Ocean plant life dropping

PhD student Daniel Boyce part of team studying why phytoplankton has declined by 40% in past century

The plant life of the oceans, called phytoplankton, is disappearing, according to Dalhousie University researchers who announced their results this week.

The three-year research project was conducted by PhD student Daniel Boyce and professors Boris Worm and Marlon Lewis. They concluded that the phytoplankton population has dropped globally by about 40 per cent since the l9th century.

The implications for the rest of the marine ecosystem could be disastrous but many questions must be answered to fully understand the causes and consequences of this trend, Boyce said.

Q: How did this research project come about?

A: It’s actually part of a larger project, as part of my PhD thesis, which is involved in investigating the effects of large predatory fish declines in the ocean. The ocean is a very highly connected system, so we reasoned that the effects of large predatory fish declines might be cascading down all the way to phytoplankton. We estimated these long-term trends in order to try and link them to what else is happening in the oceans.

Q: How did you measure the amount of phytoplankton in the water?

A: (We did it in two ways.) We went back and analyzed historical marine data. These data were collected by marine scientists who were going out on research vessels over the course of the 20th century.

But the majority of the data we collected are measurements of ocean water clarity, which is measured with a device called a Secchi disc. It’s basically a white dinner plate with a rope attached to it and the person collecting data lowers this disc into the water until he or she can no longer see the disc in the water. That depth is recorded as the water clarity or the Secchi depth. And it turns out the clarity depth correlates very well with the phytoplankton in the water, so it’s a very good indicator of the amount of algae in the upper oceans. . . . It’s very low-tech but it’s amazing. The Secchi-disk measures are very closely comparable to the satellite data.

We then combined that Secchi disk data with more current data. By doing this, we were able to build a very large database, about a half-million measurements, which extend back over the entire century and distributed throughout the oceans.

Q: What are the possible implications of the phytoplankton decline?

A: It’s difficult to answer. I don’t think anyone’s really looked at the exact consequences of a global-scale phytoplankton decline over a 100-year period. We know that phytoplankton are extremely important to the ecosystem. They perform the same function as plants on land perform. They’re the base of the food chain — they produce energy from the sun basically. A change in their abundance might have implications for the stability of our climate, the stability of our fisheries and the overall health of our oceans. We don’t know what the exact consequences will be but there will be consequences.

Q: So from the consequences, let’s turn to the cause. Is it possible this could be part of a natural cycle or could it be linked environmental changes such as global warming?

A: In our study, we examined three possible drivers of this long-term trend. Of the three that we examined, we found that rising ocean temperatures over the past century are the strongest single predictor of these declines. Phytoplankton are basically like plants, they need sunlight and nutrients to grow. And the nutrients are delivered to them from deeper ocean waters by mixing. . . . As ocean temperatures rise, the oceans become more stable or stratified. And that limits mixing, and therefore (limits) the amount of nutrients that can be delivered to phytoplankton.

As for the natural cycles, I don’t think anyone knows. This is the longest phytoplankton marker that we have right now. So in order to determine (if) this is a natural cycle, we’d have to have two or three centuries worth of data instead of just one.

Q: What might be the next step in studying this trend?

A: Firstly, we need to know how phytoplankton will change in the future. We estimated these trends over the past century but we can’t really say with any certainty what will happen in the future, like next year or 10 years down the road. So I think it’s really important given the implications of the phytoplankton decline that we continue to monitor the levels of global phytoplankton (using satellites) and see where this trend will take us into the future. . . .

There may be other things that are driving this decline that we haven’t looked at yet or haven’t considered. So I think it’s important to continue investigating what might be driving this trend so we can better halt them or better manage our global phytoplankton levels.

The ChronicleHerald.ca : Ocean plant life dropping

Image: Daniel Boyce, marine ecology student at Dalhousie: “In order to determine (if) this is a natural cycle, we’d have to have two or three centuries worth of data instead of just one.” (Eric Wynne / Staff)