Ecosystem engineering by cordgrass has weak effects in a harsh environment

FIRSTATER, FAUSTO N.; NARVARTE, MAITE; ALVAREZ, M FERNANDA; IRIBARNE, OSCAR

Mar del Plata

Conferencia; CERF 2012. The Changing Coastal and Estuarine Environment: A Comparative Approach.; 2012

Ecosystem engineers (E.E.) tend to be most dominant in stressful environments. In saltmarshes, the cordgrass Spartina spp. are considered E.E. that attenuate waves, currents and light. However, most of the studies on Spartina as E.E. come from lower latitudes, where plants are generally taller and denser. Moreover, Patagonian coasts are exposed to intense physical stress (e.g. strong winds, low rainfall and humidity, and extreme temperatures). To test the effects of aboveground structure of S. alterniflora on sediment features and on the abundance of benthic microalgae, we performedan experiment in northern Patagonia (41°S) from Nov-10 to Feb-11. The treatments consisted in 2.25 m2 plots were (i) S. alterniflora stems were completely removed (S-), (ii) stem density was enhanced by 50% by adding plastic stem mimics (S+), and (iii) left undisturbed and used as control (C). Plots were set either in the lower limit between S. alterniflora marsh and mudflat (B) or 10 m away, inside the S. alterniflora marsh (A). During the experiment we assessed sediment temperature and desiccation, and after 3 mo we measured water content, OMC, grain size and Chl-a from sediment samples. Mean stem height was 30.5 cm (SD= 4.6) and mean density 208.8 stem.m-2 (SD= 90.6). Temperature and desiccation were higher in plots S- than in plots S+ and C. There were no differences in water content and OMC between treatments. Mean grain size in B was higher than in A, but it was not related to the treatment. Finally, there was no difference in Chl-a content between treatments or location (ANOVAs). Previous studies in more benign environments,where cordgrass usually present more abundant aboveground structure, have suggested that cordgrass may alleviate physical stress by shading and stabilizing the substrate. However, it seems that in this harsh, tide-dominated system, S. alterniflora is a weak E.E., that may alter some abiotic features but is unable to modify the abundance of benthic microalgae.