Shifts in limiting nutrients in an estuary caused by mixing and biological activity

Abstract

Simple general equations are used to show that when riverine water mixes with seawater, the N : P ratio is a nonlinear function of salinity. Thus, a shift in the limiting-nutrient status at a particular salinity along a continuum is a mathematical certainty, albeit complicated by biological consumption and remineralization. The salinity where the shift occurs is a function of biological nutrient uptake ratio. Four general modes, each with a corresponding formula, are deduced here to explain the factors that govern changes in nutrient ratios vs. salinity. These formulas help to explain why changes in the seasonal nutrient limitation status in some estuaries are in some cases triggered by variations in nutrient loading, and in other cases by mixing.Limiting nutrients and nutrient budgets in aquatic systems are of wide interest because of their fundamental importance in understanding carbon fixation, coastal eutrophication, and waste management (Kelly 1998;Howarth and Marino 2006;Smith 2006). In estuaries, it is still difficult to conclude whether N or P of riverine loadings should be controlled to reduce the eutrophication and the resulting hypoxia in marine ecosystems (Howarth and Marino 2006;Conley et al. 2009). This difficulty may arise because of a general phenomenon that in rivers with relatively high dissolved inorganic nitrogen (DIN 5 [NO4 ]) molar ratios (DIN : SRP ratios), these ratios decrease as salinity increases (Fisher et al. 1992;Doering et al. 1995;Yin et al. 2004). A decrease in DIN : SRP ratios, to a certain extent, is generally consistent with the results of bioassays that there is a shift from P to N limitation (Doering et al. 1995;Yin et al. 2001;Smith 2006). Because surface seawater contains low nutrients, most previous studies have focused on changes in riverine nutrient loadings or additional sources of P in estuaries to explain the nonlinear decrease of N : P ratios across the salinity gradient (Fisher et al. 1992;Yin et al. 2004;Harrison et al. 2008).The switch from P-limited riverine water to N limitation with increasing salinity is expected, because surface seawater generally has a DIN : SRP ratio of # 3 (Fanning 1992), although in the Mediterranean Sea, DIN : SRP ratios may exceed the Redfield ratio of 16 in winter (Krom et al. 1991;Kress and Herut 2001). Previous studies have implied that a sudden shift in the limiting nutrient across estuaries is a salinity-related phenomenon (Caraco 1988;Doering et al. 1995), but whether this shift is mainly because of changing riverine loadings, increasing salinity stress, additional sources of phosphorus, or just simply through mixing remains unclear. The linear relationship between a conservative element and salinity during freshwater and seawater mixing is widely used to study biogeochemistry in estuaries and oceans. Although nutrient ratios also are widely used to determine the limiting nutrient in aquatic environments, changes in the nutrient ratio through freshwater and seawater mixing remain unstudied. Thus, whether the changes in nu...

Distribution characteristics of transparent exopolymer particles in the Pearl River estuary, China

“…Residence time of water mass was tightly related to bacterial degradation rate that finally controlled the export of TEP [ Mari et al , 2007]. Moreover, the physical mixing could lead to change of biogeochemical cycles in estuarine waters [ Harrison et al , 2008; Kasai et al , 2010; Lui and Chen , 2011]. It was suggested that mixing process was the main factor influencing p CO 2 in the mid‐estuary of the Pearl River [ Guo et al , 2009].…”

Geochemical forms and seasonal variations of phosphorus in surface sediments of the East China Sea shelf

“…Sediment is an important indicator of marine environmental changes (Meyers, 2003;Schelske et al, 1988Schelske et al, , 2006Yu et al, 2013). Phosphorus (P), an important biogenic element, plays an important role in the biological productivity in marine environments (Benitez-Nelson, 2000;Song, 2010;Lui and Chen, 2011;Yu et al, 2012;Samadi-Maybodi et al, 2013;Zhuang et al, 2014). Its cycle is markedly affected by processes taking place in the sediments and at the water-sediment interface (Fisher et al, 1982;Ruttenberg and Berner, 1993;Van Raaphorst and Kloosterhuis, 1994;Föllmi, 1996).…”

Geochemical characteristics of phosphorus in surface sediments of two major Chinese mariculture areas: The Laizhou Bay and the coastal waters of the Zhangzi Island

“…As an important biogenic element in the ocean, phosphorus (P) directly affects the primary productivity of the ocean and has a significant impact on the marine carbon cycle (Lui and Chen, 2011;Yu et al, 2012); P is also directly related to the global climate and environmental changes (Krom et al, 2004). Hecky and Kilham (1988) pointed out that P might be the element which controlled the global marine biological productivity during a long geologic time scale (>1000 years).…”

“…The average contents of P in the LB were in the order: De-P > OP > Ca-P > Fe-P > Ads-P > Al-P; in the ZI, the corresponding order was De-P > OP > Fe-P > Ca-P > Ads-P > Al-P. Due to the high nutrient loadings from the surrounding rivers, TP contents in sediments of the LB were higher than in those of the ZI. The potential bio-available P (Ads-P and OP) accounted for 14.7% and 24.2% of TP in sediments of the LB and the ZI, respectively.Ó 2014 Elsevier Ltd. All rights reserved.As an important biogenic element in the ocean, phosphorus (P) directly affects the primary productivity of the ocean and has a significant impact on the marine carbon cycle (Lui and Chen, 2011;Yu et al, 2012); P is also directly related to the global climate and environmental changes (Krom et al, 2004). Hecky and Kilham (1988) pointed out that P might be the element which controlled the global marine biological productivity during a long geologic time scale (>1000 years).…”