Dataset: Plant biomass and foliar standing crop (FSC) data from macroalgal surveys in Sitka Sound, Alaska kelp beds from 2010 to 2019

The following methodology includes a description of how these data were collected as well as how these data were used in the publication Bell and Kroeker (in review).

We conducted monthly surveys of Macrocystis pyrifera for folar standing crop (FSC) estimation in Sitka Sound, Alaska from January 2017 - February 2018 at Breast Is. (57.039 N, 135.333 W) and Harris Is. (57.032 N, 135.277 W), and seasonally in July 2018, January 2019, and July 2019 at Breast Is., Harris Is., and Samsing Pinnacle (56.988 N, 135.357 W). We surveyed all unique M. pyrifera sporophytes (hereafter, “plants” (Bolton 2016)) within two permanent 30 x 2 m transects at the 5 - 7 m depth (MLLW) contour and counted the total number of fronds extending > 1 m above the holdfast (hereafter, “frond density”). To determine the relationship between frond density and total wet mass (g), we collected and measured M. pyrifera plants (excluding their holdfasts) in summer 2017 (N = 16) and winter 2018 (N = 10) (Kroeker et al. 2020).

 In January 2022, we collected M. pyrifera stipe and blade tissue collected from the surface canopy, mid canopy, and 1m above the holdfast (N = 12 unique plants) to capture within-plant variation in tissue dry mass composition (% wet mass). We used the slopes of the zero-intercept linear regression lines generated from these relationships as conversion factors to calculate wet and dry mass for each surveyed plant from its frond density. Across all M. pyrifera tissue samples, wet biomass explained 96% of the variation in dry biomass. Although mean dry mass composition of M. pyrifera tissues varied by location along the frond, the range of total variation (8.8-12.6% of wet mass) was small. We chose to use a mean conversion value (10.3% of wet mass) to estimate dry mass for all M. pyrifera tissues, as we did not consistently collect the canopy length data necessary to incorporate within-plant variation in dry mass composition. We summed the estimated dry mass of each plant and divided by surveyed area to calculate M. pyrifera  FSC as dry mass (g · m-2) at each site for each survey.

We performed seasonal surveys of the understory stipitate kelp community, including Neoagarum fimbriatum and Hedophyllum nigripes, in July 2018 - 20, January 2019 - 2020, and March 2019 at Breast Is., Harris Is., and Samsing Pinnacle. At each site, we counted individuals of these species within two permanent 30 x 2 m transects at the 5 - 7 m depth (MLLW) contour. Starting in March 2019, we also measured a subset of individuals for total blade length and maximum blade width. When we encountered > 10 individuals of either species within a 10 x 1 m swath of a transect, we used the blade morphometrics calculated for the first 10 plants over a subsampled area to estimate total biomass for that species in the rest of that swath. To estimate total dry biomass from blade morphometrics, we collected > 10 individuals of each understory kelp species from each site in August 2018, measured each blade for maximum length and width to estimate surface area (cm-2) and weighed for wet mass (g). We dried collected N. fimbriatum and H. nigripes individuals at 60 ℃ for at least 24 hrs and reweighed for dry mass (g). For each relationship (blade surface area to wet mass, and blade wet mass to dry mass), we used the slopes of the zero-intercept linear regression lines as conversion factors to calculate a dry mass for each surveyed plant. Blade surface area explained 96% of the variability in thallus wet mass for N. fimbriatum and 97% of the variability in thallus wet mass for H. nigripes (Table S1; Bell and Kroeker, in review)). Thallus wet mass explained 99% of the variability in dry mass for both N. fimbriatum and H. nigripes.

We summed plant dry masses and divided by surveyed area to obtain the total dry mass FSC (g · m-2) of each understory species at each site for each survey. In instances where we performed surveys of both stipe counts and blade morphometrics during the same month, we used these calculated season-specific relationships to estimate total dry mass FSC of each species from their stipe densities (stipes · m-2) prior to March 2019. We also used seasonal relationships between stipe counts or blade morphometrics and the season-specific average wet mass of each understory kelp species to estimate the percent composition of understory FSC represented by each species in a survey. Stipe density in January 2020 explained 83% (N. fimbriatum) and 97% (H. nigripes) of the variability in total dry mass present in the transect, whereas stipe counts in July 2019 and 2020 explained 53% (N. fimbriatum) and 98% (H. nigripes) of the variability in total thallus dry mass during these periods.
 

Additional Funding Details:

In addition to primary funding from the NSF award OCE-1752600 additional funding was provided from The David and Lucile Packard Foundation and the North Pacific Research Board’s Graduate Student Research Award (1748-01) to Lauren Bell, PhD University of California Santa Cruz, Award title: "Fish Habitat, Fishes and Invertebrates, Lower Trophic Level Productivity Effect of substrate on herring roe response to global change."