Date Published: April 11, 2019
Publisher: Public Library of Science
Author(s): Esther F. Mvungi, Deena Pillay, Maura (Gee) Geraldine Chapman.
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.
Global change stressors pose significant threats to biodiversity and ecological resilience in marine ecosystems across the planet [1–3]. These stressors do not only impact critical biological and ecological processes ; they also negatively feed back to local communities, often in the form of impaired quality of goods and services provided [5–7]. Global warming is a particularly concerning aspect of global change and is commonly considered to be driven by increasing levels of atmospheric greenhouse gases , brought on principally by human activities [1–3,8]. The rate at which the planet is heating is alarming [2,3,9,10], with forecasts predicting a rise in global temperatures by 2–4°C by 2100 . Eutrophication, the loading of excessive nutrients into coastal ecosystems, is another dimension of global change that has severe repercussions for biodiversity and ecological integrity [8,11,12]. The intensification of eutrophication over the last few decades is considered a function of increased anthropogenic developments in coastal areas across the globe, with agricultural escalation and fertilizer runoff being particularly important drivers [13,14]. Eutrophication may also indirectly be compounded by global warming due to increased flooding associated with higher precipitation and flooding. While eutrophication management has been employed with some success in developed parts of the world , this aspect has been lagging in developing regions.
Knowledge on the individual and interactive effects of elevated temperature and nutrients on seagrass performance is rare (but see [30,31,44,45] for exceptions), despite recognitions that understanding global change impacts requires information on multiple stressor responses and whether such responses are additive, synergistic or antagonistic [2,18,46]. In addition, experimental studies on responses of Zostera capensis to abiotic stressors are also limited. In this context, our study has contributed to growing understanding of joint global change stressors on a broadly distributed African seagrass species (Z. capensis), by quantifying impacts of elevated temperature and nutrient enrichment on its physiological performance. Our results demonstrate significant effects of temperature and nutrient addition on Z. capensis growth and morphology, but that increasing nutrient levels is a more significant stressor than temperature increases. Evidence for this emanates from 14 of the 20 seagrass response variables being significantly affected by nutrient enrichment compared to 8 variables being affected by warming. Further support arises from estimates of variance explained by treatments (Table 1), which demonstrated stronger effects of nutrient addition for 13 response variables, but only two cases showed greater effects for temperature. In addition, interactions between nutrient enrichment and warming were significant for only two response variables, but even in these cases, nutrient enrichment effects were stronger. Our findings therefore suggest that of the stressors tested and their ranges used in the experiment, nutrient enrichment elicits stronger effects on Z. capensis performance and that temperature plays a secondary role. Our results also suggest that interactions are likely to be ancillary to main stressor effects, with nutrients and temperature not acting additively or synergistically to impact Z. capensis physiology. Our findings, however, must be contextualised against the high nutrient levels used in our experiment, which are potentially at the upper end of nutrient values measured in estuaries locally. In the Western Cape of South Africa, our monitoring data indicate nutrient levels as high as 12.62mg/L (PO4) and 16.31mg (NH4), which are in line with mean values used in the experiment. PO4 levels for example ranged between 1.1 and 7.7mg/L across nutrient treatments, while values for NH4 ranged between 9.6 and 12.9mg/L.