggesting sulfate‐reducing bacteria contributed to nitrogen fixation. At the bay‐wide scale, nitrogen fixation was estimated to contribute ∼430 t N/yr compared to ∼650 t N/yr from catchment and atmospheric inputs and 230 t N/yr lost through denitrification. Sensitivity analysis confirmed that while the loss of seagrass would affect the magnitude of the bay‐wide flux of nitrogen on these tidal flats, nitrogen fixation remains the dominant process.
Seagrass species form important marine and estuarine habitats providing valuable ecosystem services and functions. Coastal zones that are increasingly impacted by anthropogenic development have experienced substantial declines in seagrass abundance around the world. Australia, which has some of the world's largest seagrass meadows and is home to over half of the known species, is not immune to these losses. In 1999 a review of seagrass ecosystems knowledge was conducted in Australia and strategic research priorities were developed to provide research direction for future studies and management. Subsequent rapid evolution of seagrass research and scientific methods has led to more than 70% of peer reviewed seagrass literature being produced since that time. A workshop was held as part of the Australian Marine Sciences Association conference in July 2015 in Geelong, Victoria, to update and redefine strategic priorities in seagrass research. Participants identified 40 research questions from 10 research fields (taxonomy and systematics, physiology, population biology, sediment biogeochemistry and microbiology, ecosystem function, faunal habitats, threats, rehabilitation and restoration, mapping and monitoring, management tools) as priorities for future research on Australian seagrasses. Progress in research will rely on advances in areas such as remote sensing, genomic tools, microsensors, computer modeling, and statistical analyses. A more interdisciplinary approach will be needed to facilitate greater understanding of the complex interactions among seagrasses and their environment.
Habitat fragmentation is thought to be an important process structuring landscapes in marine and estuarine environments, but effects on fauna are poorly understood, in part because of a focus on patchiness rather than fragmentation. Furthermore, despite concomitant increases in perimeter:area ratios with fragmentation, we have little understanding of how fauna change from patch edges to interiors during fragmentation. Densities of meiofauna were measured at different distances across the edges of four artificial seagrass treatments [continuous, fragmented, procedural control (to control for disturbance by fragmenting then restoring experimental plots), and patchy] 1 day, 1 week and 1 month after fragmentation. Experimental plots were established 1 week prior to fragmentation/disturbance. Samples were numerically dominated by harpacticoid copepods, densities of which were greater at the edge than 0.5 m into patches for continuous, procedural control and patchy treatments; densities were similar between the edge and 0.5 m in fragmented patches. For taxa that demonstrated edge effects, densities exhibited log-linear declines to 0.5 m into a patch with no differences observed between 0.5 m and 1 m into continuous treatments. In patchy treatments densities were similar at the internal and external edges for many taxa. The strong positive edge effect (higher densities at edge than interior) for taxa such as harpacticoid copepods implies some benefit of patchy landscapes. But the lack of edge effects during patch fragmentation itself demonstrates the importance of the mechanisms by which habitats become patchy.
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