Why so Little Planktonic Nitrogen Fixation in Estuaries-- Limitation by Trace Metals and Grazing?

9307337豪沃思在温带，初级生产力是有限的氮在许多，如果不是大多数河口和磷在大多数湖泊。 这些差异的一个主要原因是固氮蓝藻（蓝绿藻）的差异。 在湖泊中，氮相对于磷的暂时短缺导致蓝藻的大量繁殖，蓝藻固定氮并有助于缓解短缺。 这不会发生在大多数河口，其中能够固氮的浮游蓝藻是罕见的，固氮率低。 这里提出的工作解决了这一差异与一系列的围隔实验使用水从纳拉甘塞特湾。 该提案的核心假设是，河口铁和/或钼的低可用性导致反硝化、固氮蓝藻的生长速度缓慢。 这使得蓝细菌容易被一般的放牧动物消耗，导致低种群数量和固氮率。 该假说并没有说明河口的食草动物专门以蓝藻为食。 相反，据推测，河口生态系统可能包含大量的广义食草动物，能够喂养细蓝藻细丝，并可能这样做，顺便说一句，他们对其他颗粒的喂养时，这样的细丝存在。 浮游动物、栉水母和各种双壳类（贻贝、牡蛎、蛤）是河口常见的滤食性动物的例子，它们可能以蓝藻丝状物为食。 在足够高的生长速度下，蓝藻可能会开花并固定氮，即使在高放牧率下，就像淡水中可能发生的那样。 铁和/或钼的可用性被视为核心的蓝细菌的生长速度的基础上，以前的研究，在固氮的要求都很高。 %中围实验构成了拟议研究的核心。 河口水和浮游生物组合将受到不同的添加铁，钼，磷和放牧水平将通过增加捕食者对草食性浮游动物的控制。 银汉鱼（Menidia beryllina）可能是使用的捕食者。 在重复实验中，金属添加物与鱼添加物的相互作用特别令人感兴趣。 要测量的响应变量包括固氮率，丰富的蓝藻和异形胞，浮游动物群落的组成，同化，Molimdium，苏丹和铁，和营养物质的浓度，Molibdium，和铁。 最近发表的方法，使用8-羟基喹啉复杂的不稳定形式的铁将被用来估计铁的浮游植物的可用性。 拟议的研究应导致更好地了解的因素，排除固氮蓝藻的浮游生物的大多数温带河口和沿海海域，有助于导致氮限制这些系统中的净初级生产。 这项研究还应导致更好地了解湖泊中浮游生物固氮的控制，因此，即使在接受低氮磷营养负荷的湖泊中，也要维持磷限制。 因此，研究涉及到管理策略，以控制河口和湖泊富营养化的实际问题。

9307337 Howarth In the temperate zone, primary production is limited by Nitrogen in many if not most estuaries and by Phosphorus in most lakes. One major reason for these difference is a difference in Nitrogen fixation by planktonic cyanobacteria (blue-green algae). In lakes, temporary shortages of Nitrogen relative to Phosphorus result in blooms of cyanobacteria which fix Nitrogen and help alleviate the shortage. This does not occur in most estuaries, where planktonic cyanobacteria capable of Nitrogen fixation are rare and rates of Nitrogen fixation low. The work proposed here addresses this difference with a series of mesocosm experiments using water from Narragansett Bay. The central hypothesis of the proposal is that low availability's of Iron and/or Molibdium in estuaries lead to slow growth rates of planktonic, Nitrogen-fixing cyanobacteria. This leaves the cyanobacteria vulnerable to consumption by generalized grazing animals, leading to low population numbers and rates of Nitrogen fixation. The hypothesis does not state that herbivores in estuaries specifically feed on cyanobacteria. Rather, it is hypothesized that estuarine ecosystems may contain high numbers of generalized grazers which are capable of feeding on fine cyanobacterial filaments and are likely to do so incidentally to their feeding on other particles when such filaments are present. Zooplankton, ctenophores, and a variety of bivalves (mussels, oysters, clams) are examples of such filter feeding animals common in estuaries which may fed on cyanobacterial filaments. With sufficiently high growth rates, cyanobacteria might bloom and fix Nitrogen even with high rates of grazing, as can happen in freshwater. Iron and/or Molibdium availability's are viewed as central to the growth rate of cyanobacteria based on previous research; the requirement for both in Nitrogen fixation is high. %%% Mesocosm experiments form the core of the proposed research. Estuarine water and plankton assemblages will be subje ct to different additions of Iron, Molibdium, and Phosphorus, and levels of grazing will be controlled by adding predators on herbivorous zooplankton. Silversides (Menidia beryllina) will likely be the predator used. Interactions of metal additions with the fish additions in replicated experiments are of particular interest. Response variables to be measured include rates of Nitrogen fixation, abundance's of cyanobacteria and heterocysts, composition of the zooplankton community, assimilation, of Molimdium, Sultan, and Iron, and concentrations of nutrients, Molibdium, and Iron. A recently published method using oxine to complex labile forms of Iron will be used to estimate Iron availability to phytoplankton. The proposed research should lead to a better understanding of the factors which exclude Nitrogen-fixing cyanobacteria from the plankton of most temperate estuaries and coastal seas, helping to cause Nitrogen limitation of net primary production in these systems. The research should also lead to a better understanding of the controls on Nitrogen fixation by plankton in lakes, and therefore on the maintenance of Phosphorus limitation even in lakes receiving nutrient loading with low Nitrogen:Phosphorus. Thus, the research relates to the practical issue of management strategies to control eutrophication in both estuaries and lakes.