Investigating the Climatic and Oceanographic Drivers of Spatial and Temporal Variation in Coastal Turbidity and Sedimentation

Abstract

Turbidity and sediment input in the coastal environment are greatly affected by human activities on land. Understanding the climatic, oceanographic and environmental drivers of temporal and spatial variability in coastal turbidity and sediment ux is key to understanding these processes and predicting how they may be affected by climate change. This thesis analysed monitoring data on coastal turbidity and sediment trap rates in the Hauraki Gulf to investigate how these spatio-temporal patterns relate to meteorological and oceanographic variables along an estuarine to open-coast gradient. These relationships were initially explored using a multivariate approach, and then quanti ed using a bayesian hierarchical framework with spline components. This model was developed along with two software packages to provide a predictive framework for analysing environmental monitoring data, with an emphasis on reproducibility and transparency. Turbidity declined along the estuarine to open-coast gradient. The primary driver of turbidity at the exposed open-coast regions was recent wave conditions that resuspend sediment, whereas tidal currents and daily rainfall are the primary drivers of turbidity at the harbour and estuarine sites respectively. The rate of fine sediment (< 63μ) accumulation in traps was largely governed by processes that resuspend bottom sediments, primarily wind-generated waves and tides within harbour regions, and ocean swells on the open coast. Surprisingly, there was little to no relationship with rainfall suggesting that sediment traps should not be used to document terrestrial sediment introduction on subtidal reefs. The strong coupling found between meteorological and oceanographic factors, and coastal turbidity and sediment ux highlight a number of mechanisms whereby coastal turbidity will likely change as a result of climate change. Overall, turbidity is likely to decrease based on predictions of increased offshore winds and drier conditions throughout this region. However, more frequent and intense extreme weather events will likely result in unprecedented, transient increases in turbidity, creating a highly variable coastal turbidity environment. These effects on turbidity will likely be exacerbated by sea level rise and increasing coastal erosion, therefore improvements in land management practices and coastal protection are essential to offset the likely impacts of climate change on coastal turbidity.