Short-term laboratory assessment of elevated temperature effects on two aquatic plants

Elevated water temperature, a primary driver of global climate change, may modify the performance aquatic plants in freshwater systems. Despite growing interest, the influence of warming on the growth and physiological responses of submerged macrophytes in the presence of plant–plant interactions are still not well elucidated. Thus, a two-week experimental study was conducted to assess the effects of elevated temperature on the interaction among Myriophyllum spicatum and Ceratophyllum demersum (collected in the Mediterranean region), growing individually and in coexistence. We hypothesised that (1) plant performance will be enhanced at higher temperature in a short-term exposure, and (2) species-specific responses to warming would occur, particularly under coexistence. To test these hypotheses, plant growth (apical elongation and lateral branch production) and pigment composition (chlorophylls and carotenoids) at 22 °C and 27 °C were examined in controlled laboratory experiments incorporating no-interaction, intraspecific, and interspecific interaction treatments. The studied variables indicated that increased temperature had only a minor effect on plant performance. Even though the temperature was elevated by 5 °C, it remained at a tolerable level for both species and represented only a mild warming for them. However, we observed some species-specific responses depending on the type of interaction. M. spicatum performed better at higher temperature, while C. demersum seemed resistant to change under two different temperatures at least in short-term exposure. Also, the plants were more sensitive to increased temperature under intraspecific interaction. The two-week microcosm experiment effectively captured short-term responses under controlled conditions, which is valuable for initial, exploratory investigation.