Introduction
Submerged macrophytes play vital roles in shallow aquatic ecosystems as they provide multiple key functions and ecosystem services, such as maintaining a clear water phase, providing habitat and shelter, producing food for animals, and by facilitating biodiversity (Carpenter & Lodge, 1986; O’Hare et al., 2017; Thomaz, 2021). Unfortunately, due to increased anthropogenic activities, the abundance and diversity of submerged macrophytes has declined in many shallow freshwater ecosystems worldwide (Sand-Jensen et al., 2000; Y. Zhang et al., 2017). Loss of the submerged macrophyte vegetation also means the loss of a natural feedback loop that facilitates a clear-water phase, and may result in the shift into an alternative stable state typically consisting of phytoplankton dominance and thus more turbid conditions. Consecutively, many functions and ecosystem services of aquatic ecosystems may be lost associated to this alternative turbid stable state (Sabine Hilt et al., 2017; Janssen et al., 2021; Phillips et al., 2016).
The recovery of a submerged macrophyte vegetation is one of the most important goals of lake ecological restoration (S. Hilt et al., 2018). During natural conditions, the recovery of submerged macrophytes from the turbid state should undergo different stages that may take several decades to complete (S. Hilt et al., 2018). The re-occurrence of a spring clear-water phase that can be exploited by a few macrophyte species (mainly pondweeds), after which a more diverse and abundant submerged macrophyte community can establish that stabilizes a clear-water state for the rest of growing season. In recent years, more and more knowledge has been gathered on directly transplanting macrophytes in deteriorated water bodies to speed up re-establishment and recovery of a macrophyte community, which may instead be achieved within one or two years (Y. Li et al., 2021; Z. Liu et al., 2018). However, there is little to no knowledge on which factors govern the success of re-establishment of submerged macrophytes during and after restoration projects relying on transplantation of plants. Several abiotic factors may affect the growth of submerged macrophytes, such as light, temperature and nutrients (Bornette & Puijalon, 2011). In addition, several biotic factors such as herbivory, bioturbation and effects of trophic cascades, may reduce the growth of submerged macrophytes (Elisabeth S. Bakker et al., 2016; S. Hilt et al., 2018; Phillips et al., 2016; Zhi et al., 2020).
The underwater light environment is considered to be one of the most decisive factors affecting the distribution and growth of submerged plants (Middelboe & Markager, 1997). Shading by for instance suspended particulate matter, dissolved organic matter and algae (phytoplankton and periphyton), may negatively affect the germination (Going et al., 2008; Havens et al., 2004) and growth (Bornette & Puijalon, 2011; Philbrick & Les, 1993) of submerged plants. The growth of submerged macrophytes is also (in)directly affected by nutrient loading (nitrogen, N and phosphorus, P). Generally, moderate nutrient loading would enhance the growth of macrophytes (E. S. Bakker et al., 2010; Cronin & Lodge, 2003; Ozimek et al., 1993). During eutrophic conditions phytoplankton and periphyton growth is promoted, which can substantially reduce the growth of submerged macrophytes via increased light competition (P. Zhang et al., 2020). Furthermore, changes in light availability and nutrient loading may modify plant nutrient content, the carbon (C):nutrient [nitrogen (N) and phosphorus (P)] composition, and growth rates (Gu et al., 2018; Velthuis et al., 2017; P. Zhang et al., 2020). However, during winter, when temperatures are low, algae growth may be temperature limited (Edwards et al., 2016), and eutrophication may thus have less impact during these early stages of submerged macrophyte establishment.
Herbivory is also considered an important biological factor affecting the growth of submerged plants (Elisabeth S. Bakker et al., 2016). Previous studies show potential for complex relationships between snails, periphyton and macrophytes (Bronmark, 1989; Jones et al., 1999; Underwood et al., 1992). Generally, periphyton are more palatable and the preferred food for snails over macrophytes (Bronmark, 1989; Guo et al., 2021; Koleszár et al., 2021; Mormul et al., 2010). At low density, snails can promote submerged macrophyte growth by removing periphyton from the surface of macrophytes and thus reducing the competition for light and nutrients with algae (Cao et al., 2014; Jones & Sayer, 2003; Underwood et al., 1992). At high densities, however, snails can inhibit macrophyte growth directly by grazing on the plant (Bronmark, 1990; K. Y. Li, Liu, & Gu, 2009; Zhi et al., 2020).
Herbivory may also interact with nutrient loading to affect macrophyte and algal growth. During increased nutrient loading, the quality, namely lower C:nutrient ratios, may make primary producers more palatable to herbivores (Elisabeth S. Bakker et al., 2016; Y. Liu et al., 2021). Similarly, herbivory may also interact with light, as reduced light may lower C:nutrient ratios of plants and algae also increasing palatability and nutritional quality of plants (Cronin & Lodge, 2003). Thus, during both increased nutrient loading and low light conditions effects of snails on macrophyte growth is expected to increase.
To test the effects of light, nutrient loading and snail herbivory, and their interaction, on the growth and stoichiometry of submerged macrophytes, we performed an outdoor mesocosm experiment from winter to early spring using the two common rooted submerged vascular aquatic plants Potamogeton crispus and Vallisneria spinulosa . We hypothesize that (1) Increased nutrient loading will promote periphyton growth, increasing competition for light, lowering growth of submerged macrophytes. (2) Low snail density will promote plant growth due to food preference on periphyton, lowering competition for light, and high snail biomass will inhibit plant growth under high light intensity because snails will start feeding on macrophytes as a result of food limitation.