Assessing multidecadal climate-driven shifts for Tasmanian marine species, 1992-2019

Aim: To track community-level changes of marine species associated with warming in the global ocean-warming hotspot surrounding Tasmania.

Major taxa studied: Reef fishes, mobile macroinvertebrate and macroalgal communities.

Study location: 94 reef sites (5 to 10 m depth) spanning four coastal regions around Tasmania.

Time period: Surveys of reef communities were conducted over a total of 27-years during three discrete sampling periods: the early 1990s, mid-2000s, and 2017-19.

Methods: Standardised Underwater Visual Census of reef species were consistently conducted at each of the 94 sites using 4 x 50 m long belt transects. On each 50-m transect, two blocks (each 5 m wide) were surveyed for fishes, one block (1 m wide) for macroinvertebrates, and 5 quadrats (50 x 50 cm) scored for macroalgal cover.

Results: Significant reef community changes were observed against a background of pronounced sea temperature rise (+0.80°C on average over 27-yrs). Overall, fish biomass increased, macroinvertebrate species richness and abundance decreased, and macroalgal cover decreased, mostly within the last decade. While warming was slight and reef communities relatively stable between the 1990s and mid-2000s (+0.12°C warming on average), accelerated warming and community-wide shifts during the most recent decade (+0.68°C on average) indicates critical thermal thresholds have been progressively exceeded, leading to broadscale community reorganisation. A ‘tropicalising’ shift was evidenced by increasing numbers of warm-affinity fishes, macroinvertebrates and macroalgae; conversely some cool-affinity species declined. Functionally, the ‘tropicalising’ shift was also associated with increased abundances of herbivorous fishes across most regions. Notably, theestablishment of ‘climate-invasive’ species now makes up most of the novel reef fauna in Tasmania exceeding that of exotic introduced species directly translocated by humans. Warming was associated with increased fish biomass in fished zones but had little effect on reef communities within marine reserves, demonstrating greater resilience of protected reef communities. Notably, regulation of gillnet fishing appears to have enabled some recovery of reef fishes in Tasmania, with greatest recovery of target/ bycatch species generally occurring on reefs within MPAs; where populations and ecosystem recovery via re-establishment of functional predators is still ongoing after 27-yrs and is providing resilience to climate-driven changes.

Main conclusions: Initial subtle changes in Tasmanian reef communities evident in the mid?2000s were followed by abrupt changes associated with rapid warming during the most recent decade. These changes were concomitant with accelerated warming which positively impacted warm-affinity species while negatively affecting cool-affinity species. Associated shifts towards increasing herbivores will lead to ongoing impacts on the functioning of Tasmanian reef ecosystems. Reducing non-climate stressors, including intense fishing, will help safeguard reef communities from rapid warming.