Ecological and Biogeochemical Interactions Constrain Planktonic Nitrogen Fixation in Estuaries
Ecological and Biogeochemical Interactions Constrain Planktonic Nitrogen Fixation in Estuaries
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DOI：
10.1007/s10021-002-0176-7
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发表时间：
2002-11
期刊：
影响因子：
3.7
通讯作者：
R. Marino;F. Chan;R. Howarth;M. Pace;G. Likens
中科院分区：
文献类型：
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作者：
R. Marino;F. Chan;R. Howarth;M. Pace;G. Likens
即使氮(N)强烈限制初级生产，许多类型的生态系统也很少或没有固氮作用。河口通常符合这种模式。与湖泊相反，在湖泊中，固氮蓝藻的大量繁殖通常足以缓解相对于磷(P)可用性的氮缺陷，而浮游固氮在大多数氮有限的河口中并不重要。在盐度超过8-10 ppt的河口很少观察到能够固定氮的异囊蓝藻，并且在北美这样的河口从未报道过水华。然而，当浮游动物数量保持在较低水平时，我们在河口中生态系统（盐度超过27ppt）中提供了允许异囊蓝藻生长和固定n2的条件。在河口种群密度下，大型浮游动物的放牧强烈地抑制了蓝藻种群和氮的固定。蓝藻的生长速度比在淡水中观察到的要慢，至少部分原因是硫酸盐（SO42−）的抑制作用，这种缓慢的生长速度增加了它们对放牧的脆弱性。我们得出的结论是，减缓蓝藻生长速度的生理（自下而上）因素和生态（自上而下）因素（如放牧）之间的相互作用可能是排除温带河口浮游氮固定的重要调节因子。
Many types of ecosystems have little or no N2fixation even when nitrogen (N) is strongly limiting to primary production. Estuaries generally fit this pattern. In contrast to lakes, where blooms of N2-fixing cyanobacteria are often sufficient to alleviate N deficits relative to phosphorus (P) availability, planktonic N2fixation is unimportant in most N-limited estuaries. Heterocystic cyanobacteria capable of N2fixation are seldom observed in estuaries where the salinity exceeds 8–10 ppt, and blooms have never been reported in such estuaries in North America. However, we provided conditions in estuarine mesocosms (salinity over 27 ppt) that allowed heterocystic cyanobacteria to grow and fix N2when zooplankton populations were kept low. Grazing by macrozooplankton at population densities encountered in estuaries strongly suppressed cyanobacterial populations and N2fixation. The cyanobacteria grew more slowly than observed in fresh waters, at least in part due to the inhibitory effect of sulfate (SO42−), and this slow rate of growth increased their vulnerability to grazing. We conclude that interactions between physiological (bottom–up) factors that slow the growth rate of cyanobacteria and ecological (top–down) factors such as grazing are likely to be important regulators excluding planktonic N2fixation from most Temperate Zone estuaries.