Satellite remote sensing of coastal water quality in New Zealand

ABSTRACT

Water quality in estuarine and coastal waters of Aotearoa New Zealand is of concern yet currently under-monitored, reflecting challenges of sampling at regular intervals across large areas. Satellite products are currently underutilised (globally) by management agencies but could be part of a holistic approach. NIWA-SCENZ (Seas, Coasts and Estuaries New Zealand) is a website providing access to MODIS-Aqua products at moderate spatial resolution (500 m) across relatively clear coastal waters (Case 1) to turbid coastal waters (Case 2). It also provides advanced analytical tools on an extracted area of interest for trend assessment (Shiny-SCENZ). For Case 1 areas product accuracy will be comparable to data from other websites using the same algorithms. For Case 2 and transition (blended) areas, although tuned to New Zealand conditions, accuracy of products requires validation. Data availability is mainly impacted by cloud cover, but also adjacency to land and bottom reflection. On average <5% within ∼1 km from coastlines, compared to ∼10%–30% outside 2 km. Extrapolation and blending from offshore into shorelines on month averaged maps extend estimates into many bays and estuaries. Active research efforts will deliver iterative improvements in data quality, new products, and analysis methods (e.g. variability and long-term trend maps).

KEYWORDS:

• NIWA-SCENZ
• MODIS-aqua
• ArcGIS image service
• R-Shiny app
• ocean colour
• timeseries
• climatology
• anomaly
• seasonality
• trends

Acknowledgements

We thank members of the Coastal Special Interest Group (CSIG) from regional councils and local authorities for support and feedback, particularly Megan Oliver (Greater Wellington RC) and Olive Wade (Marlborough RC). Niall Broekhuizen for Aquaculture programme support. Helen Kettles from the Department and Conservation (and other participants) from a series of workshops and feedback on policy aspects of suspended sediment management in New Zealand coastal waters. Thanks also to François Thoral, a PhD student, researching links of seaweed abundance around New Zealand to seabed light (EBED). Thank you to Rob Davies-Colley for review and suggestions for improvements in context to freshwater and coastal water quality monitoring. We also thank the journal reviewers for comments that have significantly improved this manuscript, particularly Moritz Lehmann who tested the website and provided useful feedback. We acknowledge the work of the Earth-observation community worldwide, and NASA Goddard Space Flight Center and MODIS project, for their contribution to remote sensing worldwide, without which, this work would not have been possible. Imagery from the Land Atmosphere Near real-time Capability for EOS (LANCE) system and services from the Global Imagery Browse Services (GIBS), both operated by the NASA/GSFC/Earth Science Data and Information System (ESDIS, https://earthdata.nasa.gov) with funding provided by NASA/HQ. We also acknowledge website access through NIWA-SCENZ to: NASA Worldview application (https://worldview.earthdata.nasa.gov), part of the NASA Earth Observing System Data and Information System (EOSDIS); NOAA Centre for Satellite Application and Research (STAR); European Union Copernicus Marine Environment Monitoring Service (CMEMS); and ESA Ocean Colour (OC) Climate Change Initiative (CCI).

Disclosure statement

NIWA-SCENZ is free for scientific and non-commercial use. You must acknowledge the source of this information and, where possible, provide a link to the NIWA Open Data Licence. For commercial use please contact NIWA. NIWA does not give any express or implied warranty as to the accuracy of the products. Some regional in-water sampling and laboratory analysis has been used for validation, but the user is encouraged to consider regional validation. NIWA endeavours to ensure that information on this website is accurate and that methods are fully documented. Satellite data processing and product algorithms are evolving areas of research; changes to processing algorithms will occur periodically that could change product values in some areas. Users should take note of data versions and cite in any applications or reporting. The NIWA-SCENZ website has been reviewed internally by NIWA and is subject to change.

Data availability

https://data-niwa.opendata.arcgis.com/documents/NIWA::niwa-scenz-ocean-colour-application/about

Additional information

Funding

This work was supported by the Ministry of Business, Innovation and Employment (MBIE) under NIWA's Strategic Science Investment Fund (SSIF) in the Coasts and Oceans Programme 4 (Structure and function of marine ecosystems, COES); Aquaculture Programme 2 (Aquaculture / Environmental Interactions, ACEE); and MBIE Envirolink under Grant C01X1801. The operation of the Lauder satellite receiver and first stage data processing is supported by MBIE SSIF in the Climate, Atmosphere and Hazards centre (Forecasting Weather Systems Programme, CAFS).