Pajusalu, L.; Martin, G.; Möller-Raid, T., and Paalme, T., 2024. Natural fluctuation of pH in shallow-water macrophyte habitats in the brackish Baltic Sea. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 851-855. Charlotte (North Carolina), ISSN 0749-0208.

The pH declines caused by increasing atmospheric CO₂ as defined term “ocean acidification”, are more predictable in open ocean surface waters than in coastal seas. The pH of coastal waters is inherently more variable due to the effects of different factors, for example, temperature, biological uptake and respiration, pollution, and terrestrial run-off. Benthic macrophytes are important structural components in coastal ecosystems, playing crucial roles as primary producers and habitat formers. In this coastal ecosystem, the daily pH fluctuation is also strongly affected by algal photosynthesis (increasing pH) and respiration (lowering pH). In this study, we investigated the diurnal fluctuations in pH in shallow-water macroalgal habitats in July and August 2023 in the Estonian coastal waters, NE Baltic Sea. The study sites were chosen to represent different compositions of macrophyte species and environmental conditions. Measurements were carried out at each site for a full 24-hour cycle at a 5-meter depth during the active growth period. In addition, water temperature, and photosynthetically active radiation (PAR) were measured continuously at each site. Biomass samples were collected by scuba divers from surrounding macrophyte communities. Our results showed that the pH values at different study sites exhibited large daily variations as well as large variations during the study period. Within the study period, the pH fluctuations in different sites exceeded 1 unit, which is higher than pH changes owing to ocean acidification predicted for surface ocean waters by 2100. Our results suggested that besides local environmental conditions, the magnitude of pH changes in shallow coastal waters depends on the carbon use strategies of macrophytes as well as the community biomass. Overall, this natural fluctuation in pH in shallow coastal waters is important to incorporate into future climate change prediction scenarios.