Nutrient loading and its effects on the health of the Salish Sea was a topic covered from a variety of perspectives at the recent Salish Sea Ecosystem Conference in Vancouver, BC. The most common threat from elevated nutrients is hypoxia, or dangerously low levels of dissolved oxygen as is found at times in Hood Canal. But another threat is emerging and was discussed at the conference. Excess nitrogen can add to the decrease in Salish Sea pH we are experiencing from climate change and which is threatening the shellfish industry.
Excess nitrogen in the marine environment fuels algae growth. When the algae dies and is decomposed by microbial action, CO2 is produced–the more algae, the more CO2. This CO2 from microbial respiration has the same effect as atmospheric CO2–it lowers pH. Thus CO2 in marine waters has two sources–the atmosphere and microbial respiration. And, as anthropogenic sources of nutrients increase algae growth, acidification and all its damaging consequences are accelerating.
What is most alarming is that the combination of the two sources of CO2 (atmosphere and respiration) seems to have more than an additive effect–there is some sort of synergy that lowers pH beyond what would be expected from the sum of the predictions for each of the two sources when modeled alone. In the context of the relatively limited water circulation found in the Salish Sea and especially in South Sound where nutrients tend to linger for long periods of time, this emerging science increases the urgency to control or treat all sources of excess nutrients to help compensate for the effects of climate change.