Effects of multiple disturbances in seagrass meadows: shading decreases resilience to grazing

Abstract

Abstract. Ecosystem shifts are often associated with multiple disturbances, but limited knowledge on the mechanisms involved hampers management. This study investigated how short-term shading affected the resilience of the seagrass Halophila ovalis to grazing by black swans (Cygnus atratus) -a historically dominant grazer currently recovering from hunting and habitat loss -in south-western Australian estuaries, using field surveys and a grazing-shading experiment. Black swans were heterogeneously distributed and, in a high-density site, consumed 23% of seagrass production. Seagrasses recovered rapidly from a single disturbance (i.e. short-term shading or grazing), even though shading alone halved carbohydrate content. When seagrasses were exposed to both disturbances, recovery depended on the type of grazing; where grazing was confined to leaves, leaf densities recovered within 3 weeks, but where grazing was on both leaves and rhizomes there was no recovery. Shading increased the frequency of apex initiation, as did rhizome grazing, but only if the plants had not been shaded. This indicates that shading alters the flow of energy needed to produce apices and leaves following disturbance on rhizomes. Based on the historical swan densities and continuing recurring phytoplankton blooms, management actions reintroducing swans without controlling algal blooms could have an on impact seagrass resilience and associated organisms.

Unravelling complexity in seagrass systems for management: Australia as a microcosm

“…Direct grazing or habitat to promote biodiversity • Colonising and opportunistic species that form transitory meadows are often a preferred food source of direct grazers of seagrass such as dugongs and turtles (Aragones and Marsh, 2000;Lanyon et al, 1989;Lanyon, 1991;Preen, 1995) or waterfowl (Choney et al, 2014;Eklöf et al, 2009;Jacobs et al, 1981;Nienhuis and Groenendijk, 1986). • Dugong grazing on these species can remove up to 77% of daily carbon production (McMahon, 2005) and waterfowl grazing up to 50% of seagrass production (Choney et al, 2014;Eklöf et al, 2009;Jacobs et al, 1981;Nienhuis and Groenendijk, 1986).…”

“…• Dugong grazing on these species can remove up to 77% of daily carbon production (McMahon, 2005) and waterfowl grazing up to 50% of seagrass production (Choney et al, 2014;Eklöf et al, 2009;Jacobs et al, 1981;Nienhuis and Groenendijk, 1986). • In addition many micro and macro fauna live in the habitats created by the seagrass.…”

A framework for the resilience of seagrass ecosystems

“…For example, persistent poor water quality, low lightlimited photosynthetic carbon fixation (via photosynthesis) and low energetic surpluses (low production/respiration), commonly found in coastal environments reduce the capacity of a plant to store energy reserves, thus limiting its ability to resist or recover from further stress (Eklof et al, 2009). These factors can act synergistically creating an impact that is greater than the sum of individual stressors, resulting in reduced resilience systems ) (see Fig.…”

“…Carbohydrate stores vary as a function of space and time and reflect the recent energetic balance of the plant. Disturbances that result in the depletion of carbohydrate reserves make seagrass meadows more vulnerable to additional stress (Eklof et al, 2009).…”

Effect of environmental stress on non-structural carbohydrates reserves and transfer in seagrasses

