A smoothed outline of Aotearoa New Zealand’s coast was used to construct coastal segments of roughly 10 km width (depending on complexity of the coastline), extending out to the 12 nm Territorial Sea limit. Datasets from the New Zealand Catchment Land Use for Environmental Sustainability model (CLUES), which estimate annual loads of nitrogen, phosphorus, and sediment in freshwater catchments, were used to compare nutrient and sediment loads entering estuarine and marine environments within coastal segments. Nitrogen and phosphorus are components of fertilizers used on agricultural land including pastures. Excesses of these nutrients entering aquatic systems can result in algal blooms which may lead to eutrophication and the formation of dead, or anoxic zones. Greater nutrient input can also result in poorer nutritional quality of filter feeders such as shellfish. Sedimentation is a result of soil erosion entering waterways and is naturally high in several parts of New Zealand and exacerbated by deforestation and conversion to pasture grasslands. Sedimentation can smother marine organisms, block light from photosynthetic species and clog fine filter feeding apparatuses. Greater turbidity can also reduce capacity of estuarine ecosystems to process increased nutrient inputs. Sediments also can contain a variety of contaminants such as heavy metals, which are harmful to marine organisms as well as humans that consume them. The values shown are the sum of the terminal river reach loads that overlap with the 10 km segment. Because these segments don’t occur in isolation, a 10 km buffer was used around each coastal segment to capture nitrogen, phosphorus and sediment loading in inland and adjacent coastal areas. To view a dynamic model of freshwater input along part of the South Island coast over 1 year see: https://vimeo.com/48991072

Limitations and Uncertainties:

Land-based inputs (sediments, nutrients) are calculated for individual river segments that empty to the coast, thus total inputs for each 10 km segment are quantified based on the total inputs across all rivers and streams that empty into that particular coastal segment. A major gap in our understanding of what happens to sediments, nutrients and other pollutants when they enter the coastal zone is that we typically don’t know how these inputs are dispersed both alongshore and offshore, and how dispersion varies based on local and broad scale circulation patterns, wind-wave dispersal and tidal currents. Hydrodynamic models can be used to estimate spatial and temporal variation in how these inputs are dispersed, however they are available only at large spatial scales for the whole of the EEZ. Local or regional models are available in some locations (e.g., Bay of Plenty; Montaño et al. 2023), but often don’t include conditions of extreme events such as Cyclone Gabrielle which deposited a significant amount of sediment into the coastal zone. Future improvements to our understanding of the impacts of sediments, nutrients and other land-based inputs to the coastal zone should prioritise combining terrestrial models of sediments such as SedNet (http://tools.envirolink.govt.nz/dsss/sednet/) with oceanographic circulation models such as the Moana backbone (https://www.moanaproject.org/) to better understand how these land-based inputs are dispersed and deposited in the coastal zone. Segments for Stewart Island were not included and for coastlines with large, narrow inlets, such as in Fiordland, segments do not include the inner fiords as this would increase the segment size, making comparisons between segments less consistent. CLUES models are calibrated at the national level to provide reasonable catchment scale load estimates and will include some uncertainty in capturing attenuation due to water quality improvement measures.

Source: CLUES – Catchment Land Use for Environmental Sustainability model | NIWA

Sediment: Updated suspended sediment yield estimator and estuarine trap efficiency model results 2019 - Freshwater directorate | | GIS Map Data | MfE Data Service and report.

Last updated: 3 Apr 2023

Elliott, A.H., Semadeni-Davies, A.F., Shankar, U., Zeldis, J.R., Wheeler, D.M., Plew, D.R., Rys, G.J., Harris, S.R. (2016) A national-scale GIS-based system for modelling impacts of land use on water quality. Environmental Modelling & Software, 86: 131-144.

Montaño, M. M., S. H. Suanda, and J. M. A. C. d. Souza. 2023. Modelled coastal circulation and Lagrangian statistics from a large coastal embayment: The case of Bay of Plenty, Aotearoa New Zealand. Estuarine, Coastal and Shelf Science 281:108212.

Semadeni-Davies, A., Jones-Todd, C., Srinivasan, M.S., Muirhead, R., Elliott, A., Shankar, U., Tanner, C. (2019a) CLUES model calibration and its implications for estimating contaminant attenuation. Agricultural Water Management: 105853.

