Ecological Modelling 396 (2019) 23–32 Contents lists available at ScienceDirect Ecological Modelling journal homepage: www.elsevier.com/locate/ecolmodel PCLake+: A process-based ecological model to assess the trophic state of T stratified and non-stratified freshwater lakes worldwide ⁎ Annette B.G. Janssena,b, , Sven Teurlincxb, Arthur H.W. Beusenc,d, Mark A.J. Huijbregtse, Jasmijn Rostd, Aafke M. Schipperd,e, Laura M.S. Seelenb,f, Wolf M. Mooijb,f, Jan H. Janseb,d a Water Systems and Global Change Group, Wageningen University & Research, PO Box 47, 6700 AA Wageningen, the Netherlands b Department of Aquatic Ecology, Netherlands Institute of Ecology (NIOO-KNAW), PO Box 50, 6700 AB Wageningen, the Netherlands c Department of Earth Sciences-Geosciences, Faculty of Geosciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, the Netherlands d PBL Netherlands Environmental Assessment Agency, P.O. Box 30314, 2500 GH The Hague, the Netherlands e Department of Environmental Science, Institute for Water and Wetland Research, Radboud University Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, the Netherlands f Department of Aquatic Ecology and Water Quality Management, Wageningen University, PO Box 47, 6700 AA Wageningen, the Netherlands ARTICLE INFO ABSTRACT Keywords: The lake ecosystem model PCLake is a process-based model that was developed to simulate water quality based Lake ecology on ecological interactions in shallow, non-stratifying lakes in the temperate climate zone. Here we present Food web model PCLake+, which extends the PCLake model to cover a wide range of freshwater lakes that differ in stratification Limnology regime and climate-related processes. To this end, the model was extended with a hypolimnion layer that can be Deep lake invoked and configured by forcing functions or by simple built-in empirical relationships that impose stratifi- Shallow lake cation. Further adjustments to the original PCLake model have been made with respect to the calculation of 1) Thermal stratification light irradiation in the water column, 2) evaporation processes and 3) phenology of macrophytes. The simulation output of PCLake+ for different types of lakes complies well with generally accepted limnological knowledge, thus holding promise for future contributions to ecological theory and application to lakes around the globe. 1. Introduction needed for lake water quality assessment of lake ecosystems at a global scale (Mooij et al., 2019). Earth has over 117 million lakes with a surface larger than 2000 m2 A large diversity of lake ecosystem models to assess water quality (Verpoorter et al., 2014). These lakes are of major importance for exists, each with a level of complexity that matches its own purpose and ecosystem services like drinking water, food and human health (Paerl niche (Janssen et al., 2015). For instance, there are lake ecosystem and Otten, 2013). Environmental pressures such as eutrophication, models that contain only a few equations that have shaped ecological global warming, land use change and overexploitation affect lake eco- theory such as ‘minimal models’ on alternative stable states (Scheffer systems and endanger the provision of these ecosystem services and Van Nes, 2007). On the other side of the complexity spectrum lay (Raworth, 2017; Rockström et al., 2009). As a consequence of these frameworks that allow for a high level of detail and serve the purpose of pressures, algal blooms are becoming increasingly frequent and severe simulating spatial heterogeneity in lakes (e.g. FABM (Bruggeman and (Paerl and Huisman, 2009). Bolding, 2014), Delft3D-WAQ/ ECO (Los, 2009)). In between, ecolo- Until now, we often fail to predict and prevent algal blooms (Spears gical models with an intermediate level of complexity combine relative et al., 2016, 2017). Water quality assessment of lake ecosystems is short runtimes and limited data requirements of simple models with a critical in understanding the development of algal blooms and in ulti- level of complexity suitable to answer lake water quality related mately preventing them. Global assessment studies are of great im- questions. Examples are one-dimensional lake ecosystem models such portance to achieve this goal, particularly in regions where fresh water as AQUASIM (Omlin et al., 2001), MyLake (Saloranta and Andersen, is scarce. However, for many lakes there are no data available to assess 2007) and PCLake (Janse et al., 2010). These models are in an excellent their water quality in response to environmental change (Janssen et al., position to bridge theory with application and are also suitable for 2019; Messager et al., 2016). Moreover, data themselves are not pre- global assessment studies by coupling to