Relative eelgrass leaf elongation and microalgal epiphyte load on eelgrass leaf measured through a multi-factorial in situ experiment of 10 weeks in a eelgrass bed in St. Lawrence Estuary (Baie-Saint-Ludger, Canada)

We evaluated single and interactive effects of density reduction of Zostera marina L., a habitat-forming species, shading, and sediment nutrient enrichment on the response of eelgrass relative leaf elongation (RLE) and of microalgal epiphyte load on leaves. REL was evaluated once, as a proxy for growth, using five shoots per plot that were each marked with a reference hole at the top of the sheath using a pushpin marked at the end of Period T1 (two weeks after reducing eelgrass density and at the same time than adding nutrients sediment enrichment and shades). After 19 days, we collected the shoots and brought them back to the lab where leaf elongation was measured as the displacement of the mark relative to the reference mark on the oldest nongrowing leaf (Olesen & Sand-Jensen 1994). Total leaf elongation was then divided by sheath length and the number of days of elongation. We estimated the epiphyte (microalgae) load on eelgrass by scraping the leaves of one randomly selected shoot; we then filtered the water containing the epiphytes on GF/F filters that were then kept wrapped in aluminum foil at -80 °C until analysis. Epiphyte load was assessed using chlorophyll extraction with 90% acetone, following Parsons et al. (1984). Calculated chlorophyll weight was then divided by shoot dry weight.