Data belonging to article "A mutualism between unattached coralline algae and seagrasses prevents overgrazing by sea turtles"
Date of Archiving2020
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Aquatic Ecology and Environmental Biology
Key wordssea turtle; algae; overgrazing
We conducted our study in Akumal Bay (Mexican Caribbean, 20° 23′ 44.9″ N, 87° 18′ 47.9″ W), a tropical bay protected by a fringing reef at about 100–300 m from the coast (Molina Hernández and van Tussenbroek 2014). Seagrass meadows in the lagoon cover about ~4 ha and experience high grazing pressure from a population of resident green turtles that has steadily increased over the last 2 decades (Maldonado Cuevas and others 2006). Until 2008, the slow-growing climax seagrass species Thalassia testudinum was dominant at Akumal. By 2012, however, continuous excessive grazing pressure had caused this species to strongly decline and become replaced by the more grazing-tolerant early seral species Halodule wrightii (Molina Hernández and van Tussenbroek 2014). There was serious concern that overgrazing was driving the seagrass ecosystem toward collapse (Molina Hernández and van Tussenbroek 2014). But such a collapse of the seagrass meadow has not occurred, while at the same time abundance of unattached red branched coralline algae (Neogoniolithon sp. and Amphiroa sp.) has increased greatly since 2012. Neogoniolithon sp. is a non-geniculate (not possessing non-calcified joints) alga, whereas Amphiroa sp. is geniculate (possesses uncalcified joints). These algae were very sparse in 2008 (< 1% cover); they increased to about 3% cover by 2012 with dense patches in some heavily grazed sections of the seagrass meadow and covered ~ 20% of the bay in 2016 (B. I. van Tussenbroek, unpubl. data, Figure 2). At the time of this study (Sep.–Nov. 2015), the algal aggregations formed a 3–6.8-cm-thick layer. Mean calcified weight was 5581 dry g m−2,—with Neogoniolithon sp. accounting for approximately 90–95% of coralline algae biomass (Tables S2 and S3 for characterization of the coralline algae in the bay). The algae-covered section of the bay was colonized by T. testudinum, forming a dense and high canopy with a high above—and belowground biomass (Table S4), whereas Syringodium filiforme and H. wrightii had low biomass in this area. The coralline algae-free section of the bay was dominated by H. wrightii, and T. testudinum and S. filiforme had low biomass. Experiment 1—Do Algae Facilitate Seagrass? Experiment 1.1—Removal of Coralline Algae from Ungrazed Area As a first step in testing the hypothesis that unattached branched coralline algae discourage turtles from grazing on seagrasses, we removed algae from the recovered seagrass meadow that was dominated by coralline algae. To this end, we haphazardly established 10 1 × 1-m plots from which we removed all coralline algae and 10 procedural control plots from which no algae were removed. To attract turtles to our experimental area, we mimicked turtle grazing in 20-cm-wide strips around experimental and control plots by clipping seagrass to about 3 cm height [per (Molina Hernández and van Tussenbroek 2014)] and removing algae. Maintenance of the plots consisted of keeping the removal plots free of coralline algae and continued clipping of seagrass shoots in the 20-cm ‘turtle attraction strips’ to about 3 cm height, when necessary. Prior to the start of the experiment (14-9-2015), we measured general seagrass characteristics in the area. We determined leaf length and sheath length for all seagrass species at 10 haphazardly chosen locations (aprox. 2 × 2 m in size) in the experimental area. For T. testudinum we also measured leaf width as an additional indicator. After 43 days, we terminated the experiment and re-measured all above-mentioned variables per plot. Specifically, we collected 20 shoots of each seagrass species present per location (in the beginning) or plot (at termination of the experiment) by cutting their vertical rhizome below the sediment with a knife. In the laboratory, the lengths of the sheaths and green sections of all the leaves of the collected shoots were measured with a ruler, and leaf widths were measured with a dial caliper (0.02-mm precision). The blades were cleaned of epiphytes by scraping with a razor blade and placed in a drying oven at 60°C for at least 24 h until dry. The basal 3 cm of each leaf of T. testudinum was preserved for nutrient analysis following (Molina Hernández and van Tussenbroek 2014). Five times during the experiment, we visually estimated what percentage of the surface of the plots was newly grazed by turtles, which is clearly visible by the neatly cutoff leaves. As a procedural control, we tested whether the turtle attraction strips caused an overestimation of grazing pressure within the plots. We compared shoots collected from the experimental site prior to the start of the experiment, with shoots collected from the procedural control plots at the end of the experiment. Next, we compared seagrass meadow characteristics in the removal versus procedural control plots to determine the effect of algae on grazing. Experiment 1.2—Addition of Coralline Algae to Grazed Area As a second test of the hypothesis that coralline algae deter turtles, we placed an approximately 5-cm-thick layer of algae, originating from the ungrazed seagrass meadow, on the grazed meadow in 10 haphazardly chosen 1 × 1 m plots. The same measurements were taken as in experiment 1.1, both at the start and at the end of the experiment. Maintenance of the plots consisted of maintaining the algae layer by topping up coralline algae in the plots when algae were displaced by water movement, tourists or animals, or when they were covered with sand by mantis shrimp. The experiment was initiated on 9-9-2015 and lasted for 48 days. The turtles continued to graze the areas around the plots during the experiment. We analyzed our treatment effect by comparing seagrass meadow characteristics before and after the addition treatment. Experiment 2—Do Seagrasses Facilitate Algae? To determine the effect of seagrass vegetation on establishment of unattached coralline algae, we placed thalli of Neogoniolithon sp. in ungrazed, turtle-grazed and bare sand patches in a seagrass meadow at Puerto Morelos (20° 50′ 27.2″ N, 86° 52′ 26.1″ W). Locations were undisturbed by tourists. We established 3 replicate patches per treatment, and in each patch 20 g (wet weight) of algae was placed in a 10 × 10 cm quadrat. The quadrat was removed, and small sticks were left to indicate the exact position. One week later, the 10 × 10 cm quadrat was placed back at the exact same location, with an 80 × 80 cm quadrat around it. The algae in both quadrats were collected separately, transported to the laboratory and weighed, to determine whether they persisted at the same location (10 * 10 cm quadrat) or in the general area (80 * 80 cm quadrat). After 7 days the algae were collected, as we already saw considerable removal already occurring in the plots.