In-situ assessment of the effects of periphyton on the growth of Vallisneria americana

Abstract Proliferation of nuisance algae in aquatic ecosystems is an increasingly common, global problem that can be attributed to a broad suite of anthropogenic activities. One particular concern is that algae can shade rooted vascular plants, decrease their productivity, and, in extreme cases, cause mortality and a loss of the ecological services they provide. The loss of macrophytes in Florida’s iconic spring-fed systems is especially alarming. In the Chassahowitzka River, for example, increases in epiphytic algae correlated with reductions in the abundance of important macrophytes such as Vallisneria americana. However, the relationships between loads of epiphytic algae and associated periphyton and the growth of vascular plants have not been characterized adequately. Here, we quantified the effect of epiphytic algae and associated periphyton using direct, in-situ measurements of growth rates for V. americana. Periphytic loads ranged from 11 to 15,211 mg DM per shoot (0.08 to 32.37 mg DM cm−2 of leaf), and weekly growth rates ranged from 0 to 197 mg DM or 0 to 87 cm2 per shoot (0 to 94 mg DM or 0 to 39 cm2 per leaf). Data indicated that changes in the growth rates of V. americana were the consequences of combined stresses from environmental factors (e.g., light at the water’s surface and periphytic loads) and physiological factors (e.g., leaf age). Life spans were shorter and turnover rates were faster only when shoots were exposed to higher periphytic loads throughout their lives. Boundary analyses of data for growth of leaves identified a threshold of 4–5 mg DM cm−2 of leaf in full sunlight, with heavier periphytic loads considered detrimental, and that threshold indicated that 20%–26% of incident, full sunlight was the minimum light requirement for V. americana in the Chassahowitzka River. A threshold for periphytic loads represents a valuable indicator that complements data on water quality, and the method to calculate a threshold provides a valuable tool that can improve management of all aquatic systems where the effects of periphyton on macrophytes are of concern.