After 2.5 years of graduate school my stats are as follows: 78 moose in just the first year mostly seen during my 5 drives to Homer and back, 3 whale species during 5 research cruises adding up to 43 days/nights spent on boats, 19 drafts of my thesis, and 7 more drafts to make it a manuscript. Living in Alaska far exceeded the adventure I was hoping it would be. It was colder than any human should live in, more mosquitos than should be alive on the planet, and beyond beautiful in every direction. Working in Alaska has allowed me to see and do things that are beyond any tourists’ imagination. I had islands all to myself in Katmai, I followed bear tracks to find their discarded prey carcasses, I showered on the open deck of a boat with puffins swimming next to me.
While at the University of Alaska Fairbanks (UAF) I have enjoyed being able to do outreach to students around the state. With the many jobs I’ve had in the past, I truly enjoy being able to do research and answer questions through science but another big part of it is making that science known to others. Being able to spread awareness of what is being found in the scientific field in a way that is easy to digest. Now that I’ve finished my master’s degree thesis, I’d like to do the same with my project. Of course I could easily say “it’s published!” and post the link to the article (Relative Importance of Macroalgae and Phytoplankton to Nearshore Consumers and Growth Across Climatic Conditions in the Northern Gulf of Alaska), but as my advisor would tell you, I enjoy writing more casually for the non-scientist readers. So instead, what I am going to do is re-write my thesis in my own way. It’ll be shorter, leaving out a lot of research that I did, but still has all the main points. I’m hoping this will make it more approachable and readable for everybody, especially being only 3 pages or so instead of the full 20+ pages. If you would like a more thorough read, including sources for statements made, more details on findings, and many more discussion points, please do find the published paper at the link above. If for any reason you would like to use the results or figures in this document, please cite the original published article.
TLDR: The nearshore plays a crucial role for various species, serving as nursery habitats, recreational areas, and carbon sinks. This study, done in the Northern Gulf of Alaska examines the relationship between consumers, primary producers, and temperature. In the North Pacific, macroalgae are a significant food source despite common beliefs of phytoplankton being of better quality. Climate change impacts primary producers, affecting entire food webs. Results show macroalgae are a vital food source, influencing growth rates, especially during the Pacific Marine Heatwave. Understanding these dynamics is essential for monitoring and preserving the marine ecosystem amidst climate change challenges.
Macroalgae, Temperature, and Growth in Nearshore Consumers (Katie’s Version)
Introduction
Our coasts are important environments for many species, including humans. They’re nursery habitats, provide recreational areas for tourism, and take up carbon from the atmosphere (without which, climate change would be a lot worse). All these services need healthy food webs to function, starting at the bottom with primary producers like phytoplankton and macroalgae.
As a food source, phytoplankton is often thought of as the preferred and higher quality primary producer when compared to macroalgae due to being less difficult to digest. Where this study takes place, in the North Pacific, however, macroalgae don’t have as much of a defense and are more easily digested than in other locations. On top of that, as macroalgae decompose, the few defenses they do have start to break down. This means the “better” food source may not be the same here in the North Pacific.
The climate is changing all around the world but it’s doing so at an even faster rate in the North Pacific. Changes in the climate can also cause changes in primary producers which can in turn affect entire food webs. For example, the Pacific Marine Heatwave from 2014-2016 caused a large macroalgal die-off and earlier phytoplankton blooms. This could then affect growth of consumers that rely on phytoplankton and macroalgae.
Growth is influenced by quality of food, quantity of food, temperature, and many other factors. If we go off previous studies that assumed phytoplankton to be a better food source, we’d expect to see more growth in locations and years that had more phytoplankton in their diet. Based on studies done specifically in Alaska, however, many nearshore species use the macroalgal-based pathway. As for temperature, warmer temperatures often lead to higher growth rates as long as there is enough food to sustain growth. If the temperatures get too high, growth rates can then decline as consumers put more energy into surviving rather than growing.
In this study, we look at this relationship between consumers, the primary producers at the base of their food web, and temperature. Stable isotopes of carbon and nitrogen can be used to show diet, specifically relative proportions of macroalgae vs phytoplankton in the diet/trophic pathway for each individual. Then, using growth rings on an individual, annual growth can be determined relative to other individuals. This was all done to test the hypothesis that individuals would have higher growth rates with higher macroalgal (rather than phytoplankton) contributions to the diet. With the Pacific Marine Heatwave, we then tested whether the macroalgal die-offs would lead to less macroalgae in the diet of mussels, which, when combined with the stress of the heatwave, would result in less growth.
Methods
To study the nearshore in a comprehensive but concise way, we chose three consumer species to represent three typical feeding strategies: Pacific blue mussel (Mytilus trossulus), Black Rockfish (Sebastes melanops), and Kelp Greenling (Hexagrammos decagrammus). Mussels are filter feeders, capturing particles from the water column which they can select for based on size and quality of the particles. They are also important to the food web as they are prey for sea stars, sea otters, and many species of seabirds. Black Rockfish are mostly pelagic-feeding generalists (not picky eaters) eating small fishes and zooplankton in coastal waters. Kelp Greenling prey off the sea floor eating sea cucumbers, crabs, and other benthic (sea floor) species. Both fish species occur throughout the North Pacific from the Aleutian Islands down to central California but have a small home range with individuals normally staying and feeding within ~1km2. This small home range assures us that where we catch the fish is indeed where they are feeding so any diet results would be indicative of food found in the area.
