Ecology

Has eelgrass already adapted to warmer waters?

DuBois, K., Pollard, K. N., Kauffman, B. J., Williams, S. L., Stachowicz, J. J. (2022). Local adaptation in a marine foundation species: Implications for resilience to future global change. Glob Change Biol., 00(1-15). https://doi.org/10.1111/gcb.16080.

Species resiliency

In today’s changing world, biodiversity is threatened by many different stressors, such as climate change, pollution, and species invasions. For example, climate change might be making some species’ environments warmer than they are adapted to tolerate. For a species to persist in the face of threats such as climate change, they must have what ecologists call “resiliency”.

Species resiliency refers to the capacity of a species to respond to a disturbance by resisting damage and recovering quickly. Resiliency is an important concept in today’s world because climate change is causing many species to be exposed to new disturbances or to be exposed to disturbances more often than they are used to. The better a species can resist damage during these disturbances and bounce back to pre-disturbance health and abundance, the more capable they will be to survive as climate change continues.

Eelgrass beds (Photo: National Park Service, public domain)

Many marine species will need to demonstrate their resiliency in the coming decades as climate change impacts ripple through marine environments. Ocean acidity is rising, ocean circulation patterns are changing, and marine heatwaves are becoming more common. In a recent study by researchers at the University of California, Davis, eelgrass (Zostera morina) was tested for its capacity to respond to a changing environment (its “adaptive capacity”). Adaptive capacity is one aspect of resiliency because it describes how well a species can recover from a disturbance. By measuring the eelgrass’ adaptive capacity, the researchers can determine how resilient it is to future change.

Climate change impacts on resilient populations of a marine species

Climate change can have different impacts on populations within a species. This means that one local population (for example, Z. morina eelgrass in Monterey Bay) may have higher adaptive capacity than another population of the same exact same species that lives in a different area (Z. morina in San Diego Bay).  The UC Davis researchers decided to look at how different populations of eelgrass can respond to marine heatwaves. They proposed that heat-tolerant populations of eelgrass may already exist due to frequent exposure to marine heatwaves in some areas. These populations are more likely to be locally adapted to stress caused by heat and would therefore have greater capacity to respond to higher temperatures. In other words, these populations should have higher adaptive capacity.

Many marine species, such as corals, have been identified in previous studies as having populations with varying levels of heat tolerance. This population differentiation can even occur on spatial scales as fine as meters to kilometers because temperatures can vary on scales of meters to kilometers, especially in nearshore environments. So, the UC Davis authors have strong reason to believe that populations of eelgrass can also exhibit varying levels of heat tolerance.

A map of the Tomales Bay study areas (DuBois et al., 2022)

To test their hypothesis, the researchers selected three eelgrass meadows within Tomales Bay, CA. Each meadow grows in different environmental conditions because Tomales Bay exhibits strong gradients in environmental conditions over its 16 km length. The three sites spanned the entire length of the bay to capture this gradient in environmental conditions: Nick’s Cove near the mouth of the bay, Blakes Landing mid-bay, and Millerton Point at the head of the bay.

The researchers then completed a year-long reciprocal transplant experiment, where eelgrass shoots from each site were transplanted to each other site and monitored for growth. This reciprocal transplant experiment was paired with garden experiments that could test the impacts of temperature on each of the three populations. In these experiments, shoots from each site were placed in six aquaria that allowed the researchers to control the water temperature. Three of the aquaria were kept at the average July temperature of the Millerton Point site (the warmest location in Tomales Bay) and three were kept at cooler temperatures characteristic of the Nick’s Cove and Blakes Landing sites. The researchers then monitored the growth of the shoots over one month.

A view of Tomales Bay, CA. (CC BY-SA 3.0)

Do eelgrass populations have different levels of adaptive capacity?

The results of this study indicate that eelgrass populations do have varying levels of heat tolerance based on the environmental conditions where they live. The results of both the transplant experiment and the garden experiment indicated that the eelgrass samples that originated from the site with the warmest temperatures, Millerton Point, grew best under elevated temperature conditions.

In the transplant experiment, these results appeared in the form of “homesite advantage”, meaning that the Millerton Point plants grew best at the warm Millerton Point site whereas the Nick’s Cove and Blakes Landing plants grew poorly at this site. In the garden experiment, these results appeared in the observation that the growth rate of the shoots from Millerton Point did not differ among temperature treatments. In contrast, the Nick’s Cove and Blakes Landing shoots grew 40% less under the warm temperatures compared to the cooler temperatures that are characteristic of their homesites.

This study was able to demonstrate that fine-scale environmental gradients can drive local differentiation of eelgrass populations. The higher success of Millerton Point eelgrass in warmer temperatures indicates that this population is genetically adapted to warmer temperatures. This is good news for the success of eelgrass because ocean temperatures are expected to continue to rise into the future. As temperatures rise, Millerton Point eelgrass could act as a “reservoir” for temperature-resilient genes that can spread to other locations in Tomales Bay and possibly even to other bays. By spreading these heat-tolerant genes to other eelgrass populations, the adaptive capacity of eelgrass populations across the entire region will increase.

Cover Photo: Eelgrass beds (Photo: NOAA, public domain)

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