A recent CBC article on rising sea surface temperatures in the Gulf of St. Lawrence has prompted me to share a story. It was 1985 and I was fresh out of my undergrad en route to an MSc at the National Research Council in Halifax with Jack McLachlan (father of the very talented musician Sarah McLachlan, but that’s another story). As luck would have it there was a major project surveying seaweed around Prince Edward Island that summer focused on the shifting balance between the economically preferred Chondrus crispus Stackhouse and the introduced Furcellaria lumbricalis (Hudson) Lamouroux. It was a dream job for a budding scientist with a passion for scuba.
I logged ~50 dives; most were routine underwater transects between 2-10 m depth in waters approaching 20o C. However, there were a few dives that stood out and received special attention in my dive log. An aside; dive logs are records of every dive. How deep, how long, dive buddy, air used, water temperature, noteworthy observations … you get the idea. Back to those few special dives that received extra comments in my dive log.
At about 15 m depth there was a perceptible layering of the water column – my first encounter with a thermocline! While basking in the 20o C water, you could stick your hand through and feel the colder water below. You shiver as your body slips into the cooler water, but enjoy your time knowing that a few meters above warmer water awaits your return. As a marine biologist there is the added thrill of a clear shift in biodiversity. We were just kids and the best we could do when we hit the surface was ‘did you see that big red blade…?’. We didn’t know the flora well and I certainly did not aspire to be a taxonomist at that time (I did not know what taxonomy was until stumbling into the discipline as I progressed through my MSc). However, there are those charismatic species that grab your eye and you just have to know what they are because you have never seen anything like it before. For some folks those species are whales and the like, but for me that species was Odonthalia dentata (Linnaeus) Lyngbye. You could not miss it nor mistake it for anything else and so it, like the thermocline, received mention in my dive log on these few occasions.
Fast forward three decades and I had a better grasp on taxonomy, as well as the NW Atlantic and Arctic floras. I developed a strong interest in postglacial recolonization of these regions and attracted a very talented (although lacking the vocal range of Sarah) PhD student, Trevor Bringloe, to work on this project. We collected at numerous sites from Bergen, Norway to Nome, Alaska, and from Birchy Head, Nova Scotia to Qikiqtarjuaq, Nunavut. We were targeting (sub)arctic seaweed species, which it turns out includes Odonthalia dentata. In our quest for southerly populations, I recalled my adventures in Prince Edward Island and dusted off my dive log from 1985. And so off we went in 2016 on another grand adventure – some 30 years after my summer there – to dive below the thermocline and collect these charismatic species.
Our first dive was at one of the locations carefully recorded in my dive log from 1985, which indicated the presence of a thermocline below which our treasures awaited. We did two dives, to 14.2 m and nearby to 14.4 m, but there was neither thermocline nor treasure in sight. The next day we tried another site and went to 15.9 m only to match the disappointment of the previous day. After repeated attempts, and in some desperation, we descended to 22.4 m on the final dive of the trip. Even at that depth the water was remarkably a balmy 19o C, but there, about as low as seaweed will grow in these waters, we found remnants of the once luxuriant subarctic flora of Prince Edward Island. Clearly the water was still cold enough for at least part of the year at that depth for these species to persist.
The 2016 trip had a profound impact on me. Had the water temperatures really warmed that much during the three decades between 1985 and 2016? The data in my dive log are clear, the answer is yes and the CBC story mentioned above came as no surprise.
We should have realized early in the trip that we had embarked on a difficult task. After that first unsuccessful dive I asked our young boat attendant, a local diver himself, where we could dive to find the thermocline. He responded that ‘there is no thermocline on Prince Edward Island’ and looked at me like I was some silly old fool. Although the information on which he based his opinion of me was inconsistent with the data in my dive log, it was nonetheless a sound assessment. I would encourage my cherished colleagues, other silly old fools, to explore their dive logs as well. There are data in those sheets!
Seaweed of Canada 
Terrific Gaz, a great testament to keeping information-rich and accurate dive logs (and an indictment of those of us who largely don’t, apart from logging taxonomic entries of the names and habitats of what algae we found). Reminds me of our dives in midsummer (February) at Bicheno, Tasmania, where at what?, 18 meters or so?, it was 13 degrees and not below any thermocline I can remember. And yet our waters here over the time span that you’re talking about have managed to warm sufficiently to virtually eliminate our populations of the “giant” kelp Macrocystis, among lovely forests of which I enjoyed diving in the late 80s/early 90s. You must come back before the only macroalgae you’ll see are things you already know of from Lord Howe and the Great Barrier Reef!
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Like most, in hindsight I wish I had kept much better notes. How much information have we lost? My last few years diving in Australia (~2010-2013) we managed, among others, return trips to Lord Howe and Tassie (as you know from first hand participation in the madness:-). I do recall we were starting to collect species in common between the more tropical locations and the north coast of Tassie (these not collected during our earlier trips there). I should probably get into my collection database, as well as my dive log and bring the pieces together. I should also return for a fresh look. I have confined my activities to colder waters for a decade now:-)
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