Based in the Northern Gulf of Alaska, this study spans four regions, each broken into five or six study sites (for 21 total sites). The regions include Katmai, Kachemak Bay, Kenai Fjords, and Western Prince William Sound. At the regional scale, ten of each fish species were collected from the waters in each region for each year (2018, 2019, 2021). By site, ten mussels were collected from the intertidal at each site for each year (2014-2021 for most sites), water samples were collected and filtered for each year (2012-2021) to represent particulate organic matter or phytoplankton, and the most common macroalgae were collected including red, green, and brown groups of algae for each year (2014-2021).
Muscle from the back of the fish and the inside of the mussel was collected, dried, and ran for stable isotope analysis along with dried macroalgae and phytoplankton filters. The stable isotope analysis combined with mixing models then gave us the relative proportions of macroalgae and phytoplankton at the base of the trophic pathway for each consumer.
Otoliths (ear bones) from fish along with the ridges on mussel shells contain annual rings like tree rings that can show us growth. Those growth measurements can then be compared across years, across individuals, and compared to the stable isotope analyses. Plotting these relationships allows us to see a trend, if any, between growth and diet.
Temperature loggers were also placed at each site at the 0.5m tidal elevation to record the water temperature throughout the years. Summer records were then picked out and those temperatures allowed us to more precisely define years as being before (2012-2014), during (2015-2016), or after (2017-2018, 2020-2021) the Pacific Marine Heatwave. There was a second heat spike in 2019 but since it was not part of the original heatwave, it was not included.
Results and Discussion
Diet
Similar to previous studies done in and around Alaska, we found that macroalgae were the main primary production source to consumers, above 50% of the diet for mussels, Black Rockfish, and Kelp Greenling. Comparing the focal species to each other, Black Rockfish appeared to have the lowest macroalgal contribution (most mixed diet with phytoplankton) and Kelp Greenling had the highest macroalgal contribution, with mussels falling in between. Within each species, there was minimal variation by region but Kachemak Bay consistently had lower average macroalgal contribution (still above 50%).
The Northern Gulf of Alaska is a high latitude system that has lots of macroalgae and is highly seasonal with the dark and stormy winters limiting phytoplankton availability. By consuming macroalgae or relying on a macroalgal-based trophic pathway, this could help buffer times when phytoplankton is more limited. With the fish species both being generalist feeders, and not feeding directly on macroalgae or phytoplankton, the high macroalgal contributions in their diet mean that many, if not most, of their prey species also likely rely on the macroalgal pathway.
Diet and Growth
Mussel growth by itself was different by region with Kachemak Bay and Western Prince William Sound having higher growth than Kenai Fjords. When all sites, regions, and years were combined, mussel growth significantly increased with higher macroalgal contribution. If we break it up by region, only Katmai mussels showed this significant relationship.
Growth of Black Rockfish was higher in Katmai than Kenai Fjords or Western Prince William Sound. Black Rockfish in Kachemak Bay grew significantly more with more macroalgal contribution but showed no relationship when all regions and years were combined. Kelp Greenling showed no difference in growth based on macroalgal contribution either with regions combined or by region. There was also no difference in growth by region, independent from macroalgal contribution.
With all relationships between growth and macroalgal contribution being neutral or positive, this implies that macroalgae may not be the lower quality food source as previously suggested. By having such high macroalgal contributions across samples, we can’t say for certain what this relationship would look like with less macroalgae and more phytoplankton. It could make all the relationships we tested even stronger or show a parabolic relationship with more growth with a more singular primary production source. Another study, for example, had a range of 0-60% macroalgal contribution and was significantly positively related to growth in juvenile Black Rockfish. The significance in this study compared to ours could be due to them having a larger range of macroalgal contribution values but it also brings up another factor of growth: life stages. All individuals sampled in this study were adults which could also lead to less variation in growth no matter the diet of the individuals.
Mussel Diet and Growth with the Pacific Marine Heatwave
Macroalgal contribution to mussel diet was lowest in years during the Pacific Marine Heatwave and highest after the heatwave. Mussel growth was also lowest during the heatwave and highest after the heatwave. The relationship of growth and macroalgal contribution was minimal except for during the heatwave, meaning macroalgae were more important to growth during the heatwave.
During periods of higher water temperatures (including heatwaves), macroalgae grow less and there is often less of it. Combined with the die-offs during the heatwave, this could explain the lower macroalgal contributions in mussel diet. The macroalgae that don’t survive then begin to decompose which allows them to more easily enter the food web after the heatwave, this is supported by our results. The significant relationship of macroalgal contribution and growth during the heatwave may suggest that macroalgae can provide important energy during stressful conditions that help maintain growth.
Mussel growth is driven by many factors and this study only looked at relative proportions of primary producers and temperature. The Pacific blue mussels that were used here are known to have a low heat tolerance. While we logged the water temperatures in the intertidal, the internal temperature of mussels is driven by many complex interactions. The external water temperature influences the internal temperature but with multiple other influences, the internal temperatures could be much higher or lower than the water temperatures we logged. Our results showed both lower macroalgal contribution and lower growth rates which could be related to each other or be related separately to temperature.
Conclusion
The oceans and climate are changing at a faster rate than seen before and this will likely include more frequent heatwaves. This study has shown that the resulting lower macroalgal abundance could affect the food web in terms of growth. Given the importance of macroalgae to nearshore consumers demonstrated here, understanding and anticipating nearshore responses to climate change can be beneficial in monitoring macroalgal production for the benefit of the entire food web.
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