Recent and historical sedimentation and sediment characteristics of Ōhiwa Harbour, New Zealand

The Bay of Plenty Regional Council is seeking to understand historical and recent sedimentation in Ōhiwa Harbour, a barrier-enclosed lagoon, to help establish sediment accumulation rates and water quality limits in accordance with the Resource Management Act (1991) and the NZ Freshwater Policy Statement. To provide sedimentation data that is independent of sediment plate data already being collected by the Council, and to supplement other historical data, a series of nine vibracores, nine box cores, and 73 surface sediment samples were collected from across Ōhiwa Harbour. Three key vibracores were analyzed for their radionuclide activity (²¹⁰Pb and ²²⁶Ra) in an attempt to determine their sediment accumulation rates. However, no rate could be established due to very low and irregular radionuclide activity through the cores. It is probable that the method did not work due to the combined effects of hydrodynamic mixing and bioturbation. Sedimentological analysis was undertaken on all nine vibracores and box cores, which indicated that the intertidal sediments consist of five distinctive depositional units: clast supported calci-gravel, ripple to cross bedded sand, bioturbated sand, laminated to rippled silt, and bioturbated silt. Recognition and interpretation of these units will be useful for future workers interested in reconstructing the evolution of sedimentation within the harbour. Grain size of core sediment and surficial sediment around the harbour was compared to historical surface sediment maps. They show clearly that Ōhiwa Harbour is becoming muddier through time, although in a non-linear and complex way with some parts of the harbour getting sandier. The perimeter of the harbour and the upper harbour are the main areas affected by a fining grain size trend. Loss-on-ignition (LOI) of sediment indicates that organic carbon is more concentrated in fine-grained sediment than coarser sediment across the harbour. No clear stratigraphic pattern of LOI could be established. However, surface sediments display larger LOI along the perimeter of the harbour where the proportion of mud is greater. Three recommendations are put forth based on the results of this study, as well as gaps in knowledge discovered during data collection and analysis. The first is that determining longer-term (decades–millennia) sediment accumulation rates may be achievable by carbondating the abundant shell material contained within cores. The second is that calculation of sediment compaction under its own weight ('autocompaction') may help determine the causes of sedimentation and erosion in the harbour. The third and final recommendation is for repeat surface sediment surveys to be undertaken regularly, at intervals no greater than every five years. These insights will together yield a more complete picture of past, present, and future sedimentation and its effects in Ōhiwa Harbour.