At each monitoring site on each sampling occasion, five bulked surface sediment samples (5cm diameter, 2cm deep core) are collected from random locations within a 100m by 100m monitoring plot. Five replicate surface sediment scrapes are also collected at each monitoring location for analysis of microalgal biomass . Each sample is stored frozen and then analysed for a number of chemical and physical properties. These properties are:
Sediment grain-size: Grain size analysis was carried out by wet sieving. Samples were sieved through a stack of sieves (at 2000 µm, 500 µm, 250 µm, 125 µm and 63 µm) and the fractions remaining on the sieves dried at 60°C for 48 hours. Results were reported as the percent by weight of the total sample. Grain size data were grouped into the following grain size categories: mud (<63 μm), very fine sand (63-125 μm), fine sand (125-250 μm), medium sand (250-500 μm), coarse sand (500-2000 μm) and gravel (>2000 μm) following the Wentworth sediment classification.
Sediment organic carbon and nitrogen content: Another sub-sample from each bulked sediment sample was analysed for total organic carbon and total nitrogen content using an automated CHN analyser. Samples were dried and finely ground before analysis. Sediment for total organic carbon analysis was pre-treated with acid to remove carbonate material prior to analysis.
Sediment microalgal biomass: Chlorophyll a was extracted from freeze dried sediment by boiling in 95% ethanol and the extract analysed using a spectrophotometer. Acidification was used to separate plant degradation products (phaeophytin) from chlorophyll a.
At each monitoring site (except Te Puru in the Firth of Thames) between two and six concrete plates (30cm x 30cm) have been buried at a known depth beneath the sediment surface. During each sampling event the depth of sediment over these plates is measured by inserting metal (knitting) needles into the sediment above the plates and measuring the length of needle immersed in the sediment. This provides an indication of sediment accumulation onto the sand or mudflats at the monitoring sites.
Long-term sediment accumulation rates are very variable and therefore require detailed statistical analysis to extract trends from natural variability. A recent report(external link) has analysed sediment accumulation rates and reviewed methods for the Firth of Thames and Whaingaroa (Raglan) Harbour.