Recent Marine Heatwaves Affect Marine Ecosystems from Plankton to Seabirds in the Northern Gulf of Alaska

Several decades of research and monitoring in the northern Gulf of Alaska (NGA) have revealed climate-related shifts in ocean temperature and salinity. Accompanying these shifts have been changes in the abundance and diversity of species, from single-celled plankton to fish, seabirds, and marine mammals. Research is documenting long-term change in the region and revealing the mechanisms by which recent marine heatwaves affect the ability of higher trophic levels to survive in these waters. Heatwaves in the northern Gulf of Alaska are likely to become longer, more frequent, and more intense, making long-term monitoring of ecosystem changes critical to understanding and predicting effects on valuable commercial fisheries and culturally significant native harvesting. In addition, documentation of change is necessary for projecting regional and global future climate scenarios and for informing climate-​related policy decisions.


INTRODUCTION
Several decades of research and monitoring in the northern Gulf of Alaska (NGA) have revealed climate-related shifts in ocean temperature and salinity.Accompanying these shifts have been changes in the abundance and diversity of species, from single-celled plankton to fish, seabirds, and marine mammals.Research is documenting long-term change in the region and revealing the mechanisms by which recent marine heatwaves affect the ability of higher trophic levels to survive in these waters.
Heatwaves in the northern Gulf of Alaska are likely to become longer, more frequent, and more intense, making long-term monitoring of ecosystem changes critical to understanding and predicting effects on valuable commercial fisheries and culturally significant native harvesting.In addition, documentation of change is necessary for projecting regional and global future climate scenarios and for informing climate-related policy decisions.(Phillips, 2014).

METHODS
We use a range of approaches to study the NGA, including at-sea sampling; remote observations from moorings, gliders, and satellites; and biophysical modeling.Sampling at sea occurs three times annually (spring, summer, and fall) for ocean physics, carbon cycle parameters including export flux and macro-and micronutrients, lower trophic level production rates and communities, and seabirds.
Longer cruises allow for hypothesis testing through shipboard experiments (e.g., iron enrichment).These at-sea observations have been crucial for documenting ecosystem responses to recent marine heatwaves, as they allow detailed examination of community composition shifts and provide ground truth for models that project the effects of future warming.

FINDINGS
As in many biomes worldwide, there is long-term warming in the NGA (Danielson et al., 2022) as well as increased freshening of surface waters (Figure 2).Together, these trends have increased water column stratification.Two recent marine heatwaves (2014-2016 and 2019) tested the resilience of the ecosystem (Litzow et al., 2020;Suryan et al., 2021).Heatwave effects observed by our program include a reduced spring bloom (normally the greatest primary production pulse of the year) and shifts to smaller organisms at both the phytoplankton and microzooplankton trophic levels in spring (Figure 3).This had consequences for the dominant taxa of large-bodied copepods, whose spring lipid storage (which fuels fall-winter survival and reproduction) was reduced and/or required a longer duration of surface feeding.Euphausiids (krill) decreased drastically, and other programs reported similar declines for forage fish such as capelin, sand lance, and herring (Arimitsu et al., 2021).
In contrast, warmer conditions enhanced the survival of southern zooplankton species (including copepods and pyrosomes) that are transported into our region by the Alaska Gyre current system, so fall biodiversity increased at the mesozooplankton trophic level.Seabird responses also revealed "winners" and "losers."There was a huge dieoff of common murres, normally one of the most abundant NGA species, in association with the 2015-2016 warm period, likely due to loss of their euphausiid and forage fish prey (Piatt et al., 2020).However, the abundance of planktivorous taxa such as kittiwakes and storm petrels increased over the mid and outer shelf (Cushing et al., in press).The altered food web conditions reduced finfish FIGURE 1. Sampling the Northern Gulf of Alaska.(a) Bear Glacier and Kenai Mountains are the backdrop for deployment of the "iron fish."(b) Students collect water samples from the CTD.(c) A sediment trap is attached to a cable during mooring deployment.(d) A graduate student displays a giant jelly.(e) A Research Experiences for Undergraduates (REU) participant launches a drifter in the Copper River plume.
reproductive success and led to poor body condition in adults of many species.Subsequent recovery has been highly species dependent, indicating that the specifics of diet, reproductive strategy, metabolism, and behavior are crucial to heatwave resilience.The multi-decadal length of NGA time series observations and records of known relationships among many

FIGURE 2 .FIGURE 3 .
FIGURE 2. (a) Map of the Northern Gulf of Alaska (NGA) study region.The inset shows the location of the NGA in the Northern Hemisphere.Red dots show regular station locations, including the Seward Line.(b) A time series plot of water column temperature anomalies (upper 50 m) from station GAK-1 shows a long-term warming trend of 0.22°C per decade, as well as recent marine heatwaves (orange bars).