Comparison of sediment-plate methods to measure accretion rates in an estuarine mangrove forest (New Zealand)

Abstract

Sedimentation is a fundamental process in aquatic receiving environments, including vegetated habitats such as mangroves. Selection of appropriate methods to measure sedimentation depends on time scales of interest, due to the variable nature of this process. Our study considers the appropriateness of methods to measure sediment accretion over time scales of months to years in estuarine mangroves. Sediment accretion has commonly been measured using marker horizons, sediment erosion pins and buried plates. Bioturbation and erosion can make application of these methods problematic. Here, sediment accretion rates for porous plastic-mesh and impervious ceramic plates of two sizes (0.023 and 0.16 m2) were compared. Replicate plates of each type were installed on the substrate surface at nine Rod Surface Elevation Table (RSET) sites distributed along a shore-normal sedimentation gradient. The plates were progressively buried by sedimentation (17–41 mm yr−1). Measurements were made at seasonal to annual intervals over a seven-year period (2012–2019). Comparison of accretion rates on the plastic-mesh with both types of ceramic plates, showed strong positive correlations (r2 > 0.96, with slopes close to unity, mean 0.95, range = 0.68–1.09). Sediment accretion rates measured on the large and small ceramic plates were not measurably different from rates for the plastic mesh. These results show that sediment accretion rates were spatially homogenous at sub-metre to metre scales in the mangroves and sediment accretion rates did not measurably vary between different sediment-plate methods. The sediment-plate method has a number of advantages over marker horizons for long-term measurements of sediment accretion, particularly where bioturbation alters the sediment profile. Mesh plates are easily maintained and provide a cost-effective, flexible, precise and reproducible alternative to sediment marker horizons and solid plates for long-term monitoring sediment accretion in mangroves and unvegetated intertidal habitats. Future studies, including those employing RSET, should consider incorporating mesh-plate measurements of sediment accretion.