Spatial heterogeneity and biological productivity in a large river ecosystem

我的研究项目的长期目标是模拟圣劳伦斯河（SLR）中物理和化学因素与下层食物网过程之间的相互作用。确定支持食物网的主要碳来源是了解水生生态系统结构和功能的基本步骤。一些研究认为陆地碳和异养在河流食物网中起主导作用，而另一些研究则强调原位光合作用源的更大重要性。在这两种情况下，河流中被运输的物质被光或细菌活动转化，从而影响其生物利用度并流向下游消费者。本研究旨在了解物质（如营养物质、浮游植物）和生物在栖息地（连通性）之间的转移如何控制食物供应和生物多样性，这对于确定大型河流生态系统中支持食物网和生产力的碳源非常重要。圣皮埃尔湖（LSP）是SLR最大的河流湖（约400平方公里），以其高生产力而闻名。然而，关于离开LSP进入河流河口（与河口过渡带相连）的物质的命运，目前还存在很大的空白。ETZ是光合作用的光限制区，标志着淡水与咸水的交汇处。它以其非常高的生产力而闻名，并且是几种鱼类的大型苗圃。我假设ETZ的大部分生产力依赖于河流河口的运输物质，而河流河口反过来又从LSP接收高溶解有机质和营养物质。我最近发现，在春季的破冰期，LSP可以通过大量冰藻漂移对下游生产力做出显著贡献。这一发现为理解大河的水生生产力开辟了一个全新的视角。NSERC关于SLR生态系统的项目包括：1)ETZ是大型河流的重要栖息地，通过碳和沉积物的交换充当通往海洋的门户；2)北温带河流的冬季生物活动，其中河流冰生物群可能对年初级生产力和沿食物网的碳转移有重大贡献。

The long-term objective of my research program is to model interactions between physical and chemical factors and lower food web processes in the St. Lawrence River (SLR). Identifying the primary sources of carbon supporting food webs is a fundamental step towards understanding the structure and function of aquatic ecosystems. Some studies have argued for a dominant role of terrestrial carbon and heterotrophy in river food webs while others have stressed the greater importance of in situ photosynthetic sources. In either case, the transported matter in the river is transformed by light or bacterial activity which affects its bioavailability and flow to downstream consumers. This study aims at understanding how the transfer of matter (e.g. nutrients, phytoplankton) and organisms between habitats (connectivity) is controlling food availability and biodiversity, important for determining carbon sources supporting food webs and productivity in large river ecosystems. Lac St. Pierre (LSP) is the largest fluvial lake (~400 km2) of the SLR and is well recognized for its high productivity. However, there is a large gap in knowledge about the fate of matter leaving LSP into the fluvial estuary, which connects to the estuarine transition zone (ETZ). The ETZ is light-limiting for photosynthesis and marks where freshwater meets saltwater. It is recognized for its very high productivity, and as a large nursery area for several fish species. I hypothesize that much of the productivity in the ETZ is dependent on transported material from the fluvial estuary, which in turn receives high dissolved organic matter and nutrients from LSP. I recently established that LSP can contribute significantly to downstream productivity through large populations of ice-algae drifting during the ice-break period in spring. This finding opens a totally new perspective for understanding aquatic productivity in large rivers. This NSERC program on the SLR ecosystem integrates: 1) ETZ as a key habitat of large rivers, acting as a gateway to the sea through the exchanges of carbon and sediments; and 2) winter biological activities in north-temperate rivers, in which river ice biota may substantially contribute to annual primary productivity and carbon transfer along the food web.