Ocean acidification (OA) is often demonstrated to have negative effects on marine organisms, but less is known about whether marine organisms can mediate OA effects. I examined relationships between surfgrass (Phyllospadix spp.), a foundation species and tidepool biodiversity, and its ability to mediate fluctuations in pH and dissolved oxygen (OA; DO) which are stressors in tidepools. I surveyed tidepools in northern California, where I quantified biodiversity, pH, and DO, and related those variables to surfgrass abundance. Laboratory and field experiments manipulating CO2 and surfgrass presence were done to examine surfgrass effects on day/night pH and DO fluctuations in simulated and natural tidepools. Intermediate surfgrass abundance was associated with the greatest tidepool biodiversity in the field, suggesting amelioration of abiotic conditions up to intermediate abundances, but exacerbated OA and DO stress at higher abundances. In the lab, diel pH and DO fluctuations were highest in simulated tidepools that contained surfgrass compared to pools without surfgrass, indicating the role of surfgrass photosynthesis and respiration in modulating seawater chemistry. In the field, tidepool pH and DO were higher in the day and lower at night, consistent with results from the laboratory experiment. Interestingly, day/night fluctuations in pH were highest in tidepools with intermediate rather than high surfgrass abundance, suggesting the intriguing possibility that surfgrass modulates tidepool pH both directly via metabolic activity but also indirectly by facilitating macrophyte diversity at intermediate abundances. Taken together, these results suggest that surfgrass may act as a foundation species in tidepools, by mediating tidepool pH and influencing species diversity, which has important implications for the fate of these communities in the face of rapidly-changing global climates.
Tharaldson T. M., 2018. The ability of Phyllospadix spp., a pair of intertidal foundation species, to maintain biodiversity and ameliorate CO2 stress in rocky shore tidepools. MSc thesis, Humboldt State University, 104 p. Thesis.