Spatio-temporal patterns in coastal turbidity – Long-term trends and drivers of variation across an estuarine-open coast gradient

• We analyse 22 years of coastal turbidity data along an estuarine to open-coast gradient in northern New Zealand.
• No region-wide changes in turbidity are evident, despite implementation of land management regulations.
• Temporal patterns and drivers of variation in turbidity differ along the gradient.
• This study suggests climate change is likely to increase coastal turbidity.
• Better land management and coastal protection needed to offset future increases in turbidity.

Turbidity in the coastal environment is greatly affected by human activities on the land and this is likely to be exacerbated with expanding urbanisation and climate change. Investigating the temporal and spatial drivers of variation in turbidity is key to understanding processes influencing turbidity and for developing management strategies to mitigate future increases in turbidity. We analyse 22 years of monthly turbidity data from 1992 to 2013 in New Zealand's Hauraki Gulf to determine whether turbidity has changed in response to implementation of land management regulations. We also investigate how spatial and temporal patterns in turbidity relate to meteorological and oceanographic variables along an estuarine to open-coast gradient.Turbidity, total suspended solids and chlorophyll a declined along the estuarine to open-coast gradient. Correlation analysis suggested that suspended sediment was the major determinant of turbidity along this gradient. Improvements in turbidity were evident at some harbour sites, but overall there were no consistent trends across the sites. Some cyclical patterns in turbidity were evident, but these were only weakly related to ENSO. The greatest component of temporal variation at all sites was between samples (months). The primary correlates of this variation in turbidity differed across the estuarine-open coast gradient; recent wave conditions explained the greatest variation in turbidity at open coast sites, whereas tidal currents and daily rainfall were the primary correlates at harbour channel and estuarine sites. The strong coupling found between meteorological factors and coastal turbidity highlight a number of mechanisms whereby turbidity will likely increase as a result of climate change along this coastal gradient. Improvements in land management practices, particularly in rural areas, as well as coastal protection are therefore essential to offset the likely effects of climate change on coastal turbidity.