Dissolved inorganic nutrient concentrations in the surface waters (0 to 5 m) of the Northern Gulf of Mexico (NGoM) were analyzed from 1985 to 2019 (> 10,000 observations) to determine

spatiotemporal trends and their connection to nutrients supplied from the Mississippi/Atchafalaya River (MAR). In the NGoM, annual mean dissolved inorganic P (DIP) concentrations increased

significantly over time, while dissolved inorganic N (DIN) concentrations showed no temporal trend. With greater salinity, mean DIN:DIP decreased from above the Redfield ratio of 16 to below

it, reflecting DIN losses and the more conservative behavior of DIP with salinity. Over the same time period, annual mean P (total dissolved P, DIP, dissolved organic P) loading from the

increasing MAR DIP, pointing to temporal changes in other DIP sources. The increase in NGoM DIP suggests greater N limitation for phytoplankton, with implications for N fixation and nutrient

management.

Marine primary production is often mediated and limited by the bioavailability of dissolved nutrients such as nitrogen (N) and phosphorus (P)1,2,3. Studies have shown that N limitation of

marine primary production is more widespread than P limitation3,4, though P availability may play an important role over long time scales5,6, and in certain locations, such as the Northern

Gulf of Mexico7. Surface ocean N and P concentrations are spatially and temporally variable as a result of many complex processes such as uptake by phytoplankton and bacteria, including

luxury consumption8, N fixation9, N loss through denitrification in low oxygen regions10, biological and chemical conversion of inorganic and organic N and P11,12, legacy nutrients stored in

the landscape13,14, and external anthropogenic inputs15,16. These processes can lead to deviations in organic matter production and dissolved nutrient ratios from the canonical 106C:16N:1P

of Redfield proportions3,12,17. For example, rivers, estuaries, and coastal regions typically have higher dissolved inorganic N and P (DIN and DIP) concentrations and DIN:DIP than offshore

regions18, where average surface DIN:DIP is about 135. Meanwhile, in the majority (~ 78%) of the world’s large rivers, DIN:DIP exceeds 16, and increases with DIN concentrations19. These

spatial patterns could be explained by net relative losses of DIN5,20, and/or by a relative net gain of DIP from bioconversion of dissolved organic P (DOP) to DIP as salinity increases from

nearshore to offshore waters21,22.

In this study, we focus on the spatial and temporal variability of surface DIN and DIP concentrations over the last 35 years in the Northern Gulf of Mexico (NGoM) along the salinity gradient

from the Mississippi and Atchafalaya Rivers (MAR) to offshore oceanic waters. Annual mean MAR discharge is more than 15 times that of all other rivers that drain into the NGoM23, thus we

focus on annual mean MAR discharge as the main source of nutrients and freshwater into the NGoM system. The size, direction, and location of the NGoM freshwater plume change in concert with

varying volume and timing of river discharge as well as wind speed and direction, therefore further influencing nutrient and salinity patterns24,25,26. Riverine freshwater plumes generally

extend westward in the NGoM through the Louisiana Coastal Current, though wind forcing pushes buoyant plumes eastward depending on the time of year23,27. Physical drivers such as onshore

winds and salinity cause MAR plume waters and its nutrients, sediments, and organic matter to be transported westward alongshore and eastward along the approximately 200 m depth shelf

break27,28. The majority of riverine N and P are retained in nearshore regions of the NGoM in the fall and winter29, and spread offshore in the summer, though N typically declines more

dramatically as a function of salinity than P30,31. When averaged annually, the majority of surface water DIN (70%) is retained on the shelf, while 30% of DIN is transported further

offshore32. Together the MAR are the main sources of freshwater and nutrients into the NGoM, on average delivering 80% of the freshwater, 91% of the N loading, and 88% of the P loading into

the system with a combined mean flow of approximately 21,500 m3 s−133,34,35.

Over the last 200 years, many aspects of the MAR watershed have been altered by changing water demands, fluctuating sediment yields, navigational amendments, and flood-control systems36. The

MAR’s water quality and chemistry has been substantially impacted by changes in land use, agriculture, industry, and sewage effluent37,38. From the 1950s to 1990s, TDN loading (primarily

driven by increasing DIN loading) from the MAR to the NGoM tripled, and TDP loading doubled35,36. Since then, TDN loading has not appreciably increased, and has even stabilized in some

locations35,39. Earlier studies found no temporal trends in DIP or TDP from the 1970s to 1990s40. Temporal trends in MAR nutrient loading are similar to global trends in the latter part of

the twentieth century, though MAR N fluxes increased more, and P fluxes increased less, than the global average41.

Despite the increase in N loading from the MAR, empirical studies indicate a predominance of N limitation of phytoplankton in the NGoM11, and isotopic evidence indicates that the majority of

N incorporated into planktonic biomass in the NGoM originates from MAR loading35. Nevertheless, observations of P limitation have been reported, especially at intermediate salinities within

the MAR plume during spring and summer42,43,44. Multiple studies have investigated the connections between MAR flow and NGoM nutrient concentrations31,45,46,47. Lohrenz et al. (1999) found

a positive correlation between MAR river discharge and MAR N:P, and in their 1990 study concluded that riverine nutrient supply constraints were a controlling factor of biomass and

production at high salinities. Wysocki et al. (2006) further established that the spatial distribution of NGoM nutrients changed with MAR flow, with higher NGoM nutrient concentrations

observed further offshore during periods of higher discharge. However, Cardona et al. (2016) concluded that MAR discharge alone was insufficient to predict NGoM surface nutrient

concentrations, given low nutrient concentrations observed following high flow periods.

Additionally, MAR discharge and nutrient flux are tied to the spatial and temporal variability of the summer hypoxic area, or “dead zone” in the NGoM (characterized by dissolved oxygen

content of