Unusually Warm Summer Temperatures Exacerbate Population and Plant Level Response of Posidonia oceanica to Anthropogenic Nutrient Stress

Abstract

Posidonia oceanica is a key foundation species in the Mediterranean providing valuable ecosystem services. However, this species is particularly vulnerable towards high coastal nutrient inputs and the rising frequency of intense summer heat waves, but their combined effect in situ has received little attention so far. Here, we investigated the effects of in situ nutrient addition during an unusually warm summer over a 4-month period, comparing different morphological, physiological and biochemical population metrics of seagrass meadows growing in protected areas (Ischia) with meadows already exposed to significant anthropogenic pressure (Baia – Gulf of Pozzuoli). Our study highlights that the effects of warmer than usual summer temperatures on the population level of seagrass meadows can be exacerbated if the plants are already exposed to higher anthropogenic pressures. Morphological and population level indicators mainly changed over time, possibly impacted by season and the warmer temperatures, and displayed more pronounced reductions in seagrasses from impacted sites. The additional nutrient supply had even more deleterious effects, as shown by a decrease in approximately 67% in cover in fertilized plots at high impacted sites and 33% at low impacted sites. Moreover, while rhizome starch concentration showed a seasonal increase in plants from low impacted sites it displayed a trend of a 27% decrease in fertilized plots of the high impacted sites. Epiphyte biomass was approximately four-fold higher on leaves of plants growing in impacted sites and even doubled with the additional nutrient input. Predicting and anticipating stress in P. oceanica is of crucial importance for conservation and management efforts, given the limited colonizing and reproductive ability and extremely slow growth of this ecosystem engineer. Our results suggest that monitoring efforts should focus especially on leaf area index (LAI), carbohydrate concentrations in the rhizomes, and epiphyte cover on leaves as indicators of the onset of stress in Posidonia oceanica, which can be used by decision makers to take appropriate measures before damage to the ecosystem becomes irreversible, minimize future human interference and strengthen the resilience of these important ecosystems.

Nutrient History Affects the Response and Resilience of the Tropical Seagrass Halophila stipulacea to Further Enrichment in Its Native Habitat

“…Despite these limitations, our approach was successful for highlighting the different responses to eutrophication among H. stipulacea meadows growing under different levels of anthropogenic pressures ( Mejia et al, 2016 ; Winters et al, 2017 ), while enduring summer temperatures that were higher than normal, and which potentially surpassed the threshold for H. stipulacea ’s optimal growth ( Nguyen et al, 2020 ). Our findings provide a good example of the response of a fast-growing opportunistic seagrass species undergoing various levels of local anthropogenic stressors and clearly show the difference to responses of a large-bodied, slow growing seagrass species in a previous study ( Helber et al, 2021 ).…”

“…It was shown in recent mesocosm and field experiments ( Pazzaglia et al, 2020 ; Helber et al, 2021 ) that eutrophication and higher temperatures will have detrimental effects on P. oceanica , the dominating key foundation seagrass species in the Mediterranean. Nitrogen load thresholds for this endemic seagrass were identified to range between 0.8 and 1.1 t N km –2 over 6 months ( Fernandes et al, 2019 ).…”

“…Nitrogen load thresholds for this endemic seagrass were identified to range between 0.8 and 1.1 t N km –2 over 6 months ( Fernandes et al, 2019 ). Especially in areas where plants were already exposed to higher anthropogenic pressures, P. oceanica activated strategies to cope with the additional nutrient input that probably were energetically costly as indicated by cutbacks in their carbohydrate reserves ( Pazzaglia et al, 2020 ; Helber et al, 2021 ). In contrast, H. stipulacea populations from NB in the current study seem to thrive under the additional nutrient supply, similar to invasive populations in the Caribbean ( Willette et al, 2014 ; van Tussenbroek et al, 2016 ; Winters et al, 2020 ).…”

“…of leaves, internodal distance) and biochemical (tissue nutrient content and carbohydrates) plant traits identified as potential stress response indicators ( Roca et al, 2016 ). This study is part 2 of a parallel study on Posidonia oceanica in the Mediterranean ( Helber et al, 2021 ). Through both of these studies, we aim to compare responses of H. stipulacea , a fast-growing, small-bodied seagrass, to P. oceanica , a slow-growing, large-bodied seagrass, in order to better understand how these very different seagrasses may respond under future nutrient and climate scenarios and potentially interact in areas where H. stipulacea occurs within P. oceanica meadows ( Beca-Carretero et al, 2020b ).…”

Eutrophication decreases Halophila beccarii plant organic carbon contribution to sequestration potential

“…Coastal nutrient loading is a leading threat to seagrass meadow decline (Jiang et al, 2020), which is mainly due to algal overgrowth and reduction of light availability, as well as ammonium toxicity (Burkholder et al, 2007). Meanwhile, nutrient loading can affect the carbon metabolism in seagrass plants (Touchette and Burkholder, 2000), and consequently change the organic carbon composition of seagrass plants (Pazzaglia et al, 2020;Helber et al, 2021a). Nutrient addition can always increase the content of amino acids in seagrass tissues (Udy and Dennison, 1997;Invers et al, 2004), while the response of sucrose content in seagrass to nutritional loading shows variation.…”