Study sites
We conducted field surveys and experiments near Charleston, South Carolina (SC) (Fort Johnson: 32.751305°N, 79.90142°W; Stono River: 32.75253°N, 80.0076° W) and Savannah, Georgia (GA) (Priest’s Landing: 31.96012° N, 81.01223°W; Bull’s River: 31.97458°N, 80.92287°W) in the USA. The hydro- dynamic forces in southeastern estuaries generate high turbidity and fluid soft sediments which reduce light attenuation and thus create a habitat that is largely inhospitable to macrophyte attachment and persistence (Byers et al. 2012 and references therein). Gracilaria invaded SC and GA estuaries in the early 2000s (E. E. Sotka unpubl. data), and on intertidal mudflats where Diopatra worms are common, Gracilaria presently represents 90-99% of the total macroalgal biomass (Byers et al. 2012). The green alga Ulva sp. can be found in colder months and attached to oyster shells, wooden debris, the hard calcareous tubes of the soda-straw worm and only rarely on Diopatra tubes (Berke 2012; N. M. Kollars, E. E. Sotka & C. Plante pers. obs.). Other macroalgae present in the system include red algae that are epiphytic on Gracilaria (of the genera Polysiphonia and Ceramium; Berke 2012, C. E. Gerstenmaier and E. E. Sotka pers. obs.) and rarely, Gracilaria tikvahiae (Berke 2012, N. M. Kollars & E. E. Sotka pers. obs.). The non-native Gracilaria and native Diopatra are both rare within the salt marshes and oyster beds that fringe the upper-intertidal edge of these mudflats. We performed all laboratory experiments within the Grice Marine Laboratory (College of Charleston, SC, USA). See related reference for citations.

Tidal distribution
In late August 2013, we surveyed the abundance of Diopatra and Gracilaria on the Fort Johnson mudflat, Charleston, SC at 5 tidal heights (~ +0.61, +0.09, 0.0, -0.09, and -0.91 m mean lower low water [MLLW]) that span the upper and lower tidal distributions of both species. At each tidal height, we sampled 5 replicate 3.05 × 0.5 m (~1.52 m2; long end parallel to the shore) quadrats separated by ~1.5 m. We specifically used a large sample frame in order to capture the patchy densities of Diopatra (and therefore Gracilaria) that are particularly common in the lower intertidal. Within each quadrat, we counted the number of Diopatra tube caps (the presence of a tube cap is an excellent proxy for a live worm; Peckol & Baxter 1986) and collected all Gracilaria. We removed macroscopic epifauna and epiphytes present on Gracilaria and dried Gracilaria tissue at 60°C until no change in mass occurred.

Related Reference:
Kollars, N.M., J.E. Byers and E.E. Sotka (2016) Invasive decor: an association between a native decorator worm and a non-native seaweed can be mutualistic. Marine Ecology Progress Series (DOI: 10.3354/meps11602)

Related Datasets:
MEPS_2016: Fig.2B - Gracilaria growth rate
MEPS_2016: Fig.3 - growth rate and depth
MEPS_2016: Fig.4A - worm growth
MEPS_2016: Fig.4B - stable isotopes
MEPS_2016: Fig.5A - field expt 2012
MEPS_2016: Fig.5B - field expt 2013

Kollars_fig2A.csv