Eutrophication overrides warming as a stressor for a temperate African seagrass (Zostera capensis)

Abstract

Despite knowledge that seagrass meadows are threatened by multiple global change stressors, significant gaps exist in current knowledge. In particular, little is known about the interactive effects of warming and eutrophication on seagrasses globally, or about responses of African seagrasses to global change, despite these ecosystem engineers providing critical goods and services to local livelihoods. Here, we report on laboratory experiment assessing the main and joint effects of warming and nutrient enrichment on Cape eelgrass ( Zostera capensis ) from the West coast of South Africa, in which morphological attributes, photosynthetic efficiency and elemental content were assessed. Results indicate that shoot density, leaf length, aboveground biomass and effective quantum yield were negatively impacted by both warming and nutrient enrichment. Growth rate, leaf density and leaf width decreased with increasing nutrient levels but not temperature. In addition, epiphytic fouling on seagrass leaves were enhanced by both warming and nutrient enrichment but with warming eliciting a greater response. Collectively, our findings indicate a stronger effect of enrichment on Z . capensis performance relative to warming, suggesting that the upper levels of coastal eutrophication upon which our experiment was based is likely a stronger stressor than warming. Our findings also highlight limited interaction between warming and nutrient enrichment on Z . capensis performance, suggesting that effects of these stressors are likely to be propagated individually and not interactively. Our findings raise awareness of susceptibility of Z . capensis to eutrophication and the need to manage nutrient inputs into coastal ecosystems to preserve meadows of this seagrass and the critical ecosystem functions they provide.

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

“…Our results are in line with previous studies which did not observe interactive effects of both factors on adult temperate seagrass plants (Touchette and Burkholder, 2002;Kaldy, 2014;Moreno-Marín et al, 2018;Mvungi and Pillay, 2019;Ontoria et al, 2019) and tropical seagrass plants (Viana et al, in prep). In these studies, the combination of effects of nutrient enrichment of the water column and increasing temperature did not show clear effects on seagrass plants.…”

“…In these studies, the combination of effects of nutrient enrichment of the water column and increasing temperature did not show clear effects on seagrass plants. In contrast, other previous studies showed that morphological traits, such as leaf length, growth or number of leaves per shoot, were the most variable traits under the influence of both factors (Bintz et al, 2003;Mvungi and Pillay, 2019). Interactive effects on the response in the %N of the leaves were only observed in Z. marina (Moreno-Marín et al, 2018).…”

“…In addition to being a critical stage (Ambo-Rappe and Yasir, 2015) for further seagrass development, the seedling stage response to climate change and nutrient enrichment has been little studied compared to adult seagrasses (Touchette and Burkholder, 2002;Kaldy, 2014;Moreno-Marín et al, 2018;Mvungi and Pillay, 2019;Ontoria et al, 2019). Overall, as far as we know, even less data is available on seagrass seedling responses in tropical species.…”

“…Eutrophication has been identified as one of the most important factors affecting productivity, community carbon dynamics, and seagrass growth, and is one of the major threats confronting coastal ecosystems. Direct effects occur through stability of physiological mechanisms (Burkholder et al, 1992;Bird et al, 1998;Brun et al, 2002;Invers et al, 2004;Touchette and Burkholder, 2007) causing increased nutrient uptake ability (Viana et al, 2019), nutrient imbalance (Li et al, 2019), changes in morphological indices (Mvungi and Pillay, 2019), changes in growth (Terrados et al, 1999a), changes in sexual reproduction (Duarte et al, 1997), or direct ammonium toxicity (Van Katwijk et al, 1997). Indirect effects of nutrient inputs occur through blooming algae which cause light depletion or nutrient competition (Duarte, 1995;Short et al, 1995;Moore and Wetzel, 2000;Nixon et al, 2001;Burkholder et al, 2007), through the ecological role of herbivores due to modifications in palatability and plant defenses against herbivory (Tomás et al, 2015;Jiménez-Ramos et al, 2017;Marco-Méndez et al, 2017;Campbell et al, 2018;Hernán et al, 2019), or through oxygen depletion in sediments (Terrados et al, 1999b).…”

“…The combined impacts of rising temperature and increased nutrient loading has been studied in adult species of Zostera spp. and Cymodocea nodosa (Touchette and Burkholder, 2002;Touchette et al, 2003;Kaldy, 2014;Jiménez-Ramos et al, 2017;Mvungi and Pillay, 2019;Ontoria et al, 2019), but, as far as we know, no studies were carried out in seagrass early life stages. There are few works on the combined effects of other stressors in seagrass seedlings but none in tropical species (Hernán et al, 2016;Alexandre et al, 2018;Pereda-Briones et al, 2018Yue et al, 2019).…”