Robert Owain Smith

Fiordland, Aotearoa/New Zealand (AoNZ) is host to 14 named temperate fjords, however the magnitude and timescales of oceanographic variability in the deep basins remain poorly understood due to limited oceanographic sampling in these basins over the past five decades. Fjords have been identified as globally significant carbon burial sites and oceanographic processes and properties are important components of the sequestration process. Deep‐water renewal (DWR) and vertical mixing are key processes in governing deep‐water conditions. In this study, we present a near‐monthly, 2 yr, in situ timeseries of deep‐water properties in Patea/Doubtful Sound, Fiordland, AoNZ, and determine the roles of DWR and vertical mixing in controlling deep‐water properties and timescales of their variation. In conjunction with pre‐ and post‐winter CTD casts collected in four other AoNZ fjords, these data provide the first direct observations of DWR in the southwest fjords of AoNZ. Using two years of near‐monthly repeat CTD survey data (2023–2025), we estimate vertical diffusivities at two sites using a budget method and use these to hindcast properties in the fjord between January 2012 and April 2025 using a 1‐D model. Hindcast output suggests renewal occurs every winter in both entrance and fjord‐head basins. Comparisons between hindcast output and coastal water properties reveal that annual DWR in AoNZ results from deepening and cooling of the winter mixed layer, contrasting with other fjord systems where DWR is often caused by upwelling, providing a novel renewal preconditioning mechanism.

Carbonate mineral production and dissolution regulate atmospheric carbon dioxide (CO ₂ ) concentrations via modulation of the ocean alkalinity content. The anthropogenic rise in atmospheric CO ₂ reduces calcification rates and enhances calcium carbonate dissolution, which increases ocean alkalinity, counteracts acidification, and stimulates ocean CO ₂ uptake. However, carbonate dissolution takes place primarily in the deep ocean, so this feedback is slow, maintaining ocean CO ₂ uptake over millennial timescales. Here, we present evidence that seawater alkalinity on the continental shelf is increasing on annual‐decadal timescales, at a rate that is orders of magnitude faster than the deep ocean feedback. Biogeochemical model analyses suggest this fast feedback results from calcium carbonate dissolution in the shelf seafloor driven by increasing atmospheric CO ₂ concentrations. Extrapolating these results to the global continental shelf suggests that shelf carbonate dissolution has been accelerating since the 1800s and may account for up to 10% of the missing ∼0.3 Pg C yr ⁻¹ in ocean model carbon budgets.

Carbonate mineral production and dissolution regulate atmospheric carbon dioxide (CO2) concentrations via modulation of the ocean alkalinity content. The anthropogenic rise in atmospheric CO2 reduces calcification rates and enhances calcium carbonate dissolution, which increases ocean alkalinity, counteracts acidification, and stimulates ocean CO2 uptake. However, carbonate dissolution takes place primarily in the deep ocean, so this feedback is slow, maintaining ocean CO2 uptake over millennial timescales. Here we present evidence that seawater alkalinity concentrations on the continental shelf are increasing on annual-decadal timescales, at a rate that is orders of magnitude faster than the deep ocean feedback. Biogeochemical model analyses suggest this fast feedback results from calcium carbonate dissolution in the shelf seafloor driven by increasing atmospheric CO2 concentrations. Earth-system modelling indicates that shelf carbonate dissolution has been accelerating since the 1800s and may account for up to 30% of the missing ~0.3 Pg C yr-1 in ocean model carbon budgets.

What little we know about how microbiomes change over the course of host dispersal has been gleaned from simulations or snapshot sampling of microbiomes of hosts undertaking regular, cyclical migrations. These studies suggest that major changes in both microbiome richness and turnover occur in response to long-distance movements, but we do not yet know how rare or sporadic dispersal events for non-migratory organisms might affect the microbiomes of their hosts. Here we directly examine the microbiomes of rafting seaweed, leveraging host genomic analyses, amplicon sequencing, and oceanographic modelling to study the impacts of ecological dispersal of hosts on their microbiomes. We find that once dislodged from coastal shores and adrift, kelp-associated microbial communities change profoundly—the core microbes found on attached kelp give way to a few abundant taxa and many rare taxa. Changes in microbial species richness and composition are strongly linked to variability in sea surface temperature rather than length of time spent rafting. These changes are associated with increased contributions of neutral processes shaping community assembly. These findings highlight the role of environmental predictability in triggering major community successional changes and challenge the importance of host selection in determining the microbiome.

Marine heatwaves (MHWs) are increasing in frequency, duration and intensity, disrupting global marine ecosystems. While most reported impacts have been in tropical areas, New Zealand experienced its strongest and longest MHW in 2022, profoundly affecting marine sponges. Sponges are vital to rocky benthic marine communities, with their abundance influencing ecosystem functioning. This study examines the impact of this MHW on the photosynthetic sponge Cymbastella lamellata in Fiordland, New Zealand. We describe the extent, physiological responses, mortality, microbial community changes and ecological impact of this MHW on C. lamellata. The Fiordland MHW reached a maximum temperature of 4.4°C above average, lasting for 259 days. Bleaching occurred in >90% of the C. lamellata Fiordland population. The population size exceeded 66 million from 5 to 25 m, making this the largest bleaching event of its kind ever recorded. We identified the photosynthetic symbiont as a diatom, and bleached sponges had reduced photosynthetic efficiency. Post‐MHW surveys in 2023 found that over 50% of sponges at sampling sites had died but that the remaining sponges had mostly recovered from earlier bleaching. Using a simulated MHW experiment, we found that temperature stress was a driver of necrosis rather than bleaching, despite necrosis only rarely being observed in the field (C. lamellata are key contributors of dissolved organic carbon to the water column, with their loss likely impacting ecosystem function. We demonstrate the potential for MHWs to disrupt key marine phyla in temperate regions, highlighting how susceptible temperate sponges globally might be to MHWs.

This study investigates the influence of along‐front wind forcing on chlorophyll‐a (Chl‐a) at the Otago Shelf Break (OSB) in southeast Aotearoa/New Zealand using remotely‐sensed and in situ data. Summer wind stress over the OSB was shelf‐aligned, oscillating between upfront and downfront. Surface Chl‐a concentrations along the OSB were shown to increase episodically (≤10 days duration) following upfront wind stress. This response occurred over most of the 350 km long shelf break, and was most intense north of the Otago Peninsula. Peak Chl‐a enhancement at the shelf break occurred following periods of upfront winds, exhibiting a lagged response of approximately 5 days. Moored thermistor data indicated that upfront wind events were followed by increased thermal stratification over the mid‐shelf, whereas downfront wind events were followed by a well‐mixed water column. In situ temperature, salinity, and nutrient measurements suggested an offshore movement of the surface expression of the front following upfront winds, and a reduction in nitrate over the shelf break. From these observations a model of Ekman restratification driven by upfront winds is proposed for this system, wherein off‐shelf Ekman transport converts strong horizontal isopycnal gradients at the shelf break front into vertical stratification. This stratification holds phytoplankton in the upper water column, allowing increased access to light, which is marked by the increased drawdown of nitrate at the shelf break. Downfront winds break down this stratification, move the shelf break front onshore, and mix phytoplankton through the water column, reducing the surface expression of chlorophyll.