The vulnerability and resilience of seagrass ecosystems to marine heatwaves in New Zealand: a remote sensing analysis of seascape metrics usingPlanetScopeimagery

Abstract

Seagrasses are foundation species that provide ecosystem functions and services, including increased biodiversity, sediment retention, carbon sequestration, and fish nursery habitat. However, anthropogenic stressors that reduce water quality, impose large‐scale climate changes, and amplify weather patterns, such as marine heatwaves, are altering seagrass meadow configurations. Quantifying large‐scale trends in seagrass distributions will help evaluate the impacts of climate drivers on their functions and services. Here, we quantified spatiotemporal dynamics in abundances and configurations of intertidal and shallow subtidal seagrass (Zostera muelleri) meadows in 20 New Zealand (NZ) estuaries that span a 5‐year period (mid/late 2016–early 2022) just before, during and after the Tasman Sea 2017/18 marine heatwave, the warmest summer ever recorded in NZ. We used high‐resolution PlanetScope satellite imagery to map interseasonal seagrass extent and quantify seascape metrics across 20 estuaries along a latitudinal gradient spanning 12° in NZ. We also explored the association of changes in seagrass metrics with satellite‐derived predictors such as sea surface temperature (SST), SST anomaly (SSTa), water column turbidity, and nutrient concentration. Our analyses revealed that NZ seagrass meadows varied in areal extent between years and seasons, but with no clear patterns over the 5‐year period, implying resilience to large‐scale stressors like the 2017/18 marine heatwave. Small‐scale patterns were also dynamic, for example, patch sizes and patch configurations differed across estuaries, seasons, and years. Furthermore, seagrass patches expanded in some estuaries with increasing SST and SSTa. These results highlight dynamic seagrass patterns that likely affect local processes such as biodiversity and carbon sequestration. Our analyses demonstrate that a combination of high‐resolution satellite remote sensing and seascape metrics is an efficient and novel approach to detect impacts from anthropogenic stressors, like eutrophication and climate changes, and climate extremes like cyclones and heatwaves.

Impacts of Climate Change on Marine Foundation Species

“…Warming can also influence nutrient cycling (e.g., Alexandre et al 2020) These shifts in species identity can have cascading effects on associated species and community structure and function (e.g., Viana et al 2019). Estuarine intertidal seagrass species, which naturally experience large daily and seasonal temperature fluctuations, appear to have highly variable responses to warming and heatwaves (Clemente et al 2023, Magel et al 2022. Still, heat-associated mortality has been reported on intertidal seagrass, particularly when high water and air temperatures coincide with midday low spring tides and high solar radiation (Rasheed & Unsworth 2011).…”