Standing Crop, Turnover, and Production Dynamics of Macrocystis pyrifera and Understory Species Hedophyllum nigripes and Neoagarum fimbriatum in High Latitude Giant Kelp Forests

Abstract

Production rates reported for canopy‐forming kelps have highlighted the potential contributions of these foundational macroalgal species to carbon cycling and sequestration on a globally relevant scale. Yet, the production dynamics of many kelp species remain poorly resolved. For example, productivity estimates for the widely distributed giant kelp Macrocystis pyrifera are based on a few studies from the center of this species' range. To address this geospatial bias, we surveyed giant kelp beds in their high latitude fringe habitat in southeast Alaska to quantify foliar standing crop, growth and loss rates, and productivity of M. pyrifera and co‐occurring understory kelps Hedophyllum nigripes and Neoagarum fimbriatum. We found that giant kelp beds at the poleward edge of their range produce ~150 g C · m−2 · year−1 from a standing biomass that turns over an estimated 2.1 times per year, substantially lower rates than have been observed at lower latitudes. Although the productivity of high latitude M. pyrifera dwarfs production by associated understory kelps in both winter and summer seasons, phenological differences in growth and relative carbon and nitrogen content among the three kelp species suggests their complementary value as nutritional resources to consumers. This work represents the highest latitude consideration of M. pyrifera forest production to date, providing a valuable quantification of kelp carbon cycling in this highly seasonal environment.

Season influences interspecific responses of canopy‐forming kelps to future warming and acidification at high latitude

“…Furthermore, these overlapping environmental drivers may indirectly affect higher order consumers via changes to seaweed palatability in certain seasons. Given the inherent differences in distributions, life histories, and annual production dynamics among the subtidal kelps in this study (Bell & Kroeker, 2022; Dankworth et al, 2020; Schiel & Foster, 2015), we were unsurprised to find that seasonal scenarios of OW and OA elicited distinct responses in each macroalgal species. This research demonstrates that changing environmental conditions will shift the seasonal quality and quantity of basal resources in kelp ecosystems at high latitudes, likely reducing the functional biodiversity of these communities (Schlenger et al, 2021).…”

“…In contrast to the species‐specific responses of growth rate to future environmental conditions, all three kelps in this study exhibited reduced tissue nitrogen content under winter scenarios of OW. Currently in Sitka Sound, kelp nitrogen content increases in winter due to the ample seawater nutrient supply and low energetic requirements during this season of low light and low temperature (Bell & Kroeker, 2022). While the energetic expense of nutrient acquisition can be limited by low light levels (Hurd et al, 2014; Roleda & Hurd, 2019), some kelps, including H. nigripes and M. pyrifera , readily uptake nitrate at equal or higher rates in the dark compared with the light by mobilizing carbohydrate reserves (Harrison et al, 1986; Korb & Gerard, 2000; Wheeler & Srivastava, 1984).…”

“…We focus on three large, canopy‐forming kelp species that dominate macroalgal biomass within the giant kelp forests of this region: Macrocystis pyrifera , Hedophyllum nigripes , and Neoagarum fimbriatum . The annual growth regimes of these three species are distinct in Sitka Sound (Bell & Kroeker, 2022), which may reflect underlying differences in their physiological optima and tolerances. H. nigripes is a cold‐adapted understory kelp found primarily in Arctic and sub‐Arctic waters (Dankworth et al, 2020; Grant et al, 2020; McDevit & Saunders, 2010).…”

“…These effects will be particularly evident in seasonally dynamic environments. High‐latitude marine ecosystems are characterized by large annual variations in temperature, p CO 2 , light, and nutrients that influence the seasonal dynamics of primary production and algal physiology (Bell & Kroeker, 2022; Takahashi et al, 1993; Tian et al, 2001). Increases in temperature and p CO 2 will overlay current fluctuations in temperature, p CO 2 , light, and nutrients in these systems, giving rise to novel environmental scenarios that will drive seasonally distinct effects on macroalgal physiology (Graiff et al, 2015; Gunderson et al, 2016; Harley et al, 2012; Kroeker et al, 2020).…”

“…H. nigripes is a cold‐adapted understory kelp found primarily in Arctic and sub‐Arctic waters (Dankworth et al, 2020; Grant et al, 2020; McDevit & Saunders, 2010). This species' annual growth is controlled by a strong endogenous clock, with blade elongation initiating in January and curtailing abruptly in early summer (Bell & Kroeker, 2022; Lüning, 1993). In contrast, the more temperate kelps M. pyrifera and N. fimbriatum sustain relatively high growth rates through spring, summer, and early fall (Bell & Kroeker, 2022).…”