ABSTRACT: Quantifying trophic relationships among basal food sources and primary consumers is essential to our understanding of food web structure and dynamics, but is challenging in soft shore systems where the variety of potential food sources often masks the actual pathways of energy flow. To elucidate trophic relationships among basal food sources and primary consumers, we assessed the diet composition of a deposit-feeding crab, Scopimera intermedia, an abundant primary consumer inhabiting tropical shores, using direct enumeration and stable isotope techniques. Stable isotope analyses revealed that meiofauna accounted for the majority of the crab’s diet (nematodes reaching 43% in median proportion) compared to, at most, 11% contributed by microphytobenthos (MPB). Although the abundances of MPB and meiofauna varied, meiofauna dominated over MPB in the crab’s diet at different sites and seasons. Such an important role of meiofauna in consumer diets contrasts with previous studies on sediment food webs, where MPB has often been proposed as the most plausible primary food source. The relative roles of meiofauna and MPB as food sources are, however, likely to be mediated by local physical environments. Sediment particle size, for example, is more coarse higher on the shore, where the larger interstitial spaces enable colonization of meiofauna but will limit large, motile MPB due to increased photostress. The trophic importance of commonly assumed primary food sources may thus be modified by environmental physical stress and, as a result, the role of intermediate trophic links in modulating food web dynamics should not be neglected.

中文翻译：

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沉积物特性的时空变化揭示了沙质海岸食物网中以食蟹为食的营养链缺失

摘要：量化基础食物来源与主要消费者之间的营养关系对于我们了解食物网的结构和动力学至关重要，但在软岸系统中却极具挑战性，因为潜在食物来源的多样性常常掩盖了能量流动的实际路径。为了阐明基本食物来源与主要消费者之间的营养关系，我们评估了以定居食蟹为食的中间S鱼（Scopimera intermedia）的饮食组成是使用直接枚举和稳定同位素技术居住在热带海岸的大量主要消费者。稳定的同位素分析表明，淡水动植物是蟹类食物的主要来源（线虫的中位比例达到43％），而微型底栖动物（MPB）最多贡献了11％。尽管MPB和藻类动物的丰富度各不相同，但在不同地点和季节，蟹类饮食中藻类动物比MPB占主导地位。鱼类动植物在消费者饮食中的这种重要作用与以前在沉积物食物网上的研究形成了鲜明对比，在沉积物食物网上，MPB通常被认为是最合理的主要食物来源。然而，meiofauna和MPB作为食物来源的相对作用很可能是由当地的自然环境所介导的。例如，在海岸上，沉积物的粒径更大，更粗糙，较大的间隙空间可以使藻类动物定居，但由于光胁迫的增加，它将限制大型的活动性MPB。因此，通常假定的主要食物来源的营养重要性可能会因环境物理压力而改变，因此，不应忽略中间营养环节在调节食物网动态中的作用。