Benthic composition influences daily pH patterns in Florida coastal systems

Abstract

Seagrass meadows influence local pH dynamics through photosynthesis and respiration, yet less is understood about pH dynamics in neighboring sub-tropical hardbottom habitats. Characterizing fine-scale pH dynamics is important because organisms are subject to short-term fluctuations, which can influence physiological stress or resilience under changing conditions. This study compared diel water pH fluctuations in hardbottom and seagrass habitats within the St. Martins Marsh Aquatic Preserve. We established three paired replicates — with a hardbottom and seagrass site separated by at least 100m in each pair. Loggers collected data every 30 minutes for seven days to characterize the diel change in pH at each site (3 pairs x 2 habitats x 48 pH measurements x 7 days =2,016 data points). 

We quantified daily mean pH, diel pH amplitude, and variability in the daily mean pH to assess local conditions and short-term stability. Day-to-day variability in mean pH and pH amplitude are defined as the amount that mean pH or amplitude, respectively, changes between days. Linear mixed effects modeling was used to test whether pH response variables differed between habitats and across varying percent macrophyte cover. We found that mean pH and pH amplitude did not differ between habitat types. Variability in mean pH differed between habitats and was influenced by percent macrophyte cover, primarily macroalgae percent cover in hardbottom sites. These findings improve our understanding of small-scale, habitat-level pH dynamics and the stability of subtropical coastal carbonate systems.