## 1 Introduction  
Algal biomass is widely recognized as a key feedstock for a future biobased economy and listed as a main sector of biomass supply in the EU's bioeconomy strategy [1, 2]. Due to their ability to sequester carbon dioxide, harvest light energy, and utilize municipal or agricultural wastewater, algae can become an integral part of a low-carbon circular bioeconomy [3]. In contrast to conventional crop plants, algae and especially microalgae yield higher productivities and consume less freshwater without competing for agricultural land [4; 5]. Nevertheless, feasible microalgal production processes have only been developed for few specialty chemicals, nutraceuticals or other high-value products [6; 7]. Despite these advantages, the main barrier to commercialization of microalgal commodities is the high costs of biomass feedstock production [8]. Among others, these comprise costs and energy consumption for water pumping, dewatering of dilute cultures as well as use of fertilizers for nutrient supply [9]. Therefore, low-cost open photobioreactor production systems are required that achieve high productivities and product concentrations [10; 11]. Additionally, the recycling of the used process water is necessary to recover residual nutrients, all while reducing costs for medium preparation and wastewater treatment [12; 13]. While raceway ponds are the most commonly employed open photobioreactors, they suffer from low biomass concentrations of up to 0.3-1.0 g L\({}^{-1}\) dry weight due to mutual shading and low flow turbulence in the high fluid layer depth of 15-30 cm [5; 14]. An alternate, though far less recognized, production system designed to overcome these drawbacks is the open thin-layer cascade (TLC) photobioreactor [11]. Originally developed in the 1960s in Czechoslovakia, this photobioreactor offers the advantage of a very high surface-to-volume ratio in a thin fluid layer of 5-8 mm, gravimetrically flowing down a sloped lane [15; 16]. Recently, the TLC has been redesigned and investigated indoors under a realistic physical reproduction of a southern Spain summer climate. Thereby, a maximum cell dry weight (CDW) concentration of 50 g L\({}^{-1}\) was achieved with the saline microalga strain _Microchioropsis salina_ (formerly _Nannochloropsis salina_) in a nutrient replete batch process [17], while _M. salina_ biomass with a maximum lipid content of 46% (w/w) and up to 6.6 g L\({}^{-1}\) of total lipids were produced at nutrient-limited conditions [18]. Recycling of growth medium in microalgae processes has mostly been studied in dilute cultures (<6 g CDW L\({}^{-1}\)) or cost intensive closed photobioreactors. Reported impacts of water recycling on algal cell growth and productivity appear to be divergent and system-dependent. In that context, water recycling in semi-continuous outdoor raceway pond cultivations was suggested to result in increased _Tetraselmis_ sp. biomass productivity due to mixotrophic growth on dissolved organic carbon (DOC) sources [19]. In contrast, no significant effect of water recycling on growth was observed with _Tisochrysis lutea_ and _Nannochloropsis_ sp. in a 220 L flat-panel photobioreactor [20], as well as with _M. salina_ in shake flasks [21]. Moreover, a decrease in culture performance after water recycling and nutrient replenishment was reported with _Nannochloropsis_ sp. in 120 L closed annular photobioreactors [22], as well as with _Scenedesmus_ sp. in shake flasks [23]. This decrease was attributed to the accumulation of cell debris and auto-inhibitory substances. In this respect, [PERSON] and [PERSON] [24] stated that growth of _Nannochloropsis_ sp.