Shifts in planktonic community composition due to marine heatwaves (MHWs)

Allison Dawn, GEMM Lab Master’s student, OSU Department of Fisheries, Wildlife and Conservation Sciences, Geospatial Ecology of Marine Megafauna Lab

As the first year of my Master’s is coming to an end, I am excited to have completed the first milestone of writing my research proposal. During the formation of my initial hypotheses, I have been thinking deeply about the potential drivers of zooplankton variability, and how these metrics relate to the Pacific Coast Feeding Group (PCFG) of gray whales foraging in Port Orford. One topic that continues to appear in the literature and throughout my coursework is that of the extreme marine heat wave (MHW) event (2013-2016) in the Pacific Ocean, otherwise known as the “warm blob”. In Dawn’s (now Dr. Barlow!) blog about this MHW, she discusses how whale habitat in California was compressed due to shifts in prey availability, and how this led to an increased number of whale entanglements (Santora et al., 2020). While sea surface temperature (SST) is only one of many factors that influence prey metrics, it is nevertheless an important factor to consider, especially as these heat waves are expected to increase in intensity and duration due to climate change (Joh and Di Lorenzo, 2017). As Lisa mentioned in her last blog, the “warm blob” exacerbated the loss of kelp and sea stars, which is now impacting multiple trophic levels in Port Orford. For my first thesis chapter, I plan to dive into how SST anomalies impact the mosaic of interactions at our study site in Port Orford, and ultimately try to better understand food availability for the PCFG whales.

Cavole et al., 2016 is one of the early comprehensive studies to discuss the impact of the blob on a variety of planktonic marine species. Their sea surface temperature anomaly figure (Figure 1) shows where the anomaly began in 2013 and how it migrated from the Northern Pacific to the Southern Pacific coast.

Figure 1. Plots showing the SST anomalies as the “warm blob” migrated from the Northern Pacific to the Southern Pacific from 2013 until 2016.

Among many other impacts, this MHW caused a reduction in phytoplankton, the major food source for zooplankton. The decline of this food source subsequently caused significant changes in zooplankton populations. Specifically, studies on copepod diversity and biomass show that in a typical California Current System (CCS) there is a seasonal oscillation between warm-water with subtropical species and cold-water with subarctic species. In the winter, the CCS is characterized by a high diversity of subtropical species, due to a southern water source. In the spring, northern cold water advection brings low-diversity, subarctic copepods. While the timing of these shifts is subject to change due to changes in the Pacific Decadal Oscillation (PDO), it remains that these subtropical copepod species are known to be smaller and less nutritious than subarctic copepod species regardless of arrival time (Kintisch, 2015; Leising et al., 2015). However, in 2015, this shift to cold water copepod species did not occur, but rather coastal sampling along the Oregon coast saw subtropical copepod species prevail. Specifically, there were 17 main subtropical copepod species that dominated the species composition while the nutrient-rich arctic species were rare. This occurrence of major copepod shifts alone points to the overall concern for the ecosystem imbalance, to the detriment of top predators like marine mammals and seabirds (the “losers”), and others gaining advantage (the “winners”) (Figure 2).

Figure 2. Figure showing the “losers” (right column) and “winners” (left column) of MHW impacts. Species are organized by trophic level, with top predators at the bottom. Taken from Cavole et al., 2016.

More recent studies found that in certain areas, impacts from the “warm blob” outlived the duration of the larger scale anomaly. In fact, large, positive SST anomalies have lingered on the Oregon shelf until at least September 2017 (Peterson et al., 2017). During this time period, anomalously high abundances of nearshore larval North Pacific krill (Euphausia pacifica) were collected off of the Newport Hydrographic Station (Morgan et al., 2019). Additionally, Brodeur et al. (2019) demonstrate that while indicator species in the nearshore have consistent annual variability, there were substantial differences between community composition between 2011-2014 (low diversity) and 2015-2016 (high diversity). This work also documented the shift from crustacean species (like krill and mysids) to more low-quality gelatinous taxa. As the authors acknowledge, this change in prey community assemblage could have major negative impacts on trophic interactions. This is especially true in the context of whales, as they are not known to rely on gelatinous taxa for energy.

Just like our summer sampling in Port Orford, these studies only provide a “snapshot” of plankton species abundance and composition during a particular time of year. However, even a snapshot can reveal significant changes in prey variability, which then may help us understand the drivers of PCFG habitat utilization. We are actively investigating whether there have been significant changes in the variability of several zooplankton metrics (abundance, distribution, size class, composition) relative to SST changes in Port Orford over the last 6 years (2016-2021).

We will also consider multiple other static and dynamic factors that could influence zooplankton patterns (e.g., upwelling strength, kelp health, tidal height, topography); however, given these documented strong relationships between the zooplankton community and SST across the North Pacific, we hypothesize similar impacts in our Port Orford study region. For example, in certain sampling years, net tows seemed to be comprised of smaller size classes of zooplankton than usual. We will consider how size class availability has changed and if this was driven by SST variability. Gray whales are drawn to this area for enhanced feeding opportunities, and understanding the drivers of zooplankton, especially high quality prey, is a key step to understanding whale use of the area.

Please stay tuned for more updates as we continue working towards the answer to these pressing questions!

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References

Brodeur, R. D., Auth, T. D., & Phillips, A. J. (2019). Major shifts in pelagic micronekton and macrozooplankton community structure in an upwelling ecosystem related to an unprecedented marine heatwave. Frontiers in Marine Science, 6, 212.

Cavole, L. M., Demko, A. M., Diner, R. E., Giddings, A., Koester, I., Pagniello, C. M., … & Franks, P. J. (2016). Biological impacts of the 2013–2015 warm-water anomaly in the Northeast Pacific: winners, losers, and the future. Oceanography, 29(2), 273-285.

Joh, Y., & Di Lorenzo, E. (2017). Increasing coupling between NPGO and PDO leads to prolonged marine heatwaves in the Northeast Pacific. Geophysical Research Letters, 44(22), 11-663.

Kintisch, E. (2015). ‘The Blob’ invades Pacific, flummoxing climate experts.

​​Leising, A. W., Schroeder, I. D., Bograd, S. J., Abell, J., Durazo, R., Gaxiola-Castro, G., … & Warybok, P. (2015). State of the California Current 2014-15: Impacts of the Warm-Water” Blob”. California Cooperative Oceanic Fisheries Investigations Reports, 56.

Morgan, C. A., Beckman, B. R., Weitkamp, L. A., & Fresh, K. L. (2019). Recent ecosystem disturbance in the Northern California current. Fisheries, 44(10), 465-474.

NOAA Fisheries. 2015b. California Current Integrated Ecosystem Assessment (CCIEA) State of the California Current Report, 2015. NMFS Report 2.
Santora, J. A., Mantua, N. J., Schroeder, I. D., Field, J. C., Hazen, E. L., Bograd, S. J., … & Forney, K. A. (2020). Habitat compression and ecosystem shifts as potential links between marine heatwave and record whale entanglements. Nature communications, 11(1), 1-12.

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