Seasonal and small-scale spatial variability of herbivory pressure on the temperate seagrass Posidonia oceanica

Abstract

Despite increasing evidence that herbivory on seagrasses can be substantial in nearshore systems, there is only scarce experimental data on the direct quantification of seagrass removal by herbivores. To help fill this gap, and to evaluate the hypothesis that herbivory on the Mediterranean seagrass Posidonia oceanica (L.) Delile is generally low, we assessed the patterns of grazing pressure variability and its effects on seagrass in an undisturbed meadow in the NW Mediterranean. We quantified herbivore density and grazing pressure through both direct (tethering experiment) and indirect (through marks of herbivore attacks) measurements. Although grazing varied greatly both temporally and spatially, our results show that whereas consumption by the sea urchin Paracentrotus lividus is relatively minor, P. oceanica is intensely grazed by the fish Sarpa salpa in summer. During this period, fish are very abundant at a depth of 5 m, with consumption rates that temporarily exceed seagrass production, which is at its yearly minimum. This imbalance between consumption and production causes the appearance of mowed patches that can be seen from early summer to September; seagrass biomass is reduced by 50% in such patches. Through direct measurements of consumption, our study has revealed that P. oceanica consumption by herbivores can be substantial with respect to the total annual production and much higher than previously estimated through indirect measurements. Thus, it becomes apparent that estimation of consumption rates by indirect methods can grossly underestimate the importance of herbivory in seagrass ecosystems, which leads us to strongly advocate the use of direct methods whenever possible.

Interactive effects of depth and marine protection on predation and herbivory patterns

“…This is consistent with a meta-analysis by Micheli et al (2004), which showed that herbivores commonly do not respond or respond negatively to fishing protection, particularly if they are not targeted by fisheries, and that top predators are often the most responsive trophic guild to protection. We found a general decrease in relative rates of fish herbivory with depth, confirming the results of a number of studies that show greater consumption of marine plants in relatively shallow waters around 5 m depth (Verlaque 1990, Tomas et al 2005, Vergés et al 2009). However, this bathymetric pattern was not consistent at all sites, indicating that factors other than depth are also influencing the feeding activity of herbivores.…”

“…The 3 MPAs differ in how long they have been protectedMedes Islands in Catalunya has been protected since 1983, Cabrera National Park in Mallorca since 1991 and north of Menorca since 1999 -but fish recovery has been observed in all MPAs (Garcia-Rubies & Zabala 1990, García-Charton et al 2004, Cardona et al 2007, although none are likely to have reached full recovery (Guidetti & Sala 2007). Within each region, 3 sites were sampled at 3 depths (5, 15 and 30 m) on algal-dominated rocky habitats in the summer, when fish abundance and activity tend to be highest (Sala & Zabala 1996, Tomas et al 2005, Prado et al 2007). Site selection was based on a good working knowledge of the area and taking into consideration wave exposure (all sites were in relatively waveexposed points) and slope (gentle regular slope; i.e.…”

“…When there was no information for the species of interest, allometric values from a congeneric sympatric species with similar morphology and size range were used (this happened with Labrus viridis, where we used allometric values from L. merula, and for Symphodus doderleini, where we used allometric values from Symphodus rostratus). Total herbivorous fish biomass was considered as the biomass of Sarpa salpa, the only strictly herbivorous fish in the Western Mediterranean, where it has a key role in structuring seagrass and algal communities (Tomas et al 2005, Vergés et al 2009, Tomas et al 2011 Sala 1997). Abundance of fishes was recorded for small, medium and large indivi duals of each species.…”

Experimental evidence of chemical deterrence against multiple herbivores in the seagrass Posidonia oceanica

“…Contrarily, Paracentrotus lividus is common in meadows of Posidonia oceanica, and is one of the main consumers of the plant (Tomas et al 2005), which may seem paradoxical given that it is chemically deterred by this plant. However, in decreasing order of preference P. lividus feeds on epiphyte-covered green leaves, brown dead leaf tips with epiphytes, apical tips of green leaves without epiphytes and, lastly, green leaf bases without epiphytes (Boudouresque & Verlaque 2001 and references therein).…”

“…In order to allow acclimatisation to aquarium conditions, during at least 2 d before conducting the assays all animals were fed fresh samples of the sympatric chlorophyte Ulva sp., a palatable alga that was readily eaten by all consumers and has no known deterrents. All assays were performed during the summer months, when herbivore activity peaks in the Mediterranean Sea (Tomas et al 2005).…”