“…9% 增加到 7% [13] 。 因此, 人类活动和全球气候变化已对海草床造成巨大威胁。 碳在植物体内的存在形式可分为结构性碳和非结构性碳两种。 前者( 如木质素、纤维素) 主要用于支持 植物体的形态构建;后者( 如葡萄糖、果糖、蔗糖、果聚糖、淀粉) 则是参与植株生命代谢的重要物质 [14] 。 非结 构性碳水化合物在植株体内的代谢对植株的生长有重要的影响 [15] 。 当海草整体植株的碳需求超过光合固定 碳时,海草根茎的非结构性碳水化合物就作为碳源和能源被利用,以支持海草的生长 [16] [20] 、虾海藻( Phyllospadix torreyi) [21] 、羽叶二药藻( Halodule pinifolia) 和根枝草( Amphibolis griffithii) [22] 等。 值得注意的是,二药藻( Halodule uninervis) 地上组织的非结构 性碳水化合物含量远高于地下组织的含量 [23] [24] 和大叶藻( Zostera marina) ( 美国太平洋海岸) 的淀粉所占的比例低于 5% [17] 。 同样,西班牙加 的斯海湾诺氏大叶藻( Zostera noltii) 的淀粉含量甚至比蔗糖的含量低一个数量级 [25鄄 26] 。 然而,Burke 等 [19] 却 发现,切萨皮克海湾的大叶藻淀粉含量高达 140 mg / g 干重,占非结构性碳水化合物的比例超过 65% ;而且,喜 盐草( Halophila ovalis) 叶片和根的淀粉所占的比例甚至高达 90% [27] 。 另外,在海草快速生长阶段,其可溶糖 的含量相对淀粉较高,表明海草非结构性碳水化合物的组成也会受到生长状态的影响 [28] 。 [29] 、美国加利福尼亚湾的大叶藻 [30] 、澳大利亚西南部海域的喜盐草 [31鄄 32] 及其西班牙里加特海湾的 波喜荡草( Posidonia oceanica) [16] ,其地下根茎的非结构性碳在夏季增加,在冬季和早春却减少。 这可能是由 于夏季光照强度较其他季节大,海草光合作用速率比较大,可储存更多的非结构性碳水化合物;而在冬季和早 春,地下根茎的非结构性碳水化合物却分别需用于维持海草的代谢及其支持叶片早期的生长。 然而,切萨皮 克海湾的大叶藻却在春季、秋季生长最旺盛的时候储存非结构性碳水化合物 [19] 。 2摇 环境胁迫对海草非结构性碳水化合物储存和转移的影响 2. 1摇 光强 海草的地理分布和生产力很大程度上受到光的影响 [35] 。 光限制条件下,海草不能维持正的碳平衡,其组 织中蔗糖和淀粉就会被利用以满足植株碳需求 [36] ,甚至可能会消耗地下组织的生长来支持叶的生长 [37] 。 例 如,光限制使喜盐草的叶片可溶糖在 3 d 内降低 50% [31] ,也使波喜荡草地下茎的可溶糖下降 32% -52% [38] 。 Olesen 等 [39] 研究发现,低光强条件下叶子的生长得到维持,而根茎的重量却减少了,这可能是由于地下根茎 的非结构性碳水化合物转移到叶子引起的。 而且海草地下根在光限制引起缺氧的沉积物里,需要有足够的非 结构性碳水化合物来支持夜间的厌氧代谢,这进一步降低非结构性碳水化合物的含量 [40] 。 同样,诺氏大叶藻 [18] ,其转移对于海草的生存很重要 [35] 。 另一方面,海草对光限制的响应程度与海草自身非结构性碳水化合物的季节性变化有关 [1] 。 例如,光限 制对波喜荡草的影响程度在春末夏初比在秋冬大,这是因为在夏季后期,波喜荡草储存大量的非结构性碳水 化合物,可满足秋冬季节的碳代谢,降低秋冬季节时光限制带来的不利影响 [16] 。 因此,考虑到非结构碳水化 合物的重要性及其季节变化规律,非结构性碳水化合物重新储存所需要的时间决定了海草对重复光限制的耐 受能力 [38] 。 而且那些地下茎储存较少非结构性碳水化合物的海草种,更容易受到光限制的胁迫 [35] 。 例如, 光限制对大叶藻属的 Zostera muelleri 的不利影响,比对泰来藻和齿叶海神草的影响大。 这是因为大叶藻属的 Zostera muelleri 地下茎的非结构性碳水化合物较少,较快被消耗掉,需要改变其体内结构来平衡代谢过程 …”