Structural changes of the microplankton community following a pulse of inorganic nitrogen in a eutrophic river

Abstract

Global change will increase the number and severity of extreme events resulting in strongly pulsed nutrient loading to rivers. Recent studies indicate the potential for rapid plankton shifts as short-term response to storm events and highlight the need for high-frequency methods to understand these complex processes. Here, we studied the effects of a strong short-term pulse of ammonium nitrate in a eutrophic temperate river that elevated nitrate-N concentrations from 3.1 to 10-55 mg L −1 . An intense phytoplankton bloom increased chlorophyll a concentrations from below 10 μg L −1 to over 140 μg L −1 despite of high background N concentrations and followed the N-pulse downstream. High-throughput sequencing revealed the temporal dynamics of the bacterial (16S rRNA gene) and microeukaryotic (18S rRNA gene) community throughout the event. Bacterial and microeukaryotic community richness was reduced, and community composition changed significantly during the bloom. Few microeukaryote taxa (e.g., Cyclotella meneghiniana, Chlamydomonas) dominated the bloom and replaced the rich summer phytoplankton community. Long-term monitoring data (2008-2016) using classical microscope analyses showed that seasonal shifts in phytoplankton community composition were linked to physical parameters and nutrient availability with P-limitation observed in spring but not in summer. We conclude that a disturbance in abiotic and biotic control factors, such as caused by a nutrient pulse, can induce structural shifts in the microplankton community leading to a phytoplankton bloom, even in eutrophic waters.

Spatial and temporal trends of Minnesota River phytoplankton and zooplankton

“…For phytoplankton, we observed seasonal trends similar to those reported downstream in the Mississippi River (Baker & Baker, 1981) with diatoms being dominant during spring, Cyanobacteria dominant during summer, and that other taxa (e.g., green algae, cryptophytes) are present but rarely dominant. Similar seasonal succession of phytoplankton community structure has also been reported for eutrophic Minnesota lakes (Heiskary, Hirsch, & Rantala, 2016), and numerous other studies have documented seasonal succession of phytoplankton communities in lotic systems (e.g., Kleinteich et al, 2020;Peterson & Stevenson, 1989;Salmaso & Braioni, 2008;Tavernini et al, 2011). For zooplankton, community structure differed during May and October compared with other months, and several taxa groups, including rotifers, cyclopoids, copepodites, and nauplii were notably most abundant during May.…”

“…Chlorophyll-a is a component of phytoplankton that is often measured as a surrogate for phytoplankton biomass (e.g., Basu & Pick, 1996), and we accordingly observed increases in Chl-a associated with increases in phytoplankton biovolume, particularly for Chlorophyta and Cyanobacteria. Similarly, silica concentration declines because of uptake by diatoms, and typically increases following diatom blooms (Kleinteich et al, 2020;Tavernini et al, 2011). The Minnesota River is a fertile hypereutrophic system (Dodds, Jones, & Welch, 1998), and similar to many medium to large rivers, we suspect is rarely nutrient-limited (Salmaso & Braioni, 2008;Wehr & Descy, 1998).…”

“…Total phosphorus concentrations in the Minnesota River varied from 105 to 396 μg/L during this study, likely exceeding concentrations that would limit phytoplankton growth. Water chemistry and nutrient availability have demonstrable influences on plankton communities in other lotic systems (e.g., Arora & Mehra, 2003;Kleinteich et al, 2020;Rossetti et al, 2009;Varol & Şen, 2018), but their influence on Minnesota River plankton communities is minimal at the scale of our analyses (e.g., taxa groups rather than species) and less important than the significant influences associated with seasonal succession, hydrologic condition, and dams.…”

“…Dams and impoundments can also disrupt spatial patterns and have significant influences on plankton community structure (Havel et al, 2009;Pourriot, Rougier, & Miquelis, 1997;Prygiel & Leitao, 1994). Temporal variability in plankton communities is often influenced by environmental characteristics that exhibit predictable seasonal patterns (e.g., temperature, photoperiod), but these patterns can be disrupted by flood-pulses (G orski, Collier, Duggan, Taylor, & Hamilton, 2013), extreme hydrologic conditions (e.g., droughts; Beaver et al, 2013), or anthropogenic disturbances (Kleinteich, Hilt, Hoppe, & Zarfl, 2020). Most of these influences on spatiotemporal dynamics are attributed to abiotic factors, but biotic interactions can also structure lotic plankton communities, and introduced populations of non-native planktivores have significant impacts on large river plankton communities with cascading impacts on higher trophic levels (Caraco et al, 1997;DeBoer et al, 2018;Pace, Findlay, & Fischer, 1998).…”

Episodic Nutrient Addition Affects Water Column Nutrient Processing Rates in River‐to‐Lake Transitional Zones

“…Surplus N can also be stored as phycocyanin proteins and/or cyanophycin polymers in cyanobacteria (Boussiba & Richmond, 1980;Oliver et al, 2012) and more passively in phytoplankton cellular pools (Pedersen & Borum, 1996), but without a ubiquitous mechanism of active storage, increased transient uptake may be more coupled to water column concentrations of N (Stolte & Riegman, 1995). Phytoplankton culture experiments have shown diversity and plasticity in nutrient uptake kinetics (Cáceres et al, 2019;Suttle et al, 1987) such that estuarine and nearshore phytoplankton assemblages can rapidly uptake nutrients and augment biomass from short duration inputs (Cottingham et al, 2004;Kleinteich et al, 2020;Pinckney et al, 1999). Moreover, like our finding of an increase in U TDN / Chl a • PAR , McCarthy et al (2007) observed that water column NH x uptake rates increased with stormflow inputs of NH x to a rivermouth and nearshore area of Lake Erie (Old Woman Creek).…”