The Vulnerability and Resilience of Seagrass Ecosystems to Marine Heatwaves in New Zealand: A Remote Sensing Analysis of Seascape Metrics Using PlanetScope Imagery

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

Gradients in the Number of Species at Reef-Seagrass Ecotones Explained by Gradients in Abundance

Gradients in the composition and diversity (e.g. number of species) of faunal assemblages are common at ecotones between juxtaposed habitats. Patterns in the number of species, however, can be confounded by patterns in abundance of individuals, because more species tend to be found wherever there are more individuals. We tested whether proximity to reefs influenced patterns in the composition and diversity (‘species density’ = number of species per area and ‘species richness’ = number of species per number of individuals) of prosobranch gastropods in meadows of two seagrasses with different physiognomy: Posidonia and Amphibolis. A change in the species composition was observed from reef-seagrass edges towards the interiors of Amphibolis, but not in Posidonia meadows. Similarly, the abundance of gastropods and species density was higher at edges relative to interiors of Amphibolis meadows, but not in Posidonia meadows. However, species richness was not affected by proximity to reefs in either type of seagrass meadow. The higher number of species at the reef-Amphibolis edge was therefore a consequence of higher abundance, rather than species richness per se. These results suggest that patterns in the composition and diversity of fauna with proximity to adjacent habitats, and the underlying processes that they reflect, likely depend on the physiognomy of the habitat

Resistance, Extinction and Everything in Between - The Diverse Responses of Seaweeds to Marine Heatwaves

Globally, anomalously warm temperature events have increased by 34% in frequency and 17% in duration from 1925 to 2016 with potentially major impacts on coastal ecosystems. These “marine heatwaves” (MHWs) have been linked to changes in primary productivity, community composition and biogeography of seaweeds, which often control ecosystem function and services. Here we journalarticle the literature on seaweed responses to MHWs, including 58 observations related to resistance, bleaching, changes in abundance, species invasions and local to regional extinctions. More records existed for canopy-forming kelps and bladed and filamentous turf-forming seaweeds than for canopy-forming fucoids, geniculate coralline turf and crustose coralline algae. Turf-forming seaweeds, especially invasive seaweeds, generally increased in abundance after a MHW, whereas native canopy-forming kelps and fucoids typically declined in abundance. We also found four examples of regional extinctions of kelp and fucoids following specific MHWs, events that likely have long term consequences for ecological structure and functioning. Although a relatively small number of studies have described impacts of MHWs on seaweed, the broad range of documented responses highlights the necessity of better baseline information regarding seaweed distributions and performance, and the need to study specific characteristics of MHWs that affect the vulnerability and resilience of seaweeds to these increasingly important climatic perturbations. A major challenge will be to disentangle impacts caused by the extreme temperature increases of MHWs itself from co-occurring potential stressors including altered current patterns, increasing herbivory, changes in water clarity and nutrient content, solar radiation and desiccation stress in the intertidal zone. With future increases anticipated in the intensity, duration and frequencies of MHWs, we expect to see more replacements of large long-lived habitat forming seaweeds with smaller ephemeral seaweeds, reducing the habitat structure and effective services seaweed-dominated reefs can provide