Methodology:

Sampling and analytical procedures: To assess in-situ seasonal changes in coralline algae growth rates, we collected individuals from a crustose coralline algal genus (Crusticorallina painei, C. adhaerens and C. muricata; n=40) and a geniculate coralline species (Bossiella orbigniana; n=40) from a subtidal rocky reef directly adjacent to Harris Is., Sitka Sound, AK (57.032N, 135.273W) in December 2017, July 2018, and January 2019. Crustose individuals targeted for collection were those that had a morphology that could easily be separated from the rocky substrate, often disc-shaped with distinct white growing edges. Geniculate individuals were collected to include the basal holdfast. The individuals were cleaned by removing epiphytic algae and invertebrates with tweezers, and then placed in a 100mg L-1 concentration of the membrane-permeable live-cell labeling fluorescent dye Calcein for 6h. This dye is absorbed by metabolically active meristematic tissue of the alga at the time of the stain, thus providing a growth benchmark for subsequently added tissue. After staining, each coralline individual (Crusticorallina spp.: a ‘disc’ with at least a 50% intact growing edge; B. orbigniana: a ‘floret’ containing 4-10 apical fronds) was attached to a small PVC stand by using z-spar epoxy putty to affix the older, non-meristematic tissue. Individuals on stands were then outplanted into the field on plates (2 crustose and 2 geniculate indiv. plate-1) and bolted onto rocky reef substrate at 10m depth MLLW at the edge of a giant kelp forest at Harris Is (57.032N, 135.277W). Coralline algae were retrieved after 2-3mo, for total outplant durations of 67d (Winter 2018), 66d (Summer 2018), and 103d (Winter 2019).

To measure linear growth, the coralline algae were imaged using a fluorescent lamp channel on a Ziess AxioZoom microscope at the UCSC Microscopy Center. Average growth extension from the original fluorescent stain was calculated for each individual using ImageJ (NIH v1.8.0) by analyzing measurements from up to 13 randomly selected points along the growing edge of the disc (Crusticorallina spp.) or from up to 17 randomly selected apical fronds in the floret (B. orbigniana).

Problem report:

Dataset only includes coralline algae individuals that were 1) not lost in the field, and 2) had sufficient stain uptake to enable measurements of new growth. Crustose coralline individuals that exhibited downward growth (versus primarily lateral growth along the growing edge of the disc) were more difficult to measure for accurate length extension. The parameter name “downward_growth” was used to indicate instances in which this occurred, and these individuals were not included in published analyses.