The East Australian Current (EAC) is a highly dynamic western boundary current that exerts significant influences on the marine ecosystem off southeast margin of Australia. For the first time, a quantitative mapping of the EAC system was conducted using Himawari-8 sea surface temperature data and a Topographic Position Index (TPI)-based image processing technique. The validation using Bluelink ReANalysis data suggested good reliability of our mapping results. Subsequent quantitative examination provides new insights into the EAC's cross-shelf movement which is a driving force of adjacent slope-shelf circulations. Along the coast of New South Wales (NSW), large-scale and high-frequency EAC encroachment was observed, being every 60-80 days upstream (30-32°S) and every 90-100 days downstream (33-35°S), which is associated with the EAC's intrinsic oscillation and eddy shedding. Downstream, the EAC encroachment exhibits a 20-day longer period and a double amplitude. Such dephasing is most likely due to the abrupt change of regime at the EAC separation point (32-33°S). Upstream, the EAC encroachment also exhibits seasonality, being 10-15 km closer to the coast in austral summer, which is due to the seasonal EAC broadening. Higher-frequency (16-32 days) and smaller-magnitude EAC intrusion was observed along the entire NSW coast (28-37°S), which is possibly associated with EAC's meanders and frontal eddies. In the extension zone (37.30-44°S), we observed maximum EAC encroachment in summer, which is an expression of the seasonal boundary flow off eastern Tasmania. A data set of EAC encroachment (2015-2017) is available (see Data Availability Statement). Plain Language Summary The East Australian Current (EAC) frequently intrudes landward, drives coastal water uplift, and consequently brings nearshore nutrient blooms. Such current intrusions therefore exert significant impacts on the coastal marine ecosystem. To thoroughly examine the EAC intrusions, first, we developed an accurate mapping of the EAC using satellite imagery. Next, using the mapping products, we directly measure and investigate the time-varying spatial closeness between the EAC and the coast. Our results show that the EAC is a highly dynamic and unstable eddy-current system which intrudes landward all year round with multiple periods and scales. The semiautomatic mapping presented here has many practical applications in measuring, monitoring and tracking of ocean currents and eddies.

Concurrent anthropogenic global climate change and ocean acidification are expected to have a negative impact on calcifying marine organisms. While knowledge of biological responses of organisms to oceanic stress has emerged from single‐species experiments, these do not capture ecologically relevant scenarios where the potential for multi‐organism physiological interactions is assessed. Marine algae provide an interesting case study, as their photosynthetic activity elevates pH in the surrounding microenvironment, potentially buffering more acidic conditions for associated epiphytes. We present findings that indicate increased tolerance of an important epiphytic foraminifera, Marginopora vertebralis , to the effects of increased temperature (±3°C) and p CO 2 (~1,000 µatm) when associated with its common algal host, Laurencia intricata . Specimens of M. vertebralis were incubated for 15 days in flow‐through aquaria simulating current and end‐of‐century temperature and pH conditions. Physiological measures of growth (change in wet weight), calcification (measured change in total alkalinity in closed bottles), photochemical efficiency ( Fv/Fm ), total chlorophyll, photosynthesis (oxygen flux), and respiration were determined. When incubated in isolation, M. vertebralis exhibited reduced growth in end‐of‐century projections of ocean acidification conditions, while calcification rates were lowest in the high‐temperature, low‐pH treatment. Interestingly, association with L. intricata ameliorated these stress effects with the growth and calcification rates of M. vertebralis being similar to those observed in ambient conditions. Total chlorophyll levels in M. vertebralis decreased when in association with L. intricata , while maximum photochemical efficiency increased in ambient conditions. Net production estimates remained similar between M. vertebralis in isolation and in association with L. intricata , although both production and respiration rates of M. vertebralis were significantly higher when associated with L. intricata . These results indicate that the association with L. intricata increases the resilience of M. vertebralis to climate change stress, providing one of the first examples of physiological buffering by a marine alga that can ameliorate the negative effects of changing ocean conditions.

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