Andy Pershing: How is Gulf of Maine Connected to North Atlantic?

Andy Pershing speaks about the way the Gulf of Maine is connected to the waters upstream in the North Atlantic/Arctic (which is a hotspot for climate change).

Temperature gets the most attention in climate change. The global average sea level is increasing since the beginning of the last century. As you warm seawater, it expands. That's most of it. The loss of ice packs in Greenland and Antarctica over the very most recent term have also had an impact. Since 1970/1980 we've seen a major change in the amount of snow we receive in the northern hemisphere. We still get precipitation, just not as much snow.

As we warm up, in the tropics, we will be evaporating more water in the tropics. Water vapor moving up from the water into the atmosphere has two impacts: the remaining sea water will have increased salinity. And the water vapor that evaporates has to go somewhere: we expect to see increased precipitation at the higher latitudes -- and that, in turn will make the seawater fresher.

Climate change by anecdote: we see freshening (less salinity) in the 1990s at higher latitudes. We don't really have a long enough record of data (only back to the 60s) to make great conclusions, but we can start to see patterns and form conclusions.

We are definitely seeing connections between changes in the North Atlantic to changes in the Gulf of Maine. There's about a two year time lag between salinity anomalies in the Arctic and the corresponding changes in the Gulf of Maine -- noticing this tells us something about the flow and may help us make predictions.

Salinity patterns and freshening impacts the ecosystem: phytoplankton are eaten by zooplankton, which are eaten by herring, which are eaten by right whales. Q: what's the impact of salinity on the ecosystem? The scope of the changes are about 1 part per 1000: you can't taste the difference. But salinity (and temperature) has a big impact on the density of seawater. Having slightly fresher water means we have light water sitting on top of dense water: that impacts the mixing of the water. It also keeps the phytoplankton higher up in the water column, where there's light. Fresher water = phytoplankton booms, until early 2000s when the salinity goes back to normal. When there's more phytoplankton there's an increase in their predators (copepods) and then a lot more herring.

But: Calanus -- the "big copepod" (size of a grain of rice) and a big link between phytoplankton and fish does NOT increase during the freshening in the 1990s. In fact it decreases. Why? Probably because the juvenile and adult herring are eating them up. And right whales eat Calanus. Right whale reproduction gets much more variable: boom and bust periods following the low Calanus spell.

We don't have enough data to know whether gradual salinity changes due to climate change will have the same kind of impact that a "slug" of freshwater like that in the 1990s did. We need to keep collecting historical data while recognizing its limitations.

We learn about quahogs (clams) and their growth rings. Think of those on a tree. Quahog growth changed and corresponded to the calanus and copepod changes. And, luckily, we have fossil clams, so we can look at a much longer record. If we believe the correspondence of clam growth rings and the productivity of the larger Gulf of Maine ecosystem, we can start to see longer term patterns and build more dynamic models.

What we still need to know: why are the species connected?



Andy's slides are also available online.

Q: from Patten White: Is there a difference in the nature of water runoff and salinity as the climate increases? How is snowmelt runoff different from rain runoff, if the nature of the precipitation continues to change?

A: we don't really know, but the question is very interesting. Going from a period where you no longer have an intense "spring freshet" of melting snow, to a more constant precipitation, is likely to have a

Q: from James Wilson, University of Maine. The 1990s salinity change and copepod boom wasn't connected to the productivity of the inshore fishery? Why?

A: That's a good question. What happened to the inshore herring in the late 1990s? We don't know. Are they different populations with different feeding patterns, or are the herring going where the food is and the food isn't near the coast? We need to learn more.

Q: from Priscilla Brooks, Conservation Law Foundation: there was a haddock boom in the 1990s -- how is that related?

A: We think there's a relationship between the copepod boom and haddock recruitment. It's not as strong as the relationship between copepods and herring, but we think it's there.

Q: What about cod?
A: There's a Canadian/Maine debate. Is it top down (no cod in Canada, therefore an impact on the lower feeding)? Or is it bottom up (health of the lower food chain critters impacts health of cod)? That's the $10,000 question, and there are a lot of factors. (let's supplement these notes with others in the room -- feel free to add a comment).

Q: is there any corollary with red tide events?
A: We don't know -- haven't studied that.

(Data comes from a continuous phytoplankton measurer, towed monthly beneath a ferry from Boston, MA to Yarmouth, Nova Scotia.)