Semadeni-Davies, A.F., Jones-Todd, C.M., Srinivasan, M.S., Muirhead, R.W., Elliott, A.H., Shankar, U., Tanner, C.C. (2019b) CLUES model calibration: residual analysis to investigate potential sources of model error. New Zealand Journal of Agricultural Research: 1-24. 10.1080/00288233.2019.1697708

A smoothed outline of Aotearoa New Zealand’s coast was used to construct coastal segments of roughly 10 km width (depending on complexity of the coastline), extending out to the 12 nm Territorial Sea limit. Datasets from the New Zealand Catchment Land Use for Environmental Sustainability model (CLUES), which estimate annual loads of nitrogen, phosphorus, and sediment in freshwater catchments, were used to compare nutrient and sediment loads entering estuarine and marine environments within coastal segments. Nitrogen and phosphorus are components of fertilizers used on agricultural land including pastures. Excesses of these nutrients entering aquatic systems can result in algal blooms which may lead to eutrophication and the formation of dead, or anoxic zones. Greater nutrient input can also result in poorer nutritional quality of filter feeders such as shellfish. Sedimentation is a result of soil erosion entering waterways and is naturally high in several parts of New Zealand and exacerbated by deforestation and conversion to pasture grasslands. Sedimentation can smother marine organisms, block light from photosynthetic species and clog fine filter feeding apparatuses. Greater turbidity can also reduce capacity of estuarine ecosystems to process increased nutrient inputs. Sediments also can contain a variety of contaminants such as heavy metals, which are harmful to marine organisms as well as humans that consume them. The values shown are the sum of the terminal river reach loads that overlap with the 10 km segment. Because these segments don’t occur in isolation, a 10 km buffer was used around each coastal segment to capture nitrogen, phosphorus and sediment loading in inland and adjacent coastal areas. To view a dynamic model of freshwater input along part of the South Island coast over 1 year see: https://vimeo.com/48991072

Limitations and Uncertainties:

Land-based inputs (sediments, nutrients) are calculated for individual river segments that empty to the coast, thus total inputs for each 10 km segment are quantified based on the total inputs across all rivers and streams that empty into that particular coastal segment. A major gap in our understanding of what happens to sediments, nutrients and other pollutants when they enter the coastal zone is that we typically don’t know how these inputs are dispersed both alongshore and offshore, and how dispersion varies based on local and broad scale circulation patterns, wind-wave dispersal and tidal currents. Hydrodynamic models can be used to estimate spatial and temporal variation in how these inputs are dispersed, however they are available only at large spatial scales for the whole of the EEZ. Local or regional models are available in some locations (e.g., Bay of Plenty; Montaño et al. 2023), but often don’t include conditions of extreme events such as Cyclone Gabrielle which deposited a significant amount of sediment into the coastal zone. Future improvements to our understanding of the impacts of sediments, nutrients and other land-based inputs to the coastal zone should prioritise combining terrestrial models of sediments such as SedNet (http://tools.envirolink.govt.nz/dsss/sednet/) with oceanographic circulation models such as the Moana backbone (https://www.moanaproject.org/) to better understand how these land-based inputs are dispersed and deposited in the coastal zone. Segments for Stewart Island were not included and for coastlines with large, narrow inlets, such as in Fiordland, segments do not include the inner fiords as this would increase the segment size, making comparisons between segments less consistent. CLUES models are calibrated at the national level to provide reasonable catchment scale load estimates and will include some uncertainty in capturing attenuation due to water quality improvement measures.

Source: CLUES – Catchment Land Use for Environmental Sustainability model | NIWA

Sediment: Updated suspended sediment yield estimator and estuarine trap efficiency model results 2019 - Freshwater directorate | | GIS Map Data | MfE Data Service and report.

Last updated: 3 Apr 2023

Elliott, A.H., Semadeni-Davies, A.F., Shankar, U., Zeldis, J.R., Wheeler, D.M., Plew, D.R., Rys, G.J., Harris, S.R. (2016) A national-scale GIS-based system for modelling impacts of land use on water quality. Environmental Modelling & Software, 86: 131-144.

Montaño, M. M., S. H. Suanda, and J. M. A. C. d. Souza. 2023. Modelled coastal circulation and Lagrangian statistics from a large coastal embayment: The case of Bay of Plenty, Aotearoa New Zealand. Estuarine, Coastal and Shelf Science 281:108212.

Semadeni-Davies, A., Jones-Todd, C., Srinivasan, M.S., Muirhead, R., Elliott, A., Shankar, U., Tanner, C. (2019a) CLUES model calibration and its implications for estimating contaminant attenuation. Agricultural Water Management: 105853.

Semadeni-Davies, A.F., Jones-Todd, C.M., Srinivasan, M.S., Muirhead, R.W., Elliott, A.H., Shankar, U., Tanner, C.C. (2019b) CLUES model calibration: residual analysis to investigate potential sources of model error. New Zealand Journal of Agricultural Research: 1-24. 10.1080/00288233.2019.1697708