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

9307337然而，在温带地区，初级生产力在许多河口(如果不是大多数河口)受到氮的限制，在大多数湖泊受到磷的限制。造成这些差异的一个主要原因是浮游蓝藻(蓝藻)在固氮方面的差异。在湖泊中，氮相对于磷的暂时短缺导致蓝藻大量繁殖，蓝藻固定了氮，有助于缓解这种短缺。这不会发生在大多数河口，那里的浮游蓝藻能够固氮很少，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的一系列中观实验来解决这一差异。该提案的中心假设是，河口铁和/或钼的低可获得性导致浮游、固氮蓝藻的生长速度缓慢。这使得蓝藻容易被一般放牧动物消耗，导致种群数量和固氮率较低。这一假说并没有说明河口的食草动物专门以蓝藻为食。相反，假设河口生态系统可能包含大量能够以细小的蓝藻细丝为食的广义食草动物，当这些细丝存在时，它们很可能附带地以其他颗粒为食而这样做。浮游动物、鞭毛虫和各种双壳类(贻贝、牡蛎、文蛤)是这种滤食性动物的例子，这种滤食性动物常见于可能以蓝藻细丝为食的河口。有了足够高的生长速度，蓝藻可能会繁殖并固定氮，即使在高放牧率的情况下也是如此，就像在淡水中发生的那样。根据以前的研究，铁和/或钼的有效性被认为是蓝藻生长速度的核心；在固氮方面，对两者的要求都很高。中胚层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铁、钼和磷的不同添加的影响，放牧水平将通过增加食草性浮游动物的捕食者来控制。银鱼很可能是它们的捕食者。在重复实验中，金属添加与FISH添加的相互作用特别令人感兴趣。需要测量的响应变量包括固氮率、蓝藻和异形胞的丰度、浮游动物群落的组成、对Molimdium、Sultan和Fe的同化以及营养物质、Molibdium和Fe的浓度。最近发表的一种方法，使用奥辛使铁的不稳定形式复杂化，将被用来估计浮游植物对铁的利用率。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洋的浮游生物之外的因素，从而有助于造成这些系统中净初级生产量的氮限制。这项研究还应有助于更好地了解湖泊中浮游生物固氮的控制，从而更好地理解即使在接受低氮磷营养负荷的湖泊中也维持磷限制。因此，这项研究涉及到控制河口和湖泊富营养化的管理策略的实际问题。

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.