Morphological and Physiological Responses of Enhalus acoroides Seedlings Under Varying Temperature and Nutrient Treatment

Abstract

Seagrass meadows are declining globally. In Indonesia, 75% loss has been reported in the last 5 years. The decrease of the seagrass area is influenced by the simultaneous occurrence of many factors at the local and global scale, including nutrient enrichment and climate change. This study aims to find out how increasing temperature and nutrient enrichment affect the morphological, biochemical and physiological responses of Enhalus acoroides in the seedling phase, which has not previously been studied. To achieve these aims, a laboratory experiment of combined temperature and nutrient treatments was conducted using recently-germinated seedlings of E. acoroides. The results showed that the seedlings were tolerant to an extended exposure to the current ambient maximum temperature. Under higher temperature treatment, the seedlings were observed to increase in aboveground size traits (e.g., number of leaves, leaf length, biomass, and leaf area), as well as in belowground traits, such as root length. The results in this study also showed that the initial seed size matters for morphological responses. On the contrary, nutrient responses of seedlings were practically absent, suggesting they could rely on internal reserves. Interaction between both factors was limited, with the exception of low temperature and high nutrient treatment, in which the AG:BG ratio and leaf elongation rate increased. Fluorescence parameters were not influenced by any of the water treatments. The results in this study suggest that E. acoroides seedlings rely energetically in the reserves within the seedling and that increasing temperature might result in faster seedling development, although no interactions with other organisms were tested. This is of importance when studying the resilience capacity of this species and when restoration attempts are planned, as a faster root development would provide a faster stabilization in the sediment and the survival of the whole plant.

Species-Specific Trait Responses of Three Tropical Seagrasses to Multiple Stressors: The Case of Increasing Temperature and Nutrient Enrichment

“…The experiment was carried out in the same experimental setup as Artika et al (2020). We applied low and high levels of temperature, 26 • C (LT) and 31 • C (HT), respectively, and low and high levels of nutrients by adding 2 µM NH 4 NO 3 + 0.1 µM KH 2 PO 4 (LN) and 20 µM NH 4 NO 3 + 2 µM KH 2 PO 4 (HN), respectively.…”

“…The experiment was conducted under laboratory conditions in an indoor system in the MAREE (ZMT, Bremen) with a total of 24 glass aquaria (29 × 13 × 30 cm dimensions) of 10 l volume. Each aquarium was considered as a replicate, and each of the 4 treatments had 6 replicate aquaria (see experimental design in Artika et al, 2020).…”

“…After the 3-month acclimation period, seagrasses with several ramets (i.e., iterating modular plant units) and no apparent damage or epiphyte cover were selected. Three selected ramets (here forward referred to as plant) of each species were cut and carefully planted in silicate sediment (∼7 cm depth) together with 3 seedlings of Enhalus acoroides (Artika et al, 2020) in the experimental setup previously described, where they were acclimated one more month before the start of the treatments. T. hemprichii and C. serrulata plants consisted of one shoot of 2-4 leaves, roots and a small portion of rhizome (3.6 ± 0.2 cm long).…”

“…Temperature also affects seagrass nutrient content, as lower carbon (C) and N concentrations and higher C:N ratios have been observed at increasing temperatures (Kaldy, 2014;Mvungi and Pillay, 2019;Ontoria et al, 2019b). While moderate elevations in temperature might be positive for seagrass performance (Artika et al, 2020), the individual fitness is lost once the thermal optimum range is exceeded often leading to increasing mortality rates (Collier and Waycott, 2014). Tropical seagrasses usually grow at their upper optimal temperature limit, therefore changes in mean seawater temperatures might be critical (Koch et al, 2013).…”

“…In the last years, an increasing number of papers aiming to understand cumulative impacts of stressors have exponentially increased (Gunderson et al, 2016;Adams et al, 2020;Stockbridge et al, 2020), and more empirical data on the effects of the interaction of increasing temperature and nutrient over-enrichment at an individual level has been obtained (Touchette and Burkholder, 2002;Bintz et al, 2003;Touchette et al, 2003;Burnell et al, 2013;Kaldy, 2014;Kaldy et al, 2017;Egea et al, 2018;Moreno-Marín et al, 2018;Mvungi and Pillay, 2019;Ontoria et al, 2019b). Nevertheless, responses depend on their local adaptation and life history traits (Tuya et al, 2019;Anton et al, 2020) are species-specific, and to our knowledge, there is very limited information about the combined effects of these two stressors in any tropical seagrasses species (Artika et al, 2020).…”