Coastal carbonate chemistry dynamics associated with macrophyte systems

In the coastal zone extreme variability in carbonate chemistry dynamics is driven by fluctuations in temperature, salinity, air-sea gas exchange, mixing processes, and biology. This variability could be particularly magnified in critical coastal ecosystems such as kelp beds where photosynthesis and respiration consume and release significant amounts of dissolved inorganic carbon. This variability poses a challenge for scientists assessing the impacts of climate change for nearshore ecosystems, and complicates assessment of long-term pH reduction in the presence of variability. We carried out deployments of physical & biogeochemical sensors in order to observe these processes in situ. The “SeapHOx” instruments used in this study consist of a modified Honeywell Durafet® ISFET pH sensor, an Aanderra Optode Oxygen sensor, and a SBE-37 conductivity, temperature, pressure sensor, integrated into a single package with internal power and a central microcontroller. The instruments were deployed on subsurface moorings located inside the southern portion of the La Jolla Kelp Forest and 1 km seaward at a depth of 5m. Combining pH sensor data with a conservative relationship between total alkalinity, salinity and temperature, other parameters of the carbonate system were calculated. Our analysis goals were to (a) characterize high-frequency time scales of carbonate chemistry variability (b) evaluate the amplitude and duration of variation, and (c) predict the occurrence of extreme (physiologically-significant) low pH events. Another analysis goal was to identify the corresponding drivers of the observed changes. Results thus far reveal a mean daily pH range of 0.27 units with a maximum range of 0.4 pH units of which temperature accounts for less than 30% of the variation and the rest must be attributed to changes in the chemical component. The greatest frequency of pH variability occurs twice per day corresponding to the semi-diurnal tidal cycle. Opposite to our predictions, daily ranges in pH were on average less inside the kelp forest than offshore. We hypothesize that the kelp forest is slowing currents and thus buffering the kelp forest from changes in pH associated with advection of water masses, upwelling, or shoaling of the thermocline associated with low DO and low pH. The observed variability of pH on daily time scales both inside the kelp forest and offshore is greater than open ocean predictions for global mean pH reduction by the year 2100. These results suggest that organisms on exposed coasts may be adapted to a range of pH conditions and highlight the need for scientists to consider biological response to varying scales of pH change in order to develop more realistic predictions of the impacts of climate change for the coastal zone.