Variations in primary production of northern Gulf of Mexico continental shelf waters linked to nutrient inputs from the Mississippi River

Abstract

ABSTRACT. Increases in nutrient concentrations in the Mississippi River over the past 35 yr have led to speculation that primary production of organic carbon has been elevated as a result of increased nutrient fluxes that have occurred in the northern Gulf of Mexico coastal ecosystem. However, studies thus far have not provided direct demonstration of temporal relationships between measured primary production in continental shelf waters and nver-borne nutrient fluxes. This ~nvestlgation co~npared temporal variations in primary production with associated annual and interannual changes in riverborne nutrient inputs. Prlmary production In shelf waters near the river delta were found to b e signlflcantly correlated with nitrate (NO1-) + nitrite (NO? ) concentrations and fluxes over a 6 yr period from 1988 to 1994. Although light limitation was probably an important factor during winter months, a positive correlation was demonstrated between river inputs of NO,-+NO; and primary production for data collected from other times of the year. Peak nutrient inputs generally occurred in the spring. The magnitude of the riverborne NO? +NO, inputs averaged 106% of estimated nitrogen requirements for phytoplankton in the river-impacted region, considerably greater than in Amazon shelf waters, which have been less subject to anthropogenic nutrient increases. The possibility exists that further Increases In anthropogenic nutrients in the ~i s s i s s~p p iRiver could lead to h~g h e r and more widespread primary production, and this may intensify and extend the depletion of oxygen that has already been observed in the Louisiana shelf ecosystem. However, such a prediction is difficult because relationships between increasing nutnent inputs and pnmary production are unlikely to be linear, and a complete understanding of processes intermediate between primary production of organic matter and oxygen depletion in bottom waters on the Louisiana shelf is still lacking.

Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes

“…However, the nearly linear DIC-to-salinity relationship indicates that net biological production rate, at least in a per-volume base, is much lower in the Amazon plume than in the Mississippi and Changjiang plumes. For example, NCP rates estimated from within plume DIC removal were 0.9-1.6 gC m −2 d −1 for the Amazon (Ternon et al 2000) and 1-8 gC m −2 d −1 for the Mississippi plume (Lohrenz et al 1997;Lohrenz et al 1999;Guo et al 2012), whereas the mixed layer depth of the former is nearly 10 times that of the latter. This conclusion is consistent with a comparison of measured biological production rates (see Table 4 of Guo et al 2012).…”

A coupled physical-biological model of the Northern Gulf of Mexico shelf: model description, validation and analysis of phytoplankton variability

“…Lohrenz et al (1997) used data primarily from late spring and summer in their analysis; no data points for winter and only one early fall data point were included and their only measurement from early spring (March 1991) was excluded (otherwise the resulting relationship was not significant). Lehrter et al (2009) repeated the analysis after adding 7 more data points (all spring and summer observations) and again found a significant relationship, although with a much smaller R 2 =0.20 instead of R 2 =0.58 in Lohrenz et al (1997). The question now is: how can primary production, which is not limited by nutrients in this region, be correlated with nutrient load and concentration?…”

The stoichiometry of inorganic carbon and nutrient removal in the Mississippi River plume and adjacent continental shelf

“…Intense biological processes have been reported in large river plumes and can include high uptake rates of carbon (Ternon et al, 2000;Cai, 2003) and nutrients (DeMaster and Pope, 1996;Lohrenz et al, 1997). However, DIC and nutrient concentrations in plumes are also affected by temporally variable and spatially heterogeneous river-ocean mixing dynamics, which can complicate the interpretation of the biological signals (as opposed to those within spatially bounded estuaries, Gazeau et al, 2005).…”