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

为什么河口的浮游固氮如此之少——微量金属和放牧的限制？

• 批准号: 9307337
• 负责人: Robert Howarth
• 金额: $ 30万
• 依托单位: Cornell Univ - State: AWDS MADE PRIOR MAY 2010
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1993
• 资助国家: 美国
• 起止时间: 1993-11-01 至 1996-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9307337&HistoricalAwards=false
• 关键词: Why; so; Little; Planktonic; Nitrogen

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.

9307337 Howarth 在温带地区，许多（如果不是大多数）河口的初级生产受到氮的限制，大多数湖泊的初级生产受到磷的限制。 造成这些差异的一个主要原因是浮游蓝藻（蓝绿藻）固氮的差异。 在湖泊中，氮相对于磷的暂时短缺会导致蓝藻大量繁殖，从而固定氮并有助于缓解短缺。 这种情况在大多数河口不会发生，因为能够固氮的浮游蓝藻很少，而且固氮率很低。 这里提出的工作通过使用纳拉甘西特湾的水进行一系列中生态实验来解决这一差异。 该提案的中心假设是，河口铁和/或钼的可用性较低，导致浮游固氮蓝藻生长缓慢。 这使得蓝藻很容易被普遍的放牧动物消耗，导致种群数量和固氮率较低。 该假设并未指出河口的食草动物专门以蓝藻为食。 相反，据推测，河口生态系统可能含有大量的广义食草动物，它们能够以细小的蓝藻丝为食，并且当存在此类丝时，它们很可能顺便以其他颗粒为食。 浮游动物、栉水母和各种双壳类动物（贻贝、牡蛎、蛤）是河口常见的滤食动物的例子，它们可以以蓝藻丝为食。 如果生长速度足够高，即使在高放牧率的情况下，蓝藻也可能会大量繁殖并固氮，就像淡水中那样。 根据之前的研究，铁和/或钼的可用性被认为是蓝细菌生长速度的核心；两者固氮要求都很高。 %%%中宇宙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程度的铁、钼和磷添加的影响，放牧水平将通过添加食草浮游动物的捕食者来控制。 银獾（Menidia beryllina）很可能是所使用的捕食者。 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速率、蓝细菌和异形藻的丰度、浮游动物群落的组成、钼、苏丹和铁的同化以及营养物、钼和铁的浓度。 最近发表的一种使用 oxine 复合不稳定形式铁的方法将用于估计浮游植物的铁利用率。 拟议的研究应有助于更好地了解将固氮蓝藻排除在大多数温带河口和沿海海域浮游生物之外的因素，从而有助于导致这些系统中净初级生产的氮限制。 该研究还应有助于更好地了解湖泊中浮游生物固氮的控制，从而更好地了解磷限制的维持，即使在接受低氮磷养分负荷的湖泊中也是如此。 因此，该研究涉及控制河口和湖泊富营养化的管理策略的实际问题。