N and P as ultimate and proximate limiting nutrients in the northern Gulf of Mexico: Implications for hypoxia reduction strategies

Abstract

Abstract. The occurrence of hypoxia in coastal oceans is a growing problem worldwide and clearly linked to anthropogenic nutrient inputs. While the need for reducing anthropogenic nutrient loads is generally accepted, it is costly and thus requires scientifically sound nutrient-reduction strategies. Issues under debate include the relative importance of nitrogen (N) and phosphorus (P), and the magnitude of reduction requirements.The largest anthropogenically induced hypoxic area in North American coastal waters (of 15,000+/-5,000 km Here we report the first systematic analysis of the effects of single and dual nutrient load reductions from a spatially explicit 10 physical-biogeochemical model for the northern Gulf of Mexico. The model has been shown previously to skillfully represent the processes important for hypoxic formation. Our analysis of an ensemble of simulations with stepwise reductions in N, P and N&P loads provides insight into the effects of both nutrients on primary production and hypoxia, and allows us to estimate what nutrient reductions would be required for single and dual nutrient reduction strategies to reach the hypoxia target. Our results show that, despite temporary P limitation, N is the ultimate limiting nutrient for primary production in this system. 15Nevertheless, a reduction in P load would reduce hypoxia because primary production in the region where density stratification is conducive to hypoxia development, but reduction in N load have a bigger effect. Our simulations show that, at present loads, the system is saturated with N, in the sense that the sensitivity of primary production and hypoxia to N load is much lower than it would be at lower N loads. We estimate that reduction of 63% +/-18% in total N load or 48% +/-21% in total N&P load are necessary to reach a hypoxic area of 5,000 km 2 , which is consistent with previous estimates from statistical regression models 20 and highly simplified mechanistic models.

Physical and Biogeochemical Controls on pH Dynamics in the Northern Gulf of Mexico During Summer Hypoxia

“…It has also been suggested that phytoplankton growth can be limited by phosphorus during the period of hypoxia formation (Quigg et al, ; Sylvan et al, ). By using a spatially explicit physical‐biogeochemical model, Fennel and Laurent () concluded that N and P as ultimate and proximate limiting nutrients in the nGOM, respectively. In our study, the stoichiometric ratios of the ΔDIN biol and ΔSRP biol in the two plumes (51.91 ± 3.51 and 56.6 ± 6.85; Figure c) were significantly higher than the Redfield N:P uptake ratio of 16 (potential P limitation).…”

“…The atmospheric CO 2 concentration is expected to continuously increase in the future, while the eutrophication state in the nGOM can be significantly affected by management of anthropogenic nutrient loads (Fennel & Laurent, ; Justic et al, ; Justić et al, ; Rabalais et al, ). In order to investigate the combined impact of atmospheric CO 2 increase and varying eutrophication conditions, we used the multiple end‐member mixing model to simulate the changes in pH and Ω Ar under two future scenarios relative to the present condition (atmospheric CO 2 = 405 ppm; the present eutrophication status was set as the 100% eutrophication condition).…”

Seasonal patterns in phytoplankton biomass across the northern and deep Gulf of Mexico: a numerical model study

“…The main differences between model and satellite chlorophyll are in the coastal region. Those differences can be explained (in part) by satellite chlorophyll overestimation, due to the high concentration of dissolved colored organic matter and sediments associated with river runoff (Hu et al, 2000;Del Castillo et al, 2001;Gilbes et al, 2002;D'Sa and Miller, 2003). The greatest chlorophyll concentration values are within the MS river delta, and the lowest values within the region influenced by the Loop Current.…”

“…The model does not include phosphate as a limiting nutrient for phytoplankton growth. Although previous studies have indicated the existence of phosphate limitation near the MS-A deltas during May-July (Sylvan et al 2006(Sylvan et al , 2007Laurent et al, 2012;Laurent and Fennel, 2014;Fennel and Laurent, 2017), we focus here on the role of N and Si, as observational studies suggest that N and Si can modulate phytoplankton production and composition across the northern GoM (Dortch and Whitledge, 1992;Nelson and Dortch, 1996;Lohrenz et al, 1997;Rabalais et al, 2002;Zhao and Quigg, 2014).…”

“…A key modeling aspect is the characterization of diatoms and nanophytoplankton growth. It is well known that (1) nanophytoplankton uptake nutrients more efficiently than diatoms; (2) diatoms can achieve greater growth rates than nanophytoplankton in nutrient-rich environments; and (3) diatoms require silicate as an additional nutrient for frustule formation (Litchman et al, 2006;Falkowski and Oliver, 2007). These differences should be considered when simulating phytoplankton responses to changes in nutrient availability.…”