Controlling factors of phytoplankton distribution in the river–lake transition zone of a large lake

Aquatic Sciences (2023) 85:37 https://doi.org/10.1007/s00027-023-00934-2 Aquatic Sciences RESEARCH ARTICLE Controlling factors of phytoplankton distribution in the river–lake transition zone of a large lake Gabriel Cotte1 · Frédéric Soulignac2,4 · Fabio dos Santos Correia3 · Matthieu Fallet1 · Bastiaan Willem Ibelings3 · David Andrew Barry2 · Torsten W. Vennemann1 Received: 29 December 2021 / Accepted: 10 January 2023 © The Author(s) 2023 Abstract River–lake transition zones have been identified as major drivers of phytoplankton growth. With climate change reducing the frequency of complete lake overturns, it is expected that the Rhône River, the main tributary to Lake Geneva (France/ Switzerland), will become the major source of nutrients for the lake euphotic zone. The river–lake transition zone was hence examined at the mouth of the Rhône River with the aim of understanding the complexities and controls of phytoplankton distribution in this specific deltaic ecosystem. Two field campaigns were carried out in which water samples were collected from longitudinal and transversal transects across the transition zone. These samples were analyzed for both nutrient and phytoplankton concentrations, while the fraction of Rhône River water in a lake sample was determined by the stable isotope composition of the water. The results indicate contributions in P and Si related to the Rhône intrusion into the lake. Furthermore, this river–lake transition zone appears to be a dynamic area that can locally present optimal conditions for phytoplankton growth. In early spring, a wind event broke the early and weak stratification of the lake, forcing the Rhône River and its turbidity plume to intrude deeper. Thus, this sharp drop of the turbidity within the euphotic zone allowed an increase in the phytoplankton biovolume of 44%. In early fall, outside of the turbid near field of the river mouth, the Rhône interflow, located just below the thermocline, promoted a local deep chlorophyll maximum. Keywords River intrusion · Nutrient input · Phytoplankton · Ecocline · ADCP measurements · Stable isotope tracing Introduction Large lakes of the world are habitats for diverse species and represent resources for humanity by providing many ecosystem services (Sterner et al. 2020). Yet, these ecosystems * Gabriel Cotte Frédéric Soulignac are experiencing rapid degradation as they are exposed to anthropogenic and climatic stressors (e.g., Jenny et al. 2020). One of the most common threats faced by large lakes is eutrophication (Richardson and Jørgensen 2013), characterized by recurrent algal blooms and deep-layer hypoxia 1 Institute of Earth Surface Dynamics (IDYST), University of Lausanne (UNIL), 1015 GéopolisLausanne, Switzerland 2 Ecological Engineering Laboratory (ECOL), Institute of Environmental Engineering (IIE), School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland 3 Department F.‑A. Forel for Environmental and Aquatic Sciences (DEFSE) and Institute for Environmental Sciences (ISE), University of Geneva (UNIGE), Uni Carl Vogt, 1205 Geneva, Switzerland 4 Present Address: Commission Internationale Pour la Protection des eaux du Léman, Route de Duillier 50, 1260 Nyon, Switzerland Fabio dos Santos Correia Matthieu Fallet Bastiaan Willem Ibelings David Andrew Barry Torsten W. Vennemann 13 Vol.:(0123456789) 37 Page 2 of 18 (Jenny et al. 2016). The abundance of phytoplankton within lakes has been recognized as an important component of their water quality management (Xu et al. 2001). However, understanding phytoplankton dynamics in large lakes poses many challenges, including taking into account their highly heterogeneous distribution (Ghadouani and Smith 2005; Viljanen et al. 2009; Leoni et al. 2014). Recently, several studies have been conducted to better understand the spatio-temporal heterogeneities of phytoplankton abundance in large lakes. Dynamics of algal growth can be explained by temporal and spatial variability in thermal stratification dynamics and internal wave motions (Yang et al. 2016; Soulignac et al. 2018). Furthermore, a higher abundance of phytoplankton has been reported around river inflow areas (Larson et al. 2013; Kiefer et al. 2015; Soomets et al. 2019). Another factor affecting the phytoplankton dynamics of lakes is climate change, which extends the growing season with an earlier onset of stratification and associated earlier algal blooms in spring (Anneville et al. 2018; Woolway et al. 2021). Moreover, climate warming is expected to reduce the frequency of complete lake overturns occurring at the end of winter in monomictic lakes, a process that brings bottom nutrients to the surface water (Perroud et al. 2009; Woolway and Merchant 2019). Consequently, it is expected that other sources of nutrient to the euphotic zone, such as nutrients coming from the watershed, will have a more pronounced impact on phytoplankton distribution (Anneville et al. 2013). It is then important to further evaluate the riverine inputs of the nutrients, their transport and their subsequent distribution in lakes, to help understand their metabolization and hence their role in the primary production of lakes. Chemical and biological gradients in the receiving lake are usually associated with river inflows (Schelske et al. 1980; Morrice et al. 2004; Makarewicz et al. 2012). These transition zones are defined as regions where hydrodynamic Fig. 1  Conceptual model of a negatively buoyant inflow entering a stratified lake. After plunging, the river inflow can intrude into the water column when it reaches the depth of neutral buoyancy and generate an interflow. The river mouth near field is defined as the area where the river current velocity is still measurable 13 G. Cotte et al. conditions transform from river-dominated flow to lake mixing processes (Thornton 1990). The differences in water density between the river and the receiving lake, together with the mixing processes, control the river intrusion pattern within the transition zone and determine the bioavailability of the river nutrients for phytoplankton (Rueda et al. 2007). On the one hand, if the river water is less dense than the lake water, the river water carrying the nutrients is transported as an overflow, and resources will be directly bioavailable in the surface layer. On the other hand, if the river water is denser than the lake water surface, as it is generally the case for the Rhône River in Lake Geneva, it will plunge and be distributed as an underflow. The river can flow down to the lakebed or can intrude into the water column when it reaches the depth of neutral buoyancy and generate an interflow (Fig. 1). Depending on this intrusion depth, the river nutrients can directly fuel algal growth if they are inserted within the euphotic zone, where the primary production takes place, or indirectly, if the intrusion occu (...truncated)