Invasive freshwater macroalgae are rarely described. Montagnia macrospora is a freshwater red alga introduced from South America to East Asia via the global aquarium trade. The earliest occurrence record of this alga in Taiwan is dated 2005. To determine whether M. macrospora has become invasive in Taiwan and to understand the traits that facilitated its invasion, we took a multifaceted approach that combines examination of ecological background and population genetic analysis. Our island-wide survey showed that M. macrospora is widespread in the field across Taiwan, where the climate greatly differs from that of South America, and can self-sustain for nearly a decade. Our population genetic analysis revealed a lack of genetic diversity of M. macrospora in Taiwan, consistent with the hypothesis that the alga expanded through asexual reproduction. Moreover, during our long-term ecological assessments and field surveys, we observed thatM. macrospora is an ecological generalist that can survive in a wide range of temperature, pH, illumination, and nutrient enrichment. Taken together, our data suggest that M. macrospora has successfully invaded the freshwater ecosystems ofTaiwan, likely due to its ability to disperse asexually and to grow under broad environmental conditions. We hope that our study brings attention to invasive freshwater algae, which have been overlooked in conservation planning and management.

Nutrient_levels_location_Mmacrospora_presence

For nutrient measurements, water samples of 1,000 mL were collected and stored in an ice box during transportation. Then, measurements of four nutrients (ammonia, nitrate, nitrite, and phosphate, in mg L^-1) were taken in our laboratory using a SMARTSpectro spectrophotometer (LaMotte, Maryland, USA).

Taoyuan_stream_survey_data

Field surveys between August 2012 and October 2013 were carried out at four sites along a small spring-fed stream in Longtang District, Taoyuan City, Taiwan (24°53'09.3"N, 121°13'54.1"E), once a month. The four sites were 10 m apart from each other, going from downstream (site 1) to upstream (site 4). 

The percent cover of M. macrospora at each site was calculated as the average of the percent cover of five quadrats of the site positioned along a transect perpendicular to the streamflow at each site.

Water temperature, light intensity (µmol photons m^-2 s^-1), pH, conductivity (µS cm^-1), turbidity (mg L^-1), water velocity (m s^-1), and water depth (cm) were measured at each of the four sites by using a thermometer, a portable digital photometer (DL-204 EZDO, Chi-Jui Instrument Enterprise, Taiwan), a pH meter (PH30, CLEAN Instruments Co., Taiwan), a meter for both conductivity and turbidity (CON30, CLEAN Instruments Co., Taiwan), a flow probe (FP111 Global Water, Smart Scientific Corp., Taiwan), and a ruler, respectively. The levels of four nutrients (i.e., ammonia, nitrate, nitrite, and phosphate) were measured in our laboratory from 1,000 ml water samples using a SMARTSpectro spectrophotometer (LaMotte, Maryland, USA). Precipitation data at Zhongli automatic weather station, located in Zhongli District, Taoyuan City was retrieved from the Central Weather Bureau, Taiwan (https://www.cwb.gov.tw), and hours of daylight were retrieved from the National Oceanic and Atmospheric Administration (NOAA) (https://gml.noaa.gov/grad/solcalc/).