Carbon export from the Southern Ocean exerts a strong control on the ocean carbon sink, yet recent observations from the region demonstrate poorly understood relationships in which carbon export efficiency is weakly related to temperature. These observations conflict with traditional theory where export efficiency increases in colder waters. A recently proposed “temperature‐ballast hypothesis” suggests an explanatory mechanism where the effect of temperature‐dependent respiration is masked by variation in particle‐ballast as upwelling waters move northward from Antarctica. We use observations and statistical models to test this mechanism and find positive support for the hypothesized temperature‐ballast interactions. Best fitting models indicate a significant relation between export efficiency and silica‐ballast while simultaneously revealing the expected inverse effect of temperature once ballast is accounted for. These findings reconcile model predictions, metabolic theory, and carbon export observations in the Southern Ocean and have consequences for how the ocean carbon sink responds to climate change.

Toxic phytoplankton have been detrimental to the fishing and aquaculture industry on the east coast of Tasmania, causing millions of dollars in loss due to contaminated seafood. In 2012–2017, shellfish stocks were poisoned by Alexandrium catenella, a dinoflagellate species that produces paralytic shellfish toxins (PST). Remote sensing data may provide an environmental context for the drivers of PST events in Tasmania. We conducted spatial and temporal trend analyses of the Multi-Scale Ultra-High-Resolution Sea Surface Temperature (MUR SST) and Ocean Color Climate Change Initiative chlorophyll-a (OC-CCI chl-a) to determine if SST and chl-a correlated with the major toxin increases from 2012 to 2017. Along with the trends, we compare the remotely sensed oceanographic parameters of SST and chl-a to toxin events off the east coast of Tasmania to provide environmental context for the high-toxin period. Spatial and temporal changes for chl-a differ based on the north, central, and southeast coast of Tasmania. For sites in the north, chl-a was 5.3% higher from the pre-PST period relative to the PST period, 5.1% along the central part of the coast, and by 6.0% in the south based on deviations from the coastal study area time series. Overall, SST has slightly decreased from 2007 to 2020 (tau = −0.011, p = 0.827) and chl-a has significantly decreased for the east coast (tau = −0.164, p = 1.58 × 10−3). A negative relationship of SST and PST values occurred in the north (r = −0.530, p = 5.32 × 10−5) and central sites (r = −0.225, p = 0.157). The correlation between satellite chl-a (from OC-CCI, Visible Infrared Imaging Radiometer Suite (VIIRS), and Moderate-Resolution Imaging Spectrometer (MODIS) Aqua) and in situ data is weak, which makes it difficult to assess relationships present between chl-a and toxin concentrations. Moving forward, the development of a regional chl-a algorithm and increased in situ chl-a collection and plankton sampling at a species level will help to improve chl-a measurements and toxic phytoplankton production monitoring around Tasmania.