Deleterious interaction of light impairment and organic matter enrichment on Isoetes lacustris (Lycopodiophyta, Isoetales)

Abstract

Light reduction and increased organic matter load often occur together in lakes undergoing eutrophication. We aimed at experimentally testing the relative importance of light availability, organic matter enrichment and their interactions in the collapse of healthy populations of Isoetes lacustris. We conducted an in situ shading (65 and 35% of incident light) and organic matter enrichment (10% enrichment) experiment in a Pyrenean pond (NE Spain). We followed plant performance using growth indicators, carbon balance indicators and individual survival. Severe light reduction (35%) resulted in a lengthening of the new leaves and no effects on mortality. Organic matter enrichment resulted in lower production and increased individual mortality. The combination of both stresses resulted in stronger negative effects and the highest mortality rate, which helps explaining I. lacustris die-offs observed after damming. Under severe light reduction (35%), plants used corm starch to keep growing. Consequently, starch percentage decreased and sucrose percentage increased as it was transported to the growing leaves. The most extreme changes were observed under severe light reduction and fertilization, which extremely increased the ratio between sucrose and total non-structural carbohydrates (TNC). Thus, I. lacustris sucrose:TNC ratio is a good indicator of light reduction and organic matter enrichment stresses.