Neoichnological experiments reveal key differences between marine bioturbator responses to gradual warming compared to marine heat waves

Seawater temperature fluctuations influence shallow marine ecosystems by disrupting the bioturbating (burrowing and sediment-mixing) activities of benthic species, which can, in turn, strongly affect seafloor geochemical cycling, including sediment oxygenation, organic matter degradation and nutrient regeneration. Although climate change is expected to increase water temperatures across a range of settings, particularly in mid-to high-latitude coastal environments, there is limited understanding of how the temporal dynamics of these changes (e.g., rate, duration and frequency of warming) will shape bioturbator behaviors. In this study, we used common benthic bioturbators (polychaetes and amphipods) from the Connecticut coast to investigate how conditions simulating both marine heat waves (MHWs) and gradual warming influence bioturbation and burrow features in mesocosm experiments. In addition, we use experimental observations of changes in burrow morphology to develop new criteria for diagnosing the impact of warming from trace fossil assemblages, thus providing a new window into the impact of past episodes of warming on seafloor communities. Our findings indicate that conditions simulating marine heat waves (i.e., episodic, punctuated warming) have distinct and contrasting effects on bioturbators relative to gradual warming. We observe that, for nereid polychaetes, gradual warming increased burrow area and decreased maximum burrow depth, whereas MHW conditions had limited effect. For eunicid polychaetes, gradual warming increased maximum burrow depth, whereas MHWs decreased burrow area. For amphipods, both burrow area and maximum burrow depth increased under MHW conditions. These results highlight that the effects of temperature variations on bioturbators are species- and ecology-specific; different infaunal taxa living within the same community have notably different thermal tolerances.