Stoichiometry of nitrogen, phosphorus, and silica loads in the Mississippi‐Atchafalaya River basin reveals spatial and temporal patterns in risk for cyanobacterial blooms

Abstract

Ratios of nitrogen (N), phosphorus (P), and dissolved silica (DSi) influence how algal communities respond to nutrient loading, and DSi limitation can facilitate cyanobacterial dominance. The indicator of coastal eutrophication potential (ICEP), described previously by other researchers, predicts production by diatoms vs. nonsiliceous taxa based on deviation of nutrient loads from the Redfield ratio of 106C:16 N:20Si (N‐ICEP) or 106C:1P:20Si (P‐ICEP). The ICEP was calculated for the Mississippi‐Atchafalaya River basin, and four subbasins: the Ohio‐Tennessee, Missouri, Upper Mississippi, and Arkansas‐Red basins from 1979 to 2015. The P‐ICEP indicated a stoichiometric imbalance that favored cyanobacteria for all but the Arkansas‐Red subbasin. The N‐ICEP indicated conditions favorable for cyanobacteria in the Upper Mississippi, Ohio‐Tennessee, and the northern Gulf of Mexico. Agriculture is the predominant land use in the Upper Mississippi and Ohio‐Tennessee subbasins and these subbasins controlled the stoichiometry of the nutrients delivered to the northern Gulf of Mexico. The imbalance in N, P, and DSi inputs to the Gulf was greatest during spring and early summer, and in most years transitioned to favoring diatoms by August or September. Comparing the 1980–1994 and 2001–2015 periods, there was a significant increase in the P‐ICEP for the Upper Mississippi, Ohio‐Tennessee, and Missouri subbasins that appeared to arise mainly from increased P loading to surface waters in the those basins. The ICEP revealed patterns in stoichiometry of N, P, and DSi loads among the major tributaries to the Mississippi River, and an increasing risk of cyanobacterial blooms for inland waters in much of the Mississippi‐Atchafalaya River basin.

Exploring Silica Stoichiometry on a Large Floodplain Riverscape

“…Although the complex hydrology of the UMRS may differentially influence the loads of DSi, TN, and TP (Smits et al, 2019), we only considered concentrations in our analyses because of the difficulty in calculating loads across diverse hydrogeomorphic areas. Our results agree with recent work by Royer (2019), which shows that although the UMRS represents 15% of the total Mississippi-Atchafalaya River Basin, it explains the majority (57%) of the variation in N loading at the Gulf of Mexico. This highlights the importance of studying Si:TN:TP in the UMRS, as this stoichiometric signal is reflected in the Si stoichiometry entering the Gulf of Mexico.…”

“…We can also attribute some of the longitudinal changes in DSi to downstream increases in CHL, temperature, and organic suspended material (Table S5A and Figure 6), suggesting instream uptake of Si by aquatic plants and diatoms play a role in shaping Si stoichiometry (Johnson and Hagerty, 2008;Manier, 2014;Crawford et al, 2016). Our results confirm previous work that has shown higher loads of TN relative to DSi and TP to the Gulf of Mexico, creating a more Si and P-limited system for marine phytoplankton Royer, 2019).…”

“…Globally, this type of stoichiometric imbalance in N, P, and Si has been implicated in influencing toxic and harmful algal blooms in both fresh and marine systems, including the Baltic Sea, North Sea, Great Lakes, and the California Current Ecosystem (Schelske and Stoermer, 1971;Anderson et al, 2009;Garnier et al, 2010;Davidson et al, 2012;Bargu et al, 2016). For example, agricultural activities in the Mississippi River basin, which accounts for ∼40% of the land area in the continental United States, have altered nutrient ratios of the Mississippi River and its exports to the Gulf of Mexico over the last several decades (Turner et al, 1998;Rabalais et al, 2002;Royer, 2019). These changes in stoichiometry are likely to compound already severe environmental stressors, such as massive hypoxic dead zones in the Gulf of Mexico (NOAA; Rabalais et al, 2002) caused by increases in N and P loads.…”

Global Dam‐Driven Changes to Riverine N:P:Si Ratios Delivered to the Coastal Ocean

“…Human activities have altered the absolute and relative magnitudes of riverine nutrient fluxes. Changes in riverine nutrient stoichiometry may alter the structure and functioning of downstream coastal ecosystems by modifying which macronutrient element(s) controls primary productivity (Royer, 2020). Nitrogen (N) and phosphorus (P) loads to coastal zones have at least doubled since preindustrial times (Compton et al, 2000; Filippelli, 2008; Galloway et al, 2004; Vitousek et al, 1997), largely due to enhanced fertilizer application and wastewater discharge to rivers.…”

Decoupling of silica, nitrogen and phosphorus cycling in a meromictic subalpine lake (Lake Iseo, Italy)

“…Nutrient abundance and stoichiometry in aquatic ecosystems are affected by hydrology and biogeochemical processes in the aquatic system itself, and by river runoff, which undergoes wide variations and imbalances due to the soil and its use in the watershed (Elser and Hamilton 2007;Hillebrand et al 2014). In recent decades, human activity has significantly modified the transport and storage of Si, N and P across heavily exploited watersheds by changing processes which control terrestrial inputs from natural to human-managed, including untreated sewage, partially treated wastewater, agricultural soil runoff and atmospheric deposition (Harrison et al 2009;Baker et al 2014;Serediak et al 2014;Carey and Fulweiler 2016;Maranger et al 2018;Royer 2020). Furthermore, climatic changes together with altered watershed hydro-geomorphology could increase the quantity of nutrients delivered to aquatic environments, thus changing their relative proportions (Strayer and Findlay 2010;Baron et al 2013;Barone et al 2019;Pilotti et al 2021).…”