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Intertidal temperature variability

Long-term mechanistic hindcasts predict the structure of experimentally-warmed intertidal communities

Increases in global temperatures are expected to have dramatic effects on the abundance and distribution of species in the coming years. Intertidal organisms, which already experience temperatures at or beyond their thermal limits, provide a model system in which to investigate these effects. In this project, my colleague and I take advantage of a previous study in which experimental plates were deployed in the intertidal zone and passively warmed for 12 years to a daily maximum temperature on average 2.7°C higher than control plots on the adjacent bedrock. We compared the composition of the biological communities on each experimental plate and its neighboring bedrock control. Plate communities showed decreased species richness and percent cover of filamentous algae, mussels and mobile grazers relative to the bedrock, and increased percent cover of biofilm. We then used short-term time-series measurements of plate and bedrock temperatures and a mechanistic heat-budget model to hindcast those temperatures back 12 years. Greater differences in long-term average temperature between the experimental plates and bedrock controls were correlated to lower similarity in community composition. Additionally, years with higher average differences between plate and bedrock temperatures were more predictive of compositional similarity between plate and bedrock communities, even though they occurred farther in the past than did more recent, but cooler, years. We conclude that current intertidal communities reflect their long-term, rather than short-term, thermal histories. Mechanistic heat-budget models based on short-term measurements can provide this valuable, long-term information.

This work has been submitted to PNAS. A link to the paper will be provided upon publication.

Field work during a rising tide.
Field work during a rising tide.

The intertidal zone is the region of the ocean that is exposed during low tide.

Securing thermocouples to settlement plates
Securing thermocouples to settlement plates

Thermocouples were used to determine the relative temperature of experimental plates and adjacent bedrock control plots.

Securing thermocouple wire.
Securing thermocouple wire.

Thermocouple wires were secured to eyebolts throughout our study site, to prevent their removal by waves.

Multiplexer and data logger.
Multiplexer and data logger.

Temperatures from each thermocouple were measured once per minute by a multiplexer and data logger located above the high tide line.

Intertidal diversity
Intertidal diversity

The intertidal zone is one of the world's most biological diverse environments.

A rock settlement plate
A rock settlement plate

Our warmed, experimental settlement plates had reduced percent cover and species richness relative to the adjacent bedrock.