Emma Ryan

Understanding historical coastal change over multi‐decadal timescales is fundamental for coastal hazard assessment and adaptation planning. In Aotearoa New Zealand (NZ), national‐scale coastal change information has historically been fragmented, limited in spatial resolution or constrained to relatively short timeframes. Here, we present an overview and initial analysis of New Zealand's Coastal Change Dataset (NZCCD), a nationally consistent dataset of manually mapped historical coastline positions (with relatively low positional uncertainty averaging 4.1 m, depending on scale) and coastal change rates spanning the 1940s to 2024 for most of NZ's open coast beaches and soft cliffs. Results demonstrate that NZ's coastline is highly dynamic over multi‐decadal timescales. Approximately 42% of the mapped coastline has undergone net accretion, 40% net erosion, with 18% showing no resolvable long‐term trend. Rates of erosion and accretion are generally −1 , but localised hotspots of rapid change (both erosion and accretion) exceeding ±10 m y ⁻¹ are identified. Multi‐decadal and low temporal resolution baseline datasets with national coverage, such as the NZCCD, are complementary to high temporal resolution, semi‐automated satellite‐derived studies. The dataset enables national‐scale and locally relevant decision‐making in the context of climate change and relative sea‐level rise.

How El Niño–Southern Oscillation variability and diversity have changed over the Common Era, and how corals have responded to ENSO-induced temperature anomalies, remains unclear. Here, Porites microatolls from Arno Atoll, Marshall Islands, in the western Pacific were analyzed across three periods: 484–560 ± 6 CE (Late Antique Little Ice Age; LALIA), 1273–1332 ± 4 CE (Early Little Ice Age; early LIA), and 1954–2017 CE (modern). The early LIA record indicates significantly colder conditions and reduced Sr/Ca-derived temperature seasonality compared to today, consistent with contemporaneous Central Pacific δ¹⁸O records. Negative Sr/Ca-temperature anomalies in the LALIA record likely relate to the 536 and 540 CE volcanic eruptions. Comparison with Central Pacific records suggests that Central Pacific El Niño activity increased between the first and second millennium of the Common Era, while Eastern Pacific El Niño activity remained largely stable, or may even have been stronger than today. Thermal anomalies impacted only modern coral calcification, indicating reduced tolerance when close to the population’s thermal threshold, regardless of potential adaptation to local thermal regimes.

The Estuarine Turbidity Maximum (ETM) is a biogeochemical hotspot in estuaries that regulates ecosystem health by amplifying estuarine water filtering. Convergence of elevated turbidity in an ETM is due to the intersection of repeated sediment resuspension and physical characteristics of a system. ETM formation depends on various tidal and freshwater conditions, making its ecological impact difficult to predict. Understanding the mechanisms of ETM formation has been predominantly studied in estuaries with strong tides or salt wedges. We examine how ETMs behave in a mesotidal estuary where the salt wedge is weak due to low freshwater input. Transect surveys and long-term deployments measuring salinity, suspended sediment, and currents were conducted over two summers (2024/2025) and one winter (2024) to capture spatial, tidal, and seasonal variability in ETM formation. The data revealed, for the first time, the formation of two tidally induced, intermittent ETMs. Both ETMs formed only at mid-tide, as neither gravitational circulation, tidal pumping, nor topographic trapping were strong enough to sustain cyclic/tidal sediment resuspension. The upstream ETM formed at the salt-wedge, extending further seaward in winter, whereas the downstream ETM formed due to tidal pumping, possibly assisted by topographic trapping during ebb tide. The co-occurrence of two intermittent ETMs suggests that estuaries host multiple, temporal biogeochemical hotspots. This could enhance nutrient filtering capacity but may also increase material trapping and amplify eutrophication risks. This highlights that estuarine filtering capacity may be more variable than previously assumed, with implications for water quality management within estuaries and the connected marine and freshwater systems.

Monitoring reef growth is essential for understanding sea-level rise impacts on reef-lined coasts. However, measuring coral reef vertical accretion in the field is challenging and rarely achieved. Here we present a method for measuring reef flat vertical accretion at sub-mm scale using close-range structure-from-motion photogrammetry. Comparison of repeat digital surface models constructed from field-based and lab-based photos reveal that the accuracy and precision of the method is on average 0.42 mm and 0.181 mm, respectively. We provide an example of how this method can be utilized to measure reef vertical accretion on a reef flat in the Kingdom of Tonga, South Pacific region. Results reveal the reef accreted by 8.42 ± 0.601 mm over the one-year study period. We highlight the need for future research to scale up the methodology spatially and extend the timeframe of analysis in order to track reef response to ongoing sea-level rise.

