How warmer waters affect the relationship between seagrass and oysters

Greg Howard
14th December, 2025

How warmer waters affect the relationship between seagrass and oysters

Diagrammatic representation from study of the interactions between oysters and seagrass.

Image adapted from: Schafer et al. / CC BY (Source)

Key Findings

  • This study, conducted in Narragansett Bay, Rhode Island, examined how warmer water temperatures affect eelgrass and the role oysters play in this ecosystem
  • Warmer water significantly reduced eelgrass growth and the amount of phytoplankton, demonstrating eelgrass vulnerability to rising temperatures
  • Oysters did not directly improve eelgrass growth but increased macroalgae cover, potentially creating localized shading and slightly buffering water temperatures
Shallow coastal ecosystems, including seagrass beds and oyster reefs, are vital nurseries and support a wide range of marine life. However, these habitats face increasing threats from climate change, particularly rising water temperatures. Understanding how these ecosystems respond to warming, and how different components within them interact, is crucial for effective conservation and restoration efforts. A recent study conducted by researchers at the U.S. Environmental Protection Agency and University of Sydney[1] investigated the combined effects of warming temperatures and the presence of oysters on eelgrass, a type of seagrass, and other key indicators of ecosystem health. The research employed a controlled experiment using mesocosms – essentially large outdoor tanks simulating a coastal environment. Scientists manipulated water temperature (ambient or warmed by 2°C) and the presence or absence of oysters (Crassostrea virginica) to observe the resulting effects on eelgrass (Zostera marina), macroalgae (seaweeds), chlorophyll a concentration (a measure of phytoplankton abundance), and overall water quality. The experiment ran for nine weeks. The results showed a significant negative impact of elevated water temperature on eelgrass biomass and chlorophyll a concentration. In other words, warmer water directly reduced the amount of eelgrass and the abundance of phytoplankton. Surprisingly, the presence of oysters did not have a short-term effect on eelgrass growth, chlorophyll a levels, or key nutrient concentrations. However, the study revealed more subtle, yet important, interactions. Detailed monitoring of water temperature and light levels revealed that oysters appeared to influence their surrounding environment. In the warmed mesocosms, oysters were associated with slightly lower water temperatures, and both warmed and ambient mesocosms with oysters exhibited increased light transmittance – meaning more light penetrated the water column. This suggests that oysters may offer some localized buffering against warming, possibly through shading. Another key finding was the increased abundance of macroalgae in mesocosms containing oysters, regardless of temperature. Macroalgae coverage was greater in oyster-containing tanks compared to those with eelgrass alone. This suggests a potential competitive dynamic between macroalgae and eelgrass, with oysters potentially favouring macroalgae growth. These findings build upon earlier research highlighting the vulnerability of seagrass ecosystems to climate change[2]. The Mediterranean Sea, for example, is experiencing accelerated seagrass regression, with Posidonia oceanica predicted to lose up to 75% of its suitable habitat by 2050. This loss is concerning not only for biodiversity but also because seagrasses store significant amounts of carbon, and their decline could release this carbon into the atmosphere. The current study doesn't directly address carbon storage, but it underscores the importance of understanding factors that influence seagrass health in a warming climate. Furthermore, the study’s focus on interactive effects is particularly relevant in the context of ecosystem restoration. Restoration efforts often focus on single species, such as oysters, to improve water quality and habitat structure[3]. While oyster restoration can be effective in increasing biodiversity and abundance, the study suggests that a more holistic approach, considering the interplay between multiple species like seagrass and macroalgae, may be necessary to maximize restoration success. The observed 35% shortfall in recovery identified in global oyster restoration projects[3] could be partially attributable to neglecting these complex interactions. The Tampa Bay Estuary Program provides a successful example of multi-entity watershed management that has led to seagrass recovery through nutrient load reduction[4]. The study’s findings complement this work by highlighting how biotic factors, like oyster presence, can also influence ecosystem responses to environmental changes. The increased macroalgae observed in the presence of oysters could, for instance, impact the success of seagrass restoration efforts if it outcompetes newly planted eelgrass. The research from the U.S. Environmental Protection Agency and University of Sydney emphasizes that the response of coastal ecosystems to warming is not simply a matter of temperature tolerance. The interactions between different species, such as oysters, eelgrass, and macroalgae, can play a significant role in mitigating or exacerbating the effects of climate change. This highlights the need for considering the broader ecological context when developing strategies for coastal ecosystem conservation and restoration.

EnvironmentEcologyMarine Biology

References

Main Study

1) Seagrass and oyster interactions under a warming climate scenario: A mesocosm experiment

Published 11th December, 2025

https://doi.org/10.1371/journal.pone.0337843

Related Studies

2) Dramatic loss of seagrass habitat under projected climate change in the Mediterranean Sea.

https://doi.org/10.1111/gcb.14401

3) Oyster reef restoration fails to recoup global historic ecosystem losses despite substantial biodiversity gain.

https://doi.org/10.1126/sciadv.abp8747

4) Toward reversal of eutrophic conditions in a subtropical estuary: water quality and seagrass response to nitrogen loading reductions in Tampa Bay, Florida, USA.

Journal: Environmental management, Issue: Vol 38, Issue 2, Aug 2006


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