Drastic decadal decline of the seagrass Cymodocea nodosa at Gran Canaria (eastern Atlantic): interactions with the green algae Caulerpa prolifera

Abstract

The shoot density, leaf length and biomass of the seagrass Cymodocea nodosa\n(Ucria) Ascherson were found to severely decline in the last 17 years in the oceanic\nisland of Gran Canaria (central Eastern Atlantic). Five seagrass meadows were sampled\nin summer and winter of 1994-1995 and in winter and summer 2011. The decrease in C.\nnodosa correlated with a 3-fold increase in the biomass of the green rhizophytic algae\nCaulerpa prolifera (Forsskål) J.V. Lamoroux over the same time period, although this\nincrease varied notably among meadows. We also documented a negative correlation\nbetween the biomass of C. nodosa and C. prolifera at the island-scale, sampling 16\nmeadows in 2011. Experimental evidence demonstrated that C. prolifera can cause\nsignificant negative impacts on C. nodosa: plots with total (100%) removals of C.\nprolifera had ca. 2.5 more shoots and 3.5 times more biomass of C. nodosa, after 8\nmonths, compared to plots with 50% removals and untouched control plots. Interference\nby C. prolifera appears to partially explain the decay in the abundance of C. nodosa\npopulations in Gran Canaria. This study, however, did not identify potential underlying\nprocesses and/or environmental alterations that may have facilitated the disappearance\nof C. nodosa.