Experimental analyses of diversity partitioning in southern hemisphere algal communities

Shifts in foundation species dominance and altered interaction networks after compounding seismic uplift and extreme marine heatwaves

Seismic activity, erosion, sedimentation, and extreme temperatures can cause compounding large-scale disturbances to marine organisms, like large intertidal foundational seaweeds. In November 2016, a 7.8 Mw earthquake uplifted 130 km of coastline by 0.5-6 m near Kaikōura, New Zealand and thereby increased intertidal desiccation, aerial temperatures, reef erosion, and water turbidity. Furthermore, stress on uplifted intertidal species was compounded by unprecedented marine heatwaves over the summer of 2017/18. Here we documented altered dominances of large foundational seaweed and possible flow-on effects on seaweed-associated flora and fauna, following the uplift and heatwaves. These compounding disturbances caused instant high canopy loss of the dominant primary foundation species - the large perennial canopy-forming southern bull kelp Durvillaea antarctica - and no post-disturbance recovery, suggesting a maintenance threshold has been exceeded. After canopy loss of the primary foundation species, alternative foundation species - i.e., subordinate competitors under pre-disturbance conditions (the perennial canopy-forming fucoids Carpophyllum maschalocarpum, Cystophora scalaris, and Hormosira banksii) increased in abundance. Furthermore, field observations of attachment interaction networks demonstrated that the primary and alternative foundation species facilitated different sessile and mobile taxa. For example, the smaller and more morphologically complex C. maschalocarpum, H. banksii, and C. scalaris, supported more novel attachment associations, whereas the larger Durvillaea supported longer attachment chains. Overall, our results highlight abrupt and potentially long-lasting ecological changes after compounding disturbances, which altered dominance hierarchies. Alternative foundation species are now more common than the pre-disturbance primary foundation species, with flow-on effects on wider communities that depend on biogenic habitats.

Long-term community shifts driven by local extinction of an iconic foundation species following an extreme marine heatwave

Gradual ocean warming combined with stronger marine heatwaves (MHWs) can reduce abundances of foundation species that control community structures, biodiversity, and ecosystem functioning. However, few studies have documented long-term succession trajectories following the more extreme events that cause localized extinctions of foundation species. Here, we documented long-term successional changes to marine benthic communities in Pile Bay, New Zealand, following the Tasman 2017/18 MHW, which caused localized extinctions of dominant southern bull kelp (Durvillaea sp.). Six years on, multiscale annual and seasonal surveys show no sign of Durvillaea recolonization. Instead, the invasive annual kelp (Undaria pinnatifida), rapidly colonized areas previously dominated by Durvillaea, followed by large changes to the understory community, as Durvillaea holdfasts and encrusting coralline algae were replaced by coralline turf. Between 3 and 6 years after the total loss of Durvillaea, smaller native fucoids colonized in high densities. Although Undaria initially colonized plots throughout Durvillaea's tidal range, later in the succession Undaria only retained dominance in the lower intertidal zone and only in spring. Ultimately, the tidal zone was slowly replaced by alternative foundation species, composed of different canopy-forming brown seaweeds that dominated different intertidal elevations, resulting in a net increase in canopy and understory diversity. This study is a rare example of long-term effects following an extreme MHW that caused extinctions of a locally dominant canopy-former, but these events and their associated dramatic changes to community structures and biodiversity are expected to become increasingly common as MHWs continue to increase in strength, frequency, and duration.

Functional redundancy and stability in a subtidal macroalgal community in the Southwestern Atlantic coast

Functional redundancy can stabilize ecological functions as asynchronous fluctuations among functionally similar species may buffer environmental changes. We investigated the temporal dynamics of a subtidal macroalgal community in the warm temperate Southwestern Atlantic coast (SWA) to evaluate whether functional redundancy stabilize ecosystems functions through compensatory dynamics under realistic environmental scenarios. Despite temporal variations in the community structure occurred, a high stability in macroalgal coverage was found at the community-level driven by taxa asynchronous fluctuations. No relationship between functional redundancy and stability occurred, suggesting that functional compensation cannot surpass the influence of environmental fluctuations on the performance of ecological functions. Declines in Sargassum species abundance, along with its low functional redundancy, indicate that this canopy-forming algae must be prioritized in conservation efforts in the SWA. Our study adds to the comprehension and generalization of biodiversity-stability findings in natural systems across distinct geographical areas, also contributing to their operationalization in marine ecosystems.

