Recent eutrophication and consequent hypoxia in the bottom waters of the Lower St. Lawrence Estuary: Micropaleontological and geochemical evidence

Thibodeau, Benoît; Vernal, Anne de; Mucci, Alfonso

doi:10.1016/j.margeo.2006.05.010

Cited by 89 publications

(81 citation statements)

References 50 publications

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“…Conditions in this fjord have deteriorated since the middle of the 19th Century, culminating in barren, partly laminated sediments deposited during the last 15-20 years. Similar trends were apparent in two cores from the Lower St. Lawrence Estuary, Canada (Thibodeau et al, 2006). Since ∼1960, the accumulation rate of dead benthic foraminiferal tests increased, together with the appearance and progressive increase in abundance of two buliminid species, Brizalina subaenariensis and Bulimina exilis.…”

Section: Foraminiferasupporting

confidence: 65%

Historical records of coastal eutrophication-induced hypoxia

Jorissen²,

et al. 2009

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Abstract. Under certain conditions, sediment cores from coastal settings subject to hypoxia can yield records of environmental changes over time scales ranging from decades to millennia, sometimes with a resolution of as little as a few years. A variety of biological and geochemical indicators (proxies) derived from such cores have been used to reconstruct the development of eutrophication and hypoxic conditions over time. Those based on (1) the preserved remains of benthic organisms (mainly foraminiferans and ostracods), (2) sedimentary features (e.g. laminations) and (3) sediment chemistry and mineralogy (e.g. presence of sulphides and redox-sensitive trace elements) reflect conditions at or close to the seafloor. Those based on (4) the preserved remains of planktonic organisms (mainly diatoms and dinoflagellates), (5) pigments and lipid biomarkers derived from prokaryotes and eukaryotes and (6) organic C, N and their stable isotope ratios reflect conditions in the water column. However, the interpretation of these indicators is not straightforward. A central difficulty concerns the fact that hypoxia is strongly correlated with, and often induced by, organic enrichment caused by eutrophication, making it difficult to separate the effects of these phenomena in sediment records. The problem is compounded by the enhanced preservation in anoxic and hypoxic sediments of organic microfossils and biomarkers indicating eutrophication. The use of hypoxiaCorrespondence to: A. J. Gooday (ang@noc.soton.ac.uk) specific proxies, such as the trace metals molybdenum and rhenium and the bacterial biomarker isorenieratene, together with multi-proxy approaches, may provide a way forward. All proxies of bottom-water hypoxia are basically qualitative; their quantification presents a major challenge to which there is currently no satisfactory solution. Finally, it is important to separate the effects of natural ecosystem variability from anthropogenic effects. Despite these problems, in the absence of historical data for dissolved oxygen concentrations, the analysis of sediment cores can provide plausible reconstructions of the temporal development of humaninduced hypoxia, and associated eutrophication, in vulnerable coastal environments.

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Section: Foraminiferasupporting

confidence: 65%

Historical records of coastal eutrophication-induced hypoxia

Jorissen²,

et al. 2009

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“…Downstream from Québec, tidal mixing of C, N, and P-rich fresh water with saltwater stimulates microbial production and induces the flocculation of terrigenous dissolved and colloidal organic matter. A significant fraction of the organic material brought from upstream eventually settles in the deep Laurentian channel (Annane et al 2015) where rising organic content of sediment and dropping O 2 concentrations have been reported (Gilbert et al 2005;Thibodeau et al 2006). In turn, mineralization of organic matter into CO 2 contributes to estuarine acidification (Mucci et al 2011).…”

Section: Discussionmentioning

confidence: 99%

“…Inflow of organic carbon or nutrients from terrestrial origins could thus partially account for increased O 2 consumption in deep estuarine areas, either directly (through bacterial degradation of organic matter) or indirectly (through stimulation of estuarine primary productivity) (Thibodeau et al 2006). Marine phytoplankton in temperate regions tend to be N-limited and suffer increasingly from cultural eutrophication as a result of N addition (Howarth et al 1996).…”

Section: Introductionmentioning

confidence: 99%

Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec, Canada)

et al. 2017

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Increased flux of carbon and nutrients from human activities in river basins were linked to acidification and deepwater hypoxia in estuaries and coastal areas worldwide. Annual loads (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011) of suspended particulate matter (SPM), dissolved organic carbon (DOC), total nitrogen (TN) and total phosphorus (TP) were assessed at the Lake Ontario inlet of the St. Lawrence River (SLR) (7110 m 3 s -1 ) and its estuarine outlet at Québec City (12,090 m 3 s -1 ). Internal loads from the Ottawa River (1950 m 3 s -1 ), seventeen other tributaries, urban wastewaters, atmospheric deposition and erosion were also estimated. Erosion (65% of SPM, 29% of TP), inflow from Lake Ontario (42% of DOC, 47% of TN) and Ottawa River (28% of DOC) contributed important flux to the estuary. Loads from other tributaries (20 and 27% of TN and TP at Quebec City) largely exceeded municipal sources (6% of exported TN and TP) and require future remediation. Aquatic plants fixed 277,000 t of C, 49,000 t of N and 7000 t of P (May-Sept.), delaying the nutrient flux to the estuary and turning the SLR into a nutrient sink over summers of lowest discharge. Degradation of exported organic C could consume 5.4-7.1 million t O 2 year -1 in the estuary whereas SLR flux of N and P represent 31-47% and 7-14% of total annual estuarine flux, respectively. Carbon and Nitrogen flux from freshwaters partly explain the decline in pH and oxygen concentrations in deep estuarine waters thus highlighting the need to reduce diffuse sources of nutrients in the entire watershed.

