• Estimates of primary production sources to Arctic bivalves using amino acid stable carbon isotope fingerprinting

      Rowe, Audrey G.; Wooller, Matthew; Iken, Katrin; O'Brien, Diane (2018-12)
      Benthic invertebrates are a crucial trophic link in Arctic marine food webs. However, estimates of the contribution of primary production sources sustaining these organisms are not well characterized. Potential sources could include sinking particulate organic matter from sea ice algae and phytoplankton, terrestrial organic matter eroded from the coastal environment, macroalgal material, or microbial organic matter. Proportions of these sources could also be significantly altered in the future as a result of environmental change. We measured the stable carbon isotope values of essential amino acids in muscle tissue from two common bivalve genera (Macoma spp. and Astarte spp.) collected in Hanna Shoal in the northeastern Chukchi Sea, considered an Arctic benthic hotspot. We used stable isotope mixing models in R (simmr) to compare the stable carbon isotope amino acid fingerprints of the bivalves to a suite of amino acid source endmembers, including marine phytoplankton, brown and red macroalgae, bacteria, and terrestrial plants, to estimate the proportional contributions of primary production sources to the bivalve species from Hanna Shoal. The models revealed relatively high contributions of essential amino acids from phytoplankton and bacteria averaged across both species in the region as a whole. We also examined whether stable carbon isotope fingerprints could be measured from essential amino acids preserved in bivalve shells, which could then allow proportional contributions of food sources to be estimated from ancient bivalve shells, allowing source estimates to be extended back in time. To investigate this, we measured the stable carbon isotope values of essential amino acids in a suite of paired modern bivalve shells and muscle from Macoma calcarea from the Chukchi Sea. These analyses revealed a correspondence between the fingerprints and mixing model estimates of the dominant primary production source of essential amino acids derived from analyses of these two tissue types. Our findings indicate that stable carbon isotope amino acid fingerprinting of marine bivalves can be used to examine dominant organic matter sources in the Arctic marine benthos in recent years as well as in deeper time.
    • The response of juvenile coho and chinook salmon stocks to salmon spawner abundance: marine nutrients as drivers of productivity

      Joy, Philip J.; Wipfli, Mark S.; Adkison, Milo D.; McPhee, Megan V.; Stricker, Craig A.; Rinella, Danial J. (2019-08)
      Resource subsidies from spawning Pacific salmon (Oncorhynchus spp.) in the form of marine-derived nutrients (MDN) benefit juvenile salmonids while they rear in fresh water, but it remains unclear if the abundance of spawners in a watershed affects the productivity of salmon stocks that rear in those riverine systems. This dissertation aimed to provide a better understanding of these dynamics by evaluating whether the response of juvenile salmon to MDN is sufficient to enhance overall stock productivity. In Chapter 1, I examined correlative relationships in the abundance of Pink (O. gorbuscha) and Coho (O. kisutch) salmon and simulated spawner-recruit dynamics to determine if those correlations were produced by a Coho Salmon response to marine subsidies from Pink Salmon, a shared response to marine conditions, and/or autocorrelations in the returns of both species. Results demonstrated that observed correlative patterns most closely resembled simulated freshwater effects, providing evidence that marine subsidies from Pink Salmon influence Coho Salmon productivity. In Chapter 2, I examined the relationship between spawner abundance and MDN assimilation by juvenile Coho and Chinook (O. tshawytscha) salmon in the Unalakleet River watershed. Stable isotope analysis demonstrated that after salmon spawned, MDN assimilation by juvenile salmon in the fall was a function of adult Pink and Chinook salmon spawner abundance, regardless of the habitat occupied by rearing juveniles. However, by the following summer, high retention of MDN in complex habitat masked seasonality of MDN assimilation in sloughs and river sections with abundant lentic-lotic exchanges. As such, MDN assimilation in the summer (prior to arrival of spawners) bore only a faint relationship to spawner abundance and distribution from the previous year. In chapter 3 I examined the relationship between MDN assimilation (Chapter 2) and juvenile salmon growth, size, body condition, and abundance. Prior to salmon spawning, residual MDN from past years offered little advantage to juvenile salmon. However, after the arrival of spawning salmon, MDN enhanced juvenile salmon size, growth, and condition in fall and winter. The collective results from this dissertation thus provides compelling evidence that MDN from spawning Pink Salmon may enhance the productivity of Coho and Chinook salmon. Management agencies should explore modified spawner-recruit models that incorporate MDN relationships to determine if they more accurately describe population dynamics. Where they do, such models may be used to forecast salmon returns and possibly adjust escapement goals (the number of spawners desired on the spawing grounds) to improve maximum-sustained yields (MSY).