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

Todd V Royer

doi:10.1002/lno.11300

Abstract

AbstractRatios 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.

Highlights

Nitrogen (N) and phosphorus (P) loading to inland and coastal waters relieves nutrient limitation and promotes algal growth
The N-indicator of coastal eutrophication potential (ICEP) values reported here using measured nutrient loads for the Mississippi basin agree with those estimated by Garnier et al (2010a) of 0–2 kg C km−2 d−1
The nutrient loads from the Mississippi basin drive formation of a seasonal hypoxic zone in the northern Gulf of Mexico that is among the largest in the world, often exceeding 10,000 km2 and occasionally exceeding 20,000 km2 (Matli et al 2018)

Summary

Introduction

Nitrogen (N) and phosphorus (P) loading to inland and coastal waters relieves nutrient limitation and promotes algal growth. Once DSi limits diatom production, nonsiliceous taxa may become dominant and the risk for cyanobacterial blooms is increased Others argue that increased N:Si ratios are the result of N runoff from agriculture and that reservoirs play a minor role (Downing et al 2016) In the latter case, regions of the Mississippi basin dominated by agriculture should disproportionately affect N:Si stoichiometry of nutrient loads received by the Gulf of Mexico. The ICEP can be calculated on the basis of N:Si (hereafter, N-ICEP) if N is the expected limiting nutrient for algal growth, or on the basis of P:Si if P is limiting (hereafter, P-ICEP) In both cases, values > 0 indicate excess N or P relative to DSi in the riverine load, with the value increasing in proportion to the stoichiometric imbalance. Values < 0 indicate DSi in excess of N or P and a low probability of blooms of nonsiliceous algae

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