Ocean Acidification Measurements in San Juan Channel and Suggestions for Mitigation of Causes

By Constance Sullivan, Master's Candidate

Ocean acidification refers to the decline in seawater pH over time due to anthropogenic influences. The primary cause of this condition is the addition of CO2 to the atmosphere from combustion of fossil fuels and other human activities. In coastal areas, other factors can add to the ocean acidification signal, for example through intrusion of upwelled waters or addition of acidic industrial waste products to atmosphere or ocean. The problem of ocean acidification has received substantial attention from the scientific community over the past several years, and our understanding of the problem and its consequences is growing. Despite this, relatively little is known of the acidification condition of coastal waters and their variability over time. To begin to address this need in the San Juan Archipelago, we conducted a study of carbonate chemistry and ocean acidification conditions in the San Juan Channel.

During fall 2011, we collected seawater samples on a weekly basis using FHL's R/V Centennial. Samples were collected from four depths at each of two stations over seven weeks. The North Station was located to the west of Yellow Island. Its surface waters are seasonally influenced by the Fraser River in British Columbia, Canada. The South Station was located south of Cattle Pass at the entrance to San Juan Channel, and is influenced by the upwelling regime on the outer coast. The stations are 21 km apart. Water samples were collected to measure the carbonate chemistry and dissolved oxygen, and CTD casts were made to measure salinity and temperature.

The new ocean acidification laboratory at FHL was utilized for analysis of the samples. In each sample, we measured two carbonate system variables (total alkalinity and dissolved inorganic carbon) and from these calculated two other carbonate system variables (pH and partial pressure of CO2, or pCO2). Together, these variables indicate the condition of the water with respect to ocean acidification. In addition, we calculated the saturation state of aragonite in the seawater samples. This is a measure of the ease with which calcium carbonate shells and skeletons can be precipitated or dissolved.

Seawater pH ranged from 7.63 – 7.80. These values are substantially lower than typical open ocean values of ~8.1 (remember that pH, like earthquakes, is measured on a logarithmic scale). The lowest values of pH were observed at the South Station. pCO2 (a measure of the amount of CO2 dissolved in seawater) values were relatively high, ranging from 708 – 1113, with the highest values observed at the South Station. The saturation state of aragonite never exceeded a value of 1, indicating conditions favoring dissolution of calcium carbonate structures that utilize aragonite. Additional calculations revealed that that factors other than the dissolution of atmospheric CO2 contribute to low pH in this region.

Natural sources of low pH in the San Juan Archipelago include discharge from the Fraser and Skagit Rivers, which are naturally low in pH, and intrusion of upwelled water from the outer coast. Anthropogenic sources of low pH include inputs from watersheds surrounding the Salish Sea and discharge from industrial sources such as pulp and paper mills and mines. These industrial sources contain nitrogen and sulfur oxides, which can contribute to declining pH.

The results of our work indicate that local waters are low in pH, consistent with the finding of Feely et al. (2008, 2010), and that pH and other carbonate system variables can vary on short temporal and spatial scales. These findings can help guide additional research into ocean acidification conditions in the Salish Sea, ultimately allowing investigators to determine whether local mitigation strategies can help alleviate ocean acidification in local environments.

The research was conducted as part of Sullivan's Masters degree at the UW School of Marine and Environmental Affairs. Sampling utilized the Fall Quarter cruise sequence of the FHL Research Apprenticeship Program for the Pelagic Ecosystem Function group. The water sampling at the two stations is being continued, leveraging cruises and funding from several partners, including the UW College of the Environment, Western Washington University, Northwest Indian College, and the NSF Center for Marine Observation and Prediction. Sullivan worked under Terrie Klinger, Associate Professor, School of Marine and Environmental Affairs and Jan Newton, Principal Oceanographer, Applied Physics Laboratory.