Table_6_Unusually Warm Summer Temperatures Exacerbate Population and Plant Level Response of Posidonia oceanica to Anthropogenic Nutrient Stress.docx

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.

波西多尼亚海草（Posidonia oceanica）是地中海地区的关键基础物种，能够提供极具价值的生态系统服务。然而该物种对沿海高营养盐输入以及夏季强热浪频发的态势尤为敏感脆弱，但二者的原位（in situ）联合影响迄今尚未得到充分关注。本研究在异常暖夏期间开展了为期4个月的原位营养盐添加实验，对比了保护区（伊斯基亚岛）与已遭受显著人为扰动的海域（波佐利湾巴亚区域）的海草床，分析其多项形态、生理及生化种群指标。本研究表明，若海草已处于较高人为扰动压力的环境中，夏季温度异常升高对海草床种群水平的负面影响将会被进一步加剧。形态学与种群水平指标主要随时间推移发生变化，可能受季节与高温影响，且在受扰动区域的海草种群中呈现出更为显著的下降态势。额外的营养盐输入则带来了更为严重的危害：高扰动区域的施肥样地海草覆盖度下降约67%，低扰动区域则下降33%。此外，低扰动区域海草的根茎淀粉浓度呈现季节性上升趋势，但高扰动区域的施肥样地中，该指标却表现出27%的下降趋势。受扰动区域海草叶片的附生生物量约为保护区的4倍，且在额外添加营养盐后进一步翻倍。鉴于波西多尼亚海草的定植与繁殖能力有限、生长速度极慢，对其胁迫状态进行精准预判与早期预警，对于这一生态系统工程师的保护与管理工作至关重要。本研究结果提示，监测工作应重点关注叶面积指数（LAI）、根茎碳水化合物浓度以及叶片附生覆盖度这三项指标，以此作为波西多尼亚海草胁迫发生的预警信号。决策者可借助这些指标，在生态系统遭受不可逆破坏前采取适宜措施，最大限度降低未来人类活动的干扰，增强这类重要海洋生态系统的恢复力。