Mounting evidence of global plastic pollution has prompted many studies of its potential effects on aquatic ecosystems. In particular, most research has focused on organismal responses to microplastics and the effects of microplastics when introduced as food. However, there has been far less research into the possible effects of microplastics on primary producers. In this review, we document the available evidence for possible effects from the literature and from a laboratory experiment using cyanobacteria and microplastic fragments. Our review shows that primary producer–microplastic interactions can alter algal photosynthesis, growth, gene expression, and colony size and morphology, possibly via adhesion and/or transfer of adsorbed pollutants from microplastics; and, these effects could be transferred up the food web, including to humans. We recommend that the effects of microplastics on primary producers be incorporated into microplastic research to better understand its full effects on freshwater and marine ecosystems.

Cyanobacterial blooms have substantial direct and indirect negative impacts on freshwater ecosystems including releasing toxins, blocking light needed by other organisms, and depleting oxygen. There is growing concern over the potential for climate change to promote cyanobacterial blooms, as the positive effects of increasing lake surface temperature on cyanobacterial growth are well documented in the literature; however, there is increasing evidence that cyanobacterial blooms are also being initiated and persisting in relatively cold‐water temperatures (< 15°C), including ice‐covered conditions. In this work, we provide evidence of freshwater cold‐water cyanobacterial blooms, review abiotic drivers and physiological adaptations leading to these blooms, offer a typology of these lesser‐studied cold‐water cyanobacterial blooms, and discuss their occurrence under changing climate conditions.

Microplastics are an emerging environmental pollutant, whose global ubiquity is becoming increasingly evident. Conventional wastewater treatment does not completely remove them, and there are growing concerns about microplastics in source water and post-treatment drinking water. Microplastics have been reported to alter the development, physiology, and behavior of various aquatic organisms; however, limited knowledge exists on their effect on natural phytoplankton communities. Many studies also use uniformly spherical plastic beads, while most scrub particles in consumer products and secondary microplastics in the environment have various shapes and sizes. We tested the effects of two types of microplastics, 50 µm polystyrene (PS) calibration beads and polylactic acid (PLA) plastic body wash scrub particles, and one type of plant-derived body wash scrub particle on a natural phytoplankton assemblage through a 7-day incubation experiment in a temperate, mesotrophic lake. The calibration beads and the plant-derived particles generally did not alter the taxonomic composition of the phytoplankton in the mesocosms, while the PLA body wash microplastics eliminated cryptophytes (p < 0.001) and increased chrysophytes (p = 0.041). Our findings demonstrate differential effects of irregularly shaped PLA body wash microplastics vs. PS calibration beads on lake phytoplankters and empirically support potential bottom-up alteration of the aquatic food web by secondary microplastics.

The limiting nutrient for freshwater phytoplankton productivity can vary within and between geographic regions. Understanding how local (i.e., lake and catchment) and regional (i.e., multiple catchment) factors shift the relative importance of nitrogen (N) and phosphorus (P) limitation presents a key research challenge for freshwater ecosystems and may enhance our understanding of how lakes could be managed to control eutrophication. Here, we used in situ microcosm incubations with factorial N and P amendments to determine the type of limitation in 16 lakes across the northeastern United States. Study lakes had similar climate but varied in geography, trophic status, and chemistry. Limitation by both N and P was most common (colimitation n = 5 and serial limitation n = 3); however, we also observed single nutrient N (n = 3) and P limitation (n = 4). The type of limitation was related to background phytoplankton biomass, longitude, and land use. The magnitude of response to combined nutrient enrichment was negatively related to background P concentrations. This multi-lake experiment suggests that both N and P often play a role in regulating productivity and that local and regional characteristics affect nutrient limitation patterns.