In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria exploit the abundant phosphate and fix nitrogen after other phytoplankton taxa have consumed the available nitrate. However, it remains less clear, which mechanisms favour diazotrophic cyanobacteria under more heavily stratified conditions with lower levels of excess phosphate and persistent nitrate-depletion. Here, we collected profiles of physicochemical parameters, nutrients and photo-pigments, as well as the medium to large-sized phytoplankton community during two lake-wide cruises to elucidate to what extend the abundance of diazotrophic cyanobacteria in Lake Tanganyika may be controlled by the nitrate resupply through the thermocline into the euphotic zone. At stations where nitrate was depleted but phosphate remained available near the surface, high densities of diazotrophic cyanobacteria were associated with a low nitrate supply to surface waters. Our data provide first support for two conceptual scenarios, where the relative position of the thermocline and the euphotic depth may create a functional niche for diazotrophic cyanobacteria: when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline, diazotrophic cyanobacteria, comprising Dolichospermum and Anabaenopsis, are key players in the medium- to large-sized phytoplankton community. By contrast, a thermocline located within the euphotic zone allows for the rapid vertical transport of nitrate for a thriving nitrate assimilating phytoplankton community that evidently outcompetes diazotrophic cyanobacteria. This study highlights that, under nitrogen-depleted conditions, diazotrophic cyanobacteria can also grow in response to a reduced nutrient resupply to the productive surface waters.

中文翻译：

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固氮蓝细菌与坦噶尼喀湖地表水的低硝酸盐补给有关

在坦噶尼喀湖，当上层水体经过一段时间的上升流和对流混合后重新分层时，就会出现大量固氮（固氮）蓝藻。在这个季节性过渡期间，固氮蓝细菌利用丰富的磷酸盐并在其他浮游植物类群消耗了可用的硝酸盐后固氮。然而，尚不清楚哪些机制有利于固氮蓝藻在更严重分层的条件下，过量磷酸盐和持续的硝酸盐消耗水平较低。在这里，我们收集了物理化学参数、营养素和光色素的概况，以及两次全湖航行期间的中型到大型浮游植物群落，以阐明坦噶尼喀湖中固氮蓝藻的丰度可能受到硝酸盐通过温跃层进入富光区的再供应的控制。在硝酸盐耗尽但地表附近仍有磷酸盐的站点，固氮蓝藻的高密度与地表水的硝酸盐供应量低有关。我们的数据首先为两个概念性情景提供了支持，其中温跃层的相对位置和富光深度可能为固氮蓝藻创造一个功能性生态位：当硝酸盐向上输送到富光区时，下方的温跃层减少了固氮蓝藻，包括 Dolicospermum 和 Anabaenopsis，是大中型浮游植物群落的主要参与者。相比之下，位于富光带内的温跃层允许硝酸盐快速垂直运输，以促进繁荣的硝酸盐同化浮游植物群落，这显然胜过固氮蓝细菌。这项研究强调，在氮耗尽的条件下，固氮蓝细菌也可以随着对生产性地表水的养分再供给减少而生长。