title: Empirical and semi-analytical chlorophyll a algorithms for multi-temporal monitoring of New Zealand lakes using Landsat.
abstract: The concentration of chlorophyll a (chl a; as a proxy for phytoplankton biomass) provides an indication of the water quality and ecosystem health of lakes. An automated image processing method for Landsat images was used to derive chl a concentrations in 12 Rotorua lakes of North Island, New Zealand, with widely varying trophic status. Semi-analytical and empirical models were used to process 137 Landsat 7 Enhanced Thematic Mapper (ETM+) images using records from 1999 to 2013. Atmospheric correction used radiative transfer modelling, with atmospheric conditions prescribed with Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and AIRS data. The best-performing semi-analytical and empirical equations resulted in similar levels of variation explained (r (2) = 0.68 for both equations) and root-mean-square error (RMSE = 10.69 and 10.43 μg L(-1), respectively) between observed and estimated chl a. However, the symbolic regression algorithm performed better for chl a concentrations <5 μg L(-1). Our Landsat-based algorithms provide a valuable method for synoptic assessments of chl a across the 12 lakes in this region. They also provide a basis for assessing changes in chl a individual lakes through time. Our methods provide a basis for cost-effective hindcasting of lake trophic status at a regional scale, informing on spatial variability of chl a within and between lakes.
authors.affiliation: Environmental Research Institute, The University of Waikato, Private Bag 3105, Hamilton, 3240, New Zealand, mat.g.allan@gmail.com.
authors.forename: Mathew G
authors.initials: MG
authors.lastname: Allan
authors.forename: David P
authors.initials: DP
authors.lastname: Hamilton
authors.forename: Brendan
authors.initials: B
authors.lastname: Hicks
authors.forename: Lars
authors.initials: L
authors.lastname: Brabyn
mesh_terms: D000465:Algorithms; D018533:Biomass; D002734:Chlorophyll / Q000032:analysis*; D000077194:Chlorophyll A; D017753:Ecosystem; D004784:Environmental Monitoring / Q000379:methods*; D060106:Lakes / Q000737:chemistry*; D008962:Models, Theoretical; D009520:New Zealand; D010839:Phytoplankton / Q000254:growth & development*; D063809:Satellite Imagery; D060753:Water Quality
chemical_list: D002734:Chlorophyll; D000077194:Chlorophyll A