Groundwater discharge gives periphyton a competitive advantage over macrophytes

Abstract

Lacustrine groundwater discharge (LGD) can significantly contribute to nutrient budgets of lakes. LGD usually occurs in shallow littoral zones, and thus, potentially affects periphyton and macrophytes. These benthic primary producers asymmetrically compete for resources, with periphyton and rooted macrophytes having an advantage in either light or sediment nutrient acquisition, respectively. LGD can advectively transport sediment phosphorus (P) into the water, supporting periphyton growth. We hypothesised that above a threshold level of sediment P concentrations, negative effects of periphyton shading exceed the positive effects of sediment P on macrophytes. We performed laboratory experiments comparing the growth of periphyton and a macrophyte (Stuckenia pectinata) at four different sediment P concentrations with and without simulated LGD. In LGD treatments, P mobilized from sediment promoted periphyton growth, and macrophyte growth rates negatively correlated with periphyton shading when experiments were performed with young plants. Macrophytes died when shading levels exceeded 75% of incident light. Our results confirm that LGD can significantly affect the competition between benthic primary producers at the expense of macrophytes. This process is supposed to be most relevant in oligotrophic water bodies or during periods of P deficiency in the open water, when advective transport of sediment P by LGD can prevent nutrient limitation in periphyton.