It all starts with the wind: The importance of upwelling

By Dawn Barlow, PhD student, OSU Department of Fisheries and Wildlife, Geospatial Ecology of Marine Megafauna Lab

The focus of my PhD research is on the ecology and distribution of blue whales in New Zealand. However, it has been a long time since I’ve seen a blue whale, and much of my time recently has been spent thinking about wind. What does wind matter to a blue whale? It actually matters a whole lot, because the wind drives an important biological process in many coastal oceans called upwelling. Wind blowing along shore, paired with the rotation of the earth, leads to a net movement of surface waters offshore (Fig. 1). As the surface water is pushed away, it is replaced by cold, nutrient-rich water from much deeper. When those nutrients become exposed to sunlight, they provide sustenance for the little planktonic lifeforms in the ocean, which in turn provide food for much larger predators including marine mammals such as blue whales. This “wind-to-whales” trophic pathway was coined by Croll et al. (2005), who demonstrated that off the West Coast of the United States, aggregations of whales could be expected downstream of upwelling centers, in concert with high productivity and abundant krill prey.

Figure 1. Graphic of the upwelling process, illustrating that when the wind blows along shore, surface waters are replaced by deeper water that is cold and nutrient rich. Source: NOAA
Figure 2. Map of New Zealand, with the South Taranaki Bight region (STB) denoted by the black box.

Much of what is understood today about upwelling comes from decades of research on the California Current ecosystem off the West Coast of the United States. Yet, the focus of my research is on an upwelling system on the other side of the world, in the South Taranaki Bight region (STB) of New Zealand (Fig. 2). In the case of the STB, westerly winds over Kahurangi Shoals lead to decreased sea level nearshore, forcing cold, nutrient rich waters to rise to the surface. The wind, along with the persistence of the Westland Current, then pushes a cold and productive plume of upwelled waters around Cape Farewell and into the STB (Fig. 3; Shirtcliffe et al. 1990).

Figure 3. Satellite image of the cold water plume in the South Taranaki Bight, indicative of upwelling. The origin of the upwelling at Kahurangi Shoals, Cape Farewell, and the typical path of the upwelling plume are denoted.

Through research conducted by the GEMM Lab over the years, we have demonstrated that blue whales utilize the STB region for foraging (Torres 2013, Barlow et al. 2018). Recent research on the oceanography of the STB region has further illuminated the mechanisms of this upwelling system, including the path and persistence of the upwelling plume in the STB across years and seasons (Chiswell et al. 2017, Stevens et al. 2019). However, the wind-to-whales pathway has not yet been described for this part of the world, and that is where the next section of my PhD research comes in. The whole system does not respond instantaneously to wind; the pathway from wind to whales takes time. But how much time is required for each step? How long after a strong wind event can we expect aggregations of feeding blue whales? These are some of the questions I am trying to tackle. For example, we hypothesize that some of the mechanisms and their respective lag times can be sketched out as follows:

Figure 4. The wind-to-whales trophic pathway, and hypothesized lags between steps.

All of these questions involve integrating oceanography, satellite imagery, wind data, and lag times, leading me to delve into many different analytical approaches including time series analysis and predictive modeling. If we are able to understand the lag times along this series of events leading to blue whale feeding opportunities, then we may be able to forecast blue whale occurrence in the STB based on the current wind and upwelling conditions. Forecasting with some amount of lead time could be a very powerful management tool, allowing for protection measures that are dynamic in space and time and therefore more effective in conserving this blue whale population and balancing human impacts.

Figure 5. A blue whale lunges on a patch of krill. The end of the wind-to-whales pathway. Drone piloted by Todd Chandler.

References:

Barlow DR, Torres LG, Hodge KB, Steel D, Baker CS, Chandler TE, Bott N, Constantine R, Double MC, Gill P, Glasgow D, Hamner RM, Lilley C, Ogle M, Olson PA, Peters C, Stockin KA, Tessaglia-hymes CT, Klinck H (2018) Documentation of a New Zealand blue whale population based on multiple lines of evidence. Endanger Species Res 36:27–40.

Chiswell SM, Zeldis JR, Hadfield MG, Pinkerton MH (2017) Wind-driven upwelling and surface chlorophyll blooms in Greater Cook Strait. New Zeal J Mar Freshw Res.

Croll DA, Marinovic B, Benson S, Chavez FP, Black N, Ternullo R, Tershy BR (2005) From wind to whales: Trophic links in a coastal upwelling system. Mar Ecol Prog Ser 289:117–130.

Shirtcliffe TGL, Moore MI, Cole AG, Viner AB, Baldwin R, Chapman B (1990) Dynamics of the Cape Farewell upwelling plume, New Zealand. New Zeal J Mar Freshw Res 24:555–568.

Stevens CL, O’Callaghan JM, Chiswell SM, Hadfield MG (2019) Physical oceanography of New Zealand/Aotearoa shelf seas–a review. New Zeal J Mar Freshw Res.

Torres LG (2013) Evidence for an unrecognised blue whale foraging ground in New Zealand. New Zeal J Mar Freshw Res 47:235–248.

Blown out.

By Dr. Leigh Torres, Assistant Professor, Oregon State University, Marine Mammal Institute, Geospatial Ecology of Marine Megafauna Lab

Hurry up and wait. Can’t control the weather. All set and nowhere to go.

However you want to say it, despite our best efforts to be ready to sail today, the weather has not agreed with our best-laid plans. It’s blowing 20-30 knots in the South Taranaki Bight, which makes it very difficult to spot a whale from our small (but sturdy) research vessel (NIWA’s R/V Ikatere), and practically impossible to take good photos of the whales or to deploy our hydrophones. So, we wait.

Over the last few days we have been busy tracking down gear, assembling the hydrophones, discussing project logistics, preparing equipment (Fig. 1), provisioning the vessel, getting the crew in place, and practicing vessel operations. We have flown to the other side of the world. We have prepared. We are ready. And we wait. Such is field work. I know this. I’ve been through this many times. But it is always hard to take when you feel the clock ticking on your timeline, the funds flowing from your budget, and your people waiting for action. Fortunately, I have built in contingency time so we will still accomplish our goals. We just have to wait a bit longer. As the Kiwis say, ‘Bugger!’

kristin and hydrophones small
Figure 1. Kristin Brooke Hodge of The Bioacoustics Research Program at Cornell University performs a global sound check on the hydrophones (loud bang of hammer to pipe) so that times can all be synced and any clock drift accounted for.

Below is a wind and rain forecast for New Zealand (provide by the MetService). The box in red is our study region of the South Taranaki Bight. We are currently in Wellington where the green star is, but we want to be in Pohara where the yellow star is – this will be our base during the field project, if we can just get there.

NZ wind

Wind strength and direction in these types of maps is depicted by the wind indicator lines: the wind is coming from the tail toward the flag end of the symbol, and the strength is symbolized by the number and size of the barbs on the flag end.

wind barbs

Notice how inside the red box there are lots of barbs on the indicator lines (most saying about 20 knots), but just to the west and north there are few barbs – about 5 to 10 knots. These are great survey conditions, but not where we want to be! A bit heartbreaking. But that’s how it goes, and I know we will get our weather window soon. Until then, we sit tight and watch the wind blow through the pohutukawas and cabbage trees in beautiful Wellington.