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.