Diatom Blooms and Planktonic Grazers: Paradigm or Paradox?

浮游植物水华是温带至副极地沿海海洋中上层环境中的重大事件。由于水华是食物资源的大而相对可靠的季节性脉冲，许多远洋消费者种群的生活史，包括浮游食草动物和较高营养级类型，如鱼类，似乎与水华同步。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，硅藻的一些物种，包括在浮游植物大量繁殖期间可能是生物量占优势的物种，可以抑制产卵和/或胚胎和胚后发育的几个物种的浮游悬浮液喂养桡足类。抑制的具体机制目前仍在讨论中。硅藻可能缺乏营养，但它们也含有有毒物质，对桡足类产生有害影响。然而，这些实验室研究对桡足类自然种群的一般相关性是不确定的，由于硅藻水华影响的实地调查产生了相互矛盾的结果。本研究项目的目标是在海洋中测试以下假设，即浮游植物占主导地位的浮游植物水华对产卵率和/或浮游悬浮液自然种群的胚后发育具有抑制作用-摄食桡足类太平洋哲水蚤和纽氏拟哲水蚤。这项研究将在Dabob湾（华盛顿州）进行，那里的浮游植物在三月份会大量繁殖。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产生有害影响的水平。以前在其他地方进行的硅藻水华对桡足类影响的实地研究没有满足这一条件。为了验证这一假说，提出了一种组合的方法：（1）观察桡足类、硅藻和其他潜在猎物的分布和丰度;（2）分析成年桡足类的粪便颗粒，以证实它们是原位的摄食硅藻;（3）提供足够的控制和复制实验，以确定成年桡足类对硅藻和其他猎物的摄食率;（5）硅藻对两种桡足类的产卵率、孵化率和胚后发育的直接影响。硅藻可能影响后生动物食草动物如桡足类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生态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对远洋食物网产生远远超出初级消费者水平的显著影响。也就是说，一个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取决于食草动物卵的出现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，如果是的话，这种影响的具体性质、发生的频率以及对更高营养级的影响是什么。在实际层面上，海洋鱼类的可持续捕捞量最终不仅取决于初级生产水平，而是生产如何通过食物网转移到（或不转移到）更高的（可收获的）营养水平。因此，对硅藻水华对浮游食草动物自然种群的影响进行明确的测试，可能是对远洋生物海洋学认识的一个重大进展。

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.