Diatom Blooms and Planktonic Grazers: Paradigm or Paradox?

浮游植物华是温带至亚极地沿海海域中上层环境的主要事件。因为水华是食物资源的大而相对可靠的季节性脉冲，许多远洋消费种群的生活史，无论是浮游食草动物还是更高营养水平的类型，如鱼类，似乎都与它们同步。然而，这里有一个明显的悖论。最近许多实验室研究的结果表明，某些种类的硅藻，包括在浮游植物繁殖期间可能占生物量优势的种类，可以抑制几种浮游悬浮液食性桡足类动物的产卵和/或胚胎和胚胎后发育。这种抑制作用的具体机制目前还在争论中。硅藻可能缺乏营养，但它们也含有有毒物质，对桡足类动物产生有害影响。然而，这些实验室研究与桡足类自然种群的普遍相关性是不确定的，因为对硅藻华影响的实地调查产生了相互矛盾的结果。本研究项目的目的是在海洋中验证以硅藻为主的浮游植物大量繁殖对浮游悬浮性桡足类（Calanus pacificus和pseudoalanus newmani）自然种群的产卵率和/或胚胎后发育有抑制作用的假设。这项研究将在达布湾（华盛顿州）进行，那里三月份以硅藻为主的浮游植物大量繁殖。在这些水华期间，硅藻浓度达到或超过实验室研究中已知的对桡足类动物有害的水平。以前在其他地方对硅藻华对桡足类动物的影响的实地研究没有达到这一条件。为了验证这一假设，提出了以下几种方法：(1)观察桡足类、硅藻和其他潜在猎物的分布和丰度；(2)对成年桡足类动物的粪便颗粒进行分析，以证实它们正在就地摄入硅藻；(3)提供充分控制和复制的实验，以确定成年桡足类对硅藻和其他猎物的摄食率；(4)孵化，估算两种桡足动物的原位产卵率和孵化成功率；(5)试验考察硅藻对两种桡足动物的产卵率、孵化成功率和胚胎后发育的直接影响。硅藻可能影响桡足类等后生食草动物的可能性具有巨大的基础和应用科学兴趣。在基本层面上，这个问题对于理解海洋浮游生物系统的结构和功能至关重要。浮游植物抑制食草浮游动物繁殖或发育的任何过程，都可能显著影响远洋食物网，远远超出初级消费者的水平。也就是说，影响食草动物种群反应的过程将通过食物网产生分支，直接影响其他消费物种，这些物种的繁殖和生长依赖于食草动物卵的发生和丰度以及未成熟阶段。因此，了解硅藻是否对桡足类的自然种群有害是至关重要的，如果是，影响的具体性质，发生的频率，以及对更高营养水平的影响是什么。在实际层面上，从海洋中可持续捕捞鱼类的规模最终不仅取决于初级生产的水平，还取决于这种生产如何（或不）通过食物网转移到更高（可收获）的营养水平。因此，对硅藻华对浮游食草动物自然种群影响的明确测试可能代表着对远洋生物海洋学理解的重大进步。

Phytoplankton blooms are major events in the pelagic environment of temperate to subpolar coastal seas. Because blooms are large and relatively reliable seasonal pulses of food resources, the life histories of many pelagic consumer populations, both planktonic grazers and higher trophic level types such as fish, appear synchronized with them. Yet there is an apparent paradox. Results from numerous recent laboratory studies indicate that some species of diatoms, including species that may be biomass dominants during phytoplankton blooms, can inhibit egg production and/or embryonic and postembryonic development of several species of planktonic suspension-feeding copepods. The specific mechanism underlying the inhibition is currently under debate. Diatoms may be nutritionally deficient, but they also contain toxic substances that induce deleterious effects in copepods. However, the general relevance of these laboratory studies for natural populations of copepods is uncertain, as field investigations of the effects of diatom blooms have yielded contradictory results.The goal of this research project is to test in the sea the hypothesis that diatom-dominated phytoplankton blooms have inhibitory effects on egg production rate and/or postembryonic development of natural populations of the planktonic suspension-feeding copepods Calanus pacificus and Pseudocalanus newmani. The study will be done in Dabob Bay (Washington State) where intense, diatom-dominated phytoplankton blooms occur reliably in March. During these blooms, diatom concentrations reach or exceed levels known from laboratory studies to have deleterious effects on copepods. Previous field studies elsewhere of effects of diatom blooms on copepods have not met this condition. To test the hypothesis a combination of approaches is proposed: (1) observations on the distribution and abundance of copepods, diatoms, and other potential prey; (2) analysis of fecal pellets of adult copepods to verify that they are ingesting diatoms in situ; (3) experiments providing adequate control and replication to determine feeding rates of adult copepods on diatoms and other prey; (4) incubations to estimate in situ egg production rate and hatching success of both copepod species; (5) experiments to test for the direct effects of diatoms on egg production rate and hatching success and postembryonic development of the two copepod species. The possibility that diatoms may impact metazoan grazers such as copepods is of enormous basic and applied scientific interest. At a basic level, the problem is central to understanding the structure and function of marine planktonic systems. Any process by which phytoplankton inhibit reproduction or development of grazing zooplankton could markedly affect the pelagic food web well beyond the level of primary consumers. That is, a process affecting the population response of grazers would ramify through the food web, directly impacting other consumer species whose reproduction and growth depend on occurrence and abundance of eggs and immature stages of grazers. Thus, it is crucial to know whether diatoms are deleterious to natural populations of copepods and, if so, the specific nature of the effect, how frequently it occurs, and what the implications are for higher trophic levels.On a practical level, the magnitude of sustainable harvest of fish from the ocean is ultimately dependent not just on the level of primary production, but how that production is (or is not) transferred through the food web to higher (harvestable) trophic levels. An unambiguous test of the effects of diatom blooms on natural populations of planktonic grazers could therefore represent a major advance in understanding for pelagic biological oceanography.