Variability of fine-scale chlorophyll fronts in the Tīpaka Moana Te Moana Nui a Toi Hauraki Gulf, Aotearoa New Zealand

Aotearoa New Zealand’s marine environment is heavily impacted by El Niño Southern Oscillation (ENSO). Little is known about the effect of ENSO on oceanographic properties in the area or their consequences on the distribution of marine organisms. Here we characterise the spatio-temporal variability of fine-scale fronts (< 10 km) in the area of Tīpaka Moana Te Moana Nui a Toi Hauraki Gulf (HG) and investigate the impact of dominant wind direction, seasonality, and ENSO phase. We processed Ocean-Land Color Instrument (OLCI) images from satellites between 2016-2022 with a fit-for-purpose version of the Belkin and O’Reilly Algorithm, specifically for frontal detection. We find coherent shifts in the position of fine-scale features, with ENSO phases alternatively separate and connect different parts of the Gulf during El Niño and La Niña respectively. Overall, fronts tend to co-locate with the 70 m and 40 m isobaths in the outer and inner HG respectively and their locations shift close or away from shore in response to changes in dominant wind direction. Furthermore, offshore fronts occurrences increase during winter and spring, and nearshore ones increase during summer and autumn. Our results sketch a first assessment of the distribution of fine-scale features that are likely to impact the distribution of important areas for pollutants dispersion and feeding areas for marine megafauna.