67. Quantifying the Carbonate Chemistry and pH Fluctuations of Otago Harbour, New Zealand

Katie S Nelson (1)*, Kim Currie (2), Miles D Lamare (1), Judith Murdoch (3)

1 University of Otago Department of Marine Science, Dunedin, Otago, 9016, New Zealand

2 NIWA, Dunedin, Otago, 9016, New Zealand

3University of Otago Department of Chemistry, Dunedin, Otago, 9016, New Zealand


In order to understand the impacts of Ocean Acidification on biological and ecological processes and design meaningful ex situ experimental parameters, it is necessary to quantify variability in the pH and carbonate chemistry of specific environments such as the inter-tidal zone. High resolution and continuous pH and carbonate chemistry datasets are needed to describe the variability in these complex habitats that undergo diurnal and seasonal shifts in both biotic and abiotic factors. To understand the full effect of ocean acidification it is important to answer the question: What is the current pH range and carbonate chemistry patterns experienced in situ in the inter-tidal and sub-tidal zones that is biologically relevant?


A continuous pH data set was collected over a 42 day period from May – June 2015 with a Satlantic SeaFET and compared to an ongoing light (PAR) and tidal (m) dataset collected at Portobello, Dunedin. Weekly inter-tidal and sub-tidal water samples were collected from March – October 2015 along with temperature, salinity, DO and chlorophyll concentration data. Dissolved inorganic carbon (DIC) and alkalinity (AT) were processed post collection and additional carbonate chemistry parameters were calculated using the program CO2calc.


Preliminary results are consistent with existing literature, suggesting that large natural variations in surface epipelagic “pH seascapes” are driven by local environmental conditions. The data show that Otago Harbour conditions appear to be driven by two main factors;

1. The primary frequency is driven by large light peaks followed by pH peaks caused by diurnal phytoplankton activity and,

2. The secondary frequency is driven by semi-diurnal patterns associated with tidal fluctuations bringing off-shore water in and out of the harbor.


Benthic invertebrate larvae settling in the inter-tidal or sub-tidal zones are exposed to a large range of pH conditions.