Darkness at the break of noon: Phytoplankton production in the Lower Mississippi River

Clifford A. Ochs, Orathai Pongruktham and Paul V. Zimba

Limnol. Oceanogr., 58(2), 2013, 555-568 | DOI: 10.4319/lo.2013.58.2.0555

ABSTRACT: Temporal patterns in chlorophyll a (Chl a) concentration and phytoplankton production were examined in the main channel of the Lower Mississippi River at two locations separated by ∼ 420 river km, near Tunica, Mississippi (51 sample-days, 2006–2009), and near Le Tourneau, Mississippi (15 d, 2006–2007). Phytoplankton production was estimated based on laboratory-derived photosynthesis–irradiance profiles coupled to field measurements of the light regime and temperature. Chl a concentration varied between 3 mg m−3 and 24 mg m−3, generally increasing at both sites from spring to late summer. Mean annual Chl a flux past Tunica averaged 3.6 × 106 kg in 2006–2007. During the 15 month period of sampling at both sites, there was no difference in mean Chl a flux between sites. Despite the presence of viable phytoplankton capable of gross primary production (GPP), and annually recurrent summer increases in Chl a concentration, net primary production (NPP) evaluated over the mean cross-sectional mixing depths of the river was consistently negative at both locations. Severe light limitation constrained NPP. The maximum percentage of time that phytoplankton were in the photic zone was only 25% of a 12 h day (i.e., 3 h), and the mean water-column irradiance required for NPP = 0 (GPP = phytoplankton respiration) was consistently greater than available daytime irradiance. We propose that viable phytoplankton in the main flow of the channel are subsidized from hydrologically connected regions where the light regime is favorable to positive NPP, including lateral slackwater portions of the channel and floodplain backwaters.

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