Ronald E. Thresher

Context: There are few long-term studies of the effects of land-use changes and climate change on Australia’s aquatic biota. In this study, the recent macroinvertebrate assemblages at three sites (one historically polluted by mine effluent and two ‘controls’ upstream of it) on the upper South Esk River (Tasmania) are compared with detailed surveys of the same sites conducted 50 years earlier. Aims: To assess possble long-term changes in a south-east Australian river’s aquatic biota in the face of changing climate and land use patterns. Methods: ‘Kick sampling’ was used to sample benthic aquatic animals over a 3-year period, which were sorted to the family level for most taxa and the genus level for mayflies (Ephemeroptera) and the results then compared to a similar data set collected for the same sites in the mid-1970s. Key results: At the two control sites, macroinvertebrate densities, as well as a biotic index of water quality, have approximately halved relative to the 1970s, although with increased abundances of dipterans and oligochaetes and fewer gastropods, trichopterans and acarids. The densities of the leptophlebiid mayfly Austrophlebioides, previously the most abundant mayfly genus present and an apparent indicator of clean water, have collapsed and have largely been replaced by the environmentally more tolerant Atalophlebia. Macroinvertebrate densities at the previously polluted site remain low, suggesting continuing long-term effects of metal pollution. Conclusions: The analyses overall suggest a shift from a biotically diverse and ‘clean, cold’ river in the 1970s to one that is increasingly disturbed. Implications: The source of the system stresses is unclear, given mixed land use in the catchment, but observations suggest that erosional siltation could be a contributing factor.

We compare depth and temporal distributions of sub‐fossil assemblages of two cold‐water scleractinian corals on seamounts in the Southwest Pacific to help define the temporal variations of water mass properties in the Southern Ocean (SO) during deglaciation. Peaks in the deep‐water abundance of the two species complement one another, with Desmophyllum dianthus peaking around the Antarctic Cold Reversal (ACR), and Solenosmilia variabilis briefly during the late Heinrich Stadial 1 (HS1) and during the Younger Dryas (YD). Environmental tolerances of the two species and the geochemistry of S. variabilis carbonate skeletons suggest that their secular distributions reflect complementary effects of temperature (higher at Antarctic Intermediate Water/Upper Circumpolar Deep Water depths during the YD and late HS1) and surface productivity (lower during the YD and HS1). Higher temperatures at depth we interpret as evidence of increased Zonal West Wind (ZWW)‐driven Ekman pumping during the late HS1 and YD, whereas coeval low surface production reflects poleward expansion of sub‐tropical water masses as a result of correlated poleward shifts of the ZWW belt and the Intertropical Convergence Zone. More broadly, a continuous deep coral population in the southwest Pacific that spans two species and three deglacial periods (HS1, ACR and the YD) and an early Holocene shift in coral distribution from deeper to shallower habitats appear to reflect large‐scale changes during deglaciation in SO temperature profiles and productivity.

The Little Ice Age (LIA) was one of the strongest climate perturbations in the last millennium, with widespread albeit asynchronous periods of cooling globally¹. Its effects on oceanic properties and thermohaline circulation have been questioned, however². We provide evidence of changes in the properties of Antarctic Intermediate Water (AAIW) over the last millennium that are synchronous with LIA cooling events and that appear to derive proximately from variations in the proportion of Upper Circumpolar Deep Water (UCDW) in the AAIW source mixture, and ultimately from changes in the strength and position of the zonal west winds over the Southern Ocean (SO). Export of AAIW from the SO influences climate and biogeochemical cycles globally by transporting heat, freshwater, nutrients and carbon to lower latitudes^3-5, implying widespread impacts of its LIA-correlated variability. AAIW tracking of the LIA ceased in the late 1800s, which we suggest marked the onset of over-riding impacts of anthropogenic climate change on SO circulation.

Context: Australian mayflies are hypothesised to differ from those in the Northern Hemisphere by having longer, more variable emergence seasons that overlap more widely among taxa. Aim: To test this hypothesis by comparing the behaviour of related northern and southern hemisphere species in similar habitats and at similar spatial scales. Methods: Emergence dates were recorded fortnightly over two consecutive years, one ‘warm’ and one ‘cool’, in a rocky creek in south-eastern Tasmania, and at a coarser scale for Tasmania as whole, spanning an ∼20-year period. The results are compared with emergence patterns at two sites in the northern hemisphere climatically similar to Tasmania. Key results: Durations of emergence seasons in Tasmania did not differ significantly at either the single site or whole of island scales from those in the northern hemisphere, but unlike in the latter, the start of emergence does not appear to be temperature-dependent. Conclusions: Apparent regional differences are likely to result primarily from climatically inappropriate comparisons rather than from fundamental differences in behaviour. Implications: Differences in the factor(s) that cue emergence suggest that the life histories of mayflies in Tasmania, and possibly elsewhere in Australia, are determined less by physiology than by aquatic ecology.

Metal/calcium ratios in two long‐lived deep‐sea gorgonian corals (Lepidisis and Corallium spp.) in the Southwest Pacific evidence periodic decadal variability at depths that correspond to Antarctic Intermediate Water (AAIW) and shallow Upper Circumpolar Deep Water, and a shift in the mid‐1930s to late‐1930s in mean ambient temperatures, barium/silicate concentrations and possibly pH, the rate at which these properties change over time, and the relationship between temperatures at fixed depth and the Southern Annular Mode (SAM). The decadal periodicity, which is evident in other biological indices in the study area, can be accounted for by water mass heave on the order of 100–150 m, which is consistent with observed scales of variability in the AAIW. The proximate and ultimate causes of the midcentury shifts are unclear, but could be related to suggested mid‐20th century changes in climate parameters globally and, more specifically, in the subpolar SW Pacific.