Trend Analysis of Chlorophyll-a in Hawkes Bay
Trend Analysis of Chlorophyll-a in Hawkes Bay using remote sensing data 2018-23
The Hawke’s Bay Coastal Marine Area (CMA) stretches from north of Mahia to south of Pōrangahu within the Hawkes Bay region. Hawkes Bay Regional Council (HBRC) has a responsibility for managing coastal waters and subsequently marine environmental health.
To assist in understanding changes to marine health over time, monitoring often tracks phytoplankton biomass due to their pivotal role in primary production and nutrient cycling within marine ecosystems. They are typically nutrient limited in the marine environment and as such respond strongly to the availability of nutrients making them a good indicator of trophic status. Changes in nutrient availability in the marine environment can be influenced by seasonal weather patterns as well as the presence of excess nutrients in the water column driven by land-based and/or human sources. Phytoplankton contain Chlorophyll a (Chl-a) which is utilised as a basis to measure their biomass and utilised as a tool for assessing marine environmental health.
A previous trend analysis of chl-a was undertaken in 2018[1] for the Hawke’s Bay CMA using a 15-year time series of 2002 to 2017 which utilised a locally validated remote sensing algorithm based on satellite data. In 2023, HBRC received an Envirolink Advice Grant (2332) to enable a further analysis of data from 2017-2023 to establish whether chl-a trends in the region have changed over the intervening period.
Sampling site in Hawke Bay (HBRC)
This work was undertaken by Cawthron Institute and consisted of four key analyses: an updated trend analysis, an analysis of seasonal patterns, a focus on chl-a trends in the region of the Awatoto coast, and a comparison of the Cawthron chl-a results with the NIWA-SCENZ product. The comparison between these two methods was made to allow HBRC to understand the different results and was not intended as a validation exercise. Remote sensing data in conjunction with results from HBRC State of the Environment (SOE) monitoring were utilised in the analysis.
Becky Shanahan, team leader of freshwater and marine science, of HBRC explains that this work was commissioned to see if the trends that were showing up in SOE monitoring were supported by continuous satellite data which was what the analysis indicated. Becky says “this investigation provides the knowledge to help us design more targeted work moving forward to confirm what the drivers of these trends are.”
Becky concludes by saying “the value of these studies is a good reminder of what we do on the land ultimately impacts on the sea and we need an integrated approach to management”.
[1] Knight B, Jiang W. 2018. Hawke’s Bay coastal states and trends: analysis of fifteen years of remote sensing data. Nelson: Cawthron Institute. Cawthron Report 3189. Prepared for Hawke’s Bay Regional Council.
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