Submerged Macrophytes Can Maintain Stable Dominance Over Free-Floating Competitors Through High pH

Szabó, Sándor and Fedor, Noémi and Koleszár, Gergő and Braun, Mihály and Korponai, János and Kočić, Aleksandra and Hilt, Sabine and Oláh, Viktor (2024) Submerged Macrophytes Can Maintain Stable Dominance Over Free-Floating Competitors Through High pH. FRESHWATER BIOLOGY, 70 (1). No.-e14363. ISSN 0046-5070

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Abstract

1. Submerged and free-floating macrophytes are the two main vegetation types that can alternatively dominate in small shallow ponds and ditches. Submerged macrophytes provide more aquatic ecosystem services and cause fewer problems with anoxia and greenhouse gas emissions than free-floating plants, but are inferior competitors for light. High pH values have been proposed as a contributor to submerged plant dominance, but threshold values for pH-induced growth reduction of floating plants by submerged macrophytes are not known. 2. In this study, we conducted laboratory experiments to test whether submerged plants (Ceratophyllum demersum) can effectively limit the growth of free-floating competitors (Lemna gibba) by exceeding a critical threshold pH value. We used field data to test if and when such pH values can be achieved within dense submerged macrophyte stands. 3. The pH values that caused 50% reduction in fresh weight- and chlorophyll-based growth of L. gibba were 9.6 and 9.8, respectively, and the growth was negligible above pH 10.0. The photochemical efficiency of photosystem II and chlorophyll content of the floating plants declined rapidly above pH 9 and 9.5, respectively. Similarly, phosphorus concentration decreased and dry matter content increased sharply in L. gibba fronds above pH 9.5, respectively. We thus expect a critical pH threshold for L. gibba at around 9.5. 4. Our mesocosm and field data show that the critical pH threshold for L. gibba growth is regularly exceeded within dense stands of bicarbonate-using submerged macrophytes in lentic water-bodies. Such conditions can be prevalent in the upper water layer during most of the day (12 h) in summer. 5. The literature showed that many duckweed species (and other common free-floating plants) in temperate and tropical water bodies show growth inhibition above pH 8. Therefore, high pH created by submerged macrophytes that use HCO3− in photosynthesis can be a major mechanism by which dominance over free-floating competitors is maintained in densely vegetated lentic eutrophic water bodies. We propose that alternative stable states occur between free-floating and submerged macrophytes along a pH gradient. Active management of pH may, thus, help in maintaining the desired state.

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