Beyond the current crab closures that now extend from California to Washington State, another big question on everyone's mind is whether climate variability that is directly tied to greenhouse gas emissions is influencing the increase in toxic algal blooms occurring all over the United States. We are seeing an expansion in the ranges of harmful algal bloom species (from cyanobacteria to "golden algae"), an increase in the frequency of their occurrence in inland lakes and coastal zones, and most troubling, increases in the amount of toxin they are capable of producing. What does this say for the future, and is this connected to cyclic variability or will we continue to see this trend climb in an upward direction? Without very long time series of observations (e.g. 30-50 years), it becomes a yeoman's task to answer that question with necessary scientific rigor. However, researchers have been tackling this head-on with laboratory experiments, field measurements, and reconstructions of planktonic records from sediment cores or other archival sources. As we have come to expect, the results are nuanced and do not tell a single, straightforward story. With all biological studies on climate change, there are winners and there are losers, and there are some species that appear to be impervious to broad environmental fluctuation.
One lesson learned over the last couple decades as we observe a steady rise in atmospheric CO2 concentration and the concomitant changes to ecosystems worldwide is the often, nonlinear effect these changes have on ecological dynamics. A nonlinear effect is one that deviates from our linear predictions in response to a step change in CO2, temperature, extreme weather, etc. Particularly this sort of scenario played out in the summer of 2015 along the entire West Coast of North America. That massive domoic acid event is now being referred to as a "dress rehearsal" for climate change (sensu Nick Bond @ JSIAO, Univ of Washington) because of the unexpected interaction of record high temperatures in the Pacific Ocean and the Pseudo-nitzschia bloom that has become a seasonal occurrence on the West Coast of N. America (since ~1998). While it is known that most all phytoplankton grow well in response to increases in temperature, they do have optimal ranges and some are found to occur in particular temperature regimes more than others. Pseudo-nitzschia species here on the West Coast prefer to bloom in the weeks following an "upwelling" event when northwesterly winds are strong enough to support the offshore movement of water (a phenomenon called Ekman transport). The void of water left at the coast must be filled, which leads to "pumping" of cold, nutrient-rich water from much deeper layers (usually ~200-400 m or ~650-1300 ft). Pseudo-nitzschia is a type of diatom, a group of phytoplankton that grows better than others when there is a lot of vertical shear in the water column, i.e. high mixing velocities. That means they grow quickly during upwelling events along with other diatoms, but as the waters warm in the aftermath of such an event and those nutrient-rich waters are depleted by phytoplankton at the surface, Pseudo-nitzschia finds a window of opportunity to grow even more vigorously under lower nutrient conditions (i.e. low to moderate levels of silicate, nitrate, phosphate) and warmer waters relative to a week or two prior (i.e. ~11-15 C or 54-60 F). What happened in 2015 began in this manner, with a Pseudo-nitzschia bloom starting in the usual hotspots, but then it kept growing and becoming more toxic, moving up and down in the water column with each pulse of upwelling for five months to come, defying any expectations. Average surface temperatures in places like Monterey Bay got as high as 20 C but Pseudo-nitzschia was happiest at around 15 C. Not only that, but the most toxic species we know of was blooming vigorously during these windows, leading to some of the highest DA levels we have ever measured. Was this a particular strain of P. australis that prefers slightly warmer waters? Genetic variability is a large driver behind this kind of biological and ecological diversity (see work by Holly Bowers at MBARI). This begs another important question for another post: how can we prepare for blooms of existing and emergent strains that we currently know very little about?
So, was it the anomalously warm water that caused the bloom this year? Indirectly, yes. Beyond the warmer temperatures, what happens below the surface is also very important. The warm "blob" has penetrated well into deeper waters (i.e. the subsurface). This directly affects how warm the upper and lower layers are above and below the "thermocline," that ever-important line of physical separation where a steep gradient in temperature occurs. The warmer and deeper that line is, the lower the nutrients are that make it to the surface during upwelling pulses. Perfect conditions for Pseudo-nitzschia! It appears as though the warm blob was sloshing up against the coast and creating these conditions, thereby setting up a scenario where repeated strong pulses of upwelling were not strong enough to bring the very cold, very enriched water to the phytoplankton at the surface, but rather brought up moderate levels of nutrients. That set the stage for Pseudo-nitzschia to hold onto its dominance relative to other phytoplankton (and other diatoms). Even though in the aftermath of periodic upwelling pulses, when we would expect a bloom to eventually be blown offshore, data suggest that Pseudo-nitzschia moved down below the subsurface (in some hotspots), continuing to produce toxin and awaiting the next blast of winds to inject another custom nutrient cocktail.