Additive effects of the herbicide glyphosate and elevated temperature on the branched coral Acropora formosa in Nha Trang, Vietnam

Abstract

The combined effects of the herbicide glyphosate and elevated temperature were studied on the tropical staghorn coral Acropora formosa, in Nha Trang bay, Vietnam. The corals were collected from two different reefs, one close to a polluted fish farm and one in a marine-protected area (MPA). In the laboratory, branches of the corals were exposed to the herbicide glyphosate at ambient (28 °C) and at 3 °C elevated water temperatures (31 °C). Effects of herbicide and elevated temperature were studied on coral bleaching using photography and digital image analysis (new colorimetric method developed here based on grayscale), chlorophyll a analysis, and symbiotic dinoflagellate (Symbiodinium, referred to as zooxanthellae) counts. All corals from the MPA started to bleach in the laboratory before they were exposed to the treatments, indicating that they were very sensitive, as opposed to the corals collected from the more polluted site, which were more tolerant and showed no bleaching response to temperature increase or herbicide alone. However, the combined exposure to the stressors resulted in significant loss of color, proportional to loss in chlorophyll a and zooxanthellae. The difference in sensitivity of the corals collected from the polluted site versus the MPA site could be explained by different symbiont types: the resilient type C3u and the stress-sensitive types C21 and C23, respectively. The additive effect of elevated temperatures and herbicides adds further weight to the notion that the bleaching of coral reefs is accelerated in the presence of multiple stressors. These results suggest that the corals in Nha Trang bay have adapted to the ongoing pollution to become more tolerant to anthropogenic stressors, and that multiple stressors hamper this resilience. The loss of color and decrease of chlorophyll a suggest that bleaching is related to concentration of chloro-pigments. The colorimetric method could be further fine-tuned and used as a precise, non-intrusive tool for monitoring coral bleaching in situ.

Keywords: Adaptation; Chlorophyll; Climate change; Coral bleaching; Digital image analysis; Genotype; Global warming; Pesticides; Tolerance; Zooxanthellae.

Fig. 1

Experimental time-line in days. From day 1 (Nov. 2, 2012) to day 18 (Nov. 20, 2012). Exp. 1: Experiment 1 conducted at 28 °C. Exp. 2: Experiment 2 conducted at 31 °C

Fig. 2

Vietnam is outlined and the city of Nha Trang (red box) indicated (a). The bay of Nha Trang, showing research area (red box) in more detail (b). The sampling site from where coral fragments, that were subjected to treatments, were collected (c)

Fig. 3

Experimental setup. Custom-made coral stand with a coral fragment before (a) and after exposure (b). Picture of experimental design (c)

Fig. 4

Image analysis of bleaching degree. Two opposite and perpendicular views of the same coral branch before (upper images) and after (lower images) exposure (a). Images were calibrated and integrated to a measuring scale to express areal coverage in mm² (b). Circles (white dashed line) of approximately 123 mm² were super imposed above bundle ties to quantify the bleaching intensity within the circle in two perpendicular views per coral fragment before and after exposure (c). Mean intensity of gray within circles (range 0–255) is calculated in one of the perpendicular views before (1) and after (2) exposure (d). Note the significant change in mean intensity of gray before and after exposure, i.e. bleaching severity is correlated to higher pixel values. A value of 255 is equivalent to 100% saturation (proportion of gray) or brightness (lightness and darkness)

Fig. 5

Effects of herbicide exposure at +28 and 31 °C on ln-transformed chlorophyll a content (a), for the photometrical variable MIG_E (b) and for the photometrical variable MIG_RE (c). The highest herbicide concentration (G_12.0) at the elevated temperature exposure (31 °C) differs significantly from controls for all three measured variables. No other combination of treatments differs significantly from experimental controls. Error bars are SE. In Fig. 4 b, c, values are proportional to bleaching degree

Fig. 6

Plots from multiple regression analyses where the residuals for the best model, highly influenced by zooxanthellae densities and photometric variables, are plotted against chlorophyll a. Initial model inputs were identical with the exception of photometric variables, MIG_E in (a) and MIG_RE in (b)

Fig. 7

Significant correlations and regressions between the number of zooxanthellae or chlorophyll a content and photometric variables in both experiments, total n = 54 (a, b and c, d, respectively). Note the negative relationship describing decline in the number of zooxanthellae or chlorophyll a content with increasing MIG_E or MIG_RE values