Landscape-level variation in disease susceptibility related to shallow-water hypoxia

Diel-cycling hypoxia is widespread in shallow portions of estuaries and lagoons, especially in systems with high nutrient loads resulting from human activities. Far less is known about the effects of this form of hypoxia than deeper-water seasonal or persistent low dissolved oxygen. We examined field patterns of diel-cycling hypoxia and used field and laboratory experiments to test its effects on acquisition and progression of Perkinsus marinus infections in the eastern oyster, Crassostrea virginica, as well as on oyster growth and filtration. P. marinus infections cause the disease known as Dermo, have been responsible for declines in oyster populations, and have limited success of oyster restoration efforts. The severity of diel-cycling hypoxia varied among shallow monitored sites in Chesapeake Bay, and average daily minimum dissolved oxygen was positively correlated with average daily minimum pH. In both field and laboratory experiments, diel-cycling hypoxia increased acquisition and progression of infections, with stronger results found for younger (1-year-old) than older (2-3-year-old) oysters, and more pronounced effects on both infections and growth found in the field than in the laboratory. Filtration by oysters was reduced during brief periods of exposure to severe hypoxia. This should have reduced exposure to waterborne P. marinus, and contributed to the negative relationship found between hypoxia frequency and oyster growth. Negative effects of hypoxia on the host immune response is, therefore, the likely mechanism leading to elevated infections in oysters exposed to hypoxia relative to control treatments. Because there is considerable spatial variation in the frequency and severity of hypoxia, diel-cycling hypoxia may contribute to landscape-level spatial variation in disease dynamics within and among estuarine systems.

昼夜循环低氧（diel-cycling hypoxia）广泛分布于河口与潟湖的浅水区，尤其常见于受人类活动影响导致营养盐负荷较高的水域系统。相较于深水环境中的季节性或持续性低溶解氧，学界对这类低氧胁迫的生态效应所知甚少。本研究首先调研了昼夜循环低氧的野外分布规律，随后通过野外原位实验与室内受控实验，探究其对美洲牡蛎（Crassostrea virginica）感染帕金虫（Perkinsus marinus）的初始获得与病程进展的影响，同时分析其对牡蛎生长与滤水行为的作用。帕金虫感染会引发被称为“Dermo病”的病害，该病害曾导致牡蛎种群数量大幅下降，并限制了牡蛎修复工程的实施成效。切萨皮克湾（Chesapeake Bay）各监测浅水区的昼夜循环低氧严重程度存在显著差异，且日均最低溶解氧浓度与日均最低pH值呈正相关关系。野外与室内实验均表明，昼夜循环低氧会促进帕金虫的感染获得与病程进展；其中对1龄幼牡蛎的影响显著强于2-3龄成牡蛎，且野外实验中该低氧胁迫对感染与生长的影响均比室内实验更为显著。牡蛎在短期暴露于严重低氧环境时，其滤水率会出现下降。这一效应反而促成了低氧发生频率与牡蛎生长之间的负相关关联。因此，低氧胁迫对宿主免疫反应的抑制作用，可能是暴露于低氧环境的牡蛎比对照组感染率更高的核心机制。由于低氧的发生频率与严重程度存在显著的空间异质性，昼夜循环低氧可能会导致河口系统内部及不同河口系统之间的病害流行呈现出景观尺度的空间差异。