Light compensation points and the effects of grazing and light intensity on the consumption of the native Stypopodium zonale and the non-native Grateloupia imbricata by Paracentrotus lividus in laboratory experiment on Madeira Island in 2007

Experiments were conducted from 4 June to September 2007 inside laboratory facilities at the south coast of Madeira (32°38'N, 16°54'W). The organisms used for this study were the sea urchin Paracentrotus lividus collected at Doca do Cavacas (32° 38'06 N; 16° 56'52 W), the red seaweed Grateloupia imbricata collected from the marina of Funchal (32° 38'41 N; 16° 54'46 W) at 0.5 m water depth, and the brown seaweed Stypopodium zonale collected from boulders at Reis Magos, Caniço (32° 83'45 N; 16° 49'25 W) in water depths of 3-7 m. The study consisted of three sequential stages: (i) assessing algal light compensation points, (ii) inducing light limitation and grazing, (iii) assessing grazer consumption rates in no-choice feeding assays after light limitation and grazer impact. For the latter we used the algal material from stage (ii) (see additional file 1). Pilot studies were carried out in June 2007 to identify the Light Compensation Point (LCP) for both algal species, i.e. the light intensity at which the rate of photosynthesis (measured as oxygen production) equals respiration. For this, we reduced the amount of incoming light by placing various layers of black plastic gauze material with a mesh size of 1 mm on top of each aquarium. For both macroalgae, we had a total of 12 aquaria (3.5 L each) of with each was loaded with 30 - 40 g wet weight of seaweed material. We randomly assigned two aquaria to each of six different light regimes. The number of gauze layers used was 0, 1, 2, 3, 4 and 5. After we recorded the concentration of dissolved oxygen with an oxymeter (Oxi 197, WTW Wissenschaftlich-Technische Werkstätten GmbH, Weilheim, Germany) twice a day (9am and 5pm) for the following 4 days (see additional file 2). Experiments were carried out in July 2007 for G. imbricata, and in September 2007 for S. zonale. We conducted a two-factorial experiment for each of the species in which we crossed two levels of grazing ("grazed" and "non-grazed") with six levels of light intensities (0-5 layers of gauze material) and each treatment combination was replicated eight times (n=8). Consequently, we had 6 x 8 = 48 aquaria of which each contained one sea urchin, while another 48 aquaria had no sea urchins. The latter were used to determine total algal growth rates under the different light regimes in the absence of grazers and to provide non-grazed algal material for the feeding assays. In total we had 96 aquaria for each of the two seaweed species in the study and the respective treatments, i.e. light limitation and grazing, were imposed simultaneously for 21 days. We tested for possible effects of the previously applied light limitation and grazing (see stage (ii)) on grazer consumption rates in no-choice feeding assays that lasted for 24 h. Hence, the number of replicates for the assays at stage (iii) was the same as at stage (ii). Grazer consumption rates of algal material were determined as the grazers' total consumption, which was calculated using the equation suggested by Cronin and Hay (1996-a): [Ai x (Cf / Ci) - Af ] where Ai and Af were the initial and final weight of the algae portions used in the feeding assays; Ci and Cf are the equivalent weights of the growth control algal pieces before and after the assays (Sotka et al., 2002). Finally, consumption rates were standardised for grazer wet biomass (g alga/g grazer). Negative consumption was recorded in case algal growth rates during the assays exceeded consumption rates (see additional file 3 raw data)