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: A sub-sample from each bulked sediment sample is analysed for grain size. Prior to analysis, samples were pre-treated with 10% hydrogen peroxide to remove organic material and 1M hydrochloric acid to remove carbonate material. Calgon was added as a dispersant and samples were placed in an ultrasonic bath for 10 minutes to aid disaggregation. Samples were analysed with a Malvern Mastersizer 2000 sediment analyser, which has a lower detection limit of 0.05 µm.
Every sediment is a mixture of grains of varying sizes. Sediments can be classified based on the relative proportions of different grain sizes. To find out more about the sediment grain size classification we use click here.
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 deposition onto the sand or mudflats at the monitoring sites.
Long-term sediment deposition rates are very variable and therefore require detailed statistical analysis to extract trends from natural variability. Sediment deposition rates at our REMP monitoring sites are currently being analysed.