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.

Keywords: blue carbon; carbon cycling; carbon dioxide; carbon sequestration; fringe habitat; nitrogen; nutrients; primary productivity; seaweed.

Fig. 1

Site‐level estimates (mean ± SE) by survey period of Macrocystis pyrifera (a) foliar standing crop (g dry mass · m⁻²), (b) specific growth rate (d⁻¹), (c) net rate of change (d⁻¹), and (d) production rate (g dry mass · m⁻² · d⁻¹). A missing bar indicates no data for that particular site and survey period except where noted by “(0),” in which case the data point was zero. Shaded panels indicate the months with the shortest photoperiod (October–March).

Fig. 2

Site‐level estimates (mean ± SE) by survey period of Hedophyllum nigripes (a) foliar standing crop (g dry mass · m⁻²), (b) specific growth rate (d⁻¹), (c) net rate of change (d⁻¹), and (d) production rate (g dry mass · m⁻² · d⁻¹). A missing bar indicates no data for that particular site and survey period except where noted by “(0),” in which case the data point was zero. Shaded panel indicates the months with the shortest photoperiod (October–March).

Fig. 3

Site‐level estimates (mean ± SE) by survey period of Neoagarum fimbriatum (a) foliar standing crop (g dry mass · m⁻²), (b) specific growth rate (d⁻¹), (c) net rate of change (d⁻¹), and (d) production rate (g dry mass · m⁻² · d⁻¹). A missing bar indicates no data for that particular site and survey period except when noted by “(0),” in which case the data point was zero. Shaded panel indicates the months with the shortest photoperiod (October–March).

Fig. 4

Annual variation in (a) seawater dissolved inorganic nitrogen as NO _x (μM) and in tissue (b) nitrogen and (c) carbon content of Macrocystis pyrifera surface canopy blades. Monthly from August 2018 to August 2019, kelp blades and water column seawater samples (0.5 m and 4.5 m depth) were collected on the same day at Breast Island. Outside of this time period, benthic seawater samples were collected opportunistically from kelp forest beds throughout Sitka Sound. Shaded panels indicate the months with the shortest photoperiod (October–March).

Fig. 5

Seasonal production rates by canopy level for the giant kelp Macrocystis pyrifera and understory kelps Hedophyllum nigripes and Neoagarum fimbriatum by (a) carbon mass (g C · m⁻² · d⁻¹) and (b) nitrogen mass (g N · m⁻² · d⁻¹) at Samsing Pinnacle.