Cataclysmic Disturbances to an Intertidal Ecosystem: Loss of Ecological Infrastructure Slows Recovery of Biogenic Habitats and Diversity

Understanding the resilience and recovery processes of coastal marine ecosystems is of increasing importance in the face of increasing disturbances and stressors. Large-scale, catastrophic events can re-set the structure and functioning of ecosystems, and potentially lead to different stable states. Such an event occurred in south-eastern New Zealand when a Mw 7.8 earthquake lifted the coastline by up to 6 m. This caused widespread mortality of intertidal algal and invertebrate communities over 130 km of coast. This study involved structured and detailed sampling of three intertidal zones at 16 sites nested into four degree of uplift (none, 0.4–1, 1.5–2.5, and 4.5–6 m). Recovery of large brown algal assemblages, the canopy species of which were almost entirely fucoids, were devastated by the uplift, and recovery after 4 years was generally poor except at sites with < 1 m of uplift. The physical infrastructural changes to reefs were severe, with intertidal emersion temperatures frequently above 35°C and up to 50°C, which was lethal to remnant populations and recruiting algae. Erosion of the reefs composed of soft sedimentary rocks was severe. Shifting sand and gravel covered some lower reef areas during storms, and the nearshore light environment was frequently below compensation points for algal production, especially for the largest fucoid Durvillaea antarctica/poha. Low uplift sites recovered much of their pre-earthquake assemblages, but only in the low tidal zone. The mid and high tidal zones of all uplifted sites remained depauperate. Fucoids recruited well in the low zone of low uplift sites but then were affected by a severe heat wave a year after the earthquake that reduced their cover. This was followed by a great increase in fleshy red algae, which then precluded recruitment of large brown algae. The interactions of species’ life histories and the altered physical and ecological infrastructure on which they rely are instructive for attempts to lessen manageable stressors in coastal environments and help future-proof against the effects of compounded impacts.

Missing the Forest and the Trees: Utility, Limits and Caveats for Drone Imaging of Coastal Marine Ecosystems

Coastal marine ecosystems are under stress, yet actionable information about the cumulative effects of human impacts has eluded ecologists. Habitat-forming seaweeds in temperate regions provide myriad irreplaceable ecosystem services, but they are increasingly at risk of local and regional extinction from extreme climatic events and the cumulative impacts of land-use change and extractive activities. Informing appropriate management strategies to reduce the impacts of stressors requires comprehensive knowledge of species diversity, abundance and distributions. Remote sensing undoubtedly provides answers, but collecting imagery at appropriate resolution and spatial extent, and then accurately and precisely validating these datasets is not straightforward. Comprehensive and long-running monitoring of rocky reefs exist globally but are often limited to a small subset of reef platforms readily accessible to in-situ studies. Key vulnerable habitat-forming seaweeds are often not well-assessed by traditional in-situ methods, nor are they well-captured by passive remote sensing by satellites. Here we describe the utility of drone-based methods for monitoring and detecting key rocky intertidal habitat types, the limitations and caveats of these methods, and suggest a standardised workflow for achieving consistent results that will fulfil the needs of managers for conservation efforts.

Long-term changes in macroalgae assemblages reveal a gradual biodiversity loss over the last 200 years in the hypereutrophic Guanabara Bay

Guanabara Bay, the second largest bay on the Brazilian coast, has tropical to subtropical, hypereutrophic water conditions. A survey of the macroalgae flora conducted over the past 200 years (1800-2013) yielded a list of 245 species, which represents 15.7% of the total macroalgae recorded in the Tropical Western Atlantic, while comprehends 29.9% and 50.2% of the marine flora in Brazil and Rio de Janeiro, respectively. When evaluating the macroalgal list throughout different periods, it is noticeable the gradual loss of biodiversity as anthropization increased over the last two centuries. The use of phytogeographic indexes and beta diversity confirmed the negative changes in macroalgae assemblages as a response to increasing environmental degradation. The use of floristic surveys to establish the reference conditions employing historical data and/or regionally referenced prove to be an efficient tool using macroalgae as an ecological indicator for water quality.

Unmanned Aerial Vehicles (UAVs) for Monitoring Macroalgal Biodiversity: Comparison of RGB and Multispectral Imaging Sensors for Biodiversity Assessments

Developments in the capabilities and affordability of unmanned aerial vehicles (UAVs) have led to an explosion in their use for a range of ecological and agricultural remote sensing applications. However, the ubiquity of visible light cameras aboard readily available UAVs may be limiting the application of these devices for fine-scale, high taxonomic resolution monitoring. Here we compare the use of RGB and multispectral cameras deployed aboard UAVs for assessing intertidal and shallow subtidal marine macroalgae to a high taxonomic resolution. Our results show that the diverse spectral profiles of marine macroalgae naturally lend themselves to remote sensing and habitat classification. Furthermore, we show that biodiversity assessments, particularly in shallow subtidal habitats, are enhanced using six-band discrete wavelength multispectral sensors (81% accuracy, Cohen’s Kappa) compared to three-band broad channel RGB sensors (79% accuracy, Cohen’s Kappa) for 10 habitat classes. Combining broad band RGB signals and narrow band multispectral sensing further improved the accuracy of classification with a combined accuracy of 90% (Cohen’s Kappa). Despite notable improvements in accuracy with multispectral imaging, RGB sensors were highly capable of broad habitat classification and rivaled multispectral sensors for classifying intertidal habitats. High spatial scale monitoring of turbid exposed rocky reefs presents a unique set of challenges, but the limitations of more traditional methods can be overcome by targeting ideal conditions with UAVs.