Ocean Acidification

Ocean acidification (OA) and deoxygenation, driven by anthropogenic CO₂ emissions, threaten marine biodiversity by altering seawater chemistry and disrupting ecosystems. Foundation species like marine macrophytes may locally mitigate these stressors through metabolic activity, potentially improving the average condition or providing daytime refugia and influencing community structure. Yet few studies have quantified the influence of plant structural traits, like canopy cover on the buffering potential of marine macrophytes. We investigated how the canopy cover of surfgrass Phyllospadix spp., dominant macrophytes in northern California tidepools, relates to local seawater chemistry (pH and dissolved oxygen) and biodiversity. Through instantaneous sampling of 29 tidepools and continuous sampling in focal pools, we found non-linear relationships between surfgrass cover, pH, dissolved oxygen, and community diversity. Pools with intermediate surfgrass cover (~40%) had maximum overall species diversity and macrophyte diversity, and largest diel fluctuations in pH and dissolved oxygen, while the highest surfgrass abundances were associated with decreased diversity and dampened chemical variation. A controlled mesocosm experiment corroborated the field pattern: surfgrass in mesocosms caused chemical fluctuations comparable to those in natural tidepools with similar surfgrass abundance. These results suggest that surfgrass can influence ecosystem function through direct metabolic effects and can facilitate or inhibit community diversity depending on its abundance. Our findings underscore the complexity of foundation species’ roles in coastal ecosystems and highlight the need to account for non-linear responses when predicting refugia potential of marine macrophytes.

Tharaldson Peterson T. M. & Bourdeau P. E., in press. Surfgrass canopy cover drives non-linear changes in tidepool biodiversity and seawater chemistry. Marine Ecology Progress Series. Article (subscription required).