Coastal landforms and associated archaeological records are at risk of erosion from a combination of rising sea levels and increasingly frequent high-intensity storms. Improved understanding of this risk can be gained by braiding archaeological and geomorphological methodologies with Indigenous knowledge.¹ In this article, archaeological, geomorphological and mātauranga (a form of Indigenous knowledge) are used to analyse a prograded Holocene foredune barrier in northern Aotearoa/New Zealand. Anthropogenic deposits within dune stratigraphy are radiocarbon-dated and used as chronological markers to constrain coastal evolution, alongside geomorphological analyses of topographic data, historical aerial photographs and satellite imagery. These investigations revealed that the barrier is eroding at a rate of 0.45 m/year. A midden in the foredune, which has been radiocarbon dated to 224–270 B.P. (95% Confidence), has been exposed by coastal erosion, confirming that the barrier is in the most eroded state it has been within the past ~300 years. Vertical stratigraphy reveals the presence of midden and palaeosol deposits capped by dune sand deposits in the foredune, indicating that vertical accretion of the foredune continued over the last ~200 years, despite the barrier now being in an eroding state. Mātauranga played a vital role in this project, as it was the coastal taiao (environmental) monitoring unit of Patuharakeke (a Māori sub-tribe) that discovered the midden. The ecological mātauranga shared also played a vital role in this project, adding experiential evidence to empirical observations. The work of local Indigenous groups, like Patuharakeke, demonstrates the active use of mātauranga, woven with Western science methods to preserve or capture the knowledge contained within archaeological sites at risk of being lost to coastal erosion. In this study, we present a method for weaving mātauranga, geomorphological and archaeological approaches to gain a deeper understanding of coastal landscape development.

Understanding the drivers of net coral reef calcium carbonate production is increasingly important as ocean warming, acidification, and other anthropogenic stressors threaten the maintenance of coral reef structures and the services these ecosystems provide. Despite intense research effort on coral reef calcium carbonate production, the inclusion of a key reef forming/accreting calcifying group, the crustose coralline algae, remains challenging both from a theoretical and practical standpoint. While corals are typically the primary reef builders of contemporary reefs, crustose coralline algae can contribute equally. Here, we combine several sets of data with numerical and theoretical modelling to demonstrate that crustose coralline algae carbonate production can match or even exceed the contribution of corals to reef carbonate production. Despite their importance, crustose coralline algae are often inaccurately recorded in benthic surveys or even entirely missing from coral reef carbonate budgets. We outline several recommendations to improve the inclusion of crustose coralline algae into such carbonate budgets under the ongoing climate crisis.

Understanding the drivers of net coral reef calcium carbonate production is increasingly important as ocean warming, acidification, and other anthropogenic stressors threaten the maintenance of coral reef structures and the services these ecosystems provide. Despite intense research effort on coral reef calcium carbonate production, the inclusion of a key reef forming/accreting calcifying group, the crustose coralline algae (CCA), remains challenging both from a theoretical and practical standpoint. While corals are typically the primary reef builders of today, ongoing declines in coral cover due to a range of environmental perturbations will likely increase the relative importance of CCA and other non-scleractinian calcifying taxa to coral reef carbonate production. Here, we demonstrate that CCA are important carbonate producers that, under certain conditions, can match or even exceed the contribution of corals to coral reef carbonate production. Despite their importance, CCA are often inaccurately recorded in benthic surveys or even entirely missing from coral reef carbonate budgets. We outline several recommendations to improve the inclusion of CCA into such carbonate budgets under the ongoing climate crisis.

An extremely rare example of well‐preserved emergent Holocene fossil reefs exists at Kiritimati Island, central Pacific. Fossil reefs are rich geological archives of paleoenvironmental change. The first paleoecological surveys of two fossil reefs are presented, revealing high coral cover (40–50%) and low diversity (6 genera). Fossil coral ages suggest reefs exhibited disparity in the timing of reef development (4,113 and 1,915 cal yBP) and ecological surveys show different coral compositions (Acropora or Porites dominant), between reefs. Results constrain two discrete episodes of reef shutdown (at 2,905 and 1,705 cal yBP) as lagoonal reefs thrived, and subsequently died off, through the late Holocene. Shifts in physio‐chemical conditions associated with reduced lagoon flushing following storm‐driven changes in atoll rim morphology are argued as the driver for the staged reef die‐off. The findings have implications for interpreting past and future eco‐morphological change on atolls, given projected increases in storminess with climate change.