Seasonal starch allocation of starry stonewort (Nitellopsis obtusa) growing in Lake Koronis, MN

Abstract

Starry stonewort (Nitellopsis obtusa) (Desvaux in Loiseleur) J. Groves is an invasive macro alga that can take over entire water columns and outcompete native species. Previous research has quantified seasonal life history and phenology but there is no research quantifying carbohydrate allocation. The current study utilized samples harvested from Lake Koronis, Minnesota, USA from 2020 and 2021 to quantify starch allocation patterns in N. obtusa. Starch was quantified using the amylase/amyloglucosidase method. Starch data were compared to light transmittance (%), pH and water temperature (°C) via mixed procedures models in SAS. Seasonal average low and high points of starch storage were observed to be June (3.3%) and April (9.9%) for thalli biomass then June (41.7%) and November (54.6%) for bulbils. In regard to allocation patterns, above ground thalli reallocates carbohydrate stores to the below ground rhizoids in May. Carbohydrates are then transported to bulbils from July through September as rhizoid starch decreases and the bulbil starch increases. Ultimately, the carbohydrates are converted to starch and stored in bulbils (21.0–73.7%). In its invaded range, bulbils are important for energy store, vegetative reproduction and spread, as well as temporal distribution. These bulbils spread easily, spread rapidly, can store a lot of energy (73% starch), and are a major concern for the invasion of N. obtusa. Water temperature and light availability were shown to negatively influence starch content of thalli. These findings suggest that in the invaded range, N. obtusa is capable of accumulating large amounts of starch later into the growing season. The energy reserves may give N. obtusa a competitive advantage over native species by allowing a longer growing season resulting in the production of more vegetative propagules and greater longevity in the environments it invades.