global assessment models (e.g. dictive (Cuddington et al., 2013). Therefore, a modelling approach is global NEWS or IMAGE-GNM, Beusen et al., 2016; Janse et al., 2019; ⁎ Corresponding author at: Water Systems and Global Change Group, Wageningen University & Research, PO Box 47, 6700 AA Wageningen, the Netherlands. E-mail address: [email protected] (A.B.G. Janssen). https://doi.org/10.1016/j.ecolmodel.2019.01.006 Received 17 August 2018; Received in revised form 11 January 2019; Accepted 14 January 2019 Available online 01 February 2019 0304-3800/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). A.B.G. Janssen et al. Ecological Modelling 396 (2019) 23–32 Kroeze et al., 2016). com/pcmodel. DATM files can be automatically translated to various Having an intermediate level of complexity, PCLake is potentially programming languages including Grind for Matlab, R, ACSL, Deft3D, suitable for global studies on lake ecosystems. Compared with other FORTRAN and C++ (Mooij et al., 2014; Van Gerven et al., 2015). models with an intermediate level of complexity, PCLake includes key lake food web components (Janssen et al., 2015). Ecological feedbacks 2. Methods between these food web components, such as the interaction between macrophytes and algae, are pivotal to critical nutrient loadings at which 2.1. General setup of PCLake+ the ecosystem state shifts from a clear to turbid state or vice versa. As PCLake includes these key feedbacks within the food web, the model is As in the original PCLake, PCLake+ has been developed to simulate well-suited to sudying impacts of eutrophication and oligorophication average ecological dynamics of a lake. To this end, the model was ex- on lake ecology and water quality. For instance, the model has been tended with a hypolimnion layer that can be invoked and configured by used to assess critical nutrient loading of specific lakes (Hilt et al., 2018; forcing functions or by simple build-in empirical relationships that Janse et al., 2008; Janssen et al., 2017; Kong et al., 2016; Kuiper et al., impose stratification (see for methods below). In case of continuous 2017), to simulate the effect of climate warming on these critical nu- polymictic lakes, PCLake+ resembles the original PCLake model with a trient loadings (Mooij et al., 2007; Nielsen et al., 2014) and to assess the single fully mixed water column (Janse, 2005). In case of a stratifying stability of ecosystems to disturbances around the critical nutrient lake the model was configured according to the schematization in loadings (Gsell et al., 2016; Kuiper et al., 2015). The power of PCLake Fig. 2, with the water column being divided into an epilimnion and a lies in the coherence of the ecological process formulations, minimal hypolimnion separated at mixing depth. Both water layers have their overhead and run time and limited required input. Its strong focus on own state variables for nutrients, oxygen, three groups of algae, det- ecology provides an alternative approach to physically oriented lake ritus, inorganic matter and zooplankton. Planktivorous fish, benthi- models that have their basis in hydrodynamic processes and compre- vorous fish, predatory fish and macrophytes are physically notre- hensive energy balances in lakes. stricted by stratification, thus were each captured in a single biomass PCLake was originally developed to simulate shallow, non-strati- state variable for the entire water column. River inflow and outflow of fying lake ecosystems in the temperate climate zone. While applications water were connected to the epilimnion whereas infiltration and see- of PCLake to lakes in different climate zones have shown promising page processes were connected to the hypolimnion. Similar to con- results (e.g. Janssen et al., 2017; Kong et al., 2016; Li et al., 2019), the tinuous polymictic lakes, stratifying lakes may have a marsh zone with climate-related processes need improvement. For instance, the phe- emergent macrophytes, which was connected to the epilimnion. Below nology of macrophytes in PCLake is too rigid for global applications and we give a thorough description on the implementation of stratification the light module does not yet account for latitudinal differences in light and climate related processes. intensity. Additionally, the assumption of a fully mixed water column does not hold for many lakes due to thermal stratification (Wetzel, 2.2. Implementation of stratification 2001). Applying PCLake to stratifying lakes while assuming a mixed water column will lead to incorrect estimations of relevant lake char- The presence of stratification or mixing can be initialized bythe acteristics such as chlorophyll-a and nutrient values (Janssen, 2017). user (calcMixDepth