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“…The along-channel oxygen gradient from Cabot Strait to the LSLE has steepened over the past decades (Gilbert et al, 2005). Combined with geochemical (C org :N tot ratios and δ 13 C org of the sedimentary organic matter) and micropaleontological (dinocyst abundance and assemblages) evidence (Thibodeau et al, 2006), this suggests that the remainder of the oxygen decline may be caused by increased oxygen demand within the bottom waters and the sediments of the Laurentian Channel (Benoit et al, 2006;Lehmann et al, 2009). In this paper, we reconstruct variations in the pH of the bottom waters of the LSLE using historical data as well as hydrographic and stoichiometric relationships between organic matter respiration, oxygen consumption and carbonate chemistry.…”

mentioning

confidence: 99%

Acidification of Lower St. Lawrence Estuary Bottom Waters

et al. 2011

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Accumulation of metabolic CO 2 can acidify marine waters above and beyond the ongoing acidification of the ocean by anthropogenic CO 2 . The impact of respiration on carbonate chemistry and pH is most acute in hypoxic and anoxic basins, where metabolic CO 2 accumulates to high concentrations. The bottom waters of the Lower St. Lawrence Estuary (LSLE), where persistently severe hypoxia has developed over the last 80 years, is one such case. We have reconstructed the evolution of pH in the bottom waters from historical and recent data, and from first principles relating the stoichiometry of CO 2 produced to oxygen consumed during microbial degradation of organic matter. Based on the value of the atmospheric partial pressure of CO 2 that best reproduces the preformed dissolved inorganic carbon concentration in the bottom waters, we estimate the average ventilation age of the bottom waters to be 16 ± 3 years. The pH of the bottom waters has decreased by 0.2 to 0.3 over the last 75 years, which is four to six times greater than can be attributed to the uptake of anthropogenic CO 2 . The pH decrease is accompanied by a decline in the saturation state with respect to both calcite and aragonite. As of 2007, bottom waters in the LSLE are slightly supersaturated with respect to calcite (Ω c ≈ 1.06 ± 0.04) but are strongly undersaturated with respect to aragonite (Ω a ≈ 0.67 ± 0.03).RÉSUMÉ L'accumulation de CO 2 métabolique acidifie l'eau de mer au-delà de ce qui est généré par l'ajout de CO 2 anthropique. L'impact de la respiration sur la spéciation des espèces carbonatées en solution et du pH est plus marqué dans les bassins hypoxiques et anoxiques où le CO 2 métabolique atteint de fortes concentrations. Dans l'estuaire maritime du Saint-Laurent (EMSL), des eaux profondes sévèrement hypoxiques se sont développées depuis les derniers 80 ans. Nous avons reconstitué l'évolution du pH des eaux de fond à l'aide de mesures historiques et récentes ainsi que des principes de base qui décrivent la relation stoéchiométrique entre la consommation d'oxygène et la production de CO 2 métabolique issu de la dégradation microbienne de la matière organique. Selon la valeur de la pression partielle en CO 2 qui reproduit le plus précisément les concentrations de carbone inorganique dissous préformées dans les eaux profondes, nous avons estimé l'âge moyen de ces eaux à 16 ± 3 ans. Le pH des eaux profondes s'est abaissé de 0.2 à 0.3 depuis 75 ans, ce qui est de 4 à 6 fois plus important que ce qui est attribuable au transfert du CO 2 anthropique. La décroissance du pH est accompagnée par une diminution du degré de saturation par rapport à la calcite et l'aragonite. En 2007, les eaux profondes de l'EMSL demeurent légèrement sursaturées par rapport à la calcite (Ω c ≈ 1.06 ± 0.04) mais sont sévèrement sous-saturées par rapport à l'aragonite (Ω a ≈ 0.67 ± 0.03).

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Recent eutrophication and consequent hypoxia in the bottom waters of the Lower St. Lawrence Estuary: Micropaleontological and geochemical evidence

Cited by 89 publications

References 50 publications

Historical records of coastal eutrophication-induced hypoxia

Historical records of coastal eutrophication-induced hypoxia

Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec, Canada)

Acidification of Lower St. Lawrence Estuary Bottom Waters

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